This paper carried out an observing project to map the atomic gas in a set of unusually gas-rich nearby galaxies. The main results indicate that in disk galaxies with masses of the Milky Way or higher, gas accretion may occur in an orderly fashion, without recent major interactions.
The HI environment
This paper developed a new technique to investigate HI below the detection limit for individual sources in the interferometry data. We apply the technique to the Bluedisk data and find an excess of HI mass outside the detected galaxies in the 500 kpc environment around less massive and HI-rich central galaxies. The "less massive" here refers to the 10-10.5 dex Msun stellar mass range and "HI-rich" means having a high HI mass at a fixed stellar mass or star formation rate properties in the sample. The result is consistent with the picture of lambda-CDM predicted cold-mode gas accretion for galaxies.
HI size-mass relation
This paper builds a beautiful sample of HI interferometry data for over 500 nearby galaxies and revisits the HI size-mass relation of galaxies that has been known for nearly two decades. The innovation of the paper is that it shows that there is no change in slope and scatter of the relations between dwarf and massive galaxies. The authors pointed out that this is not easily understandable, as the two types of galaxies should have different atomic-to-molecular gas conversion efficiencies. The relation also has practical applications, including predicting HI disk size distribution based on HI mass from single dish HI surveys or HI mass of unresolved galaxies in low resolution interferometry surveys.
HI radial distribution
How is cold gas accreted in galaxies? This paper combined the efforts from observers and theorists, to investigate the radial distribution of atomic gas in unusually gas-rich nearby galaxies. They found a universal shape for the radial profiles of the gas in the outer regions of the observed galaxies, and obtained remarkable agreement with simulations. In half the galaxies, the atomic gas may have been accreted in the form of "rings".
HI and galaxy/star formation
The important role of HI as an intermediate step of fueling star formation
In galaxies, stars are constantly forming, where some galaxies show more active star formation than others. Disk-dominated galaxies such as our own Milky Way have now been studied by this paper to gain new insight into disk galaxy formation and evolution. The results highlight the important role of inner HI, the HI mass within the optical radius of disk galaxies, as an intermediate step of fueling star formation in disk galaxies. It is important to focus on the inner HI instead of all the HI, because HI is typically distributed in a disk that is 2-4 times larger than the stellar disk in star-forming galaxies. The paper also present a new method to estimate inner HI from integrated HI spectra, with an uncertainty of 0.09 dex.
HI and the inside-out disc formation
This paper shows that, at a given stellar mass, a higher HI mass is associated with the bulk of star formation shifting outward in radius. It support the picture of ”inside-out“ galaxy formation, where the active star forming regions gradually move towards the outer regions of the galaxy disk as gas with ever higher angular momentum is accreted.
The weak role of HI in immediate star formation
The lack of correlation between HI and SFR was well established by studies that investigate kpc-scale regions in the inner parts of galaxies, but correlations appeared to revive on the global scales and in the outskirts of galaxies. This paper makes use of the 82 LVHIS galaxies observed at ATCA, and coherently studies the SFR-HI relations in these different regions. They find that those apparent correlations are caused by both parameters dependent on a third parameter, and are intrinsically weak. Although HI is the raw material for forming stars, many factors (e.g. metallicity, magnetic fields, turbulence) and complex physical processes have affected the formation, and as a result the formed stars have lost most of the memories about the HI gas.