Astrophysics Seminar and Colloquium Abstracts
Fall 2016

September 7 -- Colloquium

Andrew Baker (Rutgers)

Not-So-Dense Gas in Not-So-Distant Galaxies

To understand how galaxies evolve across cosmic time, we must understand not only their dark matter and their stars, but also the properties of their interstellar gas, from which new stars form and into which old stars release the products of their nucleosynthesis. In normal galaxies like the Milky Way, a majority of this gas lies at temperatures that are neither hot enough to ionize hydrogen atoms nor cold enough to permit the formation of hydrogen molecules. Such material is best traced at radio wavelengths using the 21cm spin-flip transition of neutral atomic hydrogen (HI). I will describe what we can learn from observations of HI emission in galaxies, and how we will use a powerful new array of 64 radio telescopes in South Africa to observe a single position on the sky for thousands of hours, thereby probing the evolution of galaxies' HI reservoirs over the last nine billion years. This effort, constituting the LADUMA (Looking At the Distant Universe with the MeerKAT Array) survey, will benefit from close connections with optical-wavelength observations and theoretical modelling.

September 8 -- Colloquium

Eric Gawiser (Rutgers)

Probing Dark Energy and Dark Matter with High-Redshift Galaxies in the Era of Big Data

The spatial clustering of distant galaxies is a powerful probe of both the nature of those galaxies and of fundamental physics.  I will summarize recent measurements of the physical properties of high-redshift galaxies made at Rutgers.  Understanding these galaxies allows us to mitigate systematic errors as we use them to study cosmology.   The HETDEX experiment is using our Bayesian classification method to discover 700,000 high-redshift Lyman alpha-emitting galaxies and will use their clustering to determine the dark energy equation-of-state and to measure the spatial curvature of the Universe.  As the leading example of "Big Data" in astronomy, the forthcoming Large Synoptic Survey Telescope (LSST) will produce over 50 petabytes of images.  These images will reveal billions of distant galaxies whose clustering we will use to determine the evolution of the dark energy equation-of-state, to seek evidence for modifications to General Relativity, and to measure the masses of cosmological neutrinos.  I will present a Rutgers study of improvements to the LSST experimental design that will reduce systematic errors in these measurements.

September 15 -- Juliana Kwan (U. Penn)

Galaxy-galaxy lensing for cosmology in the Dark Energy Survey

The Dark Energy Survey (DES) is an ongoing photometric survey that will cover 5000 sq deg of the Southern sky over five years with the aim of determining the origin of cosmic acceleration. Two of the probes involved in achieving this goal are the large scale clustering of galaxies and weak gravitational lensing, which are more powerful when taken in combination, since a number of parameter degeneracies can be broken by combining probes. Using measurements of large scale clustering and galaxy-galaxy lensing of red galaxies identified in the Science Verification area, we are able to provide some of the first cosmological constraints from the Dark Energy Survey.

September 29 -- Boris Leistedt (NYU)

Cosmology with large imaging surveys: from precision to accuracy

I will give an overview of the formidable science that can be achieved with modern photometric galaxy surveys, and also discuss the challenges that must be overcome in order to fully exploit these large data sets. I will present some of the latest results from the Dark Energy Survey (DES), and highlight the great opportunities that will come with the LSST. Finally, I will showcase some of the most advanced statistical methods used to analyze these surveys. I will present a new approach for robustly estimating photometric redshifts and their probability distributions even in the presence of limited, highly heterogeneous spectroscopic training data.

October 6 -- Mary Putman (Columbia)

Gas Flows on Various Scales

Gas flows into dark matter halos of all mass scales.  In this talk I will discuss the flows relative to the largest and smallest mass scales.  This is the flow of the intergalactic medium to galaxy clusters traced by Lyman-alpha absorbers and the flow of gas into a dwarf spiral galaxy at its disk-halo interface.  The latter represents one of the few direct detections of inflow.

October 13 -- Smita Mathur (OSU)

Missing baryons and missing metals in galaxies: Clues from our Milky Way

It is well-known that most galaxies are missing most of their baryonic mass.  Perhaps more surprisingly, they also seem to be missing most of their metals.  I will present Chandra observations probing our Milky Way halo in absorption. Together with XMM and Suzaku data on emission, our  results show that the Milky Way halo contains a huge reservoir of warm-hot gas that may account for a large fraction of missing baryons and metals. I'll review current status of this field, discuss implications of our results to models of galaxy formation and evolution and outline paths for future progress.

