30 research outputs found
Extremely red radio galaxies
At least half the radio galaxies at z>1 in the 7C Redshift Survey have
extremely red colours (R-K>5), consistent with stellar populations which formed
at high redshift (z>5). We discuss the implications of this for the evolution
of massive galaxies in general and for the fraction of near-IR-selected EROs
which host AGN, a result which is now being tested by deep, hard X-ray surveys.
The conclusion is that many massive galaxies undergo at least two active
phases: one at z~5 when the black hole and stellar bulge formed and another at
z~1-2 when activity is triggered by an event such as an interaction or merger.Comment: 6 pages, 2 figures, to appear in the proceedings of the workshop on
"QSO hosts and their environments", IAA, Granada, 10-12 Jan 2001, Ed. I.
Marque
Compton Thick AGN: the dark side of the X-ray background
The spectrum of the hard X-ray background records the history of accretion
processes integrated over the cosmic time. Several pieces of observational and
theoretical evidence indicate that a significant fraction of the energy density
is obscured by large columns of gas and dust. The absorbing matter is often
very thick, with column densities exceeding N_H > 1.5 10^24 cm-2, the value
corresponding to unity optical depth for Compton scattering. These sources are
called ``Compton thick'' and appear to be very numerous, at least in the nearby
universe. Although Compton thick Active Galactic Nuclei (AGN) are thought to
provide an important contribution to the overall cosmic energy budget, their
space density and cosmological evolution are poorly known. The properties of
Compton thick AGN are reviewed here, with particular emphasis on their
contributions to the extragalactic background light in the hard X-ray and
infrared bands.Comment: 28 pages, 10 figures. Review for "Supermassive Black Holes in the
Distant Universe", Ed. A. J. Barger, Kluwer Academi
The Chandra Deep Field-North survey and the cosmic X-ray background
Chandra has performed a 1.4 Ms survey centred on the Hubble Deep Field-North
(HDF-N), probing the X-ray Universe 55-550 times deeper than was possible with
pre-Chandra missions. We describe the detected point and extended X-ray sources
and discuss their overall multiwavelength (optical, infrared, submillimeter,
and radio) properties. Special attention is paid to the HDF-N X-ray sources,
luminous infrared starburst galaxies, optically faint X-ray sources, and
high-to-extreme redshift AGN. We also describe how stacking analyses have been
used to probe the average X-ray emission properties of normal and starburst
galaxies at cosmologically interesting distances. Finally, we discuss plans to
extend the survey and argue that a 5-10 Ms Chandra survey would lay key
groundwork for future missions such as XEUS and Generation-X.Comment: 16 pages; Philosophical Transactions of the Royal Society (Series A:
Mathematical, Physical, and Engineering Sciences), submitted; High-resolution
version of Figure 5 available at
http://www.astro.psu.edu/users/niel/hdf/hdf-chandra.htm
The Chandra Deep Field-North survey. VII. X-ray emission from Lymanbreak galaxies
We present results from stacking analyses, using the 1 Ms Chandra Deep Field
North data, that constrain the X-ray emission of Lyman break galaxies at z =
2-4. Stacking the counts from 24 individually undetected Lyman break galaxies
located within the Hubble Deep Field North, we have obtained average detections
of these objects in the resulting 0.5-8.0 keV and 0.5-2.0 keV images; these
images have effective exposure times of 22.4 Ms (260 days). Monte Carlo testing
empirically shows the detections to be highly significant. The average
rest-frame 2-8 keV luminosity of a Lyman break galaxy is derived to be 3.2 x
10^{41} erg s^{-1}, comparable to that of the most X-ray luminous starbursts in
the local Universe. The observed ratio of X-ray to B-band luminosity is
somewhat, but not greatly, higher than that seen from local starbursts. The
X-ray emission probably arises from a combination of high-mass X-ray binaries,
"super-Eddington" X-ray sources, and low-luminosity active galactic nuclei.Comment: 11 pages, ApJ
The fall of active galactic nuclei and the rise of star-forminggalaxies: A close look at the Chandra Deep Field X-ray number counts
We investigate the X-ray number counts in the 1-2 Ms Chandra Deep Fields
(CDFs) to determine the contributions of faint X-ray source populations to the
extragalactic X-ray background (XRB). X-ray sources were separated into Active
Galactic Nuclei (AGN), star-forming galaxies, and Galactic stars based on
several criteria. We find that AGN continue to dominate the number counts in
the 0.5-2.0 keV and 2-8 keV bands. At flux limits of ~2.5e-17 erg cm-2 s-1
(0.5-2.0 keV) and ~1.4e-16 erg cm-2 s-1 (2-8 keV), the overall AGN source
densities are 7166 (+304/-292) and 4558 (+216/-207) sources deg-2,
respectively; these are factors of ~10-20 higher than found in the deepest
optical spectroscopic surveys. While still a minority, the number counts of
star-forming galaxies climb steeply such that they eventually achieve source
densities of 1727 (+187/-169) and 711 (+270/-202) sources deg-2 at the CDF
0.5-2.0 keV and 2-8 keV flux limits, respectively. Adopting recent XRB flux
densities measurements, the CDFs resolve a total of 89.5% (+5.9%/-5.7%) and
86.9% (+6.6%/-6.3%) of the extragalactic 0.5-2.0 keV and 2-8 keV XRBs,
respectively. Extrapolation of the number-count slopes can easily account for
the entire 0.5-2.0 keV and 2-8 keV XRBs to within statistical errors. We also
revisit the reported differences between the CDF-North and CDF-South number
counts, finding that the two fields are consistent except for sources in the
2-8 keV band below F(2-8 keV)~1e-15 erg cm-2 s-1, where deviations gradually
increase to ~3.9 sigma.Comment: emulateapj.cls used, 17 pages, 11 figures (2 color) + 2 tables,
accepted for publication in AJ. Modified to replace astro-ph abstract/title,
which were inadvertantly cropped in original postin
The Chandra Deep Field North Survey. XII. The link between faint X-rayand radio source populations
We investigate the relationship between faint X-ray and 1.4 GHz radio source
populations detected within 3' of the Hubble Deep Field North using the 1 Ms
Chandra and 40 uJy VLA surveys. Within this region, we find that ~42% of the 62
X-ray sources have radio counterparts and ~71% of the 28 radio sources have
X-ray counterparts; thus a 40 uJy VLA survey at 1.4 GHz appears to be
well-matched to a 1 Ms Chandra observation. Among the different source
populations sampled, we find that the majority of the 18 X-ray detected
emission-line galaxies (ELGs) have radio and mid-infrared ISOCAM counterparts
and appear to be luminous star-forming galaxies at z=0.3-1.3. Importantly, the
radio-detected ELGs make up ~35% of the X-ray source population at 0.5-8.0 keV
X-ray fluxes between ~(1-5)x10e-16 erg/cm2/s and signal the emergence of the
luminous, high-z starburst galaxy population in the X-ray band. We find that
the locally-determined correlation between X-ray luminosities and 1.4 GHz radio
luminosity densities of the late-type galaxies can easily be extended to
include the luminous intermediate-redshift ELGs, suggesting that the X-ray and
radio emission processes are generally associated in star-forming galaxies.
