UNLV Department of Physics and Astronomy

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

DateSpeakerTitle & Abstract
Jan 14Dr. Brian Morsony
(University of Wisconsin)
(host: Zhang)
"The Impact of Environment on GRB and AGN Jets"
I will present numerical simulations of jet propagation in the context of GRB jets from massive stars and AGN jets in galaxy clusters. For GRBs, the stellar envelope collimates the jet, keeping the jet hotter than would be expected for a free expansion. This allows about 30% of the jet energy to escape as photospheric gamma-ray emission. In AGN, interaction between the jet an a dynamic cluster environment disrupt the jet, forming multiple, disconnected X-ray bubbles from a single period of AGN activity with constant luminosity. This implies that observations of multiple bubbles in a cluster are not necessarily an expression of the AGN duty cycle. Disruption also limits the size of the region that can be directly heated by the AGN, with implications for how AGN feedback in clusters may operate.
Jan 21
Jan 28Prof. Guo-Jun Qiao
(Peking University)
(host: Zhang)
"The Annular Gap Model and Pulsar High Energy and Radio Emission"
An annular gap model of pulsars will be presented. In this model, the advantages of both the polar cap/slot gap model and outer gap model are retained. The annular gap region would play an important role for both young normal pulsars and millisecond pulsars, since the potential drops in the annular region can be high enough to produce gamma-rays. This is consistent with pulsar observations by the Fermi satellite. The annular gap could be formed regardless of whether pulsars are neutron stars or quark stars. Besides the annular gap, a core gap would also form. Particles accelerated from the two gaps can explain both radio and gamma-ray emission of pulsars. The radio emission is powered by Inverse Compton scattering (ICS) in strong magnetic fields. Recent results of the annular gap model will be presented both theoretically and observationally.
Feb 4Prof. Ren-Xin Xu
(Peking University)
(host: Zhang)
"Pulsars and Quark Stars"
After a brief introduction on different manifestations of pulsar-like compact stars, the nature of pulsars and the state of realistic dense matter at supra-nuclear density are discussed. Although conventionally pulsars are thought to be normal neutron stars, I argue that it is still an open issue whether they are indeed neutron stars or alternatively are quark stars. In particular, arguments from a model independent point of view are provided, to show that realistic quark matter could be in solid state. The implications of this possibility for understanding various observational features are discussed.
Feb 11Dr. Julie McEnergy
(host: Zhang)
Feb 18Dr. Claude-Andre Faucher-Giguere
(UC Berkeley, Miller Fellow)
(host: Nagamine)
"Probing the Accretion of Gas onto Galaxies: Opportunities and Pitfalls"
Galaxies must continuously accrete gas from the intergalactic medium in order to sustain their observed star formation rates. How this accretion proceeds is a fundamental problem in galaxy formation, as it determines how rapidly galaxies can grow as a function of mass and redshift, as well as the efficiency of feedback processes. While theoretical work predicts that galaxies acquire most of their baryons via the "cold mode", observations have so far not found clear traces of the cold streams. I will discuss results from a research program aimed at robustly quantifying the observational signatures of the cold mode, focusing on Ly-alpha emission and absorption and their connection to high-redshift "Ly-alpha blobs" and halo absorbers. In doing so, I will highlight some of the theoretical challenges involved, which can jeopardize this unique opportunity to test a physical prediction of galaxy formation if not addressed properly.
Feb 25
Feb 28
Prof. Paul Wiita
(College of New Jersey)
(host: Barai)
"Testing Active Galactic Nuclei Unification Schemes via Quasar Optical Spectra and Radio Morphology"
After reviewing some key properties of AGN I will show how we can test the strength of the orientation based unification paradigm through the large numbers of radio quasars present in both the SDSS optical and FIRST radio surveys. This sample of 4714 radio quasars is over an order of magnitude larger than any such available heretofore and allows much stronger tests of correlations between line strengths and shapes and radio morphologies. Our conclusions include: radio quasars with unresolved core morphologies are redder than radio quiet quasars but other radio loud morphologies are not redder; the radio core-to-lobe ratio and the radio-to-optical ratio of the quasar core are both correlated and should act as orientation indicators; the rest equivalent widths of several lines correlate strongly and positively with the radio/optical ratio, but in contrast to earlier results, we don't see such EW correlations with the radio core/lobe ratio. However, this orientation indicator--equivalent width correlation is the opposite expected from commonly accepted orientation based theories.
