Colloquia Archives

Grant Tremblay, Yale University
Thursday, November 10, 2016
3:45 p.m.
LGRT 1033
Title:
A Galaxy-Scale Fountain of Cold Molecular Gas Pumped by a Black Hole
Abstract:
A new ALMA observation of the cool core brightest cluster galaxy in Abell 2597 reveals that a supermassive black hole can act much like a mechanical pump in a water fountain, driving a convective flow of molecular gas that drains into the black hole accretion reservoir, only to be pushed outward again in a jet-driven outflow that then rains back toward the galaxy center from which it came. The ALMA data reveal "shadows" cast by giant molecular clouds falling on ballistic trajectories towards the black hole in the innermost hundred parsecs of the galaxy, manifesting as deep redshifted continuum absorption features. The black hole accretion reservoir, fueled by these infalling cold clouds, powers an AGN that drives a jet-driven molecular outflow in the form of a 10 kpc-scale, billion solar mass expanding molecular bubble. HST reveals that the molecular shell is permeated with young stars, perhaps triggered in situ by the jet. Buoyant X-ray cavities excavated by the propagating radio source may further uplift the molecular filaments, which are observed to fall inward toward the center of the galaxy from which they came, presumably keeping the fountain long-lived. I will discuss this specific result in the larger context of galaxies as a whole, as the results show that cold molecular gas can couple to black hole growth via both feedback and feeding, in alignment with "cold chaotic accretion" models for the regulation of star formation in galaxies.
Dale Kocevski, Colby
Thursday, October 27, 2016
3:45 p.m.
LGRT 1033
Title:
Illuminating the Black Hole – Galaxy Connection with CANDELS
Abstract:
Supermassive black holes, and the active galactic nuclei (AGN) that they power, are thought to play an integral role in the evolution of galaxies by acting to regulate, and eventually suppress, the star formation activity of their host galaxies. I will discuss recent efforts to test this proposed connection by studying the demographics of galaxies undergoing active black hole growth. In particular, I will highlight recent results from the CANDELS survey, whose panchromatic Hubble ACS and WFC3 imaging is now allowing us to characterize the morphologies and stellar populations of thousands of AGN hosts out to z=2, the era when star formation activity and black hole growth in the Universe are at their peak. I will discuss what CANDELS is currently revealing about the mechanisms that fuel AGN activity at this epoch and the connection between black hole growth and the emergence of the first generation of passive galaxies in the Universe.
Arif Babul, University of Victoria
Thursday, October 20, 2016
3:45 p.m.
LGRT 1033
Title:
Cosmology with Galaxy Clusters
Abstract:
Clusters of galaxies are the largest, most massive gravitationally bound objects in the Universe. They are also the most recent of the cosmic objects to form. In the currently accepted models of cosmic structure formation, the evolution of the number density of these galaxy clusters over the recent (cosmologically speaking) past depends sensitively on the parameters describing the large-scale geometry and the expansion history of the universe For this reason, galaxy clusters are regarded as important cosmological probes. The first step in using clusters as cosmological probes involves involves starting with a large, well-defined sample of galaxy clusters and "weigh them". Here, I will discuss what "weighing them" entails and highlight some of the challenges involved. I will then summarize the outcome of the cosmological analysis, and compare the resulting values of the key cosmological parameters to those derived from the studies of the cosmic microwave background. Interestingly, the two do not agree with each other! And, this troubling tension remains unresolved. I will conclude by speculating about the implications of, and potential resolutions, of this tension.
Jason Glenn, University of Colorado
Thursday, October 13, 2016
3:45 p.m.
LGRT 1033
Title:
A Fresh Look at Molecular Gas in Galaxies
Abstract:
Carbon monoxide (CO) has been the primary tracer of cold molecular gas mass and dynamics in galaxies because H_2, the dominant species, lacks a permanent dipole moment. Prior to the Herschel Space Observatory, CO observations were limited predominantly to the lowest-lying rotational transitions in the millimeter-wave part of the spectrum: J = 1 — 0 and J = 2 — 1. Spectrometers aboard Herschel opened up observations to high-level submillimeter and far-infrared CO rotational transitions, leading to the discovery of copious warm molecular gas in galaxies. Indeed, at temperatures of hundreds of Kelvin and comprising 10% of the molecular gas in star-forming galaxies, this warm gas has 100 times the luminosity-to-mass ratio of the gas probed with the lowest-lying transitions. It appears that this warm component of molecular gas is mechanically heated (likely in shocks), and it has implications for stellar feedback into the interstellar medium in galaxies. I will discuss the results of our census of this molecular gas in 87 galaxies in the Herschel Science Archive, I will mention implications for ALMA and other ground-based observations of high-redshift galaxies, and I will describe new superconducting spectrometer technology we are developing to enable millimeter-wave multi-object spectroscopy of galaxies with CO and high-redshift [CII].
