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W. Zheng

Publications and source records attributed to W. Zheng.

At least 127 records · Page 7Linked to original sources
arXiv2007

VLT and ACS observations of RDCS J1252.9-2927: dynamical structure and galaxy populations in a massive cluster at z=1.237

We present results from an extensive spectroscopic survey, carried out with VLT FORS, and from an extensive multiwavelength imaging data set from the HST Advanced Camera for Surveys and ground based facilities, of the cluster of galaxies RDCS J1252.9-2927. We have spectroscopically confirmed 38 cluster members in the redshift range 1.22 < z < 1.25. A cluster median redshift of z=1.237 and a rest-frame velocity dispersion of 747^{+74}_{-84} km/s are obtained. Using the 38 confirmed redshifts, we were able to resolve, for the first time at z > 1, kinematic structure. The velocity distribution, which is not Gaussian at the 95% confidence level, is consistent with two groups that are also responsible for the projected east-west elongation of the cluster. The groups are composed of 26 and 12 galaxies with velocity dispersions of 486^{+47}_{-85} km/s and 426^{+57}_{-105} km/s, respectively. The elongation is also seen in the intracluster gas and the dark matter distribution. This leads us to conclude that RDCS J1252.9-2927 has not yet reached a final virial state. We extend the analysis of the color-magnitude diagram of spectroscopic members to more than 1 Mpc from the cluster center. The scatter and slope of non-[OII]-emitting cluster members in the near-IR red sequence is similar to that seen in clusters at lower redshift. Furthermore, most of the galaxies with luminosities greater than ~ K_s*+1.5 do not show any [OII], indicating that these more luminous, redder galaxies have stopped forming stars earlier than the fainter, bluer galaxies. Our observations provide detailed dynamical and spectrophotometric information on galaxies in this exceptional high-redshift cluster, delivering an in-depth view of structure formation at this epoch only 5 Gyr after the Big Bang.

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arXiv2007

Lyman break galaxies, Lya emitters and a radio galaxy in a protocluster at z=4.1

We present deep HST/ACS observations in g,r,i,z towards the z=4.1 radio galaxy TN J1338-1942 and its overdensity of >30 spectroscopically confirmed Lya emitters (LAEs). We select 66 g-band dropouts to z=27, 6 of which are also a LAE. Although our color-color selection results in a relatively broad redshift range centered on z=4.1, the field of TN J1338-1942 is richer than the average field at the >5 sigma significance, based on a comparison with GOODS. The angular distribution is filamentary with about half of the objects clustered near the radio galaxy, and a small, excess signal (2 sigma) in the projected pair counts at separations of <10" is interpreted as being due to physical pairs. The LAEs are young (a few x 10^7 yr), small ( = 0.13") galaxies, and we derive a mean stellar mass of ~10^8-9 Msun based on a stacked K-band image. We determine star formation rates, sizes, morphologies, and color-magnitude relations of the g-dropouts and find no evidence for a difference between galaxies near TN J1338-1942 and in the field. We conclude that environmental trends as observed in clusters at much lower redshift are either not yet present, or are washed out by the relatively broad selection in redshift. The large galaxy overdensity, its corresponding mass overdensity and the sub-clustering at the approximate redshift of TN J1338-1942 suggest the assemblage of a >10^14 Msun structure, confirming that it is possible to find and study cluster progenitors in the linear regime at z>4.

