The Metal-Poor Metallicity Distribution of the Ancient Milky Way

We present a low metallicity map of the Milky Way consisting of $\sim$111,000 giants with $-3.5 \lesssim$ [Fe/H] $\lesssim -$0.75, based on public photometry from the second data release of the SkyMapper survey. These stars extend out to $\sim$7kpc from the solar neighborhood and cover the main Galactic stellar populations, including the thick disk and the inner halo. Notably, this map can reliably differentiate metallicities down to [Fe/H] $\sim -3.0$, and thus provides an unprecedented view into the ancient, metal-poor Milky Way. Among the more metal-rich stars in our sample ([Fe/H] $> -2.0$), we recover a clear spatial dependence of decreasing mean metallicity as a function of scale height that maps onto the thick disk component of the Milky Way. When only considering the very metal-poor stars in our sample ([Fe/H] $< -$2), we recover no such spatial dependence in their mean metallicity out to a scale height of $|Z|\sim7$ kpc. We find that the metallicity distribution function (MDF) of the most metal-poor stars in our sample ($-3.0 <$ [Fe/H] $< -2.3$) is well fit with an exponential profile with a slope of $\Delta\log(N)/\Delta$[Fe/H] = 1.52$\pm$0.05, and shifts to $\Delta\log(N)/\Delta$[Fe/H] = 1.53$\pm$0.10 after accounting for target selection effects. For [Fe/H] $< -2.3$, the MDF is largely insensitive to scale height $|Z|$ out to $\sim5$kpc, showing that very and extremely metal-poor stars are in every galactic component.Comment: 9 pages, 5 figures; accepted for publication in ApJL. Minor corrections after acceptance addressing referee report for Chiti et al. ApJS submitte

Influence of Population III stars on cosmic chemical evolution

New observations from the Hubble ultra deep field suggest that the star formation rate at z>7 drops off faster than previously thought. Using a newly determined star formation rate for the normal mode of Population II/I stars (PopII/I), including this new constraint, we compute the Thomson scattering optical depth and find a result that is marginally consistent with WMAP5 results. We also reconsider the role of Population III stars (PopIII) in light of cosmological and stellar evolution constraints. While this input may be needed for reionization, we show that it is essential in order to account for cosmic chemical evolution in the early Universe. We investigate the consequences of PopIII stars on the local metallicity distribution function of the Galactic halo (from the recent Hamburg/ESO survey of metal-poor stars) and on the evolution of abundances with metallicity (based on the ESO large program on very metal-poor stars), with special emphasis on carbon-enhanced metal-poor stars. Our most important results show that the nucleosynthetic yields of PopIII stars lead to abundance patterns in agreement with those observed in extremely metal-poor stars. In this chemical approach to cosmic evolution, PopIII stars prove to be a compulsory ingredient, and extremely metal-poor stars are inevitably born at high redshift. (Abridged)Comment: 11 pages, 7 figures, MNRAS in pres

The Chemical Abundances Of Stars In The Halo (CASH) Project. II. A Sample Of 14 Extremely Metal-Poor Stars

We present a comprehensive abundance analysis of 20 elements for 16 new low-metallicity stars from the Chemical Abundances of Stars in the Halo (CASH) project. The abundances have been derived from both Hobby-Eberly Telescope High Resolution Spectrograph snapshot spectra (R similar to 15,000) and corresponding high-resolution (R similar to 35,000) Magellan Inamori Kyocera Echelle spectra. The stars span a metallicity range from [Fe/H] from -2.9 to -3.9, including four new stars with [Fe/H] < -3.7. We find four stars to be carbon-enhanced metal-poor (CEMP) stars, confirming the trend of increasing [C/Fe] abundance ratios with decreasing metallicity. Two of these objects can be classified as CEMP-no stars, adding to the growing number of these objects at [Fe/H]< -3. We also find four neutron-capture-enhanced stars in the sample, one of which has [Eu/Fe] of 0.8 with clear r-process signatures. These pilot sample stars are the most metal-poor ([Fe/H] less than or similar to -3.0) of the brightest stars included in CASH and are used to calibrate a newly developed, automated stellar parameter and abundance determination pipeline. This code will be used for the entire similar to 500 star CASH snapshot sample. We find that the pipeline results are statistically identical for snapshot spectra when compared to a traditional, manual analysis from a high-resolution spectrum.Physics Frontier Center/Joint Institute for Nuclear Astrophysics (JINA) PHY 02-16783, PHY 0822648Carnegie Institution of WashingtonNSF AST-0908978Astronom

