Direct Detections of Young Stars in Nearby Elliptical Galaxies
  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.
