Talks & Events
Astronomy Colloquia: 2013
Imaging instruments for the cosmic frontier
This talk reviews the current technology used for wide field optical imaging in astronomy, with emphasis in the recently commissioned Dark Energy Camera. This technology has enabled new experiments in the cosmic frontier that are starting to produce results now. Finally, during this talk, I discuss how this field is transforming to allow future astronomical instruments with unprecedented scientific potential.
Neutron Star Radii and Masses
Neutron stars offer the unique possibility of probing the equation of state of cold, ultradense matter. Understanding the properties of the neutron star interior is also important for predicting the observational appearance of short gamma-ray bursts, the end stages of neutron star coalescence, and the outcomes of supernova explosions. I will present the recent measurements of neutron star radii and masses. I will show how the combination of the tightly constrained radii and the discovery of a 2 solar mass pulsar allows for the first astrophysical inference of the pressure of cold matter above nuclear saturation density. I will discuss the implications of this measurement for nuclear theory and astrophysics.
Towards ab initio simulations of star formation in turbulent molecular clouds
Understanding how molecular clouds form in the interstellar medium and how they evolve to produce dense cores that eventually give birth to stars is an important unsolved problem in star formation. From a theoretical perspective, star formation is a challenging multi-scale problem that involves complex nonlinear interactions of gravity, turbulence, magnetic fields, radiation and feedback processes. I shall describe a self-consistent computational framework for modeling molecular cloud formation in the multiphase ISM as a first step to generate realistic initial conditions for star formation. Our approach is based on self-organization in the turbulent ISM and, with only a few control parameters, numerical experiments successfully reproduce the observed probability distributions of the molecular gas density, thermal pressure, magnetic field strength, as well as the core mass function. I shall briefly discuss the origin of Larson's scaling relations that naturally emerge in the model.
Gamma-ray detections of cosmic-ray acceleration by supernova remnants
Supernova remnants have been considered prime suspects as accelerators of cosmic rays within our Galaxy for a long time. Gamma-ray observations have offered the promise of turning that suspicion into certainty for almost as long, but only recently high-energy gamma-ray telescopes, both the Fermi Large Area Telescope and AGILE, have provided data that can be used to look for direct evidence of protons in these sources. I will present the new results from Fermi that reveal the proton signature in two supernova remnants and put this in context with the growing catalog of GeV remnants detected by LAT and the remaining questions to be answered about Galactic cosmic rays.
The Road to 100 Earths
The search for planets orbiting nearby stars has been one of the greatest success stories of the past decade, with hundreds of discoveries being made using Doppler, transit, microlensing, and direct imaging techniques. More than 2300 candidates have been detected with NASA's Kepler mission. Exoplanet detections have launched a subfield of astronomy that includes host star characterizations, measurements of planet radii and density, studies of atmospheres, interior structure, formation theory, and orbital evolution. The search for exoplanets is motivated by the question of whether life exists elsewhere. This drives our interest in the detection of planets that are similar to our own world: rocky planets with the potential for liquid surface water and plate tectonics; worlds that might harbor life that we can recognize. Importantly, we will need to discover not just a few, but hundreds of these worlds to eventually gain a statistical understanding of whether life is rare, common, or ubiquitous and ground-based telescopes offer an ideal platform for carrying out decade-long surveys. It is critical for follow-up studies (imaging, atmospheric studies) that these planets orbit nearby stars rather than stars at distances of the typical Kepler field star. In this talk, I will discuss how we plan to take what we've learned and push on to the next frontier: a search 100 Earths.
High redshift starburst galaxies revealed by SPT, ALMA, and gravitational lensing
The South Pole Telescope (SPT) has systematically identified a large number of high-redshift strongly gravitationally lensed starburst galaxies in a 2500 square degree cosmological survey of the millimeter (mm) sky. These sources are selected by their extreme mm flux, which is largely independent of redshift and lensing configuration. The flux magnification provided by the gravitational lensing enabled us to perform a spectroscopic redshift survey with the recently commissioned Atacama Large Millimeter Array (ALMA). We targeted 26 SPT sources and obtained redshifts via molecular carbon monoxide (CO) lines. We determine that roughly 40% of these sources lie at z>4, indicating the fraction of dusty starburst galaxies at high-redshift is far higher than previously thought. Two sources are at z=5.7, placing them among the highest redshift starbursts known, and demonstrating that large reservoirs of molecular gas and dust can be present in massive galaxies near the end of the epoch of cosmic reionization. These sources were additionally targeted with high resolution imaging with ALMA, unambiguously demonstrating them to be strongly gravitationally lensed by foreground structure. We are undertaking a comprehensive and systematic followup campaign to use these "cosmic magnifying glasses" to study the infrared background in unprecedented detail, inform the condition of the interstellar medium in starburst galaxies at high redshift, and place limits on dark matter substructure. I will discuss the scientific context and potential for these strongly lensed starburst galaxies, give an overview of our team's extensive followup efforts, and describe our latest science results.