Mar 4
Mar 10
(Public Lecture)
Prof. Doug Lin
(UC Santa Cruz / KIAA)
"The Search for Planets and Life Beyond the Solar System"
This public lecture will be held at the Barrick Auditorium at 7:30pm. It is intended for a general audience including enthusiasts of all backgrounds and ages.
Mar 11Prof. Doug Lin
(UC Santa Cruz / KIAA, Beijing)
(host: Zhang)
"Origin, Structure, and Evolution of Hot Jupiters and Super-Earths"
In the radial-velocity and transit searches, close-in planets are the first to be found. These planets are most likely formed at much larger distances from their host stars and migrated to their present-day location. I will discuss the implication of various recent discoveries including 1) the mass-period distribution of close-in planets, 2) their mass-radius relationship, 3) the mis-alignment of their orbital angular momentum vector and spin vector of their host stars, 4) their atmosphere and 5) the dynamical architecture of closely-packed multi-planet systems. Based on these theoretical consideration and population synthesis models, we suggest that 1) sequential core accretion is the dominant mode of planet formation process, 2) planets have considerable mobility, 3) habitable planets are common, and 4) dynamical evolution of planetary systems is an ongoing process.
Mar 25Prof. Balakrishnan Naduvalath
(UNLV, Chemistry)
(host: Nagamine)
"Quantum Dynamics of Molecular Collisions and Chemical Reactions in Astrophysical Media"
Quantitative prediction of cross sections and rate coefficients for molecular processes and chemical reactions in astrophysical environments requires accurate quantum dynamics calculations. Molecular hydrogen is the dominant coolant in primordial gas and collisional excitation of H2 rotational and vibrational levels by H, H+, He, H2, etc. is an important source of cooling. Unfortunately, uncertainties in the corresponding rate coefficients, especially at low-temperatures, continue to be a major issue in astrophysical models. I will present recent progress in our group on quantum dynamics studies of reactive and non-reactive collisions of interest in the interstellar medium. In particular, I will discuss how energy and angular momentum considerations influence rate coefficients of H2-H2 collisions at low temperatures, and how internal energy and angular momentum conservation effects can be used to dramatically simplify calculations.
Apr 1Dr. Michelle Kluwer
(host: Rhee)
"Powerful H2 Line-Cooling in Stephan's Quintet and the "Death by Debris" Phenomenon in Hickson Compact Groups"
Stephan's Quintet (SQ) is a strongly interacting compact group experiencing a group-wide shock (~ 30 kpc) due to the high velocity (~ 1000 km/s) collision of an intruder galaxy with the intragroup medium. We find in SQ the phenomenal coexistence of hot X-ray plasma and abundant excited molecular hydrogen. I will discuss the concordance of observations with a model of H2 emission driven by turbulent energy transfer. The prevalence of this pathway over other sources of cooling (such as X-rays) in fast galaxy-scale shocks, has important implications for the modeling of processes ranging from early structure formation to jet-ISM interactions in AGN hosts. In addition, our broader study of Spitzer spectroscopy of 23 Hickson Compact Groups (HCGs) reveals several systems showing enhanced H2 emission, indicative of large-scale shocks. Given recent speculations of rapid evolution occurring in HCGs, linked to gas depletion, we propose a hypothesis whereby group galaxies collide with previously stripped tidal material, shutting down star formation and accelerating the transition from actively star forming to passively evolving systems.
Apr 8NO FORUM(Saved for Thesis Defence)
Apr 15Dr. Joseph Munoz
(host: Nagamine)
"Clues to High-Redshift Star Formation"
Recent observations with the WFC3 camera aboard HST have discovered large populations of galaxies at z > 6 and out to z = 10. Yet, the star formation physics of the first galaxies remains largely unprobed. I will explore clues to the formation of the first stars and galaxies that can nevertheless be gleaned from these observations by understanding their abundance and clustering properties. Fitting a physical analytic model to the observed luminosity function can place constraints on the minimum halo mass required to form galaxies as well as the evolution of the high-redshift mass-to-light ratio. Using a similar technique, I will also explore what clues can be uncovered by examining relics of the first galaxies in the local universe orbiting the Milky Way.
