3. STAR FORMATION HISTORIES OF LOCAL GROUP GALAXIES

The current census of LG galaxies with SFH derived in some of their regions with the synthetic CMD method is impressive. SFHs have been inferred from CMDs of both the two external spiral galaxies, M31 and M33, the two Magellanic Clouds, LMC and SMC, a dozen dIrrs, 5 transition type dwarfs and about 20 early-type dwarfs (dwarf spheroidals, dSphs, and dwarf ellipticals, dEs).

In M31, long HST/ACS exposures have allowed Brown et al. (2008) to resolve stars fainter than the oldest MSTO in three regions and derive their SFH back to the earliest epochs. They find a fairly continuous activity through the whole lifetime of Andromeda. The SFH in the first M31 field studied by Brown is shown in the left-hand panel of Fig. 2: if the SFH resulting from both metal poor and rich stars is considered, it turns out to have been rather constant. In M33, HST/ACS imaging has allowed Barker et al. (2007) to study three different regions, again resolving their oldest stars. The resulting SFH (right-hand panel of Fig. 2) clearly differs from one region to the other and shows significant bumps and gasps over a rather continuous mode. In all the three regions was the SF activity already in place a Hubble time ago. It is apparent that the SF activity in the M31 field has been both stronger and more constant than in M33.

Figure 2. SFH from CMDs in (small) regions of the two LG external spirals. The left-hand panel shows the SFH of first HST/ACS field of M31 studied by Brown (2006), who divided the stars according to their metallicity. The right-hand panel shows the SFHs of the three HST/ACS fields of M33, A1, A2 and A3, studied by Barker et al. (2007).

The SFHs of several regions of the Magellanic Clouds have been studied by a number of authors, both from space and from ground (e.g. Holtzman et al. 1999, Dolphin et al. 2001, Smecker-Hane et al. 2002, Harris & Zaritsky 2004, Chiosi et al. 2006, Noel et al. 2007, Cignoni et al. 2009). Their proximity makes the oldest stars visible also from ground, with the advantage of fields of view larger than those of the HST cameras. Harris & Zaritsky (2004) even covered the whole SMC. On the other hand, the exquisite spatial resolution of HST is necessary to resolve and study the fainter stars in crowded regions, such as those of the star forming clusters. While stars at the oldest MSTOs and subgiant branches are the unique means to firmly establish the SFH at the earliest epochs, pre-MS stars are precious tools to study the details of the most recent SFH in terms of time and space behaviour (Cignoni et al. 2009). The SMC regions of intense recent star formation can provide key information on the star formation mechanisms in environments with metallicity much lower than in any Galactic star forming region. Fig. 3 shows the CMD of the young cluster NGC 602 in the Wing of the SMC, observed with HST/ACS. Both very young stars (either on the upper MS or still on the pre-MS) and old stars are found. The SFH of the cluster and the surrounding field is also shown, revealing that the cluster has formed most of its stars around 2.5 Myr ago, while the surrounding field has formed stars continuously since the earliest epochs. All the studies on the MC fields have found that the SFHs of their different regions differ from one another in the details (e.g. epoch of activity peaks, enrichment history, etc.) but are always characterized by a gasping regime, i.e. a rather continuous activity since the earliest epochs, but with significant peaks and gasps. In the LMC a clear difference has been found between the SFH of field stars and of star clusters, the latter showing a quiescence phase, several Gyr long, absent in the field.

Figure 3. Left-hand panel: CMD of the HST/ACS field around the young cluster NGC 602 in the SMC. The red sequence of pre-MS stars is easily recognizable parallel to the lower MS. The bright blue plume contains the young cluster stars while the lower MS is only populated by field stars, since the cluster stars with mass below ~ 1 M haven't yet had time to reach it. Right-hand panel: corresponding SFH as derived with the synthetic CMD method (Cignoni et al. 2009). The oldest part of the SFH is zoomed-in in the upper right inset.

Dwarf irregulars were the first systems to which synthetic CMD analyses were applied. HST has had a large impact on studies of these systems. The high spatial resolution of its cameras have allowed Dohm-Palmer et al. (1998) and Dohm-Palmer et al. (2002) to spatially resolve and measure the SF activity over the last 0.5 Gyr in all the sub-regions of the dIrrs Gr8 and Sextans A, close to the borders of the LG. The resulting space and time distribution of the SF, with lightening and fading of adjacent cells, is intriguingly reminiscent of the predictions of the stochastic self-propagating SF theory proposed by Seiden, Schulman, & Gerola (1979) 30 years ago. The HST/ACS is currently providing the deepest and tighter CMDs of dIrrs ever obtained, likely to remain unequalled for a very long time. These spectacular CMDs reach well below the oldest MSTO and allow the derivation of the SFH back to a Hubble time ago. The first of such impressive studies is that of Leo A (Cole et al. 2007), whose CMD and SFH are plotted in Fig. 4. In Leo A the star formation activity was present, although quite low, at the earliest epochs, and 90% of the activity occurred in the last 8 Gyr, with the main peak around 2 Gyr ago and a secondary peak a few hundreds Myr ago. Once again, the results obtained so far show that dIrrs experience a rather continuous star formation since the earliest epochs, but with significant peaks and gasps.

Figure 4. CMD and SFH of Leo A as derived by Cole et al. (2007) from HST/ACS data. Notice the impressive depth and tightness of the CMD, allowing to infer the SFH even at the earliest epochs.

To find SFHs peaked at earlier epochs one needs to look at early-type dwarfs: dEs, dSphs and even transition-type dwarfs clearly underwent their major activity around or beyond 10 Gyr ago. The latter also have significat activity at recent epochs (e.g. Young et al. 2007). The former have few (or no) episodes of moderate activity in the last several Gyrs (e.g. Smecker-Hane et al. 1996, Hurley-Keller et al. 1998, Hernandez, Gilmore & Valls-Gabaud 2000, Dolphin 2002, Dolphin 2005).

The beautiful CMDs from Carme Gallart's L-CID HST program on 6 dwarfs of different type (two dIrrs, two dSphs and two transition type, see Hidalgo et al. this volume) promise to provide SFHs of unprecedented time resolution for external galaxies. Another interesting project is trying to treat homogeneously all the LG galaxies observed with the HST/WFPC2, deriving the CMDs of their resolved populations in a self-consistent way (Holtzman et al. 2006) and the corresponding SFH with the same technique and assumptions (Dolphin et al. in preparation, see also Dolphin 2005). Homogeneous data sets and analyses are valuable to obtain a uniform overview of the properties of the different galaxies in the LG.