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arXiv2007

Deep GEMINI GMOS-IFU spectroscopy of BAL QSOs: I. Decoupling the BAL, QSO, starburst, NLR, supergiant bubbles and galactic wind in Mrk 231

In this work we present the first results of a study of BAL QSOs (at low and high redshift), based on very deep Gemini GMOS integral field spectroscopy. In particular, the results obtained for the nearest BAL IR QSO Mrk 231 are presented. Very deep three-dimensional spectra and maps clearly show that the BAL systems I and II are extended (reaching 1.4-1.6" = 1.2-1.3 kpc, from the nucleus) and clearly elongated at the position angle close to the radio jet PA. Which suggest that the BAL systems I and II are both associated with the radio jet, and supporting the bipolar jet-wind model for some BALs. For the nuclear region of Mrk 231, the QSO and host-galaxy components were modelled, using a new technique of decoupling 3D spectra. From this study, the following main results were found: (i) in the pure host galaxy spectrum an extreme nuclear starburst component was clearly observed, mainly as a very strong increase in the flux, at the blue wavelengths; (ii) the BAL system I is observed in the spectrum of the host galaxy; (iii) in the clean/pure QSO emission spectrum, only broad lines were detected. 3D GMOS individual spectra (specially in the IR Ca II triplet) and maps confirm the presence of an extreme and young nuclear starburst (8 < age < 15 Myr), which was detected mainly in a ring or toroid with a radius r = 0.3" = 200 pc, around the very nucleus. The physical properties of the four expanding nuclear bubbles were analysed, using the GMOS 3D spectra and maps. These results suggest that an important part of the nuclear NLR is generated by the OF process and the associated low velocity ionizing shocks.

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arXiv2006

Most hard X-ray selected quasars in Chandra Deep Fields are obscured

Measuring the population of obscured quasars is one of the key issues to understand the evolution of active galactic nuclei (AGNs). With a redshift completeness of 99%, the X-ray sources detected in Chandra Deep Field South (CDF-S) provide the best sample for this issue. In this letter we study the population of obscured quasars in CDF-S by choosing the 4 -- 7 keV selected sample, which is less biased by the intrinsic X-ray absorption. The 4 -- 7 keV band selected samples also filter out most of the X-ray faint sources with too few counts, for which the measurements of N_H and L_X have very large uncertainties. Simply adopting the best-fit L_2-10keV and N_H, we find 71% (20 out of 28) of the quasars (with intrinsic L_2-10keV > 10^44 erg/s) are obscured with N_H > 10^22 cm^-2. Taking account of the uncertainties in the measurements of both N_H and L_X, conservative lower and upper limits of the fraction are 54% (13 out 24) and 84% (31 out 37). In Chandra Deep Field North, the number is 29%, however, this is mainly due to the redshift incompleteness. We estimate a fraction of ~ 50% - 63% after correcting the redshift incompleteness with a straightforward approach. Our results robustly confirm the existence of a large population of obscured quasars.

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arXiv2006

The UV spectrum of HS1700+6416 II. FUSE observations of the HeII Lyman alpha forest

We present the far-UV spectrum of the quasar HS1700+6416 taken with FUSE. This QSO provides the second line of sight with the HeII absorption resolved into a Ly alpha forest structure. Since HS1700+6416 is slightly less redshifted (z=2.72) than HE2347-4342, we only probe the post-reionization phase of HeII, seen in the evolution of the HeII opacity, which is consistent with a simple power law. The HeII/HI ratio eta is estimated using a line profile-fitting procedure and an apparent optical depth approach, respectively. The expected metal line absorption in the far-UV is taken into account as well as molecular absorption of galactic H_2. About 27% of the eta values are affected by metal line absorption. In order to investigate the applicability of the analysis methods, we create simple artificial spectra based on the statistical properties of the HI Ly alpha forest. The analysis of the artificial data demonstrates that the apparent optical depth method as well as the line profile-fitting procedure lead to confident results for restricted data samples only (12.0 < log N(HI) < 13.0). The reasons are saturation in the case of the apparent optical depth and thermal line widths in the case of the profile fits. Furthermore, applying the methods to the unrestricted data set may mimic a correlation between eta and the strength of the HI absorption. For the restricted data samples a scatter of 10 - 15% in eta would be expected even if the underlying value is constant. The observed scatter is significantly larger than expected, indicating that the intergalactic radiation background is indeed fluctuating. In the redshift range 2.58 < z < 2.72, where the data quality is best, we find eta ~ 100, suggesting a contribution of soft sources like galaxies to the UV background.

