---
abstract: 'We describe our efforts to understand large-scale (10’s–100’s kpc) relativistic jet systems through observations of the highest-redshift quasars. Results from a VLA survey search for radio jets in $\sim$30 z$>$3.4 quasars are described along with new $Chandra$ observations of 4 selected targets.'
author:
- 'C. C. Cheung, Ł. Stawarz'
- 'A. Siemiginowska, D. E Harris, D. A. Schwartz'
- 'J. F. C. Wardle, D. Gobeille'
- 'N. P. Lee'
title: The Highest Redshift Relativistic Jets
---

Why High-redshift Jets?
=======================

It is now well established that X-ray emission is a common feature of kiloparsec-scale radio jets [see @har06 for a recent review and the associated website, http://hea-www.harvard.edu/XJET/]. The spectral energy distributions (SEDs) of the powerful quasar jets are predominantly characterized as “optically faint”, with the spectra rising between the optical and X-ray bands. Current models for this ‘excess’ X-ray emission posit either inverse Compton (IC) scattering off CMB photons in a (still) relativistic kpc-scale jet or an additional high-energy synchrotron emitting component.

In the simplest scenario, such models have diverging predictions at high redshift. Specifically, we expect a strong redshift dependence in the monochromatic flux ratio, $f_{X}/f_{r}~\propto~U_{\rm CMB}~\propto~(1+z)^{4}$ for IC/CMB, whereas in synchrotron models, we expect no such dependence, $f_{X}/f_{r}~\propto~(1+z)^{0}$. As a first order test of this simple idea, our approach is to study the highest-redshift relativistic jets. Such jets probe the physics of the earliest (first $\sim$1 Gyr of the Universe in the quasars studied) actively accreting supermassive black hole systems and are interesting for other reasons. For instance, the ambient medium in these high-redshift galaxies is probably different [e.g., @dey06] and this may manifest in jets with different morphologies, increased dissipation, and slower than their lower-redshift counterparts.

Most $Chandra$ studies of quasar jets have so far targeted known arcsecond-scale radio jets [e.g., @sam04; @mar05], as most known examples are at $z$ $\stackrel{<}{{}_\sim}$2 [@liu02]. There are currently only two high-$z$ quasars with well-established kpc-scale X-ray jet detections: GB 1508+5714 at $z$=4.3 [@sie03; @yua03; @che04] and 1745+624 at $z$=3.9 [@che06]. They are observed to have large $f_{X}/f_{r}$ values as expected in the IC/CMB model [@sch02; @che04], although the small number of high-$z$ detections preclude any definitive statements [@kat05; @che06].

We have therefore carried out a VLA survey in search of new radio jets in a sample of high-$z$ quasars (§ \[sec-vla\]) and new $Chandra$ observations of a small subset (§ \[sec-cxo\]). This contribution presents some results from these observations. For the redshifts considered, $z$=3.4 to 4.7, 1$''$ corresponds to 7.4 to 6.5 kpc ($H_{0}=70~$km s$^{-1}$ Mpc$^{-1}$, $\Omega_{\rm M}=0.3$ and $\Omega_{\rm
\Lambda}=0.7$).

Observations of a High-Redshift Quasar Sample
=============================================

VLA Imaging Survey\[sec-vla\]
-----------------------------

Using NED, we assembled a sample of z$>$3.4 flat-spectrum radio quasars for imaging with the VLA. We did not aim for our sample to be a complete one as current samples of lower-z X-ray jets are inhomogenous also. With archival [@lee05] and new VLA observations, we find that radio jets in this redshift range are common with a $\sim$50$\%$ detection rate [@che05 and in preparation]. Examples of new radio jets detected from our observations are shown in Figure \[fig-1\].

$Chandra$ Observations\[sec-cxo\]
---------------------------------

A small percentage of the radio jets from our radio study (§ \[sec-vla\]) are extended enough ($>$2.5” long) to study with $Chandra$. We observed four of them with short snapshot $Chandra$ observations (Figure \[fig-2\]). We detected bright X-ray counterparts to the jets in the quasars J1421–0643 [$z$=3.689; @ell01] and GB 1428+4217 [$z$=4.72; @hoo98]; the latter detection is currently the highest-redshift kpc-scale radio and X-ray jet known. We did not detect the X-ray counterparts to the radio jets in 1239+376 [z=3.819; @ver96] and J1754+6737 [$z$=3.6; @vil99]. The 2/4 X-ray jet detection rate of our high-$z$ sample is comparable to that of lower-$z$ samples [@sam04; @mar05].

