THE RADIO STRUCTURES OF VERY HIGH REDSHIFT QUASARS

1 John Wardle 1 , Doug Gobeille 1 and Teddy Cheung 2 Physics Department, Brandeis University, Waltham, MA 02454 2 NASA/GSFC Greenbelt, Maryland 20771 Abstract. We are making VLA observations of all known radio-loud quasars with z >2.5 in the area common to both FIRST and SDSS, and with radio fluxes > 70 mJy at either 1.4 or 5 GHz. The sample contains 134 objects ranging in redshift up to z = 4.7. When combined with previous lower redshift samples we have a unique dataset for studying the evolution of radio loud AGN and their environment back to the earliest epochs. We will present VLA images showing the variety of structures exhibited by the highest redshift sources, and also give some preliminary results concerning the density and clumpiness of the intergalactic medium at these early epochs. A parallel goal of these observations is to find new high redshift jets for observing with Chandra to test the IC/CMB mechanism.

Introduction

.

Questions of cosmic evolution require observations of sources with the largest look-back times to compare with those of more recent epochs. At redshifts of 4 to 5 we start to see clues to the origin and formation of these sources and their central engines, as well as how their properties change over cosmic time. At high redshift the environment of a radio source is quite different from that of a low redshift source. The density of the intergalactic medium scales with redshift as (1 + z) 3 and the energy density of the Cosmic Microwave Background scales as (1 + z) 4 . For a z = 5 quasar compared to a z = 1 quasar, the IGM density is 27 times higher (in simple models), and the energy density of the CMB is 81 times higher.

These should have profound and testable consequences for the morphology and x-ray emission of high redshift sources.

A high redshift quasar sample

.

We have constructed a sample of high redshift quasars in the simplest possible way. It includes every quasar in the NASA/IPAC Extragalactic Database (NED) with a redshift greater than 2.5, and a radio flux density greater than 70 mJy at 1.4 GHz or 4.9 GHz. The radio fluxes were taken from NVSS at 1.4 GHz, and from the 87GB survey at 4.9 GHz. The sample is also restricted to the area covered by both FIRST (Becker, White & Helfand 1995) and SDSS (Abazajian et al 2003) and contains 134 quasars with redshifts between 2.5 and 4.72. There are therefore excellent optical data on every source, as well as wide field radio images.

We have been able to image 36 sources deeply enough from archival VLA data, and three were observed in program AC755. We show those results here. The remaining sources are the subject of a current VLA proposal.

This is not a "complete sample" in the usual sense, but it is certainly a representative sample. The selection criteria are now well-defined enough that it will be possible to construct smaller sub-samples that are properly complete, or whose selection effects can be modeled in Monte Carlo simulations (c.f. Wardle & Aaron 1997).

The most important selection effects are due to beaming and to the increasing rest frame emission frequency at increasing redshift (e.g. Cohen 1988), and “the inevitable youthfulness of high redshift radio sources” (Blundell & Rawlings 1999). These effects must be accounted for carefully when comparing the radio structures of high redshift and low redshift samples.

Scientific goals.

(1) The first goal is simply to image these sources with arcsecond resolution and reasonably high (~1000:1) dynamic range, in order to take a fairly deep look at the high redshift radio universe. The images will be made publicly available as quickly as possible so that other astronomers can use the information to plan follow-up observations of particular sources with Chandra and HST. Here, timely dissemination is crucial, because both space observatories have limited lifetimes at this point.

(2) The most viable mechanism for the x-ray emission from many kiloparsec scale radio jets is inverse Compton scattering of the photons of the Cosmic Microwave Background radiation (the IC/CMB model; e.g. Tavecchio et al 2000). Because the energy density of the CMB increases with redshift as (1+z) 4 , the ratio of the x-ray luminosity to the radio luminosity of the jet also scales as (1+z) 4 , making high redshift radio jets potentially powerful x-ray sources. The model also requires moderate beaming (i.e. at least mildly relativistic speeds on kiloparsec scales, and that the observer is within the beaming cone). The best candidate jets are therefore flat-spectrum core-dominated sources (beamed in our direction) that reveal arcsecond-scale jets on moderately high dynamic range imaging. We estimate that we will find several tens of such jets in this sample, and the best cases will be proposed as Chandra and then HST targets. The HST observations are required because it is a low optical flux that can rule out a simple synchrotron spectrum stretching from radio to x-ray wavelengths (e.g. Sambruna et al 2004). This is a strong test of the IC/CMB mechanism, and it will also yield valuable information on the speeds of jets on kiloparsec scales. The radio/X-ray jets in two z~4 quasars have been studied in this way (Cheung 2004; Cheung et al. 2006).

(3) In an often quoted paper, Barthel and Miley (1988) showed that high redshift (z > 1.5) quasars were smaller, and had a more bent, distorted radio structure than quasars at lower redshifts. They attributed this to the effects of a denser, clumpier intergalactic medium at higher redshifts. But in a series of papers Neff and her collaborators (Neff & Hutchings 1990, and references therein) found these effects far less pronounced than did Barthel and Miley, and attributed much of them to a dependence on luminosity rather than redshift. The Barthel & Miley high redshift sample contained 80 quasars with z > 1.5, but only 6 had redshifts > 2.5. The Neff et al high redshift sample contained 58 sources with z > 2.0, but only 10 had z > 3.0. We note that our whole sample of 134 sources has z > 2.5, and 55 of them are at z > 3.0. This makes disentangling luminosity dependence from redshift dependence much easier, and the large number of very high redshift sources in our sample should make all epoch-dependent properties much more apparent. The Barthel & Miley and the Neff et al “high redshift” samples in effect become intermediate redshift samples to compare our results against.

