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Register a new user[$alpha$/Fe] Abundances of Four Outer M 31 Halo Stars
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Authors
Luis C. Vargas
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We present alpha element to iron abundance ratios, [$alpha$/Fe], for four stars in the outer stellar halo of the Andromeda Galaxy (M 31). The stars were identified as high-likelihood field halo stars by Gilbert et al. (2012) and lie at projected distances between 70 and 140 kpc from M 31s center. These are the first alpha abundances measured for a halo star in a galaxy beyond the Milky Way. The stars range in metallicity between [Fe/H]= -2.2 and [Fe/H]= -1.4. The samples average [$alpha$/Fe] ratio is +0.20+/-0.20. The best-fit average value is elevated above solar which is consistent with rapid chemical enrichment from Type II supernovae. The mean [$alpha$/Fe] ratio of our M31 outer halo sample agrees (within the uncertainties) with that of Milky Way inner/outer halo stars that have a comparable range of [Fe/H].
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We present [Fe/H] and [$alpha$/Fe] abundances, derived using spectral synthesis techniques, for stars in M31s outer stellar halo. The 21 [Fe/H] measurements and 7 [$alpha$/Fe] measurements are drawn from fields ranging from 43 to 165 kpc in projected distance from M31. We combine our measurements with existing literature measurements, and compare the resulting sample of 23 stars with [Fe/H] and 9 stars with [$alpha$/Fe] measurements in M31s outer halo with [$alpha$/Fe] and [Fe/H] measurements, also derived from spectral synthesis, in M31s inner stellar halo ($r < $26 kpc) and dSph galaxies. The stars in M31s outer halo have [$alpha$/Fe] patterns that are consistent with the largest of M31s dSph satellites (And I and And VII). These abundances provide tentative evidence that the [$alpha$/Fe] abundances of stars in M31s outer halo are more similar to the abundances of Milky Way halo stars than to the abundances of stars in M31s inner halo. We also compare the spectral synthesis-based [Fe/H] measurements of stars in M31s halo with previous photometric [Fe/H] estimates, as a function of projected distance from M31. The spectral synthesis-based [Fe/H] measurements are consistent with a large-scale metallicity gradient previously observed in M31s stellar halo to projected distances as large as 100 kpc.
We present chemical abundances of 57 metal-poor stars that are likely constituents of the outer stellar halo in the Milky Way. Almost all of the sample stars have an orbit reaching a maximum vertical distance (Z_max) of >5 kpc above and below the Galactic plane. High-resolution, high signal-to-noise spectra for the sample stars obtained with Subaru/HDS are used to derive chemical abundances of Na, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Y and Ba with an LTE abundance analysis code. The resulting abundance data are combined with those presented in literature that mostly targeted at smaller Z_max stars, and both data are used to investigate any systematic trends in detailed abundance patterns depending on their kinematics. It is shown that, in the metallicity range of -2<[Fe/H]<-1, the [Mg/Fe] ratios for the stars with Z_max>5 kpc are systematically lower (~0.1 dex) than those with smaller Z_max. This result of the lower [alpha/Fe] for the assumed outer halo stars is consistent with previous studies that found a signature of lower [alpha/Fe] ratios for stars with extreme kinematics. A distribution of the [Mg/Fe] ratios for the outer halo stars partly overlaps with that for stars belonging to the Milky Way dwarf satellites in the metallicity interval of -2<[Fe/H]<-1 and spans a range intermediate between the distributions for the inner halo stars and the stars belonging to the satellites. Our results confirm inhomogeneous nature of chemical abundances within the Milky Way stellar halo depending on kinematic properties of constituent stars as suggested by earlier studies. Possible implications for the formation of the Milky Way halo and its relevance to the suggested dual nature of the halo are discussed.
