steep spectrum radio galaxies at high redshift ilana klamer (usyd) dick hunstead, elaine sadler,...
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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
flatsteep• 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
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Using SUMSS & NVSS to search for HzRGs
• USS selection: SUMSS (843MHz) & NVSS (1400MHz)
• S(1400)>15mJy & <-1.3
• -30<<-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)
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3C295 z=0.461
0.1
1
10
100
1000
0.01 0.1 1 10 100 1000Observed Frequency (GHz)
Flux
Den
sity
(m
Jy)
Conventional wisdom for the correlation: 1
0.1
1
10
100
1000
0.01 0.1 1 10 100 1000
z=5
Negative k-correctionof concave radio spectrum
TEXAS
TEXAS
NVSSNVSS
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The k-correction is a good explanation because:
• Less significant correlation between z & rest
– 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 z<2 objects in sample
Constructed rest frame SEDs (using K-z relation to estimate z when necessary)
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• our ATCA observations confirm that high-z radio galaxy spectra are not curved
but USS spectra don’t steepen at all…
The k-correction interpretation is inconsistent with
observations
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• 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. 1981Stickel et al. 1994
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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- correlation: there is simply more gas at high redshift
Mu
rgia
et
al.
20
05
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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 -> 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)
• Knotty “frustrated” Jets– dense & clumpy IGM on scales of 85kpc (Carilli et al. 1997)
3
3
1
1
zM
zM
vir
vir
dlnB e
31 z
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SUMMARY
• The z- 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- correlation now has a plausible physical explanation