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Previous Seminars

Listed here, in reverse chronological order, are presentations that have been given recently at the University of Melbourne, by either members of this group or visiting astrophysicists.

2001

  1. The AAO/UKST H-alpha survey

    Who:
    Dr Quentin Parker, Royal Observatory, Edinburgh, Scotland
    Where:
    Rm 360, School of Physics
    When:
    Monday 26 November 2001
    Abstract:
    The AAO UK Schmidt Telescope (UKST) is undertaking an H-alpha survey of the Southern Galactic Plane and Magellanic Clouds using a single-element, narrow-band interference filter of exceptional quality. This is probably the last great photographic Schmidt survey and the first where the sole method of community access will be via an on-line digital atlas. An H-alpha survey with an unprecedented level of coverage, resolution and sensitivity is being produced, superior to any other survey of optical line emission in our Galaxy in terms of these combined properties. Many exciting discoveries have already been made including new supernovae remnants, Herbig-Haro objects and Wolf-Rayet stars. Here we provide brief results and details of a rich new vein of Galactic Planetary Nebulae. The Wide Field Astronomy Unit (WFAU) at the Royal Observatory Edinburgh will be the principal archive centre for the survey data products. The survey began in July 1997 and is due for completion in late 2001 with release to the community shortly after.

  2. First Results from the UMass/Columbia Galactic Center Chandra X-ray Survey

    Who:
    Dr Cornelia Lang, University of Massachusetts, USA
    Where:
    Rm 360, School of Physics
    When:
    3.15pm, Tuesday 27 November 2001
    Abstract:
    At a distance of only 8.0 kpc, the center of the Milky Way provides an excellent laboratory for understanding the interplay between the stellar, interstellar and high-energy components. I will highlight the first results from the new large-scale, arcsecond resolution X-ray survey of the Galactic Center carried out with the Chandra X-ray Telescope. The aim of this study is to better characterize the X-ray emission arising from the Galactic center and address the relationship of the high-energy sources to the GC stellar and interstellar features on many different spatial scales. More than 1000 X-ray point sources have now been detected and the abundant diffuse emission can therefore be cleanly separated from the point-source component. I will also touch on the multiwavelength results from the Radio Arc/Arched Filaments region, where extensive radio and near-infrared observations are now compared in detail with the new X-ray data.

  3. A Binocular Spectrograph: New Perspectives on Galaxy Kinematics

    Who:
    Dr Nigel Douglas, Kapteyn Institute, Groningen, Netherlands
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Thursday 15 November 2001
    Abstract:
    The planetary nebulae which inhabit every galaxy constitute a unique tracer of that galaxy's stellar kinematics. We are interesting in, for example, measuring the angular momentum in that part of the galaxy not accessible via conventional spectroscopy. To carry out this research we have recently commissioned a unique instrument called the "Planetary Nebula Spectrograph" at the 4.2m William Herschel Telescope on La Palma.

  4. Methanol Masers and Associated Objects

    Who:
    Dr Andrej Sobolev, Ural State University, Russia
    Where:
    Level 7 Conference Room, School of Physics
    When:
    3.15pm,Tuesday 13 November 2001
    Abstract:
    Results of current observations of shock tracers in the massive molecular cores associated with methanol masers will be described. Evolutionary, physical and chemical status of these objects will be discussed. Special attention will be paid to elucidation of how the masers are influenced by their environment and, hence, what can methanol masers tell us about the processes of massive star formation.

  5. Master equation for chemical reactions on dust grains in the interstellar medium

    Who:
    Associate Professor Ofer Biham, The Hebrew University, Israel
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Thursday 8 November 2001
    Abstract:
    Chemical reactions in the interstellar medium generate molecules that are of fundamental importance in the processes of gravitational collapse and star formation. Of particular interest are reactions such as the formation of molecular hydrogen that take place not in the gas phase but on the surfaces of dust grains. In this talk I will first describe recent analysis and interpretation of laboratory experiments trying to examine chemical processes that take plcae in the recombination of hydrogen on small dust grains in the interstellar medium. In the analysis we use rate equations that provide eld results that are appropriate for macroscopic surfaces such as those used experimentally. The implications to the interstellar medium will then be considered. I will show that in the interstellar medium the rate equation approach is not suitable due to the microscopic size of the dust grains that does not allow self averaging and requires one to take into account the discrete nature of the atoms involved in the reactions. I will then present an approach based on a master equation that provides correct results for recombination rates even on small grains. The generalization to more complex reactions on grain surfaces will also be discussed.

