Four Metal-poor Stars in the Sagittarius Dwarf Spheroidal Galaxy

We present the metallicities and carbon abundances of four newly discovered metal-poor stars with $-2.2 <$ [Fe/H] $< -1.6$ in the Sagittarius dwarf spheroidal galaxy. These stars were selected as metal-poor member candidates using a combination of public photometry from the SkyMapper Southern Sky Survey and proper motion data from the second data release from the Gaia mission. The SkyMapper filters include a metallicity-sensitive narrow-band $v$ filter centered on the Ca II K line, which we use to identify metal-poor candidates. In tandem, we use proper motion data to remove metal-poor stars that are not velocity members of the Sagittarius dwarf spheroidal galaxy. We find that these two datasets allow for efficient identification of metal-poor members of the Sagittarius dwarf galaxy to follow-up with further spectroscopic study. Two of the stars we present have [Fe/H] $< -2.0$, which adds to the few other such stars currently identified in the Sagittarius dwarf galaxy that are likely not associated with the globular cluster M54, which resides in the nucleus of the system. Our results confirm that there exists a very metal-poor stellar population in the Sagittarius dwarf galaxy. We find that none of our stars can be classified as carbon-enhanced metal-poor stars. Efficiently identifying members of this population will be helpful to further our understanding of the early chemical evolution of the system.Comment: 14 pages, 4 figures; 2 tables; Accepted for publication in Ap

Complete element abundances of nine stars in the r-process galaxy Reticulum II

We present chemical abundances derived from high-resolution Magellan/MIKE spectra of the nine brightest known red giant members of the ultra-faint dwarf galaxy Reticulum II. These stars span the full metallicity range of Ret II (-3.5 < [Fe/H] < -2). Seven of the nine stars have extremely high levels of r-process material ([Eu/Fe]~1.7), in contrast to the extremely low neutron-capture element abundances found in every other ultra-faint dwarf galaxy studied to date. The other two stars are the most metal-poor stars in the system ([Fe/H] < -3), and they have neutron-capture element abundance limits similar to those in other ultra-faint dwarf galaxies. We confirm that the relative abundances of Sr, Y, and Zr in these stars are similar to those found in r-process halo stars but ~0.5 dex lower than the solar r-process pattern. If the universal r-process pattern extends to those elements, the stars in Ret II display the least contaminated known r-process pattern. The abundances of lighter elements up to the iron peak are otherwise similar to abundances of stars in the halo and in other ultra-faint dwarf galaxies. However, the scatter in abundance ratios is large enough to suggest that inhomogeneous metal mixing is required to explain the chemical evolution of this galaxy. The presence of low amounts of neutron-capture elements in other ultra-faint dwarf galaxies may imply the existence of additional r-process sites besides the source of r-process elements in Ret II. Galaxies like Ret II may be the original birth sites of r-process enhanced stars now found in the halo.Comment: 14 pages, 5 figures, 5 tables. Accepted to Ap

SD 1313-0019 -- Another second-generation star with [Fe/H] = -5.0, observed with the Magellan Telescope

We present a Magellan/MIKE high-resolution (R ~ 35,000) spectrum of the ancient star SD 1313-0019 which has an iron abundance of [Fe/H] = -5.0, paired with a carbon enhancement of [C/Fe] ~ 3.0. The star was initially identified by Allende Prieto et al. in the BOSS survey. Its medium-resolution spectrum suggested a higher metallicity of [Fe/H] = -4.3 due to the CaII K line blending with a CH feature which is a common issue related to the search for the most iron-poor stars. This star joins several other, similar stars with [Fe/H] < -5.0 that all display a combination of low iron and high carbon abundances. Other elemental abundances of SD 1313-0019 follow that of more metal-rich halo stars. From fitting the abundance pattern with yields of Population III supernova, we conclude that SD 1313-0019 had only one massive progenitor star with 20 - 30 M_sun that must have undergone a mixing and fallback episode. Overall, there are now five stars known with [Fe/H] < -5.0 (1D LTE abundances). This population of second-generation stars strongly suggests massive first stars that almost exclusively produced large amounts of carbon through stellar winds and/or their mixing and fallback supernova explosions. As a consequence, their natal clouds -- presumably some early minihalo structures -- contained ample amounts of carbon and oxygen that likely facilitated the formation of these first low-mass stars.Comment: 7 pages and 3 figures, accepted by ApJ

