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Astronomy Colloquia at Caltech for 2011-12

Colloquia are held every Wednesday during the academic year at 4:00 pm in the Cahill Hameetman auditorium. Tea and cookies are served at 3:45 pm in the Ahmanson library located on the 1st floor of Cahill.

Date

Speaker

Talk Title

SEPTEMBER

28

Mariska Kriek Harvard-Smithsonian Center for Astrophysics/Berkeley Host: Richard Ellis

Characterizing the distant galaxy population using pseudo-spectroscopy 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 and directly witness how the relations between spatial structure, stellar population, stellar mass, and environment change with redshift. 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<z<2.0. By identifying analogous galaxies we construct composite spectral energy distributions, which are of spectroscopic quality. This composite spectrum collection opens up efficient studies of Halpha and other spectral features for large distant galaxy samples, which would otherwise require extensive near-infrared spectroscopic campaigns. I will show how we have used the composite spectra to study star formation histories of galaxies, constrain stellar population and dust models, and analyse the relation between structural and stellar population properties. Finally, I will present new stellar kinematic measurements of distant galaxies and discuss the implications for the structural evolution of galaxies.

OCTOBER

5	Lisa Kewley Hawaii Host: John Johnson	Galaxy Formation and Evolution through Metals Chemical abundances in galaxies provide a fossil record of previous generations of star formation, modulated by galactic-scale gas flows. I will present the latest results from our investigation into the chemical evolution of galaxies, both locally, and at high redshift.  Theory predicts that as a merger progresses, galaxy disks become disrupted by tidal effects, causing large radial gas flows toward the central regions where kpc-scale starbursts and AGN may be fueled. Isolated disk galaxies have strong chemical abundance gradients that may become disrupted during a merger.  We have conducted the first investigation into chemical abundance gradients across the merger sequence.  We show that abundance gradients evolve dramatically with merger progress, providing a smoking gun for galactic-scale gas flows in merging galaxies.  Moving out in redshift, I present recent results from our investigation into the cosmic chemical history of galaxies. We have measured the chemical abundances for an unprecedented number of galaxies to z~1, finding little evolution between z~1 and the present day.  At higher redshift, we exploit the power of gravitational lensing to investigate the chemical evolution in galaxies between 1<z<3. We have measured the first metallicity gradient in a normal spiral galaxy at z~2, showing a steeper gradient than observed in local spiral galaxies. I will discuss the implications of this result and the future of this field with the next generation of telescopes.
12	David Arnett Kavli Institute for Theoretical Physics, UCSB, Steward Observatory, Arizona Host: Christian Ott	Turbulence, Computers and Stars It is now feasible to numerically simulate moderately turbulent flow in the stellar context. The impact of such simulations on astrophysical theory will be illustrated, as well as a connection to deterministic chaos. There have been a number of surprises, whose implications are only beginning to be appreciated. Applications to solar abundances and helioseismology, and to gravitational collapse and supernovae will be stressed.
19	Nitya Kallivayalil Yale Host: Samaya Nissanke	A 1% Proper Motion Measurement for the Large Magellanic Cloud I present proper motions for the Large & Small Magellanic Clouds (LMC & SMC) based on three epochs of Hubble Space Telescope data, spanning a $\sim 7$ yr baseline, and centered on background QSOs. The first two epochs are provided by the ACS/HRC and were the subject of Kallivayalil et al. (2006a,b) but have been reanalyzed here. The third epoch is brand new data taken with the WFC3/UVIS, providing a completely independent check. Due to the longer timebaseline and the high quality of the WFC3 data we derive proper motions of high accuracy. We are therefore also able to derive a purely proper motion-based rotation curve for the LMC, which is in excellent agreement with the latest line-of-sight velocity-based studies, and for the first time, competitive with the kinds of accuracies typically quoted in such studies. We determine the rotation velocity at large radii a factor of 3 more accurately than previous work. I discuss the consequences of these new measurements for our understanding of the global dynamics of the Milky Way-LMC-SMC system. This work is part of a larger effort towards 6-D mapping of the Milky Way.
26	Krzysztof Gorski JPL Host: Sterl Phinney	Early Astrophysics Results from Planck Planck has been measuring the sky at nine frequencies from 30 to 857 GHz since August 2009.  Designed to extract almost all of the information contained in temperature anisotropies of the cosmic microwave background and to improve significantly our knowledge of CMB polarization, Planck can also address a large range of Galactic and extragalactic astrophysics.  I will describe some early astrophysics results from Planck, and discuss Planck's promise and schedule for cosmology.

