UNLV Department of Physics and Astronomy

Forum Schedule:  Fridays 3:45pm - 4:45pm

DateSpeakerTitle & Abstract
Jan 16
Jan 23
Jan 27
(Special Forum, 10am)
Dr. Asaf Pe'er
(host: Zhang)
A Model for Emission from Microquasar Jets: Consequences of a Single Acceleration Episode
There are strong evidence for powerful jets in the low/hard state of black-hole candidates X-ray binaries (BHXRBs). Here, we present a model in which electrons are accelerated once at the base of the jet, and are cooled by synchrotron emission and possible adiabatic energy losses. The accelerated electrons assume a Maxwellian distribution at low energies and possible energetic power law tail. These characteristic energies, combined with the decay of the magnetic field along the jet, introduce a wealth of spectra, which we study in details. We identify critical values of the magnetic field, and five transition frequencies in the spectra. In particular, we show that: (I) the decay of the magnetic field along the jet enables, for wide jets, production of flat radio spectra without the need for electrons re-acceleration along the jet. (II) An increase of the magnetic field above a critical value of ~10^5 G leads to a sharp decrease in the flux at the radio band, while the flux at higher frequencies saturates to a constant value. (III) For strong magnetic field, the flux decays in the optical/UV band as F_\nu ~ \nu^{-1/2}, irrespective of the electrons initial distribution. (IV) For B_0 ~ 10^4 G, the flux at the X-ray band gradually steepens. (V) With adiabatic energy losses, flat spectrum can be obtained only at a limited frequency range, and under certain conditions (VI) For narrow jets, r(x) ~ x^{\alpha} with \alpha < 1/2, flat radio spectrum cannot be obtained. We provide full description of the spectrum in the different scenarios, and show that our model is consistent with the key observed properties of BHXRBs.
Jan 29
(Special Forum, 3:45pm)
Dr. Daniel Ceverino
(Hebrew Univ.)
(host: Rhee)
"Simulations of Galaxy Formation: stellar feedback and galactic outflows"
I use results of a study galaxy formation in a LCDM Universe. We pay a special attention to the role of supernova explosions and stellar winds in the galaxy assembly. These processes happen at very small scales, they affect the interstellar medium (ISM) at galactic scales and regulate the formation of a whole galaxy. Once these effects are resolved in cosmological simulations, galaxy formation proceeds more realistically. For example, we do not have the overcooling problem. The angular momentum problem (resulting in a too massive bulge) is also reduced substantially: the rotation curves are nearly flat. The galaxy formation also becomes more violent. There are substantial outflows from forming and active galaxies. At high redshifts we routinely find gas with few hundred km^-1 and occasionally 1000-2000 km s^-1.
Jan 30Dr. Michael Eracleous
(Penn State Univ.)
(host: Proga)
"The Central Engines of Liners: an exercise in all of astrophysics"
Low-Ionization Nuclear Emission Regions (LINERs) are found in about 50% of nearby galaxies, but their power source is still a matter of much debate. Their defining characteristic is an emission line spectrum with moderately weak lines whose relative strengths suggest a low-ionization state in the of the emitting gas. Suggestions for the power source include shocks and photoionizaton by either very hot star or by an AGN or, more likely, combinations thereof. Therefore, figuring out the central engines of LINERs is an exercise in virtually all of astrophysics. I will describe how the quest to understand LINERs lead us to study a variety of astrophysical phenomena, ranging from the structure and dynamics of AGN accretion disks, to the tidal disruption of stars by a supermassive black holes, and to the evolution of high-mass X-ray binaries produced in a recent episode of star formation.
Feb 6
Feb 12
(Special Forum, 10am)
Prof. Qiu-He Peng
(Nanjing University)
(host: Zhang)
"Origin of ultra strong magnetic field of magnetars and their activity"
Feb 13Prof. Haozhe Liu
(Harbin Institute of Technology)
(host: Tschauner)
"Advanced characterization for materials under high pressure using synchrotrn x-ray microtomography and high energy XRD techniques"
I will present the group effort from Harbin regarding synchrotron XRD studies for metallic glasses and amorphous Se, and introduce the new technique for EOS measurement for amorphoumaterials in DAC under pressure: microtomography. Some very recently, interesting results on Be gasket via this novel microtomography will be presented as well.