  6. Meteorites, T Tauri Jets and Relativistic MHD

    Who:
    Dr Kurt Liffman, CSIRO
    Where:
    Conference Room, 7th Floor, School of Physics
    When:
    3.15pm, Friday 19 October 2001
    Abstract:
    It has been deduced, from a number of lines of evidence, that primitive meteorites are not only samples of asteroids and comets, but are also fossilized samples of the solar nebula - the disk of gas and dust that once surrounded the Sun. Many such meteorites contain small, millimetre-sized, igneous objects called chondrules and CAIs (Calcium Aluminium Inclusions) surrounded by a dark sedimentary-like substance called "matrix". The matrix has never experienced temperatures in excess of 600 K, but the chondrules and CAIs were formed at temperatures in excess of 1800 K. How such material could have formed in the solar nebula has been a subject of scientific speculation for over two centuries. Ten years ago, a new theory was published which suggested that chondrules, CAIs and other such objects were formed in or near the interaction region between the inner solar nebula and the early Sun. Moreover it was also suggested that chondrules and their kin were formed in a high-speed jet flow similar to the observed, high speed bipolar flows that are produced from young stellar objects. These small, igneous rocks could then be ejected from the inner solar nebula and subsequently ram into the cooler, outer nebula where they would agglomerate with the cooler, unprocessed nebula material to form the meteorites that we see today. This "Jet Flow Model" of chondrule/CAI formation is currently the most popular model for explaining meteorite structure. In this talk I will give an outline of the jet flow chondrule formation model. I will also present a new theory of how these flows form and the possible application of this theory to the relativistic jet flows observed from Active Galactic Nuclei.

  7. Episodic Star Formation in Disk Galaxies

    Who:
    Dr Noella D'Cruz, The University of Sydney
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Thursday 11 October 2001
    Abstract:
    There is a large amount of data present on spiral galaxies in the nearby Universe. H-alpha data from Kennicutt and collaborators provides information about recent star formation, while broad band optical colours contain the star formation histories of galaxies over their lifetimes. Radio emission at 1.4 GHz is also believed to be a tracer of star formation as the emission arises from supernova remnants. It is very likely that galaxies experience multiple episodes of star formation as they evolve. In our attempt to understand the nearby galaxies, we model their star formation history as periodic bursts with varying burst shapes, and use the PEGASE galaxy evolution code (Fioc and Rocca-Volmerange 2000) to compute the H-alpha fluxes and the optical colours. We will discuss some of the star formation histories that we use. We find that bursts which decay exponentially seem to fit the data better than other burst shapes that we explored. We also compare our models to the observed radio-to-optical luminosity function.

  8. The Highest Time resolution observations of a pulsar glitch

    Who:
    Dr Richard Dodson, The University of Tasmania
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Wednesday 3 October 2001
    Abstract:
    Pulsar glitches are the only mechanism we have to probe theinteria of a neutron star. Hobart has been monitoring, by timing, the Vela Pulsar for 20 years. The pulsar glitches about every 3 years, and is up in the sky for 18 hours a day. This gives as an excellent opportuninty to be observing when the glitch actually occurs. On the 16th of Jan 2000 this happened for only the second time, and a new system was in place to catch the event. New telescope monitoring systems functioned perfectly, allowing us to issue a IAU telegram within 12 hours. In this glitch, the largest for Vela on record, we found a new decaying term (associated with the re-coupling of the pinned superfluid) but no spinup, implying a very thin crust, tightly constraining the possible equations of state for a Neutron Star. Further follow up was undertaken using the Chandra X-ray observatory producing the beautiful `double crossbow' images, and follow up of those at the ATCA has found a (possible) radio pulsar wind nebula (PWN) around the X-ray PWN.

