Keck/DEIMOS spectroscopic redshift of S2 in the FRB 190523 field
ATel #13144; J. Xavier Prochaska (UC Santa Cruz), Sunil Simha (UC Santa Cruz), Nicolas Tejos (Pontificia Universidad Catolica de Valparaiso), on behalf of the F^4 team
on 28 Sep 2019; 20:47 UT
Credential Certification: J. Xavier Prochaska (xavier@ucolick.org)
Subjects: Optical, Fast Radio Burst
On UT 2019 July 05, the Fast and Fortunate for FRB Follow-up (F^4) team triggered a Target of Opportunity observation with the W.M. Keck Observatory to obtain a spectrum of the source named S2 located near the localization of FRB 190523 by Ravi et al. 2019. Our Keck/DEIMOS data which cover ~5000-9500 Angstroms show a faint trace at the galaxy location and a single emission line at 8949 Angstroms which we designate as Halpha. Adopting that line-identification (other common emission lines are ruled out by the lack of coincident emission at other wavelengths) gives z=0.36 for S2.
Given the DM reported by Ravi et al. 2019 for FRB 190523 (760.8 pc / cm^3), we disfavor this source as the host galaxy as the FRB. Instead, we consider it likely to intervene the sightline to FRB 190523 at an impact parameter of < 50 kpc. If its halo gas were to explain the large scattering timescale (1.4 ms) reported by Ravi et al. 2019, this would require a density n_e ~ 0.01 cm^-3 assuming fiducial sizes for the halo (see Prochaska et al. 2019 for details).
