2017ApJ...844...95K

We present the Hα intensity map of the host galaxy of the repeating fast radio burst FRB 121102 at a redshift of z = 0.193 obtained with the AO-assisted Kyoto 3DII optical integral-field unit mounted on the 8.2 m Subaru Telescope. We detected a compact Hα-emitting (i.e., star-forming) region in the galaxy, which has a much smaller angular size ( 0′′ / . 57 (1.9 kpc) at full width at half maximum (FWHM)) than the extended stellar continuum emission region determined by the Gemini/GMOS z^′ -band image (≃1′′ / . 4 (4.6 kpc) at FWHM with ellipticity b/a=0.45). The spatial offset between the centroid of the Hα emission region and the position of the radio bursts is 0′′ / . 08±0′′ / . 02 (0.26 ± 0.07 kpc), indicating that FRB 121102 is located within the star-forming region. This close spatial association of FRB 121102 with the star-forming region is consistent with expectations from young pulsar/magnetar models for FRB 121102, and it also suggests that the observed Hα emission region can make a major dispersion measure (DM) contribution to the host galaxy DM component of FRB 121102. Nevertheless, the largest possible value of the DM contribution from the Hα emission region inferred from our observations still requires a significant amount of ionized baryons in intergalactic medium (IGM; the so-called "missing" baryons) as the DM source of FRB 121102, and we obtain a 90% confidence level lower limit on the cosmic baryon density in the IGM in the low-redshift universe as Ω_IGM 0.012.