The Host Galaxy of FRB 20190520B and Its Unique Ionized Gas Distribution

The properties of host galaxies associated with Fast Radio Bursts (FRBs) provide critical information for inferring the progenitors and radiation mechanisms of these bursts. We report on the host galaxy of the repeating FRB 20190520B, a dwarf galaxy at the spectroscopic redshift z = 0.241 with a stellar mass of (6.2 ± 0.8) × 108 MJ . The emission line ratios indicate that the ionized gas in the host galaxy is powered by star formation. The total Hα-traced star formation rate (SFR) is 0.7 ± 0.1 MJ yr−1 and the metallicity is 12+log10([O/H]) ≥ 7.4±0.4. This implies an overall high specific star formation rate (sSFR) of log sSFR/yr−1 = −9.0 ± 0.1 for the whole galaxy, compared with the upper limit of log sSFR/yr−1 ≃ −9.4 observed in nearby dwarf galaxies. However, the FRB is associated with the peak of the Hα emission, which is offset from the main stellar continuum by ∼ 1.4′′ (∼ 5.5 kpc) in projection. At this location, the estimated lower limit of log sSFR/yr−1 is −8.5 ± 0.1 within the seeing-limited 1.1′′ diameter area, or more than three times the total sSFR of the entire galaxy. The Hα velocity difference between the stellar continuum center and the offset ionized gas blob is 39.6±0.4 kms−1, insignificant for us to draw the conclusion of the nature of the offset. The contribution of the inter stellar medium of the host galaxy to the dispersion measurement of the FRB is estimated to be DMhost ≈ 960 ± 220 pc cm−3 using the emission measure of Hα. The ionized gas distribution in the FRB 20190520B host galaxy is spatially associated with the locations of both the FRB and the persistent radio source, both of which are offset toward the north of the host galaxy.