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NGC 5533: decoupled nucleus and global structure
NEW ASTRONOMY ELSEWIER
New Astronomy 3 (1998) 15-26
NGC 55 33: decoupled nucleus and global structure O.K. Sil’chenko”“,
A.N. Bwenkovb’*, W
Vlasyukb,’
“Stemberg Astronomical Institute, University av. 13, Moscow, 119899 Russia “Special Astrophysical Observatory, Nizhnij Arkhyz, Zelenchukskii region, Karachaevo-Cherkessia.
357147 Russia
Received 24 April 1997; accepted 21 October 1997 Communicated by Kenneth C. Freeman
Abstract An isolated regular Sab galaxy NGC 5533 was observed at the 6m telescope of the Special Astrophysical Observatory RAS (Nizhnij Arkhyz, Russia) with the Multi-Pupil Field Spectrograph and scanning Fabry-Perot interferometer and at the 1m telescope of the Special Astrophysical Observatory through the BVRI filters with a CCD detector. Its unresolved nucleus was found to be chemically and dynamically decoupled. The central mass is estimated as 3.5. lo9 M,. The global photometric structure reveals a presence of two disk components, the inner one being typical for early-type galaxies and the outer one being more extended and with low surface brightness. An attempt is made to relate an origin of the decoupled
nucleus and that of the multi-tier structure of the global disk. 0 1998 Elsevier Science B.V. PACS: 98.52.Nr; 98.62.Dm; 98.62.J~; 98.62.L~ Keywords: Galaxies: abundances; Galaxies: individual: Galaxies: photometry; Galaxies: structure
NGC 5533; Galaxies:
1. Introduction During several years we search for chemically decoupled nuclei in disk galaxies. When we found the first three decoupled nuclei in spiral galaxies, our impression was that the host galaxies were isolated regular galaxies with strictly axisymmetric bulges. The chemically decoupled nuclei had appeared to be also dynamically decoupled, they demonstrated circular rotation much faster than that of neighboring ‘E-mail: [email protected] 2E-mail: [email protected] ‘E-mail: [email protected] 1384-1076/98/$19.00
PII
0
Sl384-1076(97)00040-7
1998 Elsevier Science
kinematics
and dynamics;
Galaxies:
nuclei;
bulge regions (Sil’chenko et al., 1992; Sil’chenko, 1993a). It seemed that the own disk gas of these galaxies at a certain moment had fallen into the center and produced a secondary star formation burst, after which a dense nuclear star cluster with enhanced metallicity leaved. But later in almost all regular spiral galaxies with chemically decoupled nuclei various signs of past gas accretion or even merging were found. NGC 7331 has a counterrotating stellar bulge (Prada et al., 1996), NGC 4826 has a counterrotating global gaseous disk (Braun et al., 1994; Rubin, 1994; Rix et al., 1995), NGC 2841 demonstrates orthogonality of star versus gas rotations in the center (Sil’chenko et al., 1997). One
B.V. All rights reserved.
O.K. Sil’chenko et al. / New Astronomy 3 (1998) 15-26
16 Table 1 Global parameters Hubble type R,, B”, MB V,(radio) Distance Inclination PA phol
extended (up to 2.5 optical radii) neutral-hydrogen disk with one of the highest central surface density (Broeils & Van Woerden, 1994). Therefore, NGC 5533, being an isolated regular spiral galaxy, provides unique possibility to investigate a relation between inner and global structure in the frame of intrinsic evolution conception.
of NGC 5533 Sab 24.6 kpc 12.14 - 21.52 3864km.s-’ 53.7Mpc(H,,=75km.s-‘.Mpc-‘) 56.6” 30”
2. Observations begins to think that, perhaps, chemically decoupled nuclei can be produced only by an intervention of another galaxy. The case of NGC 5533 (basic parameters of the galaxy are presented in Table 1) returns us to our initial impressions: this isolated Sab galaxy does not show any signs of past interaction, and its bulge looks axisymmetrical (Kent, 1984). But it has some other intrinsic peculiarities which may be related to a presence of chemically decoupled nucleus. NGC 5533 is a regular isolated Sab galaxy, but one of the first CCD photometric study by Kent (1985) has revealed an unusually low central surface brightness of its disk: k,, = 22.83. In the recent work of Sprayberry et al. (1995) the galaxy is included in the list of giant low-surface brightness spiral galaxies with the reference to Kent. In the same time Lewis with coworkers in the series of papers characterizes NGC 5533 as a supermassive spiral galaxy: though its rotation velocity, defined by 21-cm line width, namely, 270 km/s, is not too high for early-type spirals, its large extension, up to 50 kpc from the center under the long distance scale, results in a rather large total galactic mass, 8 X 1O”Mo (Lewis, 1985). And after all, it has an Table 2 Spectral observations Date 29l30.04.92 26127.02.95 9110.05.95 14/15.05.96 15/16.08.96
and data reduction
The central region of NGC 5533 has been studied by using the Multi-Pupil Field Spectrograph (MPFS) of the 6m telescope of the SAO RAS (Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhnij Arkhyz, Russia). The log of the spectral observations is presented in Table 2. The observations with MPFS (for the spectrograph description - see Afanasiev et al., 1990), which allows to obtain up to 128 spectra from a rectangular area for one exposure, provide bidimensional brightness distributions and velocity fields. A spatial element is a square with the size of 1.3”. We exposed bluegreen spectral range (4700-5400 A) to derive azimuthally averaged profiles of the absorption lines H/3, MgIh5175, FeIh5270 and h5335 and to calculate a stellar velocity field by cross-correlation with template star spectra. As the templates, we have used early K stars - main components of the binaries J 1369, STF 2788, and ADS 15470. The red spectral range (6250-6900 A) was exposed to derive ionized-gas distribution and velocity field by measuring the emission line [NII]A6583 (the Ha emission is rather weak in the nucleus of NGC 5533). The sky to be subtracted from the blue-green galaxy spectra
of NGC 5533 Configuration MPFS+IPCS MPFS + CCD MPFS + CCD MPFS + CCD MPFS + CCD
512 520 520 520 520
Exposure X X X X X
512 580 580 580 580
45 min 40 min 50 min 60 min 90min
Field
Spectral range
x x x X X
Green Green Green Green Red
12” IO” IO” IO” IO”
14” 21” 15” 16” 16”
Spectral resolution 10 A 2A 4A 5A 4A
O.K.
Sil’chenko et al. I New Astronomy 3 (1998) 15-26
was exposed separately. The seeings during the observations with MPFS were in the range of 2”2.5” (FWHM). A spectral reduction - extraction of one-dimensional spectra, linearization, construction of surface brightness maps and velocity fields - was performed by using the software developed by one of the authors (Vlasyuk, 1993). We estimate an accuracy of the absorption-line indices as 0.1 A, an accuracy of the stellar velocities as 20 km/s and an accuracy of the gaseous velocities as 15 km/s. To study a global structure of NGC 5533, we have undertaken its photometric observations at the lm telescope of the SAO RAS with a CCD 520 X 580 as a detector. The log of the photometric observations is given in Table 3. All the observations were made under satisfactory seeing conditions, FWHM I 2”. But only in June the photometric conditions were good and photometric standards were exposed in addition to the V and R exposures of the galaxy. The photometric system is Johnson (V)-Cousins (R). The sky brightness on June 13-14 was 21.48 mag/ sq.arcsec in V and 20.07 mag/sq.arcsec in R. The BVRI frames obtained on May 18-19 were not calibrated, but we have used them to derive isophote characteristics (ellipticities and major-axis position angles). A large-scale velocity field of the ionized gas in NGC 5533 has been investigated with the scanning Fabry-Perot interferometer of the 6m telescope of the SAO RAS (for the interferometer description - see Dodonov et al., 1995). The galaxy has been observed on March 27, 1996; the scanning interferometer at the prime focus of the telescope was operated in the
235th order of the Fabry-Perot etalon in the Ha A6563 line, defining the full scanned velocity range of 1300 km/s and a spectral resolution of about 100 km/s. As a detector, we have used IPCS 5 12 X 5 12. After binning, we have obtained a data cube 256 X 256 X 24 with an angular scale of 0.73” per pixel and a velocity scale of 54 km/s per channel; the total exposure time was 100 minutes. The narrow (FWHM = 20 A, A,, = 6668 A) interference filter cut a spectral range around the redshifted emission line [NII]A6583, which is stronger in the center of the galaxy than Ha; but due to a large rotation velocity, the receding north-eastern part of the galaxy is seen mainly in the Ha shifted into the filter spectral range, therefore we have used there Ha measurements instead of [NII]A6583 line to derive an extended rotation curve. A standard reduction of the data cube (correction for phase shifting, subtraction of night-sky lines, construction of velocity maps etc.) has been made by using the software ADHOC developed at the Marseille Observatory (Boulesteix, 1993).
