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Register a new userGalaxy And Mass Assembly (GAMA): The 325 MHz Radio Luminosity Function of AGN and Star Forming Galaxies
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Measurement of the evolution of both active galactic nuclei (AGN) and star-formation in galaxies underpins our understanding of galaxy evolution over cosmic time. Radio continuum observations can provide key information on these two processes, in particular via the mechanical feedback produced by radio jets in AGN, and via an unbiased dust-independent measurement of star-formation rates. In this paper we determine radio luminosity functions at 325 MHz for a sample of AGN and star-forming galaxies by matching a 138 deg sq. radio survey conducted with the Giant Metrewave Radio Telescope (GMRT), with optical imaging and redshifts from the Galaxy And Mass Assembly (GAMA) survey. We find that the radio luminosity function at 325 MHz for star-forming galaxies closely follows that measured at 1.4 GHz. By fitting the AGN radio luminosity function out to $z = 0.5$ as a double power law, and parametrizing the evolution as ${Phi} propto (1 + z)^{k}$ , we find evolution parameters of $k = 0.92 pm 0.95$ assuming pure density evolution and $k = 2.13 pm 1.96$ assuming pure luminosity evolution. We find that the Low Excitation Radio Galaxies are the dominant population in space density at lower luminosities. Comparing our 325 MHz observations with radio continuum imaging at 1.4 GHz, we determine separate radio luminosity functions for steep and flat-spectrum AGN, and show that the beamed population of flat-spectrum sources in our sample can be shifted in number density and luminosity to coincide with the unbeamed population of steep-spectrum sources, as is expected in the orientation based unification of AGN.
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The GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) is a radio continuum survey at 76-227 MHz of the entire southern sky (Declination $<+30deg$) with an angular resolution of $approx 2$ arcmin. In this paper, we combine GLEAM data with optical spectroscopy from the 6dF Galaxy Survey to construct a sample of 1,590 local (median $z approx 0.064$) radio sources with $S_{200,mathrm{MHz}} > 55$ mJy across an area of $approx 16,700~mathrm{deg}^{2}$. From the optical spectra, we identify the dominant physical process responsible for the radio emission from each galaxy: 73 per cent are fuelled by an active galactic nucleus (AGN) and 27 per cent by star formation. We present the local radio luminosity function for AGN and star-forming galaxies at 200 MHz and characterise the typical radio spectra of these two populations between 76 MHz and $sim 1$ GHz. For the AGN, the median spectral index between 200 MHz and $sim 1$ GHz, $alpha_{mathrm{high}}$, is $-0.600 pm 0.010$ (where $S propto u^{alpha}$) and the median spectral index within the GLEAM band, $alpha_{mathrm{low}}$, is $-0.704 pm 0.011$. For the star-forming galaxies, the median value of $alpha_{mathrm{high}}$ is $-0.650 pm 0.010$ and the median value of $alpha_{mathrm{low}}$ is $-0.596 pm 0.015$. Among the AGN population, flat-spectrum sources are more common at lower radio luminosity, suggesting the existence of a significant population of weak radio AGN that remain core-dominated even at low frequencies. However, around 4 per cent of local radio AGN have ultra-steep radio spectra at low frequencies ($alpha_{mathrm{low}} < -1.2$). These ultra-steep-spectrum sources span a wide range in radio luminosity, and further work is needed to clarify their nature.
We use 80922 galaxies in the Galaxy And Mass Assembly (GAMA) survey to measure the galaxy luminosity function (LF) in different environments over the redshift range 0.04<z<0.26. The depth and size of GAMA allows us to define samples split by colour and redshift to measure the dependence of the LF on environment, redshift and colour. We find that the LF varies smoothly with overdensity, consistent with previous results, with little environmental dependent evolution over the last 3 Gyrs. The modified GALFORM model predictions agree remarkably well with our LFs split by environment, particularly in the most overdense environments. The LFs predicted by the model for both blue and red galaxies are consistent with GAMA for the environments and luminosities at which such galaxies dominate. Discrepancies between the model and the data seen in the faint end of the LF suggest too many faint red galaxies are predicted, which is likely to be due to the over-quenching of satellite galaxies. The excess of bright blue galaxies predicted in underdense regions could be due to the implementation of AGN feedback not being sufficiently effective in the lower mass halos.