NOVEMBER

2	Boaz Katz IAS Host: Ott/Phinney	High energy emission from SNe and super-eccentric hot Jupiters They are out there, they are detectable and they are crucial theoretically. Detection of early high energy emission (soft X-ray to gamma-rays) at the onset of SNe will allow us to measure the radius of the progenitor, the velocity of the blast wave and signify the presence of a dense stellar wind. Detection of highly eccentric (e>~ 0.99) extra-solar gas-giants with periastron of a few stellar radii may answer the question of how hot Jupiters get so close to their host star.
9	Paul Groot Radboud University Host: Christian Ott	Ultracompact white dwarf binaries; a fresh look The evolution of ultracompact white dwarf binaries is set by the loss of angular momentum by the emission of gravitational waves. The discovery of detached and accreting systems with orbital periods as short as 5 minutes opens the possibility to test evolutionary theories, spin-orbit couplings of angular momentum, hot-slow merger events. Transient surveys such as the Palomar Transient factory are ideal to detect these systems in the Milky Way and Local Universe as they are are the (possible) sites of supernovae Type Ia, Type .Ia, helium novae and helium dwarf novae outbursts. The PTF has already discovered 6 of the 32 known interacting white dwarf ultracompact binaries and is set to revolutionize the field.
16	Enrico Ramirez-Ruiz UCSC Host: Stefanie Wachter	Tidal Disruption of Stars by Massive Black Holes A star interacting with a massive black hole cannot be treated as a point mass if it gets too close to the black hole that it becomes vulnerable to tidal distortions and even disruption. When a rapidly changing tidal force starts to compete with a star’s self-gravity, the material of the star responds on a complicated way, being stretched along the orbital direction, squeezed at right angles to the orbit and strongly shocked. This phenomenon poses an as yet unmet challenge to computer simulations. The art of modeling tidal disruption of stars, especially those entering the strong relativistic regime, forms the main theme of my talk. Detailed simulations should tell us what happen when stars of different types get tidally disrupted, and what radiation a distant observer might detect as the observational signature of such events.

23	Thanksgiving	NO COLLOQUIUM
30	Debora Sijacki CfA Host: Ott/Phinney	Moving mesh cosmology: the hydrodynamics of galaxy formation As already established in Frenk et al. 1999, entropy profiles of galaxy clusters are systematically different when simulated with SPH versus Eulerian mesh codes. This systematic difference has persisted until now, casting doubts on the ability of hydrodynamical schemes to follow complex hydrodynamical flows in full cosmological settings reliably. I will present a detailed comparison between the well-known SPH code GADGET and the new moving-mesh code AREPO on a number of hydrodynamical test problems. Through a variety of numerical experiments with increasing complexity I will establish a clear link between simple test problems with known analytic solutions and systematic numerical effects seen in cosmological simulations of galaxy formation. These tests demonstrate deficiencies of the SPH method in several sectors. An inadequate treatment of fluid instabilities in GADGET suppresses entropy generation by mixing, underestimates vorticity generation in curved shocks and prevents efficient gas stripping from infalling substructures. These accuracy problems not only manifest themselves in idealized hydrodynamical tests, but also propagate to more realistic simulation setups of galaxy formation, ultimately affecting local and global gas properties in the full cosmological framework.

December

7

Scott Gaudi Ohio State Host: John Johnson

The Demographics of Exoplanets with Gravitational Microlensing I review the landscape of microlensing searches for exoplanets, beginning with an outline of the method itself, and continuing with a review of the results that have been obtained to date. Thirteen planets have been detected with microlensing; I discuss what these detections have taught us about the frequency of cold terrestrial planets, giant planets, and solar system analogs. I then speculate on the expected returns of next-generation microlensing experiments both from the ground and from space. When combined with the results from other complementary surveys such as Kepler, next generation microlensing surveys will yield a complete picture of the demographics of planetary systems throughout the Galaxy.

14

Steve Furlanetto UCLA Host: Ott/Phinney

Piecing Together the Epoch of Reionization The era of the first galaxies and reionization is one of the final frontiers of observational cosmology.  At the moment, a wide range of observations provide tantalizing - but often confusing - clues.  I will summarize our current knowledge about this epoch and its low-redshift analog, when helium is fully ionized.  I will then describe how future observations can settle some of the important open questions that remain. When did reionization occur? What sources were responsible? How did the intergalactic medium affect the process?  And what did it do to future generations of sources?

