Galaxy kinematics from ISIS spectra

Vistas in Astronomy, Vol.34, pp. 187-200,1991 Printedin GreatBritain.Allfightsreserved.

0083--6656/91$0.00+ .50 © 1991PergamonPressplc.

GALAXY KINEMATICS FROM ISiS SPECTRA J. Castles,* C. D. McKeith* and A. GrevCf *Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast T7 INN, Northern Ireland tlnstituto de Radioastronomia Milimetrica, Avd. Divina Pastora, 7-Nucleo Central-18012, Granada, Spain

Abstract The ISIS spectrograph on the 4.2 m WHT was used to observe three galaxies NGC 2146, NGC 1569, and NGC 3034 (M82). Rotation curves along the major axes of these galaxies are presented. For NGC 3034, the rotation curve derived using the relatively new technique for measuring stellar motion from one of the the CalI IR triplet (,~8550 .~) absorption lines is compared with those for the ionized gas motion measured from Ha and [NII] (6583 ~) emission lines. The line splitting with slit oriented along the minor axis of NGC 3034 covering 4 arc rain. is examined.

Introduction The optical spectral region employed to measure the rotation of a galaxy about its nucleus should include, ideally, several absorption lines (from the stars) and several emission lines (from the hot gas). The wavelength shift of line centres, plotted as a function of distance from the nucleus constitutes a rotation curve, from which physical information about central mass distribution, and total enclosed matter, as well as stellar and gaseous kinematics may be derived. Both the stellar and gas motion require to be examined as these are not necessarily identical. In an extreme case, e.g. NGC 4546, the gas and stars rotate in opposite directions (Bettoni et al. 1991). To determine the motion of the gas, emission lines such as [SIII] 9069 ~, [SII] 6716,6731 .~, OI 8446 A, [OII] at 3727,3730 .~, and the Balmer and Paschen series from hydrogen are most appropriate if a very wide spectral range is available at high spectral resolution (R-~ 104). 187

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The determination of the motion of the stellar population is more problematic due to a lack of strong lines in early type stars. In the past (see Dressler 1984, and references therein) use was made of CaJI H and K lines (3934,3968 .~,) which are strong but intrinsically broad, CaI G band (,-~4227 A), the MgI b triplet (N5200 A), and also the Na D lines (,,,5900 A) which may be complex because of absorption by interstellar gas in the Galaxy and target galaxy. Stellar Balmer series absorption lines may be used for galactic rotation curves although they are intrinsically broad. ~)0 :~50

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Fig.1 Rotation curve measured along the long axis of NGC 2146 from Eo. The position angle of the slit was 141 °. Lately the potential in the near IR of the three Ca II lines (8498,8542,8662 .~) which are strong and intrinsically narrow has been exploited (Dressier 1984, Terlevich et al.1990). These lines are present in a wide variety of stellar types except for O,B stars( Dressler 1984). Thus for the study of the kinematics of the stellar population of galaxies these CaII lines are better suited than other absorption lines in more accessible visible spectral regions.However each of the Ca lines has a weak Paschen emission line blended in the red wing which causes a small asymmetry. This may be serious where the Paschen lines are very strong and the Ca lines weak. A general problem in the near IR region of the spectrum is the heavy contamination by

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emission lines from the earth's atmosphere (Osterbrock et al 1990). For galaxies which typically require 2000 sec exposure these sky lines may be many times stronger than the lines from the target galaxy and must be removed before rotation curves can be measured (Dressier and Pdchstone 1988). A COmmOn way to remove sky lines is to make a sky exposure at a small distance from the target. This is subtracted from the spectral image of the target and only lines from the target galaxy remain. The disadvantage with this method is that it is wasteful of time. However as the the ISIS spectrograph slit is long enough to include all the major axis of the galaxy and also a small area of dark sky on a two dimensional detector. It is then possible to eliminate specific sky observations entirely, thus making simultaneous observation of sky and targets. 06o

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Observations and reduction of ISIS data. Observations were made at the Royal Greenwich Observatory, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrophysica de Canada, using the 4.2 m WriT with the recently commissioned ISIS spectrograph. The gratings employed were 600B and 600R.

