- ID:
- ivo://CDS.VizieR/J/ApJ/765/87
- Title:
- Chandra observations of the 2QZ Cluster 1004+00
- Short Name:
- J/ApJ/765/87
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present results from a {approx}100ks Chandra observation of the 2QZ Cluster 1004+00 structure at z=2.23 (hereafter 2QZ Clus). 2QZ Clus was originally identified as an overdensity of four optically-selected QSOs at z=2.23 within a 15x15arcmin^2^ region. Narrow-band imaging in the near-IR (within the K band) revealed that the structure contains an additional overdensity of 22 z=2.23 H{alpha}-emitting galaxies (HAEs), resulting in 23 unique z=2.23 HAEs/QSOs (22 within the Chandra field of view). Our Chandra observations reveal that three HAEs in addition to the four QSOs harbor powerfully accreting supermassive black holes (SMBHs), with 2-10keV luminosities of ~(8-60)x10^43^erg/s and X-ray spectral slopes consistent with unobscured active galactic nucleus (AGN). Using a large comparison sample of 210 z=2.23 HAEs in the Chandra-COSMOS field (C-COSMOS), we find suggestive evidence that the AGN fraction increases with local HAE galaxy density. The 2QZ Clus HAEs reside in a moderately overdense environment (a factor of {approx}2 times over the field), and after excluding optically-selected QSOs, we find that the AGN fraction is a factor of {approx}3.5^+3.8^_-2.2_ times higher than C-COSMOS HAEs in similar environments. Using stacking analyses of the Chandra data and Herschel SPIRE observations at 250{mu}m, we respectively estimate mean SMBH accretion rates ((dM/dt)_BH_) and star formation rates (SFRs) for the 2QZ Clus and C-COSMOS samples.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/900/79
- Title:
- Chandra obs. of SDSS AGN pairs at z_med_~0.1
- Short Name:
- J/ApJ/900/79
- Date:
- 21 Mar 2022 09:29:38
- Publisher:
- CDS
- Description:
- We present a Chandra archival study of optically selected active galactic nucleus (AGN) pairs at a median redshift z~0.1. Out of 1286 AGN pairs (with projected separations rp<100kpc and velocity offsets {Delta}v<600km/s) optically identified from the Sloan Digital Sky Survey Seventh Data Release, we find 67 systems with archival Chandra observations, which represents the largest sample of optically selected AGN pairs studied in the X-ray. Among the 67 AGN pairs, 21 systems have both nuclei detected in the X-ray, 36 have one nucleus detected in the X-ray, and 10 have no X-ray detection. The X-ray detection rate, 78/134=58% (+/-7% 1{sigma} Poisson errors), is significantly higher than that (23/134=17%+/-4%) of a comparison sample of star-forming galaxy pairs, lending support to the optical AGN classification. In the conservative case where X-ray contamination from star formation is removed, the X-ray detection rate becomes 27%+/-4%, consistent with predictions from the latest galaxy merger simulations. The 2-10keV X-ray luminosity L2-10keV increases with decreasing projected separation in AGN pairs for rp>~15kpc, suggesting an enhancement of black hole accretion even in early-stage mergers. On the other hand, L2-10keV appears to decrease with decreasing projected separation at rp<~15kpc, which is contradictory to predictions from merger simulations. The apparent decrease in L2-10 keV of AGN pairs at rp<~15kpc may be caused by (i) enhanced absorbing columns from merger-induced gas inflows, (ii) feedback effects from early-stage mergers, and/or (iii) small number statistics. Future X-ray studies with larger samples are needed to put our results on firmer statistical ground.
- ID:
- ivo://CDS.VizieR/J/ApJ/819/162
- Title:
- Chandra obs. of ultracompact dwarf galaxies (UCDs)
- Short Name:
- J/ApJ/819/162
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the first comprehensive archival study of the X-ray properties of ultracompact dwarf (UCD) galaxies, with the goal of identifying weakly accreting central black holes in UCDs. Our study spans 578 UCDs distributed across 13 different host systems, including clusters, groups, fossil groups, and isolated galaxies. Of the 336 spectroscopically confirmed UCDs with usable archival Chandra imaging observations, 21 are X-ray-detected. Imposing a completeness limit of L_X_>2x10^38^erg/s, the global X-ray detection fraction for the UCD population is ~3%. Of the 21 X-ray-detected UCDs, seven show evidence of long-term X-ray time variability on the order of months to years. X-ray-detected UCDs tend to be more compact than non-X-ray-detected UCDs, and we find tentative evidence that the X-ray detection fraction increases with surface luminosity density and global stellar velocity dispersion. The X-ray emission of UCDs is fully consistent with arising from a population of low-mass X-ray binaries (LMXBs). In fact, there are fewer X-ray sources than expected using a naive extrapolation from globular clusters. Invoking the fundamental plane of black hole activity for SUCD1 near the Sombrero galaxy, for which archival Jansky Very Large Array imaging at 5GHz is publicly available, we set an upper limit on the mass of a hypothetical central black hole in that UCD to be <~10^5^M_{sun}_. While the majority of our sources are likely LMXBs, we cannot rule out central black holes in some UCDs based on X-rays alone, and so we address the utility of follow-up radio observations to find weakly accreting central black holes.