3. The central region of NGC 5533
3.1. Chemically
Exposure time
Zenithal distance
u cn3.5
18/19.05.96 18/ 19.05.96 18/19.05.96 18/ 19.05.96 13/ 14.06.96 13/ 14.06.96 13/ 14.06.96 13/ 14.06.96
V R I B V R R V
nucleus
4.5k 4.0
Filter
decoupled
Four green spectrum sets obtained with MPFS between 1992 and 1996 were used to derive a radial dependence of magnesium-line strength (for defini-
Table 3 Direct image observations Date
11
600 300 300 1200
s s s s
600s
600 s 600 s 600s
8.7” 10.0” 10.6” 11.5” 21” 23” 25” 27”
z 3.0 2.5
I
P I
Fig. I. Azimuthally averaged variation of the absorption-line index Mgb along radius for the central part of NGC 5533.
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et al. I New Astronomy
tion of absorption-line indices Mgb, Fe5270, and H/3 - see Worthey et al., 1994). The variation of Mgb index along radius, averaged over all the four measured ones, is presented in Fig. 1. The nucleus looks outsending at this profile: Mgb(nucleus) = 3:gl+Cl.12 A, the averaged over the radius range 2 -7 Mgb(bulge) y 3.2OkO.03, so AMgb(nucleusbulge) = 0.61+0.15 A. By applying models of old stellar populations of Worthey (1994) and under the assumption of equal (or simply large enough) ages for the nuclear and bulge stars, we can estimate metallicity difference corresponding to this value of AMgb: A[m/H] = 0.2420.06, or almost a factor of two. The circumnuclear magnesium-strength break is not the only evidence for a chemical-abundance decoupling of the nucleus in NGC 5533. Fig. 2a presents a diagram (Fe5270, Mgb) for the central part of NGC 5533; only the measurements for r 5 5” and only for the last exposure, the longest among the four, are given as the most accurate ones. The diagram (Fe5270, Mgb) has become popular after papers of Faber with coworkers (see, for example, Worthey et al., 1992) where it was shown that line indices for all the models with [Mg/Fe] =0 locate inside the narrow locus and the indices for the majority of ellipticals deviate from this locus to the right, thus demonstrating magnesium overabundance (or iron underabundance). The most probable explanation of this overabundance is a rather short time of main star formation in massive elliptical galaxies: the chemical evolution calculations (Matteucci, 1994) predict that if star formation burst duration is less than one billion years, the bulk of iron, which is produced by intermediate-mass SNI, has no time to be included into newly formed stars. Short duration of the basic star formation in ellipticals was expected. But when we (Sil’chenko, 1993b) have found that the central parts of early-type disk galaxies, lenticulars and spirals, have mostly solar Mg to Fe ratios, it became clear that the global disk presence can prolong initial nuclear star formation by some way. Fig. 2a demonstrates that the nucleus of NGC 5533 is decoupled by duration of the basic star formation as well as by the chemical abundance: in
3 (1998) 15-26
the nucleus we see magnesium overabundance which disappears with radius increasing. So, the epoch of basic star formation in the nucleus was much shorter than that in the nearest outskirts of the nucleus. It gives a reason to judge that there was a time interval between them. Fig. 2b presents a diagram (H/3, Mgb) which can in principle give an estimate of stellar population mean age: the model sequences of age and metallicity have different slopes at this diagram (Worthey, 1994). But we would delay the use of Fig. 2b for age diagnostics until the next section contained broadband photometric results: the HP absorption line may be contaminated by an emission of ionized gas.
3.2. Dynamically
decoupled
nucleus
Fig. 3 demonstrates stellar velocity field for the central part of NGC 5533. One can see one distinct spot, red, and one less prominent spot, blue, located symmetrically on different sides from the nucleus. These spots can be interpreted as local extremes on the rotation curve of NGC 5533. To relate them to the central mass concentration we must prove a circular character of rotation and, hence, an axisymmetry of the central potential in NGC 5533. One of the known ways is to study an azimuthal dependence of observed central line-of-sight velocity gradients. In the case of pure circular rotation this dependence must be a pure cosine law: dv,ldr
= w sini cos(PA - PA,,),
where o is a central angular rotation velocity, i is an inclination of the rotation plane, and PA, is an orientation of the line of nodes. And as the isophotes under the axisymmetric brightness distribution are intrinsically circular, the orientation of the line of nodes coincides with the orientation of visible isophote elongation. So, to prove a circular character of rotation, we must check two points: firstly, a dependence of dv,ldr on PA must be a cosine law, and secondly, PA, must coincide with an isophote majoraxis orientation.
O.K. Sil ‘chenko et al. I New Astronomy
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3 (1998) 15-26
3.0
‘$?.s
I
I
I
I
2.0
I
I
3.0
I
,
4.0
I
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I
5.0
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I
I
6.0
i
3.01
t
*.+.a Our data ‘++++Worthey’s War thev’s
c 1 e
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I
they
I
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. ‘6
for NGC model. mode
I. I;
mode
1
I
2.0
5533 T=17 T=12
( 1996)
T=8
I
I
ibOb
Fig. 2. The (index, index) diagrams for the central part of NGC 5533: upper observational points taken along the radius with the step of 1.3” are connected Worthey (1994) (solid lines) are marked at the values [m/H] = + 0.5, +0.25, (T = 17 billion years), squares (T= 12 billion years), and triangles (T= 8 billion
Fig. 4 presents azimuthal dependencies of central line-of-sight velocity gradients for the gaseous (a) and stellar (b) components. The form of the depen-
I 4.0
5.0
6.0
- Fe5270 versus Mgb, lower - HP versus Mgb. The by a dashed line. The mode1 metallicity sequences from - 1.0. - 1.5, - 2.0 by filled stars 0.0, - 0.25. -0.5, years).
dencies is quite perfectly cosine-like. We have approximated them by cosine laws with the help of non-linear least-square fit:
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O.K. Sil’chenko
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\
60
\ \
=O
3 (1998) 15-26
5533,
NGC
,o) 40-
ges.
r-=2-4.5”
0
4
-?A0 -
40’
I
’ 120
Position
.2
200’
’
angle.
’ 280
’
5 10
degrees
\
60 \ \ 50 @40\
-4
: -6 -4
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_ ,0,/’ L ’ 0-0 l-10-
dIGC
5533,
stars,
’
I
r=l
.4-4’
o
/ n 0
$20-
Fig. 3. Stellar velocity field for the central part of NGC 5533 obtained with MPFS. PA(uert) = 50”, so the north (direction is shown by an arrow) is in the upper right comer, and the east is in the upper left one. The plus marks the photometric center of the galaxy.
dv,ldr
= [27cos(PA - 28” k60) - 0.01 km s-’ arcsec-’
dv,ldr
-4gA0
40’
Pos It
120 ion
I
200 angle,
I
I
280
2
degrees
Fig. 4. Azimuthal dependencies of the central line-of-sight velocity gradients measured with MPFS: a - ionized gas, b - stars. Solid lines show cosine laws fitted by a least-square method (the formulae are given in the text).
