Keck Spectroscopy of Candidate Proto-globular Clusters in NGC 1275

Abstract

Keck spectroscopy of 5 proto-globular cluster candidates in NGC 1275 has been combined with HST WFPC2 photometry to explore the nature and origin of these objects and discriminate between merger and cooling flow scenarios for globular cluster formation. The objects we have studied are not HII regions, but rather star clusters, yet their integrated spectral properties do not resemble young or intermediate age Magellanic Cloud clusters or Milky Way open clusters. The clusters' Balmer absorption appears to be too strong to be consistent with any of the standard Bruzual & Charlot evolutionary models at any metallicity. If these models are adopted, an IMF which is skewed to high masses provides a better fit to the data. A truncated IMF with a mass range of 2-3 Mo reproduces the observed Balmer equivalent widths and colors at about 450 Myr. Formation in a continuous cooling flow appears to be ruled out since the age of the clusters is much larger than the cooling time, the spatial scale of the clusters is much smaller than the cooling flow radius, and the deduced star formation rate in the cooling flow favors a steep rather than a flat IMF. A merger would have to produce clusters only in the central few kpc, presumably from gas in the merging galaxies which was channeled rapidly to the center. Widespread shocks in merging galaxies cannot have produced these clusters. If these objects are confirmed to have a relatively flat, or truncated, IMF it is unclear whether or not they will evolve into objects we would regard as bona fide globular clusters.Comment: 30 pages (AAS two column style, including 9 tables and 7 figures) to appear in the AJ (August issue), also available at http://www.ucolick.org/~mkissler/Sages/sages.html (with a full resolution Fig.1) Revised Version: previous posted version was an uncorrect ealier iteration, parts of the text, tables and figures changed. The overall conclusions remain unchange