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The slit was 4 arcmin long and the observations described later cover approximately 1000 A of spectrum in each of the simultaneous blue and red (or near IR) spectral images. ~0

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Fig. 3 Rotation curve of NGC 1569, Ha measured with sllt passing through the two bright knots.

The spectral resolution measured from arc lines was 2.1 ~ in the blue, and 1.8/~ in the near IR. Spatial resolution measured from a star was 3.6 arc sec. in the blue, and 1.5 arc sec. in the near IR. Distortion free spectral images are required to ensure accuracy of measured rotation curves and also for the method of skyline subtraction, Correction specific to each spectral image was achieved as most galaxy images usually have one or more bright knots (or foreground stars) included and these were used to correct distortion along the spectral axis. Distortion along the spatial axis was corrected using sky lines. A curve was fitted to the distorted sky lines and this used to determine the degree of correction required.. Variation of pixel to pixel and large scale response across the spatial axis of the spectral images were corrected using fiat field exposures. The Starlink software packages, Figaro

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(Shortridge and Bridget) and Dips• (Howaxth) were used. The galaxies observed were (1) NGC 2146, a laxge dusty spiral with fast rotation, (2) NGC 1569, a blue irregular galaxy with little if any rotation, and NGC 3034 (M82) an irregular galaxy with some disk-like morphology and intermediate rotation speed. 140 O0

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NGC 2146 is a bright galaxy measured in the fax IR (NASA reference publication 1190). It has a disturbed morphology, with an arm llke structure 50 kpc long consisting of a string of HII regions protruding at an angle from the general disk. Several heavy dust lanes are evident, and current intense star formation is thought to be shrouded by the thick dust (Benvenuti et al. 1975, Burbidge et al. 1959, Young et al. 1988). It is possible that a very large HI gas cloud surrounds NGC 2146 (Fisher and Tully 1976). Moreover the motion measured along the major axis has been reported as highly non-circular (Young et al 1988) and there is a significant difference between the motion of the HII regions in the arm and HII regions in the maln disk.

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The starburst activity and confused morphology has Lead to the suggestion that NGC 2146 may be two objects in the process of merging (Hutchings et al. 1990). Our observations were made at several PA's ~ud three wavelength ranges. The rotation curve for Ha , with the systemic velocity subtracted, is shown in Fig.l, and is generally similar to those measured by Benvenuti et al.(1975), and Hutchings et al. (1990). Measurement of the radial velocity difference at -4-40arc sec from the centre of rotation is is ~500 km s -1, uncorrected for the inclination of the plane of NGC 2146. The rotation curve (position angle 141 °) has the overall shape of a rotating disk, but superimposed are several significant deviations. The feature at ~ 10 arc sec, Fig. 1, corresponds to an intense Ha knot, and the strong feature at N -35 arc sec corresponds to a distinct strand of material which may be merging with the main disk. 140 120

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The peculiar dwarf galaxy NGC 1569 is at a distance of 2.2 Mpc and radial velocity 40km~ -1 and has often been compared to the Magellenic clouds. It has an evolved OB stellar population and it has been suggested on the basis of intrinsic UV, U-B, B-V, and V-K colours

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,that NGC 1569 has had at least one starburst episode, about 10~ years ago and that this may be the most recent of a series of such events (see Israel 1988, and references therein, Waller 1991). There are two bright knots A and B, blue stellar-like objects, in NGC 1569 whose nature are highly controversial. Ables (1971) concluded that they were foreground stars. Hodge (1974) presented Ha images of different exposures which showed much filamentary structure, as well as the objects A and B. De Vaulcouleurs studying the Ha velocity fields (1974) later came to the conclusion (1981) that the Ha motions were consistent with explosive starburst activity centered on these two bright knots about 10~ years ago.This highlights the controversy that A and B are stellar condensations in NGC 1569 or stars in our Galaxy (see Arp and Sandage 1985). Figure 2 shows the spectrum of one of the bright knots (object A) along with the spectrum of the surrounding gas region. Spectrum (a) is from object A and shows typical stellar absorption lines, while (b) shows gas emission lines. There is no easily measured difference in the radial velocity of the absorption lines and that of the emission lines to differentiate between A and B as foreground stars or superclusters in NGC 1569. Figure 3 shows the rotation curve for NGC 1569 on the same velocity scale as NGC 2146 above to emphasize the difficulty of measuring any meaningful rotation for this galaxy. 2.2

