By Karoline Gilbert, Assistant Astronomer at STScI.
Stellar halos are built largely from the accretion and disruption of satellite galaxies. Tidal debris features from these disruption events remain identifiable for billions of years, providing observable signatures of the merger histories of individual galaxies. However, in situ star formation may also play a part in building up the inner regions of stellar halos, either via stars formed in the proto-disk of the host galaxy, or in gas deposited by disrupted satellites.
Therefore, studying stellar halos in detail provides a window into the formation histories of galaxies. With current instrumentation, the only stellar halos that can be studied in great detail are the Milky Way and Andromeda (aka M31), the two large spiral galaxies of the Local Group.
I am a member of the SPLASH collaboration (Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo). Our team has amassed a large photometric and spectroscopic dataset of red giant branch stars in M31′s halo and dwarf galaxies. Our photometric data are primarily taken with the Mosaic camera on the Mayall 4-m telescope on Kitt Peak, and include narrow band imaging that allows us to select spectroscopic targets with a high probability of being M31 stars. Our spectroscopic data are taken with the DEIMOS multi-object spectrograph on the Keck II 10-m telescope.
Figure 1: The locations of our Keck/DEIMOS spectroscopic fields in M31′s stellar halo, overlaid on the PAndAS starcount map (McConnachie et al. 2009). Our spectroscopic observations target fields on and off halo substructure, and cover a large range in radius.
My recent focus is on leveraging the full M31 halo dataset (shown in Figure 1) to learn about the global properties of M31′s stellar halo, and the ensemble of disrupted dwarf galaxies that built it. We have used this dataset to show that M31′s stellar halo extends to at least 180 kpc in projection from the center of M31. Furthermore, the density profile of stars shows no indication of a break, even though we are now tracing it to 2/3 of M31′s virial radius (Gilbert et al. 2012).
I am currently using the M31 halo dataset to analyze the metallicity distribution of M31 halo stars as a function of radius. We have spectra of over 1500 M31 stars in 32 spectroscopic fields ranging in distance from 9 to 180 kpc from M31′s center. The data show a clear gradient in metallicity that extends to 100 kpc (Figure 2). This gradient is seen in both the photometric (based on a star’s position in the color-magnitude diagram) and spectroscopic (based on the strength of the Calcium II triplet absorption feature) metallicity estimates.
Our spectra allow us to analyze the velocity distributions of stars in each field and to identify tidal debris features by their cold kinematical signatures. When we remove stars associated with tidal debris, the strength of the observed metallicity gradient increases.
Figure 2: Metallicity Distribution Functions of stars in M31′s halo: (top left) all M31 halo stars; (top right) after removal of tidal debris features; (bottom) cumulative distributions for all halo stars (solid curves) and after removal of tidal debris features (dashed curves). Arrows mark the median [Fe/H] values for each distribution. The inner halo is primarily metal-rich, while the outer halo is significantly more metal-poor. The data show evidence of a metallicity gradient in M31′s stellar halo extending from 9 kpc to ~100 kpc.
This large-scale metallicity gradient, when compared to the results of simulations of stellar halo formation, implies that the bulk of M31′s stellar halo was likely built primarily from one to a few relatively massive dwarf galaxies (>109 solar masses).
However, we also observe significant field-to-field scatter in the mean metallicities and surface brightnesses of fields at large radius. This implies that recently accreted, small dwarf galaxies have contributed substantially to the outermost regions of M31′s stellar halo.
If you are interested in learning more, a paper presenting these results is in progress. It should appear on astro-ph in the next few months! You can also check out other recent SPLASH papers, discussing the extended surface brightness profile of M31 (Gilbert et al. 2012), the properties of the inner regions of M31′s stellar halo (Dorman et al. 2012 and Dorman et al. 2013), and our spectroscopic survey of M31′s dwarf galaxies (Tollerud et al. 2012).