A Relationship Between Fast Radio Burst Dispersion Measure and Host Galaxy Inclination

Abstract

It is commonly thought that fast radio bursts (FRBs) trace star formation, and that they are produced by magnetars. Both hypotheses imply that their sources are hosted primarily (but not exclusively) in galactic disks. Although there is evidence in that direction, it has yet to be shown conclusively. In addition, a tension exists with the fact that at least one FRB source is found in a globular cluster far from the disk of the host. If FRBs originate primarily in disks, the galaxy orientation could leave an impression in the sample of observed FRBs, either as a selection bias against FRBs in edge-on galaxies or as a correlation with DMHost. Statistical tests against the inclinations of FRB host galaxies, derived from parametric morphology applied to optical and near-infrared imaging, can search for these effects. Such a search requires an adequately large sample of known FRB hosts that have been imaged to a sufficient depth, while estimates of DMHost also require the redshift of each host to be known. These conditions are best met in the Commensal Real-time ASKAP Fast Transient (CRAFT) survey sample of FRB host galaxies, for which the majority have been imaged at optical wavelengths with FORS2 on the Very Large Telescope (VLT). Here these data are fitted with the morphology software GALFIT to derive the elliptical axis ratio q (a proxy for inclination) for 34 hosts from the CRAFT sample, detected with ASKAP and followed up with VLT imaging. We find no bias against FRB detections from edge-on galaxies; instead, we uncover a new relationship between the inclination of an FRB host galaxy and DMHost. This is consistent with a primarily disk-based origin for CRAFT FRBs. Using this relationship we are able to reduce the scatter of the host sample about the Macquart relation by 40%, demonstrating a new empirical method for estimating DMHost that requires only deep optical imaging.