  9. Radiation fields in AGN with small and large scale symbiotic systems

    Who:
    Dr Alina-Catalina Donea, The University of Adelaide
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Thursday 20 September 2001
    Abstract:
    I shall discuss different external radiation fields in AGN such as : the direct radiation from the relativistic accretion disk coupled with the jet , infrared radiation from tori, the emission lines from Broad Line Region. The pair production optical depth due to the interaction of local GeV-TeV photons with external photons is analyzed for quasars and blazars. I shall review the evidence for the existence of a dust torus in blazars since the torus geometry is relevant for the TeV photon emission in these objects. The existing symbiosis between the ubiquitous accretion disk and the jets may extrapolates into a large scale symbiosis between the dusty components of blazars.

  10. Pulsar-like behaviour associated with the Planet Jupiter

    Who:
    Dr Mal Wilkinson, Monash University, Melbourne
    Where:
    School of Physics Colloquium, Hercus Theatre
    When:
    11.00am, Wednesday 19 September 2001
    Abstract:
    The enigmatic low-frequency radio emission associated with the planet Jupiter has fascinated generations of radio-physicists since its discovery in 1955 and yet the mechanisms that generate this emission and its source location are not well understood despite intensive study by ground-based observers and several spacecraft. In this presentation the basic phenomenology of this emission will be described and related to theoretical modelling studies and ground-based observations made in Melbourne by Dr Wilkinson since 1983. These observations suggest that the intense radio emission is generated in highly localised sources on magnetic field lines just above the cloud tops of Jupiter and is then focussed into multiple narrow beams which are swept past the observer by the orbital motion of Io, one of Jupiter's innermost satellites.

  11. Interstellar Scattering of Pulsar Signals as a Probe of the Interstellar Medium

    Who:
    Prof Barney Rickett, University of California at San Diego, USA
    Where:
    Room 360, School of Physics
    When:
    11.00am, Friday 7 September 2001
    Abstract:
    An understanding of radio propagation through the ionized interstellar medium is essential to the study of pulsars, since it causes a remarkable range of observable effects. Examples include dispersion, pulse broadening, arrival time wander, amplitude fluctuations and the recently discovered "arc" phenomenon. Examples of these will be shown and discussed in the context of scattering theory. The perturbations are caused by a wide range of scales in the interstellar plasma density; in turn this seems to imply the presence of magneto-hydrodynamic turbulence over more than six orders of magnitude in scale.

  12. Accretion of High-Velocity Gas onto the Galactic Halo

    Who:
    Prof Joss Bland-Hawthorn, Anglo-Australian Observatory
    Where:
    School of Physics Colloquium, Hercus Theature
    When:
    11.00am, Wednesday 5 September 2001
    Abstract:
    Since the 1960s, astronomers have been puzzled by the existence of high-velocity gas clouds which cover about a third of the sky. Their velocities do not conform to the rotation of the Galaxy which has encouraged wide ranging speculation as to their nature and origin. This rich history is reviewed in the context of new ideas and observations which may hold the key to the longstanding puzzle.

  13. Radio Afterglows of Gamma-Ray Bursts

    Who:
    Mr Edo Berger, California Institute of Technology, USA
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Tuesday 4 September 2001
    Abstract:
    The last four years have seen a revolution in the field of gamma-ray burst astronomy. With the detection of long-lived counterparts in the X-ray, optical, and radio bands, we have now firmly established the distance scale to these explosive events, we have a basic physical understanding of the afterglow emission, and it is generally believed that GRBs signal the formation of a black hole. Observational evidence is emerging that the progenitors of long-duration GRBs are massive stars, however this is yet to be firmly established. This talk concentrates on the unique contributions of radio observations to the field of GRB astronomy. In particular, I will discuss the impact of a concerted effort with an extensive network of radio/submm observatories on our understanding of the local environment as well as geometry (and hence energy) of the bursts, the macro-environment (i.e. host galaxies), and the importance of interstellar scintillation.`

  14. The Star Formation Law in CO-Bright Galaxies

    Who:
    Dr Tony Wong, Australian Telescope National Facility
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Thursday 30 August 2001
    Abstract:
    The relation between gas content and star formation rate (SFR) within galaxies is investigated using interferometric CO and HI maps of seven nearby spirals to trace the molecular and atomic gas respectively and H-alpha images to trace the SFR. We confirm the general applicability of the Schmidt law, but find that the correlation of SFR with CO emission is much stronger than with HI emission. The data are consistent with a constant SFR per unit molecular gas mass. We also raise doubts about the purported link between star formation and the gravitational stability parameter Q, which we suggest is primarily a measure of the gas fraction. Finally, we demonstrate that the ratio of HI to H_2 surface density scales with radius as R^{1.5} in the region of overlap, and suggest that the balance between the two phases is governed by the ISM pressure. We discuss how these results may be combined into a physically motivated framework for understanding star formation in disk galaxies.

