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Register a new userDissecting the Local Environment of FRB 190608 in the Spiral Arm of its Host Galaxy
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Jay S. Chittidi
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We present a high-resolution analysis of the host galaxy of fast radio burst FRB 190608, an SBc galaxy at $z=0.11778$ (hereafter HG 190608), to dissect its local environment and its contributions to the FRB properties. Our Hubble Space Telescope WFC3/UVIS image reveals that the sub-arcsecond localization of FRB 190608 is coincident with a knot of star-formation ($Sigma_{SFR} = 1.2 times 10^{-2}~ M_{odot} , kpc^{-2}$) in one of the prominent spiral arms of HG 190608. This is confirmed by H$beta$ emission present in our Keck/KCWI integral field spectrum of the galaxy with a surface brightness of $mu_{Hbeta} = (3.35pm0.18)times10^{-17};erg;s^{-1};cm^{-2};arcsec^{-2}$. We infer an extinction-corrected H$alpha$ surface brightness and compute a dispersion measure from the interstellar medium of HG 190608 of ${DM}_{Host,ISM} = 82 pm 35~ pc , cm^{-3}$. The galaxy rotates with a circular velocity $v_{circ} = 141 pm 8~ km , s^{-1}$ at an inclination $i_{gas} = 37 pm 3^circ$, giving a dynamical mass $M_{halo}^{dyn} approx 10^{11.96 pm 0.08}~ M_{odot}$. A surface photometric analysis of the galaxy using FORS2 imaging suggests a stellar disk inclination of $i_{stellar} = 26 pm 3^circ$. The dynamical mass estimate implies a halo contribution to the dispersion measure of ${DM}_{Host,Halo} = 55 pm 25; pc , cm^{-3}$ subject to assumptions on the density profile and fraction of baryons retained. The relatively high temporal broadening ($tau = 3.3 pm 0.2 ; ms$ at 1.28 GHz) and rotation measure ($ RM = 353 pm 2; rad ; m^{-2}$) (Day et al. 2020) of FRB 190608 may be attributable to both turbulent gas within the spiral arm and gas local to the FRB progenitor. In contrast to previous high-resolution studies of FRB progenitor environments, we find no evidence for disturbed morphology, emission, nor kinematics for FRB 190608.
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FRB 190608 was detected by ASKAP and localized to a spiral galaxy at $z_{host}=0.11778$ in the SDSS footprint. The burst has a large dispersion measure ($DM_{FRB}=339.8$ $pc/cm^3$) compared to the expected cosmic average at its redshift. It also has a large rotation measure ($RM_{FRB}=353$ $rad/m^2$) and scattering timescale ($tau=3.3$ $ms$ at $1.28$ $GHz$). Chittidi et al (2020) perform a detailed analysis of the ultraviolet and optical emission of the host galaxy and estimate the host DM contribution to be $110pm 37$ $pc/cm^3$. This work complements theirs and reports the analysis of the optical data of galaxies in the foreground of FRB 190608 to explore their contributions to the FRB signal. Together, the two manuscripts delineate an observationally driven, end-to-end study of matter distribution along an FRB sightline; the first study of its kind. Combining KCWI observations and public SDSS data, we estimate the expected cosmic dispersion measure $DM_{cosmic}$ along the sightline to FRB 190608. We first estimate the contribution of hot, ionized gas in intervening virialized halos ($DM_{halos} approx 7-28$ $pc/cm^3$). Then, using the Monte Carlo Physarum Machine (MCPM) methodology, we produce a 3D map of ionized gas in cosmic web filaments and compute the DM contribution from matter outside halos ($DM_{IGM} approx 91-126$ $pc/cm^3$). This implies a greater fraction of ionized gas along this sightline is extant outside virialized halos. We also investigate whether the intervening halos can account for the large FRB rotation measure and pulse width and conclude that it is implausible. Both the pulse broadening and the large Faraday rotation likely arise from the progenitor environment or the host galaxy.
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