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Register a new userElemental Abundances in M31: Alpha and Iron Element Abundances from Low-Resolution Resolved Stellar Spectroscopy in the Stellar Halo
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Measurements of [Fe/H] and [$alpha$/Fe] can probe the minor merging history of a galaxy, providing a direct way to test the hierarchical assembly paradigm. While measurements of [$alpha$/Fe] have been made in the stellar halo of the Milky Way, little is known about detailed chemical abundances in the stellar halo of M31. To make progress with existing telescopes, we apply spectral synthesis to low-resolution DEIMOS spectroscopy (R $sim$ 2500 at 7000 Angstroms) across a wide spectral range (4500 Angstroms $<$ $lambda$ $<$ 9100 Angstroms). By applying our technique to low-resolution spectra of 170 giant stars in 5 MW globular clusters, we demonstrate that our technique reproduces previous measurements from higher resolution spectroscopy. Based on the intrinsic dispersion in [Fe/H] and [$alpha$/Fe] of individual stars in our combined cluster sample, we estimate systematic uncertainties of $sim$0.11 dex and $sim$0.09 dex in [Fe/H] and [$alpha$/Fe], respectively. We apply our method to deep, low-resolution spectra of 11 red giant branch stars in the smooth halo of M31, resulting in higher signal-to-noise per spectral resolution element compared to DEIMOS medium-resolution spectroscopy, given the same exposure time and conditions. We find $langle$[$alpha$/Fe]$rangle$ = 0.49 $pm$ 0.29 dex and $langle$[Fe/H]$rangle$ = 1.59 $pm$ 0.56 dex for our sample. This implies that---much like the Milky Way---the smooth halo of M31 is likely composed of disrupted dwarf galaxies with truncated star formation histories that were accreted early in the halos formation.
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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$/Fe] abundances of stars in M31s outer halo are more similar to the abundances of Milky Way halo stars than to the abundances of stars in M31s inner halo. We also compare the spectral synthesis-based [Fe/H] measurements of stars in M31s halo with previous photometric [Fe/H] estimates, as a function of projected distance from M31. The spectral synthesis-based [Fe/H] measurements are consistent with a large-scale metallicity gradient previously observed in M31s stellar halo to projected distances as large as 100 kpc.
We measured [Fe/H] and [$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 present measurements of [Fe/H] and [$alpha$/Fe] for 128 individual red giant branch stars (RGB) in the stellar halo of M31, including its Giant Stellar Stream (GSS), obtained using spectral synthesis of low- and medium-resolution Keck/DEIMOS spectroscopy ($R sim 3000$ and 6000, respectively). We observed four fields in M31s stellar halo (at projected radii of 9, 18, 23, and 31 kpc), as well as two fields in the GSS (at 33 kpc). In combination with existing literature measurements, we have increased the sample size of [Fe/H] and [$alpha$/Fe] measurements from 101 to a total of 229 individual M31 RGB stars. From this sample, we investigate the chemical abundance properties of M31s inner halo, finding $langle$[Fe/H]$rangle$ = $-$1.08 $pm$ 0.04 and $langle$[$alpha$/Fe]$rangle$ = 0.40 $pm$ 0.03. Between 8--34 kpc, the inner halo has a steep [Fe/H] gradient ($-$0.025 $pm$ 0.002 dex kpc$^{-1}$) and negligible [$alpha$/Fe] gradient, where substructure in the inner halo is systematically more metal-rich than the smooth component of the halo at a given projected distance. Although the chemical abundances of the inner stellar halo are largely inconsistent with that of present-day dwarf spheroidal (dSph) satellite galaxies of M31, we identified 22 RGB stars kinematically associated with the smooth component of the stellar halo that have chemical abundance patterns similar to M31 dSphs. We discuss formation scenarios for M31s halo, concluding that these dSph-like stars may have been accreted from galaxies of similar stellar mass and star formation history, or of higher stellar mass and similar star formation efficiency.
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 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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