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 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), and our control galaxy, the
typical "red and dead" NGC 3379. The current star formation in our closest
targets, where we are most complete, is between 2-8 X 10^-5 Msun/yr. The star
formation history was roughly constant from 0.5-1.5 Gyr (at 3-5 X 10^-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 (at the present day) or ~10^-14/yr (when averaging over
the past Gyr) would require timescales 4-6 orders of magnitude longer than the
age of the Universe to build up the stellar mass of the galaxies, quantifying
for the first time the level of quenching they have experienced relative to
their average value. There is no obvious correlation between either the
presence or spatial distribution of postulated star formation indicators and
the star formation we detect.
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