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Identification and follow-up observations of the host galaxies of fast radio bursts (FRBs) not only help us understand the environments in which the FRB progenitors reside, but also provide a unique way of probing the cosmological parameters using the dispersion measures (DMs) of FRBs and distances to their origin. A fundamental requirement is an accurate distance measurement to the FRB host galaxy, but for some sources viewed through the Galactic plane, optical/near-infrared spectroscopic redshifts are extremely difficult to obtain due to dust extinction. Here we report the first radio-based spectroscopic redshift measurement for an FRB host galaxy, through detection of its neutral hydrogen (H i ) 21 cm emission using MeerKAT observations. We obtain an H i –based redshift of z = 0.0357 ± 0.0001 for the host galaxy of FRB 20230718A, an apparently nonrepeating FRB detected in the Commensal Real-time ASKAP Fast Transients survey and localized at a Galactic latitude of –0.°367. Our observations also reveal that the FRB host galaxy is interacting with a nearby companion, which is evident from the detection of an H i bridge connecting the two galaxies. A subsequent optical spectroscopic observation confirmed an FRB host galaxy redshift of 0.0359 ± 0.0004. This result demonstrates the value of H i to obtain redshifts of FRBs at low Galactic latitudes and redshifts. Such nearby FRBs whose DMs are dominated by the Milky Way can be used to characterize these components and thus better calibrate the remaining cosmological contribution to dispersion for more distant FRBs that provide a strong lever arm to examine the Macquart relation between cosmological DM and redshift.

Combining new H i data from a synergetic survey of Australian Square Kilometre Array Pathfinder (ASKAP) Widefield ASKAP L -band Legacy All-sky Blind surveY and Five-hundred-meter Aperture Spherical radio Telescope with the Arecibo Legacy Fast ALFA data, we study the effect of ram pressure and tidal interactions in the NGC 4636 group. We develop two parameters to quantify and disentangle these two effects on gas stripping in H i -bearing galaxies: the strength of external forces at the optical-disk edge, and the outside-in extents of H i -disk stripping. We find that gas stripping is widespread in this group, affecting 80% of H i -detected nonmerging galaxies, and that 41% are experiencing both types of stripping. Among the galaxies experiencing both effects, the two types of strengths are independent, while two H i -stripping extents moderately anticorrelate with each other. Both strengths are correlated with H i -disk shrinkage. The tidal strength is related to a rather uniform reddening of low-mass galaxies ( M _* < 10 ^9 M _☉ ) when tidal stripping is the dominating effect. In contrast, ram pressure is not clearly linked to the color-changing patterns of galaxies in the group. Combining these two stripping extents, we estimate the total stripping extent, and put forward an empirical model that can describe the decrease of H i richness as galaxies fall toward the group center. The stripping timescale we derived decreases with distance to the center, from ∼1 Gyr beyond R _200 to ≲10 Myr near the center. Gas depletion happens ∼3 Gyr since crossing 2 R _200 for H i -rich galaxies, but much quicker for H i -poor ones. Our results quantify in a physically motivated way the details and processes of environmental-effects-driven galaxy evolution, and might assist in analyzing hydrodynamic simulations in an observational way.

We report on the commensal ASKAP detection of a fast radio burst (FRB), FRB 20211127I, and the detection of neutral hydrogen (H i ) emission in the FRB host galaxy, WALLABY J131913–185018 (hereafter W13–18). This collaboration between the CRAFT and WALLABY survey teams marks the fifth, and most distant, FRB host galaxy detected in H i , not including the Milky Way. We find that W13–18 has an H i mass of M _HI = 6.5 × 10 ^9 M _⊙ , an H i -to-stellar mass ratio of 2.17, and coincides with a continuum radio source of flux density at 1.4 GHz of 1.3 mJy. The H i global spectrum of W13–18 appears to be asymmetric, albeit the H i observation has a low signal-to-noise ratio (S/N), and the galaxy itself appears modestly undisturbed. These properties are compared to the early literature of H i emission detected in other FRB hosts to date, where either the H i global spectra were strongly asymmetric, or there were clearly disrupted H i intensity map distributions. W13–18 lacks a sufficient S/N to determine whether it is significantly less asymmetric in its H i distribution than previous examples of FRB host galaxies. However, there are no strong signs of a major interaction in the optical image of the host galaxy that would stimulate a burst of star formation and hence the production of putative FRB progenitors related to massive stars and their compact remnants.