A High-Resolution View of Fast Radio Burst Host Environments

Abstract: We present 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 1.4 $r_e$ ([0.6,2.1] $r_e$; 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. We further find 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 $m_{\rm IR}\gtrsim!24.8-27.6$~AB~mag, constraining both satellite and background galaxies to luminosities well below the host luminosity of FRB121102. 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. Our 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).

An Examination of Fast Radio Burst Host Galaxies Using High-Resolution Observations

The paper "A High-Resolution View of Fast Radio Burst Host Environments" presents comprehensive data from Hubble Space Telescope (HST) observations of eight Fast Radio Burst (FRB) host galaxies. The research team, led by Mannings et al., focuses on sub-arcsecond localizations to derive insights into the spatial distribution and environments where these enigmatic events occur. This investigation includes hosts of both known repeating and apparently non-repeating FRBs.

Methodology

The study employed HST with the Wide-Field Camera 3 (WFC3) using ultraviolet and infrared channels to observe the host galaxies. These observations facilitated an intricate analysis of the FRB host galaxies, allowing the authors to conduct astrometry to tie ground-based images to the high-definition HST images, calculate spatial offsets, and determine the locations of FRBs relative to their host galaxies' light distributions.

Key Results

1. Spatial Distributions:
   • The FRBs occur at moderate host-normalized offsets, measured at 1.4 $r_e$ with a 68% interval ranging from 0.6 to 2.1 $r_e$.
   • These locations are more extended than those of long-duration gamma-ray bursts (LGRBs) and super-luminous supernovae (SLSNe), yet distinctly different from short-duration gamma-ray bursts (SGRBs) and Calcium-rich (Ca-rich) transients.
2. Light Distribution:
   • In terms of infrared light, FRBs tend to occur on fainter regions of their host galaxies. They have a median fractional flux of 0.33.
   • These distributions align with the radial distribution of infrared light in their host galaxies, particularly significant considering that they do not strongly correlate with regions of increased local star formation rate or stellar mass surface density.
3. Host Galaxy Morphology:
   • Five out of eight host galaxies showcase clear spiral arm structures. Notably, the majority of FRBs in such hosts align with these arms, though not necessarily within the brightest areas.
4. Theoretical Implications:
   • The research casts doubt on the association of FRBs with exclusively massive (engine-driven) stellar explosions, given their distinct locations compared to LGRBs and SLSNe.
   • The evidence does not support models in which FRBs primarily originate from environments with massive, short-lived star progenitors or progenitor systems involving significant time delays and kicks (e.g., SGRBs).
5. FRB Localization Error Constraints:
   • The precision offered by HST image resolutions allows for stringent evaluations of potential faint satellite or background galaxies. The observed luminosities significantly preclude these scenarios, consolidating the host-FRB associations derived from initial allocations.

Discussion and Future Directions

This high-resolution imaging study critically informs the ongoing discourse on FRB progenitors. By offering detailed statistical analyses and considerations of systematic investigations into host environments, the paper enriches the understanding of the potential mechanisms behind FRB emissions. Future research, potentially empowered by JWST and subsequent space-based or high-resolution ground-based facilities, may extend the redshift range and sensitivity to cover greater variance in FRB host features and environments.

Continued accumulation of FRB data correlated with host galaxy properties will elucidate the fundamental nature of these phenomena, refining the astrophysical models applied. The advent of larger, homogeneous samples and advanced detection methodologies will further mitigate selection biases and enable more nuanced interpretations.

In summary, this pivotal research substantiates the theory that FRBs are not primarily associated with young, massive stellar progenitors and challenges previous conjectures about their origins, advancing the field's understanding of these cosmic radio transients.