Wang, Daniel
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Professor, Department of Astronomy
Last Name
Wang
First Name
Daniel
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Astrophysics and Astronomy
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Introduction
Professor Wang is a Professor in the Department of Astronomy at the University of Massachusetts at Amherst. He received his Ph.D. in Astronomy in 1990 from Columbia University. He was awarded the ASP Robert J. Trumpler Award for Outstanding North American Ph.D Dissertation Research in Astronomy. He was then an Edwin P. Hubble postdoctoral fellow at University of Colorado and later a Lindheimer fellow at Northwestern University. He was also a member of the Institute for Advanced Study at Princeton, was the Siyuan Visiting Chair Professor at Nanjing University, and recently served on the Galactic Neighborhood Frontier Science Panel of Astro 2010 - the Decadal Survey on Astronomy and Astrophysics. He will be visiting University of Cambridge as a Raymond and Beverley Sackler Distinguished Visitor. He has published 140+ research papers in refereed journals, including four in Nature as the 1st author; about 50 over the last five years. His publication covers a broad range of topics: quark/neutron stars, pulsars, X-ray binaries, supernova remnants, superbubbles, hot gas in intracluster and intergalactic space as well as in individual galaxies, hydrodynamic simulations of hot gas, and active galactic nuclei. His current research focuses on the hot interstellar and intergalactic media, the feedback and evolution of galaxies, and galactic nuclear regions. He mainly uses infrared, ultraviolet, and X-ray observations to conduct these studies. He also carries out theoretical and computational studies with my students and collaborators.
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Publication Open Access X-raying Galaxies: A Chandra Legacy(2010) Wang, QDThis presentation reviews Chandra’s major contribution to the understanding of nearby galaxies. After a brief summary on significant advances in characterizing various types of discrete x-ray sources, the presentation focuses on the global hot gas in and around galaxies, especially normal ones like our own. The hot gas is a product of stellar and active galactic nuclear feedback—the least understood part in theories of galaxy formation and evolution. Chandra observations have led to the first characterization of the spatial, thermal, chemical, and kinetic properties of the gas in our galaxy. The gas is concentrated around the galactic bulge and disk on scales of a few kiloparsec. The column density of chemically enriched hot gas on larger scales is at least an order magnitude smaller, indicating that it may not account for the bulk of the missing baryon matter predicted for the galactic halo according to the standard cosmology. Similar results have also been obtained for other nearby galaxies. The x-ray emission from hot gas is well correlated with the star formation rate and stellar mass, indicating that the heating is primarily due to the stellar feedback. However, the observed x-ray luminosity of the gas is typically less than a few percent of the feedback energy. Thus the bulk of the feedback (including injected heavy elements) is likely lost in galaxy-wide outflows. The results are compared with simulations of the feedback to infer its dynamics and interplay with the circumgalactic medium, hence the evolution of galaxies.Publication Open Access An Optical Study Of Stellar And Interstellar Environments Of Seven Luminous And Ultraluminous X-ray Sources(2006) Ramsey, CJ; Williams, RM; Gruendl, RA; Chen, C-HR; Chu, Y-H; Wang, QDWe have studied the stellar and interstellar environments of two luminous X-ray sources and five ultraluminous X-ray sources (ULXs) in order to gain insight into their nature. Archival Hubble Space Telescope images were used to identify the optical counterparts of the ULXs Ho IX X-1 and NGC 1313 X-2, and to make photometric measurements of the local stellar populations of these and the luminous source IC 10 X-1. We obtained high-dispersion spectroscopic observations of the nebulae around these seven sources to search for He II λ4686 emission and to estimate the expansion velocities and kinetic energies of these nebulae. Our observations did not detect nebular He II emission from any source, with the exception of LMC X-1; this is either because we missed the He III regions or because the nebulae are too diffuse to produce He II surface brightnesses that lie within our detection limit. We compare the observed ionization and kinematics of the supershells around the ULXs Ho IX X-1 and NGC 1313 X-2 with the energy feedback expected from the underlying stellar population to assess whether additional energy contributions from the ULXs are needed. In both cases, we find insufficient UV fluxes or mechanical energies from the stellar population; thus these ULXs may be partially responsible for the ionization and energetics of their supershells. All seven sources that we studied are in young stellar environments, and six of them have optical counterparts with masses 7 M; thus, these sources are most likely high-mass X-ray binaries.Publication Open Access The Nonisothermality And Extent Of Galactic Diffuse Hot Gas Toward