Wang, Daniel

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Professor, Department of Astronomy
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Astrophysics and Astronomy
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.

Search Results

Now showing 1 - 10 of 100
  • Publication
    X-Ray Thread G0.13–0.11: A Pulsar Wind Nebula?
    (2002-01-01) Wang, QD; Lu, F; Lang, CC
    We 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
    Feedback from galactic stellar bulges and hot gaseous haloes of galaxies
    (2009-01-01) Tang, SK; Wang, QD; Lu, Y; Mo, HJ
    We 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
    An Optical Study of Stellar and Interstellar Environments of Seven Luminous and Ultraluminous X-Ray Sources
    (2006-01-01) Ramsey, CJ; Williams, RM; Gruendl, RA; Chen, C-HR; Chu, Y-H; Wang, QD
    We 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
    X-raying Galaxies: A Chandra Legacy
    (2010-01-01) Wang, QD
    This 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
    Studying the Nearby Universe with Chandra
    (2002-01-01) Wang, QD
    I 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
    The Nonisothermality and Extent of Galactic Diffuse Hot Gas toward Markarian 421
    (2007-01-01) Yao, Y; Wang, QD
    Diffuse 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
    A Chandra Observation of GRO J1744–28: The Bursting Pulsar in Quiescence
    (2002-01-01) Wijnands, R; Wang, QD
    We 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
    X-Ray Absorption Spectroscopy of the Multiphase Interstellar Medium: Oxygen and Neon Abundances
    (2006-01-01) Yao, Y; Wang, QD
    X-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
    Confronting feedback simulations with observations of hot gas in elliptical galaxies
    (2009-01-01) Wang, QD
    Elliptical galaxies comprise primarily old stars, which collectively generate a long-lasting feedback via stellar mass-loss and Type Ia SNe. This feedback can be traced by X-ray-emitting hot gas in and around such galaxies, in which little cool gas is typically present. However, the X-ray-inferred mass, energy, and metal abundance of the hot gas are often found to be far less than what are expected from the feedback, particularly in so-called low LX/LB ellipticals. This “missing” stellar feedback is presumably lost in galaxy-wide outflows, which can play an essential role in galaxy evolution (e.g., explaining the observed color bi-modality of galaxies). We are developing a model that can be used to properly interpret the X-ray data and to extract key information about the dynamics of the feedback and its interplay with galactic environment.
  • Publication
    Missing-iron problem and Type Ia supernova enrichment of hot gas in galactic spheroids
    (2010-01-01) Tang, S; Wang, QD
    Type Ia supernovae (Ia SNe) provide a rich source of iron for hot gas in galactic stellar spheroids. However, the expected supersolar iron abundance of the hot gas is not observed. Instead, X-ray observations often show decreasing iron abundance towards galactic central regions, where the Ia SN enrichment is expected to be the highest. We examine the cause of this missing-iron problem by studying the enrichment process and its effect on X-ray abundance measurements of the hot gas. The evolution of Ia SN iron ejecta is simulated in the context of galaxy-wide hot gas outflows, in both supersonic and subsonic cases, as may be expected for hot gas in galactic bulges or elliptical galaxies of intermediate masses. SN reverse-shock-heated iron ejecta is typically found to have a very high temperature and low density, hence producing little X-ray emission. Such hot ejecta, driven by its large buoyancy, can quickly reach a substantially higher outward velocity than the ambient medium, which is dominated by mass-loss from evolved stars. The ejecta is gradually and dynamically mixed with the medium at large galactic radii. The ejecta is also slowly diluted and cooled by in situ mass injection from evolved stars. These processes together naturally result in the observed positive gradient in the average radial iron abundance distribution of the hot gas, even if mass weighted. This trend is in addition to the X-ray measurement bias that tends to underestimate the iron abundance for the hot gas with a temperature distribution.