Astronomy Tea Talk

Why do only 10% of quasar produce powerful radio jets? The answer to this question is currently unknown but should involve fundamental properties of supermassive black holes (eg, spin or mass) as well as the evolution of their host galaxies. Modern radio surveys can provide a new opportunity to understand the origin of radio loudness, but we will have to use more information than just the radio flux at one frequency. We demonstrate the power of radio surveys with three examples. First, we show how a systematic search for radio transients can be used to compute the fraction of black holes that launch relativistic jets after disrupting and accreting a star. Second, we consider a recently constructed inventory of low-power radio galaxies (eg, Centaurus A and M87) in our local universe. These galaxies show clear evidence that the ~100 kpc scale environment is causally related to the production of powerful jets. In the third example we consider FRII radio galaxies (eg, Cygnus A). These can be used as a tracer of black hole activity throughout cosmic history as they are easy to identify in any radio survey with sufficient angular resolution. We use the FIRST survey (1.4 GHz) to obtain a homogeneous sample of hundred thousand FRIIs at z~1, by far the largest sample to date. We match these sources to optical quasars from SDSS and find a tight correlation between disk power and lobe radio luminosity. If a Blandford-Znajek governs the jet power of quasars, the small scatter in the disk-lobe correlation implies that either the range of spin of radio-loud quasars is extremely small (< 0.1 dex) or spin is not the main parameter for jet formation. From our three different methods to study active black holes (radio transients, low-z radio galaxies, and FRIIs) we conclude that a single parameter (eg, spin, halo mass, or accretion rate) may not be sufficient to explain the origin of radio loudness of supermassive black holes.
 

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