Searching for Old Stellar Populations in Candidate Proto-Spheroidals

Details Searching for Old Stellar Populations in Candidate Proto-Spheroidals Searching for Old Stellar Populations in Candidate Proto-Spheroidals

Dec 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7875b&link_type=abstract

HST Proposal ID #7875

Computer Science

Hst Proposal Id #7875 Galaxies

Scientific paper

We propose to combine NICMOS images with existing WFPC-2 images to search for underlying, old stellar components in 20 faint blue compact galaxies at z 0.2 to 0.8. These galaxies are excellent candidates for spheroidal galaxies {such as NGC 205} caught in the act of undergoing a major star-forming event: Keck spectra reveal velocity widths Sigma 50 km s^-1, WFPC2 images show extremely small sizes and high surface brightnesses {half-light radii of order 0.3 arcsec, or 1.5-5 h^-1_50 kpc}, and ground-based photometry yield very blue optical and near-infrared colors. Combined, these measurements suggest masses 1000 times less than typical L^* galaxies, yet these galaxies lie within one magnitude of L^*. Their colors and luminosities can be understood to result from a recent burst of star formation. Without additional prior or subsequent star formation, these galaxies will fade by several magnitudes after a few Gyrs. Are we witnessing, in-situ, the last major episode of star formation of today's low surface brightness spheroidals? The answer hangs critically on the amount of fading. Hence it is essential to determine the number of old stars lurking beneath the young population. We have compelling evidence from WFPC2 images that these galaxies have optical colors gradients. The near-infrared is necessary, however, to reveal color gradients large enough to separate young and old stellar populations. Only NICMOS imaging provides enough spatial resolution and depth to resolve these gradients.

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