Discussion and Summary
======================

Previous $Chandra$ imaging studies of a number of z$>$4 radio loud quasars do not reveal significant extended X-ray emission [@bas04; @lop06]. However, in these studies, there were no pre-existing information on possible radio structures in the target objects and any definitive statements regarding the nature of the X-ray emission mechanism in jets at high-redshifts may be premature. In fact, in one case where there was evidence of an extended X-ray structure [J2219–2719; @lop06], our VLA observation revealed a radio counterpart (Figure \[fig-1\]).

In our approach, we began with a VLA survey of a sample of z$>$3.4 quasars and found radio jets to be relatively common ($\sim$50$\%$ detection rate). These jets are quite luminous; with a confident detection of a 1 mJy knot at 1.4 GHz, this corresponds to luminosities of 1.5 $\times$10$^{42}$ erg s$^{-1}$ ($z$=3.4) to 3.1 $\times$10$^{42}$ erg s$^{-1}$ ($z$=4.7).

With the radio survey results, we found only a few radio jets to have sufficient angular extent to be imaged with $Chandra$. The detection rate of X-ray counterparts of the high-z radio jets (2/4) is similar to that of lower-z radio jet samples [@sam04; @mar05]. The implications of these observations for models of X-ray emission from large-scale jets will be described in forthcoming publications.

The National Radio Astronomy Observatory is operated by Associated Universities, Inc. under a cooperative agreement with the National Science Foundation. This research was funded in part by NASA through contract NAS8-39073 (A. S., D. E. H., D. A. S.) and $Chandra$ Award Numbers GO7-8114 (C. C. C., Ł. S., J. F. C. W., D .G.) issued by the $Chandra$ X-Ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-39073. Radio astronomy at Brandeis University is supported by the NSF and NASA. Ł. S. is supported by MEiN through research project 1-P03D-003-29 from 2005-2008.

Bassett, L. C., et al. 2004, , 128, 523

Cheung, C.C. 2004, , 600, L23

Cheung, C.C., Wardle, J.F.C., & Lee, N.P. 2005, in 22nd Texas Symposium on Relativistic Astrophysics, Eds. P. Chen et al., (Palo Alto: SLAC) 1613

Cheung, C.C., Stawarz, Ł., & Siemiginowska, A. 2006, , 650, 679

De Young, D. 2006, Astron. Nachr., 327, 231

Ellison, S. L., Yan, L., Hook, I. M., Pettini, M., Wall, J. V., & Shaver, P. 2001, , 379, 393

Harris, D.E., Krawczynski, H. 2006, ARAA, 44, 463

Hook, I.M., & McMahon, R.G. 1998, MNRAS, 294, L7

Hook, I.M., McMahon, R.G., Shaver, P.A., & Snellen, I.A.G. 2002, A&A, 391, 509

Kataoka, J., & Stawarz, [Ł]{}. 2005, ApJ, 622, 797

Lee, N. P. 2005, Undergraduate Honors Thesis, Brandeis University

Liu, F. K., & Zhang, Y. H. 2002, , 381, 757

Lopez, L. A., Brandt, W. N., Vignali, C., Schneider, D. P., Chartas, G., & Garmire, G. P. 2006, , 131, 1914

Marshall, H. L., et al. 2005, ApJS, 156, 13

Sambruna, R. M., Gambill, J. K., Maraschi, L., Tavecchio, F., Cerutti, R., Cheung, C. C., Urry, C. M., & Chartas, G. 2004, , 608, 698

Schwartz, D.A. 2002, , 569, L23

Siemiginowska, A., Smith, R.K., Aldcroft, T.L., Schwartz, D.A., Paerels, F., & Petric, A.O. 2003, L, 598, L15

Vermeulen, R. C., Taylor, G. B., Readhead, A. C. S., & Browne, I. W. A. 1996, , 111, 1013

Villani, D., & di Serego Alighieri, S. 1999, , 135, 299

Xu, W., Readhead, A.C.S., Pearson, T.J., Polatidis, A.G., & Wilkinson, P.N. 1995, ApJS, 99, 297

Yuan, W., Fabian, A.C., Celotti, A., & Jonker, P.G. 2003, , 346, L7
