An HST/ACS investigation of the spatial and chemical structure and sub-structure of NGC 891, a Milky Way analogue

Abstract

We present a structural analysis of NGC 891, an edge-on galaxy that has long been considered to be an analogue of the Milky Way. Using star-counts derived from deep Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) images, we detect the presence of a thick disc component in this galaxy with vertical scaleheight h_Z = 1.44 +/- 0.03kpc and radial scalelength h_R = 4.8 +/- 0.1kpc, only slightly longer than that of the thin disc. A stellar spheroid with a de Vaucouleurs-like profile is detected from a radial distance of r ~ 0.5kpc to the edge of the survey at r ~ 25kpc the structure appears to become more flattened with distance, reaching q = 0.50 in the outermost halo region probed. The halo inside of r ~ 15kpc is moderately metal-rich (median [Fe/H] ~ -1.1) and approximately uniform in median metallicity. Beyond that distance, a modest chemical gradient is detected, with the median reaching [Fe/H] ~ -1.3 at r ~ 20kpc. We find evidence for subtle, but very significant, small-scale variations in the median colour and density over the halo survey area. We argue that the colour variations are unlikely to be due to internal extinction or foreground extinction, and reflect instead variations in the stellar metallicity. Their presence suggests a startling conclusion: that the halo of this galaxy is composed of a large number of incompletely mixed sub-populations, testifying to its origin in a deluge of small accretions.

This work is based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. This publication also makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

E-mail: ibata@astro.u-strasbg.fr