The ages of disturbed field elliptical galaxies. II. Central properties

The formation of elliptical galaxies via the merger of gas-rich disks has received considerable attention in recent years, with many studies strongly supporting the merger hypothesis. When investigated in detail, the dynamics of a major merger that produces a high phase-space density of material (e.g., the center of an elliptical galaxy) invariably produces the ubiquitous signature of a centrally concentrated burst of star formation. We have searched for this central burst of star formation in a sample of field elliptical galaxies that exhibit morphological fine structure thought to be indicative of merging. Out of this sample of 32 galaxies, we find only two galaxies, NGC 3610 and NGC 5322, with sufficiently red central near-IR colors to be consistent with the asymptotic giant branch light reflective of the central burst of star formation a few gigayears ago. Using NGC 3610 and 5322 as case studies, we discuss possible astrophysical links between global and central properties and their implied constraints on elliptical galaxy formation and evolution. In particular, we conclude that the available evidence argues against mergers of disk galaxies within the last 3-4 Gyr as being the primary formation mechanism for morphologically disturbed field elliptical galaxies.