This result implies that the X-ray emission can be used as an indicator of star
formation rate for star-forming galaxies. Finally, we show that there appear to
be two statistically distinct types of ISOCAM-detected star-forming galaxies:
those with detectable radio and X-ray emission and those without. The latter
type may have stronger mid-infrared emission-line features that increase their
detectability at mid-infrared wavelengths.Comment: 12 pages, 5 figures (1 color), LaTeX emulateapj5.sty, accepted for
publication by the Astronomical Journa
The Chandra Deep Field-North Survey. XVI. The X-ray properties ofmoderate-luminosity active galaxies at z > 4
We present X-ray spectral analyses of the three z>4 Active Galactic Nuclei
(AGNs) thus far spectroscopically identified in the Chandra Deep Field-North
Survey, at redshifts of 5.186, 4.424, and 4.137. These analyses are made
possible by the extremely deep exposure (2 Ms) and the low Chandra background.
The rest-frame 2.5-40 keV spectra are the first for optically faint (two of the
three sources have I>24) z>4 AGNs. The z=5.186 quasar is well fitted by a
power-law model with photon index Gamma=1.8+/-0.3, consistent with those of
lower-redshift, unobscured AGNs. The other two AGNs have flatter effective
X-ray photon indices (Gamma approx 1.1-1.5), suggesting the presence of
intrinsic absorption (provided their underlying X-ray continua are similar to
those of lower-redshift AGNs). It is possible that the flat X-ray continuum of
the z=4.424 AGN is partially related to its radio loudness. If the z=4.137 AGN
suffers from X-ray absorption, the implied column density is N_H=2x10^23 cm^-2.Comment: 4 pages, 3 figures, 2 tables, LaTeX emulateapj5.sty. Accepted for
publication by The Astrophysical Journal Letter
The Chandra Deep Field North Survey. X. X-ray emission from very redobjects
The multi-wavelength properties of Very Red Objects (VROs; I-K>=4) are largely unknown as many of these sources are optically faint (I>=24) and undetected at most wavelengths. Here we provide constraints on the X-ray properties of VROs using the 1 Ms Chandra exposure of an 8.4'x8.4' region within the Hawaii Flanking-Field area containing the HDF-N. We find that VROs detected in the hard band (2-8 keV) have flat X-ray spectral slopes (Gamma ~ 0.9) and X-ray properties consistent with those expected from luminous obscured AGN. The fraction of such sources in the K1.4), and the X-ray emission from these sources is consistent with that expected from less energetic processes (i.e., star formation, low-luminosity AGN activity, normal elliptical galaxy emission); star-formation and low-luminosity AGN activity scenarios are favored in those sources with irregular optical morphologies. Stacking analyses of the X-ray emission from VROs not individually detected at X-ray energies yield significant detections (>=99% confidence) in the soft band and the full band (0.5-8 keV). We find this X-ray emission is produced predominantly by the optically brightest VROs. The simplest explanation of this result is that we have detected the average X-ray emission from non-active VROs with low X-ray-to-optical flux ratios [log(FX/FI) ~ -2]; this is consistent with that expected if the majority of these VROs are ~ M* elliptical galaxies. A number of VROs are also detected with mid-IR (15um) and radio emission, and we provide constraints on the nature of this emission
Resolving the source populations that contribute to the X-raybackground: The 2 Ms Chandra Deep Field-North Survey
With ~2 Ms of Chandra exposure, the Chandra Deep Field-North (CDF-N) survey provides the deepest view of the Universe in the 0.5-8.0 keV band. Five hundred and three (503) X-ray sources are detected down to on-axis 0.5-2.0 keV and 2-8 keV flux limits of ~1.5x10^{-17} erg cm^{-2} s^{-1} and ~1.0x10^{-16} erg cm^{-2} s^{-1}, respectively. These flux limits correspond to L_{0.5-8.0 keV}~3x10^{41} erg s^{-1} at z=1 and L_{0.5-8.0 keV}~2x10^{43} erg s^{-1} at z=6; thus this survey is sensitive enough to detect starburst galaxies out to moderate redshift and Seyfert galaxies out to high redshift. We present the X-ray observations and basic analysis, describe the broad diversity of X-ray selected sources, and review the prospects for deeper Chandra exposures