This page records the schedule, titles and abstracts of the JHU/STScI CAS Astrophysics Wine & Cheese Series in Fall 2015.

If not specified otherwise, the talks are a 25-min presentation plus a 5-min Q/A session.

Back to W&C Schedule

August 28th

Hans Böhringer

Testing Cosmological Models with X-ray Galaxy Clusters (full-hour)

We use a large, statistically very well characterised X-ray flux-limited sample of galaxy clusters to study the large-scale structure of the Universe out to redshifts of ~ 0.4. We use the cluster mass function to obtain tight constraints on the matter density and amplitude parameter of the density fluctuation power spectrum. We find some tension in the resulting amplitude parameters with the prediction from the PLANCK results in the frame of the standard Lambda-CDM cosmological model. The tension implies a less pronounced fluctuation amplitude of nearby large-scale structure as compared to the predictions based on Planck and a pure LCDM model. The results can be reconciled, however, by for example introducing massive neutrinos. We also use the cluster sample to study the matter distribution in the local Universe in a cosmographical fashion. One of the findings of this research is a locally underdense region in the Southern Galactic Cap region, with interesting consequences for local measurements of cosmological parameters, like the Hubble constant.

September 14th

Shadab Alam

Testing Gravity using Galaxy Redshift Surveys and CMB

The Redshift Space Distortions (RSD) in galaxy redshift surveys can probe the local dynamics at a given epoch of galaxy. I will discuss how redshift can help us learn the local dynamics and hence measure the nature of gravity at the epoch of the galaxy. I will show results from our recent analysis of SDSS-III high redshift sample (CMASS). I will then talk about combining similar RSD measurements from various other surveys to learn more about cosmology and modified gravity. I will end with a discussion on combining these measurements with CMB lensing in order to probe gravity to better precision and earlier time.

Laurent Pueyo

Discovery and spectroscopy of the young Jovian planet 51 Eri b with the Gemini Planet Imager

Directly detecting thermal emission from young extrasolar planets allows measurement of their atmospheric composition and luminosity, which is influenced by their formation mechanism. Using the Gemini Planet Imager, we discovered a planet orbiting the ~20 Myr-old star 51 Eridani at a projected separation of 13 astronomical units. Near-infrared observations show a spectrum with strong methane and water vapor absorption. Modeling of the spectra and photometry yields a luminosity of L/L⦿=1.6-4.0 × 10−6 and an effective temperature of 600-750 K. For this age and luminosity, “hot-start” formation models indicate a mass twice that of Jupiter. This planet also has a sufficiently low luminosity to be consistent with the “cold-start” core accretion process that may have formed Jupiter.

September 21st

Tim Brandt

Disrupted Globular Clusters as the Source of the Galactic Center GeV Excess

The Fermi satellite has recently detected excess gamma ray emission from the central regions of our Galaxy. This may be evidence for dark matter particles, a major component of the standard cosmological model, annihilating to produce high-energy photons. I will show that the observed signal may instead be generated by millisecond pulsars that formed in dense globular clusters in the Galactic halo. Most of these clusters were ultimately disrupted by evaporation and gravitational tides, contributing to a spherical bulge of stars and stellar remnants. The gamma ray amplitude, angular distribution, and spectral signatures of this source may be predicted without free parameters, and are in remarkable agreement with the observations. The gamma rays are then from the fossil remains of dispersed clusters, and constitute the first direct evidence for the former existence of a much larger globular cluster population.

Simeon Bird

Solving the DLA Velocity Width Problem

Matching the kinematics of strong neutral hydrogen absorbers, DLAs, has been a problem for simulations since the late 90's, and it has been suggested represents a problem for structure formation. I will explain how it was solved through a combination of modern galaxy formation models and attention to measurement details.