Eilat Glikman, Middlebury College
Thursday, October 6, 2016
3:45 p.m.
LGRT 1033
Title:
Dust-Reddened Quasars as Probes of Feedback and Galaxy Evolution
Abstract:
The study of the evolution of quasars and their influence on their host galaxies provides unique insight into how supermassive black holes became a ubiquitous feature of galaxies today. Dust-reddened quasars appear to represent a transitional phase in the merger-driven models of quasar/galaxy co-evolution. I will present results from several surveys to identify these transitional systems using infrared and radio selection. The results of these surveys reveal that red quasars are among the most intrinsically-luminous quasars in the universe, and make up ~15%-20% of the overall quasar population. They reside in actively merging galaxies, and their spectra exhibit outflows in absorption and emission. I will also present results of a Hubble Space Telescope imaging campaign of eleven red quasar host galaxies at z ~ 2, which show evidence for a recent merger in their host galaxies. I will discuss how reddened quasars fit into the larger picture of AGN evolution which includes both mergers and secular growth.
Michael McDonald, MIT
Thursday, September 29, 2016
3:45 p.m.
LGRT 1033
Title:
Galaxy Cluster Evolution over the Past 10 Billion Years
Abstract:
In recent years, the number of known galaxy clusters has grown dramatically, thanks in large part to the success of surveys utilizing the Sunyaev Zel'dovich effect. In particular, surveys like the South Pole Telescope 2500 deg^2 survey have discovered hundreds of distant clusters, allowing us to trace for the first time the evolution of clusters from shortly after their collapse (z~2) to present day (z~0). In this talk, I will highlight recent efforts to understand the observed evolution in the most massive clusters, in terms of the large-scale hot intracluster gas, the cooling gas in the very center of the cluster, the most massive central galaxy, and the supermassive black hole at the very center. In addition, I will attempt summarize the current state of galaxy cluster surveys and briefly discuss the potential of next-generation surveys.
John Johnson, CfA
Thursday, September 22, 2016
3:45 p.m.
LGRT 1033
Title:
MINERVA: Big Opportunities with Small Telescopes Searching for Small Planets
Abstract:
Just five years ago the prospect of finding temperate, rocky worlds around other stars was still the subject of science fiction: none had been found and reasonable estimates put us decades away from such a momentous discovery. All of that has changed very recently on the heels of the extraordinarily successful NASA Kepler mission, which has shown that rocky, potentially habitable planets are common throughout the Galaxy, and that the nearest inhabited planet is likely in the Solar Neighborhood. This search will require new, dedicated facilities capable of detecting the tiny signals of rocky planets around bright stars. To this end, I will describe the MINiature Exoplanet Radial Velocity Array (MINERVA), a robotic, multi-telescope facility dedicated to searching for habitable worlds and understanding the nearest planetary systems.
Jon Trump, UConn
Thursday, September 15, 2016
3:45 p.m.
LGRT 1033
Title:
The Birth and Growth of Supermassive Black Holes: Coming of Age with Space Telescope Imaging Spectrographs
Abstract:
The past 20 years have revealed that supermassive black holes play an essential role in the formation and growth of galaxies. Every massive galaxy hosts a supermassive black hole in its center, and the black hole's mass is tightly coupled to the mass of the galaxy. Remarkably, the black hole - galaxy connection has been "self-maintained" from the adolescent universe (z~2) to the current epoch, in both Milky-Way progenitors and massive cluster galaxies, governed by coupled black hole accretion and galaxy star formation. Until recently the "chicken-or-egg" birth of galaxies and supermassive black holes has remained mysterious. I will show how imaging spectrograph surveys with the Hubble Space Telescope are revolutionizing our understanding of black hole formation, revealing a fossil record of massive black hole seeds in tiny galaxies. Similar imaging spectrographs are flagship survey instruments on the upcoming JWST, WFIRST, and Euclid space telescopes, enabling an exciting future for understanding the birth of primordial galaxies and their black hole seeds.