Markarian 421(2007) Yao, Y; Wang, QDDiffuse hot gas can be traced effectively by its X-ray absorption and emission. We present a joint analysis of these tracers to characterize the spatial and temperature distributions of the Galactic hot gas along the sight line toward the nearby bright active galactic nucleus Mrk 421. We also complement this analysis with far-UV O VI absorption observations. We find that the observed absorption line strengths of O VII and O VIII are inconsistent with the diffuse background emission-line ratio of the same ions, if the gas is assumed to be isothermal in a collisional ionization equilibrium state. But all these lines as well as the diffuse keV broadband background intensity in the field can be fitted with a plasma with a power-law temperature distribution. We show that this distribution can be derived from a hot gaseous disk model with the gas temperature and density decreasing exponentially with the vertical distance from the Galactic plane. The joint fit gives the exponential scale heights as ~1.0 and 1.6 kpc and the middle plane values as 2.8 × 106 K and 2.4 × 10-3 cm-3 for the temperature and density, respectively. These values are consistent with those inferred from X-ray observations of nearby edge-on galaxies similar to our own.Publication Open Access X-ray Thread G0.13–0.11: A Pulsar Wind Nebula?(2002) Wang, QD; Lu, F; Lang, CCWe have examined Chandra observations of the recently discovered X-ray thread G0.13-0.11 in the Galactic center Radio Arc region. Part of the Chandra data was studied by Yusef-Zadeh, Law, & Wardle, who reported the detection of 6.4 keV line emission in this region. We find, however, that this line emission is not associated with G0.13-0.11. The X-ray spectrum of G0.13-0.11 is well-characterized by a simple power law with an energy slope of 1.8 (90% confidence uncertainties). Similarly, the X-ray spectrum of the pointlike source embedded in G0.13-0.11 has a power-law energy slope of 0.9. The 2-10 keV band luminosities of these two components are ~3.2 × 1033 ergs s-1 (G0.13-0.11) and ~7.5 × 1032 ergs s-1 (point source) at the Galactic center distance of 8 kpc. The morphological, spectral, and luminosity properties strongly indicate that G0.13-0.11 represents the leading edge of a pulsar wind nebula, produced by a pulsar (point source) moving in a strong magnetic field environment. The main body of this pulsar wind nebula is likely traced by a bow-shaped radio feature, which is apparently bordered by G0.13-0.11 and is possibly associated with the prominent nonthermal radio filaments of the Radio Arc. We speculate that young pulsars may be responsible for various unique nonthermal filamentary radio and X-ray features observed in the Galactic center region.Publication Open Access A Chandra Observation Of Gro J1744–28: The Bursting Pulsar In Quiescence(2002) Wijnands, R; Wang, QDWe present a Chandra/Advanced CCD Imaging Spectrometer I-array observation of GRO J1744-28. We detected a source at a position of R.A. = 17h44m33.s09, decl. = -28°44'270 (J2000.0; with a 1 σ error of ~08), consistent with both ROSAT and interplanetary network localizations of GRO J1744-28 when it was in outburst. This makes it likely that we have detected the quiescent X-ray counterpart of GRO J1744-28. Our Chandra position demonstrates that the previously proposed infrared counterpart is not related to GRO J1744-28. The 0.5-10 keV luminosity of the source is (2-4) × 1033 ergs s-1 (assuming the source is near the Galactic center at a distance of 8 kpc). We discuss our results in the context of the quiescent X-ray emission of pulsating and nonpulsating neutron star X-ray transients.Publication Open Access Studying The Nearby Universe With Chandra(2002) Wang, QDI highlight results from Chandr observations of nearby galaxies, including the Milky Way. These observations have offered insights into old mysteries and indications of new high energy astrophysical phenomena and processes that are yet to be understood.Publication Open Access Feedback From Galactic Stellar Bulges And Hot Gaseous Haloes Of Galaxies(2009) Tang, SK; Wang, QD; Lu, Y; Mo, HJWe demonstrate that the feedback from stellar bulges can, in principle, play an essential role in shaping the halo gas of galaxies with substantial bulge components by conducting 1D hydrodynamical simulations. The feedback model we consider consists of two distinct phases: (i) an early starburst during the bulge formation and (ii) a subsequent long-lasting mass and energy injection from stellar winds of low-mass stars and Type Ia supernovae. An energetic outward blastwave is initiated by the starburst and maintained and enhanced by the long-lasting stellar feedback. For a Milky Way like galactic bulge, this blastwave heats up the circum-galactic medium to a scale much beyond the virial radius, thus the gas accretion into the halo can be completely stopped. In addition to that, the long-lasting feedback in the later phase powers a galactic bulge wind that is reverse-shocked at a large radius in the presence of circum-galactic medium and hence maintains a hot gaseous halo. As the mass and energy injection decreases with time, the feedback evolves to a subsonic and quasi-stable outflow, which is enough to prevent halo gas from cooling. The two phases of the feedback thus re-enforce each