  15. A High resolution Spectrometer for the Parkes Radio telescope

    Who:
    Dr Frank Stootman, University of Western Sydney
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Thursday 16 August 2001
    Abstract:
    Parkes has a high resolution SETI spectrometer. This same instrument has been modified to take advantage of the resolution for high resolution spectroscopy of astronomical objects. This talk summarises the capacity of this instrument and invites collaboration.

  16. Distributions of Lens Image Separations

    Who:
    Dr Premana Premadi, Bandung Institute of Technology, Indonesia
    Where:
    Tute room 2.11 on the podium level, School of Physics
    When:
    11.00am, Thursday 16 August 2001
    Abstract:
    The distributions of lens image separations resulting from analytical model and from several simulated COBE-normalised CDM models are compared. This gives the following conclusions: 1. The background matter tends to slightly increase the image separations and almost independent of cosmology; 2. The simulated galaxies + background matter often produces a secondary peak in the separation distribution; 3. Large separation is predominantly caused by early type galaxies. This line of study might give indication on the mass distribution (including dark matter) within a galaxy or cluster of galaxies.

  17. The Neutrino and the Early Universe

    Who:
    Drs Nicole Bell & Yvonne Wong, University of Melbourne
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Wednesday 15 August 2001
    Abstract:
    We discuss the role neutrinos play in the early universe. In particular, we explain their part in big bang nucleosynthesis and the way in which this may be affected by the generation of relic neutrino asymmetries via active-sterile neutrino oscillations.

  18. Interaction of Solar p-modes with Magnetic Field

    Who:
    Assoc Prof Paul Cally, Monash University
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Friday 10 August 2001
    Abstract:
    p-modes with a wide range of frequency and radial order have long been observed at the solar surface. They are essentially acoustic waves being refracted by the radial temperature gradient. Analysis of observed frequencies allows us to infer a great deal about the Sun's structure, including sound speed profile with depth, convection zone depth, and differential rotation structure. Several years ago, it became apparent that p-modes incident on sunspots (and plage) are partially absorbed, up to 50% in some cases. We briefly review the possible mechanisms for this phenomenon, with particular emphasis on p-to-slow mode conversion, and show movies from simulations which indicate that magnetic field spread is an important factor.

  19. Millisecond Pulsars in 47 Tuc

    Who:
    Prof Andrew Lyne, University of Manchester & Jodrell Bank Observatory, UK
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Wednesday 8 August 2001
    Abstract:
    Twenty millisecond pulsars have been detected in the globular cluster 47 Tucanae (47 Tuc). The gravitational accelerations of the pulsars, and hence the mass distribution of 47 Tuc, have been determined by precise astrometry and measurements of the pulsars' period derivatives. By inverting this procedure, the distance to each object can be accurately determined. Differences between the dispersion measures of the twenty objects are ascribed to the presence of gas in 47 Tuc.

  20. Planetary Radio Emissions - An Overview

    Who:
    Dr Andrew Willes, The University of Sydney
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Thursday 26 July 2001
    Abstract:
    An overview is presented of the wide variety of radio emissions generated in planetary bowshocks, magnetospheres, auroral regions and moons, based on ground-based observations and recent spacecraft missions, including Galileo, Ulysses and Cassini (which recently passed Jupiter on its way to Saturn). Theoretical concepts are outlined and the applications and relevance to astrophysical phenomena are discussed.

