Talks & Events
Astronomy Colloquia: 2011
AGNs, Small-Scale Dynamo and Magnetic Fields in Galaxy Clusters
Galaxy clusters are important laboratories for cosmology and astrophysics. X-ray and radio observations of galaxy clusters have revealed a wealth of structures in association with extragalactic radio sources. Structures in the form of large-scale cavities and weak shocks provide a reliable gauge of the energy output of extragalactic radio jets launched by AGNs. Furthermore, they place interesting constraints on the nature of AGN outflows, especially on large scales. We will present 3-D MHD simulations of jets/lobes in the ICM and compare them with ~70 X-ray cavities as well as individual jet/lobe sources. In addition, we will present cosmological MHD simulations of galaxy cluster formation with AGN jets/lobes feedback and its implications for the origin and energetics of the cluster-wide magnetic fields. We demonstrate that the ICM turbulence is excited and sustained by the frequent mergers during the cluster formation. We quantify the available turbulent kinetic energy and nonlinear cascade rates. This turbulence excites a small-scale dynamo process that transports and amplifies the fields originated from the radio jet/lobe system. This process could be the primary process of populating the whole cluster with magnetic fields. We describe the properties of magnetic fields, including their strength, spatial distribution, power spectra and saturation mechanism. These simulations can be compared with observations made by VLA, LOFAR, and E-VLA.
Joys of turbulent convection and dynamos in both stellar envelopes and cores
Stellar convection zones in most settings should be able to build magnetic fields through dynamo action, especially if the flows are turbulent and the stars rotate. There is much more subtlety as to whether the resulting magnetic fields also exhibit large-scale structure and possible temporal flips and even cycles. We have been studying through 3-D global simulations the nature of both differential rotation and dynamo action that can be achieved in G-type stars like the sun by turbulent convection in their outer envelopes, and also by core convection in more massive A-type stars. The richness of structures realized will be discussed.
The physics of magnetohydrodynamics turbulence
The talk will discuss what is currently known about the properties of incompressible magnetohydrodynamic (MHD) turbulence. The talk will start with a general introduction, and then will review recent analytic developments, numerical simulations, and where possible, observations and experiments. The discussion will include the role of the guide field and the ideal conservation laws, the inherent anisotropies of the turbulent energy cascade, the physics of the "imbalance" or the Alfvenization phenomenon, and the processes of self-organization.
Blast from the Past: Hydrodynamic and X-ray Modeling of the Circumstellar Medium as Clues to Supernova Progenitors
Supernovae (SNe) are divided into many types and sub-types,but the precise progenitor of each (sub)type of SN still remains unknown. Core-collapse supernovae (SNe) arise from the explosion of massive stars. The resultant shock wave expands in the circumstellar medium formed by mass-loss from the massive star. The interaction of the shock wave with this medium gives rise to X-ray and radio emission.
In this talk we will discuss how circumstellar interaction can be used to constrain the SN progenitor. As a specific example, we will consider SN1996cr, one of the five closest SNe to explode in the past 30 years, yet which lay undiscovered for many years. Our team was awarded a 500,000 sec Chandra HETG observation of the SN, which was completed in early 2009. In order to interpret this data, we have carried out hydrodynamic simulations, followed by computations of simulated X-ray spectra under non-equilibrium ionization conditions, that can be directly compared with the observations. These
calculations allow us to infer the evolution of the SN shock wave, the density structure, and the abundances of the ejecta and surrounding medium, to reasonable accuracy. We will show how the data allow us to constrain the progenitor properties. The deep spectra even allow us to investigate the 3D morphology of this point source by studying the
detailed line shapes. We will discuss the implications for massive star mass-loss and SN evolution.
