Mannings, and 14 co-authors, reported in May 21, 2021 that the Hubble Space Telescope (HST/WFC3) ultraviolet and infrared observations of eight fast radio burst (FRB) host galaxies with sub-arcsecond localizations, including the hosts of three known repeating FRBs. We quantify their spatial distributions and locations with respect to their host galaxy light distributions, finding that they occur at moderate host normalized-offsets of 1.4re ([0.6, 2.1]re; 68% interval), occur on fainter regions of their hosts in terms of IR light, but overall trace the radial distribution of IR light in their galaxies. The FRBs in our tested distribution do not clearly trace the distributions of any other transient population with known progenitors, and are statistically distinct from the locations of LGRBs, H-poor SLSNe, SGRBs, and Ca-rich transients.
The authors further found that most FRBs are not in regions of elevated local star formation rate and stellar mass surface densities in comparison to the mean global values of their hosts. We also place upper limits to the IR flux at the FRB positions of mIR & 24.8 − 27.6 AB mag, constraining both satellite and background galaxies to luminosities well below the host luminosity of FRB 121102. We find that 5/8 FRB hosts exhibit clear spiral arm features in IR light, and that the positions of all well-localized FRBs located in such hosts are consistent with their spiral arms, although not on their brightest regions.
The authors' results do not strongly support the primary progenitor channel of FRBs being connected either with the most massive (stripped-envelope) stars, or with events which require kicks and long delay times (neutron star mergers). But the authors used high-resolution HST imaging to perform a detailed study on the locations of 8 FRBs and their environments, 6 of which are newly presented here. They used these data to place constraints on the spatial distributions (physical and host-normalized), in support of previous works based on ground-based imaging. We find a median host-normalized offset of 1.4 ([0.6, 2.1]re; 68% interval), and overall a distribution that lies between the more centrally concentrated LGRBs and SLSNe, and the extended SGRBs and Ca-rich transients.
The authors also determined the distribution of FRBs with respect to their IR (rest-frame optical) host galaxy light (fractional flux, and radial distribution), showing that FRBs are consistent with tracing the stellar mass distribution of their host galaxies. The sensitivity of HST additionally enables constraints on possible alternative host galaxy candidates; They found it to be improbable that there exists a satellite or background galaxy at the FRB locations, strengthening the associations with the brighter, putative hosts identified in ground-based imaging for this sample. They explored the FRB site properties in terms of star formation rate HST FRB Host Galaxies 21 (near-UV) and stellar mass (IR) surface densities, finding that the locations are not particularly enhanced in either property compared to the global values of their hosts (although few measurements exist for the star formation rate densities).
Finally, the authors concluded that 5 out or 8 FRB host galaxies in the sample have spiral arm features, and that these FRBs are consistent with the locations of those spiral arms (albeit inconsistent with locations on the brightest peaks of these spiral arms). If there is a dominant progenitor population among this tested distribution, They did not find strong support for a connection to the most massive (stripped-envelope) stars, or events which require kicks and long delay times.
The promise of sub-arcsecond localized FRBs in solving the progenitor question is being realized, in part, with the first population studies of their local and host galaxy environments. Such precisely-localized FRBs are and will continue to be detected at growing rates. As the number of secure associations continues to increase, we will be able to make significant progress toward understanding their progenitors, as well as connecting the properties of FRBs to those of their host galaxies. The current sample of well-localized FRBs is admittedly small, much less those with high-resolution imaging. Moreover, the current sample with secure host galaxies is subject to various selection biases which have been mentioned throughout this paper.
Looking toward the future, the authors indicated that upcoming FRB experiments and upgrades to existing ones will deliver larger, more uniform samples of sub-arcsecond localized FRBs, which importantly will push beyond the current DM or redshift horizons. Equipped with a large sample of FRBs with high resolution imaging, we will be able to identify trends between the locations of FRBs in known repeaters and apparent non-repeaters, in host galaxies of different morphological types (e.g., prevalence of spiral sub-structure, star-forming vs. quiescent), and make more statistically significant statements about their similarities or differences to other transient populations.
Matched to the increased sensitivities of discovery experiments, we will also explore evolution of the local properties of FRBs with redshift. All of these studies will provide important clues to their origins. Larger samples will also enable tighter constraints to be placed on local contributions to the dispersion measure in DM and IGM studies, optimizing the use of FRBs as a cosmological probe. Finally, HST and soon JWST will also aid in our understanding of whether FRBs originate from a single, dominant progenitor channel or multiple contributing channels, a central question in FRBs.
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