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Much of our understanding of the physics of AGN

  1. Jul 12, 2003 #1


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    Bill Keel posted this note to sci.astro on Nov 4, 1996. I've edited it slightly. MWR

    Having just lost most of my carefully written post on this (blast it, I hate
    hitting the caps lock key at the wring time), I'll just throw in the
    relevant bit of TeX notes from last time I taught my grad class on
    galaxies - this summarizes the main arguments with references:

    Bill Keel
    Astronomy, University of Alabama

    Much of our understanding of the physics of AGN depends on knowing their absolute properties (luminosities, size scales) and thus their distances. There is a small but vocal school which claims that much of the redshift of QSOs (at least) arises not in the Hubble flow but in exotic physical processes, and thus that redshift distances to (some?) QSOs are nonsense. This point of view has been defended in Arp's book ( Quasars, Redshifts, and Controversies, Interstellar Media, Berkeley), with some of his best cases.

    The observational suspicion that some AGN might be at noncosmological distances seems to have first arisen when Arp (1967 ApJ 146, 321) noted an association between several low-redshift peculiar galaxies and quasars in a rough pairing sense, with pairs of QSOs on each side of the galaxy. Specific two-color searches led to identification of numerous QSOs in te fields of nearby galaxies (see, for example, Arp 1981 ApJ 250, 31 and references therein).

    There has been much fruitless discussion of what might appear a straightforward statistical problem - are there or are there not excess QSOs in the directions of bright galaxies? The difficulties lie in the fact that QSO searches are still quite inhomogeneous over the sky, and thus a search may be deep enough to tell us something but cover too little solid angle, or cover the whole sky with too few QSOs. For example, there are four close galaxy-QSO pairs in the 3C catalog (Burbidge, Burbidge, Solomon, and Strittmatter 1971 ApJ 170, 233). But with only about 100 quasars over half the sky, the statistics are too sparse to do more. Perhaps large-scale automated surveys will be able to resolve this. The methodology Arp has frequently adopted doesn't help - starting from a galaxy and searching outward until a quasar shows up, then if it's ``interestingly" close keep on going outward. This is guaranteed to produce an apparent excess, on the ``seek and ye shall find" principle. A final problem with a statistical analysis is that it is not always clear what it is whose likelihood we want to assess. Some papers talk about QSO-galaxy pairs, some about QSO pairs with discordant redshift, lines of quasars... Statistics after the fact has a bad reputation.

    Any of the above phenomena would require explanation through some sort of new physics, the sort that people get Nobel Prizes for working out. Some of the original impetus for noncosmological redshifts arose, oddly enough, from conventional physics - the ``synchrotron catastrophe", in which quasar luminosities would be too high to sustain against their own synchrotron self-absorption. However, Seyfert galaxies know how to do this perfectly well at smaller and better-determined distances, so this seems to be our problem and not the universe's. Furthermore, people such as Hoyle who found a staedy-state universe appealing on philosophical grounds needed some other avenue to make objects that appear at first glance to show cosmological evolution. What do we require of any mechanism that can mimic Doppler shifts? It must

    conserve wavelength ratios
    preserve basic emission- and absorption-line physics\cr
    not need to conserve energy\cr
    act as a sort of screen over whole galaxies\cr
    give systematic redshifts but not blueshifts\cr
    Some tentative explanations for various pieces have appeared. We need not require a complete theoretical framework to establish an empirical effect, but little is to be gained by jumping up and shouting ``Oh no it's not" to every aspect of established theory without some new scheme. Arp and Hoyle have discussed ideas involving creation of mass and a finite sphere of graviton exchange, perhaps producing a homogeneous microwave background while they're at it. Some ideas involving backwards beaming from moving quasars have also been discussed to avoid blueshifts.
    In his book, Arp sets out an evolutionary scheme that he finds acceptable from his interpretation. Objects are ejected from galactic nuclei, possible at very high velocities, with initially large density, high temperature, and large redshifts (quasars and BL Lac objects). As they age, stars appear starting with early-type ones and the redshift decreases. Finally, extended halos or spiral features appear, and the noncosmological redshift nearly vanishes. This gives sort of a fireworks-display view of galactic history. Most QSOs then are not very large or bright - more like the brightest supergiants than galaxy-hiding monstrosities.

    So what are the arguments directly favoring conventional cosmological distances for quasars? We may examine associated and host galaxies, gravitational lenses, and absorption-line systems.

    Galaxies are known to be associated with low-redshift QSOs both around the QSO and nearby (see p. 108). It seems too much to ask that whole groups of galaxies can share the same disease and exactly mimic distance, or that there be two populations of QSOs so contrived as to have no observable distances but be of vastly different luminosity. Oddly enough, the resolved fuzz around high-redshift QSOs recently reported by Heckman et al (ApJ in press) doesn't strengthen this argument - the $(1+z)^4$ dimming in surface brightness makes normal galaxies unobservable at large redshifts, so these must be something that is peculiar by any standard. We are slowly learning that a QSO host galaxy need not look exactly like a quiescent counterpart (as in 3C 48). The broad relation between host galaxy magnitide and redshift may be construed as suggesting that the galaxies have distances related to redshift, and many are certainly galaxies containing stars as shown by direct spectroscopy.
    Gravitational lensing will work only if the lens and QSO are at approximately their Hubble-law distances; this argument has been set out explicitly by Dar 1991 (ApJLett 382, L1). At the least, the QSO must be beyond the lens galaxy, which already has redshifts of order 0.5. Again, one must invoke quite a coincidence otherwise. Huchra has admitted orally that his first thought on discovering the Einstein cross was the chilling thought that Arp might have been right all these years.
    Absorption-line systems again require that the QSO be beyond all the absorbing material unless all he intervening material has noncosmological redshifts as well. In this case, a strong coincidence is needed to make the redshift distributions of various kinds of absorber make any sense at all in a conventional model.
    Last edited by a moderator: Feb 5, 2013
  2. jcsd
  3. Jul 13, 2003 #2
    Hoyle said somewhere that we badly need to resolve the questions of the exact physical natures of QSOs and AGNs before any real progress can be made.

    He also said (again--somewhere) the desires of high energy (particle) physicists for some kind of super-laboratory for mulling their favorite theories do not comprise sufficient reason for avoiding the exploration of alternatives to the prevailing totally-cosmological interpretation of red shifts.

    It sounds good to me.

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