R-process enrichment from a single event in an ancient dwarf galaxy

Elements heavier than zinc are synthesized through the (r)apid and (s)low neutron-capture processes. The main site of production of the r-process elements (such as europium) has been debated for nearly 60 years. Initial studies of chemical abundance trends in old Milky Way halo stars suggested continual r-process production, in sites like core-collapse supernovae. But evidence from the local Universe favors r-process production mainly during rare events, such as neutron star mergers. The appearance of a europium abundance plateau in some dwarf spheroidal galaxies has been suggested as evidence for rare r-process enrichment in the early Universe, but only under the assumption of no gas accretion into the dwarf galaxies. Cosmologically motivated gas accretion favors continual r-process enrichment in these systems. Furthermore, the universal r-process pattern has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultra-faint dwarf galaxies assembled shortly after the first stars formed, and are ideal systems with which to study nucleosynthesis events such as the r-process. Reticulum II is one such galaxy. The abundances of non-neutron-capture elements in this galaxy (and others like it) are similar to those of other old stars. Here, we report that seven of nine stars in Reticulum II observed with high-resolution spectroscopy show strong enhancements in heavy neutron-capture elements, with abundances that follow the universal r-process pattern above barium. The enhancement in this "r-process galaxy" is 2-3 orders of magnitude higher than that detected in any other ultra-faint dwarf galaxy. This implies that a single rare event produced the r-process material in Reticulum II. The r-process yield and event rate are incompatible with ordinary core-collapse supernovae, but consistent with other possible sites, such as neutron star mergers.Comment: Published in Nature, 21 Mar 2016: http://dx.doi.org/10.1038/nature1742

Stellar Metallicities from SkyMapper Photometry II: Precise photometric metallicities of ∼\sim280,000 giant stars with [Fe/H] <−0.75< -0.75 in the Milky Way

The Milky Way's metal-poor stars are nearby ancient objects that are used to study early chemical evolution and the assembly and structure of the Milky Way. Here we present reliable metallicities of $\sim280,000$ stars with $-3.75 \lesssim$ [Fe/H] $\lesssim -0.75$ down to $g=17$ derived using metallicity-sensitive photometry from the second data release (DR2) of the SkyMapper Southern Survey. We use the dependency of the flux through the SkyMapper $v$ filter on the strength of the Ca II K absorption features, in tandem with SkyMapper $u,g,i$ photometry, to derive photometric metallicities for these stars. We find that metallicities derived in this way compare well to metallicities derived in large-scale spectroscopic surveys, and use such comparisons to calibrate and quantify systematics as a function of location, reddening, and color. We find good agreement with metallicities from the APOGEE, LAMOST, and GALAH surveys, based on a standard deviation of $\sigma\sim0.25$dex of the residuals of our photometric metallicities with respect to metallicities from those surveys. We also compare our derived photometric metallicities to metallicities presented in a number of high-resolution spectroscopic studies to validate the low metallicity end ([Fe/H] $< -2.5$) of our photometric metallicity determinations. In such comparisons, we find the metallicities of stars with photometric [Fe/H] $< -2.5$ in our catalog show no significant offset and a scatter of $\sigma\sim$0.31dex level relative to those in high-resolution work when considering the cooler stars ($g-i > 0.65$) in our sample. We also present an expanded catalog containing photometric metallicities of $\sim720,000$ stars as a data table for further exploration of the metal-poor Milky Way.Comment: 15 pages, 9 figures, 2 tables; submitted to ApJS and revised after one round of referee feedback. Full version of Table 2 in sourc

Can Supermassive Black Holes Form in Metal-Enriched High-Redshift Protogalaxies ?

Primordial gas in protogalactic dark matter (DM) halos with virial temperatures Tvir > 10^4 K begins to cool and condense via atomic hydrogen. Provided this gas is irradiated by a strong ultraviolet (UV) flux and remains free of H2 and other molecules, it has been proposed that the halo with Tvir ~10^4 K may avoid fragmentation, and lead to the rapid formation of a supermassive black hole (SMBH) as massive as M=10^5-10^6 Msun. This ``head--start'' would help explain the presence of SMBHs with inferred masses of several x 10^9 Msun, powering the bright quasars discovered in the Sloan Digital Sky Survey at redshift z>~6. However, high-redshift DM halos with Tvir~10^4K are likely already enriched with at least trace amounts of metals and dust produced by prior star-formation in their progenitors. Here we study the thermal and chemical evolution of low-metallicity gas exposed to extremely strong UV radiation fields. Our results, obtained in one-zone models, suggest that gas fragmentation is inevitable above a critical metallicity, whose value is between Zcr~3x10^{-4} Zsun (in the absence of dust) and as low as Zcr~ 5 x 10^{-6} Zsun (with a dust-to-gas mass ratio of about 0.01 Z/Zsun). We propose that when the metallicity exceeds these critical values, dense clusters of low--mass stars may form at the halo nucleus. Relatively massive stars in such a cluster can then rapidly coalesce into a single more massive object, which may produce an intermediate-mass BH remnant with a mass up to M <~10^2-10^3 Msun.Comment: 39 pages, 10 figures, ApJ in pres

