Fast radio bursts (FRBs) are millisecond-duration radio pulses of extragalactic origin. Their dispersive sweeps, caused by the interaction of the radio waves with free electrons in the interstellar and intergalactic medium, have been used to probe the ionized baryon content of the intergalactic medium (IGM). While host-galaxy contributions to the dispersion measure (DM) are generally considered small, some FRBs show evidence of extreme magneto-ionic local environments and associated compact persistent radio sources. This study focuses on FRB 20190520B, a repeating FRB, to investigate its properties and the implications for understanding FRB origins and using them to probe cosmological distances. Understanding the contribution of the host galaxy to the total dispersion measure is crucial for accurately estimating the redshift of FRBs and thus their distances. A miscalculation of host galaxy DM can lead to significant errors in the redshift estimation, impacting our understanding of the FRB's cosmological distribution and its utility as a cosmological probe. This research contributes to a more complete understanding of FRBs by analyzing a source with an unusually high host galaxy DM contribution, which challenges the existing assumptions about FRB redshift estimation.

Previous research has established the use of FRB dispersion measures to probe the IGM. However, the relative contributions of the IGM, host galaxy, and Milky Way to the total DM remain a subject of ongoing investigation. Studies have shown that for most FRBs, the host-galaxy DM contribution is relatively small. However, exceptions exist, such as FRB 121102, which exhibits an extreme magneto-ionic environment and an associated compact persistent radio source. The discovery of repeating FRBs, like FRB 121102 and FRB 20180916B, has provided valuable insights into the nature of FRB emission mechanisms and their associated environments. Studies have attempted to correlate FRB properties, such as repetition rates and morphology, with the presence of persistent radio sources and the magnitude of host-galaxy DM. This work builds upon these previous findings by investigating another repeating FRB with a surprisingly high host-galaxy DM contribution.

FRB 20190520B was initially detected using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in drift-scan mode. Follow-up observations were conducted with FAST to characterize the burst rate and properties. Localization of the FRB was achieved using the Karl G. Jansky Very Large Array (VLA) using the 'realfast' system. Deep radio images revealed a persistent radio continuum counterpart co-located with the FRB. Optical and near-infrared imaging, using the Canada-France-Hawaii Telescope (CFHT) and Subaru Telescope, were employed to identify the host galaxy. Optical spectroscopy, using the Palomar 200-inch Hale Telescope and Keck I Telescope, determined the redshift of the host galaxy. Data analysis included modeling the FRB burst rate using a Weibull distribution, fitting the radio continuum counterpart spectrum with a power-law, and estimating the chance coincidence probability of the host galaxy association. The dispersion measure (DM) was analyzed to separate the contributions from the Milky Way, the intergalactic medium (IGM), and the host galaxy. Modeling of the IGM DM contribution as a function of redshift was used to infer the host galaxy DM. The scattering time was also considered to assess the plausibility of the host galaxy being a foreground object. The properties of the host galaxy, such as stellar mass and star-formation rate, were estimated based on its observed magnitudes and emission line fluxes.

FRB 20190520B was localized to a dwarf galaxy at a redshift of 0.241 ± 0.001. A compact persistent radio source was detected co-located with the FRB. The host galaxy has a high specific star-formation rate. The estimated host-galaxy DM is approximately 903 pc cm⁻³, nearly an order of magnitude higher than the average for FRB host galaxies. This large DMhost value significantly exceeds the IGM DM contribution, highlighting the importance of considering host-galaxy DM when estimating FRB redshifts. The FRB exhibits a high burst repetition rate (R = 4.5^+1.3_−1.2 h⁻¹). The observed scattering time (10 ± 2 ms) is much smaller than what would be expected if the identified galaxy were a foreground object, further supporting its identification as the host galaxy. The radio continuum counterpart is compact (<0.36″ or <1.4 kpc) and has a spectral luminosity of L_3GHz = 3 × 10²⁹ erg s⁻¹ Hz⁻¹. The properties of FRB 20190520B and its environment strongly resemble those of FRB 20121102A, another repeating FRB associated with a compact persistent radio source. The similarity between FRB 20190520B and FRB 20121102A suggests that these repeating FRBs may represent a younger population of sources still embedded within their complex natal environments, characterized by high DMhost and persistent radio emission.

The unusually high host-galaxy DM of FRB 20190520B challenges the common assumption that host-galaxy DM contributions are negligible in FRB redshift estimations. This finding necessitates a reevaluation of the methods used to determine FRB redshifts and highlights the need for precise host-galaxy identifications in future studies. The co-location of FRB 20190520B with a compact persistent radio source and its high repetition rate further support the growing evidence suggesting a connection between FRB activity, the presence of a persistent radio source, and a dense, magnetized environment. The observed similarities between FRB 20190520B and FRB 20121102A, both characterized by high DMhost and associated persistent radio sources, suggest the possibility of a subclass of FRBs that are associated with younger, actively star-forming environments. However, more data are needed to fully characterize the relationship between FRB properties, host-galaxy characteristics, and the presence of persistent radio emission.

This study presents a detailed analysis of FRB 20190520B, a repeating FRB associated with a dwarf host galaxy and a compact persistent radio source. The exceptionally high host-galaxy DM of this source emphasizes the importance of accurate host-galaxy identification in FRB redshift estimations. The similarities between FRB 20190520B and FRB 20121102A suggest that certain FRBs may originate from young sources residing in dense, magnetized environments. Further studies with larger samples of precisely localized FRBs and their host galaxies are needed to fully characterize the distribution of host-galaxy DM and to investigate the connection between FRB properties and their environments.

While the observed properties strongly support the identification of J160204.31–111718.5 as the host galaxy, the possibility of a chance coincidence, though small (0.8%), cannot be completely ruled out. The exact contribution of the circum-source medium versus the interstellar medium of the host galaxy to the large DMhost remains uncertain without further high-resolution observations. The study relies on current models for the IGM DM contribution, which have inherent uncertainties. The limited number of well-localized FRBs with measured host-galaxy redshifts restricts the statistical power to draw definitive conclusions about the overall distribution of host-galaxy DM.