To visualize how the SFH affects the CMD morphology, a few representative cases are displayed in Fig. 1. The six panels of the Figure show the effect of different SFHs on the synthetic CMD of a hypothetical galactic region with number of resolved individual stars, photometric errors, blending and incompleteness factors typical of a region in the SMC imaged with HST/WFPC2. If the SFH of the studied region has been one of the following six cases, then, according to stellar evolution models, the CMD of its stars is one of those shown in Fig. 1. The top three panels show examples of CMDs typical of late-type galaxies, with ongoing or recent star formation activity. If the star formation rate (SFR) has been constant for all the galaxy lifetime, the CMD of the region is expected to have the morphology of the top-central panel, with a prominent blue plume mostly populated by main-sequence (MS) stars and an equally prominent red plume resulting from the overposition of increasingly bright and massive stars in the red giant branch (RGB), asymptotic giant branch (AGB) and red supergiant phases. At intermediate colours, for decreasing brightness, stars in the blue loops and subgiant phases are visible, as well stars at the oldest MS turnoff (MSTO) and on the faint MS of low mass stars. Stars of all ages are present, from those as old as the Hubble time to the brightest ones a few tens Myr old.
Figure 1. The effect of the SFH on the theoretical CMD of a hypothetical galactic region with (m-M)0 = 19, E(B-V) = 0.08, and with the photometric errors and incompleteness typical of HST/WFPC2 photometry. All the shown synthetic CMDs contain 50000 stars and are based on the Padova models (Fagotto et al. 1994a, Fagotto et al. 1994b) with the labelled metallicities. Top-central panel: the case of a SFR constant from 13 Gyr ago to the present epoch. Top-left panel: the effect of adding a burst 10 times stronger in the last 20 Myr to the constant SFR. The CMD has a much brighter and thicker blue plume. Top-right panel: same constant SFR as in the first case, but with a quiescence interval between 3 and 2 Gyrs ago; a gap appears in the CMD region corresponding to stars 2-3 Gyr old, which are completely missing. Bottom-central panel: SF activity only between 13 and 10 Gyr ago with Z = 0.004. Bottom-right panel: SF activity only between 13 and 10 Gyr ago with Z = 0.0004: notice how colour and luminosity of turnoff, subgiant and red giant branches differ from the previous case. Bottom-left panel: SF activity between 13 and 11 Gyr ago, followed by a second episode of activity between 5 and 4 Gyr ago: a gap separates the two populations in the CMD, but less evident than in the top-right panel case, when the quiescent interval was more recent.
If we leave the SFH unchanged except for the addition of a burst ten times stronger concentrated in the last 20 Myr, the CMD (top-left panel) has a much brighter and more populated blue plume, now containing also stars a few Myr old. In the top-right panel the same constant SFR as in the first case is assumed, but with a quiescent interval between 3 and 2 Gyrs ago: a gap is clearly visible in the CMD region corresponding to the age of the missing stars.
The three bottom panels of Fig. 1 show CMDs typical of early-type galaxies, whose SF activity is concentrated at the earliest epochs. If only one SF episode has occurred from 13 to 10 Gyr ago, with a constant metallicity Z = 0.004 as in the top panel cases, the resulting CMD is shown in the bottom-central panel. If the SF has occurred at the same epoch, but with a metallicity ten times lower, the evolutionary phases in the resulting CMD (bottom-right panel) have colours and luminosities quite different from the previous case. Finally, the bottom-left panel shows the case of two bursts, the first from 13 to 11 Gyr ago and the second from 5 to 4 Gyr ago. The gap corresponding to the quiescent interval is evident in the CMD, although not as much as the more recent gap of the top-right panel.
The tight dependence of the CMD morphology on the SFH is the cornerstone of the synthetic CMD method, which consists in comparing the observational CMD of a galactic region with synthetic CMDs, such as those of Fig. 1, created via Monte Carlo extractions on stellar evolution tracks or isochrones for a variety of SFHs, IMFs, binary fractions and age-metallicity relations (see e.g. Tosi et al. 1991, Tolstoy 1996, Greggio et al. 1998, Aparicio & Gallart 2004 for detailed descriptions of different procedures). The synthetic CMDs take into account the number of stars, photometric errors, incompleteness and blending factors of the observational CMD (or portions of it). Hence, a combination of assumed parameters is acceptable only if the resulting synthetic CMD reproduces all the features of the observational one: morphology, colours, luminosity functions, number of stars in specific evolutionary phases. The method does not provide unique solutions, but significantly reduces the possible SFH scenarios.
At its first applications to photometric data from ground-based, moderate size telescopes the synthetic CMD method demonstrated its power, showing that even in tiny galaxies such as Local Group dwarf irregulars (dIrrs) the SFH varies from one region to the other and that their star formation regime is rather continuous, with long episodes of moderate activity, separated by short quiescent intervals (the so-called gasping regime, Ferraro et al. 1989, Tosi et al. 1991, Marconi et al. 1995, Gallart et al. 1996, Tolstoy 1996) and not the bursting regime (short episodes of strong SF activity separated by long quiescent phases) that most people attributed to late-type dwarfs at the time.
When the first non-aberrated images were acquired with HST, the impressive improvement in the achievable photometric resolution and depth, and the corresponding quantum leap in the quality of the CMDs, triggered a worldwide burst of interest in the derivation of the SFHs of nearby galaxies and in the synthetic CMD method. Many people developped their own procedures and to date a large fraction of Local Group galaxies have had the SFH of at least some of their regions derived with the synthetic CMD method. Nowadays, in LG galaxies it is possible to resolve individual stars down to faint/old objects in all galactic regions and we can thus infer the SFHs over long lookback times (up to the Hubble time), with an average time resolution around (0.1 - 0.2).