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 X-Ray Spectroscopy of Galactic Hot Gas along the PKS 2155$-$304 Sight Line(2010-01-01) Hagihara, T; Yao, Y; Yamasaki, NY; Mitsuda, K; Wang, QD; Takei, Y; Yoshino, T; McCammon, DWe present a detailed spectroscopic study of the hot gas in the Galactic halo toward the direction of a blazer PKS 2155304 ( 0.117). The OVII and OVIII absorption lines were measured with the Low and High Energy Transmission Grating Spectrographs aboard Chandra, and the OVII, OVIII, and NeIX emission lines produced in an adjacent field of the PKS 2155304 direction were observed with the X-ray Imaging Spectrometer aboard Suzaku. Assuming vertically exponential distributions of the gas temperature and the density, we performed a combined analysis of the absorption and emission data. The gas temperature and the density at the galactic plane were determined to be (2.5 ) 10K and () 10cm, and the scale heights of the gas temperature and density were 5.6 kpc and 2.3 kpc, respectively. These values are consistent with those obtained in the LMC X-3 direction.Publication The Apparent Host Galaxy of PKS 1413+135: Hubble Space Telescope,* ASCA, and Very Long Baseline Array ** Observations(2002-01-01) Perlman, ES; Stocke, JT; Carilli, CL; Sugiho, M; Tashiro, M; Madejski, G; Wang, QD; Conway, JPKS 1413+135 (z = 0.24671) is one of very few radio-loud active galactic nuclei (AGNs) with an apparent spiral host galaxy. Previous authors have attributed its nearly exponential infrared cutoff to heavy absorption but have been unable to place tight limits on the absorber or its location in the optical galaxy. In addition, doubts remain about the relationship of the AGN to the optical galaxy given the observed lack of reemitted radiation. We present new Hubble Space Telescope (HST), ASCA, and Very Long Baseline Array observations, which throw significant new light on these issues. The HST observations reveal that the active nucleus of PKS 1413+135 has an extremely red color, (V-H) = 6.9 mag, requiring both a spectral turnover at a few microns because of synchrotron aging and an absorbing region the size of a giant molecular cloud. Combining constraints from the HST and ASCA data, we derive an intrinsic column NH = 4.6 × 1022 cm-2 and covering fraction f = 0.12. The spin temperature of the molecular absorption lines found by previous authors suggests that the cloud is located in the disk of the optical galaxy, making our sight line rather unlikely (P ~ 2 × 10-4). The properties of this region appear typical of large giant molecular clouds in our own Galaxy. The H I absorber appears centered 25 mas away from the nucleus, while the X-ray and nearly all of the molecular absorbers must cover the nucleus, implying a rather complicated geometry and cloud structure, in particular requiring a molecular core along our line of sight to the nucleus. Interestingly, the HST/NICMOS data require the AGN to be decentered relative to the optical galaxy by 13 ± 4 mas. This could be interpreted as suggestive of an AGN location far in the background compared with the optical galaxy, but it can also be explained by obscuration and/or nuclear structure, which is more consistent with the observed lack of multiple images.Publication Probing the Local Group Medium toward Markarian 421 with Chandra and the Far Ultraviolet Spectroscopic Explorer(2005-01-01) Williams, RJ; Mathur, S; Nicastro, F; Elvis, M; Drake, JJ; Fang, T; Fiore, F; Krongold, Y; Wang, QD; Yao, YWe report the detection of highly ionized gas at z ~ 0 seen in resonant UV and X-ray absorption lines toward the z = 0.03 blazar Mrk 421. A total of 13 X-ray and three UV lines were measured (or upper limits derived), including three lines in the O VII K series and Kα transitions from neon, carbon, and nitrogen. From the three O VII lines we derive a 2 σ Doppler parameter constraint of 24 km s-1 < b < 55 km s-1. The FUSE spectrum shows strong Galactic low-velocity O VI λ1032 absorption and a possible weak O VI high-velocity component (HVC). The Doppler parameter of the low-velocity O VI measured with FUSE is ~3 σ higher than that derived from the O VII line ratios, indicating that the O VII and Galactic O VI arise in different phases. This velocity dispersion along with limits on the gas temperature and density from the X-ray line ratios (assuming a single phase with collisional ionization equilibrium plus photoionization) are all consistent with an extragalactic absorber. However, the O VII Doppler parameter is inconsistent with the high temperature required to produce the observed O VIHVC/O VII ratio, implying that the HVC is probably not related to the O VII. In addition, the O VIKα line detected by Chandra implies a column density ~4 times higher than the λ1032 absorption. Although an extragalactic absorber is fully consistent with the measured column density ratios, a Galactic origin cannot be ruled out given the uncertainties in the available data.Publication THE RADIO PROPERTIES AND MAGNETIC FIELD CONFIGURATION IN THE CRAB-LIKE PULSAR WIND NEBULA G54.1+0.3(2010-01-01) Lang, CC; Wang, QD; Lu, F; Clubb, KIWe present a multifrequency radio investigation of the Crab-like pulsar wind nebula (PWN) G54.1+0.3 using the Very Large Array. The high resolution of the observations reveals that G54.1+0.3 has a complex radio structure which includes filamentary