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

The formation of elliptical galaxies via the merger of gas-rich disks has r eceived 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 sear ched for this central burst of star formation in a sample of field elliptic al galaxies that exhibit morphological fine structure thought to be indicat ive of merging. Out of this sample of 32 galaxies, we find only two galaxie s, NGC 3610 and NGC 5322, with sufficiently red central near-IR colors to b e consistent with the asymptotic giant branch light reflective of the centr al burst of star formation a few gigayears ago. Using NGC 3610 and 5322 as case studies, we discuss possible astrophysical links between global and ce ntral properties and their implied constraints on elliptical galaxy formati on and evolution. In particular, we conclude that the available evidence ar gues against mergers of disk galaxies within the last 3-4 Gyr as being the primary formation mechanism for morphologically disturbed field elliptical galaxies.