Tammy Smecker-Hane 45 min Review of nearby dSphs and the LMC Taft Armandroff 10 min Andromeda I dE - HST photometry of giant/horizontal branches which illustrate the classic second parameter problem Mario Mateo 10 min Leo II dSph - HST deep CMD and the age distribution Discussion 25 min
We will open the meeting by reviewing what is currently known about the star formation and chemical evolution histories of the dwarf galaxies in the Local Group. For if we are to claim to understand the physics of galaxy evolution, we must first be able to successfully model these, the simplest of galaxies. Because of their close proximity, we can determine ages and chemical compositions on a star-by-star basis and accurately determine their evolution from t=0 to the present day (with varying degrees of time resolution). The density of stars in regions of the color-magnitude diagram (CMD), particularly at main-sequence turnoff, gives us the star formation history. We can hope to disentangle the degeneracy of age-metallicity in these galaxies (as opposed to more distant ones for which only integrated light is available) by determining metallicity from the color of the giant branch, or by explicitly determining the metallicity distribution from spectra of individual stars. Using these methods, we are finding that, contrary to past prejudices, even the dwarf spheroidals in the Local Group have had complex evolutionary histories.
The evolution of the Carina, Fornax, Leo I and Leo II dwarf spheroidals and the Large Magellanic Cloud, which span a wide range in galaxy luminosity from M_v=-9 to -18, will be discussed in detail. (I have chosen these because CMDs reaching the critical main-sequence turnoff region have been obtained for each.) In these galaxies, episodic "bursts" of star formation appear to be common. I use quotes around bursts to highlight the fact that we do not yet know how bursty these were. Theoreticians have argued that low luminosity dwarfs form at early times on a cloud cooling timescale, typically 10^7 yrs. However, we will argue that evidence points to these bursts happening on timescales of greater than, or of order, 10^8 yrs with duty cycles of ~5 Gyr. The duration of the bursts and the duty cycle have obvious implications for the contribution of intrinsically low luminosity dwarfs to the faint blue galaxy counts.
My review talk will consist of: 1) Carina dSph (M_V=-9) The Carina dSph has had bursts of star formation 3, 5, 6 to 8 and ~15 Gyr ago with little or no star formation between these episodes. Roughly 70% of the stars are of intermediate-age and 15% are ~15 Gyr. The colors of stars on the giant branch imply the mean metallicity is [Fe/H]=-1.8 and the dispersion is low (~< 0.2 dex). The lack of chemical enrichment and the high M/L ratio (32 Mo/Lo), suggests that galactic winds ejected gas through out the evolution of the galaxy and lead to an inefficient conversion of gas to stars. references: Mighell (1990) A&AS Mighell & Butcher (1992) A&A Smecker-Hane, et al. (1994, 1996) - AJ and in progress Mateo et al (1993) AJ Carina illustrates some puzzling problems that are ubiquitous to dwarf galaxies: - The energy input from the ~10^3 supernovae that exploded during the first episode of SF is much larger than the binding energy of the galaxy (even with 90% of the mass in a dark matter halo) -- naively, this galaxy should have ejected its ISM during the first SF episode. - But it did not. Multiple episodes of SF occurred so either ejection of gas was inefficient, or new gas was accreted. The observed metallicity argue for pristine gas, rather than gas that had been ejected during the first burst of SF. However, accretion would be difficult because the total mass is very low (10^7 Mo), the cross section is small (r_t=660 pc) and the kinematic speed in the outer halo is ~220 km/s, hence for gas to successfully merge with the galaxy it would have had to be moving in nearly the same orbit. I will briefly discuss my objections to the idea put forth by Lin and Murray (1994) that Carina is the remains of a piece ripped off a larger-LMC galaxy, in which a second episode of SF was triggered by subsequent accretion of gaseous tidal debris. - I will argue that the best explanation for such an episodic SF rate is that cooling and SF operates slowly (on ~ 10^8 yr) and ejection of gas is inefficient. Supernovae drive winds which rapidly accelerate the low density ISM. However, because of the small physical size