A generative model of everything in the Universe (including astrophysicists)
Radio astronomical studies of galaxy formation: the dense gas history of the Universe and the ALMA/EVLA revolution
Deep optical and near-IR surveys have traced the star formation history of the Universe as a function of environment, stellar mass,and star formation rate, back to cosmic reionization and the first galaxies (z > 6). While progress has been truly impressive, near-IR studies of early galaxies are fundamentally limited in two ways: (i) obscuration of rest-frame UV emission by dust, and (ii) near-IR studies reveal only the stars and ionized gas, thereby missing the evolution of the cool gas in galaxies, the fuel for star formation. Line and continuum studies at radio wavelengths (cm through submm) address both these issues, by probing deep into the earliest, most active, and dust obscured, phases of galaxy formation, and by revealing the molecular and cool atomic gas. I will summarize the techniques of radio astronomy to perform these studies, then present two recent examples. The first will consider the atomic and molecular gas, dust, and star formation, in the host galaxies of z ~ 6 quasars. The host galaxies are under-going extreme starbursts, with star formation rates > 1000 Mo/year, and molecular gas masses in excess of 1e10 Mo. Through gas dynamics, we can estimate the ratio of the bulge mass to black hole mass. These observations imply that we are witnessing the co-eval formation of massive elliptical galaxies, and super-massive black holes, within 1 Gyr of the Big Bang. The second entails observations of normal galaxy formation during the 'epoch of galaxy assembly' (z ~ 1.5 to 2.5). These observations reveal massive gas reservoirs without hyper-starbursts, and that active star formation occurs over a wide range in galaxy stellar mass. We find that the peak epoch of star formation in the Universe also corresponds to an epoch when the baryon content of galaxies was dominated by molecular gas, not stars. I will conclude with a description and status report of the Atacama Large Millimeter Array, and the Expanded Very Large Array. These telescopes represent an order of magnitude, or more, improvement over existing observational capabilities from 1 GHz to 1 THz, promising to revolutionize our understanding of galaxy formation.
Bringing our Galaxy's Supermassive Black Hole and its Environs into Focus with Laser Guide Star Adaptive Optics
The proximity of our Galaxy's center presents a unique opportunity to study a galactic nucleus with orders of magnitude higher spatial resolution than can be brought to bear on any other galaxy. After more than a decade of astrometry from diffraction-limited speckle imaging on large ground-based telescopes, the case for a supermassive black hole at the Galactic center has gone from a possibility to a certainty, thanks to measurements of individual stellar orbits. The advent of adaptive optics technology has significantly expanded the scientific reach of our high-spatial-resolution infrared studies of the Galactic center. In this talk, I will present the results of several new adaptive optics studies on (1) our current understanding of the galaxy's central gravitational potential, (2) the puzzling problem of how young stars form in the immediate vicinity of the central black hole, (3) the surprising, apparent absence of the predicted central stellar cusp around the central supermassive black hole (an essential input into models for the growth of nuclear black holes), and (4) how future large ground-based telescope may allow these studies to test general relativity and cosmological models.
The Circumgalactic Medium: 50 Years of Intellectual History and the Latest News from HST/COS
In recent years we have come to appreciate the great importance of the gas just outside galaxies - the circumgalactic medium - to galaxy formation and evolution. I will survey the intellectual history of this subject going back to the 1960s before presenting the latest results from two large HST projects that are using the new Cosmic Origins Spectrograph to systemically characterize the gaseous halos of low-redshift galaxies. COS has uncovered a clear link between star formation and hot gas in gaseous halos beyond 100 kpc, and strong indications of winds propagating from star-forming galaxies. These results show that the circumgalactic medium likely does participate in the ongoing star formation and evolution of modern galaxies.
The diverse, yet orderly lives of galaxies
At first glance the galaxy population today and even more so at earlier times exhibits a huge diversity. However, the well-known correlations between different galaxy properties, such as spatial structure, stellar population, stellar mass, stellar dynamics, and environment suggest that galaxy formation is actually an orderly process. With the recent large photometric and spectroscopic surveys and new instrumentation on the Hubble Space Telescope, it is now finally possible to study galaxies in a systematic way at earlier times, so that we can see directly how these relations change over cosmic time and what the physical processes are that drive them. Until very recently, these studies were hampered by the small sizes of spectroscopic galaxy samples, whereas much larger photometric samples lack the required spectroscopic information. I will discuss a novel approach, that makes use of medium-band photometry to perform detailed spectroscopic studies of ~3500 galaxies at 0.5
Infrared Extragalactic Backgrounds Near and Far
The extragalactic background light (EBL) provides an integral measure of the radiation produced by galaxy formation over cosmic history. Precise measurements of the spatial structure of the EBL promise to reveal the formation of galaxies from dark matter over-densities, and tease out signatures from the epoch of reionization. The ESA Herschel Space Observatory, with unprecedented sensitivity and mapping speed, has opened a new window on the far-infrared extragalactic universe. I will discuss recent results from the Hermes/Herschel guaranteed-time extragalactic survey, a large nested far-infrared photometric survey using the SPIRE and PACS instruments. In the near-infrared, the CIBER experiment is conducting a search for the EBL component produced by starlight from the epoch of reionization. CIBER uses a new Fraunhofer line measurement to improve absolute EBL photometry, and a multi-wavelength fluctuations measurement to probe for the reionization component based on spatial power spectra. Finally this is a field with rich potential for future measurements, and I will discuss plans for combining EBL measurements with CMB and 21 cm surveys.
