The faint source population at 15.7 GHz - I. The radio properties

Abstract

We have studied a sample of 296 faint (> 0.5 mJy) radio sources selected from an area of the Tenth Cambridge (10C) survey at 15.7 GHz in the Lockman Hole. By matching this catalogue to several lower frequency surveys (e.g. including a deep GMRT survey at 610 MHz, a WSRT survey at 1.4 GHz, NVSS, FIRST and WENSS) we have investigated the radio spectral properties of the sources in this sample; all but 30 of the 10C sources are matched to one or more of these surveys. We have found a significant increase in the proportion of flat spectrum sources at flux densities below ≈1 mJy -the median spectral index between 15.7 GHz and 610 MHz changes from 0.75 for flux densities greater than 1.5 mJy to 0.08 for flux densities less than 0.8 mJy. This suggests that a population of faint, flat spectrum sources is emerging at flux densities 1 mJy.The spectral index distribution of this sample of sources selected at 15.7 GHz is compared to those of two samples selected at 1.4 GHz from FIRST and NVSS. We find that there is a significant flat spectrum population present in the 10C sample which is missing from the samples selected at 1.4 GHz. The 10C sample is compared to a sample of sources selected from the SKADS Simulated Sky by Wilman et al. and we find that this simulation fails to reproduce the observed spectral index distribution and significantly underpredicts the number of sources in the faintest flux density bin. It is likely that the observed faint, flat spectrum sources are a result of the cores of FRI sources becoming dominant at high frequencies.These results highlight the importance of studying this faint, high frequency population.

Observational evidence that positive and negative AGN feedback depends on galaxy mass and jet power

“…This would also agree with recent findings (e.g. Whittam et al 2013Whittam et al , 2015. We note that almost half of these SMA observations are upper limits.…”

The ATLAS 5.5 GHz survey of the extendedChandraDeep Field South: the second data release

“…Norris et al 2011). In general there is good agreement between the observed counts and that of semi-empirical simulations of Wilman et al 2008, but there maybe an excess in observed sources in the ∼0.5 -2 mJy flux density range, which may be related to flat-spectrum sources detected at sub-mJy levels at higher frequencies (>10 GHz, Whittam et al 2013;Franzen et al 2014). The 1.4 -5.5 GHz spectral index has also been determined for the 5.5 GHz sample.…”

“…There is now emerging evidence that the spectral index flattens again at sub-mJy levels, but the nature and properties of these faint radio sources is still unclear. The flattening of the average spectral index at sub-mJy levels has been observed in faint 5 GHz selected samples (Prandoni et al 2006;Huynh et al 2012b) and recently confirmed in sub-mJy samples selected at even higher frequencies (>10 GHz, Whittam et al 2013;Franzen et al 2014). However sub-mJy sources selected at 1.4 GHz or 610 MHz do not appear to exhibit a flattening in their average spectral index (Ibar et al 2009).…”

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“…The 10C survey is complete to 1 mJy in ten different fields covering a total of ≈ 27 deg 2 ; deep areas covering ≈ 12 deg 2 , contained within these fields, are complete to 0.5 mJy, making the 10C survey the deepest high-frequency radio survey published to date. Whittam et al (2013) used data at a range of frequencies to study the spectral indices of 10C sources in the Lockman Hole. They found a significant change in spectral index with flux density; the median spectral index α (where S ∝ ν −α for a source with flux density S at frequency ν) calculated between 1.4 and 15.7 GHz changes from 0.75 for sources with S 15.7 GHz > 1.5 mJy to 0.08 for sources with S 15.7 GHz < 0.8 mJy.…”