Gravitational lensing in LoTSS DR2 -- Extremely faint 144-MHz radio emission from two highly magnified quasars

TL;DR

This study detects extremely faint, gravitationally lensed radio emissions from two quasars at high redshift using LOFAR, revealing the lowest luminosity at such distances and suggesting star formation as the emission source.

Contribution

First detection of ultra-faint 144 MHz radio emission from highly magnified quasars, pushing the limits of low-frequency radio astronomy and lensing studies.

Findings

SDSS J1004+4112 is the faintest low-frequency radio source detected so far.

The intrinsic luminosities are consistent with star formation rates of 5.5 and 73 solar masses per year.

Detection extends the known low-luminosity regime of high-redshift quasars at 144 MHz.

Abstract

We report extremely faint 144 MHz radio emission from two gravitationally lensed quasars, SDSS J1004+4112 (z = 1.730) and SDSS J2222+2745 (z = 2.803), using the LOFAR Two Metre Sky Survey (LoTSS) data release 2. After correcting for the lensing magnifications, the two objects have intrinsic flux-densities of 13+/-2 and 58+/-6 uJy, respectively, corresponding to 144 MHz rest-frame luminosities of 10^(23.2+/-0.2) and 10^(24.42+/-0.05) W / Hz, respectively. In the case of SDSS J1004+4112, the intrinsic flux density is close to the confusion limit of LoTSS, making this radio source the faintest to be detected thus far at low frequencies, and the lowest luminosity known at z > 0.65. Under the assumption that all of the radio emission is due to star-formation processes, the quasar host galaxies are predicted to have star-formation rates of 5.5^(+1.8)_(-1.4) and 73^(+34)_(-22) M / yr, respectively. Further multi-wavelength observations at higher angular resolution will be needed to determine if any of the detected radio emission is due to weak jets associated with the quasars.