Wang, Daniel
Loading...
Email Address
Birth Date
Research Projects
Organizational Units
Job Title
Professor, Department of Astronomy
Last Name
Wang
First Name
Daniel
Discipline
Astrophysics and Astronomy
Expertise
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.
Name
100 results
Search Results
Now showing 1 - 10 of 100
Publication Open Access Confronting Feedback Simulations With Observations Of Hot Gas In Elliptical Galaxies(2009) Wang, QDElliptical 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 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 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 Ultraluminous X-ray Source 1e 0953.8+6918 (m81 X-9): An Intermediate-mass Black Hole Candidate And Its Environs(2002) Wang, QDWe present a ROSAT and ASCA study of the Einstein source X-9 and its relation to a shock-heated shell-like optical nebula in a tidal arm of the M81 group of interacting galaxies. Our ASCA observation of the source shows a flat and featureless X-ray spectrum well described by a multicolour disc blackbody model. The source most likely represents an optically thick accretion disc around an intermediate-mass black hole (M∼102 M⊙) in its high/soft state, similar to other variable ultraluminous X-ray sources observed in nearby disc galaxies. Using constraints derived from both the innermost stable orbit around a black hole and the Eddington luminosity, we find that the black hole is fast-rotating and that its mass is between ∼80 M⊙–1.5×102 M⊙. The inferred bolometric luminosity of the accretion disc is ∼(1.1×1040 erg s−1)/(cos i). Furthermore, we find that the optical nebula is very energetic and may contain large amounts of hot gas, accounting for a soft X-ray component as indicated by archival ROSAT PSPC data. The nebula is apparently associated with X-9; the latter may be powering the former and/or they could be formed in the same event (e.g. a hypernova). Such a connection, if confirmed, could have strong implications for understanding both the birth of intermediate-mass black holes and the formation of energetic interstellar structures.Publication Open Access Extra-planar Diffuse Hot Gas Around Normal Disk Galaxies(2005) Wang, QDI review results from Chandra observations of nearby normal edge-on galaxies (Sd to Sa types). These galaxies have a broad range of star formation rate, but none of them is dominated by a nuclear starburst. The galaxies are all in directions of low Galactic foreground absorption (NHI ≤ 4 × 1020 cm−2). Extra-Planar diffuse soft X-ray emission is detected unambiguously from all the galaxies, except for N4244 (Sd), which is low in both stellar mass and star formation rate. The thermal nature of the X-ray-emitting gas is well established, although its chemical and ionization states remain largely uncertain. The X-ray luminosity of the gas is proportional to the star formation rate and to the stellar mass of the galaxies. But the luminosity accounts for at most a few percent of the expected supernova mechanical energy input. Therefore, there is a "missing" energy problem for spiral galaxies. Much of the energy in late-type spirals may be converted and radiated in lower energy bands. But early-type ones most likely have outflows, which are powered primarily by Type Ia supernovae in galactic bulges. These galactic outflows may strongly affect both the dynamics and cooling of the intergalactic gas accretion, hence the evolution of the galaxies.Publication Open Access Title: Chandra Observations Of The Galactic Center And Nearby Edge-on Galaxies(2002) Wang, QDI review our recent Chandra surveys of the center region of the Milky Way and other nearby edge-on galaxies. Our Galactic center survey provides an unprecedented high-resolution, broad-band X-ray panorama of a 0.8x2 square degree swath along the Galactic plane. Our preliminary analysis has led a detection of about 1000 discrete sources. We find that the diffuse X-ray emission dominates over the contribution from faint discrete sources and is globally associated with distinct interstellar structures observed at radio and mid-infrared wavelengths. We study how high-energy activities in the center region affect the immediate vicinity and may influence other aspects of the Galaxy. We have further observed nearby edge-on late-type disk galaxies in fields of low foreground Galactic extinction to gain external perspectives of the global disk/halo interaction. We have detected a giant diffuse X-ray-emitting corona around the galactic disk of NGC 4631. Extraplanar diffuse X-ray emission is also detected around NGC 3556. These X-ray-emitting coronae morphologically resemble the radio halos of these