Star-forming, dI galaxies represent the largest fraction by number of galaxies in the LG, and it is clear from deep imaging surveys that this number count dominance appears to increase throughout the Universe with lookback time (Ellis 1997). The large numbers of ``Faint Blue Galaxies'' (FBG) found in deep imaging-redshift surveys appear to be predominantly intermediate redshift (z < 1, or a look-back time out to roughly half a Hubble time), intrinsically small late type galaxies, undergoing strong bursts of star formation (Babul & Ferguson 1996). Thus we can assume that the dIs we see in the LG are a cosmologically important population of galaxies which can be used to trace the evolutionary changes in the sfr of the Universe with redshift. The ``Madau-diagram'' (Madau et al. 1998) uses the results of redshift surveys to plot the SFH of the Universe against redshift. It predicts that most of the stars that have formed in the Universe have done so at redshifts, z ~ 1-2. If it is correct, then the MSTOs from the most active period of star formation in the Universe will be easily visible as 7-9 Gyr old MSTOs in the galaxies of the LG (e.g. Rich 1998). Determining accurate SFHs for all the galaxies in the local Universe using CMD analysis provides an alternate route to and thus check upon the Madau-diagram.
Recent detailed CMDs of several nearby galaxies and self-consistent grids of theoretical stellar evolution models have transformed our understanding of galactic SFHs. Most of the dI CMDs to date suggest that the sfr was higher in the past, although the peak in the sfr has occured at relatively recent times as defined by Madau-diagram (the peaks occur at z = 0.1-0.2, within the first bin). The Mateo review of all LG dwarf galaxies (Mateo 1998) and studies of M31 and our Galaxy (Renzini 1998), on the other hand, suggest that the LG had its most significant peak in star formation > 10 Gyr ago (i.e at z > 3), the epoch of halo formation. Many galaxies contain large numbers of RR Lyr variables (or HB) and/or globular clusters which can only come from a significant older population. It is possible that dI galaxies have quite different SFHs to the more massive galaxies. Thus although the small dI galaxies in the LG have been having short, often intense, bursts of star formation in comparatively recent times this is not representative of the majority of the star formation in the LG. However direct observations of the details of the oldest star forming episodes in any galaxy are limited at best. This is an area where advanced CMD analysis techniques have been developed (e.g. Tolstoy & Saha 1996) and telescopes with sufficient image quality exist and the required deep, high quality imaging are observations are waiting to be made.
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Figure 4. In the upper panel is a rough summation of the sfrs of the LG dwarf galaxies with time (data taken from Mateo 1998) to obtain the integrated SFH of all the LG dwarfs. The redshifts corresponding to lookback times (for H0 = 50, q0 = 0.5). In the middle panel, a wild extrapolation is made; the assumption that the integrated SFH of the LG dwarfs in the upper panel is representative of the Universe as a whole. The resulting star formation density of the LG versus redshift is plotted using the same scheme as Madau et al. (1998) and Shanks et al. (1998), and these two models are also plotted and the LG curve is arbitrarily, and with a very high degree of uncertainty, normalised to the other two models. In the lowest panel the The LG dwarf sfr as a fraction of the total star formation integrated over all time is plotted versus redshift, and the Madau curve is also replotted in this form, for the volume of the LG. This highlights the totally different distribution of star formation with redshift found from galaxy redshift surveys and what we appear to observe in the stellar population of the LG. |
Figure 4 summarizes what can currently be said about the SFH of the LG and how this compares with the Madau et al. (1998) and Shanks et al. (1998) redshift survey predictions. We have not included the dominant large galaxies in the LG, the Galaxy and M 31, but the SFH of the combined dwarfs is broadly consistent with what is known about the SFH of these large systems. They have, as far as we can tell, had a global sfr that has been gradually but steadily declining since their (presumed) formation epoch > 10 Gyr ago. There is currently no evidence for a particular peak in sfr around 7-9 Gyr ago or any other time, as predicted by the Madau-diagram for either large galaxies or dwarfs. The dominant population by mass in the LG dwarfs are dE, if dIs are singled out a population with a star formation peak in the Madau-diagram range can be found. But at present the statistics are too limited to determine the typical fraction of old population in LG dIs. There is clearly a total mismatch between the SFH of the LG and the results from the redshifts surveys. This might hint at serious incompleteness problems in high redshift galaxy surveys, which appear to miss passively evolving systems in favour of small bursting systems.
The recent HST CMD results give much cause for optimism that we can hope to sort out in detail the SFH of all the different types of galaxies within in the LG if only HST would point at them occasionally. There is also great potential for ground based imaging using high quality imaging telescopes with large collecting areas, such as VLT is clearly going to be.