other's impact on the gas dynamics. The simulation results demonstrate that the stellar bulge feedback may provide a plausible solution to the long-standing problems in understanding the Milky Way type galaxies, such as the ‘missing stellar feedback’ problem and the ‘overcooling’ problem. The central point of the present model is that the conspiracy of the two-phase feedback keeps a low density and a high temperature for the circum-galactic medium so that its X-ray emission is significantly lowered and the radiative cooling is largely suppressed. The simulations also show that the properties of the hot gas in the subsonic outflow state depend sensitively on the environment and the formation history of the bulge. This dependence and variance may explain the large dispersion in the X-ray to B-band luminosity ratio of the low LX/LB elliptical galaxies.Publication Open Access X-ray Absorption Spectroscopy Of The Multiphase Interstellar Medium: Oxygen And Neon Abundances(2006) Yao, Y; Wang, QDX-ray absorption spectroscopy provides a powerful tool in determining the metal abundances in various phases of the interstellar medium (ISM). We present a case study of the sight line toward 4U 1820-303, based on Chandra grating observations. The detection of O I, O II, O III, O VII, O VIII, and Ne IX Kα absorption lines allows us to measure the atomic column densities of the neutral, warm ionized, and hot phases of the ISM through much of the Galactic disk. By comparing these measurements with the 21 cm hydrogen emission and with the pulsar dispersion measure, we estimate the mean oxygen abundances in the neutral and total ionized phases as 0.3(0.2, 0.6) and 2.2(1.1, 3.5) in units of Anders & Greversse's solar value (90% confidence intervals). This significant oxygen abundance difference is apparently a result of molecule/dust grain destruction and recent metal enrichment in the warm ionized and hot phases. We also measure the column density of neon from its absorption edge and obtain a solar value of the Ne/O ratio accounting for the expected oxygen contained in molecules and dust grains. From a joint analysis of the O VII, O VIII, and Ne IX lines, we obtain the Ne/O abundance ratio of the hot phase as 1.4(0.9, 2.1) solar, which is not sensitive to the exact hot gas temperature distribution assumed. These comparable ISM Ne/O ratios for the different phases are thus considerably less than the value recently inferred from corona emission of solar-like stars.Publication Open Access A Large-scale Survey Of X-ray Filaments In The Galactic Centre(2009) Johnson, SP; Dong, H; Wang, QDWe present a catalogue of 17 filamentary X-ray features located within a 68 × 34 arcmin2 view centred on the Galactic Centre region from images taken by Chandra. These features are described by their morphological and spectral properties. Many of the X-ray features have non-thermal spectra that are well fitted by an absorbed power law. Of the 17 features, we find six that have not been previously detected, four of which are outside the immediate 20 × 20 arcmin2 area centred on the Galactic Centre. Seven of the 17 identified filaments have morphological and spectral properties expected for pulsar wind nebulae (PWNe) with X-ray luminosities of 5 × 1032 to 1034 erg s−1 in the 2.0–10.0 keV band and photon indices in the range of Γ= 1.1 to 1.9. In one feature, we suggest the strong neutral Fe Kα emission line to be a possible indicator for past activity of Sgr A*. For G359.942−0.03, a particular filament of interest, we propose the model of a ram pressure confined stellar wind bubble from a massive star to account for the morphology, spectral shape and 6.7 keV He-like Fe emission detected. We also present a piecewise spectral analysis on two features of interest, G0.13−0.11 and G359.89−0.08, to further examine their physical interpretations. This analysis favours the PWN scenario for these features.Publication Open Access The Xmm-newton/chandra Monitoring Campaign Of The Galactic Center Region(2006) Wijnands, R; in't Zand, JJM; Rupen, M; Maccarone, T; Homan, J; Cornelisse, R; Fender, R; Grindlay, J; van der Klis, M; Kuulkers, E; Markwardt, CB; Miller-Jones, JCA; Wang, QDWe present the first results of our X-ray monitoring campaign on a 1.7 square degree region centered on Sgr A* using the satellites XMM-Newton and Chandra. The purpose of this campaign is to monitor the behavior (below 10 keV) of X-ray sources (both persistent and transient) which are too faint to be detected by monitoring instruments aboard other satellites currently in orbit (e.g., Rossi X-ray Timing Explorer; INTEGRAL). Our first monitoring observations (using the HRC-I aboard Chandra) were obtained on June 5, 2005. Most of the sources detected could be identified with foreground sources, such as X-ray active stars. In addition we detected two persistent X-ray binaries (1E 1743.1-2843; 1A 1742-294), two faint X-ray transients (GRS 1741.9-2853; XMM J174457-2850.3), as well as a possible new transient source at a luminosity of a few times 1034 erg s-1. We report on the X-ray results on these systems and on the non-detection of the transients in follow-up radio data using the Very Large Array. We discuss how our monitoring campaign can help to improve our understanding of the different types of X-ray transients (i.e., the very faint ones).