galaxies, indicating a close connection between outflows of hot gas, cosmic rays, and magnetic field from the galactic disks. There is only marginal evidence for extraplanar diffuse X-ray emission in NGC 4244 -- a galaxy with an extremely low star formation rate. In general, the extraplanar diffuse X-ray emission is evidently related to recent massive star forming activities in the galactic disks, especially in their central regions.Publication Open Access Title: Recent X-ray Observations Of Disk Galaxies: Tracing The Dynamic Interstellar Medium(2000) Wang, QDI review recent results from our deep ROSAT and Chandra observations of two galaxies, M101 and NGC 4631, in fields of exceptionally low Galactic extinction. Large amounts of X-ray-emitting gas are detected in these galaxies. Such gas is produced primarily in massive star forming regions and have an average characteristic temperature of a few times $10^6$ K. Cooler gas ($\sim 10^6$ K) is found typically outside galactic disks and may represent outflows from blown-out superbubbles. Propagation of star formation, driven by the expansion of hot gas, appears to be operating in giant HII complexes. A substantial fraction of photo-evaporated gas in such complexes may be mass-loaded into hot gas, which explains their large X-ray luminosities. These processes likely play an important role in determining the global properties of the interstellar medium, especially the disk/halo interaction.Publication Open Access Correction For The Flux Measurement Bias In X-ray Source Detection(2004) Wang, QDWith a high spatial resolution imaging instrument such as the Chandra ACIS, one can confidently identify an X-ray source with only a few detected counts. The detection threshold of such sources, however, varies strongly across the field of view of the instrument. Furthermore, the low-detection counting statistics, together with a typical steep source number-flux relation, causes more intrinsically faint sources to be detected at apparently higher fluxes than the other way around. We quantify this "X-ray Eddington bias" as well as the detection threshold variation and devise simple procedures for their corrections. To illustrate our technique, we present results from our analysis of X-ray sources detected in the fields of the large-scale hierarchical complex A2125 at z = 0.247 and the nearby galaxy NGC 4594 (Sombrero). We show that the sources detected in the A2125 field, excluding 10 known complex members, have a number-flux relation consistent with the expected from foreground or background objects. In contrast, the number-flux relation of the NGC 4594 field is dominated by X-ray sources associated with the galaxy. This galactic component of the relation is well characterized by a broken power law.Publication Open Access Chandra Observations Of The Galactic Center: High Energy Processes At Arcsecond Resolution(2006) Wang, QDAbout 2 million seconds of Chandra observing time have been devoted to the Galactic Center (GC), including large-scale surveys and deep pointings. These observations have led to the detection of about 4000 discrete X-ray sources and the mapping of diffuse X-ray emission in various energy bands. In this review, I frst summarize general results from recent studies and then present close-up views of the three massive star clusters (Arches, Quintuplet, and GC) and their interplay with the Galactic nuclear environment.Publication Open Access Global Hot Gas In And Around The Galaxy(2009) Wang, QDThe hot interstellar medium traces the stellar feedback and its role in regulating the eco-system of the Galaxy. I review recent progress in understanding the medium, based largely on X-ray absorption line spectroscopy, complemented by X-ray emission and far-UV O VI absorption measurements. These observations enable us for the first time to characterize the global spatial, thermal, chemical, and kinematic properties of the medium. The results are generally consistent with what have been inferred from X-ray imaging of nearby galaxies similar to the Galaxy. It is clear that diffuse soft X-ray emitting/absorbing gas with a characteristic temperature of ~106 K resides primarily in and around the Galactic disk and bulge. In the solar neighborhood, for example, this gas has a characteristic vertical scale height of ~1 kpc. This conclusion does not exclude the presence of a larger-scale, probably much hotter, and lower density circum-Galactic hot medium, which is required to explain observations of various high-velocity clouds. This hot medium may be a natural product of the stellar feedback in the context of the galaxy formation and evolution. ©2009 American Institute of Physics