JANUARY

4	Annika Peter UC Irvine Host: Samaya Nissanke	What the #*!$ is dark matter? One astrophysicist's perspective The nature of dark matter is one of the major "known unknowns" of physics of the Universe.  From astronomical observations, we know that dark matter exists, makes up 23% of the mass budget of the Universe, clusters strongly to form the load-bearing frame of structure for galaxy formation, and hardly interacts with ordinary matter.  This information is not enough to identify the particle specie(s) that make up dark matter, though.  As such, the problem of determining the identity of dark matter has largely shifted from astronomy to the fields of astroparticle and particle physics.  In this talk, I will give an overview of the ecosystem of astroparticle and particle-physics searches for dark matter, and show what its prospects are for dark-matter detection in the next decade.  I will also highlight some of the pitfalls of this particle-physics-oriented search.  This will lead me to advocate a return of the problem of dark-matter identification to astronomy, and show what kinds of theoretical and observational work might be used to pin down the nature of dark matter once and for all.
11	AAS	NO COLLOQUIUM
18	Andrei Beloborodov Columbia/Kingsley Visitor Host: Sterl Phinney	Emission Mechanism of Gamma-Ray Bursts Gamma-ray bursts are emitted by super-powerful ultra-relativistic jets from compact engines, most likely just born black holes or magnetars. A long debated problem is how the jet emits the observed gamma rays. A simplest model resembles the big bang: the opaque hot plasma expands to transparency and releases its thermal photons. The expected spectrum from a radiation-dominated jet is Planckian with a peak around 1 MeV. The observed burst spectra do peak around 1 MeV, however they have non-Plankian shapes, with extended high-energy tails. Physical processes generating non-Plankian radiation will be discussed. The old phenomenological model of synchrotron emission from internal shocks is currently experiencing a crisis -- it appears to contradict both shock physics and observations. I will describe a different emission mechanism: internal motions in the neutron-proton jet generate electron heat via nuclear and Coulomb collisions, and the electrons radiate the received energy. This mechanism invokes no phenomenological parameters, and the produced radiation can be calculated from first principles. Remarkably, the predicted spectra agree with observations. I will also discuss the central engine and the mechanism driving the jet. The jet may be a strongly magnetized outflow driven by rotation of the central object. Alternatively, the jet may be driven by thermal pressure due to neutrino heating around the central object. Both mechanisms can supply the observed luminosities, but require extreme rotation rates and magnetic fields.
25	Julianne Dalcanton University of Washington Host: Nick Scoville	Galaxies Viewed as Collections of Individual Stars In extragalactic astronomy, we routinely observe galaxies in broad-band filters, and then interpret the resulting spectral energy distribution to learn about the galaxies' masses, star formation rates, ages, and metallicities.  The fidelity of this interpretation relies on having a detailed understanding of the stellar populations within the galaxy, and on accurately characterizing the luminosities and colors of the billions of stars which contribute to a galaxy's light.  In this talk I will discuss several large programs which use the Hubble Space Telescope to resolve millions of the most luminous stars in nearby galaxies.  I will highlight results using near infrared observations, focusing on implications for the evolution of stars, the integrated NIR light of distant galaxies, and the structure of the dusty interstellar medium.

FEBRUARY

1	Ruth Murray-Clay Harvard CfA Host: John Johnson	Planet Formation at Wide Separations Several giant planets have now been directly imaged, offering the first view of extrasolar planets at wide separations from their host stars.  Formation of these objects, either by core accretion or gravitational instability, presents substantial theoretical difficulties.  In this talk, I will discuss the challenges and opportunities posed by wide-separation planets for theories of planet formation and orbital  evolution.  I will demonstrate how to use upcoming constraints from direct imaging to distinguish between theories of planet formation, and I will present a new theory of planetary core growth in the presence of gas that extends the reach of core accretion to large stellocentric distances.  Finally, I will briefly discuss how the atmospheres of planets at all stellocentric distances are shaped by their formation histories, including their protoplanetary disk environments and atmospheric escape.
8	Robert Williams STScI/Kingsley Visitor Host: Richard Ellis	The Nova Outburst:  An Evolving Paradigm?
15	Volker Bromm University of Texas Host: Samaya Nissanke	The First Stars and Galaxies How and when did the cosmic dark ages end? I present simulations of the formation of the first stars and galaxies, discuss their feedback on the intergalactic medium, and describe ways to probe their signature with missions such as WMAP and the James Webb Space Telescope. The properties of the first stars are determined by the interplay between cold dark matter and the atomic and molecular physics of hydrogen. I will identify the key processes and outline the major remaining uncertainties.
22	Claude-André Faucher-Giguère      UC Berkeley Host: Sterl Phinney	Inflows, outflows, and the physics of galaxy formation Galaxies must continuously accrete gas from the intergalactic medium in order to maintain their star formation rates. At the same time, star-forming galaxies and their black holes drive powerful winds that strongly affect their evolution. Inflows and outflows are thus among the key physical processes in galaxy formation, tying together the vastly different scales involved. I will first summarize the inflow predictions from cosmological simulations and show how they can be tested observationally. In doing so, I will address the physical nature of Ly-alpha blobs and Lyman limit absorbers. Motivated by breakthrough discoveries of massive, galaxy-scale outflows driven by active galactic nuclei in the last year, I will then address two puzzles raised by the data: the extreme multiphase structure traced by cool absorbers, and the origin of the large outflow momentum fluxes. The talk will conclude by outlining advances that will dramatically improve the realism and predictive power of theoretical studies of galaxy formation in the next few years.
29	Risa Wechsler  KIPAC/Stanford Host: Samaya Nissanke	The Galaxy-Halo Connection Across Mass and Time Dark matter halos are the fundamental building blocks in the growth of structure and they provide the framework for our modern understanding of galaxy  formation.  I will discuss the current state of the art in our understanding of the connection between galaxy properties and their dark matter hosts over a range of masses and redshifts.  In the context of a given cosmological model, I will show how the galaxy-halo relation can be tightly constrained at low redshift, and how it can be used to infer the full star formation histories of galaxies.   This model for the co-evolution of galaxies and LCDM halos is in excellent agreement with a wide range of data, including the evolution of the stellar mass function, galaxy clustering statistics, and the statistics of satellites around Milky Way mass hosts.  The faintest dwarf galaxies still present challenges to this picture, and may hold the key to new insight into galaxy formation or the properties of dark matter on small scales.

