HIghMass -- High HI Mass, HI-rich Galaxies at z~0: Sample Definition, Optical and Halpha Imaging, and Star Formation Properties

TL;DR

This study characterizes HI-rich, massive galaxies at z~0, analyzing their star formation, gas properties, and disk structure to understand their evolution and the role of gas accretion and halo spin.

Contribution

It provides a detailed analysis of a new sample of HI-rich galaxies, linking their high gas fractions to star formation activity, disk growth, and dark matter halo properties.

Findings

HIghMass galaxies have active star formation with moderate efficiency.

They exhibit lower past SFRs and inside-out disk growth.

Downbending disks suggest gas distribution thresholds influence star formation.

Abstract

We present first results of the study of a set of exceptional HI sources identified in the 40% ALFALFA extragalactic HI survey catalog alpha.40 as being both HI massive (M_HI > 10^10 Msun) and having high gas fractions for their stellar masses: the HIghMass galaxy sample. We analyze UV- and optical-broadband and Halpha images to understand the nature of their relatively underluminous disks in optical and to test whether their high gas fractions can be tracked to higher dark matter halo spin parameters or late gas accretion. Estimates of their star formation rates (SFRs) based on SED-fitting agree within uncertainties with the Halpha luminosity inferred SFRs. The HII region luminosity functions have standard slopes at the luminous end. The global SFRs demonstrate that the HIghMass galaxies exhibit active ongoing star formation (SF) with moderate SF efficiency, but relative to normal spirals, a lower integrated SFR in the past. Because the SF activity in these systems is spread throughout their extended disks, they have overall lower SFR surface densities and lower surface brightness in the optical bands. Relative to normal disk galaxies, the majority of HIghMass galaxies have higher Halpha equivalent widths and are bluer in their outer disks, implying an inside-out disk growth scenario. Downbending double exponential disks are more frequent than upbending disks among the gas-rich galaxies, suggesting that SF thresholds exist in the downbending disks, probably as a result of concentrated gas distribution.