Talks & Events
Astronomy Colloquia: 2007
Some Recent Developments in the Theory of MHD Turbulence
There are many situations of astrophysical interest, like the solar wind or the interstellar medium, in which the turbulent motions of an electrically conducting fluid are affected by a background magnetic field. The resulting coupled system, which goes under the name of magneto-hydrodynamic (MHD) turbulence,has been the subject of study for over forty years. There are two competing theories of MHD turbulence, following Kolmogorov, both describe an energy cascade process from large to small scales, but their phenomenological underpinning and their predictions are drastically different. The first, developed independently by Iroshnikov and Kreichnan in the sixties, is based on the assumption that the energy cascade is isotropic at all scales. It predicts that the nonlinear interactions weaken at small scales and that the slope of the energy spectrum in the inertial range is -3/2. By contrast, the second theory, developed jointly by Goldreich and Shreidar in the nineties, assumes that the energy cascade is anisotropic, and that the degree of anisotropy increases at small scales. It assumes further that the nonlinear interactions remain strong at all scales and that the slope of the energy spectrum in the inertial range is -5/3 in the directions transverse to the local magnetic field. Interestingly, numerical simulations over the last decade, rather than clarifying the issues, have produced a picture in which the energy cascade is clearly anisotropic, in agreement with the Goldreich-Shridar phenomenology, but the slope of the energy spectrum in the inertial range is close to -3/2 as predicted by the Iroshnikov-Kreichnan theory. In my talk I will show how the numerical results and the Goldreich-Shridar theory can be reconciled by the introduction of the idea of dynamical alignment. According to this picture, the polarization vectors of the velocity and magnetic field fluctuations become increasingly more aligned at small scales. I will show that this tendency is associated with the conservation of cross-helicity in the ideal problem, and that one of the consequences is that the nonlinear interactions are depleted so that the slope of the energy spectrum agrees with the observed results.
The First Supermassive Black Holes
The existence of a supermassive black hole in nearly every galactic nucleus is no longer in doubt, but the question of how these black holes formed is wide open. I will argue that they could have formed via the direct infall and collapse of gas in pregalactic haloes at redshifts ~10-20, without the intermediate stage of Pop III star formation. Global gravitational instabilities get rid of excess angular momentum, and the infalling gas forms a self-gravitating, optically thick structure - a "quasistar". As matter piles on, the core of the quasistar heats up until it undergoes runaway neutrino cooling and collapses to form a 10 solar mass black hole. The black hole then grows by accreting from the quasistar at an extremely super-Eddington rate, reaching thousands of solar masses in less than a million years. Concurrently, the quasistar expands to form a radiation pressure-dominated, convective envelope reminiscent of a red giant. I will discuss the structure and evolution of quasistars and their detectability with the James Webb Space Telescope.
The Fossil History of the Solar System: Links to Interstellar Chemistry
In a few key instances the chemical composition of meteorites and comets provides a fossil record of the physical conditions present at their creation. In this talk I will explore lines of evidence that link the chemistry of the solar nebula to that seen in the interstellar medium today. †In particular, I will present results that address two long-standing questions in solar system chemistry: (1) the overall nitrogen deficiency in comets (when compared to carbon and oxygen) and† (2) the origin of oxygen isotopic anomalies seen in primitive meteorites. I will show how an answer to these questions has implications for the origin of other anomalies seen in comets and meteorites, but also implies the birth of the Sun in a large star cluster.
An Explosion of Cosmic Explosions
Only about a hundred years ago astronomers came to recognize cosmic explosive events. What was once termed as Stella Nova are now divided into two major families, novae and supernovae (with real distinct classes in each). The speaker will first summarize new developments in the field of cosmic explosions (namely gamma-ray bursts) and proceed to speculate on (and announce) new classes of cosmic explosions. Such a discussion is timely given the imminent commissioning of wide-angle optical surveys for transient sources (e.g. SkyMapper, PS1, LSST).
How Collisionless Shocks Work (And How They Don't)
Collisionless shocks are ubiquitous in astrophysics. Besides their primary role of terminating supersonic flows, collisionless shocks are commonly inferred to accelerate nonthermal particles and cosmic rays,and to generate significant magnetic fields. How and if this actually happens remains largely a realm of speculations. I will present the results of a systematic study of relativistic collisionless shocks through ab-initio particle-in-cell simulations, focusing on the basic physics that mediates a shock. I will show how shock properties depend on magnetization and composition of the flow and describe which parameter regimes are conducive to particle acceleration. In particular, I will show the first evidence for self-consistent Fermi particle acceleration in simulations, and address the issue of the electron-ion temperature ratio in relativistic shocks. These simulations begin to place constraints on the composition and magnetization of relativistic outflows in astrophysics.
From Protostars to Planets to Debris Around Young Suns
This talk will summarize our basic understanding of the evolutionary time scales associated with Circumstellar material. I will cover both the dissipation of primordial dust and gas, which is associated with the formation of planets, and the generation and evolution of secondary or debris dust, which is the most readily apparent astronomical signature of our own mature planetary system.
Star Formation as probed by Spitzer
Recent science results from the Spitzer Infrared Nearby Galaxies Survey (SINGS) will be reviewed. In particular, the unprecedented angular resolution of Spitzer has enabled the investigation of the mid-IR emission as a tracer of star formation both of and within galaxies. One immediate application of the mid-IR tracers is to the spatially-resolved Schmidt Law, which I will discuss for the specific case of the nearby galaxy M51. Applicability of the mid-IR SFR tracers to samples of galaxies at cosmological distances will be also discussed.