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A straight line fitted to this curve shows a small rotation of -~ 1Okras -1 difference between positions -35 arc sec and +35 arcsec.

N G C 3034 This classical irregular starburst galaxy (M82) at a distance of 3.2 Mpc has been the subject of intense study and equally intense controversy. Evidence has been presented (Gottesman and Weliachew 1977) and (Cottrel 1977) of interaction with M81, ie a neutral hydrogen cloud around NGC 3034 with an arm extending to M81. 2.4

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radial velocity from the main trend of the curve. Some regions show 120 km s -1 difference. The features at -10 arc sec and +15 arc sec approximately, correspond with the tangential points of the IR and CO ring (see compilation by Gc3tz et al. 1990). O'Connel] and Mangano (1978) present an Ha rotation curve for the major axis of NGC 3034 where the overall velocity curve is similar to Fig. 4, but there is little indication of the small scale high/low velocity features. Nitrogen lines (6548,6583 ~) extend much further from the nucleus than Ha and are measurable right out to the edge of the spectral image. The rotation curve for [NII](6583 .~) is reproduced in Fig. 5. Comparison with Fig. 4 shows a corresponding number of fine scale kinematic features, though in the case of nitrogen, the greater extent of the measurable emission line reveals even greater velocity features, eg. the slow moving region at position ÷50 arc seconds. 0.60

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[SIII] line the Paschen series lines are evident, as are the stellar CaII IR absorption lines in this cleaned spectrum. An enlarged section of this spectrum is given in Fig. 8 to show the region around the triplet of absorption lines. The Paschen lines are small and although they do cause some asymmetry in the profile of the calcium lines this has a negligible effect on the measured line centre. The rotation of the stellar population measured from the CaII (8542 .~) line along the major axis is shown in Fig. 9. Comparison of the rotation curve measured from Ca II (8542 ~ ) with the curves measured from H~, shown in Fig.4, and [NII] (6583 ~[) Fig.

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similarities between the kinematic behaviour of the stellar population of NGC 3034 and the gas constituent. Firstly, the central steep gradient is found in all lines, as are the less steep regions beyond positions -20 arc sec and +20 arc sec. Secondly, some of the small scale features noted in the gas rotation curves are present in the stellar absorption line curve. Essentially the stellar population and gas are rotating together. This is a significant finding as earlier work (Burhidge et al. 1964, O'Connell and Mangano 1978, Saito et al. 1984) indicated that the stellar rotation curve derived from the Na D spectral absorption line did not exhibit the steep central gradient of the gas curve. 160 140 120 100 80 oOO 60 40

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Ca K and H absorption lines (3934,3968),) have often been used to determine stellar rotation (Saito et al.1984). The profile of Ca K at 3934 ~ is complex having at least two main blended components, see Fig. 10 (asterisks). Multiple components are seen in many interstellar resonance lines in our Galaxy, making CaII H and K more ambiguous lines than those of the IR CaII triplet for rotation measurements in external galaxies. Images of NGC 3034 in H~ show an apparent outflow of gas along the minor axis above and below the plane of the galaxy. The emission lines in these areas along the minor axis are spUt.The explanation of these features has been the subject of controversy since the 1960's (Bland and Tully 1988). One view is that there is gas flowing outward from the nucleus along the minor axis. The line splitting indicates radial velocity difference of about 250 km 8-1, implying a velocity away from the nucleus along the minor axis of about 600 km 8-1 (See G6tz et al. Fig. 5 1990). The alternative view is that the line splitting is produced by gas close to the nucleus and the radiation is reflected into our line af sight by dust which forms a large halo surrounding NGC 3034.In support of this it has been shown that indeed there is much polarized radiation from the regions involved (Visvanathan 1972) The splitting of H-alpha measured along the minor axis is presented in Fig. 11. 21