and diffuseness of the galaxy, the wind rapidly breaks through holes in the ISM -- leaking out metals, heat, energy and momentum -- but only slowly disrupt the densest molecular gas clouds. I argue that SF in the first "burst" occurs slowly over ~10^8 yr, some gas remains in the galaxy after SF is quenched, and the gas stays below the threshhold for SF for a few Gyr. Exactly what causes the few Gyr timescale between bursts remains to be answered -- maybe the remaining gas is optically thin to the UV background and remains photoionized for long times, or maybe tidal torques or increased ambient pressure can repeatedly nudge it over the threshold for SF (the suspected orbital peroid around the Galaxy is ~ few Gyr). 2) Leo I dSph (M_V=-10) A CMD of Leo I obtained with HST compliments the ground-based CMD and shows that Leo I also has experienced two distinct epochs of SF. Roughly 90% of the stars have an age of ~3 Gyr, and ~10% have ages of ~> 12 Gyr. references: Lee et al (1993), AJ Mateo et al (1994) BAAS 3) Leo II dSph (M_V=-12) The CMD of Leo II obtained with HST shows a period of extended SF from 7 to 14 Gyr ago. However, because of its larger distance, subsequently larger photometric errors, and the narrowing of the isochrones for older ages, it is unclear whether how constant the SFR was during this interval. reference: Mighell et al (1996), ApJ 4) Fornax dSph (M_V=-14) Fornax dSph has a higher luminosity, mass, surface density and a retinue of 5 globular clusters. The latest CMDs of Fornax show that it has had an extended SF history. Stars began to form early, ~15 Gyr ago. There are hints that the SFR rose between 5 and 8 Gyr ago, and a trickle of stars continued to form until very recently. Indeed, there is a small population of very young (few 10^8 yr old) stars. Unlike lower mass dSphs, Fornax stars show a wide range of metal abundances from -1.7 < [Fe/H] < -0.7. Its lower M/L (~10 Mo/Lo) suggests a more efficient conversion of gas to stars. references: Buonanno et al (1985), A&A Beauchamp et al (1995), AJ Smecker-Hane et al (1996), in preparation Stetson et al (1996), in preparation Mateo et al (1991), AJ 5) Large Magellanic Cloud (M_V=-18) For many years, the uneven distribution of LMC star cluster ages (0.1 Gyr, ~3 to 4 Gyr, and ~12 to 15 Gyr) hinted at a non-constant SFR. Ground-based CMDs of LMC field stars have been analyzed. Because of the limitations of the photometry (driven by crowding), an unknown mix of metallicities, and possible inadequacies of stellar evoln models, the Padova group have advocated "region-fitting" to quantify the LMC's SFR. They count stars in key regions of the CMD that are age sensitive to test simple models of the SFR. Assuming a constant SFR + one burst, they determine that the best fit model is one in which a burst that increased the SFR by a factor of 10 occurred ~ 2 to 4 Gyr ago (with mean age possibly varying with position across the LMC) and having a duration of ~10^9 yr or more. Although the data hint that this simple model (a single burst) may be oversimplified. In support of these results, a new CMD of field stars obtained with HST shows structure in the main-sequence turnoff region - evidence of an episodic SFR. A burst with an age of ~2 Gyr, duration 10^8 yr, superimposed on a nearly constant SFR in the last 1 to 3 Gyr is derived. Of order 25% of the stars may have formed in the 2 Gyr burst with 80% of the stars forming from 1 to 3 Gyr ago. This was preceded by low SFR in the preceeding few Gyr. The mean age of the older disk population is ~8 Gyr and with a very low fraction of disk stars being ~15 Gyr old. Complimenting this, new results on the age and chemical abundances of planetary nebula in the LMC show that a rapid increase in the chemical abundances (factor of 2) occurred 2 Gyr ago. But these data/analysis do not preclude an more complex evolution at ages ~> 4 Gyr. In fact, they hint that the single burst picture is not complete. It is imperative that we use HST to overcome crowing problems at faint mags, in combination with ground based photometry and spectroscopic metallicity determinations of bright evolved stars, if we are to resolve the evolutionary history of one of our nearest, most luminous, neighbors. references: Bertelli et al (1992), ApJ Westerlund, Linde & Lynga (1995), A&A Vallenari et al (1996a, 1996b), A&As, in press Gallagher et al (1996), ApJ, in press Smecker-Hane et al (1996), in progress Dopita et al (1996), in progress