Hubble Space Telescope Observations of Candidate Young Globular Clusters and Stellar Associations in the Recent Merger Remnant NGC 3921

Abstract

New, high-resolution images of NGC 3921 obtained with the Wide Field and Planetary Camera 2 of the refurbished HST are presented. NGC 3921 is the remnant of two merged disk galaxies and thought to be a 0.7±0.3 Gyr old protoelliptical. A search for star clusters has yielded 102 candidate globular clusters (GC) and 49 separate, more extended stellar associations (A) brighter than 7=26.0, the estimated completeness limit (corresponding to M v = — 8.5 for # 0 = 75 km s -1 Mpc -1). Most of these objects are newly discovered. Among the best-observed objects [7^25, error cr(7—/)=^0.15], the majority are blue and have remarkably uniform colors, with median (7—/) Cousins =0.65 for GCs and 0.56 for As. The color indices of the GCs yield estimates of 250-750 Myr for the median age, depending on the adopted metallicity (1.0-0.2 Z 0), and suggest a prolonged formation period (several 100 Myr) roughly in agreement with the estimated duration of the merger. The most luminous candidate GCs have absolute magnitudes M v = — 12 to -13 and are predicted to fade by 3-4 mag over the next 15 Gyr, whence they will then have luminosities typical of luminous old GCs. The median half-light radius of candidate GCs is Reff—S pc, compared with a median i?eff = 3 pc for Milky Way globulars. Since from their colors most of these clusters appear to be dozens of core crossing times old, they must be gravitationally bound. The luminosity functions of both the GCs and the As are well represented by a single power law, (f){L)dL^L~ 2A±03 dL (for —8.5). Various arguments suggest that the progenitors of these objects may have been normal Giant Molecular Clouds rather than Supergiant Molecular Clouds. The projected radial distribution of GCs is strongly centrally concentrated and follows closely the 7-light profile of the galaxy (~r 1/4 law), suggesting that these clusters or their progenitors experienced the same violent relaxation as did the average luminous matter of the two merging galaxies. In contrast, the radial distribution of As appears relatively flat. We estimate that the total number of GCs in NGC 3921 has increased by ^40% during the merger, from about 270±60 GCs before to ^380 GCs after the merger. The inferred specific GC frequency is 5^^ 0.7 at present and will, barring further mergers or significant cluster destruction, increase to S w 2= 1.4, 1.8, and 2.9 over the next 2, 5, and 15 Gyr as the galaxy fades. Hence, within ^7 Gyr NGC 3921 may begin to appear as a normal elliptical with a GC population typical for Es in low-density environments.