Radiogalaxies in the Sloan Digital Sky Survey: spectral index-environment correlations

CARLOS G.BORNANCINI; ANA LAURA O'MILL; SEBASTIÁN GUROVICH; DIEGO GARCÍIA LAMBAS

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2010

0035-8711

We analyze optical and radio properties of radiogalaxies detected inthe Sloan Digital Sky Survey (SDSS). The sample of radio sources areselected from the catalogue of Kimball & Ivezić (2008) with fluxdensities at 325, 1400 and 4850 MHz, using WENSS, NVSS and GB6 radiosurveys and from flux measurements at 74 MHz taken from VLALow-frequency Sky Survey (Cohen et al. 2006). We study radiogalaxy spectralproperties using radio colour-colour diagrams and find that our samplefollows a single power law from 74 to 4850 MHz. The spectral index vs. spectroscopic redshift relation (alpha-z) is not significant for our sample of radio sources.We analyze a subsample of radio sources associated with clusters ofgalaxies identified from the maxBCG catalogue and find that about 40%of radio sources with ultra steep spectra (USS, alpha<-1, are associated with galaxy clusters orgroups of galaxies.  We construct a Hubble diagram of USS radiosources in the optical r band up to z~0.8 and compare ourresults with those for normal galaxies selected from different opticalsurveys and find that USS radio sources are around as luminous asthe central galaxies in the maxBCG cluster sample and typically morethan 4 magnitudes brighter than normal galaxies at z~0.3.We study correlations between spectral index, richness and luminosityof clusters associated with radio sources. We find that USS at lowredshift are rare, most of them reside in regions of unusually high ambientdensity, such of those found in rich cluster of galaxies.  Our resultsalso suggest that clusters of galaxies associated with steeper than the average spectra have higher richness counts and are populated byluminous galaxies in comparison with those environments associated to radio sources with flatter than the average spectra. A plausible explanation for ourresults is that radio emission is more pressure confined in higher gasdensity environments such as those found in rich clusters of galaxiesand as a consequence radio lobes in rich galaxy clusters will expandadiabatically and lose energy via synchrotron and inverse Comptonlosses, resulting in a steeper radio spectra.