Lorenzo Sorbo, UMass Amherst Physics Department
Thursday, September 8, 2016
3:45 p.m.
LGRT 1033
Title:
“Inflation, the polarized CMB, and the Universe in a mirror”
Abstract:
While the WMAP and Planck experiments have provided the ultimate measurement of the temperature fluctuations of the Cosmic Microwave Background for a large set of scales, an improved measurement of the polarization of the CMB is still the goal of several experiments. The polarization of the CMB contains precious information about the gravitational waves that should have been produced during primordial inflation, which occurred during the first 10ˆ(-30) or seconds of life of the Universe. In a large class of models, the inflaton - the agent responsible for inflation - is not invariant under parity, the mathematical operation equivalent to looking at the Universe in a mirror. After an introduction to inflation and the CMB, I will discuss how parity violation might be detectable in the polarization of the CMB. Then I will discuss other unusual signatures that might appear in the CMB in this class of models. Finally, I will mention how this class of models might even lead to gravitational waves that might directly detected by advanced LIGO in the next few years.
John Johnson, CfA
Monday, August 29, 2016
3:45 p.m.
LGRT 1033
Title:
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Abstract:
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Juna Kollmeier (Carnegie Observatory)
Thursday, December 10, 2015
3:45 p.m.
LGRT 1033
Title:
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Jo Bovy (University of Toronto)
Thursday, December 3, 2015
3:45 p.m.
LGRT 1033
Title:
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Nicolas Bouche (IRAP)
Thursday, November 19, 2015
3:45 p.m.
LGRT 1033
Title:
The 3D View of Galaxy Formation and Gas Flows
Abstract:
TBA
Daniela Calzetti (University of Massachusetts Amherst)
Thursday, November 12, 2015
3:45 p.m.
LGRT 1033
Title:
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Abstract:
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Daniela Calzetti
Thursday, November 12, 2015
3:45 p.m.
LGRT 1033
Title:
Star Formation Across Space
Abstract:
Fundamental questions remain unanswered in understanding galactic-scale star formation, despite many decades of investigation and progress. These include: how do stars cluster in galaxies, and how do these structures evolve in time? Do we actually have a `clustered' and `diffuse' mode of star formation? When structures remain bound (star clusters), how do their populations evolve? How are they related to the galactic-scale star formation? Is the stellar Initial Mass Function universal? How are popular star formation rate indicators affected by the recent star formation history of a galaxy? How are these effects impacting our understanding of the scaling laws of star formation with the gas reservoir? The answers to these questions inform our theories for the evolution of galaxies through cosmic times. Many of these questions are being addressed by recent projects that combine UV and high-angular resolution with the Hubble Space Telescope, and which I will describe together with the results they have obtained so far.
Cara Battersby (CfA)
Thursday, November 5, 2015
3:45 p.m.
LGRT 1033
Title:
The Milky Way Laboratory for Star Formation
Abstract:
Our home Galaxy, the Milky Way, is our closest laboratory for understanding physical processes throughout the Universe. Submillimeter observations of the cool, dense gas and dust in our Milky Way provide insights on universal processes including how stars form in both 'regular' and 'extreme' environments and how gas is organized on galactic scales. On a tour through our Milky Way Laboratory, I will discuss 1) how we can use long, skinny molecular clouds, potential "Bones of the Milky Way," to trace our Galaxy's spiral structure, 2) how large surveys of our Galaxy have revealed that star clusters continue to grow even as they are forming, and 3) how observing the Central Molecular Zone (the inner few hundred parsecs of our Galaxy) can help us learn more about the conversion of gas into stars during the peak of cosmic star formation (z~2).
Francesca Civano (Yale University)
Thursday, October 29, 2015
3:45 p.m.
