Laboratoire d'Astronomie Spatiale du CNRS, Traverse du Siphon, B.P.8, 13376 Marseille Cedex 12, France
Keywords: stellar population
The dwarf spheroidal galaxy Leo I has been observed in the ultraviolet with the Faint Object Camera (FOC) in an attempt to resolve UV-bright stars intrinsically fainter than the hot stars resolved in the bulge of the galaxy M31 and identified as Post-Asymptotic Giant Branch (P-AGB) stars (King et al. 1992, Bertola et al. 1995). That stars intrinsically less luminous than the P-AGB stars may significantly contribute to the UV emission of an old population is suggested by several lines of evidence:
(i) The FOC images of the central bulge of M31 show a diffuse UV background due to unresolved UV-bright stars (the two above references differ by the amount of light ascribed, respectively, to unresolved true UV light and contamination through filter red leak).
(ii) Before the HST results, theoretical arguments and the interpretation of IUE data (e.g., Greggio & Renzini 1990, Brocato et al. 1990) favored less luminous and longer lived UV-bright stars than P-AGB stars as the most important UV producers, especially in metal-rich environment. The different evolutionary paths of low-mass stars at hot temperature have been further investigated by several authors (Dorman et al. 1995 and references therein).
(iii) The integrated UV spectra obtained down to 912Å with the Hopkins Ultraviolet Telescope on the elliptical galaxy NGC 1399 (Ferguson et al. 1991), the bulge of M31 (Ferguson & Davidsen 1993 ), and recently six more elliptical (Brown et al. 1995) are best explained by composite star population (not simple P-AGB star population).
The present observation is one of several possible approaches for better understanding the nature of UV-bright stars in elliptical-like population. It takes advantage of the short distance of local dwarf spheroidal galaxies: in Leo I (selected for its relatively high surface brightness) stars are expected to be resolved down to the horizontal branch. There is, however, a difficulty, as discussed in the following, with the complex star formation history in most of these objects, especially the relatively recent star formation in Leo I.
The images reported here were taken on 1995 April 16, using the 5121024 pixel imaging mode (pixel size 0.0280.014 arcsec, FOV 1414 arcsec) of the f/96 camera of the FOC. A total exposure time of 8386 s was obtained with the filter F175W and 4193 s with the filter F342W. Stellar fluxes were computed using the standard IRAF aperture photometry package.
Since the UV image reveals only a handful of faint stars at the limit of detection, we have preferred to run the daofind star finding routine in the deeper frame F342W (2.6 mag deeper than F175W for an energy-flat distribution) and to set there the detection threshold. We then measure the stellar flux in the F342W frame and the counterpart, if any, in the UV frame. This procedure has the drawback of giving a lot of false detections in the UV frame. By visual inspection, we have set a flux limit at magnitude 24 below which any detection is very likely to be spurious (mag 24 corresponds to a signal-to-noise ratio of about 4). Another difficulty is the value of the detection threshold: if too high, the counterparts in the UV are only due to the red leak of the F175W filter and we may miss faint blue objects; if too low, we increase the number of false stars and especially of false blue stars. We have made a trade-off by trials, allowing a few blue stars, eventually discarded as spurious by visual inspection (they were also fainter than the limit of mag 24). After a final verification that no significant signal was left over in the UV frame by the procedure used (both by visual inspection and running the daofind routine), we conclude that the sample displayed in the m vs. mm color-magnitude diagram (Figure 1) is reasonably complete down to magnitude 24 and should not contain many false UV detections. The magnitudes m and m of a star were calculated as
where I is the inverse sensitivity of the modes used (6.12 10 and 2.80 10 ergs cm sÅ 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 (aperture radius of three pixels, background annulus with inner and outer radii of four and seven pixels, respectively) and = 0.72 (aperture radius of five pixels, background annulus with inner and outer radii of 10 and 15 pixels, respectively).
Figure: Color-magnitude diagram with the isochrones (solid lines) from Bertelli et al. (1994) for the ages log t = 6.6, 9.0 years and the blue part of the zero age horizontal branch (dashed line) from Dorman et al. (1993)
The theoretical isochrones of Bertelli et al. (1995) for Z=0.0004 (appropriate for the metallicity of Leo ranging from [Fe/H] of -1 to -2, Demers et al. 1994) have been transported into the m vs. mm plane, using the stellar atmosphere models of Kurucz (1979) and the synphot synthetic photometry package. A distance modulus of and a foreground extinction with a color excess E(B-V)= 0.02 were adopted (Lee et al. 1993).
Given the various uncertainties, the observed stars are reasonably consistent, except for the bluest object, with the isochrone for an age of 1Gyr displayed in Figure 1. These stars would be the youngest stars of Leo I, formed in a star formation episode that ended some 1Gyr ago. This conclusion is in agreement with the previous finding that Leo I has a substantial population of MS turn-off stars with ages of 3Gyr and is the youngest Milky Way dwarf spheroidal galaxy (Lee et al. 1993).
However, a number of the stars in the CMD may be Horizontal Branch (HB) stars of an older population as shown by the ZAHB (Z=0.0006) of Dorman et al. (1993) the blue part of which is plotted in Figure 1. Quantitatively, the number of possible BHB (Blue HB) stars appears to be between 0 (the detection of the bluest star in the CMD is uncertain) and, let say, four in the field of view.
How does this result compare with the UV observations of elliptical galaxies and does it tell something on the nature of the UV-bright stars in elliptical galaxies? Let us take the central bulge of M31 for comparison and scale down its UV flux to the case of Leo I, according to the V central surface brightness of the objects. For M31, we adopt the IUE data and the V magnitude referring to the same diaphragm, as given by Burstein et al. (1988). The observed V surface brightness of Leo I is taken as 22.3 mag arcsec (Lee et al. 1993). We predict 16 BHB stars with an average m magnitude of 23 in our 1414 arcsec field of view at the center of Leo I. There are three possible explanations for the fact that we observe less BHB stars (if any) than predicted.
(i) The (1550-V) color of elliptical galaxies is known to get redder with the Mg absorption line strength decreasing (Burstein et al. 1988). Because of its low metallicity, Leo I would compare better with a galaxy at least 1 magnitude redder than M31, which means predicting 2.5 times less BHB stars in the field.
(ii) Another category of stars, more luminous but in smaller number than the BHB stars would be contributing to the UV flux of elliptical-like population. They would be statistically absent in our small field of view.
(iii) Since the observed V surface brightness of Leo I includes the contribution of a recent generation ( 1Gyr) of stars in addition to that of the older population, the prediction actually gives an upper limit to the number of BHB stars in the field.
In conclusion, the peculiar star formation history of Leo I makes uncertain the number of BHB stars detected (if any) and prevents any general conclusion on the nature of UV-bright stars in old stellar populations to be reached.
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