(4) While a higher IGM density at higher redshift is expected to lead to smaller overall linear sizes, it will not by itself cause bends and distortions. Those suggest any or all of the following: a clumpy IGM, significant host galaxy velocities with respect to the IGM, and changing axes of the central engines. This is the natural scenario of galaxy and cluster formation through mergers at early epochs. Bent and distorted radio structure may therefore be one of the best signatures of very young systems that are still in the process of formation (see Overzier, Miley & Ford 2007, for such a system at z = 2.2, and Overzier et al 2008, for another at z = 4.1). We suspect that our observations will yield many such systems, which will then be excellent candidates for follow-up observations with HST and Chandra to study the emission line gas, the hot IGM and the properties of the other objects in the field.

(5) It is now thought that the formation and evolution of supermassive black holes (SMBHs) in AGN is intimately linked to the formation and evolution of their host galaxies (e.g. Richstone et al 1998, Ho 2004, and references therein). The highest redshift radio sources in our sample are inevitably extremely young (Blundell & Rawlings 1999), and our observations may allow us to investigate the evolution of SMBHs through the radio sources they make. Reasonable estimates of the black hole mass can be made using (for our redshift range) the continuum luminosity and the CIV (l1549) line width (Vestergaard & Peterson 2006). This adds an extra dimension to investigating the evolution of source properties at the earliest times.

X-rays from kiloparsec scale jets at high redshift.

The detection of x-ray jets at large redshifts is expected as a natural consequence of the inverse Compton (IC) emission off the cosmic microwave background (CMB) model (e.g., Tavecchio et al.

2000 ; Celotti, Ghisellini, & Chiaberge This is because the (1+z) 4 2001 ).

dependence of the CMB energy density compensates for cosmological dimming of radiation, so that IC/CMB X-ray jets should remain detectable out to large cosmological distances (Schwartz 2002). The model has been successfully applied to account for X-ray jets in many other powerful quasars at more modest redshifts (e.g., Sambruna et al. 2004). The model requires that the jets are still relativistic on kiloparsec scales, in order that the electrons in the jet frame see an adequately boosted photon source. It is unclear whether this is consistent with other estimates of jet speeds on kiloparsec scales (e.g. Wardle & Aaron 1997) or if a more complicated model is required.

Further detections in x-rays of high redshift jets will be a stringent test of the model.

Plot of the ratio of the jet X-ray to radio monochromatic luminosity vs. redshift (taken from Cheung (2004) and updated). Only jet features interpreted by the authors as IC/CMB X-ray emission are plotted. The curves indicate the expected ratio for given combinations of

B

and δ, which scale as (1 + z) 4 . For reference, the μG and case derived for GB 1508+5714, which used the additional equipartition constraint, defines the dotted line that lies between the other two curves. Light vertical lines connect features from the same source, i.e. different knots in the same jet. Red points are for lobe dominated sources, and are consistent with lower Doppler factors, as expected.

Results

This is a progress report on an on-going project to explore the high redshift radio universe. About one half of the 134 quasars with z > 2.5

have been imaged from data in the VLA archives, and some general comments can be made.

(1) The range of structures seen among the resolved sources mimics that seen in lower redshift samples. We see classical triples, doubles and jets.

(2) The fraction of resolved sources (41 %) is far lower than among the z < 2.5 sources in the same area, studied by Barthel & Miley (1988) and by Neff & Hutchins (1990). For 1.0 < z < 2.5 the fraction of resolved sources is 79 %, and for z < 1 it rises to 90 %. This can be attributed to the increasing emitted frequency at higher redshifts, making steep spectrum extended structure fainter relative to the cores.

(3) The great majority of the sources in our sample (84 %) are selected by their core flux. Since the cores are assumed to be beamed, this favors sources whose jets make a small angle to the line of sight and whose cores are Doppler boosted (Cohen 1988, Lister and Marscher 1997). This sample will have a strongly anisotropic distribution of orientations, making the core to lobe flux ratios larger and the projected linear sizes smaller.

(4) Superficially the median projected linear size of the resolved sources at z > 2.5 is smaller (~50 kpc) than at low redshift (~100 kpc). In view of the bias towards small angles to the line of sight mentioned above, and the “youthfulness effect” (Blundell & Rawlings 1999), it would be premature to draw any conclusion at all. Similar comments can be made about “bending angles” for the triple sources. The 13 sources above include one source with a bend of 90 degrees and others that are remarkably straight.

(5) Perhaps the most important result is that two of our highest redshift quasars, at redshifts of 3.89 and 3.82, are large triple radio sources with total projected linear extents of 149 kpc and 128 kpc respectively.

Evidently, even by the tender age of 1.6 Gy the universe had already formed high luminosity radio loud AGN, containing, presumably, supermassive black holes of 108 – 109 solar masses.

Acknowledgements We are very grateful to Jennifer Carson, who compiled the original list of all 307 radio sources with z > 2.5 from NED.

This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

It has also made extensive use of the VLA archives of the National Radio Astronomy Observatory. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work has been supported by grant AST-0607453 from the Nation Science Foundation.

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