Star formation histories of resolved galaxies

4. STAR FORMATION HISTORIES OF GALAXIES OUTSIDE THE LOCAL GROUP

In galaxies beyond the LG, distance makes crowding more severe, and even HST cannot resolve stars as faint as the MSTO of old populations. The higher the distance, the worse the crowding conditions, and the shorter the lookback time tau reachable even with the deepest, highest resolution photometry. Depending on distance and intrinsic crowding, the reachable tau in galaxies more than 1 Mpc away ranges from several Gyrs (in the best cases, when the RGB or even the HB are clearly identified), to several hundreds Myr (when AGB stars are recognized), to a few tens Myr (when only the brightest supergiants are resolved).

The effect of distance on the possibility of resolving individual stars, and therefore on the reachable tau , is shown in Fig. 5, where the CMDs obtained from WFPC2 photometry of three late-type galaxies are shown: the LMC bar (Smecker-Hane et al. 2002), with a distance modulus of 18.47 (50 kpc) and a CMD reaching several mags below the old MSTO; NGC 1705 (Tosi et al. 2001), with distance modulus 28.54 (5.1 Mpc) and a CMD reaching a few mags below the tip of the RGB; and I Zw 18 (Aloisi et al. 1999), with the new distance modulus 31.3 (18 Mpc) derived by Aloisi et al. (2007). Notice that the latter modulus is inferred from the periods and luminosities of a few classical Cepheids measured from HST/ACS data which also allowed us to reach the RGB, but the WFPC2 data shown in Fig. 5 allow to reach only the AGB. The CMD obtained from the ACS is shown in Fig. 6.

Figure 5. Effect of distance on the resolution of individual stars and on the corresponding lookback time tau for the SFH. CMD in absolute magnitude and colour of systems observed with the HST/WFPC2 and analysed with the same techniques, but at different distances; from left to right: 50 Kpc (LMC bar), 5.1 Mpc (NGC 1705) and 18 Mpc (I Zw 18). The central panel also shows stellar evolution tracks from Fagotto et al. (1994b) for reference: red lines refer to low-mass stars, green lines to intermediate mass stars, and blue lines to massive stars.

Figure 6. CMD of I Zw 18, obtained from HST/ACS imaging (Aloisi et al. 2007). Overimposed are the Z = 0.0004 isochrones by Bertelli et al. (1994) with the RGB in red. Also shown is the average position of the 4 classical Cepheids with reliable light-curves obtained from these data.

Since the Local Group doesn't host all types of galaxies, with the notable and unfortunate absence of both the most and the least evolved ones (ellipticals and Blue Compact Dwarfs, BCDs, respectively), a few people have tackled the challenging task of deriving the SFH of more distant galaxies. In spite of the larger uncertainties and the shorter lookback time, these studies have led to quite interesting results, which wouldn't have been possible without HST.

First of all, all the galaxies, including BCDs, where individual stars have been resolved by HST, and the SFH has been derived with the synthetic CMD method, have turned out to be already active at the lookback time reached by the photometry (see e.g. Lynds et al. 1998, Aloisi et al. 1999, Schulte-Ladbeck et al. 2000, Schulte-Ladbeck et al. 2001, Annibali et al. 2003, Rejkuba et al. 2004, Vallenari et al. 2005). None of them appears to be experiencing now its first star formation activity, including the most metal poor ones, such as SBS 1415 and I Zw 18 (see Aloisi et al. 2005 and Aloisi et al. 2007). Fig. 7 sketches the SFHs derived by various authors for some of the starburst dwarfs studied so far and one low surface brightness dwarf, UGC 5889. The lookback time is indicated and in all cases stars with that age were detected.

Figure 7. SFHs in late-type galaxies derived with the synthetic CMD method. In all panels the SF rate per unit area as a function of time is plotted. The thin vertical line in each panel indicates the reached lookback time. References: NGC 1569, Greggio et al. (1998), Angeretti et al. (2005); NGC 1705, Annibali et al. (2003); I Zw 18, Aloisi et al. (1999); I Zw 36, Schulte-Ladbeck et al. (2001); Mrk 178, Schulte-Ladbeck et al. (2000); UGC 5889, Vallenari et al.(2005).

All the late-type dwarfs of Fig. 7 present a recent SF burst, which is what let people discover them in spite of the distance, and none of them exhibits long quiescent phases within the reached tau . It is interesting to notice that the SFH of the low surface brightness dwarf UGC 5889 (Vallenari et al. 2005) is also qualitatively similar to that of starburst dwarfs, except that the SFR is definitely moderate. In all the shown galaxies the strongest SF episodes are overimposed over a rather continuous, moderate SF, already in place at the tau reached by the photometry. Indeed, no one has ever found yet a galaxy without stars as old as tau from the CMDs of the resolved populations.

The SF rate differs significantly from one galaxy to the other. The two most powerful bursts measured so far are the recent ones in NGC 1705 and NGC 1569, with SFR per unit area a factor 10-100 higher than in the other starbursting dwarfs studied through their CMDs. Intriguingly, the strongest of all is not a BCD, but the dwarf irregular NGC 1569, suggesting that the morphological classification of these faint small galaxies was possibly affected by their distance and the capability of resolving their shape with ground-based small telescopes, at the time of their discovery. Had it been at 20 Mpc, NGC 1569 would have probably been classified as a BCD.