Unveiling the Boxy Bulge and Bar of the Andromeda Spiral Galaxy

A new, 2.8 deg^2 J,H,K_s infrared survey from the 2MASS 6x program across theextent of the optical disk of the Andromeda (M31) galaxy provides a clear viewof the M31 center almost completely unfettered by dust extinction, and revealsa high contrast bulge with extremely boxy isophotes dominating the NIR light toa semi-major axis of ~700''(2.6 kpc). The inner bulge (<~50'') is relativelycircular, but shows some isophotal twisting. Beyond this, the ellipticity andboxiness of the bulge increase with radius -- achieving a boxiness that rivalsthat of any other known disk galaxy observed in the near infrared -- and theposition angle is constant at ~50 deg, which is about 10 deg higher than theposition angle of the M31 disk. Boxy bulges in highly inclined disks have beenshown to be the vertical structure of bars, and self-consistent, N-bodymodeling specific to the NIR images presented here can reproduce the observedNIR M31 features with a combination of a classical bulge and a boxy bulge/bar.Beyond the boxy bulge region and nearly along the 40 deg position angle of thedisk a narrow ridge of infrared flux, which can be identified with the thinpart of the bar, more or less symmetrically extends into the inner disk atsemi-major axis radii of 700'' to 1200'' or more. Little variation in themorphology or relative brightnesses of these various M31 structures is seenacross the NIR bands (i.e., no color gradients are seen). These new data verifythat M31 is a barred spiral galaxy like the Milky Way.Comment: Suibmitted to Astrophysical Journal Letter