A dim host galaxy for FRB 20251229A: a FRB 20121102A host twin?

ATel #17822; Luciano Nicastro (INAF-OAS), Andrea Rossi (INAF-OAS), Eliana Palazzi (INAF-OAS), Gabriele Bruni (INAF-IAPS), Sandra Savaglio (UniCal)
on 29 May 2026; 18:13 UT
Credential Certification: Luciano Nicastro (luciano.nicastro@inaf.it)

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In ATel #17577, using CHIME/FRB Outriggers data, an elongated arcsecond-scale localization of the potentially hyperactive fast radio burst FRB 20251229A was reported (ATel #17574). The localization error ellipse centre is: RA (ICRS): 20h41m23.82s, Dec (ICRS): +16^o01′51.34′′ with a semi-major / -minor axis of 20′′/ 0.07′′ and position angle of 13.2 degrees (East of North). Based on this, the authors investigate the Pan-STARRS1 (PS1) candidate host galaxy J204123.23+160126.54 (CWISE J204123.22+160126.1), whose centroid is at 26′′ from the FRB error ellipse centre. It has a Kron r-mag of 18.78 +/− 0.02 and a photometric redshift of 0.08 from PS1-WISE-STRM (Beck et al. 2021).
We performed CIGALE SED fitting of this galaxy and confirmed the SDSS DR16 catalogue estimate z = 0.12 +/− 0.02, which is compatible with the FRB DM. Using PATH (Aggarwal et al. 2021) the probability of host association is found to be P(O|x) ∼ 0.8. In ATel #17577, the authors conclude that it is the most likely host, but note that the actual host galaxy could be below the detection threshold of PS1.
Here we report on optical LBT/LBC deep observations in the g- and r-bands of the FRB 20251229A region. Our 20-min observations, carried out on May 19^th, 2026 with a seeing of 1.1 arcsec, reached 3-sigma limiting magnitudes of 25.7 and 26.0 in the r- and g-band, respectively.
We detect a weak irregular galaxy on the North-West side of the FRB centroid. The presence of multiple interacting galaxies is also possible.
Using a 3 arcsec aperture photometry, we obtain:

g = 24.20 +/− 0.09, centroid at RA (J2000): 20h41m23.756s, Dec: +16^o01′53.26′′ (310.348983, 16.031461 deg)
r = 24.08 +/− 0.12, centroid at RA (J2000): 20h41m23.716s, Dec: +16^o01′53.62′′ (310.348817, 16.031561 deg)

The galaxy centroid is then about 2.4 arcsec from the FRB centroid.
Attached are the images covering an area of about 1.1 x 1.1 arcmin² centred on the FRB position (top panels). The FRB centered zoomed version is shown in the bottom panels. Differences in luminosity in the two filters are clearly visible. The red box marks the FRB centroid. The 3 arcsec radius white circle shows the host galaxy. The image resolution is 0.226 arcsec/pix.
Assuming an FRB redshift of z ∼ 0.1 (ATel #17574) we obtain a scale of 1.9 kpc/arcsec. Therefore, the entire galaxy is apparently contained (in optical) within a circle with a radius of 6 kpc. At this redshift, once corrected for the foreground Galactic extinction A_r = 0.16, Schlafly & Finkbeiner 2011, the observed r-band magnitude gives an absolute r-band magnitude M_r ∼ −14.2. This is approximately 2 mags fainter than the very faint host associated with FRB 20121102. Therefore, the host galaxy we just identified is possibly the faintest FRB host candidate known to date.

Spectral observations of the host galaxy have been planned.

Note:
In ATel #17723, a putative PRS associated with the FRB was reported based on uGMRT observations. This claim was not confirmed by higher spatial resolution VLA observations (ATel #17735), pointing to significant contribution to the continuum radio emission from integrated star formation.

Link to LBT/LBC images

We acknowledge excellent support from the LBTO and LBT-INAF staff, particularly S. Tosi, C. Ventura and D. Paris