  21. Scintillation of Extragalactic Radio Sources

    Who:
    Dr Jean-Pierre Macquart, The University of Sydney
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Thursday 19 July 2001
    Abstract:
    Intraday variable (IDV) radio sources refer to a class of extragalactic objects that exhibit hourly to daily flux variations. The variations occur in total intensity and often the polarized intensity. If the variability were intrinsic to the sources it would imply brightness temperatures in the range 10^18 - 10^21 K. Such high brightness temperatures are extremely difficult to explain if the emission is due to incoherent synchrotron radiation. However, several independent lines of evidence have now established that most - if not all - of the hourly to daily variability observed in IDV sources is due to interstellar scintillation (ISS). Under such an interpretation the source brightness temperatures are in the range 10^13-10^15 K, but still require uncomfortably large doppler boosting in order to be consistent with standard synchrotron source models. I discuss (i) the evidence that the short-timescale variability is due to ISS, (ii) the brightness temperature problems with these sources, (iii) how ISS can be used to infer detailed source properties, such as the circular polarization structure on microarcsecond scales, and (iv) what these measurements are telling us about our Galaxy's interstellar medium.

  22. Dare a Scientist Believe in Design?

    Who:
    Professor Owen Gingerich, Harvard University, USA
    Where:
    Hercus Theatre, School of Physics
    When:
    3.15pm, Tuesday 17 July 2001
    Abstract:
    The natural universe seems full of amazing features that lead us to exclaim "What wonderful design!" Yet many scientists argue vehemently against the notion of supernatural design and a Designer. Can entertaining the idea of superintelligent design cripple a scientist in his work? Professor Gingerich will spiral in on the multi-faceted question, using both modern and historical examples to explain why he believes that a scientist can contribute effectively while holding a belief in a purposefully designed universe.

    Professor Gingerich is also speaking on "Galileo: Hero or Heretic?" on July 18 at Trinity College, University of Melbourne: Dinner ($30.00) 6.15 for 6.30 pm Lecture ($15.00, Conc. $10.00); enquiries and registrations to Dr Helen Joynt, Ph.: 9836 6871 Email: iscastv@alphalink.com.au

  23. Particle Acceleration at Relativistic Shocks

    Who:
    Prof Matthew Baring, Rice University, USA
    Where: Room 360, School of Physics
    When:
    3.15pm, Thursday 12 July 2001
    Abstr act:
    Relativistic shocks find natural application in astrophysics in active galaxies and gamma-ray bursts. While modest to moderate Lorentz factors are associated with the former environment, it is probable that extremely high values of 100-1000 are attained in bursts. Studies of diffusive acceleration at such relativistic shocks are more sparse than those pertaining to their non-relativistic counterparts. This is in part due to the inapplicability of the diffusion approximation to relativistic shocks, where particle isotropy is never realized. This talk presents results of acceleration properties of such relativistic shocks over a wide range of Lorentz factors of the upstream flow. Our tool is a well-known and successful Monte Carlo simulation that possesses versatility to probe a variety of scattering properties, and determine acceleration spectra, anisotropies, spatial gradients and acceleration times. Here we present results from our simulation of quasi-parallel shocks that relate to these various quantities, demonstrating close agreement with semi-analytic convection-diffusion equation results in the limit of pitch angle diffusion. We also explore the spectral flattening that arises when scattering angles are finite. Furthermore, it is determined that acceleration times can never become arbitrarily short in ultrarelativistic shocks, but are dominated by diffusion in the downstream region and couple to the particle's gyroperiod. The implications of such results for theories of gamma-ray bursts and active galaxies are discussed.

  24. Gravitational Microlensing in the Einstein Cross

    Who:
    Dr Stuart Wyithe, Princeton University, USA
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Wednesday 20 June 2001
    Abstract:
    The Einstein Cross comprises a galaxy at low-redshift that lies directly along the line of sight to a background quasar, producing a 4 image gravitational lens. The small red-shift of the lens galaxy places the quasar images in the bulge, and gravitational lensing by individual stars differentially magnify and de-magnify the quasar images on timescales of months. The 4 images of the Einstein Cross have been monitored by various groups for 15 years, and intensively by OGLE for the past 4 years. I will describe a monte-carlo approach to analysing the variability record that yields probabilities for the transverse component of the galactic velocity, the average mass of compact objects in the bulge, and the quasar source sizes in both optical light and mid-IR. I will then describe how this information can be used to predict High Magnification Events, observations of which will in-directly resolve the brightness profile of the quasars accretion disc. This forewarning will facilitate triggering of target of opportunity observations (TOO), and I will discuss the likelihood of success for a planned TOO experiment using the Hubble Space Telescope.

