Optical and Radio Properties of Extragalactic Sources Observed by the FIRST Survey and the Sloan Digital Sky Survey

Abstract

We discuss the optical and radio properties of ∼30,000 FIRST (radio, 20 cm, sensitive to 1 mJy) sources positionally associated within 1.5 arcsec with an SDSS (optical, sensitive to r * ∼22.2) source in 1230 deg 2 of sky. The matched sample represents ∼30% of the 108,000 FIRST sources and 0.1% of the 2.5 × 10 7 SDSS sources in the studied region. SDSS spectra are available for 4,300 galaxies and 1,154 quasars from the matched sample, and for a control sample of 140,000 galaxies and 20,000 quasars in 1030 deg 2 of sky. This large and unbiased catalog of optical identifications provides much firmer statistical footing for existing results and allows several new findings.The majority (83%) of the FIRST sources identified with an SDSS source brighter than r * =21 are optically resolved; the fraction of resolved objects among the matched sources is a function of the radio flux, increasing from ∼50% at the bright end to ∼90% at the FIRST faint limit. Nearly all optically unresolved radio sources have non-stellar colors indicative of quasars. We estimate an upper limit of ∼5% for the fraction of quasars with broad-band optical colors indistinguishable from those of stars. The distribution of quasars in the radio flux -optical flux plane supports the existence of the "quasar radio-dichotomy"; 8±1% of all quasars with i * <18.5 are radio-loud and this fraction seems independent of redshift and optical luminosity. The radio-loud quasars have a redder median color by 0.08±0.02 mag, and show a 3 times larger fraction of objects with extremely red colors.FIRST galaxies represent 5% of all SDSS galaxies with r * <17.5, and 1% for r * <20, and are dominated by red (u * − r * >2.22) galaxies, especially those with r * >17.5. Magnitude and redshift limited samples show that radio galaxies have a different optical luminosity distribution than non-radio galaxies selected by the same criteria; when galaxies are further separated by their colors, this result remains valid for both blue and red galaxies. For a given optical luminosity and redshift, the observed optical colors of radio-galaxies are indistinguishable from those of all SDSS galaxies selected by identical criteria. The distributions of radio-to-optical flux ratio are similar for blue and red galaxies in redshift-limited samples; this similarity implies that the difference in their luminosity functions, and resulting selection effects, are the dominant cause for the preponderance of red radio galaxies in flux-limited samples. The fraction of radio galaxies whose emission line ratios indicate an AGN (30%) rather than a starburst origin is 6 times larger than the corresponding fraction for all SDSS galaxies (r * <17.5). We confirm that the AGN-to-starburst galaxy number ratio increases with radio flux, and find that radio emission from AGNs is more concentrated than radio emission from starburst galaxies.

Determining the fraction of reddened quasars in COSMOS with multiple selection techniques from X-ray to radio wavelengths

“…By definition, only the radio-loud quasars will be targeted by this approach, so that the sample in itself is not complete (see, e.g., Urry & Padovani 1995;Sikora et al 2007, for a description of the radio emission from quasars and the empirical observations of the two sub-populations, respectively). Two recent radio selections of optical spatially unresolved quasars (Ivezić et al 2002;Baloković et al 2012) used data from the Faint Images of the Radio Sky at Twenty centimeters (FIRST; Becker et al 1995) mission together with the SDSS program. A similar approach was executed based on source detection with the FIRST matched to the NIR counterparts of the Two Micron All Sky Survey (2MASS; Skrutskie et al 2006) by Glikman et al (2007Glikman et al ( , 2012; Urrutia et al (2009).…”

Classifying radio emitters from the Sloan Digital Sky Survey

“…As an observational proof of this scenario, Ivezić et al (2002) have shown that a sample of optically unresolved radio sources from the FIRST survey has bluer colors than do other SDSS objects, and Richards et al (2002) find that the SDSS quasar candidates -which are likely to have a radio counterpart -display blue colors, especially at z > 1, indicative of ongoing star formation. To understand the importance of the AGN-feedback in the current scenarios of galaxy evolution, a study of the AGN populations, the properties of the host galaxies, and the physical mechanisms that trigger the production of the emission lines or the radio-activity, is necessary.…”

“…Obric et al (2006) find strong correlations between the fraction of detected AGNs at other wavelengths and optical properties such as flux, colors, and emission-line strengths. Ivezić et al (2002) discuss the optical and radio properties of ∼30 000 FIRST (Becker et al 1995) sources positionally associated with a SDSS source by analyzing their colors, while Best et al (2005) compare the optical survey to both FIRST and NVSS radio surveys in order to derive the local radio luminosity functions of radioloud AGNs and star-forming galaxies. In the SDSS Early Data Release (Ivezić et al 2002), ∼70% of FIRST sources do not have an optical counterpart within 3 .…”

Probing the radio loud/quiet AGN dichotomy with quasar clustering

“…This division into RLQs and RQQs still remains a point of discussion. Some authors advocate the idea that radio-loudness (R, radio-to-optical flux ratio) distribution for optical-selected quasars is bimodal (Kellermann et al 1989;Miller et al 1990;Ivezić et al 2002;Jiang et al 2007a), while others have confirmed a very broad range for the radioloudness parameter, questioning its bimodality nature (Cirasuolo et al 2003;Singal et al 2011Singal et al , 2013). …”