Central UV Spikes in Two Galactic Spheroids

Summary

FOS spectra and FOC photometry of two centrally located, UV-bright spikes in the elliptical galaxy NGC 4552 and the bulge-dominated early spiral NGC 2681, are presented. These spectra reveal that such point-like UV sources detected by means of HST within a relatively large fraction (∼ 15%) of spheroids can be related to radically different phenomena. While the UV unresolved emission in NGC 4552 represents a transient event likely induced by an accretion event onto a supermassive black hole, the spike seen at the center of NGC 2681 is not variable and it is stellar in nature.

Introduction

HST UV images of nearby galaxies presented by Maoz et al. (1996) and Barth et al. (1998), as well as analogous space-borne optical images of early-type galaxies discussed by Lauer et al. (1995) and Carollo et al. (1997) have shown that about 15% of imaged galaxies show evidence of unresolved central spikes.

In the following we discuss two ‘prototype’ galactic spheroids, NGC 2681 and NGC 4552, that we properly monitored with HST–which host UV-bright, unresolved spikes at their center. While the early-spiral (Sa) galaxy NGC 2681 shows a nonvariable unresolved cusp, the UV spike which became visible at the center of the Virgo Elliptical NGC 4552 is a UV flare caught in mid-action, presumably related to a transient accretion event onto a central supermassive black hole (Renzini et al. 1995; Cappellari et al. 1998).