Laboratoire d'Astronomie Spatiale du CNRS, Traverse du Siphon, B.P.8, 13376 Marseille Cedex 12, France
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Institut d'Astrophysique, CNRS, 98 bis Boulevard Arago, 75014 Paris, France
Keywords: galaxy, stellar population, young stars
NGC 5102 is a gas-rich S0 galaxy, where recent star formation has been inferred in the inner bulge from abnormally blue color (van den Bergh 1976, Pritchet 1979) and the ultraviolet spectral energy distribution (Rocca-Volmerange & Guiderdoni 1987, Burstein et al. 1988, Kinney et al. 1993). Large-scale plates have also shown a small number of faint resolved stars and two HII regions in the outer part of the galaxy (van den Bergh 1976). We report here the first detection of individual blue stars in the central region, using the Faint Object Camera (FOC) with the filters F175W and F342W. Following is a preliminary discussion on the nature of these stars in a color-magnitude diagram. The FOC images also reveal several dust patches. Their presence is not unusual in early-type galaxies and S0 galaxies (Kormendy & Djorgovski 1989) and dust has been detected in the specific case of NGC 5102 (Danks et al. 1979). The HST resolution is now known to enhance the detection capability of dust lanes close to the center of early-type galaxies (Van Dokkum & Franx 1995).
The images reported here were taken on 1995 May 26, using the 512512 pixel imaging mode (pixel size 0.014 arcsec, field of view 7 arcsec) of the f/96 camera of the FOC (Jedrzejewski et al., 1994). Total exposure times of 4075 s and 1789 s were obtained with the filter F175W and F342W respectively. Stellar fluxes were computed using the standard IRAF aperture photometry package, following basically the procedure of Paresce et al. (1995).
The main difficulty here comes from the unresolved galaxy light which increases by a factor of the order of 20 from the edge of our field to the inner bulge. We have run the daofind star finding routine with different threshold values in order to keep a relatively homogeneous significance of star detection. These threshold values were kept at approximately 6 above the average sky level and changed accordingly in different domains following isophotal shapes. The limiting magnitude is lower in the central regions. The procedure was run on the F175W image and results in a limiting m magnitude of 23 in the outer parts of the field. The photometric measurements are displayed in a color-magnitude diagram (Figure 1). The magnitudes m and m of a star were calculated as
where I is the inverse sensitivity of the modes used (7.65 10 and 3.50 10 ergs cm s Å per counts/s respectively for the F175W and F342W frames), c is the total number of counts attributable to the star, t the exposure time in s and the energy fraction associated with the aperture photometry. The latter factor accounts for the amount of light missed by the aperture as well as unduly subtracted out in the background reference annulus. We have adopted = 0.46 and = 0.51 (aperture radius of 3 pixels, background annulus with inner and outer radii of 4 and 7 pixels respectively).
Figure: Color-magnitude diagram with (a) the isochrones of Bertelli et al. for the ages log t = 6.6, 7.0, 7.2, 7.4 years (from top to bottom), (b) the P-AGB evolutionary tracks of Vassiliadis and Wood for the core masses 0.900, 0.677, 0.569 M (from top to bottom). The discontinuity in two of these tracks is due to the use of black body curves in place of stellar atmosphere models at high effective temperatures.
The isochrones of Bertelli et al. (1994) for the initial chemical composition [Z=0.02, Y=0.28] and the ages = 6.6, 7.0, 7.2 and 7.4 years have been superposed on the data of Figure 1. The absolute magnitude and effective temperature at different steps along the isochrones have been transformed in the m vs. mm plane, using the synphot synthetic photometry package and the stellar atmosphere models of Kurucz (1979). A distance modulus of 27.47 and a foreground extinction with E(B-V)= 0.05 have been adopted (McMillan, Ciardull, & Jacoby 1994).
Most of the blue stars detected and plotted in the color-magnitude diagram appear as resulting from a recent star formation episode that ended 1.5 10 years ago. Integrated light spectra and population synthesis models (e.g., Bica & Alloin 1987) have suggested a burst age (if time-peaked) of a few 10 years.
The stellar population responsible for the bulk of the visible light of the bulge is presumably old enough to include Post-Asymptotic Giant Branch (P-AGB) stars; a number of planetary nebulae have indeed been identified by McMillan et al.(1994). The P-AGB stars are known to be the most UV luminous of the various hot and low-mass stars that can be anticipated in an old population (e.g., Greggio & Renzini 1990, Dorman et al. 1995).
We have, therefore, superposed the evolutionary tracks for P-AGB stars of Vassiliadis & Wood (1994) (H-burning PNN evolutionary models for core masses of 0.569, 0.677 and 0.900 M and metallicity Z=0.016) on the data. The calculations are the same as before but use bolometric luminosities and black-body curves when the effective temperatures are larger than encompassed by Kurucz's models.
Figure 1 shows that some of the blue stars may be P-AGB stars and not young stars. However, their number should not be large because the distribution of the stars in the color-magnitude diagram does not match the time evolution pattern on the P-AGB tracks. First, they appear concentrated on the red part of the evolutionary tracks while the time evolution is not faster at high temperature; it has been verified that the lack of very blue objects does not result from a selection effect (the F342W frame is deep enough). Second, the time evolution strongly decreases when the stellar core-mass decreases, i.e., from upper to lower tracks.
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