Infalling groups and galaxy evolution in the IMACS Cluster Building Survey
From a photometric/spectroscopic study of galaxy clusters at z~0.4, using the wide field of the IMACS instrument on the Magellan-Baade telescope, we have studied infalling galaxies that were building typical rich clusters 4 billion years ago, and compared them to field galaxies at that epoch over the full range of galaxy environment. The results emphasize the important role of infalling groups of galaxies in building a cluster, and provide evidence that significant quenching of starforming galaxies occurs in such groups --- in both the cluster and field environment. Our study also suggests a new interpretation of the role that starbursts play in galaxy evolution.
Status and Future of the APOGEE Project
The Apache Point Observatory Galactic Evolution Experiment (APOGEE) in Sloan Digital Sky Survey III (SDSS-III) is a large-scale, near-infrared, high-resolution (R ≥ 22,500) spectroscopic survey of the Milky Way (MW) using a 300-fiber, cryogenic spectrograph operating over 1.51-1.70 µm (i.e., a large fraction of the H-band). Because of the lower H band dust extinction compared to that at optical wavelengths (AH / AV = 0.16), APOGEE effectively pierces through dust obscuration and will provide a vast, uniform database of chemical abundances and radial velocities for stars across all Galactic populations (bulge, thin and thick disks, halo). APOGEE started observations in May 2011 and in 3 years of SDSS-III bright time intends to observe of order 100,000 giant star candidates selected from the Two Micron All-Sky Survey (2MASS) across hundreds of sight lines with field limits ranging over H=11-13.5. With its high resolution and S/N (>100 per Nyquist-limit-sized pixel), APOGEE will determine precision radial velocities (presently at 100 m/s absolute accuracy and 30 m/s relative precision) and accurate abundances for numerous chemical species, including C, N, O and Fe, as well as other α, odd-Z, and iron-peak elements for its primary targets. About 5% of the APOGEE targeting is dedicated to a number of ancillary science programs that are already yielding interesting results. To date the APOGEE survey has collected more than 350,000 spectra of over 65,000 distinct stars. I will give an overview of APOGEE and some of its initial findings, and lay out our plans to expand the APOGEE survey for another 6 years and in both the Northern and Southern Hemispheres in SDSS-IV.
Making Hay with ALFALFA
The recently completed ALFALFA blind HI survey has produced an unprecedented catalogue of 21-cm detected objects in the local universe out to z = 0.06. With a sensitivity more than an order of magnitude better than previous wide-field blind HI surveys, ALFALFA represents the current state-of-the-art in neutral hydrogen searches. In this talk I will give a brief overview of the ALFALFA survey, then will describe two optical follow-up projects led by our group at Indiana. The first is a large narrow-band H-alpha imaging study of a volume-limited sample of ALFALFA sources. The second is a deep imaging study of nearby HI clouds that have the characteristics of being low-mass dark matter halos. One of the objects uncovered in this latter study is a nearby dwarf galaxy with fairly unique properties that we dubbed Leo P.
Feedback from AGN In Massive Galaxies: The Importance of Momentum Driving
Black holes, resident in the centers of galaxies, will be fed by accretion of ambient gas whenever gas reaches those central regions. This can be due to mergers, but even without mergers the evolution of the stellar populations of normal galaxies provides very large amounts of gas, as stars pass through the planetary nebula stage, the total mass release being greater than 1011 Msolar for massive ellipticals. Much of that gas will cool and fall to the centers of the systems, where it will induce starbursts and accretion events onto the central black holes with resultant AGN outbursts. The mass, momentum and energy in these outbursts can have dramatic consequences for the growth of the BH and for the ambient galaxy. Most AGN feedback treatments do not include the mass and momentum components. We follow these events with 1D, 2D and 3D hydrodynamic codes. BH growth is similar to what has been found by others, but the momentum driving produces much more energetic winds than does thermal feedback reducing star formation and thermal X-ray emission. Observable consequences include the narrow line AGN absorption lines, shock accelerated synchrotron emitting particles and wind driven bubbles in the IGM. In addition, we find that the feedback strongly inhibits inflow, causing episodic accretion and a low “duty cycle”. The simulations help us to understand many phenomena including the black hole stellar mass relation, “quenching” of the mass growth, the X-Ray luminosity of ellipticals, the incidence of the “E+A” phenomena and the observed fact that most of the black holes found in galactic centers are found in the “off” state.
