Wide Field Planetary Camera 2 Observations of the Polar Ring Galaxy NGC 4650A

Abstract

We present multicolor Wide Field Planetary Camera 2 images of the polar ring galaxy NGC 4650A (D = 35 Mpc), taken with the Hubble Space Telescope. These data allow a detailed examination of the morphology of the central S0 component and the surrounding polar ring of stars, gas, and dust, which are spatially extended and form a polar disk. The nuclear complex, located near the common center of the two components, consists of an unresolved compact source less than 20 pc across, embedded in a more extended light distribution about 60 pc in radius. A complicated pattern of dust lanes crosses the high surface brightness central region of the S0, where dust features appear to extend inward to small radii. In areas of the S0 exterior to the projected polar disk, the light distribution is relatively smooth on small spatial scales, with few candidates for old star clusters or signs of internal dust features. Outer regions of the S0 galaxy have uniformly red colors of a stellar population where major star formation ceased ~3-5 Gyr in the past. The polar disk appears to be warped, with a complex morphology. On small spatial scales, features are produced mainly by dust and clumps of star formation; gas seems to be piling up in a dusty ring that encircles the central S0 at a radius of ~1 kpc (6''). The polar disk is inclined by about 63° near its center, twists toward edge-on at intermediate radii, and again becomes more nearly face-on further out, where two spiral arms are marked by young stars. We do not detect any regions with a prominent Baade sheet of older red stars in the polar disk; most of the optical light is supplied by moderately young stars, with lifetimes of ~1 Gyr. The polar material appears relatively unevolved, especially when compared to the S0 component of NGC 4650A. Either the polar disk is a late addition to the system or the outer gas has formed very few stars until recently.