MARCH

7	Andreas Burkert LMU Munich Host: Nick Scoville	Watching a Little Gas Cloud on its Way into the Galactic Supermassive Black Hole The Galactic center is one of the most fascinating and extreme places in the Galaxy. Harboring a supermassive black hole with a mass of order 4 million solar masses it experiences cycles of activity and star formation, separated by periods of quiescence that last of order a million years. The Milky Way's SMBH currently is inactive. However a small, diffuse gas cloud (G2) has recently been detected (Gillessen et al. 2012) on an orbit almost straight into the Galactic SMBH. Within the next 2 years, G2 will pass the SMBH at a small distance of just 3000 Schwarzschild radii, corresponding to 4e15 cm. Depending on its nature it might break up and feed the SMBH, triggering a phase of AGN activity. The next years will therefore provide a unique opportunity to investigate directly the processes that drive and regulate gas accretion onto the Galactic SMBH as well as the onset of activity cycles in the Galactic center. This talk will summarize the observations of G2 and current models about its nature. The existence of such a tiny, cold gas cloud in the hostile vicinity of the SMBH raises numerous fascinating questions. Is G2 a diffuse gas clump that originates from winds of high-mass stars in the surrounding stellar disk or is it the atmosphere of an evaporating, invisible protostellar disk, planet or star? Or is it something completely different? Where did it come from and where will it go? Why is it on such a highly eccentric orbit? Which physical processes constrain its properties like its size, mass, density, temperature and geometrical shape? How many clouds like G2 are currently orbiting Sgr A* and how do they affect its activity and gas accretion rate? Like comet Shoemaker Levy's 1994 collision with Jupiter, the big challenge has started for astrophysicists to predict the outcome of G2's close encounter with the SMBH in the year 2013 and beyond. Their models will be validated directly by observations within the next couple of years.
14	Katrien Kolenberg Harvard CfA Host: Ott/Cohen	Kepler and the RR Lyrae stars   The spectacular data delivered by NASA's Kepler mission not only boost the discoveries of planets orbiting other stars, but they also open a window on the inner workings of the stars themselves. Kepler has been a breakthrough for the study of RR Lyrae stars and the still mysterious Blazhko effect.  I will present some of the most interesting results obtained so far. To date, over 40 RR Lyrae stars have been found in the Kepler field. The outstandingly high-precision data of these stars are investigated within the RR Lyrae working group as part of the Kepler Asteroseismic Science Consortium (KASC).
21	SPRING BREAK	NO COLLOQUIUM
28	SPRING BREAK	NO COLLOQUIUM

APRIL

4	Christian Ott Caltech
11	Scott Ransom NRAO Host: Christian Ott
18
25	Ewine van Dishoeck   Leiden   Host: Hillenbrand	(14th annual Greenstein Lecturer)

MAY

2	John Johnson Caltech
9	Laurant Vigroux AIP/Kingsley Visitor Host: Anneila Sargent
16
23
30

JUNE

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Previous Astronomy Colloquia:
1997
1998
1999
2000
2001
2002
2003
2004

2005

2006

2007

2008

2009

2010

2011

Updated: 2/13/12 gma