Chemical Abundances of new member stars in the Tucana II dwarf galaxy

We present chemical abundance measurements for seven stars with metallicities ranging from [Fe/H] = $-$3.3 to [Fe/H] = $-$2.4 in the Tucana II ultra-faint dwarf galaxy (UFD), based on high-resolution spectra obtained with the MIKE spectrograph on the 6.5 m Magellan-Clay Telescope. For three stars, we present detailed chemical abundances for the first time. Of those, two stars are newly discovered members of Tucana II and were selected as probable members from deep narrow band photometry of the Tucana II UFD taken with the SkyMapper telescope. This result demonstrates the potential for photometrically identifying members of dwarf galaxy systems based on chemical composition. One new star was selected from the membership catalog of Walker et al. (2016). The other four stars in our sample have been re-analyzed, following additional observations. Overall, six stars have chemical abundances that are characteristic of the UFD stellar population. The seventh star shows chemical abundances that are discrepant from the other Tucana II members and an atypical, higher strontium abundance than what is expected for typical UFD stars. While unlikely, its strontium abundance raises the possibility that it may be a foreground metal-poor halo star with the same systemic velocity as Tucana II. If we were to exclude this star, Tucana II would satisfy the criteria to be a surviving first galaxy. Otherwise, this star implies that Tucana II has likely experienced somewhat extended chemical evolution.Comment: 20 pages, 6 figures, 5 tables; Accepted for publication in ApJ. Changes w.r.t. v1: corrected coordinates for TucII-078 in Table

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

Stellar metallicities from SkyMapper photometry I: A study of the Tucana II ultra-faint dwarf galaxy

We present a study of the ultra-faint Milky Way dwarf satellite galaxy Tucana II using deep photometry from the 1.3m SkyMapper telescope at Siding Spring Observatory, Australia. The SkyMapper filter-set contains a metallicity-sensitive intermediate-band $v$ filter covering the prominent Ca II K feature at 3933.7A. When combined with photometry from the SkyMapper $u, g$, and $i$ filters, we demonstrate that $v$ band photometry can be used to obtain stellar metallicities with a precision of $\sim0.20$dex when [Fe/H] $> -2.5$, and $\sim0.34$dex when [Fe/H] $< -2.5$. Since the $u$ and $v$ filters bracket the Balmer Jump at 3646A, we also find that the filter-set can be used to derive surface gravities. We thus derive photometric metallicities and surface gravities for all stars down to a magnitude of $g\sim20$ within $\sim$75 arcminutes of Tucana II. Photometric metallicity and surface gravity cuts remove nearly all foreground contamination. By incorporating Gaia proper motions, we derive quantitative membership probabilities which recover all known members on the red giant branch of Tucana II. Additionally, we identify multiple likely new members in the center of the system and candidate members several half-light radii from the center of the system. Finally, we present a metallicity distribution function derived from the photometric metallicities of likely Tucana II members. This result demonstrates the utility of wide-field imaging with the SkyMapper filter-set in studying UFDs, and in general, low surface brightness populations of metal-poor stars. Upcoming work will clarify the membership status of several distant stars identified as candidate members of Tucana II.Comment: 24 pages; 12 figures, 4 tables; Accepted for publication in Ap

