Close Major-merger Pairs at z = 0: Bulge-to-total Ratio and Star Formation Enhancement

Abstract

We present a study of the bulge-to-total ratio (B/T) of a K s-band-selected sample of 88 close major-merger pairs of galaxies (H-KPAIR), based on two-dimensional decompositions of SDSS r -band images with galfit . We investigate the dependence of the interaction-induced specific star formation rate enhancement (sSFR enh ) on the B/T ratio, and the effects of this dependence on the differences between star-forming galaxies (SFGs) in spiral+spiral (S+S) and spiral+elliptical (S+E) pairs. Of all the 132 spiral galaxies in H-KPAIR, the 44 in S+E pairs show higher B/T than those in the 44 S+S pairs, with means of B/T = 0.35 ± 0.05 and B/T = 0.26 ± 0.03, respectively. There is a strong negative dependence of sSFR enh on the B/T ratio, and only paired SFGs with B/T < 0.3 show significant (>5 σ ) enhancement. Paired SFGs in S+S pairs show a similar trend, and many disky SFGs (B/T < 0.1) in S+S pairs have strong sSFR enhancements (sSFR enh > 0.7 dex). For SFGs in S+E pairs, the sSFR has no clear B/T dependence, nor any significant enhancement in any B/T bin. Disky SFGs in S+S pairs show significant (>4 σ ) enhancement in molecular gas content ( M H 2 / M star ), while SFGs in S+E pairs have no such enhancement in any B/T bin. No significant enhancement of the total gas content ( M gas / M star ) is found in any B/T bin for paired galaxies. The star formation efficiency of either the total gas (SFE gas = SFR/ M gas ) or molecular gas ( SFE H 2 = SFR / M H 2 ) does not depend on the B/T ratio. The only significant (> 4 σ ) SFE enhancement found for paired SFGs is the SFE gas for disky SFGs in S+S pairs.