LGRT 1033
Title:
X-rays from the COSMOS Survey
Abstract:
Observations indicate that supermassive black holes (SMBHs) ordinarily dwell at the centers of local galaxies. Scaling relations have been identified between SMBHs and many large scale properties of the host galaxies that point to a joint cosmic evolution. Is this evolution regulated by the galaxy or by the black hole? Is star-formation triggering black hole activity or vice versa? For more than 30 years, X-ray surveys have provided a unique and powerful tool to find and study accreting SMBHs in the distant Universe. In the past decade alone, dozens of surveys in the 0.5-10 keV band with XMM-Newton and Chandra have covered a wide range in area and X-ray flux, corresponding to a similarly wide range of volume covered in the Universe. The advent of the NuSTAR telescope, with its 3-80 keV energy response, allows us to now complement the "soft" surveys, providing the ability to characterize the whole population, including highly obscured sources. In this talk, I will focus on the results I obtained using the COSMOS field X-ray surveys. I will highlight the importance of X-ray studies to compile a (quasi-) complete and unbiased sample of active SMBHs, suitable to study the mutual relation between the supermassive black hole and its host galaxy across the Universe. I will briefly discuss about the future and the idea of achieving fainter X-ray fluxes over large areas on legacy fields to match with upcoming deep optical and near-infrared data (JWST, HyperSuprimeCam, WFIRST).
Jeyhan Kartaltepe (RIT)
Thursday, October 22, 2015
3:45 p.m.
LGRT 1033
Title:
TBA
Abstract:
TBA
Rosalba Perna (Stony Brook)
Thursday, October 15, 2015
3:45 p.m.
LGRT 1033
Title:
Highly magnetic neutron stars: bewildering astrophysical laborat
Abstract:
Anomalous X-ray Pulsars and Soft Gamma-Ray Repeaters (SGRs) are young neutron stars characterized by high X-ray quiescent luminosities, outbursts, and, in the case of SGRs, sporadic giant flares. They are believed to be magnetars, that is neutron stars powered by ultra-strong magnetic fields. However, the diversity of their behaviours, and, especially, the observation of magnetar-like bursts from 'low-field' neutron stars, has been a theoretical puzzle. In the first part of the talk, I will discuss results of long-term MHD simulations which, by following the evolution of magnetic stresses within the neutron star crust, have allowed to relate the observed magnetar phenomenology to the physical properties of the neutron stars, and in particular to their age and magnetic field strength and topology. The dichotomy of 'high-B' field pulsars versus magnetars is naturally explained, and occasional outbursts from old, low B-field neutron stars are predicted. In the second part of the talk, I will discuss how observations of highly magnetized neutron stars can be handy tools in the cosmological domain, and in particular as a way to set constraints on the hypothetical particle 'axion'.
Dan McIntosh (UMKC)
Thursday, October 8, 2015
3:45 p.m.
LGRT 1033
Title:
TBA
Abstract:
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Fred C. Adams (University of Michigan)
Thursday, October 1, 2015
3:45 p.m.
LGRT 1033
Title:
Effects of Clusters on their Constituent Solar Systems and Possi
Abstract:
Most stars -- and most solar systems -- form within groups and clusters. One objective of this work is to explore how these star forming environments affect the solar systems forming within them via three channels: dynamical interactions, elevated radiation fields, and increased particle fluxes. The discussion starts with the dynamical simulations, which are used to study how cluster evolution depends on system size and initial conditions. Multiple realizations of equivalent cases are used to build up a statistical description of these systems, e.g., distributions of closest approaches and radial locations. These results provide a framework from which to assess the effects of clusters on solar system formation. Distributions of radial positions are used in conjunction with UV luminosity distributions to estimate the radiation exposure of circumstellar disks. Photoevaporation models determine the efficacy of radiation in removing disk gas and compromising planet formation. The distributions of closest approaches are used in conjunction with scattering cross sections to determine probabilities for solar system disruption. Finally, we determine the distributions of radioactive nuclei that are provided to circumstellar disks, where they enhance ionization and heating. This work provides a quantitative determination of the effects of clusters on forming solar systems. In the second part of the talk, these results are used to place constraints on the possible birth environments for our own solar system.
Lorenzo Sironi (Harvard University)
Thursday, September 24, 2015
3:45 p.m.
