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Register a new userThe Faintest Dwarf Galaxies
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Joshua D. Simon
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The lowest luminosity (L < 10^5 L_sun) Milky Way satellite galaxies represent the extreme lower limit of the galaxy luminosity function. These ultra-faint dwarfs are the oldest, most dark matter-dominated, most metal-poor, and least chemically evolved stellar systems known. They therefore provide unique windows into the formation of the first galaxies and the behavior of dark matter on small scales. In this review, we summarize the discovery of ultra-faint dwarfs in the Sloan Digital Sky Survey in 2005, and the subsequent observational and theoretical progress in understanding their nature and origin. We describe their stellar kinematics, chemical abundance patterns, structural properties, stellar populations, orbits, and luminosity function, and what can be learned from each type of measurement. We conclude that: (1) in most cases, the stellar velocity dispersions of ultra-faint dwarfs are robust against systematic uncertainties such as binary stars and foreground contamination; (2) the chemical abundance patterns of stars in ultra-faint dwarfs require two sources of r-process elements, one of which can likely be attributed to neutron star mergers; (3) even under conservative assumptions, only a small fraction of ultra-faint dwarfs may have suffered significant tidal stripping of their stellar components; (4) determining the properties of the faintest dwarfs out to the virial radius of the Milky Way will require very large investments of observing time with future telescopes. Finally, we offer a look forward at the observations that will be possible with future facilities as the push toward a complete census of the Local Group dwarf galaxy population continues.
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We provide a comprehensive description and offer an explanation for the sizes of the faintest known galaxies in the universe, the dwarf spheroidal (dSph) satellites of the Milky Way and Andromeda. After compiling a consistent data set of half-light radii (r_{1/2}) and luminosities, we describe the size-luminosity relation of dSphs by a log-normal distribution in r_{1/2} with a mean size that varies as a function of luminosity. Accounting for modest number statistics, measurement uncertainties and surface brightness limitations, we find that the size-luminosity relations of the Milky Way and Andromeda dSph populations are statistically indistinguishable, and also very similar: their mean sizes at a given stellar luminosity differ by no more than 30%. In addition, we find that the mean size, slope and scatter of this log-normal size description of Local Group dSphs matches onto the relation of more massive low-concentration galaxies. This suggests that the stellar sizes of dSphs are ultimately related to their overall initial baryonic angular momentum. To test this hypothesis we perform a series of high resolution N-body simulations that we couple with a semi-analytic model of galaxy formation. These predict the same mean size and slope as observed in dSph satellites. At the same time, these models predict that the size-luminosity distributions for satellite galaxies around similar host-halos must be similar providing a natural explanation as to why the size distributions of Milky Way and Andromeda satellites are similar. Although strong rotation is currently not observed in dSphs, this may well be consistent with our angular-momentum-based explanation for their sizes if the disks of these galaxies have become sufficiently stirred through tidal interaction.
We present high-resolution of spectroscopy of four stars in two candidate ultra-faint dwarf galaxies (UFDs) Grus I (Gru I) and Triangulum II (Tri II). Neither object currently has a clearly determined velocity dispersion, placing them in an ambiguous region of parameter space between dwarf galaxies and globular clusters. No significant metallicity difference is found for the two Gru I stars, but both stars are deficient in neutron-capture elements. We verify previous results that Tri II displays significant spreads in metallicity and [$alpha$/Fe]. Neutron-capture elements are not detected in our Tri II data, but we place upper limits at the lower envelope of Galactic halo stars, consistent with previous very low detections. Stars with similarly low neutron-capture element abundances are common in UFDs, but rare in other environments. This signature of low neutron-capture element abundances traces chemical enrichment in the least massive star-forming dark matter halos, and further shows that the dominant sources of neutron-capture elements in metal-poor stars are rare. In contrast, all known globular clusters have similar ratios of neutron-capture elements to those of halo stars, suggesting that globular clusters form as part of relatively massive galaxies rather than in their own dark matter halos. The low neutron-capture element abundances may be the strongest evidence that Gru I and Tri II are (or once were) galaxies rather than globular clusters, and we expect future observations of these systems to robustly find non-zero velocity dispersions or signs of tidal disruption. However, the nucleosynthetic origin of this low neutron-capture element floor remains unknown.
$Lambda$-Warm Dark Matter (WDM) has been proposed as alternative scenario to $Lambda$ cold dark matter (CDM), motivated by discrepancies at the scale of dwarf galaxies, with less small-scale power and realized by collisionless particles with energies in the range $1-3$ keV. We present a new approach to constrain the viability of such WDM models using star formation histories of the dwarf spheroidal galaxies (dSphs) in the Local Group. We compare their high time-resolution star formation histories (SFHs) obtained with HST-based color magnitude diagrams with the range of possible collapse redshifts of their dark matter halos expected in CDM and in different WDM scenarios. The collapse redshift is inferred after determining a plausible infall mass of the subhalo. This is based on the current mass of individual dwarf inferred from stellar kinematics combined with results of cosmological simulations providing information on the subhalo evolution. Since WDM subhalos close to the filtering mass scale form significantly later than CDM, we show that they are in the first place difficult to reconcile with a truncation of star formation occurring as early as $zgeq 3$. The Ultra-Faint Dwarfs (UFDs) provide the most stringent constraints. Using 6 UFDs with the best determination of the SFHs, we show that we can exclude a 1 keV warm particle to a 2-$sigma$ confidence interval consistently with other methods reported in the literature. For some objects the $2$ keV model is also excluded. We discuss the various caveats of the method, most notably the low number of dwarfs with accurately determined star formation histories and the uncertainties in the determination of the infall mass of the subhalos. Our preliminary analysis serves as a pathfinder for future investigations that will combine upcoming accurate SFHs for more local dSphs with direct analysis of WDM cosmological simulations with baryons.
The current Lambda CDM cosmological model predicts that galaxy evolution proceeds more slowly in lower density environments, suggesting that voids are a prime location to search for relatively pristine galaxies that are representative of the building blocks of early massive galaxies. To test the assumption that void galaxies are more pristine, we compare the evolutionary properties of a sample of dwarf galaxies selected specifically to lie in voids with a sample of similar isolated dwarf galaxies in average density environments. We measure gas-phase oxygen abundances and gas fractions for eight dwarf galaxies (M_r > -16.2), carefully selected to reside within the lowest density environments of seven voids, and apply the same calibrations to existing samples of isolated dwarf galaxies. We find no significant difference between these void dwarf galaxies and the isolated dwarf galaxies, suggesting that dwarf galaxy chemical evolution proceeds independent of the large-scale environment. While this sample is too small to draw strong conclusions, it suggests that external gas accretion is playing a limited role in the chemical evolution of these systems, and that this evolution is instead dominated mainly by the internal secular processes that are linking the simultaneous growth and enrichment of these galaxies.
Star forming dwarf galaxies (SFDGs) have a high gas content and low metallicities, reminiscent of the basic entities in hierarchical galaxy formation scenarios. In the young universe they probably also played a major role in the cosmic reionization. Their abundant presence in the local volume and their youthful character make them ideal objects for detailed studies of the initial stellar mass function (IMF), fundamental star formation processes and its feedback to the interstellar medium. Occasionally we witness SFDGs involved in extreme starbursts, giving rise to strongly elevated production of super star clusters and global superwinds, mechanisms yet to be explored in more detail. SFDGs is the initial state of all dwarf galaxies and the relation to the environment provides us with a key to how different types of dwarf galaxies are emerging. In this review we will put the emphasis on the exotic starburst phase, as it seems less important for present day galaxy evolution but perhaps fundamental in the initial phase of galaxy formation.
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