October 20 -- Shy Genel (CCA)

Lagrangian analysis of cosmological simulations

Galaxies form hierarchically and at any given cosmic epoch z>0, the material making up z=0 galaxies may be spread over many Megaparsecs and in numerous progenitor galaxies. Cosmological simulations provide us with the ability to follow the time evolution of the individual 'mass elements' that make up galaxies, referred to as Lagrangian analysis. I will discuss two applications of such an approach, one focusing on the evolution in the gas phase with results on the angular momentum content of galaxies, and one focusing on the evolution in the stellar phase with implications for our understanding of stellar IMF variations.

October 27 -- Steve Boada (Rutgers)

Measuring the scatter in the cluster optical richness-mass relation with machine learning

I will discuss the work that I did for my PhD thesis while at Texas A&M. I studied the applicability of the upcoming Hobby Eberly Dark Energy Experiment (HETDEX) to calibrating the optical richness-mass relationship of galaxy clusters. The study was conducted in two parts: Firstly, I used a set of detailed mock observations to estimate the number of clusters observed and their associated parameters. Then I used a traditional scaling relation and a machine learning approach to estimate each cluster's total mass. Secondly, I used a set of real observations to apply the methods developed previously to ten clusters selected from the SDSS. I will discuss the results of this work and look forward to HETDEX.

November 3 -- Marla Geha (Yale)

The Satellite Populations of Milky Way-like Galaxies

The properties of the Milky Way's satellite galaxies provide critical clues to how galaxies form and test the nature of dark matter. However, the number of Milky Way satellites and their properties do not fully agree with well-established cosmological models. The SAGA (Satellites Around Galactic Analogs) Project is a long-term program to find satellites around 100 Milky Way analogs. Our survey has so far determined complete luminosity functions down to M_r = -12 for eight Milky Way analogs. We find a wide range of satellite number and properties. I will discuss how these results potentially change the physical interpretation of measurements based only on the Milky Way's satellites.

Thursday Nov. 10 -- Special Seminar
Karl Gebhardt
Rutgers 250 Fellow

From Black Holes to Dark Energy, with a journey from New Jersey to Texas

The nature of dark energy and the roles of black holes and dark matter have been and will continue to be major outstanding issues for understanding our universe. I will summarize initial results and prospect for three major observing campaigns regarding each of these issues. HETDEX is our large dark energy program that is recently starting taking data. The main focus is to measure any evolution of dark energy. It is a 4-year observing campaign. I will present the initial analysis and demonstrate we are on our way to making the most precise measure of the universe expansion rate.

December 1 -- Francis-Yan Cyr-Racine

ETHOS - From dark particle physics to the matter distribution of the Universe and beyond

The fundamental properties of dark matter can affect structure formation in our Universe in subtle ways. Unfortunately, many dark matter theories make similar predictions for how structure assembles on large and small scales, leading to a large degeneracy in model space. Motivated by this, we formulate an effective theory of structure formation (ETHOS) that enables cosmological structure formation to be computed in a vast array of microphysical model of dark matter physics. This framework maps the detailed microphysical theories of particle dark matter interactions into the physical effective parameters that shape the linear matter power spectrum and the self-interaction transfer cross section of non-relativistic dark matter. These are the input to structure formation simulations, which follow the evolution of the cosmological and galactic dark matter distributions.  These effective parameters in ETHOS allow the classification of dark matter theories according to their structure formation properties rather than their intrinsic particle properties, paving the way for future simulations to span the space of viable dark matter physics relevant for structure formation. We discuss how this framework can help understanding how dark matter microphysics affects structure formation on small scales.

December 7 -- Jo Dunkley

December 8 -- Michael Tremmel

How to Quench a Galaxy

Galaxy "Genetic Modification" (GM) is a breakthrough method that provides the ability to conduct controlled experiments with fully cosmological simulations (Roth+ 2016). This approach is implemented, along with a new sub-grid model for Supermassive Black Hole (SMBH) physics that I have developed, into a series of controlled experiments on a halo with virial of 10^12 Msun at z = 2. I show that the interaction between SMBH feedback and merger history determine whether and for how long star formation is quenched at high redshift. I further test these results in the Romulus Simulations, a new set of  large-scale cosmological simulations run at the same resolution and with the same physics as the GM runs, showing how merger history informs the evolving population of quenched galaxies. I also predict the the observable properties of `rejuvenated' galaxies, that quench for a time and then come back onto the main sequence.