(gas),
= [27cos(PA - 41” k4s) + 2.41 km s-l arcsec-’
--30-
(stars).
As the orientation of the outermost isophotes is PA(phot) = 30” (see Table 1) - the orientation of inner isophotes will be specified in the next section we can conclude that the nuclear gas rotation looks quite circular. The deviation of the stellar dynamical line of nodes from the photometrical major axis by some 10” remains to be understood. In any case, if some triaxiality takes place, the gas being a collisive system would respond to it more strongly than the stars. So we have reasons to consider mass distribution in the center of NGC 5533 as axisymmetric.
Fig. 5 presents a stellar rotation curve calculated in the frame of the circular rotation model under the best-fit parameters i = 50” and PA, = 38” from the bidimensional velocity field given in Fig. 3. Red and blue velocity extrema from Fig. 3 have given a rise to a prominent rotation-velocity local maximum at R = 3” followed by a steep rotation-velocity drop. This rotation curve resembles that of M 3 1 (Kormendy, 1988; Dressler & Richstone, 1988) which has a dynamically decoupled nucleus. So we conclude that NGC 5533 has a dynamically decoupled nucleus too. By using a gas rotation curve, qualitatively similar to the stellar one (Fig. 6), we can estimate a mass of the central condensation: M 5 3.5 * lo9 M, (an upper limit is caused by our finite
O.K. Sil’chenko
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i -
et al. / New Astronomy
NGC
MPFS.
5533,
stars
/
J
41
21 R,
Fig. 5. Stellar rotation
6I
arcsec
curve for the central part of NGC 5533.
Fig. 6. Combined rotation curve for the ionized gas of NGC 5533 obtained with MPFS (asterisks) and the scanning Fabry-Perot interferometer (filled circles).
spatial resolution - the real radius of the decoupled nucleus must be smaller than 3”-5”).
4. Global structure
of NGC 5533
4. I. Two disks ? Fig. 6 shows a full measured rotation curve of the ionized gas of NGC 5533. The points for R < 10” are provided by the MPFS observations (measurements of the emission line [NII]A6583), the points in the radius range 10”-60” are obtained from the interferometric observations (measurements of the emission lines [NII] and Ha). Two parts of the rotation curve perfectly meet together. We see two local extremes on this curve: a sure maximum at R = 20” and another possible one at R 5 5” which may be related to the neighbouring rotation velocity maximum of the stellar component (Fig. 5). As 20” at the galaxy’s distance corresponds to about of 5 kpc, we can state an existence of two distinct dynamical subsystems in NGC 5533, extended enough. The analogous conclusion was obtained by
Broeils & Van Woerden (1994) who have measured velocity distribution along the major axis of NGC 5533 in the neutral-hydrogen line 21 cm. The more inner component is not a bulge: according to Kent (1985), the characteristic scale of the de Vaucouleurs’ bulge is r, = 23” and related rotationvelocity maximum, if exists, must be at R = O.l3r,. (and it really exists at R = 3”). Hence, what are these two extended subsystems? From our photometric observations we have derived azimuthally averaged surface brightness profiles in V and R. Close-to-R profiles for NGC 5533 were obtained earlier more than once (Kent, 1984; Broeils & Knapen, 1991; Courteau, 1996). The V profile is obtained for the first time. We show a comparison of our R-profile with that of Courteau (1996) in Fig. 7. One can see that the profiles are almost parallel to each other up to R = 45” (the systematic shift by - 0.5 mag is explained by a photometric system difference), and for R 2 20” the profiles can be well described by an exponential law. But at R = 50” the r-profile of Courteau (1996) becomes flatter; the whole radius range gives an
22
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NGC
5533,
V( J) , June
1996
NGC
5533,
R(C),
1996
n. x m
16
June
0
a cn 0 L 0
Fig. 7. Azimuthally averaged V- and R,-band profiles for NGC 5533. The comparison of our R,-data (circles) with the data of Courteau (1996) (crosses and asterisks) is given. Solid line is the best-fit exponential approximation in the radius range 20”-50” (the formulae are given in the text).
impression of two exponential disks with different scales meeting at R = 50”. Our V and R profiles can be fitted in the radius range 20”-50” by exponential laws:
range 20”-5O”, would be:
v=
The agreement between the two disk-scale estimates is impressive. On the other hand, if we approximate the r-profile of Courteau (1996) in the radius range 70”-130” the exponential formula is:
20.20 - Rl( lS.O,,- 6..)
R, = 19.40 - Rl( 17.6;,,,) with the r.m.s. scatter less than 0.03 mag. It seems that the discrepancy with the previous disk-scale estimates, 34”-48” (Kent, 1985; Broeils & Knapen, 1991; Courteau, 1996), is great. But if we approximate the r-profile of Courteau (1996) in the radius
r =
r =
as we have done with ours, the fit
19.85 -R/(17.7;,.,,,).
21.4 -R/(36.4;,,,,,).
These characteristics, the central surface brightness ,uO,*= 21.4 which is weaker than the mean central surface brightness for the whole sample of Courteau
O.K. Sil’chenko
et al. I New Astronomy
(1996) - (CL,,,) = 20.1 - by more than one magnitude, and the characteristic radius of the outer disk, 9 kpc, can be reasons to treat NGC 5533 as a giant LSB galaxy. However let us to consider the “inner” disk of NGC 5533 within borders of 20”-50” (5.213 kpc). The parameters of this disk, ,x,,, = 19.85 and h = 4.2 kpc, are quite typical for early-type spiral galaxies. So, if a photometric investigation of NGC 5533 was extended only to the radius of 13 kpc - as the majority of spiral galaxies are investigated - it would look like a quite typical Sab galaxy, not a low surface brightness one. Unfortunately, we cannot relate the two photometric disks to the two extended dynamical subsystems which define a non-monotonic character of the rotation curve. For a flat exponential density distribution the maximum rotation velocity is reached at R = 2.15/z, so the maximum at R ^I 20” is not related to the “inner” disk, and on the contrary, there is no any maximum of the rotation velocity at R = 2.15hinner = 35”. So we may conclude that, untypically for a spiral galaxy, the inner rotation curve of NGC 5533 is dominated by an unseen halo, not by a luminous component. The very high maxi-
20
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23
2 300 km/s, is also conmum rotation velocity, sistent with such an interpretation: it overtops the mean Tully-Fisher relation by some 80 km/s (30%) if we take M, of NGC 5533 from Courteau (1996) and the last calibration of red TF relation from Courteau & Rix ( 1997). 4.2. Colour distribution Fig. 8 presents a global colour map of NGC 5533. The most noticeable features of this map are prominent red lanes which are located to the south-east from the center. Obviously, it is dust. We can state that the south-eastern half of the galactic disk is nearer to us than the north-western one. So, the spiral pattern is a trailing one that is typical for noninteracting spiral galaxies. Fig. 9 demonstrates a radial profile of the colour V- R, obtained by averaging in concentric elliptical rings. One can see two colour maxima in the inner part of the galaxy related to the dust lanes: the first one at R = 3” and the second one at R = 1.3”. Interestingly, outside the zone of dust reddening a colour gradient is practically absent in NGC 5533
0
-20
-40
AC/,arcsec Fig. 8. The (V-
Rc )
colour map of the inner part of NGC 5533. Isophotes in Rc are also given for the orientation.
24
O.K. Sil’chenko
et al. I New Astronomy
1: ::: cj0.65 z
0. 60
20.55
..‘Q
.*...*.*
***. . . . . *a..
i
. . . ..Q.....*
*...***.. . l. .._.
l
*...*. .
0.50
” 450
I
I
10
I
I
20
I
30
I
I
50
60
70
arcsec
R,
Fig. 9. Azimuthally 5533.
