Unveiling the Boxy Bulge and Bar of the Andromeda Spiral Galaxy

Abstract

A new, 2.8 deg^2 J, H, Ks near-infrared (NIR) survey from the 2MASS 6X program across the extent of the optical disk of the Andromeda (M31) galaxy provides a clear view of the M31 center almost completely unfettered by dust extinction and reveals a high-contrast bulge with very boxy isophotes dominating the NIR light to a semimajor axis of ~700" (2.6 kpc). The inner bulge (≾50") isophotes are relatively circular but show some twisting. Beyond this, (1) the M31 bulge ellipticity increases, (2) its position angle is constant at ~50°, or about 10° higher than the position angle of the M31 disk, and (3) its boxiness increases to a degree (~3%-4%) comparable to other renowned examples of boxy bulges observed in the NIR. In a companion paper, self-consistent N-body simulations of a classical bulge plus a bar with a boxy bulge are shown to reproduce the observed NIR M31 features presented here. Beyond the boxy bulge region and nearly along the 40° position angle of the disk a narrow ridge of NIR flux, which can be identified with the thin part of the bar, more or less symmetrically extends into the inner disk at semimajor axis radii of 700"-1200" or more. Little variation in the morphology or relative brightnesses of these various M31 structures is seen across the NIR bands (e.g., no color gradients are seen). These new data verify that M31 is a barred spiral galaxy like the Milky Way.