- Email: [email protected]
UV (2000Å) observations of nearby well resolved spiral galaxies
Ada. Space Res. Vol.11, No. ll,pp. (11)143—(11)147, 1991 Printed in Great Britain. Allrights reserved.
0273—1177/91 $0.00 +.50 Copyright © 1991 COSPAR
UV (2000A) OBSERVATIONS OF NEARBY WELL RESOLVED SPIRAL GALAXIES .* A. Vuillemin, V. Buat, * J. ** M. Golay *
Donas,* B. Milhard, * G. Courtes * and ..
Laboraro ire d’Astronomie Spatiale, Traverse du Siphon, Les trois Lucs,
F-13012 Marseille, France ** Observatoire de Genève, Ch. des Mailletes 51, CH-1290 Sauverny, Switzerland
ABSTRACT UV (2000A) images of nearby well resolved galaxies (M31, M33, M51, M81, MiOl and NGC4258) have been obtained during repetitive flights of a balloon-borne 40-cm, 12 20 arcsec resolution telescope. In the case of M5 1, the ultraviolet flux was derived for 36 circular regions where published gas contents exist. After corrections for dust extinction, the UV luminosity per unit area is found to correlate the HI+H2 surface density and thus confirms that the total gas density is a driving parameter for large scale star formation processes. -
INTRODUCTION The UV emission of spiral galaxies is a tracer of their young stars population. Under some reasonable hypotheses, this emission is simply proportional to the current star formation rate (SFR) in their disk (Donas et aL) /1/. In this context several galaxies(M31, M33, M51, M81, Miol and NGC4258) have been observed in UV with a resolution of 12 or 20 arcsec depending on the optics used. After a description of observations and data reduction, the resulting UV emission of M51 and MiOl will be presented with their mean features. In the case of M51, the preliminary results of a comparison with the (HI+H2) gas content will be shown. OBSERVATIONS AND DATA REDUCTIONS The instrument is briefly described by B. Milliard et al. /2/. The telescope has a primary mirror of 39cm diameter, a field of view of 1.5 or 2.3 degrees, the bandpass is l5OAwide, and observing times are typically of 1000 sec. All frames were digitized on a PDS microdensitometer using 25jsm square aperture. Flat field correction, intensity conversion and detection of objects (stellar and non-stellar) with a precision of 4 arcsec were performed with a software developed in the laboratory. The conversion of celestial coordinates to plate coordinates were performed by the use of a selection of 75 stars in common with the GSC catalogue on the M51 frame. MAIN FEATURES OF M51 and MiOl M51 (Figure 1) is associated with a companion (NGC5 195) well observed in IR and classified
(11)143
(11)144
A. Vuillemjn
e~al.
SB Olp, (barred galaxy). These two galaxies are in gravitational interaction, a comparison from different colours (IR to UV) shows that progressively the image of the companion dims. A possible interpretation is that UV light is absorbed by a cloud of interstellar matter in the prolongation of an M51 arm. This offers a way to study the extinction law in a galaxy different than our. The bright dots near the companion would perhaps be indicators of star formation. M101(Figure 2), seen almost face on like M51, shows well defined arms; it is a good candidate for star formation and spiral arms studies. The arms are narrower in UV than in visible light where the dominant stars are older and have been dispersed.(Golay et al. /3/) M51: A COMPARISON BETWEEN THE UV EMISSION AND GAS CONTENT To compare UV data to the gas content, we have used CO measurements of Scoville and Young /4/ in 36 fields of 50 arcsec (HPBW), for which the pointing precision is 4 arcsec. These fields were positioned on our frame with a comparable accuracy. Integration in circular fields (50 arcsec diameters), centred on Scoville positions were done by the use of MIDAS software system. Before any quantitative interpretation, correction for dust extinction must be performed. The main difficulty is that the UV emission is largely affected by dust extinction reaching very large values especially in the arms (van der Hulst et al./5/). As a first step, the extinction correction have been estimated using the total gas content ( Buat et al. /6/). A very good correlation is found between the corrected UV luminosity per unit area (Figure 3) and the total gas surface content which is largely dominated by the molecular component(correlation coefficient R=0.9). This correlation also holds with the corrected UV emission (Figure 3, correlation coefficient RerO.8). Thus the SFR