September 28th

Paul La Plante

Helium Reionization Simulations: Seeing the Lyman-alpha Forest for the Trees

Helium reionization is an important epoch in the Universe’s history, and the most recent large-scale transition of the intergalactic medium (IGM). Reionization is driven largely by quasars, and has important implications on the thermal history of the IGM. Due to the biased nature of sources and the large degree of photoheating, numerical simulations are ideally suited to investigating this problem. Recently we have run a new suite of large-scale cosmological simulations that solve N-body, hydrodynamics, and radiative transfer simultaneously in order to study the impact of helium reionization on the IGM. Specifically, we make predictions for the temperature density relation of the IGM and observables related to the Lyman-alpha forest. We show that aspects of reionization such as the timing and duration are visible in the helium II Lyman-alpha forest, and might be detectable in the hydrogen forest as well.

Mohammadtaher Safarzadeh

What shapes the far-infrared spectral energy distribution of galaxies

We study the Far-Infrared (FIR) Spectral Energy Distributions(SEDs) of a set of hydro-dynamically simulated galaxies that are post-processed with dusty-radiative transfer simulation to account for dust extinction of the UV light and re-emission of the absorbed light by dust in the FIR. We perform PCA analysis on the SEDs. We find that the first two PCs can explain 97% of the variance in the FIR SEDs with the first PC describing the peak of the FIR SED and the second PC describing its width. Both PCs are well predicted by IR luminosity and dust mass. Our results suggest that the observed redshift evolution in the effective dust temperature at fixed IR luminosity is not driven by geometry: the SEDs of z~2-3 ultra-luminous IR galaxies (ULIRGs) are cooler than those of local ULIRGs not because the high-redshift galaxies are more extended but rather because they have higher dust masses at fixed IR luminosity. Finally, based on our simulations, we introduce a two-parameter set of SED templates that depend on both IR luminosity and dust mass.

October 5th

Jorge Barrera

Mapping star formation and metallicity in CALIFA merging galaxies

Tidal induced flows have been thought to be the primary mechanism to enhanced the star formation rate (SFR) in interacting galaxies. Despite the large evidence of the global SFR enhancement in these galaxies, little is known on how is its spatial distribution. Thanks to the CALIFA survey, we are able to study the (specific) SFR and the ionized gas metallicity at different scales in 103 galaxies, covering different stages of interaction - from pairs to remnants. To quantify the impact of the interaction, we compare our results with a sample of 80 non-interacting galaxies (Barrera-Ballesteros et al. 2014 {link2}). Although enhancement of the stellar activity is observed in the central region of interacting objects, at extended regions, the SFR from both samples is similar. We also find similar central metallicities between the interacting and isolated galaxies. Our results suggest that even though central SFR and lower metallicities for interacting galaxies have been attributed to tidally induced inflows, other processes such as stellar feedback can contribute to the metal enrichment in interacting galaxies.

Additional paper

Salvatore Cielo

The physics of AGN jets from 3D simulations

Among the diffeent feedback mechanisms that AGN provide to their host galaxies and clusters, relativistic, collimated jets show a very rich physics. The interaction of such jets with the hot gas present in galaxy groups or clusters up to Megaparsec scales may explain several well-observed but complex features such as creations of bow-shocks fronts, complex sound waves structures, fast secondary winds, inflation of X-ray cavities (often seen in multiple pairs). Another important question concerns whether the jets may couple efficiently to the host (mainly because of the jets' high directionality), thus being energetically relevant for galaxy groups and clusters. We explore this physics by running a series of high-resolution 3D numerical simulations of the jet/hot gas interaction, including also multiple (non-coplanar) jet events. Such simulations offer good insight on the energetics of jet feedback and may capture the observed complex structures, as it is visible from animated flow maps and synthetic x-ray images produced from the simulation output.

