H. Alyson Ford, Joel N. Bregman
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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http://arxiv.org/abs/1205.1066
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