Asymmetric HI 21 cm lines of fast radio burst hosts: connection with
  galaxy interaction
  Fast radio bursts (FRBs) are enigmatic transients with very short radio
emission. Their nature is still widely debated. I provide the first analysis of
atomic gas properties of a small sample of FRB hosts to constrain their nature.
HI observations exist for NGC 3252, the host of FRB 181030A, M81, the host of
FRB 200120E, and the Milky Way, the host of FRB 200428. I report three
observables: i) all three FRB hosts are interacting galaxies; ii) the Hi
spectra of both FRB hosts with such data available are highly asymmetric,
several standard deviations above the general population of galaxies; iii) two
FRB hosts have normal atomic gas properties and one is strongly deficient in
atomic gas. This indicates that nearby and repeating FRBs are connected with a
recent enhancement of star formation due to interaction. This supports fast FRB
channels, for example a massive star with a short delay time so that
interaction signatures giving rise to the birth of the progenitor are still
visible. Long gamma-ray burst (GRB) and broad-lined type Ic supernova (SN)
hosts exhibit much more symmetric spectra, even though they were claimed to
experience gas inflow from the intergalactic medium. The difference can be
explained by the interactions experienced by FRB hosts being more disruptive
than these gas inflows, or by the mass effect, with GRB/SN hosts at lower
masses having less organized gas motions, so with HI lines closer to a
symmetrical Gaussian. This also suggests that the emission mechanisms of FRBs
and GRBs are different.
