Spectroscopic Confirmation and Physical Characterization of the Most Probable Host Galaxy of Repeating FRB 20251229A

ATel #17856; Mohit Bhardwaj (Indian Institute of Technology Kanpur), Aida Kirichenko (Universidad Nacional Autonoma de Mexico), Ashwin Kharat (Indian Institute of Technology Kanpur), Ajay Kumar (Indian Institute of Technology Kanpur)
on 27 Jun 2026; 18:29 UT
Credential Certification: Mohit Bhardwaj (work.mohitbhardwaj@gmail.com)

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We report optical spectroscopic observations of the field surrounding the hyperactive repeating fast radio burst FRB 20251229A (ATels #17574, #17591, #17606, #17671, #17709) obtained with the OSIRIS spectrograph on the 10.4-m Gran Telescopio Canarias (GTC) on 2026 June 9 and 11. Our observations targeted the optical galaxy SDSS J204123.23+160126.54, identified by the CHIME/FRB Collaboration (ATel #17709) as the most probable host galaxy of FRB 20251229A. The galaxy is also spatially coincident with the 650 MHz radio source reported in ATel #17723, with a flux density of 64 +/- 13 microJy.

Simultaneous multi-component Gaussian profile fitting using lmfit (Newville et al. 2014) reveals strong nebular emission lines, yielding a secure spectroscopic redshift of z = 0.1275 +/- 0.0002. The emission-line ratios place the galaxy in the star-forming region of the Baldwin-Phillips-Terlevich (BPT) diagram (Baldwin et al. 1981), with no evidence for AGN activity. Using the O3N2 calibration of Marino et al. (2013), we estimate a gas-phase oxygen abundance of 12 + log(O/H) ~8.4, corresponding to a moderately sub-solar metallicity (~0.5 Zsun). The Galactic-extinction-corrected Halpha luminosity, corrected for slit losses using SDSS r-band photometry, implies a star formation rate of 0.7 +/- 0.2 Msun/yr. Using ALLWISE W1 and W2 photometry together with the WISE stellar-mass calibration of Jarrett et al. (2023), we estimate a stellar mass of ~1 x 10^10 Msun. The resulting specific star formation rate, log(sSFR/yr) ~-10.2, is consistent with a normal main-sequence star-forming galaxy. Using the aperture-corrected Halpha luminosity and the empirical Halpha-radio continuum calibration of Murphy et al. (2011), we estimate an integrated 650 MHz flux density of ~25-40 microJy from star formation. Given the intrinsic scatter of the radio-SFR relation, the 64 +/- 13 microJy emission reported by the uGMRT (ATel #17723) can plausibly be explained, at least in substantial part, by integrated star formation within the host galaxy. Assuming a synchrotron spectral index of alpha = -0.75, the corresponding 6 GHz flux density is ~5-8 microJy, consistent with the VLA 3-sigma upper limit of 10 microJy (ATel #17735).

The CHIME/FRB Collaboration reported an observed dispersion measure of DM_obs = 192.14 pc cm^-3 (ATel #17574) and inferred z <= 0.13 from the Macquart relation (Macquart et al. 2020). Our measured redshift is fully consistent with this prediction. After subtracting the maximum expected Galactic disk and halo contributions, we estimate the FRB host dispersion measure of only <~ 30 pc cm^-3 (Bhardwaj et al. 2021), indicating a modest host-galaxy plasma contribution. Together with the very small residual rotation measure reported by FAST (|RM| <=20 rad m^-2; ATel #17591), these observations suggest that FRB 20251229A resides in a relatively clean magneto-ionic environment.

We also obtained ~3 hours of deep GTC spectroscopy of the faint irregular galaxy candidate identified in recent LBT observations (ATel #17822) at an alternative position within the CHIME baseband and outrigger localization regions (ATels #17606, #17709). The extracted spectrum is dominated by a very faint continuum. A forced extraction at the target position reveals a marginal emission feature near 6680 Angstrom with an estimated flux of ~6 x 10^-17 erg s^-1 cm^-2. However, no additional significant spectral features are detected, preventing a secure line identification or spectroscopic redshift. Deeper spectroscopy will be required to determine the nature of this source.