I1
40
averaged
(V - Rc) colour
profile
of NGC
unlike the majority of spiral galaxies. It means that gradients of star formation rate and metallicity in the disk of NGC 5533 are absent too. We have tried to use this averaged colour profile for age determination by combining it with the Mgb and HP profiles. Fig.
3 (1998)
10 presents a diagram (Mgb, V - R,) for the observations of the very central part of NGC 5533 together with the model calculations of Worthey (1994). All the points except the nucleus fall well below the model sequences; obviously, the measurements are affected by the dust. Then we have measured the colour strictly along the minor axis to the north-west from the nucleus. The new magnesium-colour trend is aligned with the model sequences; we think it is because these measurements are less affected by the dust. The comparison of Fig. 10 with Fig. 2b has shown their full consistency: both diagrams give for the nucleus an age - 10 billion years and a nearly solar metallicity and for the bulge an age - 17 billion years and a metallicity about of - 0.4. So, besides the metallicity drop, there may be an age jump between the nucleus and the nearest bulge. 4.3. Isophotal
4.0< ;3.0F
I
‘840
0.45
I,
I
0.50
I
I
I
I
0.55
V-R(
I1
I
Cl
I
0.60
I
I
I
0.65
0.70
Fig. 10. The diagram (Mgb, V- R<) for the very central part of NGC 5533. Observational points taken along the radius with the step of 1.3” are connected by dashed lines: circles with dots azimuthally averaged data, circles with pluses - measurements along the NW part of minor axis. The model metallicity sequences from Worthey (1994) (solid lines) are marked at the values [m/H] = +0.5, +0.25, 0.0, - 0.25, - 0.5, - 1.0, - 1.5, - 2.0 by points (T= 17 billion years), triangles (T = 12 billion years), squares (T = 8 billion years), asterisks (T = 5 billion years), and open stars (T= 3 billion years).
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analysis
Fig. 11 presents results of the isophotal analysis of the images of NGC 5533 in all four bands BVR,Ic. They agree very well. Probably, a line of nodes of the inner disk is oriented at PA = 28”; two prominent PA minima at R = 5 ” and R = 13 ” may be related to the dust lanes at these radii. The possible turn of the isophote major axis toward the nucleus must be restricted by 3”-4”, so Fig. 1 la confirms a result on axisymmetric brightness distribution in the inner part of NGC 5533. Ellipticity trend along the radius (Fig. 1 lb) is quite monotonous which is also consistent with an axisymmetric brightness distribution. Moreover, it gives a strong argument in favour of flatness of the photometric subsystem dominating between R = 20” and 50”: the ellipticity reaches its maximum at R = 15” and is nearly constant outside this radius up to the boundary of the galaxy. It means that photometric influence of the bulge is negligible outside R = 15”. The behaviour of the fourth coefficient of the azimuthal Fourier decomposition, a4 la, in the central part of NGC 5533 is somewhat chaotic due to the dust lanes: it oscillates between 0 and +O.Ol; but there is no signs of a boxy bulge or bar in the galaxy.
O.K. Sil’chenko
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!50
arcsec
Fig. Il. The results of isophotal analysis for NGC 5533: upper position angle of the major axis, lower - axis ratio. The isophotes are fitted by ellipses.
5. Conclusions NGC 5533 is a regular isolated spiral galaxy which was included earlier into the list of giant low surface brightness galaxies (Sprayberry et al., 1995) and into the sample of galaxies with extended HI disks (Broeils & Van Woerden, 1994). We have undertaken a detailed spectral study of the central part of NGC 5533. We have found that an unresolved nucleus of this galaxy is chemically decoupled. Firstly, there exists a magnesium-line strength break between the nucleus and the nearest bulge corresponding to a metallicity difference of 0.24*0.06dex. Secondly, only in the nucleus the ratio [Mg/Fe] > 0, while in the bulge a solar magnesium to iron ratio is observed. The nucleus of
3 (1998) 15-26
2s
NGC 5533 is also dynamically decoupled: the stellar rotation velocity has a local maximum at R - 3” with its noticeable drop outside this radius. We have estimated a mass of the decoupled nucleus: M 5 3.5 . 10yMo. As NGC 5533 is an isolated galaxy with a quite regular structure, an origin of the decoupled nucleus can scarcely be related to interaction or merging. So we try to find any intrinsic peculiarities which may be related to the origin of the decoupled nucleus. The photometric study of NGC 5533 has shown that though a disk (flat) component dominates surface brightness distribution at R 2 15”, we cannot fit the whole brightness radial profile by a single exponential law. We propose a model of two embedded disks. The inner one is quite typical for earlytype spiral galaxies, with a characteristic scale of 4.2 kpc and a central surface brightnesses ,u,,,~ = 19.4 and I-+,~ = 20.2. The outer one, which dominates at R > 13’kpc, has a characteristic scale larger by a factor of two, 9 kpc, and rather low central surface brightness; it is just this disk that was measured earlier and gave a start to including NGC 5533 into the list of low surface brightness galaxies. If we also take into account an extended HI disk which is detected up to 2.5 optical radii (Broeils & Van Woerden, 1994), we can state an existence of three tiers in the global disk of NGC 5533. It is not the only case of such a multi-tier structure of global disks in isolated spiral galaxies. Just the same threetier structure with extended HI disk was recently found in NGC 157(Ryder et al., 1997). It would be natural to suggest that each stellar disk had a proper star formation epoch. Obviously, each disk formation epoch was preceded by a gas density re-distribution in the global disk. Perhaps, one of such events had also provoked a formation of the decoupled nucleus in NGC 5533. A possible nature of such gas redistributions needs a further investigation.
Acknowledgements We are very grateful to the astronomers Special Astrophysical Observatory RAS
-
of the V.L.
26
O.K. Sil’chenko et al. I New Astronomy 3 (1998) 15-26
Afanasiev, S.N. Dodonov, and S.V Drabek providing the observations at the 6 m telescope and V.H. Chavushyan for the help at the lm telescope. During the data analysis we have used the Lyon-Meudon Extragalactic Database (LEDA) supplied by the LEDA team at the CRAL-Observatoire de Lyon (France) and the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. For reduction of Fabry-Perot data, we have used the software ADHOC developed at the Marseille Observatory. The work was supported by the grant of the Russian Foundation for Basic Research No. 95-02-04480. The 6 m telescope is operated under the financial support of Science Department of Russia (registration number 01-43).
References Afanasiev,V.L., Dodonov, S.N., Sil’chenko, O.K., & Vlasyuk,VV., 1990, preprint SAO, N54. Boulesteix, J., 1993, ADHOC Reference Manual. Marseille: Publ. de l’observatoire de Marseille. Braun, R., Walterbos, R., Kennicutt, R.C., Jr., & Tacconi, 1994, ApJ, 420, 558. Broeils, A.H. & Knapen, J.H., 1991, A&AS, 91, 469. Broeils, A.H. & Van Woerden H., 1994, A&AS, 107, 129.