  25. The Search for Extrasolar Planets with Microlensing

    Who:
    Mr Daniel Kubas, University of Potsdam, Germany
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Friday 15 June 2001
    Abstract:
    A compact object (like a star) passing near the line of sight to a distant source, can act as a gravitational lens which magnifies the source. Since lens, source and observer are in relativ motion, the magnification changes with time, generating a socalled "Microlensing Event". A single lens gives rise to very symmetric and simple lightcurve. Possible planetary companions however can cause a detectable signature in these lightcurves, causing short lived deviations from the single-lens-lightcurve. Since 1995 the PLANET Group (Probing Lensing Anomalies NETwork) led by Penny Sacket et al. is monitoring Microlensing events within our galaxy to look for extrasolar planets. The talk shall explain the Microlensing effect and give an insight into the observational efforts and results achieved so far.

  26. Supernova Remnants, Molecular Clouds and OH Masers

    Who:
    Dr Mark Wardle, The University of Sydney
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Thursday 14 June 2001
    Abstract:
    When an expanding supernova remnant encounters an interstellar cloud, both the remnant and the cloud are strongly affected. The cloud is compressed, accelerated and heated, the remnant's expansion is inhibited, and it's structure and evolution is further modified by evaporation of cloud material into the remnant's hot interior. This interaction produces maser emission from OH molecules at 1720 MHz. I shall outline recent progress towards understanding how these masersare produced. The rich mixture of physical and chemical processes that are responsible has surprisingly broad implications for molecular clouds, shock waves, and supernova remnants.

  27. The Highest Redshift Neutral Hydrogen

    Who:
    Prof Frank Briggs, Kapteyn Institute, Groningen, Netherlands
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Wednesday 13 June 2001
    Abstract:
    The neutral hydrogen that fills the Universe after recombination at z~1000 suffers `reionization' once the first sources of ionizing radiation form. The study of these first objects is one of the frontiers in observational cosmology, since the formation of the first stars is both a direct consequence of the nature of dark matter and of the astrophyics of material of primordial composition. The disappearance of the neutral intergalactic medium through this Epoch of Reionization will be monitored through radio observations of the redshifted 21cm line with next generation radio telescopes.

  28. The HIPASS Bright Galaxy Catalog

    Who:
    Dr Baerbel Koribalski, Australian Telescope National Facility
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Tuesday 29 May 2001
    Abstract:
    The distribution of the brightest galaxies in the southern sky as obtained from the HI Parkes All-Sky Survey (HIPASS) will be presented. This survey is expected to eventually catalog about ten thousand galaxies, about 30% of which were either previously uncataloged or had no velocity measurement. The "HIPASS Bright Galaxy Catalog" contains the thousand HI brightest galaxies. Most of the cataloged HI sources can be identified with individual galaxies, but some correspond to pairs/groups where one or several galaxies as well as tidal tails/bridges contribute to the HI emission. We found nearly 10% previously uncataloged galaxies, most of which lie in the Zone-of-Avoidance The "HIPASS Bright Galaxy Catalog" is the biggest homogeneous HI catalog, so far, only to be superceded by the deep HIPASS catalog in near future.

  29. Detecting Isolated Black Holes through Accretion

    Who:
    Dr Eric Agol, California Institute of Technology, USA
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Thursday 24 May 2001
    Abstract:
    Long timescale microlensing events indicate the presence of about 10^8 isolated black holes in our galaxy, which is consistent with the number of black holes estimated from stellar population synthesis. We predict that of order 100 isolated black holes within dense interstellar clouds in our galaxy should be accreting at rates comparable to black holes in X-ray binaries. Depending on the efficiency of accretion, about 10 to 1000 of these should be detectable with Chandra or EXIST.