Apr 22Dr. Naveen Reddy
(NOAO, Hubble Fellow)
(host: Nagamine)
"How Dust Affects Our View of the Distant Universe"
The primary by-products of stellar nucleosynthesis include heavy elements that are released into the interstellar medium where they can coagulate to form dust particles. I will discuss why dust is important for studying galaxy evolution and for uncovering the fundamental physics that governs the Universe. Our results suggest that galaxy growth is intimately connected with the evolution of dark matter during the first ~3 billion year history of the Universe. I will go on to show that dust in the distant Universe has a direct affect on the observed luminosity distribution of galaxies, the dustiness of a galaxy is directly correlated with its emergent UV luminosity, and that faint galaxies dominate the luminosity and star formation rate density at high redshift. I then use this information to resolve one of the outstanding problems that has plagued cosmologists over the last few years, namely the discrepancy between the star formation history and the evolution in stellar mass density of the Universe. I conclude with an overview of how future facilities like JWST and TMT, as well as forthcoming dark energy experiments, can be used to uncover the physics of faint galaxies at high redshift and unveil the complex connection between galaxies and the surrounding intergalactic medium.
Apr 29Prof. Kris Stanek
(Ohio State Univ.)
(host: Proga)
"Drinking Coffee, Writing Papers"
Coffee is essential to human life, but at OSU Astronomy Department, coffee is also an important contributor to science. OSU's daily coffees seeds research across boundaries, and in this talk I will discuss three new ideas that I have worked on resulting from OSU's astro-ph coffee discussions. The first idea, "Protecting Life in the Milky Way: Metals Keep the GRBs Away" formulates a strong hypothesis that gamma-ray bursts can only explode if the metallicity of parent star is below about 0.1 solar. The second, "The Future is Now: the Formation of Single Low Mass White Dwarfs in the Solar Neighborhood" discusses new aspects of stellar evolution when the stellar metallicity is highly super-solar. The third, "A Survey About Nothing: Monitoring a Million Supergiants for Failed Supernovae" examines how we could verify that all massive stars do indeed explode as core-collapse supernovae.
May 6
(1:30pm, BPB-102)
Robert K. Weiss
(President, X-Prize Foundation)
"Encouraging & Supporting Innovation through Competition"
What are some of the best ways that new discoveries happen? Government agencies like NASA, NSF and DARPA play a big role -- as does industry. But what about a model that encourages innovation through competition and increases the effectiveness of public/private partnerships? Come learn about the exciting work of the X Prize Foundation and their role in finding solutions to some of our world's biggest challenges. The Foundation has active competitions in Genomics and Lunar Exploration, and is now developing X PRIZEs in Healthcare, Energy, and Exploration (Ocean & Deep Space). Robert K. Weiss is Vice Chairman and President of the X PRIZE Foundation. A veteran Hollywood producer, Mr. Weiss was the executive producer for the television series WEIRD SCIENCE and SLIDERS (which he co-created). Weiss executive produced many films at Paramount including TOMMY BOY, A NIGHT AT THE ROXBURY, SUPERSTAR, and THE LADIES MAN. He went on to produce the hit movies, SCARY MOVIE 3 & 4.
May 13Dr. Sung Keun Lee
(Seoul National University)
(host: Tschauner)
"Universality in Pressure-Induced Structural Disorder in Amorphous Oxides: Insights From Non-Resonant Inelastic X-Ray Scattering & Solid-State NMR"
Full understanding of atomic structure and the degree of disorder in oxide glasses at varying pressure and composition (from archetypal single component to multi-component) has been a fundamental problem in modern physical sciences due to lack of suitable experimental probes and inherent topological disorder. Recent advances in element-specific experimental probe of local structures including non-resonant synchrotron inelastic x-ray scattering (IXS) and solid-state NMR unveil previously unknown structural details of the pressure-induced changes in the bonding nature of the diverse oxide glasses. IXS, combined with advances in diamond anvil cell technique and x-ray optics, allowed us to study bonding changes in amorphous oxides up to about 40 GPa. The results suggest the formation of the triply coordinate oxygen and highly coordinated framework cations at high pressure with complex compositional dependence. We account for these differences with a conceptual model that utilizes pressure rigidity (the resistance to structural changes with increased pressurization) defined by the variance of the ratio of energy difference between high and low pressure states to its pressure gradient. The first high resolution NMR spectrum for amorphous alumina reveal that four and fivecoordinated species are predominant (about 95 percent), while six coordinated species are minor. Such a species distribution is similar to what has been predicted theoretically for alumina melts and requires that there exists in the amorphous alumina a significant fraction of triply bonded oxygen. Finally, contrary to an expected complexity in densification for multicomponent oxide glasses, multi-nuclear 2-dimensional NMR spectra for multi-component amorphous oxide at high pressure also demonstrate that the pressure-induced changes in melt structures show a universality where all the experimental non-bridging oxygen (NBO) fractions at high pressure converge into a single decaying function.