Star formation regulated by magnetic reconnection
Recent years have been marked by a notable change in the star formation paradigm. Instead of quasi-static molecular clouds slowly evolving under the influence of gravity and ambipolar diffusion a new picture with more action and dynamics emerged. Within new understanding of interstellar processes, the molecular clouds are associated with turbulent density fluctuations and the structure of the interstellar medium evolves fast on the sound crossing times. I appeal to the advances in understanding of magnetic reconnection in turbulent medium and demonstrate that fast reconnection can cause of the efficient magnetic field diffusion that does not depend on the degree of media ionization. I shall show that a process that I term "reconnection diffusion" can be responsible for efficient removing magnetic flux during star formation. For the giant molecular clouds (GMCs) and for many cloud cores the resulting rates of magnetic field removal dominate the ambipolar diffusion rates in partially ionized gas. I shall show that numerical simulations validate the concept of "reconnection diffusion" and that this process can successfully explain the existing observational data on magnetic field -- density correlations in diffuse media, removal of magnetic fields from clouds and accretion disks on the time scale less than the ambipolar diffusion as well as the recent results by Crutcher et al. on the magnetization of cores and envelops. I shall discuss the implications of the reconnection diffusion process for the theory of star formation.
The Galactic Center: Unveiling the Heart of our Galaxy
The proximity of the center of our Galaxy has presented us with 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. This advantage, along with the recent advances in high angular resolution imaging technologies, has allowed the first observations of individual stars at the very heart of a galaxy. After more than a decade, such observations have transformed the case for a supermassive black hole at the Galactic center from a possibility to a certainty, thanks to measurements of individual stellar orbits. The rapidity with which these stars move on small-scale orbits indicates that 4 million times the mass of the sun resides within a region comparable to the size of our solar system and provides the best evidence yet that supermassive black holes, which confront and challenge our knowledge of fundamental physics, do exist in the Universe. Subsequent high-resolution imaging studies of the Galactic center have shown that the stellar population near our Galaxy's supermassive back hole is quite different from the predications of theoretical models for the interaction between central black holes and their environs (an essential input into models for the growth of nuclear black holes). In particularly, the observations have revealed an abundance of young stars in a region that is inhospitable to star formation and, conversely, a dearth of old stars where as a stellar cusp is expected. Further improvements in measurement precision should enable tests of Einstein's theory of General Relativity in the extreme environment near a supermassive black hole.
One of the main avenues for understanding the formation and evolution of galaxies is through studying their present day stellar populations. A new generation of population synthesis tools that we have been developing are now capable of extracting an unprecedented amount of information from high quality spectra of galaxies. In this talk I will present results from an ongoing program aimed at measuring the stellar initial mass function and detailed elemental abundance patterns of early-type galaxies over the interval 0 < z < 1. Current data suggest that the IMF varies systematically across the galaxy population, with implications both for star formation theory and the inferred dark matter content in the central regions of galaxies. Constraints on the abundances of the alpha elements, iron peak elements, and neutron capture elements offer the promise of reconstructing the detailed star formation histories of these now dormant galaxies. Measuring the evolution of these quantities through cosmic time will provide new constraints on the assembly histories of galaxies and will open up a new era of 'extragalactic chemical tagging'.
Origins of Gas Giant Planets
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 by either leading theory--core accretion or gravitational instability--presents substantial difficulties. These challenges may be reinterpreted as opportunities. In this talk, I will demonstrate how to use upcoming constraints from direct imaging to distinguish between theories of giant planet formation. Along the way, I will discuss whether gravitational instability could have formed the iconic directly-imaged planetary system HR 8799, present a new theory of planetary core growth in the presence of gas that extends the reach of core accretion to large stellocentric distances, and show how the atmospheric compositions of giant planets record signatures of their formation locations.
Cosmological simulations of the formation of galaxies
Cosmological hydro simulations can give unique insight into the formation and evolution of galaxies and their interplay with the intergalactic medium. They have realistic initial conditions, they can provide representative samples of galaxies that span a wide range of mass and environment, they can follow their evolution over time, and they can simultaneously model both the galaxies the intergalactic medium around them. The drawback of cosmological simulations is, however, their limited resolution and the importance of subgrid models. I will give an overview of the ingredients of the simulations and discuss recent developments, current issues and bottlenecks. I will then use the simulations to highlight the key role that self-regulation plays in the evolution of galaxies. Finally, I will demonstrate that the feedback processes that appear to be needed to match the observations have a dramatic impact on observational cosmology.
Large-Scale Structure in the intergalactic medium and the formation of galaxies: What we are learning from BOSS in SDSS-III
The Baryon acoustic OScillations Survey in SDSS-III is obtaining more than one hundred thousand absorption spectra of quasars at redshift z>2. This opens a new window for studies of large-scale structure at high redshift, which has already resulted in the first detection of the Baryon Acoustic Oscillation peak at these redshifts, using absorption of intergalactic gas. Cross-correlations of the Lyman alpha absorption with other objects, among them Damped Lyman Alpha absorption systems as tracers of high-redshift galaxies, reveals their spatial distribution and provides new clues on how galaxies were forming at this epoch, when the universe was about 3 billion years old.
Image: Contours of the redshift-space cross-correlation of damped Lyman alpha systems with the Lyman alpha forest absorption, comparing the observation (left) with the linear theory prediction (right).