Violent Star Formation in Merger Remnants

Summary

In mergers of gas-rich spirals powerful starbursts are triggered, in the course of which a secondary population of globular clusters (GCs) may be formed. We present results from our chemical and spectrophotometric evolutionary models and show that even in the case of an old merger remnant like NGC 7252 the star formation (SF) history can be determined quite exactly, if only enough observational data are available. About a Gyr ago, NGC 7252 went through a starburst that, over (1–5)10⁸ yr, increased its stellar mass by 20–50% and created a number of new GCs detected with HST. Young GCs may serve as a tracer for star formation efficiency (SFE). Our models predict metallicities for a secondary population of GCs which should allow to identify Sp-Sp merger remnants among ellipticals. In the case of NGC 7252, follow-up spectroscopy of the two brightest young GCs confirmed our metallicity prediction. We show that once the metallicity is known, very exact age dating of these GCs becomes possible.

Introduction

A most violent mode of SF is observed in mergers of massive gas-rich spirals. These starbursts have been considered as the near-by analogues of violent SF during the initial collapse at galaxy formation. Luminosities of 10¹²−10¹⁴L_⊙ are observed, predominantly emitted in the IR. All the IR-UL galaxies and many of the luminous IRAS galaxies have by now been shown to be in an advanced stage of merging.