Thanks to deep, high-resolution photometry and synthetic color-magnitude diagram techniques, fairly detailed knowledge of the star formation histories of Local Group dSphs is now available. The resulting picture is one of high complexity: No two dSphs exhibit the same star formation history (Grebel 1997). As mentioned already, all dSphs studied in detail so far were found to contain old Population II stars. Some dSphs are dominated by ancient stars, others only have a small old population and a dominant intermediate-age population, and there is one example of a dSph that experienced star formation as recently as a few hundred Myr ago (Fornax, see Grebel & Stetson 1999). Generally, star formation has proceeded continuously in these galaxies, although the amplitude varied and eventually declined at intermediate or younger ages (e.g., Grebel et al. 2003). Only one dSph with clearly episodic star formation is known (Carina, Smecker-Hane et al. 1994 and Monelli et al. 2003). DSphs with several populations typically show population gradients in the sense that more metal-rich and/or younger populations are more centrally concentrated (Harbeck et al. 2001). Substructure of this kind is not necessarily symmetrically distributed (e.g., Stetson et al. 1998).
While the past decade was mainly one of photometrically derived star formation histories, we are now entering an era where the age-metallicity degeneracy, which is inherent to purely photometric determinations, can be broken by adding spectroscopic abundance information (e.g., Tolstoy et al. 2001; Pont et al. 2004, Cole et al. 2005, Koch et al. in these proceedings). This will ultimately permit us to derive detailed age-metallicity relations for these galaxies.
4.1. Comparing stellar populations and star formation histories
Comparing star formation histories of dwarf galaxies in general and dSphs in particular (e.g., Grebel 1997, 1999), one finds variations in the duration of star formation, in the star formation rates as a function of time, and in the enrichment. In spite of being overall metal-poor, all dSph galaxies that were studied spectroscopically so far show a spread of metallicities of typically 1 dex in [Fe/H] or more (e.g., Shetrone et al. 2001, 2003; Bonifacio et al. 2004). There appears to be a trend of increasing intermediate-age population fractions with increasing distance from the Milky Way among the Galactic dSphs (van den Bergh 1994; Grebel 1997), which may be due to the environmental impact of the Milky Way.
If environment was indeed the governing factor determining the evolution of these low-mass galaxies, then one should expect to find a similar trend among the dSph companions of M31. However, this is not observed. Although M31's dSphs cover a comparable range of distances as their Galactic counterparts, they all appear to be dominated by old populations and lack the indicators of prominent intermediate-age populations present in the more distant Milky Way dSphs (Harbeck et al. 2001, 2004, 2005).
Considering what we can infer from present-day dSphs about their star formation histories, how do they fit in as potential building blocks? With respect to stellar populations, dSphs dominated by old populations are compatible with the stellar content of the Galactic halo. DSphs with substantial intermediate-age populations seem less likely to have made a major contribution to the build-up of the halo of our Milky Way (Unavane et al. 1996). On the other hand, this problem would be diminished if most of the minor merger events took place at very early epochs. Comparing the old, metal-poor stellar populations in M31's dSphs to M31's halo indicates that the dSphs cannot have been primary building blocks of M31's halo since it was found to contain a substantial contribution from intermediate-age, comparatively metal-rich populations (Brown et al. 2003). An old, metal-poor halo population, however, has been detected as well (Brown et al. 2004), and again the population differences would be less severe if most of the dSph accretion had taken place at very early epochs, whereas the remainder of the younger halo of M31 would have been formed through the later accretion of more massive and more evolved galaxies. - These statements assume that dSphs have not changed appreciably over time (e.g., did not lose substantial amounts of mass) and that their observed stellar content permits one to arrive at a fair representation of their evolutionary history.
 |
Figure 2. Mean stellar metallicity of Population II stars versus baryonic luminosity for different classes of dwarf galaxies as indicated in the legend. Note the offset between gas-deficient (filled symbols) and gas-rich (open symbols) dwarfs. At the same galaxy luminosity, the old populations of dSphs are more metal-rich than those of dIrrs. Thus in contrast to dIrrs, dSphs must have experienced comparatively rapid early enrichment. Note the location of the so-called dIrr/dSph transition-type galaxies, which combine population properties of dSphs with ongoing star formation and a measurable gas content in the diagram. Their properties make them plausible dSph progenitors. For details, see Grebel, Gallagher, & Harbeck (2003). |
4.2. Are DSph Abundance Patterns Consistent with the Building Block Scenario?
During the past few years more and more detailed, high-resolution abundance
analyses of individual red giants in dSphs have become available, leading
to a growing, yet still limited body of knowledge about their detailed
element abundance ratios. In particular,
[
/Fe] ratios, r- and
s-process abundances are being measured.