LGRT 1033
Title:
The Journey of High-Energy Photons in Blazar Jets
Abstract:
We investigate the origin and the fate of high-energy photons in blazar jets, by means of analytical theory and first-principles particle-in-cell (PIC) kinetic simulations. In magnetically-dominated jets, magnetic reconnection is often invoked as a mechanism to transfer the jet magnetic energy to the emitting particles, thus powering the observed non-thermal emission. With 2D and 3D PIC simulations, we show that magnetic reconnection in blazar jets satisfies all the basic conditions for the emission: extended non-thermal particle distributions (with power-law slope between -2 and -1), efficient dissipation and rough equipartition between particles and magnetic field in the emitting region. TeV photons emitted by the highest energy electrons accelerated by reconnection will interact in the intergalactic medium (IGM) with the extragalactic background light, producing a dilute beam of ultra-relativistic pairs. It is a matter of recent debate whether the energy of the pair beam is lost due to inverse Compton scattering off the CMB -- resulting in ~10-100 GeV photons -- or heats the IGM via collective plasma instabilities. The astrophysical stakes are very high because of the large amount of energy and extensive cosmic volume involved in this process. We study the relaxation of blazar-induced beams in the IGM, by means of 2D and 3D PIC simulations. We find that at most 10% of the beam energy is deposited into the IGM plasma, so that at least 90% of the beam energy will be ultimately re-processed in the multi-GeV band.
Darby Dyar (Mt. Holyoke College)
Thursday, September 17, 2015
3:45 p.m.
LGRT 1033
Title:
Laser-Induced Breakdown Spectroscopy on Mars: The Long Road towa
Abstract:
Laser-induced breakdown spectroscopy (LIBS) is being used almost daily on Mars by the ChemCam instrument on the Curiosity rover. The technique uses UV, VIS, and VNIR atomic emission spectra of surface rocks and soils to quantify elemental abundances. Because only 67 standards were run on the flight instrument before launch, calibration efforts are still ongoing. LIBS is different from other types of spectroscopy because atoms interact in the plasma, such that the ratio of intensity or area of an emission line to the abundance of the element producing that line is not a direct correlation. Thus LIBS quantitative analysis is complicated by chemical matrix effects related to abundances of neutral and ionized species in the plasma, collisional interactions within plasma, laser-to-sample coupling efficiency, and self-absorption. Atmospheric composition and pressure also influence plasma intensity. This talk will detail our group's contributions to calibrating Chemcam data, both through dramatic expansion of the laboratory calibration database and development of advanced machine learning tools for their analysis. We find that many previously-unappreciated steps in spectral processing, such as baseline/continuum removal, wavelength calibration, and normalization, have dramatic effects on prediction accuracy; many of these new insights have broader applications to other types of spectroscopy as well.
Anna Sajina (Tufts University)
Thursday, September 10, 2015
3:45 p.m.
LGRT 1033
Title:
Modelling dusty galaxies
Abstract:
I will present current work on comparing GADGET+Sunrise hydrodynamic simulations with observations of IR-luminous galaxies at z~0.3-3. I will especially focus on the relative roles of stars and AGN in heating the dust, the role of merger stage on the emergent SED, and the most likely initial gas fractions of the merger progenitors. In all cases, I will compare the results of the simulations with earlier, more direct, measurements. I will address the successes and limitations of the current generation of hydrodynamic simulations.
Jason Tumlinson (STScl)
Tuesday, September 8, 2015
3:45 p.m.
LGRT 1033
Title:
The Circumgalactic Medium: A New Window on Galactic Fueling, Que
Abstract:
Why do some galaxies quench while others continue to form stars? Where is all the normal matter that galaxies should have, but don't? What happens to all the heavy elements that stars produce? The gas flows that feed galaxies and return their enriched products back to their environments are arguably the most important and least understood processes driving galaxy evolution. I will survey our group's results from Hubble's Cosmic Origins Spectrograph on the diffuse "Circumgalactic Medium" surrounding galaxies that reveal it to be a massive and richly structured medium with important roles as the mediator of galaxy accretion and feedback and a potential answer to some of these open questions about galaxies.
Meredith Hughes (Wesleyan University)
Thursday, April 23, 2015
3:45 p.m.
LGRT 1033
Title:
Circumstellar disks and planet formation with ALMA
Abstract:
Circumstellar disks provide the raw material and initial conditions for planet formation. Millimeter-wavelength interferometry is a powerful tool for studying gas and dust in planet-forming regions, and it is undergoing an immense leap in sophistication with the advent of the ALMA interferometer. I will discuss some ways in which we are using millimeter-wavelength interferometry to study the process of planet formation in circumstellar disks, with particular emphasis on the kinematics of turbulence in protoplanetary disks and the surprising presence of gas in debris disks around main-sequence stars.
