Robert Antonucci Research Summary
I. Geometrical Unified Models and Accretion Modes for Radio Galaxies and Quasars
Radio Galaxies were discovered in the 1950s, and they generally comprise fairly ordinary elliptical galaxies in optical images, but giant (10^6 light-year) dumb-bells in radio images. Quasars came along in the 1960s, and are mostly similar, but with the very important addition of extremely powerful tiny (<1 light-year) optical/UV sources in the exact centers.
By the early 1980s. the best wisdom was that quasars are accreting black holes, with the brilliant optical/UV light being thermal emission from optically thick accreting matter. The radio galaxies seemed to lack the accreting matter, so were by default assumed to be non-accreting, and powered by extraction of rotational energy from spinning black holes perhaps left over from previous quasar-like stages. Their energy output seemed to be restricted to kinetic energy funneled through "jets" powering the dumb-bells.
My group discovered about that time that many of the most powerful radio galaxies (and the analogous "radio-quiet active galactic nuclei") in fact contain hidden quasars which we could detect in light reflected off scattering gas clouds via spectro-polarimetric observations. In fact the quasars and hidden quasars are surrounded by opaque dusty gas tori, with the visibility of the accretion flow depending on viewing orientation. A cartoon indicating this geometry for the radio-loud case is attached.
There was another twist when various authors pointed out that this all works out very well among very luminous objects but that statistical arguments exist (both ways) for a subpopulation of radio galaxies at lower luminosities which don't participate in this "unified model," and thus may lack hidden quasars.
In the case for this we may be back to the original picture of non-accretion- powered radio sources for this subset. The distinction is extremely important theoretically because it's fundamental to determine whether some black holes are powered by rotation rather than accretion, and whether some produce just kinetic energy (in the radio jets producing the radio dumb-bells) without the thermal optical/ UV emission. This was roughly the situation when I wrote a long review article (Ann Rev Astron Astrophys 31, 473, 1993). (A quasi interview about that article can be found at http://www.in-cites.com/papers/DrRobertAntonucci.html).
The polarized light trick requires a fortuitous placed cloud of scattering gas, so UCSB student Dave Whysong and I have pursued an alternative and more robust method of detecting hidden quasars in radio galaxies that just became possible with high angular resolution with the Keck telescopes: using mid-IR imaging to constrain the reradiation of the optical/UV light absorbed from any hidden quasars by the putative dusty toroidal gas configurations. the results of this ongoing project have been very enlightening and have lead to some conference papers and one recently published journal paper: we find that the radio galaxies with small projected linear sizes in fact tend to lack hidden quasars and thus very likely represent true nonthermal, non-accretion-powered, pure kinetic-energy emitting black holes!
We were recently granted substantial access to the fabulous fourth Great Observatory, the Spitzer infrared telescope which was launched last year. This is basically the franchise on the canonical "3C" extragalactic radio source list. The data will be provide ultrasensitive (revolutionary) spectra of all ~50 of our targets.
II. Whysong and I have also worked closely with the Adaptive Optics team creating this groundbreaking and extremely valuable instrumentation for Lick and Keck observatories; the leader, Claire Max, works at Livermore and Santa Cruz. Adaptive optics measures in real time the wavefront shape from a bright star, real or created with a laser beam in Earth's upper atmosphere. It uses this information to correct at millisec response times the shape of a "rubber mirror" element in the optical path with many pistons working simultaneously, and the results are awesome for near-IR work (but poor for visible work). In the near-IR, it's much sharper than the Hubble Space Telescope. Our first published paper, Canalizo et al (Astrophysical Journal 597, 823, 1993, has the best A. O. images I've even seen. They reveal the very central regions of the nearby powerful radio galaxy Cygnus A, with the cones of scattered light showing that this radio galaxy does have a hidden quasar as deduced by Ogle et al from spectropolarimetry.
III. We (former postdoc Makoto Kishimoto - now at Edinburgh
Observatory, Omer Blaes, and Catherine Boisson - Paris Observatory) have use
spectropolarimetry at Keck and the European Very Large Telescope to exploit
scattering gas on scales 5 orders of magnitude close to the black hole than that
discussed above.
This allows us to pick off light probably on scales of
<100 Schwarzchild radii, free from the surrounding and intensely problematic
atomic emission that renders the observed total-flux continuum virtually
uninterpretable. We are the only group doing this. For the first time, we have
shown that the intrinsic emission has an optically thick thermal nature
(although the theorist on the team, Omer, is still doing detailed modeling), a
decisive confirmation of the thermal accretion process which couldn't be
obtained in any other way.
IV. The journal paper recently published by Gaskell,
Goosman, Antonucci, and Whysong, Astrophysical Journal, in press) is radical and
must be widely applicable and important, or else wrong. For decades theorists
have been fitting quasar spectra to accretion models, for example to derive
accretion
rates and black hole masses. We believe that most of the observed
spectra are
qualitatively affected by wavelength-selective absorption by dust
in the very nuclear regions of the host galaxies. I had the pleasure of being
wrong on
this: I'd written an argument against the idea in a conference
review based on the idea that in such cases broadband spectra should show
continuous curvature except in the "unlikely" case of a lack of small grains. I
now believe we've shown that that's exactly what's going on. This affects quasar
modeling greatly, and also studies of quasar demographics, e.g. comoving black
hole densities in the Universe.
V. We are doing several more projects including high-redshift masers, searches for ultrahigh redshift molecules (blind searches on cluster lensing caustics as well as the recently discovered redshift 10 galaxy), very long baseline interferometry of radio quiet quasars, X-ray spectroscopy (P Ogle, PI), etc,
Primordial nucleosynthesis requires a baryon density of 0.05 times the critical density, and this is thought to reside in the Inter-Galactic Medium (IGM). Two reasons for this conclusion are 1) lack of alternatives, and 1) favorable results from modeling the ~10^-5 neutral gas fraction actually observable via absorption lines. It'd be of great interest to measure the dominant ionized component directly however, and the only way I know how to do this is to try to detect the halos of Thomson-scattered radio waves which must surround high-redshift radio galaxies. (The high redshift is needed to get an Inter-Galactic Medium of detectable density.) This would provide another precious cosmological observable.
The project is extremely difficult and in our first attempt we measured the
density of the intergalactic medium as -0.1+/-0.3! This was published as R
Geller et al, Astrophysical Journal 539, 73. The amazing thing is that I was
happy with the result. I knew we were on a long learning curve, and as often
happens in my research, we were and are the only group in the world doing
it.
That observation was done with the Australia Telescope Compact Array,
which has some technical advantages, but we've more recently gotten a very large
data set with the Very Large Array telescope in New Mexico. We are now analyzing
this data - a most non-trivial task. Dave Whysong is leading this effort. We are
entirely limited by systematics, so we could in principle get down to really
interesting levels.
The "Allen (radio) Telescope Array" is currently being built mainly to search for extraterrestrials. It turns out to be phenomenally appropriate for our purpose. We could detect the IGM, map it as a function of redshift, do "tomography" at a given redshift looking of the "Swiss-cheese" structure, use the sizes of the halos to determine radio galaxies' ages, use the azimuthal dependence of the scattering halos to test models of radio jets that necessarily imply special-relativity headlight effects, and do other new science. -----
We're often in the pleasant position of being the only group in the world pursuing some potentially attractive research path. However it should be noted that in this small space I have not done ANY justice to other workers' contributions to all this!
Picture of a unified Model of an active galactic nucleus