- ID:
- ivo://CDS.VizieR/J/ApJS/239/13
- Title:
- Chandra PHAT X-ray catalog of the M31 disk
- Short Name:
- J/ApJS/239/13
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The X-ray source populations within galaxies are typically difficult to identify and classify with X-ray data alone. We are able to break through this barrier by combining deep new Chandra ACIS-I observations with extensive Hubble Space Telescope (HST) imaging from the Panchromatic Hubble Andromeda Treasury of the M31 disk. We detect 373 X-ray sources down to 0.35-8.0keV flux of 10^-15^erg/cm^-2^/s over 0.4deg^2^, 170 of which are reported for the first time. We identify optical counterpart candidates for 188 of the 373 sources, after using the HST data to correct the absolute astrometry of our Chandra imaging to 0.1". While 58 of these 188 are associated with point sources potentially in M31, over half (107) of the counterpart candidates are extended background galaxies, 5 are star clusters, 12 are foreground stars, and 6 are supernova remnants. Sources with no clear counterpart candidate are most likely to be undetected background galaxies and low-mass X-ray binaries in M31. The hardest sources in the 1-8keV band tend to be matched to background galaxies. The 58 point sources that are not consistent with foreground stars are bright enough that they could be high-mass stars in M31; however, all but 8 have optical colors inconsistent with single stars, suggesting that many could be background galaxies or binary counterparts. For point-like counterparts, we examine the star formation history of the surrounding stellar populations to look for a young component that could be associated with a high-mass X-ray binary. About one-third of the point sources are not physically associated with a young population, and are therefore more likely to be background galaxies. For the 40 point-like counterpart candidates associated with young populations, we find that their age distribution has two peaks at 15-20Myr and 40-50Myr. If we only consider the 8 counterpart candidates with typical high-mass main-sequence optical star colors, their age distribution peaks mimic those of the sample of 40. Finally, we find that intrinsic faintness, and not extinction, is the main limitation for finding further counterpart candidates.
- ID:
- ivo://CDS.VizieR/J/ApJ/625/633
- Title:
- Chandra sources in B1600+434 and B1608+656 fields
- Short Name:
- J/ApJ/625/633
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We observed B1600+434 and B1608+656 with Chandra ACIS, detecting both quasar images in B1600+434 and three of four images in B1608+656. We also detected 157 serendipitous X-ray sources in the two Chandra fields and identified the brighter optical counterparts using the Sloan Digital Sky Survey and Palomar Digital Sky Survey.
- ID:
- ivo://CDS.VizieR/J/ApJ/770/98
- Title:
- Chandra study of NGC 6791
- Short Name:
- J/ApJ/770/98
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the first X-ray study of NGC 6791, one of the oldest open clusters known (8Gyr). Our Chandra observation is aimed at uncovering the population of close interacting binaries down to L_X_~1x10^30^erg/s (0.3-7keV). We detect 86 sources within 8' of the cluster center, including 59 inside the half-mass radius. We identify 20 sources with proper-motion cluster members, which are a mix of cataclysmic variables (CVs), active binaries (ABs), and binaries containing sub-subgiants. With follow-up optical spectroscopy, we confirm the nature of one CV. We discover one new, X-ray variable candidate CV with Balmer and He II emission lines in its optical spectrum; this is the first X-ray-selected CV in an open cluster. The number of CVs per unit mass is consistent with the field, suggesting that the 3-4 CVs observed in NGC 6791 are primordial. We compare the X-ray properties of NGC 6791 with those of a few old open (NGC 6819, M67) and globular clusters (47 Tuc, NGC 6397). It is puzzling that the number of ABs brighter than 1x10^30^erg/s normalized by cluster mass is lower in NGC 6791 than in M 67 by a factor ~3-7. CVs, ABs, and sub-subgiants brighter than 1x10^30^erg/s are under-represented per unit mass in the globular clusters compared to the oldest open clusters, and this accounts for the lower total X-ray luminosity per unit mass of the former. This indicates that the net effect of dynamical encounters may be the destruction of even some of the hardest (i.e., X-ray-emitting) binaries.
- ID:
- ivo://CDS.VizieR/J/ApJ/696/47
- Title:
- Chandra study of Rosette star-forming complex. II.
