Morphology and Environments of X-ray/
Radio-Loud AGN in the GOODS Fields
A. M. Koekemoer (STScI), B. Mobasher (STScI), R. P. Norris (ATNF), B. Chan (U. Sydney), L. Cram (ARC), J. Afonso (Lisbon),
C. Conselice (Caltech), N. A. Grogin (JHU), E. Chatzichristou (Yale), C. Jackson (RSAA), S. Jogee, P. Padovani (STScI),
R. Lucas (STScI), E. J. Schreier (STScI/AUI), C. M. Urry (Yale), R. Fosbury (ST-ECF), S. Ettori (ESO), GOODS Team
Abstract
The advent of ultra-deep X-ray surveys with Chandra, together with deep HST/ACS
imaging and microJy-level radio surveys, allows us to directly probe the detailed properties
of the environments, host galaxies and central accretion disks of the most distant radio
galaxies. Here we present results from a combined program of our ultra-deep 20cm radio
survey with the Australia Telescope of the Chandra Deep Field South, together with data
from the multi-band GOODS/ACS imaging program with HST. We describe a sample of
radio-loud AGN in this field and present results on the optical morphological properties of
their hosts and environments, as well as their X-ray properties. We discuss the results in the
context of previous studies on lower-redshift radio galaxies, and also present a comparison
with the properties of radio-quiet AGN to examine whether radio-loudness represents the
high end of a continuum of properties or is instead a phenomenon that is quite distinct from
radio-quiet sources. We examine the implications of these results for the properties of black
holes in the early universe.
Figure 1. Photograph
showing 5 of the 6
antennas of the Australia
Telescope Compact Array
(ATCA), located at
Narrabri, NSW, Australia.
The array has a maximum
baseline of 6km; the
dishes are each 22m
across and the back-end
receiver systems can
observe at wavelengths
of 20, 13, 3, 6, 1.2 and
0.3cm, with a bandwidth
up to 2x128MHz.
Introduction
A major question in modern astrophysics involves the understanding
of how galaxies are built up over cosmic time, and how this process is
in turn related to the properties of their central supermassive black
holes (SBH). An important clue to this process is the relationship
between central SBH mass and bulge mass (e.g., Magorrian et al.
2000; Ferrarese & Merritt 2000), which clearly indicates a connection.
However, the causal nature of this connection remains unknown – do
the SBHs simply follow the gradual growth of their hosts via
merger/accretion events, or do they instead directly govern the
evolution of their hosts by means of the energy deposited from them
into their surroundings?
To address these questions requires large, multi-faceted
observational campaigns of many galaxies over a wide range of
wavelengths. The GOODS program is aimed at combining ultra-deep
space-based X-ray, HST, and SIRTF observations with matching
ground-based optical, NIR and radio observations to provide welldefined samples of active and non-active galaxies up to the
reionization epoch.
Here we present results from the Ultra-Deep 20cm radio survey of
the Chandra Deep Field South, which we have been carrying out using
the Australia Telescope Compact Array (ATCA). More details about
the survey are presented in Koekemoer et al. (2002). The principal
goals of this survey are:
• Detect radio emission from the sub-mJy starforming population to
lookback times at least half the age of the universe, to probe the
evolution of star-formation via mergers
• Cross-correlate with the HST/ACS data, so that detailed
morphological information can be obtained about the hosts of all the
radio sources
• Cross-correlate with the X-ray data, to reveal not only X-ray
emission from starbursts but also from accretion disks around active
galactic nuclei (AGN), and thereby probe the X-ray/radio properties
of AGN up to high redshifts.
By providing information on the radio emission from these sources,
we are thus able to probe the full range of physical emission
mechanisms related to star formation and active nuclei.
Figure 3. ACS morphologies (B,V,i,z) of the CDFS radio/Xray sources.
Figure 4. Relationship between P(1.4GHz) and Lx(0.2-10keV). Soft
sources are light, hard sources are dark. Note that the hard sources, likely
dominated by AGN X-ray emission, display less scatter than soft sources,
which are probably more dominated by star formation.
Observations
The radio survey was designed to provide uniform sensitivity to levels
of ~10µJy r.m.s. across a relatively wide region, ~30’ in diameter,
sufficiently large to cover not only the Chandra pointing itself but also
the larger XMM field surrounding it. Since the primary beam of both
the ATCA and the VLA drop to about their half-power point on these
scales, it is necessary to use a compact mosaicong pattern to ensure
that uniform sensitivity is fully achieved across a 30’ field of view.
