Elemental Abundances in M31: Properties of the Inner Stellar Halo. (arXiv:2009.00529v1 [astro-ph.GA])

Elemental Abundances in M31: Properties of the Inner Stellar Halo. (arXiv:2009.00529v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Escala_I/0/1/0/all/0/1">Ivanna Escala</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kirby_E/0/1/0/all/0/1">Evan N. Kirby</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gilbert_K/0/1/0/all/0/1">Karoline M. Gilbert</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wojno_J/0/1/0/all/0/1">Jennifer Wojno</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cunningham_E/0/1/0/all/0/1">Emily C. Cunningham</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Guhathakurta_P/0/1/0/all/0/1">Puragra Guhathakurta</a>

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 M31’s 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 M31’s 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 M31’s 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 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 M31’s 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 M31’s 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 M31’s 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.

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