An IUE Search for Star Formation in Ellipticals with Gaseous Disks

Abstract

New IUE observations are presented of NGC 5077, an elliptical galaxy known to have a Hα bright gaseous disk in its central region. The ultraviolet spectral energy distribution of NGC 5077 is compared to those of the bulges of two galaxies, M31 and NGC 3115, that are of similar Mg_2_ absorption-line strength and are known not to contain obvious gaseous disks. No significant differences are seen between the UV spectrum of NGC 5077 and those of M31 and NGC 3115, leading us to conclude that essentially no recent star formation (< 3 Gyr old) is observed coming from the gaseous disk of NGC 5077. Further analysis of the full sample of 34 early-type galaxies studied with IUE shows that the presence of recent star formation is not simply predicted by recent merger activity, as evidenced either by the presence of a gaseous disk or the degree to which the galaxy has optical structural peculiarities. On the other hand, those galaxies known to be "active," i.e., with strong emission lines, have energy distributions within the IUE wavelength range (1250-3200 A) that suggest recent star formation. Apparently the relationship between the kinds of stellar populations in ellipticals and evidence of recent merger activity is more complex than a simple merger scenario would predict.