October 12th

Moritz Münchmeyer

Oscillations in the CMB bispectrum

Oscillating signatures in the correlation functions of the primordial density perturbations are predicted by a variety of inflationary models. A theoretical mechanism that has attracted much attention in recent years is a periodic shift symmetry in the inflaton potential, which allows to protect large field models from quantum corrections, as implemented in axion monodromy inflation. This symmetry leads to so called resonance non-gaussianities, whose key feature are logarithmically stretched oscillations in the power spectrum and bispectrum. Oscillations are also a generic consequence of excited (non Bunch-Davies) states during inflation. A number of possible excited states have been proposed, leading to a variety of possible power spectrum and bispectrum shapes. As a third example, sharp features in the potential induce oscillations as the inflaton relaxes back to its attractor solution. Oscillating shapes are therefore a very interesting experimental target. After giving an overview of these theoretical motivations, I will discuss how to search for these signatures in the CMB data. Fast oscillations are very difficult to search for with traditional estimation techniques, and I will demonstrate how targeted expansions, that exploit the symmetry properties of the underlying shapes, allow to circumvent these difficulties. As a member of the Planck collaboration, I will discuss the Planck results that have been obtained using these methods in the bispectrum, as well as a joint search combining bispectrum and power spectrum. Due to their low overlap with well constrained non-gaussian shapes, as well as their low overlap among each other, oscillating bispectrum shapes are not exhaustively constrained and a potential discovery is therefore not yet ruled out. My talk will be based in particular on arxiv:1412.3461, arxiv:1505.05882 and Planck publications on inflation and non-gaussianities.

Johannes Sahlmann

Exploring the giant planet - brown dwarf connection with astrometry

Modern surveys of isolated brown dwarfs and of extrasolar planets around Sun-like stars reveal a continuous mass distribution across the deuterium-burning mass limit. This challenges the mass criterion sometimes used to distinguish between these objects and calls for methods that can trace the different formation paths instead. Astrometric measurements are a way forward because they yield both the orbital parameters and the planet mass. I will show results from ground- and space-based astrometric surveys that explore the giant planet - brown dwarf connection. These include the systematic determination of true masses for substellar objects found in radial-velocity surveys and the discovery of brown dwarfs orbited by very low-mass substellar objects. Furthermore, I will outline how the Gaia astrometry mission will help to describe the substellar demographics.

October 19th

Roman Gold

Electromagnetic counterparts to Gravitational Waves from accreting Super-Massive Black-Hole (SMBH) binaries

Accreting black holes (BHs) are at the core of relativistic astrophysics as messengers of the strong-field regime of General Relativity and prime targets of several observational campaigns, including imaging the black hole shadow in Sgr A* and M87 with the Event Horizon Telescope. The robust association with a supermassive BH (SMBH) at every AGN core together with the inferred SMBH masses today implies that BH-BH mergers must have occurred in the universe. Binary Black Holes probe the strong field regime of GR as one of the most promising gravitational wave sources for adLIGO (stellar mass) and Pulsar Timing Arrays (SMBHs). In the SMBH binary case it can be expected that the binary is accreting from its magnetized, gaseous environment and thereby providing an electromagnetic counterpart to the gravitational wave signal. The recent surge in proposed candidate SMBH binaries such as PG 1302-102 (Graham et al 2015 arXiv:1501.01375), PSO J334.2028+01.4075 (Liu et al 2015 arXiv:1503.02083), and Graham et al 2015 arXiv:1507.07603 call for advancing our understanding of the structure and evolution of these systems. I will present results from studies involving global GRMHD simulations of both single and binary BHs embedded in a hot, magnetized disk. The discussion will feature binary orbital evolution, gravitational wave emission, disk structure/dynamics, jets, and hints at distinguishing observational features.

Stephen Kent

The Dark Energy Survey (DES): Status and Early Science Results

The Dark Energy Survey (DES) is a project to to map 5000 square degrees of the southern sky in 5 bands to 24th magnitude, measuring positions and photometric redshifts for over 300 million galaxies and detecting thousands of Type Ia supernovae out to redshfit 1. The survey utilizes the Dark Energy Camera (DECam), a 570 mega-pixel optical and near-infrared camera with a 3 square degree field of view that is installed at the prime focus of the Blanco 4 meter telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile. The main goal of the survey is to probe the nature of the dark energy using four probes that measure the expansion history of the universe. The talk will cover some of the first scientific results of the survey, including the detection of new dwarf galaxies and tidal streams in the Milky Way, identification of strongly lensed galaxies and quasars, and mass maps constructed from weak lensing shear measurements.

