Interaction-induced HI gas concentration with centrally-enhanced star formation in ALFALFA-SDSS galaxies

Abstract

We present a statistical analysis for the interaction-induced central concentration of HI gas distributions and its connection with interaction-induced central star formation enhancement, using a large sample of $\sim 10^4$ galaxies from the ALFALFA and SDSS surveys. By adopting the HI profile parameter $K$, an indicator of gas concentration inferred from the integrated 21 cm emission line, we find that galaxies with more centrally concentrated HI (higher $K$ values) or enhanced specific star foramtion rate (sSFR) exhibit significantly stronger clustering and higher probability of hosting a nearby neighbor on scales below $100h^{-1}\mathrm{kpc}$, which is more pronounced in low-mass galaxies. Furthermore, by utilizing the enhancement functions for a sample of galaxy pairs, we directly trace the evolution of HI concentration and sSFR enhancement as a function of projected separation. Our findings indicate that tidal interactions drive a statistical synchrony between the central concentration of atomic gas and the enhancement of central star formation. Gas concentration appears to be a necessary condition for central star formation enhancement in interacting systems at all but the smallest separations. Compared to satellite galaxies, central galaxies exhibit stronger enhancement of gas fraction, gas concentration and sSFR, suggesting the role of environmental regulation.

Figures (9)

• Figure 1: The distribution of ALFALFA galaxies in the plane of stellar mass versus SFR (upper panel) and sSFR (lower panel). The left panels show the scatter plot, where galaxies are classified into star-forming (blue) and quenched (red) populations based on the dividing threshold of $\log_{10} \mathrm{sSFR} = -11.0 ~\mathrm{yr^{-1}}$. The black contours indicate the number density distribution of SDSS galaxies as the background. The corresponding right panels show the same distributions, with each pixel color coded by the median value of $K$.
• Figure 2: Projected cross-correlation functions (left panel) and close neighbor counts (right panel) for the two $K$-divided subsamples (shown by symbols), compared with the corresponding results for the $\mathrm{sSFR}$-divided subsample, as indicated.
• Figure 3: Same as \ref{['fig:2ppcf_sf_k_allmass']}, for different mass intervals of star-forming sample, as indicated.
• Figure 4: The distribution of ALFALFA galaxies in the $K$–sSFR plane with the contours representing the number density distribution of galaxies. Each pixel is color coded with the median value of stellar mass. The dotted line shows the sSFR threshold used to separate star-forming and quenched galaxies.
• Figure 5: Left: Enhancement of $K$ (black solid line, normalized by the corresponding enhancement values of $r_p=15h^{-1}\text{kpc}$, referenced to the left y-axis) and sSFR (dotted gray line, referenced to the right y-axis) for the close galaxy pairs as a function of pair separations. Middle: The fraction of galaxies with enhanced sSFR as a function of pair separations, for the entire pair sample (black dotted lines), as well as for subsamples with gas concentration ($\Delta K>0$, blue dotted lines) and relatively diffuse gas ($\Delta K<0$, red dotted lines). Right: Similar as the middle panel, but for the fraction of galaxies with enhanced $K$ and the condition of $\Delta \log_{10}\mathrm{sSFR}$, as indicated.