An abundance study of red-giant-branch stars in the Hercules dwarf spheroidal galaxy

Using high-resolution spectroscopy, we provide a determination of [Fe/H] and [Ca/H] for confirmed red-giant branch member stars of the Hercules dwarf spheroidal galaxy. Based on this we explore the ages of the prevailing stellar populations in Hercules, and the enrichment history from supernovae. Additionally, we provide a new simple metallicity calibration for Stromgren photometry for metal-poor, red giant branch stars. We find that the red-giant branch stars of the Hercules dSph galaxy are more metal-poor than estimated in our previous study that was based on photometry alone. Additionally, we find an abundance trend such that [Ca/Fe] is higher for more metal-poor stars, and lower for more metal-rich stars, with a spread of about 0.8 dex. The [Ca/Fe] trend suggests an early rapid chemical enrichment through supernovae of type II, followed by a phase of slow star formation dominated by enrichment through supernovae of type Ia. A comparison with isochrones indicates that the red giants in Hercules are older than 10 Gyr.Comment: 12 pages, 11 figures. Accepted for publication in A&

HE 0557-4840 - Ultra-Metal-Poor and Carbon-Rich

We report the discovery and high-resolution, high S/N, spectroscopic analysis of the ultra-metal-poor red giant HE 0557-4840, which is the third most heavy-element deficient star currently known. Its atmospheric parameters are T_eff = 4900 K, log g = 2.2, and [Fe/H]= -4.75. This brings the number of stars with [Fe/H] < -4.0 to three, and the discovery of HE 0557-4840 suggests that the metallicity distribution function of the Galactic halo does not have a "gap" between [Fe/H] = -4.0, where several stars are known, and the two most metal-poor stars, at [Fe/H] ~ -5.3. HE 0557-4840 is carbon rich - [C/Fe] = +1.6 - a property shared by all three objects with [Fe/H] < -4.0, suggesting that the well-known increase of carbon relative to iron with decreasing [Fe/H] reaches its logical conclusion - ubiquitous carbon richness - at lowest abundance. We also present abundances (nine) and limits (nine) for a further 18 elements. For species having well-measured abundances or strong upper limits, HE 0557-4840 is "normal" in comparison with the bulk of the stellar population at [Fe/H] ~ -4.0 - with the possible exception of Co. We discuss the implications of these results for chemical enrichment at the earliest times, in the context of single ("mixing and fallback") and two-component enrichment models. While neither offers a clear solution, the latter appears closer to the mark. Further data are required to determine the oxygen abundance and improve that of Co, and hence more strongly constrain the origin of this object.Comment: Submitted to Astrophysical Journal. 52 pages (41 text, 11 figures

A single low-energy, iron-poor supernova as the source of metals in the star SMSS J 031300.36-670839.3

The element abundance ratios of four low-mass stars with extremely low metallicities indicate that the gas out of which the stars formed was enriched in each case by at most a few, and potentially only one low-energy, supernova. Such supernovae yield large quantities of light elements such as carbon but very little iron. The dominance of low-energy supernovae is surprising, because it has been expected that the first stars were extremely massive, and that they disintegrated in pair-instability explosions that would rapidly enrich galaxies in iron. What has remained unclear is the yield of iron from the first supernovae, because hitherto no star is unambiguously interpreted as encapsulating the yield of a single supernova. Here we report the optical spectrum of SMSS J031300.36- 670839.3, which shows no evidence of iron (with an upper limit of 10^-7.1 times solar abundance). Based on a comparison of its abundance pattern with those of models, we conclude that the star was seeded with material from a single supernova with an original mass of ~60 Mo (and that the supernova left behind a black hole). Taken together with the previously mentioned low-metallicity stars, we conclude that low-energy supernovae were common in the early Universe, and that such supernovae yield light element enrichment with insignificant iron. Reduced stellar feedback both chemically and mechanically from low-energy supernovae would have enabled first-generation stars to form over an extended period. We speculate that such stars may perhaps have had an important role in the epoch of cosmic reionization and the chemical evolution of early galaxies.Comment: 28 pages, 6 figures, Natur