and loop-like structures that are magnetized, a diffuse extent similar to the associated diffuse X-ray emission. But the radio and X-ray structures in the central region differ strikingly, indicating that they trace very different forms of particle injection from the pulsar and/or particle acceleration in the nebula. No spectral index gradient is detected in the radio emission across the PWN, whereas the X-ray emission softens outward in the nebula. The extensive radio polarization allows us to image in detail the intrinsic magnetic field, which is well-ordered and reveals that a number of loop-like filaments are strongly magnetized. In addition, we determine that there are both radial and toroidal components to the magnetic field structure of the PWN. Strong mid-infrared (IR) emission detected in Spitzer Space Telescope data is closely correlated with the radio emission arising from the southern edge of G54.1+0.3. In particular, the distributions of radio and X-ray emission compared with the mid-IR emission suggest that the PWN may be interacting with this interstellar cloud. This may be the first PWN where we are directly detecting its interplay with an interstellar cloud that has survived the impact of the supernova explosion associated with the pulsar's progenitor.Publication The Origin of the Dust Arch in the Halo of NGC 4631: An Expanding Superbubble?(2003-01-01) Taylor, CL; Wang, QDWe study the nature and the origin of the dust arch in the halo of the edge-on galaxy NGC 4631 detected by Neininger & Dumke. We present CO observations made using the new on-the-fly mapping mode with the Five College Radio Astronomy Observatory 14 m telescope and find no evidence for CO emission associated with the dust arch. Our examination of previously published H I data shows that, if previous assumptions about the dust temperature and gas/dust ratio are correct, then there must be molecular gas associated with the arch, below our detection threshold. If this is true, then the molecular mass associated with the dust arch is between 1.5 × 108 M and 9.7 × 108 M, and likely toward the low end of the range. A consequence of this is that the maximum allowed value for the CO-to-H2 conversion factor is 6.5 times the Galactic value, but most likely closer to the Galactic value. The kinematics of the H I apparently associated with the dust arch reveals that the gas here is not part of an expanding shell or outflow but is instead two separate features (a tidal arm and a plume of H I sticking out into the halo) that are seen projected together and appear as a shell. Thus there is no connection between the dust "arch" and the hot X-ray–emitting gas that appears to surround the galaxy.Publication Hiccups in the Night: X-ray Monitoring of the Two Crab-like LMC Pulsars(2002-01-01) Gotthelf, EV; Zhang, W; Marshall, FE; Middleditch, J; Wang, QDWe are undertaking an extensive X-ray monitoring campaign of the two Crab-like pulsars in the Large Magellanic Clouds, PSR B0540-69 and PSR J0537-6910. We present our current phase-connected timing analysis derived from a set of 50 pointed X-ray observations spanning several years. From our initial 1.2 yr monitoring program of the young 50 ms pulsar PSR B0540-69, we find the first compelling evidence for a glitch in its rotation. This glitch is characterized by Delta nu / nu = (1.90+-0.05) x 10-9 and Delta nudot / nudot = (8.5+-0.5) x 10-5. Taking into account the glitch event, we derive a braking index of n = 1.81+-0.07, significantly lower than previous reported. For the 16 ms pulsar, PSR J0537-6910, we recorded 6 large glitch events during a period of nearly 3 years, the highest rate of all known Crab-like systems. Despite the extreme timing activity, the long term spin-down of this pulsar continues to average -1.9743 x 10-10 Hz/s.Publication X-Raying A2125: A Large-Scale Hierarchical Complex of Galaxies and Hot Gas(2004-01-01) Wang, QD; Owen, F; Ledlow, MWe present an 82 ks Chandra ACIS-I observation of a large-scale hierarchical complex, which consists of various clusters/groups of galaxies and low surface brightness X-ray emission at z = 0.247. This high-resolution Chandra observation allows us for the first time to separate unambiguously the X-ray contributions from discrete sources and large-scale diffuse hot gas. We detect 99 X-ray sources in a 17' × 17' field. Ten of these sources are identified as members of the complex and are mostly radio-bright. Whereas unresolved X-ray sources tend to be associated with galaxies in intermediate density environments, extended X-ray emission peak at bright radio galaxies in the central cluster. In particular, a distinct X-ray trail appears on one side of the fast-moving galaxy C153, clearly due to ram pressure stripping. The diffuse X-ray emission from the central cluster can be characterized by a thermal plasma with a characteristic temperature of 3.2 keV and a heavy element abundance of 0.24 solar (90% confidence uncertainties). In comparison, a patch of low surface brightness X-ray emission apparently originates in relatively low density intergalactic gas with a characteristic temperature of 0.98 keV and an abundance of 0.09 solar. The Chandra observation, together with extensive multiwavelength data, indicates that the complex represents a projection of several galaxy substructures, which may be undergoing major mergers. We