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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Now showing 1 - 10 of 100
  • Publication
    Title: Properties of the Compact HII Region Complex G-0.02-0.07
    (2011-01) Mills, E; Morris, MR; Lang, C; Dong, H; Wang, QD; Cotera, A; Stolovy, S
    Abstract: We present new extinction maps and high-resolution Paschen alpha images of G-0.02-0.07, a complex of compact HII regions located adjacent to the M-0.02-0.07 giant molecular cloud, 6 parsecs in projection from the center of the Galaxy. These HII regions, which lie in projection just outside the boundary of the Sgr A East supernova remnant, represent one of the most recent episodes of star formation in the central parsecs of the Galaxy. The 1.87 micron extinctions of regions A, B and C are almost identical, approximately 1.5 magnitudes. Region D, in contrast, has a peak 1.87 micron extinction of 2.3 magnitudes. Adopting the Nishiyama et al. (2008) extinction law, we find these extinctions correspond to visual extinctions of A_V = 44.5 and A_V = 70, respectively. The similar and uniform extinctions of regions A, B and C are consistent with that expected for foreground extinction in the direction of the Galactic center, suggesting that they lie at the front side of the M-0.02-0.07 molecular cloud. Region D is more compact, has a higher extinction and is thus suspected to be younger and embedded in a dense core in a compressed ridge on the western edge of this cloud.
  • Publication
    XMM-Newton Spectra of Intermediate-Mass Black Hole Candidates: Application of a Monte Carlo Simulated Model
    (2004-01) Wang, QD; Yao, Y; Fukui, W; Zhang, SN; Williams, R
    We present a systematic spectral analysis of six ultraluminous X-ray sources (NGC 1313 X-1 and X-2, IC 342 X-1, Ho IX X-1, NGC 5408 X-1, and NGC 3628 X-1) observed with the XMM-Newton observatory. These extranuclear X-ray sources in nearby late-type galaxies have been considered as intermediate-mass black hole candidates. We have performed Monte Carlo simulations of Comptonized multicolor blackbody accretion disks. This unified and self-consistent spectral model assumes a spherically symmetric, thermal corona around each disk and accounts for the radiation transfer in the Comptonization. We find that the model provides satisfactory fits to the XMM-Newton spectra of the sources. The characteristic temperatures of the accretion disks (Tin), for example, are in the range of ~0.05-0.3 keV, consistent with the intermediate-mass black hole interpretation. We find that the black hole mass is typically about a few times 103 M and has an accretion rate of ~10-6 to 10-5 M yr-1. For the spectra considered here, we find that the commonly used multicolor blackbody accretion disk model with an additive power-law component, although not physical, provides a good mathematical approximation to the Monte Carlo simulated model. However, the latter model provides additional constraints on the properties of the accretion systems, such as the disk inclination angles and corona optical depths.
  • Publication
    A Chandra Catalog of X-Ray Sources in the Central 150 pc of the Galaxy
    (2006-01) Muno, MP; Bauer, FE; Bandyopadhyay, RM; Wang, QD
    We present the catalog of X-ray sources detected in a shallow Chandra survey of the inner 2° × 08 of the Galaxy and in two deeper observations of the Radio Arches and Sgr B2. The catalog contains 1352 objects that are highly absorbed (NH 4 × 1022 cm-2) and are therefore likely to lie near the Galactic center (D 8 kpc), and 549 less absorbed sources that lie within 6 kpc of Earth. On the basis of the inferred luminosities of the X-ray sources and the expected numbers of various classes of objects, we suggest that the sources with LX 1033 ergs s-1 that comprise 90% of the catalog are cataclysmic variables and that the 100 brighter objects are accreting neutron stars and black holes, young isolated pulsars, and Wolf-Rayet and O (WR/O) stars in colliding-wind binaries. We find that the spatial distribution of X-ray sources matches that of the old stellar population observed in the infrared, which supports our suggestion that most of the X-ray sources are old cataclysmic variables. However, we find that there is an apparent excess of 10 bright sources in the Radio Arches region. That region is already known to be the site of recent star formation, so we suggest that the bright sources in this region are young high-mass X-ray binaries, pulsars, or WR/O star binaries. We briefly discuss some astrophysical questions that this catalog can be used to address.
  • Publication
    A Chandra Observation of GRO J1744–28: The Bursting Pulsar in Quiescence
    (2002-01) 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-raying Galaxies: A Chandra Legacy
    (2010-01) 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
    The Nonisothermality and Extent of Galactic Diffuse Hot Gas toward Markarian 421
    (2007-01) 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
    Studying the Nearby Universe with Chandra
    (2002-01) 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
    Feedback from galactic stellar bulges and hot gaseous haloes of galaxies
    (2009-01) 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
    X-Ray Absorption Spectroscopy of the Multiphase Interstellar Medium: Oxygen and Neon Abundances
    (2006-01) 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
    Chandra and XMM–Newton detection of large-scale diffuse X-ray emission from the Sombrero galaxy
    (2007-01) Li, Z; Wang, QD; Hameed, S
    We present an X-ray study of the massive edge-on Sa galaxy, Sombrero (M 104; NGC 4594), based on XMM–Newton and Chandra observations. A list of 62 XMM–Newton and 175 Chandra discrete X-ray sources is provided, the majority of which are associated with the galaxy. Spectral analysis is carried out for relatively bright individual sources and for an accumulated source spectrum. At energies ≳2 keV, the source-subtracted X-ray emission is distributed similarly as the stellar K-band light and is primarily due to the residual emission from discrete sources. At lower energies, however, a substantial fraction of the source-subtracted emission arises from diffuse hot gas extending to ∼20 kpc from the galactic centre. The galactic disc shows little X-ray emission and instead shadows part of the X-ray radiation from the bulge. The observed diffuse X-ray emission from the galaxy shows a steep spectrum that can be characterized by an optically thin thermal plasma with temperatures of ∼0.6–0.7 keV, varying little with radius. The diffuse emission has a total luminosity of ∼3 × 1039 erg s−1 in the 0.2–2 keV energy range. This luminosity is significantly smaller than the prediction by current numerical simulations for galaxies as massive as Sombrero. However, such simulations do not include the effect of quiescent stellar feedback (e.g. ejecta from evolving stars and Type Ia supernovae) against the accretion from intergalactic medium. We argue that the stellar feedback likely plays an essential role in regulating the physical properties of hot gas. Indeed, the observed diffuse X-ray luminosity of Sombrero accounts for at most a few per cent of the expected mechanical energy input from Type Ia supernovae. The inferred gas mass and metal content are also substantially less than those expected from stellar ejecta. We speculate that a galactic bulge wind, powered primarily by Type Ia supernovae, has removed much of the ‘missing’ energy and metal-enriched gas from the region revealed by the X-ray observations.