L.J.,
Courteau, S., 1996, ApJS, 103, 363. Courteau, S. & Rix, H-W., 1997, ApJ Letters, submitted (astro-phi 9707290). Dodonov, S.N., Vlasyuk, VV., & Drabek. S.V., 1995, Fabry-Perot Interferometer Manual. Nizjnij Arkhyz: SAO Publ. Dressier, A. & Richstone, D.O., 1988, ApJ, 324, 701. Kent, S.M., 1984, ApJS, 56, 105. Kent, S.M., 1985, ApJS, 59, 115. Kormendy, J., 1988, ApJ, 325, 128. Lewis, B.M., 1985, ApJ, 292, 451. Matteucci, F., 1994, A&A, 288, 57. Prada, F., Gutierrez, C.M., Peletier, R.F., & McKeith, CD., 1996, ApJ, 463, L9. Rix, H.-W. R.. Kennicutt, R.C., Jr., Braun, R., & Walterbos, R.A.M., 1995, ApJ, 438, 155. Rubin,V.C., 1994, AJ, 107, 173. Ryder, S., Zasov, A., McIntyre, V., Walsh, W., & Sil’chenko, 0.. 1997, MNRAS, accepted. Sil’chenko, O.K., 1993a, in: Galactic Bulges, IAU Symp. 153, eds. H. Habing & H. Dejonghe (Dordrecht, Kluwer Academic Publishers), p. 427. Sil’chenko, O.K., 1993b. PAZh, 19, 701. Sil’chenko, O.K., Afanasiev,V.L., & Vlasyuk,VV., 1992, AZh, 69, 1121. Sil’chenko, O.K., Vlasyuk, VV., & Burenkov, A.N., 1997, A&A, 326, 941. Sprayberry, D., Impey, C.D., Bothun, G.D., & Irwin, M.J., 1995, AJ, 109, 558. Vlasyuk, VV., 1993, Astrofiz. issled. (Izv. SAO RAS), 36, 107. Worthey, G., 1994, ApJS, 95. 107. Worthey, G., Faber, S.M., & Gonzalez, J.J., 1992, ApJ, 398, 69. Worthey, G., Faber, S.M., Gonzalez. J.J., & Burstein D., 1994, ApJS. 94, 687.
New Astronomy 3 (1998) 15-26
NGC 55 33: decoupled nucleus and global structure O.K. Sil’chenko”“,
A.N. Bwenkovb’*, W
Vlasyukb,’
“Stemberg Astronomical Institute, University av. 13, Moscow, 119899 Russia “Special Astrophysical Observatory, Nizhnij Arkhyz, Zelenchukskii region, Karachaevo-Cherkessia.
357147 Russia
Received 24 April 1997; accepted 21 October 1997 Communicated by Kenneth C. Freeman
Abstract An isolated regular Sab galaxy NGC 5533 was observed at the 6m telescope of the Special Astrophysical Observatory RAS (Nizhnij Arkhyz, Russia) with the Multi-Pupil Field Spectrograph and scanning Fabry-Perot interferometer and at the 1m telescope of the Special Astrophysical Observatory through the BVRI filters with a CCD detector. Its unresolved nucleus was found to be chemically and dynamically decoupled. The central mass is estimated as 3.5. lo9 M,. The global photometric structure reveals a presence of two disk components, the inner one being typical for early-type galaxies and the outer one being more extended and with low surface brightness. An attempt is made to relate an origin of the decoupled
nucleus and that of the multi-tier structure of the global disk. 0 1998 Elsevier Science B.V. PACS: 98.52.Nr; 98.62.Dm; 98.62.J~; 98.62.L~ Keywords: Galaxies: abundances; Galaxies: individual: Galaxies: photometry; Galaxies: structure
NGC 5533; Galaxies:
1. Introduction During several years we search for chemically decoupled nuclei in disk galaxies. When we found the first three decoupled nuclei in spiral galaxies, our impression was that the host galaxies were isolated regular galaxies with strictly axisymmetric bulges. The chemically decoupled nuclei had appeared to be also dynamically decoupled, they demonstrated circular rotation much faster than that of neighboring ‘E-mail: [email protected] 2E-mail: [email protected] ‘E-mail: [email protected] 1384-1076/98/$19.00
PII
0
Sl384-1076(97)00040-7
1998 Elsevier Science
kinematics
and dynamics;
Galaxies:
nuclei;
bulge regions (Sil’chenko et al., 1992; Sil’chenko, 1993a). It seemed that the own disk gas of these galaxies at a certain moment had fallen into the center and produced a secondary star formation burst, after which a dense nuclear star cluster with enhanced metallicity leaved. But later in almost all regular spiral galaxies with chemically decoupled nuclei various signs of past gas accretion or even merging were found. NGC 7331 has a counterrotating stellar bulge (Prada et al., 1996), NGC 4826 has a counterrotating global gaseous disk (Braun et al., 1994; Rubin, 1994; Rix et al., 1995), NGC 2841 demonstrates orthogonality of star versus gas rotations in the center (Sil’chenko et al., 1997). One
B.V. All rights reserved.
O.K. Sil’chenko et al. / New Astronomy 3 (1998) 15-26
16 Table 1 Global parameters Hubble type R,, B”, MB V,(radio) Distance Inclination PA phol
extended (up to 2.5 optical radii) neutral-hydrogen disk with one of the highest central surface density (Broeils & Van Woerden, 1994). Therefore, NGC 5533, being an isolated regular spiral galaxy, provides unique possibility to investigate a relation between inner and global structure in the frame of intrinsic evolution conception.
of NGC 5533 Sab 24.6 kpc 12.14 - 21.52 3864km.s-’ 53.7Mpc(H,,=75km.s-‘.Mpc-‘) 56.6” 30”
2. Observations begins to think that, perhaps, chemically decoupled nuclei can be produced only by an intervention of another galaxy. The case of NGC 5533 (basic parameters of the galaxy are presented in Table 1) returns us to our initial impressions: this isolated Sab galaxy does not show any signs of past interaction, and its bulge looks axisymmetrical (Kent, 1984). But it has some other intrinsic peculiarities which may be related to a presence of chemically decoupled nucleus. NGC 5533 is a regular isolated Sab galaxy, but one of the first CCD photometric study by Kent (1985) has revealed an unusually low central surface brightness of its disk: k,, = 22.83. In the recent work of Sprayberry et al. (1995) the galaxy is included in the list of giant low-surface brightness spiral galaxies with the reference to Kent. In the same time Lewis with coworkers in the series of papers characterizes NGC 5533 as a supermassive spiral galaxy: though its rotation velocity, defined by 21-cm line width, namely, 270 km/s, is not too high for early-type spirals, its large extension, up to 50 kpc from the center under the long distance scale, results in a rather large total galactic mass, 8 X 1O”Mo (Lewis, 1985). And after all, it has an Table 2 Spectral observations Date 29l30.04.92 26127.02.95 9110.05.95 14/15.05.96 15/16.08.96
and data reduction
The central region of NGC 5533 has been studied by using the Multi-Pupil Field Spectrograph (MPFS) of the 6m telescope of the SAO RAS (Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhnij Arkhyz, Russia). The log of the spectral observations is presented in Table 2. The observations with MPFS (for the spectrograph description - see Afanasiev et al., 1990), which allows to obtain up to 128 spectra from a rectangular area for one exposure, provide bidimensional brightness distributions and velocity fields. A spatial element is a square with the size of 1.3”. We exposed bluegreen spectral range (4700-5400 A) to derive azimuthally averaged profiles of the absorption lines H/3, MgIh5175, FeIh5270 and h5335 and to calculate a stellar velocity field by cross-correlation with template star spectra. As the templates, we have used early K stars - main components of the binaries J 1369, STF 2788, and ADS 15470. The red spectral range (6250-6900 A) was exposed to derive ionized-gas distribution and velocity field by measuring the emission line [NII]A6583 (the Ha emission is rather weak in the nucleus of NGC 5533). The sky to be subtracted from the blue-green galaxy spectra
of NGC 5533 Configuration MPFS+IPCS MPFS + CCD MPFS + CCD MPFS + CCD MPFS + CCD
512 520 520 520 520
Exposure X X X X X
512 580 580 580 580
45 min 40 min 50 min 60 min 90min
Field
Spectral range
x x x X X
Green Green Green Green Red
12” IO” IO” IO” IO”
14” 21” 15” 16” 16”
Spectral resolution 10 A 2A 4A 5A 4A
O.K.