- Short Name:
- J/ApJ/696/47
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We explore here the young stellar populations in the Rosette Molecular Cloud (RMC) region with high spatial resolution X-ray images from the Chandra X-ray Observatory, which are effective in locating weak-lined T Tauri stars as well as disk-bearing young stars. A total of 395 X-ray point sources are detected, 299 of which (76%) have an optical or near-infrared (NIR) counterpart identified from deep FLAMINGOS images. From X-ray and mass sensitivity limits, we infer a total population of ~1700 young stars in the survey region. Based on smoothed stellar surface density maps, we investigate the spatial distribution of the X-ray sources and define three distinctive structures and substructures within them. Structures B and C are associated with previously known embedded IR clusters, while structure A is a new X-ray-identified unobscured cluster. A high-mass protostar RMCX 89=IRAS 06306+0437 and its associated sparse cluster are studied.
- ID:
- ivo://CDS.VizieR/J/A+A/655/A109
- Title:
- Chandra view of the LX-LUV relation in quasars
- Short Name:
- J/A+A/655/A109
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- We present a study of the relation between X-rays and ultraviolet emission in quasars for a sample of broad-line, radio-quiet objects obtained from the cross-match of the Sloan Digital Sky Survey DR14 with the latest Chandra Source Catalog 2.0 (2332 quasars) and the Chandra COSMOS Legacy survey (273 quasars). The non-linear relation between the ultraviolet (at 2500{AA}, LUV) and the X-ray (at 2keV, LX) emission in quasars has been proved to be characterised by a smaller intrinsic dispersion than the observed one, as long as a homogeneous selection, aimed at preventing the inclusion of contaminants in the sample, is fulfilled. By leveraging on the low background of Chandra, we performed a complete spectral analysis of all the data available for the SDSS-CSC2.0 quasar sample (i.e. 3430 X-ray observations), with the main goal of reducing the uncertainties on the source properties (e.g. flux, spectral slope). We analysed whether any evolution of the LX-LUV relation exists by dividing the sample in narrow redshift intervals across the redshift range spanned by our sample, z~=0.5-4. We find that the slope of the relation does not evolve with redshift and it is consistent with the literature value of 0.6 over the explored redshift range, implying that the mechanism underlying the coupling of the accretion disc and hot corona is the same at the different cosmic epochs. We also find that the dispersion decreases when examining the highest redshifts, where only pointed observations are available. These results further confirm that quasars are 'standardisable candles', that is we can reliably measure cosmological distances at high redshifts where very few cosmological probes are available.
- ID:
- ivo://CDS.VizieR/J/ApJ/588/1009
- Title:
- Chandra X-ray observations of NGC 2516
- Short Name:
- J/ApJ/588/1009
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a comprehensive study of the Chandra X-ray observations of the young open cluster NGC 2516. We have analyzed eight individual Chandra observations. We have combined the data to achieve the greatest sensitivity, reaching down to log f_X_=-14.56(erg/s/cm^2^), or logLX=28.69(erg/s) at the distance of NGC 2516. Out of 284 X-ray sources detected, 155 are identified with photometric cluster members, with very little ambiguity.
- ID:
- ivo://CDS.VizieR/J/ApJS/238/23
- Title:
- Chandra X-ray point sources in Abell 133
- Short Name:
- J/ApJS/238/23
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- As an evolutionary phase of galaxies, active galactic nuclei (AGNs) over a large range of redshifts have been utilized for understanding cosmic evolution. In particular, the population and evolution of AGNs have been investigated through the study of the cosmic X-ray background in various fields. As one of the deep fields observed by Chandra, with a total of 2.8Ms exposures, Abell 133 is a special region for investigating AGNs, providing a testbed for probing the environmental effects on AGN triggers, since cluster environments can be different from field environments. The achieved flux limits of data at the 50% completeness levels of 6.95x10^-16^, 1.43x10^-16^, and 1.57x10^-15^erg/s/cm^2^ are 0.5-8, 0.5-2, and 2-8keV. Using the wavdetect and no-source binomial probability (i.e., p<0.007), we analyze the combined Chandra image, detecting 1617 (in 0.5-8keV), 1324 (in 0.5-2keV), and 1028 (in 2-8keV) X-ray point sources in the Abell 133 region. Here, we present the X-ray point source catalog with the source fluxes, which can be combined with multiwavelength data for future works. We find that the number count distribution of the X-ray point sources is well reproduced with a broken power-law model, while the best-fit model parameters are sensitive to the fitting range of the number count distribution. Finally, we find an excess of number density (a decrease of AGN fraction) at the central region of the cluster, which reflects the effect of dense environments on AGN triggers, a finding similar to those of other studies of galaxy clusters.
