Fall 2014






Yue Shen

The diversity of quasars unified by accretion and orientation

Quasars are rapidly accreting supermassive black holes at the centers of massive galaxies. They display a broad range of properties across all wavelengths, reflecting the diversity in the physical conditions of the regions close to the central engine. These properties, however, are not random, but form well-defined trends. The dominant trend is known as 'Eigenvector 1', in which many properties correlate with the strength of optical iron and [O III] emission. The main physical driver of Eigenvector 1 has long been suspected to be the quasar luminosity normalized by the mass of the hole (the 'Eddington ratio'), which is an important parameter of the black hole accretion process. But a definitive proof has been missing. Here we report an analysis of archival data that reveals that the Eddington ratio indeed drives Eigenvector 1. We also find that orientation plays a significant role in determining the observed kinematics of the gas in the broad-line region, implying a flattened, disk-like geometry for the fast-moving clouds close to the black hole. Our results show that most of the diversity of quasar phenomenology can be unified using two simple quantities: Eddington ratio and orientation.

Adrian Liu

HERA—The Next Step in Hydrogen Cosmology

Despite their importance in the evolution of cosmic structure, the Epoch of Reionization (EoR) and the preceding Dark Ages remain poorly constrained.  The first luminous structures formed during the dark ages, and during the EoR, ionized the intergalactic medium (IGM) around them.  To date, there have only been indirect observations of this period.  In recent years, however, 21cm cosmology has emerged as a promising direct probe of reionization.  By measuring fluctuations in the redshifted 21cm hyperfine transition of hydrogen, 21cm cosmology will provide a new window into the clustering, heating, and ionization properties of the IGM during the EoR.  In this talk, I will highlight some recent progress from current-generation 21cm instruments, such the Precision Array for Probing the Epoch of Reionization (PAPER), the MIT Epoch of Reionization experiment (MITEoR), and the Murchison Widefield Array (MWA).  Recent upper limits have begun to rule out "cold reionization" scenarios, and I will discuss the technical challenges that have been overcome to make these limits possible.  I will conclude by describing the next steps in 21cm cosmology, with special emphasis on the recently funded Hydrogen Epoch of Reionization Array (HERA) as an example of a next-generation instrument.

Aleks Diamond-Stanic

Extreme Outflows and the Gas Around Galaxies

Our understanding of galaxy evolution centers around questions of how gas gets into galaxies, how it participates in star formation and black hole growth, and how it is returned to its galactic surroundings via feedback.  On a global scale, measurements of the baryon density and the stellar mass function indicate that only 5% of baryons have formed stars by the present day, and this suggests that feedback from massive stars and supermassive black holes must prevent gas from forming stars in both low-mass and high-mass dark matter halos.  I will present observational results on ejective feedback that is capable of quenching star formation by removing the cold gas supply.  These results have broader implications for how gas is consumed and expelled at the centers of massive galaxies and for the limits of feedback from stellar radiation and supernovae.  I will also discuss prospects for characterizing the physical properties of gas in and around galaxies using multi-wavelength spectroscopy with existing and future facilities.

Astrid Lamberts

Impact of inhomogeneous blazar heating on the intergalactic medium

The intergalactic medium (IGM)  contains 90 % of the baryons of the Universe and is the reservoir for structure formation. Acting as a calorimeter, its thermal evolution traces the conditions for structure formation and evolution. It was recently shown that TeV blazar heat up the IGM as the gamma-rays they produce turn into pairs which lose their kinetic energy to the surrounding medium through plasma instabilities. Assuming uniform heating, TeV blazar heating increases the temperature of the IGM and produces an inverted temperature-density relation in underdense regions. In this talk I will review the main features of TeV blazar heating. Then, I will detail the method we recently developed to take into account heating fluctuations due to clustering.  We find that blazar heating is more complex than initially assumed. The resulting temperature-density relation presents a wide scatter, which is suggested by some recent Ly alpha observations.

Wladimir Lyra

Evolution of circumstellar disks and planet formation

During the first million years of evolution, nascent planetary
systems are embedded in dense disk­-shaped clouds of gas. These
circumstellar disks are home to a myriad of hydrodynamical processes,
which bring about turbulence and the emergence of viscous­-like behavior,
enabling accretion of gas onto the protostar. Meanwhile, micron­size dust
grains embedded in the disk are growing through coagulation onto pebbles
and rocks. Turbulence has a positive effect on these small solids,
concentrating them into transient high pressure regions for long enough to
achieve gravitational collapse into km­-sized bodies, forming the first
planetesimals. Giant storm systems in the disk, similar to Jupiter's Great
Red Spot, may exist in quiescent zones of the disk. These are even more
prone to collecting solid material, producing the first terrestrial
planets and cores of giant planets. In this talk I will discuss the state
of the art and recent advances in the field of planet formation, as well
as pressing problems such as the asymmetries observed in ALMA images of
circumstellar disks, and how to interpret them.