Feedback and Galaxy Formation
Feedback from young stars plays a critical role in shaping the galaxy mass function, particularly at the low mass end, while feedback from supermassive black holes appears to shape the high mass end, statements supported by both numerical and semi-analytic models of galaxy formation. However, the exact form of the feedback is not certain. I will describe recent work shedding light on this problem. First I will describe three dimensional radiative magnetohydrodynamics calculations of the effects of young stars and supernovae on giant molecular clouds, showing that star formation is rapid, but that feedback halts star formation when ~5-10% of the cloud is turned into stars; supernovae play only a minor role. Next I will describe high resolution (~1 parsec) SPH simulations of star forming galaxies employing momentum feedback from young stars, as well as heating from supernovae, O star winds, and HII regions, showing that all these forms of feedback have a role to play, with different forms of feedback coming to the fore in different galaxies. These simulations naturally produce galactic scale superwinds, with mass loss rates from 1-10 times the star formation rate. Finally, I will briefly describe recent results on quasar feedback, including observational constraints one the launching mechanism of BAL winds, one of the more promising forms of "quasar mode" feedback.
Ultra-Faint Galaxies Around the Milky Way
In the past several years, fourteen new satellite galaxies have been discovered around the Milky Way, more than doubling the known population. These "ultra-faint" galaxies have emerged as the least luminous and most dark-matter-dominated galaxies in the known Universe. They are dramatically reshaping our understanding of galaxy formation and may hold the keys to deciphering the nature of dark matter. I will review our current understand of the ultra-faint galaxies, focusing on the constraints these objects provide on dark matter.
Gas Outflow and Inflow in redshift 0.2<z<2 galaxies
We present new results on the kinematics of metal-enriched gas near galaxies over a period in which star formation activity and gas accretion mechanism are thought to evolve strongly. We focus on constraints provided by near-ultraviolet spectroscopy of star-forming galaxies between redshift 2.0 and 0.4 obtained over a several year period with multislit spectroscopy at Keck. In particular, we investigate the properties of galactic outflows via interstellar absorption lines, resonance emission, and fluorescent emission. We compare the properties of the galaxies that host the outflows to those that do not; and we examine how outflow velocity varies with galactic mass and star formation rate. The most interesting aspect of this analysis, however, may be the discovery of infalling, metal-enriched gas in a small, but significant, fraction of the galaxies. Because the properties of these 'inflow galaxies' appear to be otherwise indistinguishable from the outflow galaxies; it is possible that they mark a ubiquitous phenomemon observed at an optimal orientation. From this more quantitative understanding of how metals circulate in and out of galaxies, we discuss both the escape of metals from galaxies and the mixing of gas ejected by winds with the "cold flows" that characterize cosmological models of galaxy formation.
Damped Lyman alpha Absorption Systems: Neutral Gas Reservoirs for Star Formation in Early Galaxies
Damped Lyman-alpha absorption systems (DLAs) are gas layers that dominate the neutral-gas content of the Universe in the redshift interval z=[0,5] and serves as neutral-gas reservoirs for star formation in early galaxies. I first discuss results of our recent Keck survey for the DLAs out to z=5. I focus on evolution of metal content of the gas. I emphasize the connection between DLAs and the galactic stellar populations, in particular the galactic halo. I next discuss our recent study of the star formation efficiency of DLA neutral gas. I focus on two independent tests indicating that the efficiency of star formation in DLAs is far below that indicated by the standard Kennicutt-Schmidt (K-S) Law. Finally, I discuss DLA thermodynamics. I emphasize the importance of detecting the principal coolant of neutral gas in DLAs, i.e., the [C II] 158 micron emission line.