The FRB20190520B Sightline Intersects Foreground Galaxy Clusters

The repeating fast radio burst FRB20190520B is an anomaly of the FRB population thanks to its high dispersion measure (DM$=1205\,pc\,cm^{-3}$) despite its low redshift of $z_\mathrm{frb}=0.241$. This excess has been attributed to a host contribution of ${DM_{host}} \approx 900\,\mathrm{pc\,cm^{-3}}$, far larger than any other known FRB. In this paper, we describe spectroscopic observations of the FRB20190520B field obtained as part of the FLIMFLAM survey on the 2dF/AAOmega facility, which yielded 701 galaxies redshifts in a field of $\approx 3\,\mathrm{deg}^2$. Applying a friends-of-friends group finder reveals multiple galaxy groups and clusters, for which we then estimated halo masses by comparing their richness with forward-modeled mocks from numerical simulations. We discover two separate $M_\mathrm{halo} >10^{14}\,M_\odot$ galaxy clusters, at $z=0.1867$ and $z=0.2170$, respectively, that are directly intersected by the FRB sightline within their characteristic radius $r_{200}$. Subtracting off their estimated DM contributions as well that of the diffuse intergalactic medium, we estimate a host contribution of $DM_{host}=467^{+140}_{-230}\,\mathrm{pc\,cm^{-3}}$ or ${DM_{host}} = 339^{+122}_{-174}\,\mathrm{pc\,cm^{-3}}$ (observed frame) depending on whether we assume the halo gas extends to $r_{200}$ or $2\times r_{200}$. This significantly smaller $DM_{host}$ -- no longer the largest known value -- is now consistent with H$\alpha$ emission measure estimates of the host galaxy without having to invoke unusually high gas temperatures. We also re-estimate the turbulent fluctuation and geometric amplification factor of the scattering layer to be $FG \approx 3.9 - 7.5\,(\mathrm{pc^2\;km})^{-1/3}$. This result illustrates the importance of incorporating foreground data for FRB analyses, both for understanding the nature of FRBs and to realize their potential as a cosmological probe.
Comment: 14 pages, 5 figures, 2 tables. Interactive figure (link in text). Submitted to ApJL