Karen Olsen

Predictions for the CO emission of main-sequence z~2 galaxies

The star-forming cold gas in galaxies can be probed through CO rotational line emission. More gas leads to higher CO line luminosity, while density and temperature work together to set the shape of the CO Spectral Line Energy Distribution (SLED). At z~2, when the cosmic star formation rate density peaks, CO emission has been observed in a range of starburst galaxies, but only in a handful of main-sequence galaxies. The details in of the molecular gas remain a mystery, which is unfortunate, as they habour the bulk of the cosmic star formation. I will present a new method, called SÍGAME, for simulating the CO emission of galaxies, and report on tests made on 3 main-sequence galaxies at z~2, with stellar masses and star formation rates (SFRs) in the range 0.5-2×1011 M⊙ and 40-140 M⊙/yr respectively. We find CO SLEDs that peak at CO(4-3) to CO(6-5) transitions, which is higher than observations of similar galaxies at z~0, but in agreement with other model predictions at z~2. Global αCO factors of our galaxies are Milky Way like, while CO(3-2) luminosities are low compared to z~2 observations but explainable due to low total and molecular gas masses in our model galaxies compared to observations. SÍGAME derives the molecular gas properties by applying physically well-motivated recipes and sub-grid procedures in post-processing of an SPH simulation. The initial hot SPH gas phase is cooled with atomic and ionic emission lines, as well as recombination and free-free electron-ion interactions, and the subsequent denser molecular gas is cooled via molecular emission lines and gas-dust interactions. In both cases, heating is caused by impinging far-UV and cosmic ray fields, that vary across the galaxy according to SFR density. The final CO spectra are derived with the radiative transfer code LIME, applied to a grid of suitable individual GMCs in order to determine the global spectrum by summing up across the galaxy. The prediction of warmer and denser gas in z~2 main-sequence galaxies, and thereby higher excitation of CO, is interesting for future observations. I will finish by looking into the future in which we hope to apply SÍGAME to more diverse galaxy types and to predict the intensities of other emission lines.




Nov 26

Day before Thanksgiving



Soma De

Cosmological Probes after Planck- CMB and neutral hydrogen in the low frequencies

I will discuss how the lower frequency regime of the EM spectrum can be rich with the cosmological information. The knowledge of this regime is important to learn as we propose to probe lower values of r and therefore inflation, primordial magnetic field from the CMB B-modes. Many effects such as Faraday rotation become important as we go lower in frequency, so as the sky foregrounds. I will show all sky models of polarized and unpolarized foregrounds, generalized for all frequency range, but focussing on lower frequencies. These models were generated with HAMMURABI simulations. Additionally, I will propose how to develop an instrument to detect circular polarization, in the context of detection of First stars, which can trigger a Stokes V (circular polarization) component in otherwise linearly polarized CMB. I will refine my previous calculation of this Stokes V by introducing a random direction of the magnetic field along a given line of sight. To summarize, I will present foregrounds (in both polarized and unpolarized values), methods of how to remove them and cosmological signal underneath, in the low frequency (a few hundred MHz-a few GHz) regime.

Jeffrey Fung

Disk-Planet Interaction: From 2D to 3D

The flow of disk material close to a planet, at separations of order r_H, the Hill radius, generates the largest transfer of angular momentum and mass between the planet and the disk. For large, Jupiter-size planets, they typically have r_H larger than h, the disk's scale height, hence the flow structure is well approximated in 2D. In this regime we study how the balance between planetary and viscous torque opens gaps in disks, and derive scaling relations for how empty these gaps are. Small, Earth-size planets, on the other hand, have r_H < h, and consequently one expects significant vertical variations in the flow. Our new, GPU-accelerated hydrodynamics code PEnGUIn allows for high-resolution 3D simulations of the global flow around the planet's orbit. Our results demonstrate that the 3D treatment for disk-planet interaction gives rise to new, and potentially dominant, aspects of both planetary migration and accretion.

Tuesday, Dec 16

Finals Week

Kevin Bundy

How do galaxies grow?

Understanding galaxy growth is not only fundamental but motivates many of the most important questions in galaxy formation, including how galaxies are tied to dark matter, how star formation is fueled, regulated, and quenched, and how mergers drive mass assembly. I will begin this talk with a puzzle: the most massive galaxies are predicted to grow significantly since z~1. Indeed, we observe a dramatic increase in their sizes, but strangely, we detect no growth in stellar mass. I will show how precise limits from new wide-field surveys combined with targeted observations of extreme progenitors suggests a possible resolution. To gain deeper insight on this question and all aspects of growth, we must resolve the internal structure and evolutionary history of galaxies. In the second half of this talk, I will present a new SDSS-IV survey designed to do exactly that. MaNGA (Mapping Nearby Galaxies at APO) began on July 1st and will obtain resolved spectroscopy for 10,000 galaxies, providing maps of internal composition and dynamics and resolving distinct subcomponents. MaNGA is already revealing exciting phenomena in the outskirts of galaxies that may be crucial to understanding their dominant modes of growth.

Dec 17

Finals Week

Alexie Leauthaud

Weak Lensing to the Limit

A precise understanding of the interplay between galaxies and their dark matter halos remains a key challenge. In contrast to the potential for detailed observations of the baryonic components of galaxies, their dark components are elusive and much more difficult to constrain. One of the most powerful methods to directly probe the dark side of galaxies is gravitational lensing. Traditionally, strong lensing is used to probe dark matter distributions on scales of a few kilo-parsecs while weak lensing is used on scales greater than a few hundred kilo-parsecs. Instead, in this talk, I will explore the possibility of probing the very inner regions of galaxy/halo density profiles by measuring stacked weak lensing on scales of only a few tens of kilo-parsecs. High signal-to-noise weak lensing measurements at these scales, in combination with stellar kinematics, offer the exciting possibility of directly measuring mass-to-light ratios, providing independent constraints on the Stellar Initial Mass Function (IMF), as well as on the inner slope of the dark matter density profile.