Feb 20Prof. James Stone
(Princeton University)
(host: Proga)
"Three New MHD Instabilities in Weakly Collisional Plasmas"
Many astrophysical plasmas are so dilute that the mean free path of particles is long compared to their gyro-radius, and is comparable to macroscopic length scales in the system. In this regime, the simplest description of the plasma dynamics is given by the equations of MHD, supplemented with anisotropic transport coefficients for heat (conduction) and momentum (viscosity). Remarkably, this produces qualitative changes in the dynamics. For example, in this regime the convective stability criterion depends only on the temperature (rather than the entropy) gradient. The physics of this, and two other MHD instabilities, will be described, with application to the X-ray emitting plasma in clusters of galaxies, and hot accretion flows onto compact objects.
Feb 26
(Special Forum, 3:45pm)
Prof. Tomonori Totani
(Kyoto Univ.)
(host: Nagamine)
Topic 1: "Delay Time Distribution of Type Ia Supernovae and Implications for the Progenitor", Topic 2: "Lyman Alpha Emitters in Hierarchical Galaxy Formation"
[Topic 1:] The distribution of delay time from star formation to type Ia supernova (SN Ia) events gives an important hint for the still unknown progenitor of SNe Ia. We recently derived the delay time distribution by using faint variable objects found in the Subaru XMM-Newton Deep Survey (SXDS) data set. We will report the result with implications obtained for the SN Ia progenitor. [Topic 2:] Lyman Alpha emitters (LAEs) are very interesting population of galaxies, being used to probe the highest redshift universe. We present our latest model of luminosity function of LAEs in terms of Lyman alpha luminosity and UV continuum luminosity, as well as predictions of equivalent width (EW) distributions. These model predictions are carefully compared with a variety of observed data. We then address about some interesting issues, including the origin of very large EW LAEs (EW > 240 A at rest), which cannot be explained by normal stellar populations having a normal stellar initial mass function and the solar metallicity.
Feb 27
(Cancelled; will be rescheduled.)
Dr. Andy Fruchter
(host: Zhang)
"Gamma-Ray Bursts from a Safe Distance"
Gamma-Ray Bursts are explosions of nearly unrivalled brilliance. They can be bright enough to be seen at cosmological distances with the naked eye and can appear to emit the energy of the rest mass of the sun in high-energy photons in a matter of seconds. I will show evidence that most of these bursts, the so-called long, soft bursts, are produced by the collapse of extremely massive stars in galaxies unlike our own, but similar in many ways to our neighbors, the Magellanic clouds. The astrophysical origin of a smaller subset of bursts, the short, hard bursts, remains a mystery, though these seem to be distributed much more like the general population of stars in the local universe. Even if uncertainties surrounding their formation and emission mechanisms remain, both types of bursts may prove useful cosmological probes in the years ahead.
Mar 6
Mar 12
(Special Forum, 1pm)
Prof. Yusheng Zhao
(host: Nicol)
"High Pressure Nano-Mechanics and Molecular-Physics"
Nano-crystalline materials show drastic differences in physical properties compared with their bulk counterparts under high pressure (P) and temperature (T) conditions. We show a model to explain the observed contrasts between nano-metals and nano-ceramics, in the sense that the surface tension and compression of nanocrystals are the underlying cause of the differences in elasticity, yield strength, and work hardening and weakening. This nano-mechanics model has been tested by the comparative study of constitutive property and elastic modulus of nano-/micron- crystalline materials under high pressure and high temperature conditions. We demonstrate that the severe surface strains in the nano-grains present a good crystallographic model to interpret the mechanical performances of nanostructured materials. Our study on molecular inclusion compounds consisting of host frameworks with guest hydrogen molecules trapped inside nano-scale cages reveals that the clathrate cage can store up to four hydrogen molecules with an intermolecular H2-H2 distance of only 2.93 A. This distance is much shorter than that in the solid hydrogen (3.78 A) obtained at higher pressure and/or lower temperatures. This result suggests a strong densification effect that the nano-scale molecular cages may exert on the enclosed hydrogen molecules. We demonstrate that the striking framework pressurizing/freezing effect also exist in the metal-organic frameworks (MOFs). Owing to the enormous variety and flexibility of their frameworks, the molecular inclusion compounds may offer superior properties for storage of hydrogen and/or hydrogen-rich molecules.
