A Uniformly Selected Sample of Low-mass Black Holes in Seyfert 1 Galaxies. III. Radio Sources from the SKA Pathfinders and Beyond

Abstract

Occupying the intermediate-mass regime of the accretion-jet parameter space, radio continuum emission from active galactic nuclei (AGNs) with black hole mass M _BH ≲ 10 ^6 M _⊙ (low-mass AGNs) is a valuable probe to the physics of relativistic jets. Yet the number of low-mass AGNs with radio detection is rather limited so far (≈40 in total). In this work, we make two efforts to search for radio counterparts for the largest sample of optically selected low-mass AGNs. First, we collect counterparts from the recent data releases of Square Kilometre Array (SKA) pathfinders such as LOFAR Two-meter Sky Survey (LoTSS). Additionally, we deeply mine in Faint Images of the Radio Sky at Twenty-Centimeters (FIRST), fitting the FIRST images of the optical AGNs with an elaborate procedure optimized to detect faint radio sources. We have obtained 151 radio sources (mainly from the SKA pathfinders), including 102 new reliable sources (signal-to-noise ratio, hereafter S/N, ≥ 5) and 23 new candidates (3.5 ≤ S/N < 5). The majority of these new sources (119 of 125) have flux densities lower than the threshold of the official FIRST catalog. The new sources have rest-frame 20 cm power ( P _20 cm ) from 1.98 × 10 ^20 to 1.29 × 10 ^23 W Hz ^−1 . For low- z Seyfert galaxies, P _20 cm correlates with M _BH intrinsically and positively, yet only marginally with Eddington ratio L / L _Edd . In terms of the log N –log S relation for the expanding Universe, the limiting flux density for the completeness of our LoTSS sources turns out to be 0.45 mJy at 1.4 GHz; i.e., complete to such a flux-density level that is 4 times deeper than the official FIRST catalog.

Keywords

• Active galactic nuclei
• Accretion
• Intermediate-mass black holes
• Seyfert galaxies
• Radio continuum emission
• Relativistic jets