3 Responses to “Why are QSO radio fluxes almost redshift independent? Holmqvist”

1. admin says:

   July 7 at 4:33 am

   In article <mt2.0-9039-1047901…@star.bris.ac.uk>,
   

    rh4170…@juno.com (rhmd) writes:
   > If you take any catalog containing QSOs (Veron-Cetty Veron, Bauer,
   > etc.) and plot optical magnitude or x-ray flux vs redshift you get the
   > usual sort of magnitude/redshift relationship.  However, radio flux vs
   > redshift doesn’t produce the same type of curve: radio flux decreases
   > much less with redshift.

   The "negative K-correction" is probably the biggest effect.
   Non-thermal radio flux densities rise towards lower frequency.  If
   you observe at a given frequency, the radiation you detect was
   emitted at a lower frequency the larger the redshift was.  For
   example, if you observe at 20 cm, for a z=1 object you are detecting
   radiation emitted at 40 cm, whereas for a z=3 object, the radiation
   was originally emitted at 80 cm, where the emission was greater.

   Another effect is that the QSO population was, on average, more
   luminous in the past, but this affects optical and X-ray fluxes as
   well.

   The negative K-correction is especially important at submillimeter
   wavelengths, where for high redshift objects, the rest wavelengths
   may be between 20 and 200 microns.  Dust emission gives very rapid
   falloff with frequency in this range.

   –
   Steve Willner            Phone 617-495-7123     swill…@cfa.harvard.edu
   Cambridge, MA 02138 USA                
   (Please email your reply if you want to be sure I see it; include a
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2. admin says:

   July 7 at 4:33 am

   In article <mt2.0-29315-1048067…@star.bris.ac.uk>, I wrote some
   complete nonsense, as was kindly pointed out to me by email.
   (Thanks!)  Apologies to all readers for that earlier post.  Maybe
   this one will clear things up.
   

   > In article <mt2.0-9039-1047901…@star.bris.ac.uk>,
   >  rh4170…@juno.com (rhmd) writes:
   > > If you take any catalog containing QSOs (Veron-Cetty Veron, Bauer,
   > > etc.) and plot optical magnitude or x-ray flux vs redshift you get the
   > > usual sort of magnitude/redshift relationship.  However, radio flux vs
   > > redshift doesn’t produce the same type of curve: radio flux decreases
   > > much less with redshift.

   > The "negative K-correction" is probably the biggest effect.

   I should have stopped here!  Although "negative" is not really right
   for ordinary radio observations.  Let’s start from the beginning.

   Suppose we observe in visible light, 500 nm wavelength.  If an object
   has redshift z=1, we are seeing light that was emitted at 250 nm in
   the object’s rest frame.  For a z=3 object, the light was emitted at
   125 nm.  Most objects have emission that falls very steeply between
   250 and 125 nm, so objects at z=3 will tend to be quite a lot fainter
   than objects at z=1.  One way of expressing this is to say the
   "K-correction" is large for visible light and redshift range 1 to 3.

   Now suppose we observe in the radio at 20 cm wavelength.  The radio
   waves from our z=1 object started out at 10 cm, whereas for the z=3
   object, the waves started at 5 cm.  It’s true that for typical
   objects the emitted flux density decreases from 10 to 5 cm but only
   by something less than a factor of 2.  So z=3 radio sources will tend
   to be fainter than z=1 radio sources but not by nearly so much as in
   visible light.  One may say the K-correction for radio sources in
   this redshift range is positive but is much smaller than for visible
   light observations.  I _think_ source evolution may also be stronger
   in radio emission than in the visible, but I’m not sure of that or
   which effect is more important.

   Now let’s consider submillimeter observations, 800 microns.  Emission
   comes from rest 400 microns at z=1, rest 200 microns at z=3.  In
   contrast to other wavelengths, intrinsic flux density for typical
   sources _rises_ from 400 to 200 microns because dust emission peaks
   around 200 microns or so.  The rise is quite steep, in fact.  Thus
   the submillimeter K-correction for redshifts 1 to 3 is negative and
   is quite large in magnitude.

   I hope this sets things right, but if a real expert wanted to put in
   further corrections (or a better explanation altogether), I wouldn’t
   object.  Again, sorry for the original nonsense.

   –
   Steve Willner            Phone 617-495-7123     swill…@cfa.harvard.edu
   Cambridge, MA 02138 USA                
   (Please email your reply if you want to be sure I see it; include a
   valid Reply-To address to receive an acknowledgement.  Commercial
   email may be sent to your ISP.)
3. admin says:

   July 7 at 4:33 am

   >>>>> "rhmd" == rhmd  <rh4170…@juno.com> writes:

   rhmd> If you take any catalog containing QSOs (…) and plot optical
   rhmd> magnitude or x-ray flux vs redshift you get the usual sort of
   rhmd> magnitude/redshift relationship.  However, radio flux vs
   rhmd> redshift doesn’t produce the same type of curve: radio flux
   rhmd> decreases much less with redshift.  I’m sure there’s a
   rhmd> well-accepted and well-known explanation for this, but I can’t
   rhmd> find it!  Is it just the Holmqvist effect and if so, why is
   rhmd> radio flux different from X-ray flux?

   Steve Willner’s already posted a good explanation of what happens in
   the radio.  He didn’t quite say it, but there is a well-known effect
   for radio galaxies: More distant galaxies have steeper spectra.

   At radio wavelengths, the typical radio galaxy has a spectrum such
   that the radio emission decreases as one goes to higher frequencies.
   Moreover, the amount by which the radio emission decreases becomes
   larger and larger at higher frequencies.  (The reason for this is that
   radio galaxies emit synchrotron radiation.  In order to produce the
   higher radio frequency emission, ever more energetic relativistic
   electrons are required.  However, the more energetic relativistic
   electrons also lose energy faster, so the radio emission falls off
   faster at higher frequencies.)

   If one now redshifts this spectrum, but observes at the same
   frequency, one is looking at a higher frequency in the *rest frame* of
   the radio galaxy.  Thus, the radio galaxy does look dimmer.

   Typically, this is quantified by the spectral index.  Synchrotron
   radiation usually produces a power-law spectrum, so the flux density
   from a radio galaxy behaves in the following manner: S ~ f^(alpha),
   where S is the flux density, f is the frequency, and alpha is the
   spectral index.  For a typical radio galaxy, alpha ~ -0.7 at
   frequencies around 1 GHz.

   Consider observing at two different radio frequencies.  Then

        (S1/S2) = (f1/f2)^alpha

   or alpha = log(S1/S2)/log(f1/f2).  So we can measure the flux density
   of radio galaxies at two different frequencies and determine their
   spectral indices.  The empirical relation I mentioned earlier is that,
   as one goes to higher and higher redshift, alpha becomes more and more
   negative.  Thus, in the local Universe, alpha ~ -0.7.  For radio
   galaxies at a redshift of about 1, alpha ~ -1.  For distant radio
   galaxies, at redshifts of 3 and greater, alpha ~ -1.5.

   For more information, one might want to consult the ADS; Carlos de
   Breuck, Wil van Breugel, and Huub Rottgering have been quite active in
   trying to find distant radio galaxies by looking at steep spectrum
   radio sources.

   –
   Lt. Lazio, HTML police   | e-mail: jla…@patriot.net
   No means no, stop rape.  | http://patriot.net/%7Ejlazio/
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