科研之友
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摘要
We select a sample of radio galaxies at high redshifts (z greater than or similar to 1) in the COSMOS field by cross-matching optical and infrared (IR) images with the FIRST radio data. The aim of this study is to explore the high-z radio-loud (RL) active galactic nuclei (AGN) population at much lower luminosities than the classical samples of distant radio sources, which are similar to those of the local population of radio galaxies. Precisely, we extended a previous analysis focused on low-luminosity radio galaxies. The wide multiwavelength coverage provided by the COSMOS survey allows us to derive their spectral energy distributions (SEDs). We model them with our own developed technique 2SPD that includes old and young stellar populations and dust emission. When added to those previously selected, we obtain a sample of 74 RL AGN. The SED modeling returns several important quantities associated with the AGN and host properties. The resulting photometric redshifts range from z similar to 0.7 to 3. The sample mostly includes compact radio sources but also 21 FR IIs sources; the radio power distribution of the sample covers similar to 10(31.5)-10(34.3) erg s(-1) Hz(-1), thus straddling the local FR I/FR II break. The inferred range of stellar mass of the hosts is similar to 10(10)-10(11.5) M-circle dot. The SEDs are dominated by the contribution from an old stellar population with an age of similar to 1-3 Gyr for most of the sources. However, UV and mid-IR (MIR) excesses are observed for half of the sample. The dust luminosities inferred from the MIR excesses are in the range, L-dust similar to 10(43)-10(45.5) erg s(-1), which are associated with temperatures approximately of 350-1200 K. Estimates of the UV component yield values of similar to 10(41.5)-10(45.5) erg s(-1) at 2000 angstrom. The UV emission is significantly correlated with both IR and radio luminosities; the former being the stronger link. However, the origin of UV and dust emission, whether it is produced by the AGN of by star formation, is still unclear. Our results show that this RL AGN population at high redshifts displays a wide variety of properties. Low-power radio galaxies, which are associated with UV- and IR-faint hosts are generally similar to red massive galaxies of the local FR Is. At the opposite side of the radio luminosity distribution, large MIR and UV excesses are observed in objects consistent with quasar-like AGN, as also proved by their high dust temperatures, which are more similar to local FR IIs.
- 出版日期2014-7
