JW

CGM cooling and diffuse HI formation induced by tidal interaction

In this paper, making use of HI data from FEASTS, we discover a large-scale, low-density, and diffuse HI in the group NGC 4631 which was missed in previous deepest type of interferometric HI observation. This diffuse HI may represent a condensing phase of the hot and ionized IGM. Active tidal interactions on-going and in the past may have produced the wide-spreading HI distribution, and triggered the gas accretion to NGC 4631 through the phase of the diffuse HI.

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.

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 develops a new method, based on the statistical properties of HI distribution, to estimate inner HI from integrated HI spectra, with an uncertainty of 0.09 dex.

Scatter of the gas-phase metallicity-stellar mass relation

Using the method outlined in this paper, we derive the inner HI mass for the MaNGA-HI sample. In this paper, we find that the inner HI mass is strongly correlated with the scatter of the gas-phase metallicity-stellar mass relation derived at the effective radius of galaxies. The relation is much tighter than if replacing the inner HI mass with the integral HI mass. It indicates that the inflow of HI gas dilutes the metal which is mainly produced in the stellar disks.

HI disk thickness

In this paper, we find that the thickness of HI disks depends much more strongly on the ratio of inner HI mass over the stellar mass than the ratio of the total HI mass. Theoretical studies predict that, the HI disk thickness is supported by turbulent pressure, while the turbulence is produced by energetic processes including stellar feedbacks and gas inflow. Thus it is reasonable that the inner HI mass fraction correlates better than the total HI with the HI disk thickness.

Not directly forming stars

This paper coherently studies the SFR-HI relations globally, in the inner disks, and on the disk outskirts. They find that those apparent SFR-HI correlations are caused by both parameters dependent on the stellar mass surface density, 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.

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.

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 morphologis

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.