seems to depend strongly on the gas content as already seen. In fact, dust extinction correction affects mainly the power-law exponent, that is to say the slope of the linear regression of the log of SFR per unit area on that of total gas content which is lowered from 1.53 (with correction) to 0.72 (without dust correction). /7/ Better estimates of dust extinction corrections are needed to obtain a more accurate determination of the power-law dependence of the SFR on the gas content. This might involve complementary data like FIR emission, Hc~and/or radio measurements. ACKNOWLEDGEMENTS Thanks to people of FOCA team (LAS) and “ Observatoire cle Genève” who have made this experiment possible. REFERENCES 1.- J. Donas, .J.M. Deharveng, M. Laget, B. Milliard and D. Huguenin, Ultraviolet Observation and star formation rate in galaxies, Astron. Astrophys 180, 12-26 (1987) 2.- B. Milliard, 3. Donas and M. Laget, A 40-cm UV (2000A) balloon-borne imaging telescope results and current works, this symposium 3.- M. Golay, D. Huguenin, A. Blecha, N. Cramer, Images insolites de quelques galaxies bien connues, ORION 233/234 4.- N. Scoville and 3.5. Young, The molecular gas distribution in M51, Ap.J. 265, 148-165(1983) 5.- J.M. van der Huist, R.C. Kennicut, P.C. Crane, A.H. Rots, Radio properties and extinction of the HII regions in M51, Astron. Astrophys 195, 38-52 (1988)
Nearby Well Resolved 6.-
V. Bunt,, J.M. Deharveng
type galaxies,
Astron.
and J. Donas,
Astrophys
7.- Y. Bust, Star formation IAU symposium 146
223,
(11)145
Spiral Galaxies
Star formation
rate and gas surface
density
in Iate
42-46 (1988)
rate from UV emission
and gas surface density
Fig. 1. M5 1 at 2000A.
in late type galaxies,
(11)146
A. Vuillemjnetal.
.0
~
;
~
~~JIF
~~
Fig. 2. MiOl at 2000A.
Nearby Well Resolved Spiral Galaxies
M51 No correction
(11)147
•
M51 corrected
•
•
• S
S.
•
•S
•
o o o (N
•,
~ ~j
-
•
0
•
~
S
•
0
•
~
5
•S.•S
.
~—4
0 0 0
S
••
-
•:
55 S
$
(N •S
.9
.9
10 -s
S S
•S
S
S
I
I
lot
10
Iog(Hk-H2)
Fig. 3. UV Luminosity versus (H1+H2).
102 Iog(Hl+H2)
0273—1177/91 $0.00 +.50 Copyright © 1991 COSPAR
UV (2000A) OBSERVATIONS OF NEARBY WELL RESOLVED SPIRAL GALAXIES .* A. Vuillemin, V. Buat, * J. ** M. Golay *
Donas,* B. Milhard, * G. Courtes * and ..
Laboraro ire d’Astronomie Spatiale, Traverse du Siphon, Les trois Lucs,
F-13012 Marseille, France ** Observatoire de Genève, Ch. des Mailletes 51, CH-1290 Sauverny, Switzerland
ABSTRACT UV (2000A) images of nearby well resolved galaxies (M31, M33, M51, M81, MiOl and NGC4258) have been obtained during repetitive flights of a balloon-borne 40-cm, 12 20 arcsec resolution telescope. In the case of M5 1, the ultraviolet flux was derived for 36 circular regions where published gas contents exist. After corrections for dust extinction, the UV luminosity per unit area is found to correlate the HI+H2 surface density and thus confirms that the total gas density is a driving parameter for large scale star formation processes. -
INTRODUCTION The UV emission of spiral galaxies is a tracer of their young stars population. Under some reasonable hypotheses, this emission is simply proportional to the current star formation rate (SFR) in their disk (Donas et aL) /1/. In this context several galaxies(M31, M33, M51, M81, Miol and NGC4258) have been observed in UV with a resolution of 12 or 20 arcsec depending on the optics used. After a description of observations and data reduction, the resulting UV emission of M51 and MiOl will be presented with their mean features. In the case of M51, the preliminary results of a comparison with the (HI+H2) gas content will be shown. OBSERVATIONS AND DATA REDUCTIONS The instrument is briefly described by B. Milliard et al. /2/. The telescope has a primary mirror of 39cm diameter, a field of view of 1.5 or 2.3 degrees, the bandpass is l5OAwide, and observing times are typically of 1000 sec. All frames were digitized on a PDS microdensitometer using 25jsm square aperture. Flat field correction, intensity conversion and detection of objects (stellar and non-stellar) with a precision of 4 arcsec were performed with a software developed in the laboratory. The conversion of celestial coordinates to plate coordinates were performed by the use of a selection of 75 stars in common with the GSC catalogue on the M51 frame. MAIN FEATURES OF M51 and MiOl M51 (Figure 1) is associated with a companion (NGC5 195) well observed in IR and classified
(11)143
(11)144
A. Vuillemjn
e~al.