  30. Small-Scale Structure of HI Absorption from Interstellar Scintillation

    Who:
    Prof. Carl Gwinn, University of California at Santa Barbara, USA
    Where:
    Conference Room, Level 7, School of Physics
    When:
    3.15pm, Thursday 17 May 2001
    Abstract:
    The 21-cm line of HI varies by tens of percent, over scales as small as a few AU, when observed via absorption of compact background sources. This suggests that much of the interstellar gas resides in small cloudlets, with densities (and pressures) many times the average values. I'll discuss an alternative interpretation: that the variation arises from the interaction of interstellar scattering with velocity gradients in the HI. In other words, the variations arise from a propagation effect. I'll discuss the underlying optics, conditions required in the HI, and observational tests.

  31. The Dark Matter Distribution in Low Surface Brightness galaxies: Cores or Cusps?

    Who:
    Dr. Erwin de Blok, Australia Telescope National Facility
    Where:
    Conference Room, Level 7, School of Physics
    When:
    3.15pm, Tuesday 15 May 2001
    Abstract:
    Using high-resolution optical rotation curves we show that the dark matter mass distributions in Low Surface Brightness (LSB) galaxies are inconsistent with those predicted by Cold Dark Matter (CDM) models. Classical pseudo-isothermal halos provide a superior description, suggesting LSB galaxy halos are dominated by a core. Previous literature results, indicating cusps in LSB galaxies, only probed the edges of the cores, giving the (false) impression of a steep slope in the mass density. CDM in its current form cannot describe LSB galaxies.

  32. The Topography of HI in the Local Universe

    Who:
    Dr. Jessica Rosenberg, CASA, University of Colorado, USA
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Thursday 10 May 2001
    Abstract:
    In the last few years several large, blind 21 cm surveys have been completed. These studies are beginning to outline the distribution of high column density HI in the local universe. Additionally, a lot of progress has been made in understanding the distribution of HI from absorption-line studies and detailed N-body simulations. I will outline what we know about the topography of the gas in the local universe by (1) reviewing the results of the Arecibo Dual-Beam survey which probed high column density gas; (2) discussing how these blind survey results shape our interpretation of absorption-line study data; (3) outlining a project that is designed to make the connection between the high and low column density gas.

  33. The Environments of Active Galactic Nuclei

    Who:
    Dr. Michael Brown, National Optical Astronomy Observatory, Arizona, USA
    Where:
    Room 360, School of Physics
    When:
    4.00pm, Tuesday 24 April 2001
    Abstract:
    While there is strong evidence that Active Galactic Nuclei (AGN) are massive accreting black holes, it is unclear how material is fed towards the AGN. If galaxy mergers significantly increase the accretion rate and the AGN luminosity, AGN should occur in over-dense environments compared to most galaxies. In contrast, if starformation plays a significant role, AGN should occur in environments comparable to weakly clustered starforming galaxies. Previous estimates of AGN-galaxy clustering have often had very low signal-to-noise and there is no consensus on the environments of radio-quiet AGN. The resulting constraints on models of AGN have therefore been poor. A new study of AGN-galaxy clustering using the Panoramic Deep Fields galaxy catalog is discussed. Photometric redshifts and colour selection of galaxies have been used to improve the signal-to-noise of estimates of AGN-galaxy clustering by almost an order of magnitude. Accurate estimates of the environments of radio-loud and radio-quiet AGN environments are presented and correlations between AGN properties and environment are discussed.

  34. The Origin and Evolution of Quasars and Radio Galaxies

    Who:
    Dr. John Stocke, CASA, University of Colorado, USA
    Where:
    Room 360, School of Physics
    When:
    3.15pm, Wednesday 11 April 2001
    Abstract:
    To be discussed: (1) A ground-based imaging study to identify those intermediate redshift radio galaxies in clusters at z=0.15-0.65. (2) An HST WF/PC + NICMOS imaging study of the nearest examples of the Compact Symmetric Object (CSO) phenomenon. CSOs are thought to be the very youngest radio galaxies, only a few thousand years old.

  35. Simulated and Observed Cosmic Shear

    Who:
    Dr Andrew Barber, University of Sussex, UK
    Where:
    Room 360, School of Physics
    When:
    2.15pm, Thursday 29 March 2001
    Abstract:
    The detection of "cosmic shear", arising from weak gravitational lensing by the large-scale structure in the universe, provides important information about the distribution of dark matter rather than that of luminous matter. In this talk we will explore the comparisons between the recently observed cosmic shear signal and values obtained from cosmological $N$-body simulations. The numerical method, developed from a code for the evaluation of the shear in three dimensions, will be described and we will discuss how the results may be interpreted in terms of the cosmological parameters.

