STAR FORMATION IN PARTIALLY GAS-DEPLETED SPIRAL GALAXIES

摘要

Broadband B and R and Ha images have been obtained with the 4.1 m SOAR telescope atop Cerro Pachon, Chile,for 29 spiral galaxies in the Pegasus I galaxy cluster and for 18 spirals in non-cluster environments. Pegasus I is aspiral-rich cluster with a low-density intracluster medium and a low galaxy velocity dispersion. When combinedwith neutral hydrogen (H I) data obtained with the Arecibo 305 m radio telescope, acquired by Levy et al. (2007)and by Springob et al. (2005b), we study the star formation rates in disk galaxies as a function of their Hdeficiency. To quantify H I deficiency, we use the usual logarithmic deficiency parameter, DEF. The specific starformation rate (SSFR) is quantified by the logarithmic flux ratio of Ha flux to R-band flux, and thus roughlycharacterizes the logarithmic SFR per unit stellar mass. We find a clear correlation between the global SFR perunit stellar mass and DEF, such that the SFR is lower in more H 1-deficient galaxies. This correlation appears toextend from the most gas-rich to the most gas-poor galaxies. We also find a correlation between the central SFRper unit mass relative to the global values, in the sense that the more H 1-deficient galaxies have a higher centralSFR per unit mass relative to their global SFR values than do gas-rich galaxies. In fact, approximately half of theH I-depleted galaxies have highly elevated SSFRs in their central regions, indicative of a transient evolutionarystate. In addition, we find a correlation between gas depletion and the size of the Ha disk (relative to the R-banddisk); HI-poor galaxies have truncated disks. Moreover, aside from the elevated central SSFR in many gas-poorspirals, the SSFR is otherwise lower in the Ha disks of gas-poor galaxies than in gas-rich spirals. Thus, both disktruncation and lowered SSFR levels within the star-forming part of the disks (aside from the enhanced nuclearSSFR) correlate with H i deficiency, and both phenomena are found to contribute equally to the global suppression ofstar formation. We compare our results found in the low-richness Pegasus I cluster and in non-cluster environmentswith SFRs found for H i-deficient spirals in the Virgo cluster by Koopmann Kenney and Fumagalli Gavazzi.