Direct Detections of Young Stars in Nearby Elliptical Galaxies

Abstract

Small amounts of star formation in elliptical galaxies are suggested by several results: surprisingly young ages from optical line indices, cooling X-ray gas, and mid-infrared dust emission. Such star formation has previously been difficult to directly detect, but using ultraviolet Hubble Space Telescope Wide Field Camera 3 imaging, we have identified individual young stars and star clusters in four nearby ellipticals. Ongoing star formation is detected in all galaxies, including three ellipticals that have previously exhibited potential signposts of star-forming conditions (NGC 4636, NGC 4697, and NGC 4374), as well as the typical "red and dead" NGC 3379. The current star formation in our closest targets, where we are most complete, is between 2.0 and 9.8 × 10^-5 M _⊙ yr^-1. The star formation history was roughly constant from 0.5 to 1.5 Gyr (at (3-5) × 10^-4 M _⊙ yr^-1), but decreased by a factor of several in the past 0.3 Gyr. Most star clusters have a mass between 10² and 10⁴ M _⊙. The specific star formation rates of ~10^-16 yr^-1 (at the present day) or ~10^-14 yr^-1 (when averaging over the past Gyr) imply that a fraction 10^-8 of the stellar mass is younger than 100 Myr and 10^-5 is younger than 1 Gyr, quantifying the level of frosting of recent star formation over the otherwise passive stellar population. There is no obvious correlation between either the presence or spatial distribution of postulated star formation indicators and the star formation we detect.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program 11583.