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arXiv2006

HST/ACS Multiband Coronagraphic Imaging of the Debris Disk around Beta Pictoris

(Abridged.) We present F435W (B), F606W (Broad V), and F814W (Broad I) coronagraphic images of the debris disk around Beta Pictoris obtained with HST's Advanced Camera for Surveys. We confirm that the previously reported warp in the inner disk is a distinct secondary disk inclined by ~5 deg from the main disk. The main disk's northeast extension is linear from 80 to 250 AU, but the southwest extension is distinctly bowed with an amplitude of ~1 AU over the same region. Both extensions of the secondary disk appear linear, but not collinear, from 80 to 150 AU. Within ~120 AU of the star, the main disk is ~50% thinner than previously reported. The surface-brightness profiles along the spine of the main disk are fitted with four distinct radial power laws between 40 and 250 AU, while those of the secondary disk between 80 and 150 AU are fitted with single power laws. These discrepancies suggest that the two disks have different grain compositions or size distributions. The F606W/F435W and F814W/F435W flux ratios of the composite disk are nonuniform and asymmetric about both projected axes of the disk. Within ~120 AU, the m_F435W-m_F606W and m_F435W-m_F814W colors along the spine of the main disk are ~10% and ~20% redder, respectively, than those of Beta Pic. These colors increasingly redden beyond ~120 AU, becoming 25% and 40% redder, respectively, than the star at 250 AU. We compare the observed red colors within ~120 AU with the simulated colors of non-icy grains having a radial number density ~r^-3 and different compositions, porosities, and minimum grain sizes. The observed colors are consistent with those of compact or moderately porous grains of astronomical silicate and/or graphite with sizes >0.15-0.20 um, but the colors are inconsistent with the blue colors expected from grains with porosities >90%.

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arXiv2006

X-ray spectral properties of AGN in the Chandra Deep Field South

We present a detailed X-ray spectral analysis of the sources in the 1Ms catalog of the Chandra Deep Field South (CDFS) taking advantage of optical spectroscopy and photometric redshifts for 321 sources. As a default spectral model, we adopt a power law with slope Gamma with an intrinsic redshifted absorption N_H, a fixed Galactic absorption and an unresolved Fe emission line. For 82 X-ray bright sources, we perform the X-ray spectral analysis leaving both Gamma and N_H free. The weighted mean value is ~ 1.75+-0.02, with an intrinsic dispersion of sigma~0.30. We do not find hints of a correlation between Gamma and the N_H. We detect the presence of a scattered component at soft energies in 8 sources, and a pure reflection spectrum, typical of Compton-thick AGN, in 14 sources (Compton-thick AGN candidates). The intrinsic N_H distribution shows a lognormal shape, peaking around log(N_H)~23.1 and with sigma~1.1. We find that the fraction of absorbed sources (with N_H>10^{22} cm^{-2}) in the sample is constant (at the level of about 75%) or moderately increasing with redshift. Finally, we compare the optical classification to the X-ray spectral properties, confirming that the correspondence of unabsorbed (absorbed) X-ray sources to optical Type I (Type II) AGN is accurate for at least 80% of the sources with spectral identification (1/3 of the total X-ray sample).

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arXiv2006

Seventy-One New L and T Dwarfs from the Sloan Digital Sky Survey

We present near-infrared observations of 71 newly discovered L and T dwarfs, selected from imaging data of the Sloan Digital Sky Survey (SDSS) using the i-dropout technique. Sixty-five of these dwarfs have been classified spectroscopically according to the near-infrared L dwarf classification scheme of Geballe et al. and the unified T dwarf classification scheme of Burgasser et al. The spectral types of these dwarfs range from L3 to T7, and include the latest types yet found in the SDSS. Six of the newly identified dwarfs are classified as early- to mid-L dwarfs according to their photometric near-infrared colors, and two others are classified photometrically as M dwarfs. We also present new near-infrared spectra for five previously published SDSS L and T dwarfs, and one L dwarf and one T dwarf discovered by Burgasser et al. from the Two Micron All Sky Survey. The new SDSS sample includes 27 T dwarfs and 30 dwarfs with spectral types spanning the complex L-T transition (L7-T3). We continue to see a large (~0.5 mag) spread in J-H for L3 to T1 types, and a similar spread in H-K for all dwarfs later than L3. This color dispersion is probably due to a range of grain sedimentation properties, metallicity, and gravity. We also find L and T dwarfs with unusual colors and spectral properties that may eventually help to disentangle these effects.

