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We present point-source catalogs for the ~2 Ms exposure of the Chandra Deep Field-South (CDF-S); this is one of the two most-sensitive X-ray surveys ever performed. The survey covers an area of ~436 arcmin^2 and reaches on-axis sensitivity limits of ~1.9x10^{-17} and ~1.3x10^{-16} ergs/cm^2/s for the 0.5-2.0 and 2-8 keV bands, respectively. Four hundred and sixty-two X-ray point sources are detected in at least one of three X-ray bands that were searched; 135 of these sources are new compared to the previous ~1 Ms CDF-S detections. Source positions are determined using centroid and matched-filter techniques; the median positional uncertainty is ~0.36". The X-ray-to-optical flux ratios of the newly detected sources indicate a variety of source types; ~55% of them appear to be active galactic nuclei while ~45% appear to be starburst and normal galaxies. In addition to the main Chandra catalog, we provide a supplementary catalog of 86 X-ray sources in the ~2 Ms CDF-S footprint that was created by merging the ~250 ks Extended Chandra Deep Field-South with the CDF-S; this approach provides additional sensitivity in the outer portions of the CDF-S. A second supplementary catalog that contains 30 X-ray sources was constructed by matching lower significance X-ray sources to bright optical counterparts (R, 18 pages, 15 figures, ApJS in press. Data and images available at http://www.astro.psu.edu/users/niel/cdfs/cdfs-chandra.html

We investigate morphological structure parameters and local environments of distant moderate-luminosity active galactic nucleus (AGN) host galaxies in the overlap between the HST/ACS observations of the Great Observatories Origins Deep Survey (GOODS) and the two Chandra Deep Fields. We compute near-neighbor counts and BViz asymmetry (A) and concentration (C) indices for ~35,500 GOODS/ACS galaxies complete to z_850 ~ 26.6, including the resolved hosts of 322 X-ray-selected AGNs. Distributions of (1) z_850 asymmetry for 130 AGN hosts at z_850 < 23 and (2) near-neighbor counts for 173 AGN hosts at z_850 < 24 are both consistent with non-AGN control samples. This implies no close connection between recent galaxy mergers and moderate-luminosity AGN activity out to appreciable look-back times (z < 1.3), approaching the epoch of peak AGN activity in the universe. The distribution of z_850 concentrations for the AGN hosts is offset by +0.5 compared to the non-AGN, a 6.4-sigma discrepancy much larger than can be explained by the possible influence of unresolved emission from the AGN or a circumnuclear starburst. The local universe association between AGN and bulge-dominated galaxies thus persists to substantial look-back time. We discuss implications in the context of the low-redshift supermassive central black hole mass correlation with host galaxy properties, including concentration., Astrophysical Journal Letters, in press. Figure 1 available at full resolution upon request

Using early data from the Infrared Array Camera (IRAC) on the Spitzer Space Telescope, taken for the Great Observatories Origins Deep Survey (GOODS), we identify and study objects that are well-detected at 3.6um, but are very faint (and in some cases, invisible) in the Hubble Ultra Deep Field (HUDF) ACS and NICMOS images and in very deep VLT Ks-band imaging. We select a sample of 17 objects with f_nu(3.6um)/f_nu(z_{850})>20. The analysis of their spectral energy-distributions (SEDs) from 0.4 to 8.0um shows that the majority of these objects cannot be satisfactorily explained without a well-evolved stellar population. We find that most of them can be well fitted by a simple two-component model, where the primary component represents a massive, old population that dominates the strong IR emission, while the secondary component represents a low amplitude, on-going star-formation process that accounts for the weak optical fluxes. Their estimated photometric redshifts (z_p) range from 1.6 to 2.9, with the median at z_p=2.4. For the simple star formation histories considered here, their corresponding stellar masses range from 0.1 to 1.6e11 M_sun for a Chabrier Initial Mass Function (IMF). Their median rest-frame Ks-band absolute magnitude is -22.9 mag in AB system, or 1.5xL*(K) for present--day elliptical galaxies. In the scenario of pure luminosity evolution, such objects may be direct progenitors for at least 14 to 51% of the local population of early--type galaxies. Due to the small cosmic volume of the HUDF, however, this simple estimate could be affected by other effects such as cosmic variance and the strong clustering of massive galaxies. A full analysis of the entire GOODS area is now underway to assess such effects.

We studied the X-ray variability of sources detected in the Chandra Deep Field South (Giacconi et al. 2002), nearly all of which are low to moderate z AGN (Tozzi et al. 2001). We find that 45% of the sources with >100 counts exhibit significant variability on timescales ranging from a day up to a year. The fraction of sources found to be variable increases with observed flux, suggesting that >90% of all AGNs possess intrinsic variability. We also find that the fraction of variable sources appears to decrease with increasing intrinsic absorption; a lack of variability in hard, absorbed AGNs could be due to an increased contribution of reflected X-rays to the total flux. We do not detect significant spectral variability in the majority (~70%) of our sources. In half of the remaining 30%, the hardness ratio is anti-correlated with flux, mimicking the high/soft-low/hard states of galactic sources. The X-ray variability appears anti-correlated with the luminosity of the sources, in agreement with previous studies. High redshift sources, however, have larger variability amplitudes than expected from extrapolations of their low-z counterparts, suggesting a possible evolution in the accretion rate and/or size of the X-ray emitting region. Finally, we discuss some effects that may produce the observed decrease in the fraction of variable sources from z=0.5 out to z=2., 24 pages, including 15 figures and 1 table. In press on ApJ

