Star Formation in Interacting and Normal Galaxies over the last 7 Gigayears

Astronomy and Astrophysics – Astronomy

Scientific paper

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Scientific paper

We present a study of the star formation properties of strongly disturbed interacting/merging systems and normal undisturbed (E/S0, Sa-Sd, Irr) galaxies over z 0.24 to 0.80 using Hubble Space Telescope ACS images, Combo-17 redshifts, and Spitzer 24 micron data of 4500 galaxies in the GEMS survey. For high mass (M/Msun >= 2.5 x 10^10) galaxies, the observed fraction of strongly disturbed galaxies varies from 9% to 12% over z 0.24 to 0.80. The corresponding interaction rate varies from a few times 10^-4 to 10^-3 galaxies per Gyr per Mpc^3. The UV-based star formation rate (SFR) typically ranges from 0.01 to 25 Msun yr-1, but the ratio of IR-based to UV-based SFR has a median value of 4, indicating a substantial amount of obscured star formation. The average SFR of strongly disturbed systems is comparable to, or only moderately enhanced with respect to that of normal galaxies. This suggests that extreme enhancements in SFRs during mergers and interactions may be relatively rare. For intermediate mass systems, strongly disturbed galaxies only contribute a small fraction (typically below 20 %) of the total SFR density, compared to normal undisturbed galaxies. This suggests that the variation of the cosmic SFR density over z 0.24 to 0.80 is primarily influenced by the shutdown of star formation in normal undisturbed galaxies, rather than by the SF activity in interacting systems. We acknowledge support from NASA LTSA grant NAG5-13063, NSF grant AST-0607748, and HST grants G0-9500, GO-10395 and GO-10861.

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