Extragalactic Source Populations

Jim Condon (National Radio Astronomy Observatory)

Extragalactic Source Populations

Almost all discrete radio sources are extragalactic and so distant (median redshift <z> ~ 1) that the radio sky is extremely isotropic. Sources with log[L(W Hz−1)] > 23 at cm wavelengths are usually powered by AGNs in elliptical radio galaxies, Seyfert galaxies, and QSOs; star-forming spiral and irregular galaxies dominate at lower luminosities. Flux density reflects luminosity more than distance; fainter samples are not “deeper.” Most radio sources stronger than S 1 mJy are powered by AGNs. Galaxies obeying the far-infrared/radio correlation probably dominate below S ~ 0.1 mJy and provide extinction-free tracers of recent star formation. “Radio quiet” QSOs are rarely radio silent, with starbursts powering radio emission at the log[L(W Hz−1)] ~ 23 level.  About 96% of the sky background is contributed by sources stronger than S ~ 1 μJy. The median angular size of faint sources is only <Ø> ≤ 1", small enough to allow reliable identifications with LSST galaxies by position coincidence alone. Compact radio cores place supermassive black holes with milli-arcsec absolute accuracy within optical emission from host galaxies. Known Extragalactic slow radio transients and variables include SNe, microquasars, gamma-ray burst afterglows, and AGN outbursts caused by tidal disruption events.

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