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The velocity of neither component of Ha is constant along the minor axis, and the separation between the two components also varies with distance from the nucleus. This favours motion along the line of sight. Beyond position +80 arc sec in Fig 11 three components of Ho are detected. Further,the two main components of Ho show a variation in the ratio of their intensities. H~ profiles from north and south of the nucleus are shown in Fig. 12. This additional factor will have to be accounted for in the interpretation of the evidence for bipolar outflow. 300, 25O 200 150

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Conclusion These preliminary observations with the ISIS spectrograph on the WriT are part of an ongoing program on Excitation, Extinction, and Kinematics in dusty IRAS/CO galaxies. This spectral investigation includes many other diagnostic lines eg. [OII] (3727,3729 ~), [OII] (7320,7330 .~), [SII] (6717,6731 .~), [OIII] (5007 .~), for determination of Te, and ne, and extinction measurements of the internal galaxy dust content from comparison of the predicted

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and observed emission line ratios (see Greve et al.1989). Here are presented one aspect of this investigation, the general rotation dynamics of three galaxies. 3.6

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Ables,tt.D.,(1971),Publ.US.Naval Obs.Sec.Ser XX(IV),61. Arp.ti.C.,Sandage,A.R.,(1985),Astrom J.,90,1163. Benvenuti,P.,Capaccioli,M.,D'Odorico,S.,(1975),Astr.Ap.,41,91 (BCD). Bettoni.D.,G alletta,G.,Osterloo,T.,(1991),MNRAS,248,544. Bland,J.,Tully, R.B.,(1988) Nature 334,43. Burbidge,E.M.,Burbidge.E.R.,Prendergast,K.H.,(1959),Ap.J.,130,739. Burbidge,E.M. ,Burbidge,G.R.,Rubin,V. C.,(1964),Astrophys.J., 140,942. Cottrell,G.A.,(1977),MNRAS,bf 178,577. De Vaucouleurs.G.,De Vaucouleurs,A.,Pence,W.,(1974),Astrophys.J.Letters, 194,Ll19. De Vaucouleurs.G.,(1981),Sky Te1.,62,406. Dressler,A.,(1984) Ap.J., 286,97. Dressler,A.,Richstone,D.O.,(1988),Ap.J.,324,701. Fisher,J.R.,Tully, R.B.,(1976),Astr.Ap.,53,397. Gottesman,S.T.,Weliachew,L.,(1977),Ap.J.,bf 211,47. GStz,M.,McKeith,C.D.,Downes.D.,Greve,A.,(1990),Astron.Astrphys.,240, 52. Greve,A.,McKeith,C.D.,Barnet t,E.W.,GSt z,M.,(1989),Astron.Astrophys., 215,113. ttodge,P.W.,(1974),Astrophys.J.Letters,191,L21. Itut chings,J.B.,Neff,S.G.,Stanford,S.A.,Lo,E.,Unger,S.W.,(1990),Ap.J.100, 60. Israel,F.P.,(1988),Astron.Astrophys.,194,24. N.A.S.A. Reference Publication 1190 (IRAS Catalogue 1988). O'ConneU,R.W.,Mangano,J.J.,(1978),Ap.J.,221,62. Osterbrock,D.E.,Shaw,A.,Veilleux,S.,(1990),Ap.J.,352,561. Saito,M.,Sasaki,M.,Kaneko,N.,Nishimura,M.,Toyama,K.:(1984) Publ. Astron. Soc. Japan,33, 305. Terlevich,E.,Diaz,A.I.,Terlevich,R.,(1990)MNRAS,242,271. Visvanathan,N.,Sandage,A.,(1972),Astrophys.J.176,57. Waller,W.ti.,(1991),Ap.J.,370,144. Young,J.S.,Claussen,T.J.,Kleinmann,S.G., (1988),Ap.J.,331,L81.