October 22

Alexandre Lazarian

Star formation mediated by turbulent reconnection

Magnetic fields are known to play an important role at different stages of star formation, including the evolution of molecular cloud and circumstellar disk formation. The theory, however, was based on the assumption of perfect magnetic flux freezing in ionized plasmas. This assumption is not right in the presence of turbulent reconnection. I shall discuss how the processes of star formation evolve in realistic turbulent molecular clouds and accretion disks.

Irina Dvorkin

The origin of dispersion in DLA metallicities

Damped Lyman-alpha absorbers (DLA) dominate the neutral gas content of the Universe in the redshift range z=0-5 and are likely the progenitors of low redshift galaxies. The chemical properties of DLAs can be determined with great precision, and provide a unique probe of the properties of cold neutral gas out of which stars form at high redshifts. Recent chemical abundance measurements of DLAs revealed a large intrinsic scatter in their metallicities. In this talk I will discuss a semi-analytic model that was specifically designed to study this scatter. This model accurately traces the chemical evolution of the interstellar matter in small regions of the Universe with different mean density, from over- to underdense regions. I will show that the different histories of structure formation in these regions, namely halo abundance, mass and stellar content are reflected in the chemical properties of the proto-galaxies, and that the dispersion arising from this environmental effect is an important contribution to the overall intrinsic scatter. I will also show our predictions for the dispersion in deuterium abundance and discuss how it can be used to constrain the star formation history at high redshifts.

Rebekka Bieri

Galaxy formation and AGN feedback

October 26th

Tom Brown

The Quenching of the Ultra-faint Dwarf Galaxies in the Reionization Era

I will present results from a Hubble Space Telescope survey of the ultra-faint dwarf galaxies. These Milky Way satellites, discovered in the Sloan Digital Sky Survey, appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in the efforts to understand the missing satellite problem. Because they are the least luminous, most dark matter dominated, and least chemically evolved galaxies known, the ultra-faint dwarfs are the best candidate fossils from the early universe. The primary goal of the survey is to measure the star-formation histories of these galaxies and discern any synchronization due to the reionization of the universe.

November 2nd

Marc Rafelski

The Star Formation Rate Efficiency of Atomic-dominated Hydrogen Gas from z~1 to z~3

Current observational evidence suggests that the star formation rate (SFR) efficiency of neutral atomic hydrogen gas measured in Damped Lyman-alpha System (DLAs) at z~3 is a factor of 10 lower than predicted by the Kennicutt-Schmidt relation. To understand the origin of this deficit, we measure the SFR efficiency of atomic gas at z~1, z~2, and z~3 to investigate possible coevolution with galactic properties. We use new robust photometric redshifts to create galaxy stacks in these three redshift bins, and measure the SFR efficiency by combining DLA absorber statistics with the observed rest-frame UV emission in the galaxies' outskirts. Contrary to simulations and models that predict a reduced SFR efficiency with decreasing metallicity and thus with increasing redshift, we find no significant evolution in the SFR efficiency. We conclude that the reduced SFR efficiency is driven by the low molecular content of this atomic-dominated phase, with metallicity playing a second order effect in regulating the conversion between atomic and molecular gas. This interpretation is supported by the similarity between the observed SFR efficiency and that observed in local atomic-dominated gas, such as in the outskirts of local spiral galaxies or local dwarf galaxies.

Ori Fox

Infrared Observations of Dusty Supernovae

Abstract: While most supernovae (SNe) tend to fade over the course of several months, a growing fraction of SNe are now observed to remain bright for years post-explosion due to the presence of warm dust. The origin and heating mechanism of the dust, however, is not always clear. Here I present infrared (IR) observations of SNe from a number of different subclasses, including both core-collapse and thermonuclear events. I will discuss the impact of these observations on our understanding of the progenitor systems, and discuss future work that can only now be performed with JWST.

November 9th