Evidence for an Outer Disk in the Prototype “Compact Elliptical” Galaxy M32

M32 is the prototype for the relatively rare class of galaxies referred to as`compact ellipticals'. It has been suggested that M32 may be a tidallydisturbed r^(1/4) elliptical galaxy, or the remnant bulge of a disk-strippedearly-type spiral galaxy. This paper reveals that the surface brightnessprofile, the velocity dispersion measurements, and the estimated supermassiveblack hole mass in M32 are inconsistent with the galaxy having, and probablyever having had, an r^(1/4) light profile. Instead, the radial surfacebrightness distribution of M32 resembles an almost perfect (bulge + exponentialdisk) profile, which is accompanied by a marked increase in the ellipticityprofile and an associated change in the position angle profile where the `disk'starts to dominate. Compelling evidence that this bulge/disk interpretation isaccurate comes from the best-fitting r^(1/n) bulge model which has a Sersicindex n=1.5, in agreement with the recently discovered relation between abulge's Sersic index and the mass of its supermassive black hole. An index n>4would also be inconsistent with the stellar velocity dispersion of M32. Thebulge-to-disk size ratio r_e /h equals 0.20, and the logarithm of thebulge-to-disk luminosity ratio log(B/D) equals 0.22, typical of lenticulargalaxies. The effective radius of the bulge is 27" (~100 pc), while thescale-length of the disk is less well determined: due to possible tidal-stripping of the outer profile beyond 220-250", the scale-length may be aslarge as 1.3 kpc. M32 is a relatively face-on, nucleated, dwarf galaxy with alow surface brightness disk and a high surface brightness bulge. This findingbrings into question the existence of the compact elliptical class of galaxies.Comment: Accepted for publication in ApJ Letter