The FRB 20190520B Sight Line Intersects Foreground Galaxy Clusters

Abstract

The repeating fast radio burst FRB 20190520B is an anomaly of the FRB population thanks to its high dispersion measure (DM = 1205 pc cm ^−3 ) despite its low redshift of z _frb = 0.241. This excess has been attributed to a large host contribution of DM _host ≈ 900 pc cm ^−3 , far larger than any other known FRB. In this paper, we describe spectroscopic observations of the FRB 20190520B field obtained as part of the FLIMFLAM survey, which yielded 701 galaxy redshifts in the field. We find multiple foreground galaxy groups and clusters, for which we then estimated halo masses by comparing their richness with numerical simulations. We discover two separate M _halo > 10 ^14 M _⊙ galaxy clusters at z = 0.1867 and 0.2170 that are directly intersected by the FRB sight line within their characteristic halo radius r _200 . Subtracting off their estimated DM contributions, as well that of the diffuse intergalactic medium, we estimate a host contribution of ${{\rm{D}}{\rm{M}}}_{{\rm{h}}{\rm{o}}{\rm{s}}{\rm{t}}}={430}_{-220}^{+140}$ or ${280}_{-170}^{+140}\,{\rm{p}}{\rm{c}}\,{{\rm{c}}{\rm{m}}}^{-3}$ (observed frame), depending on whether we assume that the halo gas extends to r _200 or 2 × r _200 . This significantly smaller DM _host —no longer the largest known value—is now consistent with H α emission measures of the host galaxy without invoking unusually high gas temperatures. Combined with the observed FRB scattering timescale, we estimate the turbulent fluctuation and geometric amplification factor of the scattering layer to be $\widetilde{F}G\approx 4.5\mbox{--}11{({\mathrm{pc}}^{2}\,\mathrm{km})}^{-1/3}$ , suggesting that most of the gas is close to the FRB host. 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.

Keywords

• Radio transient sources
• Intergalactic gas
• Circumgalactic medium
• Redshift surveys