Chemical Abundances of New Member Stars in the Tucana II Dwarf Galaxy

We present chemical abundance measurements for seven stars with metallicities ranging from Fe/H] =-3.3 to [Fe/H] =-2.4 in the Tucana II ultra-faint dwarf galaxy (UFD), based on high-resolution spectra obtained with the MIKE spectrograph on the 6.5 m Magellan-Clay Telescope. For three stars, we present detailed chemical abundances for the first time. Of those, two stars are newly discovered members of Tucana II and were selected as probable members from deep narrowband photometry of the Tucana II UFD taken with the SkyMapper telescope. This result demonstrates the potential for photometrically identifying members of dwarf galaxy systems based on chemical composition. One new star was selected from the membership catalog of Walker et al. The other four stars in our sample have been reanalyzed, following additional observations. Overall, six stars have chemical abundances that are characteristic of the UFD stellar population. The seventh star shows chemical abundances that are discrepant from the other Tucana II members and an atypical, higher strontium abundance than what is expected for typical UFD stars. While unlikely, its strontium abundance raises the possibility that it may be a foreground metal-poor halo star with the same systemic velocity as Tucana II. If we were to exclude this star, Tucana II would satisfy the criteria to be a surviving first galaxy. Otherwise, this star implies that Tucana II has likely experienced somewhat extended chemical evolution.A.C. and A.F. are supported by NSF CAREER grant AST-1255160. A.F. acknowledges partial support from PHY 14-30152; and Physics Frontier Center/JINA Center for the Evolution of the Elements (JINA-CEE), awarded by the US National Science Foundation. A.P.J. is supported by NASA through Hubble Fellowship grant HST-HF2-51393.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. H.J. and J.E.N. acknowledge the support of the Australian Research Council through Discovery project DP150100862. The national facility capability for SkyMapper has been funded through ARC LIEF grant LE130100104 from the Australian Research Council, awarded to the University of Sydney, the Australian National University, Swinburne University of Technology, the University of Queensland, the University of Western Australia, the University of Melbourne, Curtin University of Technology, Monash University and the Australian Astronomical Observatory. SkyMapper is owned and operated by The Australian National University’s Research School of Astronomy and Astrophysics. This work made use of NASAs Astrophysics Data System Bibliographic Services. This work has also made extensive use of the astropy package (Astropy Collaboration et al. 2013)

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

Portrait of a Dark Horse: a Photometric and Spectroscopic Study of the Ultra-faint Milky Way Satellite Pegasus III

Pegasus III (Peg III) is one of the few known ultra-faint stellar systems in the outer halo of the Milky Way. We present the results from a follow-up campaign with Magellan/IMACS and Keck/DEIMOS. Deep stellar photometry down to {r}0≈ 25 mag at 50% completeness level has allowed accurate measurements of its photometric and structural properties. The color-magnitude diagram of Peg III confirms that the stellar system is well described by an old (≳12 Gyr) and metal-poor ([Fe/H] ≲ -2.0 dex) stellar population at a heliocentric distance of 215 ± 12 kpc. The revised half-light radius {r}h=53+/- 14 pc, ellipticity ɛ ={0.38}-0.38+0.22, and total luminosity {M}V=-3.4+/- 0.4 are in good agreement with the values quoted in our previous paper. We further report on the spectroscopic identification of seven, possibly eight, member stars of Peg III. The Ca ii triplet lines of the brightest member stars indicate that Peg III contains stars with metallicity as low as [Fe/H] = -2.55 ± 0.15 dex. Peg III has a systemic velocity of -222.9 ± 2.6 km s-1 and a velocity dispersion of {5.4}-2.5+3.0 km s-1. The inferred dynamical mass within the half-light radius is {1.4}-1.1+3.0× {10}6{M}⊙ and the mass-to-light ratio M/{L}V={1470}-1240+5660 {M}⊙ /{L}⊙ , providing further evidence that Peg III is a dwarf galaxy satellite. We find that Peg III and another distant dwarf satellite Pisces II lie relatively close to each other ({{Δ }}{d}{spatial}=43+/- 19 kpc) and share similar radial velocities in the Galactic standard-of-rest frame ({{Δ }}{v}{GSR}=12.3+/- 3.7 km s-1). This suggests that they may share a common origin