Asymmetries in spatially unresolved 21-cm emission line profiles of isolated galaxies
The origin of asymmetry in the ${\rm H}\,{\scriptsize{\rm I}}$ of galaxies remains an elusive problem, largely due to the difficulties associated with distinguishing between its secular and external contributors. We have compiled a sample of local isolated galaxies from the UNAM-KIAS and the latest AMIGA samples with near-complete beam coverage and robust estimates of ${\rm H}\,{\scriptsize{\rm I}}$ line asymmetry. The ${\rm H}\,{\scriptsize{\rm I}}$ measurements are based on single-dish spectra sourced from five surveys: ${\rm H}\,{\scriptsize{\rm I}}$-MaNGA, NIBLES, KLUN, ALFALFA, and xGASS. Our galaxies tend to be late-type, star-forming centrals at all masses but exhibit slightly lower specific star formation rates (sSFRs) and higher ${\rm H}\,{\scriptsize{\rm I}}$ contents than expected. The latter is likely driven by lower star formation efficiency, weaker outflows, and additionally, higher accretion rate of gas onto the galaxy at $M_\star\lesssim 10^{10.3}\,\mathrm{M}_\odot$. AMIGA, however, shows systematically higher ${\rm H}\,{\scriptsize{\rm I}}$ masses than UNAM-KIAS, which we attribute to higher local densities probed by the latter. Furthermore, both samples show bar frequencies similar to normal spirals, indicating that the gravitational instabilities leading to bars predominantly stem from internal processes, as suggested by recent works. Our galaxies show unexpectedly high merger fractions, possibly due to sampling bias and/or the inability of the classification method to distinguish between flybys and encounters leading to coalescence. We also find higher sSFRs for asymmetric galaxies below $M_\star\sim 10^{10.3}\,\mathrm{M}_\odot$, in agreement with the predictions for centrals from the ${\scriptsize{\rm EAGLE}}$ simulation. We release the ${\rm H}\,{\scriptsize{\rm I}}$ measurements along with ancillary galaxy and halo properties for public use.