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Register a new userThe Chandra Deep Field-North Survey. XVI. The X-ray properties of moderate-luminosity Active Galaxies at z>4
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C. Vignali
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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.
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The ~1 Ms Chandra Deep Field North observation is used to study the extended X-ray sources in the region surrounding the Hubble Deep Field North (HDF-N), yielding the most sensitive probe of extended X-ray emission at cosmological distances to date. A total of six such sources are detected, the majority of which align with small numbers of optically bright galaxies. Their angular sizes, band ratios, and X-ray luminosities -- assuming they lie at the same distances as the galaxies coincident with the X-ray emission -- are generally consistent with the properties found for nearby groups of galaxies. One source is notably different and is likely to be a poor-to-moderate X-ray cluster at high redshift (i.e., z > 0.7). We are also able to place strong constraints on the optically detected cluster of galaxies ClG 1236+6215 at z=0.85 and the wide-angle-tail radio galaxy VLA J123725.7+621128 at z~1-2. With rest-frame 0.5--2.0 keV X-ray luminosities of <(3-15)e42 ergs s^{-1}, the environments of both sources are either likely to have a significant deficit of hot intra-cluster gas compared to local clusters of galaxies, or they are X-ray groups. We find the surface density of extended X-ray sources in this observation to be 167 (+97,-67) deg^{-2} at a limiting soft-band flux of approximately 3e-16 ergs s^{-1} cm^{-2}. No evolution in the X-ray luminosity function of clusters is needed to explain this value. (Abridged)
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.
We present an analysis of the X-ray emission from a large sample of ultraviolet (UV) selected, star forming galaxies with 0.74<z<1.32 in the Hubble Deep Field North (HDF-N) region. By excluding all sources with significant detected X-ray emission in the 2 Ms Chandra observation we are able to examine the properties of galaxies for which the emission in both UV and X-ray is expected to be predominantly due to star formation. Stacking the X-ray flux from 216 galaxies in the soft and hard bands produces significant detections. The derived mean 2-10 keV rest-frame luminosity is 2.97+/-0.26x10^(40) erg/s, corresponding to an X-ray derived star formation rate (SFR) of 6.0+/-0.6 Msolar/yr. Comparing the X-ray value with the mean UV derived SFR, uncorrected for attenuation, we find that the average UV attenuation correction factor is ~3. By binning the galaxy sample according to UV magnitude and colour, correlations between UV and X-ray emission are also examined. We find a strong positive correlation between X-ray emission and rest-frame UV emission. A correlation between the ratio of X-ray-to-UV emission and UV colour is also seen, such that L(X)/L(UV) increases for redder galaxies. Given that X-ray emission offers a view of star formation regions that is relatively unaffected by extinction, results such as these can be used to evaluate the effects of dust on the UV emission from high-z galaxies. For instance we derive a relationship for estimating UV attenuation corrections as a function of colour excess. The observed relation is inconsistent with the Calzetti et al. (2000) reddening law which over predicts the range in UV attenuation corrections by a factor of ~100 for the UV selected z~1 galaxies in this sample (abridged).
We have analyzed optically bright, X-ray faint [OBXF; i.e., log(fX/fR) < -2] sources identified in an 178.9 square arcminute area within the Chandra Deep Field-North (CDF-N) 2 Ms survey. We find 43 OBXF sources in this area, comprising ~15% of the X-ray sources above a 0.5--2 keV flux of 2.3e-17 erg cm^-2 s^-1. We present spectroscopic identifications for 42 of the OBXF sources and optical spectra for 25, including 5 previously unpublished redshifts. Deep optical imaging data (either HST or ground-based) are presented for all the OBXF sources. The OBXF population consists mainly of normal and starburst galaxies detected out to cosmologically significant distances (i.e., to a median redshift of z=0.297 and a full redshift range z=0.06-0.845). This is notable since these distances equate to look-back times of up to ~8 Gyr; we are thus provided with a window on the X-ray emission from galaxies at redshifts much closer to the cosmic star formation peak than was possible prior to Chandra. The X-ray luminosity distribution of OBXF sources extends to higher luminosity than does that of normal galaxies indicating that a significant fraction are likely dominated by low-luminosity AGN (LLAGN) or vigorous star formation. By combining the detected X-ray counts, we find the average OBXF X-ray spectrum to be consistent with a Gamma=2.0 power law. The 0.5--2 keV log N-log S for the OBXF galaxies is much steeper (alpha=-1.7) than for the general X-ray source population. Indeed, the number of OBXF sources has doubled between the 1~Ms and 2~Ms survey, rising sharply in numbers at faint fluxes. The extragalactic OBXF sources are found to contribute ~1-2% of the soft extragalactic X-ray background.
We study the clustering properties of about 1200 z~4 Lyman Break Galaxy (LBG) candidates with i<26 which are selected by color from deep BRi imaging data of a 618 arcmin^2 area in the Subaru/XMM-Newton Deep Field taken with Subaru Prime Focus Camera. The contamination and completeness of our LBG sample are evaluated, on the basis of the Hubble Deep Field North (HDFN) objects, to be 17% and 45%, respectively. We derive the angular correlation function over theta = 2-1000, and find that it is fitted fairly well by a power law, omega(theta)=A_omega theta^{-0.8}, with A_omega = 0.71 +/- 0.26. We then calculate the correlation length r0 (in comoving units) of the two-point spatial correlation function xi(r) = (r/r0)^{-1.8} from A_omega using the redshift distribution of LBGs derived from the HDFN, and find r0=2.7 (+0.5/-0.6) h^{-1} Mpc in a Lambda-dominated universe (Omega_m=0.3 and Omega_Lambda=0.7). This is twice larger than the correlation length of the dark matter at z~4 predicted from an analytic model by Peacock & Dodds but about twice smaller than that of bright galaxies predicted by a semi-analytic model of Baugh et al. We find an excess of omega(theta) on small scales (theta < 5) departing from the power law fit over 3 sigma significance levels. Interpreting this as due to galaxy mergers, we estimate the fraction of galaxies undergoing mergers in our LBG sample to be 3.0 +/- 0.9%, which is significantly smaller than those of galaxies at intermediate redshifts.
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