Active Galactic Nuclei

Active Galactic Nuclei

Since arriving at Caltech in 1992, I have been heavily involved with two large Caltech--Jodrell Bank VLBI surveys that have now been completed (CJ1 -- Polatidis et al. 1994; CJ2 -- Taylor et al. 1994). Together with the Pearson--Readhead survey (1988) these provide ~1 mas resolution images for a flux limited sample of 321 sources. One of the most interesting findings of these surveys was the discovery of three compact symmetric objects (CSOs) and forty additional candidate CSOs in the nuclei of galaxies and quasars. These are compact (size ~100 pc) sources with emission on both sides of the central engine that is thought to be free of beaming effects (example: 1946+708, 2352+495) To account for their small sizes the CSOs must be either young or severely confined by a dense neutral medium. If these objects are young (ages ~3000 yrs) and growing at rates typical of equally luminous, but 1000 times larger, radio galaxies like Cygnus A then they must be a common phase in the evolution of galaxies, or perhaps a recurrent one. Alternatively, if the CSOs are strongly confined and longer lived then the large amount of material required for their confinement should have several observational consequences -- large amounts of atomic and molecular gas, high induced Faraday rotations, and possibly severe reddening and distortions of the starlight from the host galaxy.

In an effort to discriminate between the above models we have performed deep infrared imaging of a number of CSOs and CSO candidates using the 200'' telescope at Palomar. We have also made high-dynamic range radio continuum imaging with the VLA to look for large Faraday rotation measures, or for extended components that might be the result of a previous active phase (Taylor et al. 1995). We have also observed one nearby CSO candidate in CO 1-0 with the Owens Valley millimeter array to search for molecular gas.

In addition to studying the newly discovered CSOs and the morphological classification of AGN on the pc-scale, the CJ surveys have three cosmological goals: (1) to populate the proper motion--redshift diagram for superluminal sources; (2) to populate the size--redshift diagram for compact sources (both diagrams can be used to estimate the deceleration parameter, q$_0$); and (3) to search for small-separation gravitationally-lensed systems and hence to look directly for mass concentrations in the unexplored range of $10^6 - 10**9 solmass. Six such candidate systems have been found and are reported on by Wilkinson et al. (1994). All three cosmological goals of the CJ surveys require knowing the redshifts of the sources. We are finishing the first year of a highly successful campaign using the INT 1.5 m and the Palomar 5 m telescopes. More time has been granted at Palomar and we hope to have redshifts for over 90% of the objects in our sample by mid-1995. Second epoch Global VLBI observations of over 100 sources were taken in September of 1994 and should dramatically improve the proper motion-- redshift statistics.


Lees, J.F., Knapp, G.R., Rupen, M.P., & Phillips, T.G. 1991, ApJ, 379, 177.

Mazzarella, J.M., Graham, J.R., Sanders, D.B., & Djorgovski, S. 1993, ApJ, 409, 170

Pearson, T.J. & Readhead, A.C.S. 1988, ApJ, 328, 114

Polatidis, A.G., Wilkinson, P.N., Xu, W., Readhead, A.C.S., Pearson, T.J., Taylor, G.B., & Vermeulen, R.C. 1995 ApJS, in press.

Sanders, D.B., Scoville, N.Z., & Soifer, T.B. 1991, ApJ, 370, 158

Scoville, N.Z., Padin, S., Sanders, D.B., Soifer, B.T., & Yun, M.S. 1993, ApJ, 415, L75.

Taylor, G.B., Vermeulen, R.C., Pearson, T.J., Readhead, A.C.S., Henstock, D.R., Browne, I.W.A., & Wilkinson, P.N. 1994, ApJS 95, 345.

Wilkinson, P.N. et al. 1994, in {Compact Extragalactic Radio Sources}, eds. J.A. Zensus and K.I. Kellerman (NRAO), p. 259.