  36. The Power of Galaxy Merging

    Who:
    Dr Kenji Bekki, University of NSW
    Where:
    Room 360, School of Physics
    When:
    11.00am, Friday 23 March 2001
    Abstract:
    Galaxy merging is generally considered to play major roles in morphological transformation of galaxies, activation of nuclear starbursts and AGNs, and determination of spectral energy distribution of dusty galaxies. Based on my recent numerical simulations, I demonstrate how important it is to understand physical processes of minor, unequal-mass, and major galaxy mergers at low and high z. In particular, I discuss the origin of elliptical and S0 galaxies, the formation of ultra-luminous infrared galaxies, an evolutionary link between high-z SCUBA sources and extremely red objects (EROs), the formation sites of young globular clusters, the evolution from nuclear starbursts to AGNs, and the formation of the Galactic stellar halo.

  37. Transition Edge Sensors for the IR through UV: A New Tool for Astrophysics

    Who:
    Dr Roger Romani, Stanford University, USA & ATNF and University of Sydney
    Where:
    Laby Theatre, School of Physics
    When:
    11.00am, Wednesday 21 March 2001
    Abstract:
    Many astrophysical observations will remain photon-starved even inthe era of 8-m class telescopes. Advanced cryogenic detector technology can provide important capabilities for the pursuit of faint object astrophysics. The detectors being developed by our Stanford/NIST TES program, for example, provide time- and energy-resolved photon counting at high QE across a wide wavelength range. I summarise the operation of the TES sensor, describe our initial instrumentation for ground-based IR/optical and show early results from observations of pulsars and other compact objects. The future potential of these devices is illustrated by sketching the capabilities of a next generation space-based array for astrophysics and cosmology.

  38. A New View of Cooling Flows

    Who:
    Dr Alistair Edge, Department of Physics University of Durham, UK
    Where:
    Room 360, School of Physics
    When:
    3.00pm, Thursday 8 March 2001
    Abstract:
    The recent detection of dust and CO emission from 16 central cluster galaxies in strong cooling flows has shed new light on the long-running debate over cooling flows. I shall review these new results, those in the optical and near-infrared and results still to come.

  39. Gravitational Waves from Spinning Neutron Stars

    Who:
    Dr Yuri Levin, Research Fellow, Department of Astronomy University of California at Berkeley, USA
    Where:
    Room 360, School of Physics
    When:
    3.00pm, Thursday 1 March 2001
    Abstract:
    Neutron Stars are the densest objects in the universe, save for black holes. They could, in principle, be spun up to rotational frequencies exceeding 1kHz. However, such high rotational frequencies have not been observed in nature so far. In this talk I outline possible scenarios in which gravitational waves play an important role in setting the spins of both young and old (recycled) neutron stars. I will discuss future radio, x-ray, and gravitational-wave observations which might allow us to distinguish between these scenarios.

  40. Overview of Gravitational Wave Research

    Who:
    Dr Yuri Levin, Research Fellow, Department of Astronomy University of California at Berkeley, USA
    Where:
    Room 360, School of Physics
    When:
    3.00pm, Tuesday 27 February 2001
    Abstract:
    The first generation of interferometric gravitational wave telescopes, e.g. LIGO and LISA, is expected to come on line during this decade or the next. This talk will describe the design and state of readiness of these instruments, as well as the basic physics of gravitational waves themselves and their astronomical sources.

  41. Electron Heating and Acceleration by Shock Waves

    Who:
    Dr Stuart Bale, Space Sciences Laboratory, University of California at Berkeley, USA
    Where:
    Room 360, School of Physics
    When:
    3.00pm, Tuesday 30 January 2001
    Abstract:
    Shock acceleration is an important mechanism for generating high-energy particles in astrophysical contexts as diverse as supernova blast waves, jets from active galaxies, ultra-high energy cosmic rays, and coronal mass ejections associated with Solar activity. This talk will introduce the physics of shock energization, focusing on in situ spacecraft observations, and discuss some of the outstanding puzzles in the theory and its application to the above systems.


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