Deep observations of the Super-CLASS super-cluster at 325 MHz with the GMRT: the low-frequency source catalogue

Abstract

We present the results of 325 MHz GMRT observations of a super-cluster field, known to contain five Abell clusters at redshift $z\sim 0.2$. We achieve a nominal sensitivity of $34\mu$Jy beam${}^{-1}$ toward the phase centre. We compile a catalogue of 3257 sources with flux densities in the range $183\mu \mathrm{Jy}-1.5\mathrm{Jy}$ within the entire $\sim 6.5$ square degree field of view. Subsequently, we use available survey data at other frequencies to derive the spectral index distribution for a sub-sample of these sources, recovering two distinct populations -- a dominant population which exhibit spectral index trends typical of steep-spectrum synchrotron emission, and a smaller population of sources with typically flat or rising spectra. We identify a number of sources with ultra-steep spectra or rising spectra for further analysis, finding two candidate high-redshift radio galaxies and three gigahertz-peaked-spectrum radio sources. Finally, we derive the Euclidean-normalised differential source counts using the catalogue compiled in this work, for sources with flux densities in excess of $223\mu$Jy. Our differential source counts are consistent with both previous observations at this frequency and models of the low-frequency source population. These represent the deepest source counts yet derived at 325 MHz. Our source counts exhibit the well-known flattening at mJy flux densities, consistent with an emerging population of star-forming galaxies; we also find marginal evidence of a downturn at flux densities below $308\mu$Jy, a feature so far only seen at 1.4 GHz.

arXiv comment25 pages, 18 figures, 10 tables. Accepted for publication in MNRAS

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