Disentangling the Cosmic Web toward FRB 190608

Abstract

FRB 190608 was detected by ASKAP and localized to a spiral galaxy at\n$z_{host}=0.11778$ in the SDSS footprint. The burst has a large dispersion\nmeasure ($DM_{FRB}=339.8$ $pc/cm^3$) compared to the expected cosmic average at\nits redshift. It also has a large rotation measure ($RM_{FRB}=353$ $rad/m^2$)\nand scattering timescale ($\\tau=3.3$ $ms$ at $1.28$ $GHz$). Chittidi et al\n(2020) perform a detailed analysis of the ultraviolet and optical emission of\nthe host galaxy and estimate the host DM contribution to be $110\\pm 37$\n$pc/cm^3$. This work complements theirs and reports the analysis of the optical\ndata of galaxies in the foreground of FRB 190608 to explore their contributions\nto the FRB signal. Together, the two manuscripts delineate an observationally\ndriven, end-to-end study of matter distribution along an FRB sightline; the\nfirst study of its kind. Combining KCWI observations and public SDSS data, we\nestimate the expected cosmic dispersion measure $DM_{cosmic}$ along the\nsightline to FRB 190608. We first estimate the contribution of hot, ionized gas\nin intervening virialized halos ($DM_{halos} \\approx 7-28$ $pc/cm^3$). Then,\nusing the Monte Carlo Physarum Machine (MCPM) methodology, we produce a 3D map\nof ionized gas in cosmic web filaments and compute the DM contribution from\nmatter outside halos ($DM_{IGM} \\approx 91-126$ $pc/cm^3$). This implies a\ngreater fraction of ionized gas along this sightline is extant outside\nvirialized halos. We also investigate whether the intervening halos can account\nfor the large FRB rotation measure and pulse width and conclude that it is\nimplausible. Both the pulse broadening and the large Faraday rotation likely\narise from the progenitor environment or the host galaxy.