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arXiv2005

An Overdensity of Galaxies near the Most Distant Radio-Loud Quasar

A five square arcminute region around the luminous radio-loud quasar SDSS J0836+0054 (z=5.8) hosts a wealth of associated galaxies, characterized by very red (1.3 < i_775 - z_{850} < 2.0) color. The surface density of these z~5.8 candidates is approximately six times higher than the number expected from deep ACS fields. This is one of the highest galaxy overdensities at high redshifts, which may develop into a group or cluster. We also find evidence for a substructure associated with one of the candidates. It has two very faint companion objects within two arcseconds, which are likely to merge. The finding supports the results of a recent simulation that luminous quasars at high redshifts lie on the most prominent dark-matter filaments and are surrounded by many fainter galaxies. The quasar activity from these regions may signal the buildup of a massive system.

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arXiv2005

The Ionized Gas and Nuclear Environment in NGC 3783 V. Variability and Modeling of the Intrinsic Ultraviolet Absorption

We present results on the location, physical conditions, and geometry of the outflow in the Seyfert 1 galaxy NGC 3783 from a study of the variable intrinsic UV absorption. Based on 18 observations with HST/STIS and 6 observations with FUSE, we find: 1) The absorption from the lowest-ionization species in each of the three strong kinematic components varied inversely with the continuum flux, indicating the ionization structure responded to changes in the photoionizing flux over the weekly timescales sampled by our observations. 2) A multi- component model with an unocculted NLR and separate BLR and continuum line-of-sight covering factors predicts saturation in several lines, consistent with the lack of observed variability. 3) Column densities for the individual metastable levels are measured from the resolved C III *1175 absorption complex observed in one component. Based on our computed metastable level populations, the electron density of this absorber is ~3x10^4 cm^-3. Photoionization modeling results place it at ~25 pc from the central source. 4) Using time-dependent calculations, we are able to reproduce the detailed variability observed in this absorber, and derive upper limits on the distances for the other components of 25-50 pc. 5) The ionization parameters derived for the higher ionization UV absorbers are consistent with the modeling results for the lowest-ionization X-ray component, but with smaller total column density. They have similar pressures as the three X-ray ionization components. These results are consistent with an inhomogeneous wind model for the outflow in NGC 3783. 6) Based on the predicted emission-line luminosities, global covering factor constraints, and distances derived for the UV absorbers, they may be identified with emission- line gas observed in the inner NLR of AGNs. (abridged)

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arXiv2005

A Dynamical Simulation of the Debris Disk Around HD 141569A

We study the dynamical origin of the structures observed in the scattered-light images of the resolved debris disk around HD 141569A. We explore the roles of radiation pressure from the central star, gas drag from the gas disk, and the tidal forces from two nearby stars in creating and maintaining these structures. We use a simple one-dimensional axisymmetric model to show that the presence of the gas helps confine the dust and that a broad ring of dust is produced if a central hole exists in the disk. This model also suggests that the disk is in a transient, excited dynamical state, as the observed dust creation rate applied over the age of the star is inconsistent with submillimeter mass measurements. We model in two dimensions the effects of a fly-by encounter between the disk and a binary star in a prograde, parabolic, coplanar orbit. We track the spatial distribution of the disk's gas, planetesimals, and dust. We conclude that the surface density distribution reflects the planetesimal distribution for a wide range of parameters. Our most viable model features a disk of initial radius 400 AU, a gas mass of 50 M_earth, and beta = 4 and suggests that the system is being observed within 4000 yr of the fly-by periastron. The model reproduces some features of HD 141569A's disk, such as a broad single ring and large spiral arms, but it does not reproduce the observed multiple spiral rings or disk asymmetries nor the observed clearing in the inner disk. For the latter, we consider the effect of a 5 M_Jup planet in an eccentric orbit on the planetesimal distribution of HD 141569A.

