Martin Haas, Scientist
  Adress: Astronomisches Institut
Ruhr-Universität Bochum
Universitätsstr. 150
Gebäude NA 7/173
D-44780 Bochum
Germany
  Telephone: +49 - (0)234-32-28673
  Fax: +49 - (0)234-32-14412
  Email: haas@astro.rub.de

 
Unified schemes

 
Unified schemes (Orr and Browne 1982, Barthel 1989) try to explain quasars and radio galaxies as intrinsically similar sources where the observed differences are due to aspect-angle effects: a quasar with flat radio spectrum is observed nearly pole-on so that a beamed jet with superluminal motion causes the bright radio and FIR appearance; a quasar with steep spectrum is seen at an intermediate angle between pole-on and edge-on;   a radio galaxy is viewed nearly edge-on, whereby the nucleus contains an ``optical quasar'' which is obscured behind a dust torus. Radio quiet quasars share the thermal properties of the radio loud ones, but the lack of magnetic fields near the central engine prohibits the development of a powerful jet which seems to be the origin of the strong synchroton emission.   A check of these schemes is offered via the FIR: If the FIR emission of the dust torus is optically thin and isotropic, then it should show up in the SEDs of all members of a unified class, irrespective of aspect angle.The SEDs reveal signatures of thermal dust as well as synchroton emission. In most cases one of the two components is so dominant that the other remains hidden.
 
  The SEDs of the radio quiet (e.g. PG 0050+124) and steep spectrum quasars (e.g. 3C48) show a bump around 60µm and a decline longward of 100µm. These spectra provide an impressive confirmation of the hypothesis that the SED of a typical steep spectrum and radio quiet quasar contains a strong far infrared bump due to thermal emission by dust. It can be described as a superposition of several modified blackbodies, showing the broad variety of temperatures from hot (600K) to cool (30K) dust present in these objects. Ultraluminous IR galaxies are generally cooler and provide much less IR power than quasars, this argues against the interpretation of every ULIG being a hidden quasar.   Unification between flat spectrum and steep spectrum quasars: While the FIR spectrum of the steep spectrum quasar 3C48 closely resembles that of other quasars in our sample, the FIR emission of the flat spectrum quasars (e.g. FR 0234+28) is dominated by a strong synchroton component, smoothly concatenating the mm flux to that observed in the MIR. In one flat spectrum radio quasar we might see thermal and synchroton emission with similar FIR strength: for the source 3C279 a bump around 20-100µm is prying above the synchroton spectrum interpolated between the mm and NIR range. The upper limits for thermal emission are such that the dust emission of any but the brightest quasars could lie underneath the dominating synchroton spectra.   Therefore Doppler boosted synchroton emission of a relativistic jet seen almost end-on is consistent with our observed FIR spectra of flat spectrum quasars and does not contradict this unification.
Unification between strong radio galaxies and radio loud quasars: The detection of a strong thermal component in Cyg A similar to 3C48 is evidence of a hidden quasar in this key radio galaxy.


Further reading:
Haas M., Chini R., Meisenheimer K., et al., 1998, ApJ Letters 503, L109
Haas M., Müller S., Chini R., et al., 2000, A&A 453, 354
Meisenheimer K., Haas M., Müller S., et al., 2001, A&A 372, 719