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Register a new userElemental abundances in M31: [Fe/H] and [{alpha}/Fe] in M31 Dwarf Galaxies Using Coadded Spectra
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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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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 deep spectroscopy from Keck/DEIMOS of Andromeda I, III, V, VII, and X, all of which are dwarf spheroidal satellites of M31. The sample includes 256 spectroscopic members across all five dSphs. We confirm previous measurements of the velocity dispersions and dynamical masses, and we provide upper limits on bulk rotation. Our measurements confirm that M31 satellites obey the same relation between stellar mass and stellar metallicity as Milky Way (MW) satellites and other dwarf galaxies in the Local Group. The metallicity distributions show similar trends with stellar mass as MW satellites, including evidence in massive satellites for external influence, like pre-enrichment or gas accretion. We present the first measurements of individual element ratios, like [Si/Fe], in the M31 system, as well as measurements of the average [alpha/Fe] ratio. The trends of [alpha/Fe] with [Fe/H] also follow the same galaxy mass-dependent patterns as MW satellites. Less massive galaxies have more steeply declining slopes of [alpha/Fe] that begin at lower [Fe/H]. Finally, we compare the chemical evolution of M31 satellites to M31s Giant Stellar Stream and smooth halo. The properties of the M31 system support the theoretical prediction that the inner halo is composed primarily of massive galaxies that were accreted early. As a result, the inner halo exhibits higher [Fe/H] and [alpha/Fe] than surviving satellite galaxies.
We present the first measurements of [Fe/H] and [$alpha$/Fe] abundances, obtained using spectral synthesis modeling, for red giant branch stars in M31s giant stellar stream. The spectroscopic observations, obtained at a projected distance of 17 kpc from M31s center, yielded 61 stars with [Fe/H] measurements, including 21 stars with [$alpha$/Fe] measurements, from 112 targets identified as M31 stars. The [Fe/H] measurements confirm the expectation from photometric metallicity estimates that stars in this region of M31s halo are relatively metal-rich compared to stars in the MWs inner halo: more than half the stars in the field, including those not associated with kinematically identified substructure, have [Fe/H] abundances $> -1.0$. The stars in this field are $alpha$-enhanced at lower metallicities, while [$alpha$/Fe] decreases with increasing [Fe/H] above metallicities of [Fe/H] $gtrsim -0.9$. Three kinematical components have been previously identified in this field: the giant stellar stream, a second kinematically cold feature of unknown origin, and M31s kinematically hot halo. We compare probabilistic [Fe/H] and [$alpha$/Fe] distribution functions for each of the components. The giant stellar stream and the second kinematically cold feature have very similar abundance distributions, while the halo component is more metal-poor. Although the current sample sizes are small, a comparison of the abundances of stars in the giant stellar stream field with abundances of M31 halo and dSph stars from the literature indicate that the progenitor of the stream was likely more massive, and experienced a higher efficiency of star formation, than M31s existing dSphs or the dEs NGC147 and NGC185.
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 analyze existing measurements of [Fe/H] and [$alpha$/Fe] for individual red giant branch (RGB) stars in the Giant Stellar Stream (GSS) of M31 to determine whether spatial abundance gradients are present. These measurements were obtained from low- ($R sim 3000$) and moderate- ($R sim 6000$) resolution Keck/DEIMOS spectroscopy using spectral synthesis techniques as part of the Elemental Abundances in M31 survey. From a sample of 62 RGB stars spanning the GSS at 17, 22, and 33 projected kpc, we measure a [Fe/H] gradient of $-$0.018 $pm$ 0.003 dex kpc$^{-1}$ and negligible [$alpha$/Fe] gradient with M31-centric radius. We investigate GSS abundance patterns in the outer halo using additional [Fe/H] and [$alpha$/Fe] measurements for 6 RGB stars located along the stream at 45 and 58 projected kpc. These abundances provide tentative evidence that the trends in [Fe/H] and [$alpha$/Fe] beyond 40 kpc in the GSS are consistent with those within 33 kpc. We also compare the GSS abundances to 65 RGB stars located along the possibly related Southeast (SE) shelf substructure at 12 and 18 projected kpc. The abundances of the GSS and SE shelf are consistent, supporting a common origin hypothesis, although this interpretation may be complicated by the presence of [Fe/H] gradients in the GSS. We discuss the abundance patterns in the context of photometric studies from the literature and explore implications for the properties of the GSS progenitor, suggesting that the high $langle$[$alpha$/Fe]$rangle$ of the GSS (+0.40 $pm$ 0.05 dex) favors a major merger scenario for its formation.
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