June 6
(Monday 2-3pm)
Dr. Mark Dijkstra
(Max Planck Instituite for Astrophysics)
(host: Nagamine)
"Probing the Epoch of Reionization with Lyman Alpha Emitting Galaxies"
The next generation of telescopes aim to directly observe the first generation of galaxies that initiated the reionization process in our Universe. The Lyman Alpha (Lya) emission line of atomic hydrogen is robustly predicted to be the most prominent intrinsic spectral feature of these galaxies, making it an ideal target to search for and study high-redshift galaxies. I briefly discuss why Lya emitting galaxies (LAEs) provide a good probe of the intergalactic medium (IGM) during the Epoch of Reionization (EoR). Current observations indicate that 'something' is happening at z>5.7. However, if we wish to properly interpret this data, and fully exploit LAEs as a probe of the EoR, it is important to understand what drives their observed redshift evolution after reionization is completed. An important uncertainty in interpreting existing LAE observations relates to the impact of the ionized IGM on Lya photons emitted by galaxies, which in turn is strongly connected to the effects of scattering through HI in the ISM and galactic outflows. Understanding this scattering process provides insights into the distribution (and kinematics) of cold gas in the interstellar and circum galactic medium of star forming galaxies. Resonant scattering of Lya photons in the ionized IGM can also introduce non-gravitational effects on the clustering of LAEs after reionization has been completed.
June 21
(Tue 3-4pm)
Dr. Brian Maple
(UC San Diego)
(host: Kumar)
"High-Pressure Studies of Strongly Correlated Electron Phenomena in Novel Materials"
Complex multinary d- and f-electron compounds have proven to be a rich reservoir of strongly correlated electron ground states and phenomena: e.g., valence fluctuations, hybridization gap semiconductivity, heavy fermion and non-Fermi liquid behavior, quantum criticality, unconventional superconductivity in which electrons are paired in states with non-zero angular momentum, high temperature superconductivity, magnetic order, coexistence of superconductivity and magnetic order, quadrupolar order, etc. In these materials, the occurrence of such a wide range of ground states and phenomena arises from a delicate interplay between competing interactions that can be tuned by chemical substitution, as well as the application of pressure and magnetic fields, resulting in complex electronic phase diagrams in the hyperspace of temperature, chemical composition, pressure and magnetic field. In this talk, we review selected examples in which pressure has been employed as a tuning parameter to access and study various strongly correlated electron phenomena in novel d- and f-electron materials. Phenomena of particular interest include unconventional superconductivity, high temperature superconductivity, magnetic and quadru-polar order, charge and spin density waves, quantum critical behavior, heavy fermion behavior and non-Fermi liquid behavior. The support of the US NNSA, DOE and NSF is gratefully acknowledged.
June 22
(Wed 2-3pm)
Dr. Amit Kashi
(Technion, Israel Institute of Technology)
(host: Nagamine)
"Optical transients as accretion powered events: from BD-planet mergerbursts to LBV major eruptions"
Intermediate Luminosity Optical Transients (ILOTs) are erupting objects with luminosities between those of novae and supernovae. While new ILOTs are being discovered in an accelerated rate, the physical mechanism that powers the eruption is still poorly understood. Similarities between ILOTs and major eruptions of Luminous Blue Variables (like the 19th century great eruption of Eta Carinae), suggest that a similar mechanism powers both. We suggest that this process is a short duration release of gravitational energy in a binary system. We further propose that destruction of a planet y a brown dwarf can lead to a transient event that will also be accretion powered and will have a light curve resembling that of a stellar ILOT, with lower luminosity.

Past forums: Fall '08   Spring '09   Fall '09   Spring '10   Fall '10