Abstract
X-ray emission from AGN is dominated by the accretion disk around a SMBH. The radio luminosity, however, has not such a clear origin except in the most powerful sources where jets are evident. The origin (and even the very existence) of the local bi-modal distribution in radioloudness is also a debated issue. By analysing X-ray, optical and radio properties of a large sample of type 1 AGN up to z>2, where the bulk of this population resides, we aim to explore the interplay between radio and X-ray emission in AGN, in order to further our knowledge on the origin of radio emission, and its relation to accretion. We analyse a large (~800 sources) sample of type 1 AGN and QSOs selected from the 2XMMi X-ray source catalogue, cross-correlated with the SDSS DR7 spectroscopic catalogue, covering a redshift range from z~0.3 to z~2.3. SMBH masses are estimated from the Mg II emission line, bolometric luminosities from the X-ray data, and radio emission or upper limits from the FIRST catalogue. Most of the sources accrete close to the Eddington limit and the distribution in radioloudness does not appear to have a bi-modal behaviour. We confirm that radioloud AGN are also X-ray loud, with an X-ray-to-optical ratio up to twice that of radioquiet objects, even excluding the most extreme strongly jetted sources. By analysing complementary radio-selected control samples, we find evidence that these conclusions are not an effect of the X-ray selection, but are likely a property of the dominant QSO population. Our findings are best interpreted in a context where radio emission in AGN, with the exception of a minority of beamed sources, arises from very close to the accretion disk and is therefore heavily linked to X-ray emission. We also speculate that the RL/RQ dichotomy might either be an evolutionary effect that developed well after the QSO peak epoch, or an effect of incompleteness in small samples.
Highlights
Strong X-ray and radio emissions are properties that distinguish active galactic nuclei (AGN) from the whole population of galaxies
Radio emission is most apparent in a fraction of AGN, in particular in those classified as “radio-loud” (RL), which constitute 10−20% of the local AGN population, recent work shows that even radioquiet (RQ) AGN exhibit a radio-emitting core, which might result from some sort of radio plasma arising in the vicinity of the supermassive black hole (SMBH)
While optical emission in AGN is due to the superposition of thermal components coming from different distances from the nucleus, with a contribution of radiation reprocessed outside the AGN central engine, both X-rays and radio emission can be used to probe the immediate environment of the SMBH
Summary
Strong X-ray and radio emissions are properties that distinguish active galactic nuclei (AGN) from the whole population of galaxies. The release of large catalogues of fairly deep X-ray and radio sources, along with the optical photometry and spectroscopy provided by the Sloan Digital Sky Surveys (SDSS; see Abazajian et al 2009, for the final public data release from SDSS-II), calls for a study of the relation between accretion and radio properties in large samples of AGN. This is of particular interest to infer the physical origin of the radio emission.
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