Radio SEDs of highredshift radio galaxies

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Radio SEDs of high-redshift radio galaxies

• Ilana Klamer (CSIRO ATNF)
• Ron Ekers (ATNF), Carlos De Breuck (ESO)
• Julia Bryant, Dick Hunstead, Elaine Sadler
  (Sydney Uni)

with thanks to Avi Loeb (Harvard)
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Observational evidence for Positive Feedback in
the early universe

• UV (rest frame)
• UV (rest frame)
• Ly-a

Unpolarised shows absorption lines P-Cygni
type profiles similar to SF galaxies
van Breugel (1999)
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Theyre rare, but relevant

• Out to z5, radio galaxies form a lower envelope
  to the K-z diagram they are the brightest, and
  therefore the most massive, galaxies. Total
  baryonic masses 1012 Msun (the so-called
  monsters)

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The z-? correlation

• The average value of ??is -0.8. At high-z, its
  more like ?lt-1.
• Radio spectral index culling of radio catalogues
  is an efficient way to find high-z RGs.
• A majority are found by exploiting this
  correlation. Its critical that its origin is
  understood.
• More than 300 zgt2 USS-selected known to date.
• Chambers et al. 1996, Blundell et al. 1998, De
  Breuck et al. 2000, 2004, Cohen et al. 2004.

flat
steep
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z5
TEXAS 325MHz
NVSS 1.4GHz
k-correction of concave radio spectrum
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k-correction?
No.
Rest flux density (mJy)
Rest frequency (GHz)
There is NO k-correction for spectral curvature
Klamer et al. 2006
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Intrinsic? Yes.

• The z-? correlation is not caused by curved radio
  SEDs, because steep spectrum radio galaxies have
  power law SEDs.
• The z-? correlation is intrinsic known high-z
  radio galaxies have steeper intrinsic ??than the
  typical RGs.
• In addition, the radio galaxies with the steepest
  ? show the least amount of spectral curvature.
  The same effect is seen in 3CR (Mangalam Gopal
  Krishna 1995), in preliminary results from 6C
  7C, and even 4C 41.17 has a perfect power law SED
  (Chambers et al. 1990)

colour-colour diagram

• Q what drives the z-? correlation?
• Q why do the steepest RGs also have power law
  SEDs?

Courtesy of Katherine Blundell
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Environments of nearby RGs

• Nearby radio galaxies do not typically reside in
  the richest galaxy clusters!
• Auriemma et al. 1977, Ledlow Owen 1995, McLure
  Dunlop 2001
• The number of RGs detected simply scales with
  the number of galaxies surveyed
• Ledlow Owen 1995

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but those that do have unusually steep ?

• It has been known for 3 decades or so (Slingo
  1973, Slee et al. 1983) that RGs at the centre of
  rich clusters, or compact radio sources within
  the ISM of their host, have steeper radio SEDs
  than the usual.
• This has been interpreted (Komissarov Gubnov
  1994, Jones Preston 2001) as confinement of the
  radio plasma by dense ambient gas, effectively
  halting the adiabatic expansion of the source and
  keeping the brightness of the radio emission
  above a detectable limit.
• As a radio source ages, its SED steepens. So if
  you halt the adiabatic expansion, you will see
  steeper SED for a longer time. Slowing the
  expansion keeps the magnetic fields high, which
  further speeds up the spectral ageing.

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Using radio SEDs to probe the ambient gas density

• If RGs occupy regions of increasingly higher gas
  density as a function of redshift, then there
  would be more RGs with steep SEDs as a function
  of redshift.
• If this is the mechanism driving the z-?
  correlation, then radio SEDs could be used as
  density probes in the early universe.

Fornax A Fomalont Ekers
Klamer et al. 2006
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Environment encountered by a radio jet

• Assume that radio jets advancing away from their
  host galaxies encounter an ambient density given
  by NFW (1997) so that
• within the galaxy's DM halo, and then
• where

VIRGO simulation. Credit Volker Springel
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• A radio jet/hotspot at z0.5 advancing from its
  host galaxy, will encounter an ambient gas
  density 3.4x higher than a z0 jet/hotspot from a
  non-cluster RG.
• One at z4 encounters 125 times higher gas
  density.
• The virial radius of the DM halos at z4 is
  300kpc, at z0 is 1Mpc

high-z
??r?
low-z
Loeb 2006
r
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And if you dont believe me yet

• Gas and Dust Reservoirs
• Stevens et al 2003, Kurk et al 2004
• Rotation Measures
• 1000 -18350 rad m2 -gt X-ray cluster scale
  densities
• (Carilli et al. 1997, Pentericci 2000, Athreya
  1998, Benn 2005)
• Clustering Environments
• e.g. Kurk et al. 2000, Venemans et al. 2002, 2004
  Miley et al. 2004
• Proto-cluster Masses (next talk)
• 2-9 x 1014 Msun -gt rich clusters
• (Venemans et al. 2005)

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The take-home message

• Radio galaxies with the steepest radio colours
  have power law SEDs.
• The z-? correlation may be driven by evolution in
  the ambient gas density around RGs as a function
  of redshift.
• If this is the case, then we may be able to use
  radio SEDs to probe the density of the Universe.

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The Cygnus question

• Q But Cygnus A is in a rich cluster it has a
  normal spectral index and a curved SED

• A Cygnus A resides near but offset from the
  center of a RC 1 cluster that appears to be
  merging with another cluster of similar richness.
  Ledlow, Owen Miller, 2005

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4C41.17 z3.91

• Power law SED with ??1.3 from 26MHz to at least
  5GHz (rest frame 128MHz-25GHz).
• Chambers et al. (1990) concluded that the
  k-correction does nothing to steepen the SED.

• UV (rest frame)
• UV (rest frame)
• Ly-a

van Breugel 1999
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Why are steep spectrum sources so straight?
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• The virial radius of a galaxy with baryonic mass
  1012 Msun is 1Mpc at z0 and 330kpc at z3.

• The jets are advancing well within the virial
  radius of the DM halos.
• RGs inside and outside of clusters have the same
  masses and so the it is the density of the RGs
  immediate environment which seems to play a role
  in the radio SED.

• A radio jet/hotspot at z4 advancing from its
  host galaxy, will encounter an ambient gas
  density 125x higher than a z0 jet/hotspot from a
  non-cluster RG.

Loeb 2006
VIRGO simulation. Credit Volker Springel
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