We present a wide-field study of the globular cluster (GC)/low-mass X-ray binary (LMXB) connection in the giant elliptical NGC 1399. The large field of view of the Advanced Camera for Surveys/WFC, combined with Hubble Space Telescope and Chandra high resolution, allow us to constrain the LMXB formation scenarios in elliptical galaxies. We confirm that NGC 1399 has the highest LMXB fraction in GCs of all nearby elliptical galaxies studied so far, even though the exact value depends on galactocentric distance due to the interplay of a differential GC versus galaxy light distribution and the GC color dependence. In fact, LMXBs are preferentially hosted by bright, red GCs out to > 5R(eff) of the galaxy light. The finding that GCs hosting LMXBs follow the radial distribution of their parent GC population argues against the hypothesis that the external dynamical influence of the galaxy affects the LMXB formation in GCs. On the other hand, field-LMXBs closely match the host galaxy light, thus indicating that they are originally formed in situ and not inside GCs. We measure GC structural parameters, finding that the LMXB formation likelihood is influenced independently by mass, metallicity, and GC structural parameters. In particular, the GC central density plays a major role in predicting which GCs host accreting binaries. Finally, our analysis shows that LMXBs in GCs are marginally brighter than those in the field, and in particular the only color-confirmed GC with L(X) > 10(39) erg s(-1) shows no variability, which may indicate a superposition of multiple LMXBs in these systems.

We present a comprehensive high spatial-resolution imaging study of globular clusters (GCs) in NGC1399, the central giant elliptical cD galaxy in the Fornax galaxy cluster, conducted with HST/ACS. Using a novel technique to construct drizzled PSF libraries for HST/ACS data, we accurately determine the fidelity of GC structural parameter measurements from detailed artificial star cluster experiments. The measurement of rh for the major fraction of the NGC1399 GC system reveals a trend of increasing rh versus galactocentric distance, Rgal, out to about 10 kpc and a flat relation beyond. This trend is very similar for blue and red GCs which are found to have a mean size ratio of rh(red)/rh(blue)=0.82+/-0.11 at all galactocentric radii from the core regions of the galaxy out to ~40 kpc. This suggests that the size difference between blue and red GCs is due to internal mechanisms related to the evolution of their constituent stellar populations. Modeling the mass density profile of NGC1399 shows that additional external dynamical mechanisms are required to limit the GC size in the galaxy halo regions to rh~2 pc. We suggest that this may be realized by an exotic GC orbit distribution function, an extended dark matter halo, and/or tidal stress induced by the increased stochasticity in the dwarf halo substructure at larger Rgal. We match our GC rh measurements with radial velocity data from the literature and find that compact GCs show a significantly smaller line-of-sight velocity dispersion, =225+/-25 km/s, than their extended counterparts, =317+/-21 km/s. Considering the weaker statistical correlation in the GC rh-color and the GC rh-Rgal relations, the more significant GC size-dynamics relation appears to be astrophysically more relevant and hints at the dominant influence of the GC orbit distribution function on the evolution of GC structural parameters., Comment: 22 pages, 17 figures, accepted for publication in The Astrophysical Journal, a high-quality PDF version is available at http://www.astro.puc.cl/~tpuzia/PUC/Home.html

We present initial results from an exploratory X-ray monitoring project of two groups of comparably luminous radio-quiet quasars (RQQs). The first consists of four sources at 4.10, 18 pages (emulateapj), 8 figures. Accepted for publication in ApJ

We study the statistical properties of the normalized excess variance of variability process characterized by a "red-noise" power spectral density (PSD), as in the case of active galactic nuclei (AGNs). We perform Monte Carlo simulations of light curves, assuming both a continuous and a sparse sampling pattern and various signal-to-noise ratios (S/Ns). We show that the normalized excess variance is a biased estimate of the variance even in the case of continuously sampled light curves. The bias depends on the PSD slope and on the sampling pattern, but not on the S/N. We provide a simple formula to account for the bias, which yields unbiased estimates with an accuracy better than 15%. We show that the normalized excess variance estimates based on single light curves (especially for sparse sampling and S/N < 3) are highly uncertain (even if corrected for bias) and we propose instead the use of an "ensemble estimate," based on multiple light curves of the same object, or on the use of light curves of many objects. These estimates have symmetric distributions, known errors, and can also be corrected for biases. We use our results to estimate the ability to measure the intrinsic source variability in current data, and show that they could also be useful in the planning of the observing strategy of future surveys such as those provided by X-ray missions studying distant and/or faint AGN populations and, more in general, in the estimation of the variability amplitude of sources that will result from future surveys such as Pan-STARRS and LSST. © 2013. The American Astronomical Society. All rights reserved.