Steep spectrum radio galaxies at high redshift

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Steep spectrum radio galaxies at high redshift

• Ilana Klamer (USYD)
• Dick Hunstead, Elaine Sadler, Julia Bryant, Helen
  Johnston, Jess Broderick, Carlos De Breuck, Ron
  Ekers

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How to find a HzRG

• A trend/correlation exists between the redshift
  of a radio galaxy and its radio spectral index
  measured in the observed frame.
• Spectral index culling of existing radio sky
  surveys preferentially selects HzRGs.

flat
steep
e.g. Rottgering et al 1994, Blundell et al 1998,
De Breuck et al 2000, 2004
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Using SUMSS NVSS to search for HzRGs

• USS selection SUMSS (843MHz) NVSS (1400MHz)
• S(1400)gt15mJy a lt-1.3
• -30ltdlt-40
• Parent sample 76 sources
• (De Breuck et al. 2004)
• 35 spectroscopic redshifts so far including 5
  with zgt3
• (De Breuck et al. 2005, in press)

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Conventional wisdom for the correlation 1
z5
TEXAS
NVSS
Negative k-correction of concave radio spectrum
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The k-correction is a good explanation because

• Less significant correlation between z arest
• e.g. Carilli et al 1999, Blundell et al. 1999,
  Lacy et al 1993, Gopal-Krishna et al 1989
• But, a correlation still exists ...
• e.g. Carilli et al 1999, Blundell et al. 1999,
  Lacy et al 1993

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ATCA observations of the SUMSS-NVSS USS radio
galaxies
Matched low resolution ATCA observations at
2.4GHz (12.5cm), 4.8GHz (6.3cm), 6.2GHz (4.8cm)
Further ATCA observations at 8.6GHz (3.5cm)
18GHz (1.7cm) for zlt2 objects in
sample Constructed rest frame SEDs (using K-z
relation to estimate z when necessary)
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but USS spectra dont steepen at all

• our ATCA observations confirm that high-z radio
  galaxy spectra are not curved

The k-correction interpretation is inconsistent
with observations
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• The number of nearby USS radio galaxies in 5GHz
  selected surveys is lt1.
• So USS HzRGs are still extreme in some way. They
  do not represent a typical radio galaxy in
  energy loss regime

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Learning from the neighbours

• It is well known that local USS sources are rich
  cluster sources (e.g. Slee et al 1983)
• This is interpreted as pressure confinement of
  the radio lobes which keeps the oldest (steepest)
  radio emission above a given surface brightness
• Nearby USS sources are very RARE, but majority
  reside in regions of unusually high ambient gas
  density
• This explains the z-a correlation there is
  simply more gas at high redshift

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The Gaseous Environments of Distant Radio
Galaxies

• Linear Sizes
• Cosmological expansion
• 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
• 2-9 x 1014 Msun -gt rich clusters (Venemans et
  al. thesis)
• Knotty frustrated Jets
• dense clumpy IGM on scales of 85kpc (Carilli et
  al. 1997)

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SUMMARY

• The z-a correlation is exploited to find high-z
  radio galaxies by data mining radio all sky
  surveys
• We have selected 76 USS sources selected from the
  SUMSS and NVSS
• So far we have discovered 4 new radio galaxies at
  zgt3
• The USS galaxies DO NOT have concave SEDs
• The nearby USS galaxies reside in dense gaseous
  environments
• Observations show similar environments around
  high-z radio galaxies
• The z-a correlation now has a plausible physical
  explanation