Mar 13Hidenobu Yajima
(Tsukuba Univ.)
(host: Nagamine)
"The escape of ionizing photons from supernova-dominated primordial galaxies"
In order to assess the constribution of Lyman alpha emitters (LAEs) and Lyman break galaxies (LBGs) to the ionization of intergalactic medium (IGM), we investigate the escape fraction of ionizing photons from high-z primordial galaxies by solving the 3D radiative transfer. The model galaxy is employed from the ultra-high-resolution chemodynamic simulation of a primordial galaxy by Mori & Umemura (2006). According to the chemical enrichment, we incorporate the effect of dust extinction. As a result, we find that, in contrast to the previous analyses, LAEs and LBGs allow a large escape fraction of ionizing photons, which is ~17% - 47%. The result comes from the fact that the interstellar gas in LAEs and LBGs can be highly ionized owing to not only UV radiation from young stars but also the heating by supernavae. Confronting our results with observational data, we find that high-z LAEs and LBGs can ionize the IGM at z=3-5. However, UV radiation from LAEs as well as LBGs falls short to ionize the IGM at z>6. That implies that there must be additional ionization sources at z>6. Moreover, as a discussion I will show the infrared property of our model galaxies and the vision of future works.
Mar 16
(Special Forum, 1pm)
Dr. Christian Mailhoit
(host: Nicol)
"Creating Planets and Stars in the Laboratory: Scientific Opportunities on the National Ignition Facility"
In 2009, the Department of Energy (DOE) will commission the most energetic laser system ever built -- the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory. The NIF consists of 192 high-fluence laser beams that can deliver nearly 2 MJ of ultraviolet laser energy at 351 nm (3w) in nanosecond pulses to the target chamber center. The NIF will significantly and uniquely contribute to the nation's needs in national security, energy independence, and discovery-class fundamental science. Specifically, as a high-energy-density (HED) science experimental platform, the NIF will advance the scientific frontiers in the fields of the physics and chemistry of planetary interiors, astrophysical hydrodynamics, laser-plasma interactions, and nuclear physics. This presentation will highlight the scientific opportunities afforded by the NIF to create extreme states of matter in the laboratory that are akin to those found in the core of large planets -- thereby enhancing our understanding of the fundamental physical mechanisms governing planetary formation and evolution. Scientific challenges and opportunities germane to the creation, interrogation, and theoretical prediction of the behavior of strongly driven extreme states of matter in the NIF-class HED regime will also be discussed.
Mar 20
Mar 27Sarira Sahu
(UNAM, Mexico)
(host: Zhang)
"Neutrino oscillation in the Gamma-Ray Burst Fireball"
The central engine which powers the Gamma-Ray Burst (GRB) fireball, produces neutrinos in the energy range of about 5-20 MeV. Fractions of these neutrinos may propagate through the fireball which is far away from the central engine. We have studied the propagation of these neutrinos through the fireball (with and with out the magnetic field) which is contaminated by baryons and have shown that, resonant conversion of neutrinos are possible for the oscillations of nu_e to nu_{mu,tau}, nu_e to nu_s and anti-nu_(mu,tou) to anti-nu_s if the neutrino mass square difference and mixing angle are in the atmospheric and/or LSND range. On the other hand it is probably difficult for neutrinos to have resonant oscillation if the neutrino parameters are in the solar neutrino range. From the resonance condition we have estimated the fireball temperature and the baryon load in it.
Apr 3
Apr 10No Forum --- Spring Break
Apr 17Dr. Huirong Yan
(Univ. of Arizona)
(host: Zhang)
"Atomic Alignment- A New Diagnostics of Astrophysical Magnetic Field"
I would like to present a new technique of studying magnetic fields in interstellar and intergalactic gas/plasma. This technique is based on the alignment (in terms of their angular momentum in the ground state) of atoms and ions with fine or hyperfine splitting of the ground state. A unique feature of this technique is that the properties of the polarized radiation (both absorption and emission) depend on the 3D geometry of the magnetic field as well as the direction and anisotropy of incident radiation. I shall outline the prospects of the technique and its possible application to studies magnetic fields within circumstellar regions, interplanetary medium, interstellar medium, intergalactic medium. Both spatial and temporal variations of turbulent magnetic field can be traced with this technique as well. In addition, I shall demonstrate that atomic alignment induced by anisotropic radiation can cause polarization of the radio/far-infrared magnetic dipole transitions within the ground state, thus providing a possible way to study magnetic fields, e.g. at the epoch of Universe reionization.