Sil’chenko et al. I New Astronomy 3 (1998) 15-26
was exposed separately. The seeings during the observations with MPFS were in the range of 2”2.5” (FWHM). A spectral reduction - extraction of one-dimensional spectra, linearization, construction of surface brightness maps and velocity fields - was performed by using the software developed by one of the authors (Vlasyuk, 1993). We estimate an accuracy of the absorption-line indices as 0.1 A, an accuracy of the stellar velocities as 20 km/s and an accuracy of the gaseous velocities as 15 km/s. To study a global structure of NGC 5533, we have undertaken its photometric observations at the lm telescope of the SAO RAS with a CCD 520 X 580 as a detector. The log of the photometric observations is given in Table 3. All the observations were made under satisfactory seeing conditions, FWHM I 2”. But only in June the photometric conditions were good and photometric standards were exposed in addition to the V and R exposures of the galaxy. The photometric system is Johnson (V)-Cousins (R). The sky brightness on June 13-14 was 21.48 mag/ sq.arcsec in V and 20.07 mag/sq.arcsec in R. The BVRI frames obtained on May 18-19 were not calibrated, but we have used them to derive isophote characteristics (ellipticities and major-axis position angles). A large-scale velocity field of the ionized gas in NGC 5533 has been investigated with the scanning Fabry-Perot interferometer of the 6m telescope of the SAO RAS (for the interferometer description - see Dodonov et al., 1995). The galaxy has been observed on March 27, 1996; the scanning interferometer at the prime focus of the telescope was operated in the
235th order of the Fabry-Perot etalon in the Ha A6563 line, defining the full scanned velocity range of 1300 km/s and a spectral resolution of about 100 km/s. As a detector, we have used IPCS 5 12 X 5 12. After binning, we have obtained a data cube 256 X 256 X 24 with an angular scale of 0.73” per pixel and a velocity scale of 54 km/s per channel; the total exposure time was 100 minutes. The narrow (FWHM = 20 A, A,, = 6668 A) interference filter cut a spectral range around the redshifted emission line [NII]A6583, which is stronger in the center of the galaxy than Ha; but due to a large rotation velocity, the receding north-eastern part of the galaxy is seen mainly in the Ha shifted into the filter spectral range, therefore we have used there Ha measurements instead of [NII]A6583 line to derive an extended rotation curve. A standard reduction of the data cube (correction for phase shifting, subtraction of night-sky lines, construction of velocity maps etc.) has been made by using the software ADHOC developed at the Marseille Observatory (Boulesteix, 1993).
3. The central region of NGC 5533
3.1. Chemically
Exposure time
Zenithal distance
u cn3.5
18/19.05.96 18/ 19.05.96 18/19.05.96 18/ 19.05.96 13/ 14.06.96 13/ 14.06.96 13/ 14.06.96 13/ 14.06.96
V R I B V R R V
nucleus
4.5k 4.0
Filter
decoupled
Four green spectrum sets obtained with MPFS between 1992 and 1996 were used to derive a radial dependence of magnesium-line strength (for defini-
Table 3 Direct image observations Date
11
600 300 300 1200
s s s s
600s
600 s 600 s 600s
8.7” 10.0” 10.6” 11.5” 21” 23” 25” 27”
z 3.0 2.5
I
P I
Fig. I. Azimuthally averaged variation of the absorption-line index Mgb along radius for the central part of NGC 5533.
18
O.K. Sil’chenko
et al. I New Astronomy
tion of absorption-line indices Mgb, Fe5270, and H/3 - see Worthey et al., 1994). The variation of Mgb index along radius, averaged over all the four measured ones, is presented in Fig. 1. The nucleus looks outsending at this profile: Mgb(nucleus) = 3:gl+Cl.12 A, the averaged over the radius range 2 -7 Mgb(bulge) y 3.2OkO.03, so AMgb(nucleusbulge) = 0.61+0.15 A. By applying models of old stellar populations of Worthey (1994) and under the assumption of equal (or simply large enough) ages for the nuclear and bulge stars, we can estimate metallicity difference corresponding to this value of AMgb: A[m/H] = 0.2420.06, or almost a factor of two. The circumnuclear magnesium-strength break is not the only evidence for a chemical-abundance decoupling of the nucleus in NGC 5533. Fig. 2a presents a diagram (Fe5270, Mgb) for the central part of NGC 5533; only the measurements for r 5 5” and only for the last exposure, the longest among the four, are given as the most accurate ones. The diagram (Fe5270, Mgb) has become popular after papers of Faber with coworkers (see, for example, Worthey et al., 1992) where it was shown that line indices for all the models with [Mg/Fe] =0 locate inside the narrow locus and the indices for the majority of ellipticals deviate from this locus to the right, thus demonstrating magnesium overabundance (or iron underabundance). The most probable explanation of this overabundance is a rather short time of main star formation in massive elliptical galaxies: the chemical evolution calculations (Matteucci, 1994) predict that if star formation burst duration is less than one billion years, the bulk of iron, which is produced by intermediate-mass SNI, has no time to be included into newly formed stars. Short duration of the basic star formation in ellipticals was expected. But when we (Sil’chenko, 1993b) have found that the central parts of early-type disk galaxies, lenticulars and spirals, have mostly solar Mg to Fe ratios, it became clear that the global disk presence can prolong initial nuclear star formation by some way. Fig. 2a demonstrates that the nucleus of NGC 5533 is decoupled by duration of the basic star formation as well as by the chemical abundance: in
3 (1998) 15-26
the nucleus we see magnesium overabundance which disappears with radius increasing. So, the epoch of basic star formation in the nucleus was much shorter than that in the nearest outskirts of the nucleus. It gives a reason to judge that there was a time interval between them. Fig. 2b presents a diagram (H/3, Mgb) which can in principle give an estimate of stellar population mean age: the model sequences of age and metallicity have different slopes at this diagram (Worthey, 1994). But we would delay the use of Fig. 2b for age diagnostics until the next section contained broadband photometric results: the HP absorption line may be contaminated by an emission of ionized gas.
3.2. Dynamically
decoupled
nucleus
Fig. 3 demonstrates stellar velocity field for the central part of NGC 5533. One can see one distinct spot, red, and one less prominent spot, blue, located symmetrically on different sides from the nucleus. These spots can be interpreted as local extremes on the rotation curve of NGC 5533. To relate them to the central mass concentration we must prove a circular character of rotation and, hence, an axisymmetry of the central potential in NGC 5533. One of the known ways is to study an azimuthal dependence of observed central line-of-sight velocity gradients. In the case of pure circular rotation this dependence must be a pure cosine law: dv,ldr
= w sini cos(PA - PA,,),
where o is a central angular rotation velocity, i is an inclination of the rotation plane, and PA, is an orientation of the line of nodes. And as the isophotes under the axisymmetric brightness distribution are intrinsically circular, the orientation of the line of nodes coincides with the orientation of visible isophote elongation. So, to prove a circular character of rotation, we must check two points: firstly, a dependence of dv,ldr on PA must be a cosine law, and secondly, PA, must coincide with an isophote majoraxis orientation.
O.K. Sil ‘chenko et al. I New Astronomy
19
3 (1998) 15-26
3.0
‘$?.s
I
I
I
I
2.0
I
I
3.0
I
,
4.0
I
I
I
I
5.0
I
I
I
I
6.0
i
3.01
t
*.+.a Our data ‘++++Worthey’s War thev’s
c 1 e
0-L9.0
1.0
I
Nor
I
they
I
1.0
. ‘6
for NGC model. mode
I. I;
mode
1
I
2.0
5533 T=17 T=12
( 1996)
T=8
I
I
ibOb
Fig. 2. The (index, index) diagrams for the central part of NGC 5533: upper observational points taken along the radius with the step of 1.3” are connected Worthey (1994) (solid lines) are marked at the values [m/H] = + 0.5, +0.25, (T = 17 billion years), squares (T= 12 billion years), and triangles (T= 8 billion
Fig. 4 presents azimuthal dependencies of central line-of-sight velocity gradients for the gaseous (a) and stellar (b) components. The form of the depen-
I 4.0
5.0
6.0
- Fe5270 versus Mgb, lower - HP versus Mgb. The by a dashed line. The mode1 metallicity sequences from - 1.0. - 1.5, - 2.0 by filled stars 0.0, - 0.25. -0.5, years).
dencies is quite perfectly cosine-like. We have approximated them by cosine laws with the help of non-linear least-square fit:
20
O.K. Sil’chenko
et al. I New Astronomy
6
\
60
\ \
=O
3 (1998) 15-26
5533,
NGC
,o) 40-
ges.
r-=2-4.5”
0
4
-?A0 -
40’
I
’ 120
Position
.2
200’
’
angle.