Recent results from the SDSS Lens + ACS survey
I'll present some of the latest results from our (SLACS) survey for galaxy-galaxy strong gravitational lenses discovered by following up the spectroscopic survey of SDSS with direct ACS imaging. As the largest lens survey, we have confirmed more than 87 multiply imaged systems with a myriad of image configurations. This unique and homogeneous data set constrains the intrinsic properties of the lensing galaxies and allows us to construct a more "fundamental" plane of early-type galaxies.
Injection of Small Bodies into the Interstellar Medium by Planetary Nebulae
Recent studies of the nearest planetary nebulae using the Hubble Space Telescope indicate much and perhaps most of the material ejected by intermediate mass stars during the planetary nebula phase is trapped into dense knots of planetary mass. I will report on a detailed study of these knots in the nearest bright planetary nebula, NGC 7293-the Helix Nebula. Using data spanning x-ray to radio wavelengths, we have been able to establish the basic characteristics and the physics governing the knots. We have been able to establish the source of the intense luminosity of these knots in the ubiquitous H2 molecule. The tantalizing question that remains is whether or not these condensations survive in the Interstellar Medium and play a role in the formation of stars and planets.
Echoes of Galaxy Assembly: Faint Light Around Nearby Galaxies
Galaxies are not simple superpositions of disks and spheroids. Instead, most disk galaxies host multiple faint stellar components. Studies within the Milky Way suggest that these additional components are old and chemically unevolved, and that they trace distinct epochs in the early history of the Galaxy. I will discuss recent work on the structure, kinematics, and stellar populations of thick disks and stellar halos, and the important constraints that they place on the assembly of disk galaxies.
Probing the Extremes of Surface Brightness at the Faint End of the Galaxy Luminosity Function
Models of structure formation based on cold dark matter generate a mass spectrum of dark-matter halos that is much steeper than the mass function of observed galaxies. In environments comparable to the Local Group, this “missing satellite” problem is particularly acute for satellite masses below approximately a hundred million solar masses, where the evolution of satellite galaxies is complex, and where the observations are least certain. I will discuss how two ongoing surveys targeting galaxies at opposite extremes of surface brightness are probing the evolution of faint satellite galaxies in environments ranging from low-mass groups to the most massive clusters of galaxies.
Some Uses of Gravitational Lensing
The first proposed use of gravitational lenses was to measure the Hubble constant. Recent measurements are consistent with those obtained from other determinations and the challenge is to reduce the systematic error on selected sources. Attempts to do this for the source B1608+656 will be discussed and prospects for application to a larger sample will be considered. A more recent proposal to use weak lensing of the clustering of the faintest "galaxies" -- to help understand their redshift distribution, physical properties and role in galaxy formation -- will be described.
Dark Matter and Galaxies
I will discuss various aspects of the relation between galaxies and dark matter and more general issues in the theory of galaxy formation, including: disk galaxy scaling relations and the fraction of baryons in galaxies, the relation between satellite galaxies and dark matter substructures, the universality of the relation between central galaxies and dark matter halos,the relative accretion/merger rates of central and satellite galaxies, and the origin of bimodality in the galaxy color distribution and the shape of the galaxy luminosity function. While recent theoretical and observational developments have shed light on many of these issues, many puzzles remain.
Weak Lensing by Large Scale Structure
Intervening structures in the universe give rise to small distortions in the shapes of distant galaxies. By measuring this tiny coherent signal, we can study the mass distribution in the universe directly, without relying on baryonic tracers. This makes weak lensing by large-scale structures a powerful probe of cosmology. I will review the topic of "cosmic shear" and discuss how the signal is extracted from the data. I will present results from recent surveys, most notably the CFHT Legacy Survey. Finally I will discuss what will be required to significantly improve constraints on the properties of dark energy.
Large Scale Structures and Galaxy Evolution in the COSMOS Survey
The COSMOS survey is the largest high redshift galaxy evolution survey ever done -- imaging 2 square degrees with all major space-based and ground based observatories. I will describe the key data in the survey and then present recent results on large-scale structures, the dark matter distributions and galaxy evolution.
Magnetic Fields in Star-Forming Clouds
The "strong field" picture of star formation involves a slow drift of neutrals through a magnetic field into the core of a self-gravitating cloud until magnetic support can no longer prevent dynamical collapse. A contrasting "weak field", picture involves support primarily by turbulence with clumps sometimes gaining sufficient mass to become self-gravitating. Attempts to compare observations with theoretical simulations have not been adequate to provide a convincing model.
I will discus the outlook for measuring relevant properties of the magnetic field such as the angular dispersion, the relative magnitudes of the systematic and turbulent components, and the size of the turbulent eddies.
Star Formation and Neutral Gas in Normal Galaxies
Great strides have been made in galaxy evolution in the last decade through a combination of improved observations, semi-analytic modeling of galaxy evolution, and simulations. Nevertheless, one of the biggest gaps in our knowledge has to do with how stars form on galactic scales. How does the number and distribution of new stars in galaxies get established? How does this change as a function of redshift? What is the role of "feedback," i.e. mass expulsion? How ubiquitous is the IMF? I will take the position that the distribution and state of the interstellar gas is paramount in answering these questions. I will describe several well-known observational facts about the distribution of atomic gas in spiral galaxies that have no agreed upon explanation. We will see that the neutral gas, the atomic plus the molecular, follows an approximate scaling relation. I will discuss the molecular gas depletion time problem in spiral galaxies, and some of the implications for galaxy evolution, and a possible solution. I will show that what determines how the star forming material, the dense molecular gas is formed in galaxies, is hydrostatic pressure, and how this relates to the star formation within them.