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 Ks-band-selected sample of 88 close major-merger pairs of galaxies (H-KPAIR) based on 2-D decomposition of SDSS r-band images with \textsc{galfit}. We investigate the dependence of the interaction-induced specific star formation rate enhancement ($\mathrm{sSF}{\mathrm{R}}_{\mathrm{enh}}$) 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 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 $B/T=0.35\pm 0.05$ and $B/T=0.26\pm 0.03$, respectively. There is a strong negative dependence of $\mathrm{sSF}{\mathrm{R}}_{\mathrm{enh}}$ on the B/T ratio and only paired SFGs with $B/T<0.3$ show significant ($>5\sigma$) enhancement. Paired SFGs in S+S pairs show a similar trend, and many disky SFGs ($B/T<0.1$) in S+S have strong sSFR enhancements ($\mathrm{sSF}{\mathrm{R}}_{\mathrm{enh}}>0.7$~dex). For SFGs in S+E, the sSFR has no clear B/T dependence, nor any significant enhancement in any B/T bin. Disky SFGs in S+S show significant ($>4\sigma$) enhancement in the molecular gas content (${\mathrm{M}}_{{\mathrm{H}}_2}/{\mathrm{M}}_{\mathrm{star}}$), while SFGs in S+E have no such enhancement in any B/T bin. No significant enhancement on total gas content (${\mathrm{M}}_{\mathrm{gas}}/{\mathrm{M}}_{\mathrm{star}}$) is found in any B/T bin for paired galaxies. The star formation efficiency of either the total gas ($\mathrm{SF}{\mathrm{E}}_{\mathrm{gas}}=SFR/{\mathrm{M}}_{\mathrm{gas}}$) or the molecular gas ($\mathrm{SF}{\mathrm{E}}_{{\mathrm{H}}_2}=SFR/{\mathrm{M}}_{{\mathrm{H}}_2}$) does not depend on the B/T ratio. The only significant ($>4\sigma$) SFE enhancement found for paired SFGs is the $\mathrm{SF}{\mathrm{E}}_{\mathrm{gas}}$ for disky SFGs in S+S pairs.

arXiv commentAccepted for publication in ApJS, 18 pages, 13 figures, 1 table

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