We measured [Fe/H] and [$alpha$/Fe] using spectral synthesis of low-resolution stellar spectroscopy for 70 individual red giant branch stars across four fields spanning the outer disk, Giant Stellar Stream (GSS), and inner halo of M31. Fields at M31-centric projected distances of 23 kpc in the halo, 12 kpc in the halo, 22 kpc in the GSS, and 26 kpc in the outer disk are $alpha$-enhanced, with $langle$[$alpha$/Fe]$rangle$ = 0.43, 0.50, 0.41, and 0.58, respectively. The 23 kpc and 12 kpc halo fields are relatively metal-poor, with $langle$[Fe/H]$rangle$ = $-$1.54 and $-$1.30, whereas the 22 kpc GSS and 26 kpc outer disk fields are relatively metal-rich with $langle$[Fe/H]$rangle$ = $-$0.84 and $-$0.92, respectively. For fields with substructure, we separated the stellar populations into kinematically hot stellar halo components and kinematically cold components. We did not find any evidence of an [$alpha$/Fe] gradient along the high surface brightness core of the GSS between $sim$17$-$22 kpc. However, we found tentative suggestions of a negative [$alpha$/Fe] gradient in the stellar halo, which may indicate that different progenitor(s) or formation mechanisms contributed to the build up of the inner versus outer halo. Additionally, the [$alpha$/Fe] distribution of the metal-rich ([Fe/H] $>$ $-$1.5), smooth inner stellar halo (r$_{rm{proj}}$ $lesssim$ 26 kpc) is inconsistent with having formed from the disruption of progenitor(s) similar to present-day M31 satellite galaxies. The 26 kpc outer disk is most likely associated with the extended disk of M31, where its high $alpha$-enhancement provides support for an episode of rapid star formation in M31s disk, possibly induced by a major merger.
We obtain a new determination of the metallicity distribution function (MDF) of stars within $sim5$-$10$ kpc of the Sun, based on recently improved co-adds of $ugriz$ photometry for Stripe 82 from the Sloan Digital Sky Survey. Our new estimate uses the methodology developed previously by An et al. to study in situ halo stars, but is based on a factor of two larger sample than available before, with much-improved photometric errors and zero-points. The newly obtained MDF can be divided into multiple populations of halo stars, with peak metallicities at [Fe/H] $approx -1.4$ and $-1.9$, which we associate with the inner-halo and outer-halo populations of the Milky Way, respectively. We find that the kinematics of these stars (based on proper-motion measurements at high Galactic latitude) supports the proposed dichotomy of the halo, as stars with retrograde motions in the rest frame of the Galaxy are generally more metal-poor than stars with prograde motions, consistent with previous claims. In addition, we generate mock catalogs of stars from a simulated Milk Way halo system, and demonstrate for the first time that the chemically- and kinematically-distinct properties of the inner- and outer-halo populations are qualitatively in agreement with our observations. The decomposition of the observed MDF and our comparison with the mock catalog results suggest that the outer-halo population contributes on the order of $sim35%$-$55%$ of halo stars in the local volume.
We present chemical abundances of red giant branch (RGB) stars in the dwarf spheroidal (dSph) satellite system of Andromeda (M31), using spectral synthesis of medium resolution (R $sim 6000$) spectra obtained with the Keck II telescope and DEIMOS spectrograph via the Spectroscopic and Photometric Landscape of Andromedas Stellar Halo (SPLASH) survey. We coadd stars according to their similarity in photometric metallicity or effective temperature to obtain a signal-to-noise ratio (S/N) high enough to measure average [Fe/H] and [$alpha$/Fe] abundances. We validate our method using high S/N spectra of RGB stars in Milky Way globular clusters as well as deep observations for a subset of the M31 dSphs in our sample. For this set of validation coadds, we compare the weighted average abundance of the individual stars with the abundance determined from the coadd. We present individual and coadded measurements of [Fe/H] and [$alpha$/Fe] for stars in ten M31 dSphs, including the first [$alpha$/Fe] measurements for And IX, XIV, XV, and XVIII. These fainter, less massive dSphs show declining [$alpha$/Fe] relative to [Fe/H], implying an extended star formation history. In addition, these dSphs also follow the same mass-metallicity relation found in other Local Group satellites. The conclusions we infer from coadded spectra agree with those from previous measurements in brighter M31 dSphs with individual abundance measurements, as well as conclusions from photometric studies. These abundances greatly increase the number of spectroscopic measurements of the chemical composition of M31s less massive dwarf satellites, which are crucial to understanding their star formation history and interaction with the M31 system.
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