Direct Detections of Young Stars in Nearby Elliptical Galaxies

TL;DR

Using sensitive UV imaging from HST, this study directly detects ongoing star formation in nearby elliptical galaxies, revealing that these galaxies are not entirely passive but experience low levels of recent star formation.

Contribution

First direct detection of individual young stars and clusters in elliptical galaxies using UV imaging, demonstrating ongoing star formation at levels previously undetectable.

Findings

Star formation rates are between 1-8x10^(-5) Msun/yr in the closest galaxies.

Star formation history was roughly constant over 0.5-1.5 Gyr and declined in the last 0.3 Gyr.

Most star clusters have masses between 10^2 and 10^4 Msun.

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-IR dust emission. Such star formation has previously been difficult to directly detect, but using UV Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) imaging, we have identified individual young stars and star clusters in four nearby ellipticals. This technique is orders of magnitude more sensitive than other methods, allowing detections of star formation to 10^(-5) Msun/yr. 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 1-8x10^(-5) Msun/yr. The star formation history was roughly constant from 0.5-1.5 Gyr (at 3-5x10^(-4) Msun/yr), but decreased by a factor of several in the past 0.3 Gyr. Most star clusters have a mass between 10^2 - 10^4 Msun. 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.