Keck DEIMOS Spectroscopy of a GALEX UV-Selected Sample from the Medium Imaging Survey

Creators: Mallery, Ryan P.; Rich, R. Michael; Salim, Samir; Small, Todd; Charlot, Stéphane; Seibert, Mark; Wyder, Ted; Barlow, Tom A.; Forster, Karl; Friedman, Peter G.; Martin, D. Christopher; Morrissey, Patrick; Neff, Susan G.; Schiminovich, David; Bianchi, Luciana; Donas, José; Heckman, Timothy M.; Lee, Young-Wook; Madore, Barry F.; Milliard, Bruno; Szalay, Alex S.; Welsh, Barry Y.; Yi, Sukyoung

Abstract

We report results from a pilot program to obtain spectroscopy for objects detected in the Galaxy Evolution Explorer (GALEX) Medium Imaging Survey (MIS). Our study examines the properties of galaxies detected by GALEX fainter than the Sloan Digital Sky Survey (SDSS) spectroscopic survey. This is the first study to extend the techniques of Salim and coworkers to estimate stellar masses, star formation rates (SFRs), and the b (star formation history) parameter for star-forming galaxies out to z ~ 0.7. We obtain redshifts for 50 GALEX MIS sources reaching NUV = 23.9 (AB mag) having counterparts in the SDSS Data Release 4 (DR4). Of our sample, 43 are star-forming galaxies with z < 0.7, 3 have emission-line ratios indicative of active galactic nuclei with z < 0.7, and 4 objects with z > 1 are QSOs, 3 of which are not previously cataloged. We compare our sample to a much larger sample of ~50,000 matched GALEX/SDSS galaxies with SDSS spectroscopy; while our survey is shallow, the optical counterparts to our sources reach ~3 mag fainter in SDSS r than the SDSS spectroscopic sample. We use emission-line diagnostics for the galaxies to determine that the sample contains mostly star-forming galaxies. The galaxies in the sample populate the blue sequence in the NUV − r versus Mr color-magnitude diagram. The derived stellar masses of the galaxies range from 10^8 to 10^(11) M_⊙, and derived SFRs are between 10^(−1) and 10^2 M_⊙ yr^(−1). Our sample has SFRs, luminosities, and velocity dispersions that are similar to the samples of faint compact blue galaxies studied previously in the same redshift range by Koo and collaborators, Guzmán and collaborators, and Phillips and collaborators. However, our sample is ~2 mag fainter in surface brightness than the compact blue galaxies. We find that the star formation histories for a majority of the galaxies are consistent with a recent starburst within the last 100 Myr.

Additional Information

© 2007 The American Astronomical Society. Received 2006 January 25; accepted 2006 July 25. GALEX is a NASA Small Explorer, launched in 2003 April. We gratefully acknowledge NASA's support for construction, operation, and science analysis for the GALEX mission, developed in cooperation with the Centre National d'Etudes Spatiales of France and the Korean Ministry of Science and Technology. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. The analysis pipeline used to reduce the DEIMOS data was developed at University of California, Berkeley, with support from NSF grant AST 00-71048. The authors thank an anonymous referee for extremely helpful comments. Facilities: GALEX, Keck:II.

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