Deep multi-frequency radio imaging in the Lockman Hole — II. The spectral index of submillimetre galaxies

Abstract

We have employed the Giant Metre‐wave Radio Telescope and the Very Large Array to map the Lockman Hole. At 610 and 1400 MHz, we reach noise levels of 15 and 6 μJy beam−1, respectively, with well‐matched resolutions (∼5 arcsec). At this depth, we obtained reliable detections for about half of the known sub‐mm galaxies (SMGs) in the field. For radio‐identified SMGs, which are typically at z∼ 2, we measure a mean radio spectral index of α1400610=−0.75 ± 0.06 (where Sν∝να) and standard deviation of 0.29, between approximate rest‐frame frequencies of 1.8 and 4.2 GHz. The slope of their continuum emission is indistinguishable from that of local star‐forming galaxies and suggests that extended optically thin synchrotron emission dominates the radio output of SMGs. Cooling effects by synchrotron emission and Inverse Compton scattering off the cosmic microwave background do not seem to affect their radio spectral energy distributions. For those SMGs judged by Spitzer mid‐infrared colours and spectroscopy to host obscured active galactic nuclei (AGN), we find a clear deviation from the rest of the sample – they typically have steeper radio spectral indices, α1400610≲−1.0. These findings suggest these mid‐IR‐/AGN‐selected SMGs may have an intrinsically different injection mechanism for relativistic particles, or they might reside in denser environments. This work provides a reliable spectral template for the estimation of far‐IR/radio photometric redshifts, and will enable accurate statistical K‐corrections for the large samples of SMGs expected with SCUBA‐2 and Herschel.

Hidden starbursts and active galactic nuclei at 0 < z < 4 from theHerschel-VVDS-CFHTLS-D1 field: Inferences on coevolution and feedback

“…An α of 0.68 was chosen as this is the median spectral index derived from those sources which were detected at >5σ in both the VLA and GMRT images. This value is also reasonably consistent with values used in other studies (e.g., Prandoni et al 2006;Hickox et al 2009;Smolčić 2009) and is consistent with values found in other submillimeter galaxy (SMG) studies (e.g., Ibar et al 2010). In practice, our choice of α used for the bulk k-correction makes little difference on our results.…”

“…However, more recent work has found that radio emission originating from an AGN shows a wide variety of spectral indices (e.g., Ho & Ulvestad 2001;Clemens et al 2008;Ibar et al 2010). Because there is imaging in the radio of the CFHTLS-D1 field in two bands, we initially attempted to select radio AGNs based on a spectral slope (α) criterion alone.…”

The far-infrared/radio correlation as probed byHerschel

“…Such AGN are more common at z ∼ 2 than today (e.g. Wall et al 2005); moreover, radio emission from faint starbursts (with α = −0.8, although see Ibar et al 2010) becomes more difficult to detect at higher redshifts, such that the fraction of radio-loud AGN in a flux-limited sample will rise, driving down q IR . Indeed, Fig.…”

GOODS-Herschel: radio-excess signature of hidden AGN activity in distant star-forming galaxies

“…We also extended the five SFG templates to the radio band with a power-law slope S ν ∝ ν −α , with α = 0.7 (e.g., Ibar et al 2009Ibar et al , 2010see Fig. 2).…”