Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Quasar Redshift

  1. Apr 24, 2007 #1
    Does anyone know how much percentage of the total redshift of a quasar is contributed by Gravitational redshift considering light from the Quasar is emitted by heavy active galactic nuclei ?

    If there is a significant contribution from gravitational redshift then how will the Hubbles law hold true for calculating distance of a Quasar based on redshift ? Are we removing Gravitational redshift contribution before calculating its distance ?

    Pls explain.
     
    Last edited: Apr 24, 2007
  2. jcsd
  3. Apr 24, 2007 #2

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    In the spectra of quasars - no matter what waveband - the gravitational redshift component is effectively zero; it's certainly smaller than the wavelength resolution of any spectroscope that I know of that's been used to obtain quasar spectra.

    It's pretty easy to see why this is so: the EM we see comes from regions far enough from the SMBH at the heart of a quasar that redshift due to its depth in the SMBH's gravitational well is negligible ... if it didn't (come from such regions), then at least the line profiles would be quite different than what we actually observe.

    AFAIK (as far as I know), the only gravitational redshifts, due to SMBH, that have been observed are some x-ray line profiles, and those are from normal, nearby galaxies (gravitational redshift due to SgrA* may also have been observed).
     
  4. Apr 25, 2007 #3

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    As Nereid noted, radiation emitted near enough to a black hole to be noticeably gravitationally redshifted would be swamped by radiation emitted by infalling particles too distant from the blackhole to be gravitationally redshifted. Visualize the black hole as an intensely hot speck of matter surrounded by a diffuse cloud of matter. Most of the matter would 'burn up' far from the hot spark at the center. Near the center, mostly ashes would remain with very little mass left to convert to energy. It would be carried off in the form of extremely energetic photons, meaning far fewer photons are needed to carry off the same amount of mass as reactions occurring at distances where gravitational redshift is negligible. A candle flame is a fair analogy. The center of the flame is hotter, yet far dimmer than the plasma envelope.
     
  5. Apr 26, 2007 #4
    Thanks for explaining. If the entire redshift is attributed to its distance then why time dilation has not been observed similar to 1a supernova ? Is there something fundamental with Qusars that we didnt understand ?
     
  6. Apr 26, 2007 #5

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    How would you know you were observing time dilation in the light curve of a quasar? Or even in the light curves of a million quasars?

    Perhaps one way to answer this is by comparing quasars with Type1a SNe; how do we observe time dilation in such?

    By comparing an observed characteristic time with the 'rest frame' equivalent. What 'characteristic time'? Let's call it the decay of a radioisotope of iron* - we know it's x days, here on Earth; we observe it to be y days, in a particular Type 1a SNe ... we attribute the difference to time dilation.

    Is there something similar in quasars? Unfortunately, AFAIK, no.

    For a start, quasars are, to all intents and purposes, point sources. However, their spectra tell us that the light (EM in general) comes from at least four different physical regimes - an accretion disk, jets, a broad line region, a narrow line region, and (for some wavebands) a dusty torus. On top of that, we expect at least a component of the variability of quasars to be due to microlensing, of the accretion disk (say) by stars in the galaxy surrounding that disk. How to tease apart an apparent magnitude vs time curve into intrinsic variability of any particular component?

    It gets worse.

    We also know that quasars evolve - not only were there more of them a few billion years ago, but their physical characteristics seem to have changed, over (cosmological) time. How to attribute any observed variability to the parts that are due to the particular evolutionary stage a quasar is at vs that due to time dilation?

    (There's more, but that will do for now).

    *it doesn't matter what it actually is, for the purposes of my explanation here, just so long as we have a high degree of confidence that there is an unambiguous characteristic time signature in the light curve.
     
  7. Apr 26, 2007 #6

    Wallace

    User Avatar
    Science Advisor

    We can actually see evidence of gravitational redshift in QSO's. As pointed out above, the vast majority of the redshift is cosmological, however the gravitational redshifted in imprinted onto the shape of some of the emission lines of QSO's. There are lines that broadened, due to some material emitting radiation very close to the Black Hole having a greater gravitational shift than material further out.

    This broadening is in addition to other broadening such as thermal effects, but theoretical models can piece the two apart, plus the broadening is not symmetric so has a tell tale shape.

    So the position of various emission lines in a QSO in determine almost entirely by the cosmological redshift but we can see the evidence of gravitational redshift in the shape of some of those lines.
     
  8. Apr 27, 2007 #7

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Some details.

    Wallace, do you have references to any papers on analyses of quasar line profiles (other than in nearby AGNs such as Seyferts), showing gravitational redshifting?
     
  9. Apr 27, 2007 #8

    Wallace

    User Avatar
    Science Advisor

    Errm doesn't your link show some nice spectra doing just that?

    The confusion may lie in my sloppy use of terminology. I've never really grasped the difference between QSO's, Quasars and AGNs and use the terms interchangeably, though I'm sure this is not correct. Is this effect only seen in AGNs but not the others? By my understanding of these things I can't see why it would be only the AGNs but as I say, I've never understood the rationale for the difference boxes we like to put these things into.
     
  10. Apr 27, 2007 #9

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You're right - if you consider Seyferts and quasars (and ...) as AGNs, then case closed.

    However, historically, and still to some extent observationally, quasars and Seyferts are quite distinct classes of objects (and even today, SDSS, for example, makes a clear distinction between type 2 quasars - where the torus hides the accretion disk - and 'normal' quasars).

    I suspect that the term 'quasars' is still quite widely understood to have its historical meaning of 'quasi-stellar object' (i.e. no resolved galaxy that it sits at/as the nucleus of). I took it that the 'quasar' talksabcd meant, in the OP, was this historical meaning.
     
  11. Apr 28, 2007 #10

    Garth

    User Avatar
    Science Advisor
    Gold Member

    As an 'old hand' I always understood
    QSO to mean 'Quasi-Stellar Object' and
    Quasar to mean 'Quasi-Stellar Radio Object'.

    When they were first discovered in the 1960's the QSO's were recognised as being point like objects with large red shift and Quasar's as being the same but with radio lobes. Often with a quasar it was the radio emission that was the first to be observed and noticed.

    AGN's were observed as extended galactic objects that had high emissions in other parts of the spectrum from their nucleii.

    Of course both quasars and QSOs are now known to be embedded in galaxies, but that came later.

    Garth
     
  12. Apr 28, 2007 #11

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Given the importance of SDSS to studies of quasars, it may be of interest to quote what SDSS DR5 has to say about "quasar candidates":
    Similarly, somewhat older, is 2dF's 2QZ ("The 2dF QSO Redshift Survey"):
    So, somewhere between the 2QZ and SDSS DR5, "QSOs" became "quasars"! :surprised
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Quasar Redshift
  1. Quasar feedback (Replies: 2)

  2. Quasars and Blazars (Replies: 2)

  3. Quasar images (Replies: 6)

  4. Redshift (Replies: 1)

Loading...