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
How to find a HzRG
steep
flat
• 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.
e.g. Rottgering et al 1994, Blundell et al 1998, De Breuck et al 2000, 2004
Using SUMSS & NVSS
to search for HzRGs
• USS selection: SUMSS (843MHz) & NVSS (1400MHz)
• S(1400)>15mJy & a <-1.3
• -30<d<-40
• Parent sample 76 sources
– (De Breuck et al. 2004)
• 35 spectroscopic redshifts so far including 5 with z>3
– (De Breuck et al. 2005, in press)
Conventional wisdom for
the correlation: 1
3C295 z=0.461
z=5 z=2
3C295
100
10
1
1000
Flux Density (mJy)
Flux Density (mJy)
1000
0.1
0.01
Negative k-correction
of concave radio spectrum
100
10
1
0.1
0.01
0.1
0.1
1
1
10
10
Observed
Frequency
Observed
Frequency
(GHz) (GHz)
100
100
1000
1000
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, GopalKrishna et al 1989
• But, a correlation still exists ...
– e.g. Carilli et al 1999, Blundell et al. 1999, Lacy et al 1993
ATCA observations of the SUMSSNVSS 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 z<2 objects in sample
Constructed rest frame SEDs (using K-z relation to estimate
z when necessary)
but USS spectra don’t steepen at all…
• our ATCA
observations confirm
that high-z radio
galaxy spectra are
not curved
The k-correction
interpretation is
inconsistent with
observations
•
•
The number of nearby
USS radio galaxies in
5GHz selected surveys is
<1%.
So USS HzRGs are still
extreme in some way.
They do not represent a
‘typical’ radio galaxy in
energy loss regime
Kuehr et al. 1981
Stickel et al. 1994
• 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
Murgia et al. 2005
Learning from the neighbours…
The Gaseous Environments of
Distant Radio Galaxies
• Linear Sizes
• Cosmological expansion
• Gas and Dust Reservoirs
–
  1  z 
3

 Bne dl
Stevens et al 2003, Kurk et al 2004
• Rotation Measures
– 1000 -18350 rad m2 -> 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 -> rich clusters (Venemans et al. thesis)
  M vir 1  z 3
–
M vir  1  z 
• Knotty “frustrated” Jets
dense & clumpy IGM on scales of 85kpc (Carilli et al. 1997)
3
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 z>3
• 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