The low-frequency size distribution of radio sources in the Lockman Hole

ABSTRACT Active galactic nuclei (AGNs) can launch powerful jets that can affect the gas properties in their host galaxies and influence their star formation activity. Depending on their powers and lifetimes and the properties of the surrounding medium, these can remain confined within or close to the galaxy at kiloparsec scales, or grow to giant radio galaxies on megaparsec scales. We measure the projected angular extents of a complete sample of 2110 radio sources ($z < 2.5$; $S_{\rm 144\, MHz}> 600\, \mu$Jy) using $\nu _{\rm obs}=144$ MHz images over a $6.6\ \mathrm{deg}^2$ area of the Lockman Hole field from the International LOw Frequency Array (LOFAR) Telescope (ILT) at resolutions of $6$, $1.8$, and $0.45\,\mathrm{ arcsec}$. Using these measurements, we derive the first radio source size distribution at a frequency below $200\ \mathrm{MHz}$ and present a power-linear size diagram for the objects. We then focus on the 1205 sources not identified as star-forming galaxies based on spectral energy distribution classifications from previous work. These have linear sizes in the range $\ell = 0.7\ \mathrm{kpc} {\!-\!} 1\ \mathrm{Mpc}$, radio powers in the range $P_\mathrm{144\,\mathrm{MHz}} \approx 10^{21} {\!-\!} 10^{29}\, \mathrm{W}\, \mathrm{Hz}^{-1}$, and a linear size distribution in qualitative agreement with that of radio AGNs in the LOFAR Two-metre Sky Survey. While the sample is limited to radio powers $P_\mathrm{144\,\mathrm{MHz}} \ge 10^{24}\, \mathrm{W}\, \mathrm{Hz}^{-1}$ at higher redshifts due to selection effects, such radio AGNs appear to prefer more compact projected lengths $\ell \lesssim 20\ \mathrm{kpc}$, which could indicate that more short-lived, high accretion activity was present in the Early Universe.