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arXiv2005

The Chandra Deep Field South/GOODS survey. Optically faint X-ray sources

We provide important new constraints on the nature and redshift distribution of optically faint (R>25) X-ray sources in the Chandra Deep Field South Survey. We show that we can derive accurate photometric redshifts for the spectroscopically unidentified sources thus maximizing the redshift completeness for the whole X-ray sample. Our new redshift distribution for the X-ray source population is in better agreement with that predicted by X-ray background synthesis models; however, we still find an overdensity of low redshift (z<1) sources. The optically faint sources are mainly X-ray absorbed AGN, as determined from direct X-ray spectral analysis and other diagnostics. Many of these optically faint sources have high (>10) X-ray-to-optical flux ratios. We also find that ~71% of them are well fitted with the SED of an early-type galaxy with ~1.9 and the remaining 29% with irregular or starburst galaxies mainly at z_phot>3. We estimate that 23% of the optically faint sources are X-ray absorbed QSOs. The overall population of X-ray absorbed QSOs contributes a ~15% fraction of the [2-10] keV X-ray Background (XRB) whereas current XRB synthesis models predict a ~38% contribution.

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arXiv2005

IR mergers and IR QSOs with galactic winds. III. Mrk 231: composite outflow with multiple expanding bubbles and BALs (an exploding young QSO?)

A study of outflow (OF) and BAL systems in Mrk 231 (and in similar IR QSOs) is presented. This study is based mainly on 1D and 2D spectroscopy (obtained at La Palma/WHT, HST, IUE, ESO/NTT, KPNO, APO and CASLEO observatories) plus HST images. For Mrk 231 we report evidence that the extreme nuclear OF process has at least 3 main components, which are probably associated with: (i) the radio jet, at pc scale, and (ii(the extreme starburst at pc and kpc scale. This OF has generate 4 or more concentric expanding bubbles and the BAL systems. The variability of the short lived BAL-III system was studied. Finally, the IR colour diagram and the UV BAL system of IR+OF+FeII QSOs are analysed. We propose that the phase of young QSO is associated with:accretion of large amount of gas (by the SMBH)+extreme starburst+extreme composite OFs/BALs.

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arXiv2004

The Luminosity Functions of the Galaxy Cluster MS1054-0321 at z=0.83 based on ACS Photometry

We present new measurements of the galaxy luminosity function (LF) and its dependence on local galaxy density, color, morphology, and clustocentric radius for the massive z=0.83 cluster MS1054-0321. Our analyses are based on imaging performed with the ACS onboard the HST in the F606W, F775W and F850LP passbands and extensive spectroscopic data obtained with the Keck LRIS. Our main results are based on a spectroscopically selected sample of 143 cluster members with morphological classifications derived from the ACS observations. Our three primary findings are (1) the faint-end slope of the LF is steepest in the bluest filter, (2) the LF in the inner part of the cluster (or highest density regions) has a flatter faint-end slope, and (3) the fraction of early-type galaxies is higher at the bright end of the LF, and gradually decreases toward fainter magnitudes. These characteristics are consistent with those in local galaxy clusters, indicating that, at least in massive clusters, the common characteristics of cluster LFs are established at z=0.83. We also find a 2sigma deficit of intrinsically faint, red galaxies (i-z>0.5, Mi>-19) in this cluster. This trend may suggest that faint, red galaxies (which are common in z<0.1 rich clusters) have not yet been created in this cluster at z=0.83. The giant-to-dwarf ratio in MS1054-0321 starts to increase inwards of the virial radius or when Sigma>30 Mpc^-2, coinciding with the environment where the galaxy star formation rate and the morphology-density relation start to appear. (abridged)

