Photometry and the Metallicity Distribution of the Outer Halo of M31

Abstract

We have conducted a wide-field CCD-mosaic study of the resolved red-giant branch (RGB) stars of M31, in a field located 20 kpc from the nucleus along the SE minor axis. In our (I, V-I) color-magnitude diagram, RGB stars in the top three magnitudes of the M31 halo are strongly present. Photometry of a more distant control field to subtract field contamination is used to derive the `cleaned' luminosity function and metallicity distribution function (MDF) of the M31 halo field. From the color distribution of the foreground Milky Way halo stars, we find a reddening E(V-I)= 0.10 +/- 0.02 for this field, and from the luminosity of the RGB tip, we determine a distance modulus (m-M)_o = 24.47 +/- 0.12 (= 783 +/- 43 kpc). The MDF is derived from interpolation within an extensive new grid of RGB models (Vandenberg et al. 2000). The MDF is dominated by a moderately high-metallicity population ([m/H]~ -0.5) found previously in more interior M31 halo/bulge fields, and is much more metal-rich than the [m/H]~ -1.5 level in the Milky Way halo. A significant (~30% - 40%, depending on AGB star contribution) metal-poor population is also present. To first order, the shape of the MDF resembles that predicted by a simple, single-component model of chemical evolution starting from primordial gas with an effective yield y=0.0055. It strongly resembles the MDF recently found for the outer halo of the giant elliptical NGC 5128 (Harris et al. 2000), though NGC 5128 has an even lower fraction of low-metallicity stars. Intriguingly, in both NGC 5128 and M31, the metallicity distribution of the globular clusters in M31 does not match the halo stars; the clusters are far more heavily weighted to metal-poor objects. We suggest similarities in the formation and early evolution of massive, spheroidal stellar systems.Comment: to appear in the Astronomical Journal; 43 pages, including 15 figure

A close look at the Centaurus A group of galaxies

“…First, we have a small (∼22%, see e.g. Durrell et al 2001) contamination from old ( > ∼ 10 Gyr), low-luminosity AGB stars that overlap with our RGB. With the available photometry it is impossible to distinguish them from RGB stars, and our main conclusions are not affected by this inevitable contamination since it would introduce a systematic bias towards slightly lower metallicities within the investigated metallicity range.…”

“…In this way we are able to derive the metallicity of each star on the RGB via interpolation among the isochrones. This method is widely used for studies of predominantly old populations, for which spectroscopy is not available (e.g., Durrell et al 2001;Sarajedini et al 2002;Rejkuba et al 2005;Harris et al 2007;Richardson et al 2009), to derive photometric metallicity distribution functions.…”

Direct detection of galaxy stellar halos: NGC 3957 as a test case

“…The metallicity distribution derived by Ibata et al (2007) from the diffuse light of M 31 halo translates into V − R ∼ 0.51. The M2 field of Durrell et al (2001), located at 20 kpc along the M 31 minor axis, peaks at [M/H] ∼ −0.5 with a long tail of more metal-poor population down to −2.5, leading to an integrated V − R ∼ 0.54. In contrast, the spectroscopic study of Koch et al (2008) In summary, V − R from ∼0.45 to ∼0.6 constitutes the range of expected integrated colours of old and preferentially metalpoor galactic halos.…”

Exploratory C star search in the disk of M 31 beyond 30 kpc

“…The first two points correspond to counts in regions of the size of DOLORES in SW2 (Battinelli et al 2003). The dashed line represents the counts in Durrell et al (2001) region R1 located 7.5 degrees from M 31. We assume that these counts represent the foreground contribution.…”