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S.C. Chapman (Caltech), R. Ibata (Observatoire de Strasbourg), A. Ferguson (ROE, Edinburgh), M. Irwin (IoA, Cambridge), G. Lewis (IoA, University of Sydney), N. Tanvir (Univ. of Hertfordshire)
We present the discovery of an inhomogenous, low-surface brightness, extended disk-like structure around the Andromeda galaxy (M31) based on a large kinematic survey with the Keck/DEIMOS multi-object spectrograph. The structure spans radii from 15 kpc out to ~ 40 kpc, with possible detections out to 70 kpc. The constituent stars lag the expected velocity of circular orbits in the plane of the M31 disk by ~ 40 kms and have a velocity dispersion of ~ 30 kms . The colour range on the upper RGB shows a large spread indicative of a population with a significant range of metallicity. The mean metallicity of the population, deduced from Ca~II equivalent widths, is {[Fe/H] = -0.9 ±0.2}. The morphology of the structure is irregular at large radii, and shows a wealth of substructures which must be transitory in nature, and are almost certainly tidal debris. The presence of these substructures indicates that the global entity was formed by accretion. This extended disk follows an exponential density law from the inner regions of the M31 disk at 15 kpc out to ~ 40 kpc, with a scale length of 16.2 ± 1.8 kpc. However, the population possesses almost identical kinematic and abundance properties over the entire region where it is detected in the survey. We estimate that the structure has a total luminosity comparable to the dwarf spiral galaxy M33, and accounts for approximately 10% of the total luminosity of M31. Given the huge scale of the structure, this means that ~ 30% of the total angular momentum of Andromeda resides in the extended disk component. This finding indicates that at least some galactic disks are vastly larger than previously thought and are formed, at least in their outer regions, primarily by accretion.
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The author(s) of this abstract have provided an email address for comments about the abstract: schapman@astro.caltech.edu
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Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.