Although the ATCA is nominally ~2x less sensitive than the VLA, it is
far more efficient at mosaicing, hence the ATCA was the instrument of
choice for carrying out this survey. In addition, the southern location
of the field (-28) is more optimal for the ATCA than for the VLA,
providing improved phase stability. Finally, we used the ATCA in its
widest bandwidth mode at 256 MHz, while retaining a large number of
channels (64) which greatly reduces chromatic smearing and helps to
improve calibration.
The observations were obtained over a total of 15x 12h blocks,
during April and August 2002. We employed 2 IFs (128 MHz each),
32 channels per IF, all 4 Stokes polarization products, and 10s
integration times. Both IFs were in the 20cm band, with one centered
at 1344 MHz and the other at 1432 MHz, which are optimal for
avoiding radio frequency interference. The primary flux calibrator used
was the source PKS 1934-638, which was observed once at the start of
each 12h run for bandpass and phase delay calibration. The secondary
calibrator used was PKS 0237-233, which is a strong source within 10o
from the field and therefore provides robust phase calibration
information.
The data were calibrated using the AIPS and miriad reduction
packages, which contain routines specific to ATCA data. After initial
calibration and multi-frequency synthesis cleaning, self-calibration was
carried out on the data to improve the quality of phase and amplitude
calibration and increase the dynamic range. A preliminary image of the
resulting field is shown in Figure 2. Further work on self-calibration is
still ongoing to remove residual sidelobes and improve dynamic range.
Figure 2. Preliminary image of the CDFS, extending 1o across, made from the first 5x12h set of observations from the ATCA at 20cm.
The data have been calibrated and passed through multi-frequency synthesis cleaning, to reach an initial r.m.s. of ~25-30 µJy. Adding the
remaining 10x 12h of observations, together with further cleaning and self-calibration, will allow us to reach our ultimage goal of ~10µJy.
Figure 5. Radio/X-ray flux as a function of redshift, tracing the ratio of
kinetic/radiative output from the AGN. There is no sign of any strong
evolution up to at least z~2, indicating that these properties are likely
determined much earlier in the universe.
Discussion and Conclusions
After creating the radio image, our first step consisted of creating a catalog of radio sources within the central 30’ diameter region of the CDFS, down to
the 5-sigma detection threshold of 150 µJy. This catalog was then cross-correlated with the X-ray catalog from Giacconi et al. (2002), which is based on
the 1Ms Chandra dataset, using a 2” matching tolerance (corresponding to the joint positional uncertainties from the X-ray and radio catalogs combined).
This yielded a total of 49 radio sources that are detected in either the hard or soft X-ray band. We next identified these sources either in the ground-based
optical/NIR catalogs or the HST/ACS imaging, and for redshift information we generally used the photometric redshifts (Mobasher et al. 2003, in
preparation). In Figure 3 we show the mprphologies of the sources in the ACS field. In Figure 4 we show the relationship between P(1.4GHz) and Lx(0.210keV). The souces generally obey the radio/X-ray correlation known for active galaxies, but it is interesting to note that the hard sources tend to show a
tighter correlation than the softer sources which are likely more dominated by starburst emission.
We next investigate the evolution of the ratio of P(1.4GHz) vs hard X-ray luminosity as a function of redshift for the sources (Figure 5). The hard Xrays are likely to be dominated by radiation from the accretion disk which is penetrating through the obscuring torus, and therefore provide a relatively
unbiased means of selecting the AGN, although there may be some relatively minor contributions from inverse Compton emission associated with the jets,
or hard X-ray binaries in the host galaxies themselves. The radio emission, on the other hand, predominantly traces the kinetic output extracted by the jets
from the black hole, hence this provides a means of exploring whether the ratio of kinetic/radiative output (or Eddington luminosity) has evolved during
the cosmic timescale that is probed by these sources. The figure appears to indicate very little evolution in this property since redshift 2, possibly indicating
that the intrinsic radio/X-ray ratio, hence the degree of radio loudness of the sources, is determined relatively early on in the universe and is not much
affected by the subsequent merger history of the host galaxies.
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