We present an analysis of the properties of the lowest Halpha-luminosity galaxies (L_Halpha<4x10^32 W; SFR<0.02 Msun/yr) in the Galaxy And Mass Assembly (GAMA) survey. These galaxies make up the the rise above a Schechter function in the number density of systems seen at the faint end of the Halpha luminosity function. Above our flux limit we find that these galaxies are principally composed of intrinsically low stellar mass systems (median stellar mass =2.5x10^8 Msun) with only 5/90 having stellar masses M>10^10 Msun. The low SFR systems are found to exist predominantly in the lowest density environments (median density ~0.02 galaxy Mpc^-2 with none in environments more dense than ~1.5 galaxy Mpc^-2). Their current specific star formation rates (SSFR; -8.5 < log(SSFR[yr^-1])<-12.) are consistent with their having had a variety of star formation histories. The low density environments of these galaxies demonstrates that such low-mass, star-forming systems can only remain as low-mass and forming stars if they reside sufficiently far from other galaxies to avoid being accreted, dispersed through tidal effects or having their gas reservoirs rendered ineffective through external processes.
We study the environments of low- and high- excitation radio galaxies (LERGs and HERGs respectively) in the redshift range $0.01 < z < 0.4$, using a sample of 399 radio galaxies and environmental measurements from the Galaxy And Mass Assembly (GAMA) survey. In our analysis we use the fifth nearest neighbour density ($Sigma_{5}$) and the GAMA galaxy groups catalogue (G3Cv6) and construct control samples of galaxies matched in {update stellar mass and colour} to the radio-detected sample. We find that LERGs and HERGs exist in different environments and that this difference is dependent on radio luminosity. High-luminosity LERGs ($L_{rm NVSS} gtrsim 10^{24}$ W Hz$^{-1}$) lie in much denser environments than a matched radio-quiet control sample (about three times as dense, as measured by $Sigma_{5}$), and are more likely to be members of galaxy groups ($82^{+5}_{-7}$ percent of LERGs are in GAMA groups, compared to $58^{+3}_{-3}$ percent of the control sample). In contrast, the environments of the HERGs and lower luminosity LERGs are indistinguishable from that of a matched control sample. Our results imply that high-luminosity LERGs lie in more massive haloes than non-radio galaxies of similar stellar mass and colour, in agreement with earlier studies (Wake et al. 2008; Donoso et al. 2010). When we control for the preference of LERGs to be found in groups, both high- and low- luminosity LERGs are found in higher-mass haloes ($sim 0.2$ dex; at least 97 percent significant) than the non-radio control sample.
How do galaxy properties (such as stellar mass, luminosity, star formation rate, and morphology) and their evolution depend on the mass of their host dark matter halo? Using the Galaxy and Mass Assembly (GAMA) group catalogue, we address this question by exploring the dependence on host halo mass of the luminosity function (LF) and stellar mass function (SMF) for grouped galaxies subdivided by colour, morphology and central/satellite. We find that spheroidal galaxies in particular dominate the bright and massive ends of the LF and SMF, respectively. More massive haloes host more massive and more luminous central galaxies. The satellite LF and SMF respectively show a systematic brightening of characteristic magnitude, and increase in characteristic mass, with increasing halo mass. In contrast to some previous results, the faint-end and low-mass slopes show little systematic dependence on halo mass. Semi-analytic models and simulations show similar or enhanced dependence of central mass and luminosity on halo mass. Faint and low-mass simulated satellite galaxies are remarkably independent of halo mass, but the most massive satellites are more common in more massive groups. In the first investigation of low-redshift LF and SMF evolution in group environments, we find that the red/blue ratio of galaxies in groups has increased since redshift $z approx 0.3$ relative to the field population. This observation strongly suggests that quenching of star formation in galaxies as they are accreted into galaxy groups is a significant and ongoing process.
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