Steep spectrum radio galaxies at high redshift

1. 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

2. 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. steep flat e.g. Rottgering et al 1994, Blundell et al 1998, De Breuck et al 2000, 2004

3. 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)

4. Conventional wisdom for the correlation: 1 z=5 TEXAS NVSS Negative k-correction of concave radio spectrum

5. 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

6. 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)

7. 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

8. 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

9. 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…

10. 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)

11. 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