’ 280
’
5 10
degrees
\
60 \ \ 50 @40\
-4
: -6 -4
-2
0 AX [orcsec]
2
:::
_ ,0,/’ L ’ 0-0 l-10-
dIGC
5533,
stars,
’
I
r=l
.4-4’
o
/ n 0
$20-
Fig. 3. Stellar velocity field for the central part of NGC 5533 obtained with MPFS. PA(uert) = 50”, so the north (direction is shown by an arrow) is in the upper right comer, and the east is in the upper left one. The plus marks the photometric center of the galaxy.
dv,ldr
= [27cos(PA - 28” k60) - 0.01 km s-’ arcsec-’
dv,ldr
-4gA0
40’
Pos It
120 ion
I
200 angle,
I
I
280
2
degrees
Fig. 4. Azimuthal dependencies of the central line-of-sight velocity gradients measured with MPFS: a - ionized gas, b - stars. Solid lines show cosine laws fitted by a least-square method (the formulae are given in the text).
(gas),
= [27cos(PA - 41” k4s) + 2.41 km s-l arcsec-’
--30-
(stars).
As the orientation of the outermost isophotes is PA(phot) = 30” (see Table 1) - the orientation of inner isophotes will be specified in the next section we can conclude that the nuclear gas rotation looks quite circular. The deviation of the stellar dynamical line of nodes from the photometrical major axis by some 10” remains to be understood. In any case, if some triaxiality takes place, the gas being a collisive system would respond to it more strongly than the stars. So we have reasons to consider mass distribution in the center of NGC 5533 as axisymmetric.
Fig. 5 presents a stellar rotation curve calculated in the frame of the circular rotation model under the best-fit parameters i = 50” and PA, = 38” from the bidimensional velocity field given in Fig. 3. Red and blue velocity extrema from Fig. 3 have given a rise to a prominent rotation-velocity local maximum at R = 3” followed by a steep rotation-velocity drop. This rotation curve resembles that of M 3 1 (Kormendy, 1988; Dressler & Richstone, 1988) which has a dynamically decoupled nucleus. So we conclude that NGC 5533 has a dynamically decoupled nucleus too. By using a gas rotation curve, qualitatively similar to the stellar one (Fig. 6), we can estimate a mass of the central condensation: M 5 3.5 * lo9 M, (an upper limit is caused by our finite
O.K. Sil’chenko
”
3 (1998) 15-26
200
i -
et al. / New Astronomy
NGC
MPFS.
5533,
stars
/
J
41
21 R,
Fig. 5. Stellar rotation
6I
arcsec
curve for the central part of NGC 5533.
Fig. 6. Combined rotation curve for the ionized gas of NGC 5533 obtained with MPFS (asterisks) and the scanning Fabry-Perot interferometer (filled circles).
spatial resolution - the real radius of the decoupled nucleus must be smaller than 3”-5”).
4. Global structure
of NGC 5533
4. I. Two disks ? Fig. 6 shows a full measured rotation curve of the ionized gas of NGC 5533. The points for R < 10” are provided by the MPFS observations (measurements of the emission line [NII]A6583), the points in the radius range 10”-60” are obtained from the interferometric observations (measurements of the emission lines [NII] and Ha). Two parts of the rotation curve perfectly meet together. We see two local extremes on this curve: a sure maximum at R = 20” and another possible one at R 5 5” which may be related to the neighbouring rotation velocity maximum of the stellar component (Fig. 5). As 20” at the galaxy’s distance corresponds to about of 5 kpc, we can state an existence of two distinct dynamical subsystems in NGC 5533, extended enough. The analogous conclusion was obtained by
Broeils & Van Woerden (1994) who have measured velocity distribution along the major axis of NGC 5533 in the neutral-hydrogen line 21 cm. The more inner component is not a bulge: according to Kent (1985), the characteristic scale of the de Vaucouleurs’ bulge is r, = 23” and related rotationvelocity maximum, if exists, must be at R = O.l3r,. (and it really exists at R = 3”). Hence, what are these two extended subsystems? From our photometric observations we have derived azimuthally averaged surface brightness profiles in V and R. Close-to-R profiles for NGC 5533 were obtained earlier more than once (Kent, 1984; Broeils & Knapen, 1991; Courteau, 1996). The V profile is obtained for the first time. We show a comparison of our R-profile with that of Courteau (1996) in Fig. 7. One can see that the profiles are almost parallel to each other up to R = 45” (the systematic shift by - 0.5 mag is explained by a photometric system difference), and for R 2 20” the profiles can be well described by an exponential law. But at R = 50” the r-profile of Courteau (1996) becomes flatter; the whole radius range gives an
22
O.K. Sil ‘chenko
15
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et al. I New Astronomy
3 (1998)
15-26
NGC
5533,
V( J) , June
1996
NGC
5533,
R(C),
1996
n. x m
16
June
0
a cn 0 L 0
Fig. 7. Azimuthally averaged V- and R,-band profiles for NGC 5533. The comparison of our R,-data (circles) with the data of Courteau (1996) (crosses and asterisks) is given. Solid line is the best-fit exponential approximation in the radius range 20”-50” (the formulae are given in the text).
impression of two exponential disks with different scales meeting at R = 50”. Our V and R profiles can be fitted in the radius range 20”-50” by exponential laws:
range 20”-5O”, would be:
v=
The agreement between the two disk-scale estimates is impressive. On the other hand, if we approximate the r-profile of Courteau (1996) in the radius range 70”-130” the exponential formula is:
20.20 - Rl( lS.O,,- 6..)
R, = 19.40 - Rl( 17.6;,,,) with the r.m.s. scatter less than 0.03 mag. It seems that the discrepancy with the previous disk-scale estimates, 34”-48” (Kent, 1985; Broeils & Knapen, 1991; Courteau, 1996), is great. But if we approximate the r-profile of Courteau (1996) in the radius
r =
r =
as we have done with ours, the fit
19.85 -R/(17.7;,.,,,).
21.4 -R/(36.4;,,,,,).
These characteristics, the central surface brightness ,uO,*= 21.4 which is weaker than the mean central surface brightness for the whole sample of Courteau
O.K. Sil’chenko
et al. I New Astronomy
(1996) - (CL,,,) = 20.1 - by more than one magnitude, and the characteristic radius of the outer disk, 9 kpc, can be reasons to treat NGC 5533 as a giant LSB galaxy. However let us to consider the “inner” disk of NGC 5533 within borders of 20”-50” (5.213 kpc). The parameters of this disk, ,x,,, = 19.85 and h = 4.2 kpc, are quite typical for early-type spiral galaxies. So, if a photometric investigation of NGC 5533 was extended only to the radius of 13 kpc - as the majority of spiral galaxies are investigated - it would look like a quite typical Sab galaxy, not a low surface brightness one. Unfortunately, we cannot relate the two photometric disks to the two extended dynamical subsystems which define a non-monotonic character of the rotation curve. For a flat exponential density distribution the maximum rotation velocity is reached at R = 2.15/z, so the maximum at R ^I 20” is not related to the “inner” disk, and on the contrary, there is no any maximum of the rotation velocity at R = 2.15hinner = 35”. So we may conclude that, untypically for a spiral galaxy, the inner rotation curve of NGC 5533 is dominated by an unseen halo, not by a luminous component. The very high maxi-
20
3 (1998)
15-26
23
2 300 km/s, is also conmum rotation velocity, sistent with such an interpretation: it overtops the mean Tully-Fisher relation by some 80 km/s (30%) if we take M, of NGC 5533 from Courteau (1996) and the last calibration of red TF relation from Courteau & Rix ( 1997). 4.2. Colour distribution Fig. 8 presents a global colour map of NGC 5533. The most noticeable features of this map are prominent red lanes which are located to the south-east from the center. Obviously, it is dust. We can state that the south-eastern half of the galactic disk is nearer to us than the north-western one. So, the spiral pattern is a trailing one that is typical for noninteracting spiral galaxies. Fig. 9 demonstrates a radial profile of the colour V- R, obtained by averaging in concentric elliptical rings. One can see two colour maxima in the inner part of the galaxy related to the dust lanes: the first one at R = 3” and the second one at R = 1.3”. Interestingly, outside the zone of dust reddening a colour gradient is practically absent in NGC 5533
0
-20
-40
AC/,arcsec Fig. 8. The (V-
Rc )
colour map of the inner part of NGC 5533. Isophotes in Rc are also given for the orientation.