discuss the dynamic states of the complex and its substructures, as well as properties of X-ray-emitting galaxies and the relationship to their environments.Publication Diffuse X-Rays from the Arches and Quintuplet Clusters(2005-01-01) Rockefeller, G; Fryer, CL; Melia, F; Wang, QDThe origin and initial mass function of young stellar clusters near the Galactic center are still poorly understood. Two of the more prominent ones, the Arches and Quintuplet clusters, may have formed from a shock-induced burst of star formation, given their similar age and proximity to each other. Their unusual mass distribution, however, may be evidence of a contributing role played by other factors, such as stellar capture from regions outside the clusters themselves. Diffuse X-ray emission from these sources provides us with a valuable, albeit indirect, measure of the stellar mass-loss rate from their constituents. Using recent data acquired with Chandra, we can study the nature and properties of the outflow to not only probe the pertinent physical conditions, such as high metallicity, the magnetic field, and so forth, but also to better constrain the stellar distribution within the clusters, in order to identify their formative history. In this paper, we present a set of three-dimensional smoothed particle hydrodynamics simulations of the wind-wind interactions in both the Arches and Quintuplet clusters. We are guided primarily by the currently known properties of the constituent stars, though we vary the mass-loss rates in order to ascertain the dependence of the measured X-ray flux on the assumed stellar characteristics. Our results are compared with the latest observations of the Arches cluster. Our analysis of the Quintuplet cluster may be used as a basis for comparison with future X-ray observations of this source.Publication Chandra Observation of the Edge-on Galaxy NGC 3556 (M108): Violent Galactic Disk-Halo Interaction Revealed(2003-01-01) Wang, QD; Chaves, T; Irwin, JAWe present a 60 ks Chandra ACIS-S observation of the isolated edge-on spiral galaxy NGC 3556, together with a multiwavelength analysis of various discrete X-ray sources and diffuse X-ray features. Among 33 discrete X-ray sources detected within the IB = 25 mag arcsec-2 isophote ellipse of the galaxy, we identify a candidate for the galactic nucleus, an ultraluminous X-ray source that might be an accreting intermediate-mass black hole, a possible X-ray binary with a radio counterpart, and two radio-bright giant H II regions. We detect large amounts of extraplanar diffuse X-ray emission, which extend about 10 kpc radially in the disk and 4 kpc away from the galactic plane. The diffuse X-ray emission exhibits significant substructures, possibly representing various blown-out superbubbles or chimneys of hot gas heated in massive star-forming regions. This X-ray-emitting gas has temperatures in the range of ~(2-7) × 106 K and has a total cooling rate of ~2 × 1040 ergs s-1. The energy can be easily supplied by supernova blast waves in the galaxy. These results show NGC 3556 to be a galaxy undergoing vigorous disk-halo interaction. The halo in NGC 3556 is considerably less extended, however, than that of NGC 4631, in spite of many similarities between the two galaxies. This may be due to the fact that NGC 3556 is isolated, whereas NGC 4631 is interacting. Thus, NGC 3556 presents a more pristine environment for studying the disk-halo interaction.Publication An XMM–Newton and Chandra study of the starburst galaxy IC 10(2005-01-01) Wang, QD; Whitaker, KE; Williams, RWe present an X-ray study of our nearest starburst galaxy IC 10, based on XMM–Newton and Chandra observations. A list of 73 XMM–Newton and 28 Chandra detections of point-like X-ray sources in the field is provided; a substantial fraction of them are likely stellar objects in the Milky Way due to the low Galactic latitude location of IC 10. The brightest source in the IC 10 field, X-1, has a mean 0.3–8.0 keV luminosity of ∼1.2 × 1038 erg s−1 and shows a large variation by a factor of up to ∼6 on time-scales of ∼104 s during the XMM–Newton observation. The X-ray spectra of the source indicate the presence of a multicolour blackbody accretion disc with an inner disc temperature Tin≈ 1.1 keV. These results are consistent with the interpretation of the source as a stellar mass black hole (BH), probably accreting from a Wolf–Rayet (W–R) star companion. We infer the mass of this BH to be ∼4 M⊙ if it is not spinning, or a factor of up to ∼6 higher if there is significant spinning. We also detect an apparent diffuse soft X-ray emission component of IC 10. An effective method is devised to remove the X-ray CCD-readout streaks of X-1 that strongly affect the study of the diffuse component in the XMM–Newton and Chandra observations. We find that the diffuse X-ray morphology is oriented along the optical body of the galaxy and is chiefly associated with starburst regions. The diffuse component can be characterized by an optically thin thermal plasma with a mean temperature of ∼4 × 106 K and a 0.5–2 keV luminosity of ∼8 × 1037 erg s−1, representing only a small fraction of the expected mechanical energy inputs from massive stars in the galaxy. There is evidence that the hot gas is driving outflows from the starburst regions; therefore, the bulk of the energy inputs may be released in a galactic wind.