The Metal-poor Halo of the Andromeda Spiral Galaxy (M31)

By J. Kalirai, K. Gilbert, P. Guhathakurta, S. Majewski, J. Ostheimer, R. Rich, M. Cooper, D. Reitzel, R. Patterson. 2006

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We present spectroscopic observations of red giant branch (RGB) stars over a large expanse in the halo of the Andromeda spiral galaxy (M31), acquired with the DEIMOS instrument on the Keck II 10 m telescope. Using a combination of five photometric/spectroscopic diagnostics—(1) radial velocity, (2) intermediate-width DDO51 photometry, (3) Na I equivalent width (surface gravity-sensitive), (4) position in the color-magnitude diagram, and (5) comparison between photometric and spectroscopic [Fe/H] estimates—we isolate over 250 bona fide M31 bulge and halo RGB stars located in 12 fields ranging from R = 12 to 165 kpc from the center of M31 (47 of these stars are halo members with R > 60 kpc). We derive the M31 spheroid (bulge and halo) metallicity distribution function and find it to be systematically more metal-poor with increasing radius, shifting from ⟨[Fe/H]⟩ = -0.47 ± 0.03 (σ = 0.39) at R < 20 kpc to ⟨[Fe/H]⟩ = -0.94 ± 0.06 (σ = 0.60) at R ~ 30 kpc to ⟨[Fe/H]⟩ = -1.26 ± 0.10 (σ = 0.72) at R > 60 kpc, assuming [α/Fe] = 0.0. These results indicate the presence of a metal-poor RGB population at large radial distances out to at least R = 160 kpc, thereby supporting our recent discovery of a stellar halo in M31 (structural component with an R-2 power-law surface brightness profile). This component has a distinct metallicity distribution from M31's bulge. If we assume an α-enhancement of [α/Fe] = +0.3 for M31's halo, we derive ⟨[Fe/H]⟩ = -1.5 ± 0.1 (σ = 0.7). Therefore, the mean metallicity and metallicity spread of this newly found remote M31 RGB population are similar to those of the Milky Way halo.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/a14380fb2c32669401b1a6fa7933862f0bb08e52

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THE SPLASH SURVEY: SPECTROSCOPY OF 15 M31 DWARF SPHEROIDAL SATELLITE GALAXIES (2011, background): “Thus, selecting targets in e.g., the M −DDO51 versus M − T2 color–color diagram allows for far more efficient selection of spectroscopic targets that have high probabilities of being giants (Kalirai et al. 2006; Guhathakurta et al. 2006; Gilbert et al. 2006).”
Shredded Galaxies as the Source of Diffuse Intrahalo Light on Varying Scales (2007, background): “…the Milky Way (MM31 1012 M ; see Klypin et al. 2002; Seigar et al. 2006), the recently discovered, metal-poor stellar halo of M31 may contain a significantly higher fraction of that galaxy’s light, fIHL 2:5%Y5% (Irwin et al. 2005; Guhathakurta et al. 2005; Kalirai et al. 2006; Chapman et al. 2006).”
STRUCTURE AND KINEMATICS OF THE STELLAR HALOS AND THICK DISKS OF THE MILKY WAY BASED ON CALIBRATION STARS FROM SLOAN DIGITAL SKY SURVEY DR7 (2009, background): “Photometric and spectroscopic observations by Kalirai et al. (2006b), and more recently by Koch et al. (2008a), have demonstrated that a metalpoor halo of M 31 does indeed exist, but is located beyond 40 kpc from its center (substantially different than observed for the Milky Way, where the outer…”
THE DUAL ORIGIN OF STELLAR HALOS (2009, background): “M31 has been shown to have a distinct stellar halo with a surface brightness profile similar to the Milky Way’s, and a metallicity gradient with the inner regions of its halo (R < 30 kpc) more metal-rich than the outer regions (Guhathakurta et al. 2005; Kalirai et al. 2006).”
The Haunted Halos of Andromeda and Triangulum: A Panorama of Galaxy Formation in Action (2007, background): “Comparison to Kalirai et al. (2006b) Our discovery of a smooth very extended halo component covering the entire southern quadrant of Andromeda was anticipated by the kinematic study of Kalirai et al. (2006b).”
THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY (2012, methodology): “…its bulge, and sub-solar metallicity stars in its outskirts (e.g., Sil’chenko et al. 1998; Lauer et al. 2012; Brown et al. 2003; Worthey et al. 2005; Kalirai et al. 2006a; Chapman et al. 2006; Fan et al. 2008, and references therein), making M31 a superb laboratory for constraining stellar…”
Cosmological simulations of the formation of the stellar haloes around disc galaxies (2011, methodology): “…measurements (by applying a constant offset) to account for differences in the assumed solar abundances in the studies of Gilbert et al. (2009a), Kalirai et al. (2006), Richardson et al. (2009), and Koch et al. (2008) from the solar abundances of Asplund et al. (2005), which we use to…”
THE LARGE-SCALE STRUCTURE OF THE HALO OF THE ANDROMEDA GALAXY. I. GLOBAL STELLAR DENSITY, MORPHOLOGY AND METALLICITY PROPERTIES (2013, background): “…in §4.3 are in reasonable agreement with the results of earlier spectroscopic analyses that sampled the outer halo of M31 at a small number of locations (Chapman et al. 2006; Kalirai et al. 2006; Koch et al. 2008), and which reported mean metallicities in the range 〈[Fe/H]〉 ∼ −1.26 to −2.0.”
Galactic stellar haloes in the CDM model (2009, background): “Recent surveys of M31 (e.g. Ferguson et al. 2002; Kalirai et al. 2006; Ibata et al. 2007; McConnachie et al. 2009) have revealed an extensive halo (to ∼150 kpc) also displaying abundant substructure.”
THE M31 VELOCITY VECTOR. II. RADIAL ORBIT TOWARD THE MILKY WAY AND IMPLIED LOCAL GROUP MASS (2012, background): “Also, LOS velocity studies of the GSS have revealed a secondary cold component (in addition to the GSS and base galaxy; e.g., Kalirai et al. 2006b; Gilbert et al. 2007; Gilbert et al. 2009) which is not reproduced by our model.”
The Detailed Star Formation History in the Spheroid, Outer Disk, and Tidal Stream of the Andromeda Galaxy (2006, methodology): “The minor-axis population at 11 kpc from the center has kinematics that are inconsistent with a rotationally supported disk (Kalirai et al. 2006b; Rich et al. in prep.)”
Tracing Galaxy Formation with Stellar Halos. II. Relating Substructure in Phase and Abundance Space to Accretion Histories (2008, background): “…(metallicity∼ −0.7 Ferguson et al. 2005), while there is increasing evidence for a smoother, more metal-poor extended component at larger radii (metallicity ∼-1.3, see Irwin et al. 2005; Chapman et al. 2006; Kalirai et al. 2006), extending as far as 165kpc from its center (Gilbert et al. 2006).”
The stellar halo of the Galaxy (2008, background): “However, Kalirai et al. (2006) and Chapman et al. (2006) recently discovered a metal-poor halo out to 160 kpc that is similar to that of the Galaxy, both in terms of its metallicity, radial profile, as well as the amount of substructure.”