Elaine M. Sadler, Ronald D. Ekers, Elizabeth Mahony, Tom Mauch, Tara Murphy
We study a sample of 202 radio sources from the Australia Telescope 20 GHz (AT20G) survey which are identified with nearby galaxies from the 6dF Galaxy Survey (6dFGS). Our sample includes many of the youngest and most active radio galaxies in the local universe, and around 65% of the sample are candidate Compact Steep-Spectrum (CSS) and Gigahertz-Peaked Spectrum (GPS) radio sources. The AT20G-6dFGS galaxies have a median redshift of z=0.058 and span a wide range in radio luminosity. The local radio luminosity function (RLF) of galaxies at 20 GHz roughly matches the local 1.4 GHz RLF for radio-loud active galaxies if we make a simple shift in radio spectral index. While most of the AT20G-6dFGS galaxies are massive ellipticals, at least 30% of the radio sources in our sample are hosted by galaxies with WISE infrared colours characteristic of spiral galaxies with ongoing star formation. We see a strong dichotomy in the WISE colours of the host galaxies of FR-1 and FR-2 radio sources in our sample, with the FR-1 systems found almost exclusively in `WISE early-type' galaxies with [4.6]-[12] < 2.0 mag and the FR-2 radio galaxies in `WISE late-type' galaxies with [4.6]-[12] > 2.0 mag. This strongly suggests that some factor related to the host-galaxy morphology or large-scale environment helps to determine whether a young radio source evolves into an extended FR-1 radio galaxy or an FR-2 system. The host galaxies of flat- and steep-spectrum sources in our sample have a similar distribution in both galaxy stellar mass and WISE colours (though galaxies with flat-spectrum radio sources are more likely to show weak emission lines in their optical spectra). This is consistent with a picture in which these flat-spectrum and steep-spectrum radio sources represent different stages in radio-galaxy evolution, rather than beamed and unbeamed radio-source populations.
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http://arxiv.org/abs/1304.0268
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