Tracy Webb (McGill University)
Thursday, April 16, 2015
3:45 p.m.
LGRT 1033
Title:
The Growth of the Most Massive Galaxies in the Highest Density R
Abstract:
The most massive galaxies in the local universe reside at the centres of galaxy clusters. Often called Brightest Cluster Galaxies (BCGs), they exhibit, as a class, highly uniform properties and are distinct from the general galaxy population. This suggests formation processes which are themselves distinct from those which dominate in massive galaxies outside of cluster cores. The mass growth of BCGs is likely linked to the overall physics of hierarchical structure formation on galaxy cluster scales, including the fundamental processes of gas cooling, star formation, energy feedback and galaxy mergers, at the centers of giant dark matter halos. In this talk I will present new results from the largest study of high-redshift BCGs conducted to date, drawn from the SpARCS optical/NIR cluster survey. Using archival infrared data we show the star formation rate within BCGs increases to z~2, and can add as much mass to the BCG population as the previous standard model of growth by dry mergers. At low redshifts, and in X-ray/SZ selected clusters, the rare examples of star forming BCGs appear to be fed by large-scale cooling flows. However, the first of the SpARCS systems we have studied in detail, SpARCS1049, has revealed a very different phenomenon - a train-wreck of a galaxy merger at the center of the cluster. This is the first example of such a process in high-redshift cluster cores and may represent a new phase of BCG evolution, previously unaccounted for.
Pieter van Dokkum (Yale University)
Thursday, April 9, 2015
3:45 p.m.
LGRT 1033
Title:
A history of the dense cores of massive galaxies over the past
Abstract:
Owing to several large surveys with new instruments on HST we are now able to measure the basic properties of galaxies over most of cosmic history. This talk will highlight one such survey, 3D-HST, which has obtained spectra and images of many thousands of faint galaxies. I will also discuss the next step, that is, how to reconstruct the histories of different kinds of galaxies from this wealth of data. This reconstruction has now been done for galaxies like the Milky Way, enabling us to look at "baby pictures" of galaxies like our own.
Katja Poppenhaeger (CfA)
Thursday, April 2, 2015
3:45 p.m.
LGRT 1033
Title:
A high-energy view of exoplanets and their host stars
Abstract:
Many exoplanets orbit their host stars at close distances, with orbital periods of only a few days. For such systems, the magnetic activity of the planet-hosting star determines the high-energy environment of its exoplanet and is an important factor in understanding the exoplanetary evolution. X-ray and UV observations allow us to determine the high-energy input into the exoplanetary atmosphere, which is the main driver of planetary evaporation. X-ray emission is also directly connected to the stellar rotation; without external input of angular momentum, the star spins down over time due to magnetic braking. However, if there is tidal interaction between a star and its Hot Jupiter, the spin-down of the host star may not follow the usual age/rotation/activity relations. I will discuss recent X-ray and UV observations which provide insights into both the evaporation of close-in exoplanets and a possibly altered age-activity relationship for host stars of such planets.
Rob Simcoe (MIT)
Thursday, March 26, 2015
3:45 p.m.
LGRT 1033
Title:
Heavy Element Enrichment in Early Intergalactic and Circumgalact
Abstract:
Infrared photometric surveys are discovering numerous quasars at z > 6.5, enabling absorption investigations of neutral Hydrogen and its associated heavy elements at the tail end of the reionization epoch. I will describe the status of my group's ongoing IR and optical spectroscopic surveys targeting metal pollution in the first few Gyr. Beginning with a systematic study of absorption candidates for "cold flows" at z~3, I will move on to describe a 100-sightline survey of circumgalactic MgII pollution with the Magellan/FIRE spectrometer, extending prior optical measurements (restricted to z<2) out to z~6.5. I will also describe our latest constraints on the CIV mass density and intergalactic carbon enrichment at z = 4.5-6.5. Finally, I will outline our first attempt at measuring actual chemical abundances in the z > 7 universe, and discuss their significance for reionization and the formation of the first stars.

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