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arXiv2004

Classification and Characterization of Objects from GALEX and SDSS surveys

We use the GALEX (Galaxy Evolution Explorer) Medium Imaging Survey (MIS) and All-Sky Imaging Survey (AIS) data available in the first internal release, matched to the SDSS catalogs in the overlapping regions, to classify objects by comparing the multi-band photometry to model colors. We show an example of the advantage of such broad wavelength coverage (GALEX far-UV and near-UV, SDSS ugriz) in classifying objects and augmenting the existing samples and catalogs. From the MIS [AIS] sample over an area of 75 [92] square degrees, we select a total of 1736 [222] QSO candidates at redshift less than 2, significantly extending the number of fainter candidates, and moderately increasing the number of bright objects in the SDSS list of spectroscopically confirmed QSO. Numerous hot stellar objects are also revealed by the UV colors, as expected.

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arXiv2004

The spatial clustering of X-ray selected AGN and galaxies in the Chandra Deep Field South and North

We investigate the spatial clustering of X-ray selected sources in the two deepest X-ray fields to date, namely the 2Msec Chandra Deep Field North (CDFN) and the 1Msec Chandra Deep Field South (CDFS). The projected correlation function w(r_p), measured on scales ~0.2-10 h^-1 Mpc for a sample of 240 sources with spectroscopic redshift in the CDFN and 124 sources in the CDFS at a median redshift of z~0.8, is used to constrain the amplitude and slope of the real space correlation function xi(r)=(r/r0)^-gamma. The clustering signal is detected at high confidence (>~ 7 sigma) in both fields. The amplitude of the correlation is found to be significantly different in the two fields, the correlation length r0 being 8.6 +- 1.2 h^-1 Mpc in the CDFS and 4.2 +- 0.4 h^-1 Mpc in the CDFN, while the correlation slope gamma is found to be flat in both fields: gamma=1.33 +- 0.11 in the CDFS and gamma=1.42 +- 0.07 in the CDFN (a flat Universe with Omega_m=0.3 and Omega_L=0.7 is assumed; 1 sigma Poisson error estimates are considered). The correlation function has been also measured separately for sources classified as AGN or galaxies. In both fields AGN have a median redshift of z~0.9 and a median 0.5-10 keV luminosity of L_x~10^43 erg s^-1, i.e. they are generally in the Seyfert luminosity regime. As in the case of the total samples, we found a significant difference in the AGN clustering amplitude between the two fields, the best fit correlation parameters being r0=10.3 +- 1.7 h^-1 Mpc, gamma=1.33 +- 0.14 in the CDFS, and r0=5.5 +- 0.6 h^-1 Mpc, gamma=1.50 +- 0.12 in the CDFN. Within each field no statistically significant difference is found between soft and hard X-ray selected sources or between type 1 and type 2 AGN. (abridged)

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arXiv2004

The Luminosity Function of Early-Type Galaxies at z~0.75

We measure the luminosity function of morphologically selected E/S0 galaxies from $z=0.5$ to $z=1.0$ using deep high resolution Advanced Camera for Surveys imaging data. Our analysis covers an area of $48\Box\arcmin$ (8$\times$ the area of the HDF-N) and extends 2 magnitudes deeper ($I\sim24$ mag) than was possible in the Deep Groth Strip Survey (DGSS). At $0.5 1.7$, E/S0 galaxies at brighter luminosities ($M_B<-20.1$), but are increasingly different at fainter magnitudes where `blue' galaxies are both smaller and have lower Sersic parameters. Fits of the colors to stellar population models suggest that most E/S0 galaxies have short star-formation time scales ($τ<1$ Gyr), and that galaxies have formed at an increasing rate from $z\sim8$ until $z\sim2$ after which there has been a gradual decline.

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