If dSphs were dominant contributors to the build-up of the Galactic halo,
their abundance patterns should match those of the halo. However, the
existing measurements show pronounced differences to the abundance ratios
in our Galactic halo: Dwarfs are characterized by slower star formation
rates, leading to reduced
[/Fe] ratios at lower
metallicity
([Fe/H]) than found in the Galactic halo. This can be interpreted as a
signature of a larger contribution of supernovae of Type Ia early on, such
that a solar [/Fe] is
reached sooner (e.g.,
Matteucci 2003).
In contrast, the Galactic halo experienced comparatively rapid star
formation accompanied by gas removal, leading to low metallicities with
higher
element ratios. These
different properties lead to the conclusion that dSphs cannot have
contributed in a major way to the build-up of the Galactic halo
(Shetrone et al. 2001),
unless the majority of the minor
merger events occurred at very early epochs when the abundance ratios in
the Milky Way and in the dSphs were still very similar.
4.3. Morphological segregation and the metallicity-luminosity relation for dwarf galaxies
Going back to global metallicities ("[Fe/H]"), what can these tell us about galaxy evolution and interrelations between different galaxies? Now we do not consider dSphs as building blocks of larger galaxies, but dSphs as the stripped remnants of initially more massive galaxies. Clearly, dSphs must once have been more massive and considerably more gas-rich in order to have formed the stars we observe in them today. Their present-day gas deficiency still lacks a satisfactory explanation (Gallagher et al. 2003, Grebel et al. 2003). What were the progenitors of dSphs? The first type of galaxies that comes to mind are dIrrs, gas-rich, irregularly shaped dwarfs with ongoing star formation yet also with very old populations.
Could dSphs simply be stripped dIrrs? Taken at face value, the
morphological segregation observed in the Local Group (as well as in other
groups) would seem to support this idea (see, e.g., Fig. 1 in
Grebel 1995):
Gas-deficient dwarf galaxies (dEs, dSphs) are usually found within
300 kpc around more massive galaxies, while gas-rich dwarfs (esp. dIrrs)
are also (and predominantly) found at larger distances. When plotting
distance from the nearest primary vs. HI content, there is a clear
tendency to find dSphs with HI mass limits below 105
M
within 300 kpc, while dIrr galaxies with typical HI masses >
107
M tend to lie
at distances > 400 kpc
(Grebel et al. 2003,
their Fig. 3).
The proximity to massive galaxies and interactions with
these may be an efficient agent in removing material from the dwarfs (e.g.,
Mayer et al. 2001).
On the other hand, the luminosity-metallicity relations of dSphs and dIrrs have long been known to differ. While for both classes of galaxies the metallicity increases with luminosity (and hence with mass), the two relations are offset from one another (e.g., Skillman & Bender 1995) in the sense that dSphs are more metal-rich for a given luminosity. However, the luminosity-metallicity relations are based on different tracers: For dIrrs, usually the present-day oxygen abundances as measured in HII regions are used, while for dEs and dSphs, metallicities of old populations (and occasionally oxygen abundances of intermediate-age planetary nebulae) are used. Thus the metallicities of populations of very different ages as well as nebular abundances versus stellar abundances are compared.
This mixture of different evolutionary stages and different metallicity indicators is unsatisfactory. Therefore we decided to attempt to compare apples with apples: In order to compare not only mean stellar metallicities in dIrrs and dSphs, but also the metallicities of the same populations (i.e., of stars of similar age), we chose old Population II giants, which are found in all LG dwarf galaxies. We used (1) old red giants in dSphs and in the outskirts of dIrrs (where old populations dominate), (2) spectroscopic abundances wherever available (from our own and literature data), and (3) photometric abundances elsewhere. The resulting data set may not yet have an ideal degree of homogeneity, but is the best and most comprehensive one currently available (Grebel et al. 2003). In the coming years, undoubtedly stellar spectroscopic measurements will also become available for those dwarfs for which we only have photometric estimates at present.
Interestingly, even when confining the comparison of luminosity-metallicity relations to old populations, the differences continue to exist. Thus at the same galaxy luminosity, the old populations of dSphs are more metal-rich than those of dIrrs. This indicates that in contrast to dIrrs, dSphs must have experienced fairly rapid early enrichment (Grebel et al. 2003). This and several other factors make dIrrs unlikely progenitors of dSphs. If dSphs are stripped remnants of more massive galaxies, then the fact that they do follow a baryonic luminosity-metallicity relation indicates that they must have continued to form stars and to experience enrichment even after the main mass removal occurred. Grebel et al. (2003) present a series of arguments why dIrr/dSph transition-type galaxies appear to be fairly plausible dSph progenitors, suggesting a gentle and slow transition from one kind of low-mass galaxy to another.