24
O.K. Sil’chenko
et al. I New Astronomy
1: ::: cj0.65 z
0. 60
20.55
..‘Q
.*...*.*
***. . . . . *a..
i
. . . ..Q.....*
*...***.. . l. .._.
l
*...*. .
0.50
” 450
I
I
10
I
I
20
I
30
I
I
50
60
70
arcsec
R,
Fig. 9. Azimuthally 5533.
I1
40
averaged
(V - Rc) colour
profile
of NGC
unlike the majority of spiral galaxies. It means that gradients of star formation rate and metallicity in the disk of NGC 5533 are absent too. We have tried to use this averaged colour profile for age determination by combining it with the Mgb and HP profiles. Fig.
3 (1998)
10 presents a diagram (Mgb, V - R,) for the observations of the very central part of NGC 5533 together with the model calculations of Worthey (1994). All the points except the nucleus fall well below the model sequences; obviously, the measurements are affected by the dust. Then we have measured the colour strictly along the minor axis to the north-west from the nucleus. The new magnesium-colour trend is aligned with the model sequences; we think it is because these measurements are less affected by the dust. The comparison of Fig. 10 with Fig. 2b has shown their full consistency: both diagrams give for the nucleus an age - 10 billion years and a nearly solar metallicity and for the bulge an age - 17 billion years and a metallicity about of - 0.4. So, besides the metallicity drop, there may be an age jump between the nucleus and the nearest bulge. 4.3. Isophotal
4.0< ;3.0F
I
‘840
0.45
I,
I
0.50
I
I
I
I
0.55
V-R(
I1
I
Cl
I
0.60
I
I
I
0.65
0.70
Fig. 10. The diagram (Mgb, V- R<) for the very central part of NGC 5533. Observational points taken along the radius with the step of 1.3” are connected by dashed lines: circles with dots azimuthally averaged data, circles with pluses - measurements along the NW part of minor axis. The model metallicity sequences from Worthey (1994) (solid lines) are marked at the values [m/H] = +0.5, +0.25, 0.0, - 0.25, - 0.5, - 1.0, - 1.5, - 2.0 by points (T= 17 billion years), triangles (T = 12 billion years), squares (T = 8 billion years), asterisks (T = 5 billion years), and open stars (T= 3 billion years).
15-26
analysis
Fig. 11 presents results of the isophotal analysis of the images of NGC 5533 in all four bands BVR,Ic. They agree very well. Probably, a line of nodes of the inner disk is oriented at PA = 28”; two prominent PA minima at R = 5 ” and R = 13 ” may be related to the dust lanes at these radii. The possible turn of the isophote major axis toward the nucleus must be restricted by 3”-4”, so Fig. 1 la confirms a result on axisymmetric brightness distribution in the inner part of NGC 5533. Ellipticity trend along the radius (Fig. 1 lb) is quite monotonous which is also consistent with an axisymmetric brightness distribution. Moreover, it gives a strong argument in favour of flatness of the photometric subsystem dominating between R = 20” and 50”: the ellipticity reaches its maximum at R = 15” and is nearly constant outside this radius up to the boundary of the galaxy. It means that photometric influence of the bulge is negligible outside R = 15”. The behaviour of the fourth coefficient of the azimuthal Fourier decomposition, a4 la, in the central part of NGC 5533 is somewhat chaotic due to the dust lanes: it oscillates between 0 and +O.Ol; but there is no signs of a boxy bulge or bar in the galaxy.
O.K. Sil’chenko
0.i+
101
20’ 8,
30’
I
40’
et al. I New Astronomy
/
!50
arcsec
Fig. Il. The results of isophotal analysis for NGC 5533: upper position angle of the major axis, lower - axis ratio. The isophotes are fitted by ellipses.
5. Conclusions NGC 5533 is a regular isolated spiral galaxy which was included earlier into the list of giant low surface brightness galaxies (Sprayberry et al., 1995) and into the sample of galaxies with extended HI disks (Broeils & Van Woerden, 1994). We have undertaken a detailed spectral study of the central part of NGC 5533. We have found that an unresolved nucleus of this galaxy is chemically decoupled. Firstly, there exists a magnesium-line strength break between the nucleus and the nearest bulge corresponding to a metallicity difference of 0.24*0.06dex. Secondly, only in the nucleus the ratio [Mg/Fe] > 0, while in the bulge a solar magnesium to iron ratio is observed. The nucleus of
3 (1998) 15-26
2s
NGC 5533 is also dynamically decoupled: the stellar rotation velocity has a local maximum at R - 3” with its noticeable drop outside this radius. We have estimated a mass of the decoupled nucleus: M 5 3.5 . 10yMo. As NGC 5533 is an isolated galaxy with a quite regular structure, an origin of the decoupled nucleus can scarcely be related to interaction or merging. So we try to find any intrinsic peculiarities which may be related to the origin of the decoupled nucleus. The photometric study of NGC 5533 has shown that though a disk (flat) component dominates surface brightness distribution at R 2 15”, we cannot fit the whole brightness radial profile by a single exponential law. We propose a model of two embedded disks. The inner one is quite typical for earlytype spiral galaxies, with a characteristic scale of 4.2 kpc and a central surface brightnesses ,u,,,~ = 19.4 and I-+,~ = 20.2. The outer one, which dominates at R > 13’kpc, has a characteristic scale larger by a factor of two, 9 kpc, and rather low central surface brightness; it is just this disk that was measured earlier and gave a start to including NGC 5533 into the list of low surface brightness galaxies. If we also take into account an extended HI disk which is detected up to 2.5 optical radii (Broeils & Van Woerden, 1994), we can state an existence of three tiers in the global disk of NGC 5533. It is not the only case of such a multi-tier structure of global disks in isolated spiral galaxies. Just the same threetier structure with extended HI disk was recently found in NGC 157(Ryder et al., 1997). It would be natural to suggest that each stellar disk had a proper star formation epoch. Obviously, each disk formation epoch was preceded by a gas density re-distribution in the global disk. Perhaps, one of such events had also provoked a formation of the decoupled nucleus in NGC 5533. A possible nature of such gas redistributions needs a further investigation.
Acknowledgements We are very grateful to the astronomers Special Astrophysical Observatory RAS
-
of the V.L.
26
O.K. Sil’chenko et al. I New Astronomy 3 (1998) 15-26
Afanasiev, S.N. Dodonov, and S.V Drabek providing the observations at the 6 m telescope and V.H. Chavushyan for the help at the lm telescope. During the data analysis we have used the Lyon-Meudon Extragalactic Database (LEDA) supplied by the LEDA team at the CRAL-Observatoire de Lyon (France) and the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. For reduction of Fabry-Perot data, we have used the software ADHOC developed at the Marseille Observatory. The work was supported by the grant of the Russian Foundation for Basic Research No. 95-02-04480. The 6 m telescope is operated under the financial support of Science Department of Russia (registration number 01-43).
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