Apr 21
(Special Forum, 4pm)
Dr. Huirong Yan
(Univ. of Arizona)
(host: Zhang)
"Nonlinear Cosmic Ray Transport and Acceleration in MHD turbulence"
Cosmic ray (CR) transport and acceleration are essential for many astrophysical problems, e.g., CMB foreground, ionization of molecular clouds and all high energy phenomena. Recent advances in MHD turbulence call for revisions in the paradigm of cosmic ray transport. We use the models of magnetohydrodynamic turbulence that were tested in numerical simulations, in which turbulence is injected at large scale and cascades to to small scales. I shall demonstrate that compressible modes instead of Alfven modes dominate the transport of CRs. I shall introduce a non-linear formalism that extends the Quasi-Linear Theory (QLT) that is routinely used for the purpose. This allows us to avoid the usual problem of 90 degree scattering and enable our computation of the mean free path of cosmic rays. The result is supported by recent test particle simulations. Implications for particle transport and acceleration in Solar flare and interstellar medium will be discussed. In addition, I address the issue of the transport of CRs perpendicular to the mean magnetic field and show that the concept of cosmic ray subdiffusion is not applicable in 3D turbulence. As a result, this work provides a formalism that can be applied for CR transport and acceleration in a wide variety of circumstances.
Apr 24Dr. James Jee (LSST Fellow)
(Univ. of California, Davis)
(host: Choi / Nagamine)
"Dark Matter Substructures in Galaxy Clusters Revealed by HST/ACS Gravitational Lensing"
Gravitational lensing analysis of galaxy clusters with HST/ACS provides an unique opportunity to paintinvisible dark matter with unprecedented resolution. After a brief review on the methodology, I will talk about peculiar dark matter substructures revealed in merging clusters, which together with galaxies and X-ray gas distribution enable us to study the dynamical history of the clusters and perhaps the properties of dark matter. In addition, I will brief on our recent weak-lensing analysis of the galaxy cluster XMM2235 at z=1.4, the most distant cluster yet measured with lensing. Because the mass turns out to be surprisingly high for a cluster at such a high redshift, the likelihood of finding such a massive cluster in the survey and cosmological implications will be discussed. Finally, if time allows, I will give an introduction on the Large Synoptic Survey Telescope (LSST) project and where it stands now.
Apr 30
(Special Forum)
Dr. Masami Ouchi
(Carnegie Observatories)
(host: Nagamine)
"Subaru Deep Wide-Field Surveys of Lya Emitters at z=3-9 for Galaxy Formation and Reionization"
Lya Emitters (LAEs) at high redshifts are useful probes of low-mass galaxy formation and cosmic reionization. I present the recent results from our Subaru deep wide-field surveys for LAEs at z=3-7 in a total area of 1.2 deg^2. I show statistical properties of LAEs with luminosity functions, correlation functions, and composite spectra which are obtained from our large photometric and spectroscopic samples of 1301 and 178 LAEs, respectively. I discuss galaxy formation and evolution based on these measurements including errors of statistics and field variance robustly estimated from our large samples. Cosmic reionization at z=6-7 is characterized by the evolution of luminosity function and clustering amplitudes. At the last part of my talk, I introduce our on-going z=8.8 LAE search to study galaxy formation and reionization at z=7-9.
May 1Prof. John Stocke
(Univ. of Colorado)
(host: Rhee)
"Hubble's UV Spectrographs Study and Galaxy Infall and Outflows"
In this talk, I will review what we have recently learned about the local intergalactic medium (IGM), the gas which surrounds galaxies, using the diagnostic power of UV spectroscopy from the Hubble Space Telescope (HST). Specifically, I will show GHRS and STIS (the previous Hubble high-resolution spectrographs) and ground-based results which focus on the question of gas accretion (infall) and winds (outflows) from galaxies. I will end with an introduction to the Cosmic Origins Spectrograph (COS), which will revolutionize these types of studies and which will be installed on HST next month.

Past forums: Fall '08