SB Olp, (barred galaxy). These two galaxies are in gravitational interaction, a comparison from different colours (IR to UV) shows that progressively the image of the companion dims. A possible interpretation is that UV light is absorbed by a cloud of interstellar matter in the prolongation of an M51 arm. This offers a way to study the extinction law in a galaxy different than our. The bright dots near the companion would perhaps be indicators of star formation. M101(Figure 2), seen almost face on like M51, shows well defined arms; it is a good candidate for star formation and spiral arms studies. The arms are narrower in UV than in visible light where the dominant stars are older and have been dispersed.(Golay et al. /3/) M51: A COMPARISON BETWEEN THE UV EMISSION AND GAS CONTENT To compare UV data to the gas content, we have used CO measurements of Scoville and Young /4/ in 36 fields of 50 arcsec (HPBW), for which the pointing precision is 4 arcsec. These fields were positioned on our frame with a comparable accuracy. Integration in circular fields (50 arcsec diameters), centred on Scoville positions were done by the use of MIDAS software system. Before any quantitative interpretation, correction for dust extinction must be performed. The main difficulty is that the UV emission is largely affected by dust extinction reaching very large values especially in the arms (van der Hulst et al./5/). As a first step, the extinction correction have been estimated using the total gas content ( Buat et al. /6/). A very good correlation is found between the corrected UV luminosity per unit area (Figure 3) and the total gas surface content which is largely dominated by the molecular component(correlation coefficient R=0.9). This correlation also holds with the corrected UV emission (Figure 3, correlation coefficient RerO.8). Thus the SFR seems to depend strongly on the gas content as already seen. In fact, dust extinction correction affects mainly the power-law exponent, that is to say the slope of the linear regression of the log of SFR per unit area on that of total gas content which is lowered from 1.53 (with correction) to 0.72 (without dust correction). /7/ Better estimates of dust extinction corrections are needed to obtain a more accurate determination of the power-law dependence of the SFR on the gas content. This might involve complementary data like FIR emission, Hc~and/or radio measurements. ACKNOWLEDGEMENTS Thanks to people of FOCA team (LAS) and “ Observatoire cle Genève” who have made this experiment possible. REFERENCES 1.- J. Donas, .J.M. Deharveng, M. Laget, B. Milliard and D. Huguenin, Ultraviolet Observation and star formation rate in galaxies, Astron. Astrophys 180, 12-26 (1987) 2.- B. Milliard, 3. Donas and M. Laget, A 40-cm UV (2000A) balloon-borne imaging telescope results and current works, this symposium 3.- M. Golay, D. Huguenin, A. Blecha, N. Cramer, Images insolites de quelques galaxies bien connues, ORION 233/234 4.- N. Scoville and 3.5. Young, The molecular gas distribution in M51, Ap.J. 265, 148-165(1983) 5.- J.M. van der Huist, R.C. Kennicut, P.C. Crane, A.H. Rots, Radio properties and extinction of the HII regions in M51, Astron. Astrophys 195, 38-52 (1988)
Nearby Well Resolved 6.-
V. Bunt,, J.M. Deharveng
type galaxies,
Astron.
and J. Donas,
Astrophys
7.- Y. Bust, Star formation IAU symposium 146
223,
(11)145
Spiral Galaxies
Star formation
rate and gas surface
density
in Iate
42-46 (1988)
rate from UV emission
and gas surface density
Fig. 1. M5 1 at 2000A.
in late type galaxies,
(11)146
A. Vuillemjnetal.
.0
~
;
~
~~JIF
~~
Fig. 2. MiOl at 2000A.
Nearby Well Resolved Spiral Galaxies
M51 No correction
(11)147
•
M51 corrected
•
•
• S
S.
•
•S
•
o o o (N
•,
~ ~j
-
•
0
•
~
S
•
0
•
~
5
•S.•S
.
~—4
0 0 0
S
••
-
•:
55 S
$
(N •S
.9
.9
10 -s
S S
•S
S
S
I
I
lot
10
Iog(Hk-H2)
Fig. 3. UV Luminosity versus (H1+H2).
102 Iog(Hl+H2)