Wine and Cheese Spring 2015

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

Spring 2015 Schedule

Date	Speaker	Title
January 26	Ingyin Zaw (NYU Abu Dhabi)	Probing the Central Parsec of Active Galactic Nuclei with Water Masers
	Naoki Bessho (NASA/UMD)	Particle acceleration during magnetic reconnection in ultrarelativistic electron-positron plasmas
February 02	Marius Millea (UC Davis)	Planck 2015 Constraints on the Cosmic Neutrino(-like) Background
	Colin Hill (Columbia)	Cosmology from the One-Point Function
February 09	Katherine Lee (Harvard)	CARMA Large Area Star Formation Survey (CLASSy)
	Rongmon Bordoloi (STScI)	Investigating the Milky Way’s Nuclear Outflow Kinematics
February 16	Yacine Ali-Haïmoud (JHU)	Perturbative interaction approach to cosmological structure formation
	Nao Suzuki (IPMU)	Future SNIa surveys and Blackbody Spectra
February 23	Rubab Khan (GSFC)	Massive Star Geriatrics
	Jon Bird (Vanderbilt)	Clues to Galaxy Formation from the Milky Way's Cosmological Context
March 02	Alexie Leauthaud (IPMU, Full seminar)	Evolving Galaxies in a Dark Universe
March 09	Nicole Czakon (ASIAA)	Scaling Sunyaev-Zel'dovich Observables to Dark Matter Halos for Cluster Cosmology
	Kate Daniel (JHU)	Constraints on the Efficiency of Radial Migration in Spiral Galaxies
March 23	Kendrick Smith (Perimeter)	Primordial Non-Gaussianity in the CMB and Large-Scale Structure
	Sanch Borthakur (JHU)	Probing the Connection Between the Circumgalactic Medium and the Interstellar Medium of Galaxies
March 30	Katie Harrington (JHU)	CLASS: The Cosmology Large Angular Scale Surveyor
	Peter Polko (UMD)	From Accretion Flow to Particle Acceleration: New Relativistic Magnetohydrodynamic Jet Solutions
April 06	Tomohiro Nakama	On whether supermassive black holes can be explained by primordial black holes
	Brooks Kinch (JHU)	Fe K-alpha Emission Lines from Simulations of Black Hole Accretion
April 13	Xavier Dumusque (CfA)	Pushing the radial-velocity precision to unveil Earth-mass exoplanets
	Samantha Hoffmann (JHU)	Mega-SH0ES: Searching for Cepheid Variables in Type Ia Supernova Host Galaxies
April 20	Liang Dai (JHU)	Relativistic clustering and separate universes
	Jennifer Sobeck (UVa)	APOGEE I/O: Efforts to Effectively Harness the Large Scale Data Set of the APOGEE 1+2 Survey
April 27	Johan Mazoyer (STScI)	Extrasolar Planetary Systems Imaging
	Daan Meerburg (CITA)	Optimal estimator for resonant bispectra in the Cosmic Microwave Background
May 04	Agnieszka Cieplak (Brookhaven National Laboratory)	Theoretical Predictions of Large Scale Clustering in the Lyman-alpha Forest
	Lixin Dai (UMD)	X-ray Temperature Tidal Disruption Events from Stars on Deep Plunging Orbits
May 11	Tim Brandt (IAS)	Extended Main Sequence Turnoffs in ~1-1.5 Gyr-old Clusters: Age Ranges or Stellar Rotation?
	Alvise Raccanelli	Testing cosmological models with galaxy surveys

26 Jan 2015

Ingyin Zaw

Probing the Central Parsec of Active Galactic Nuclei with Water Masers
Determining the geometry and dynamics of the inner-most parsec of active galactic nuclei (AGN) is critical for understanding accretion and the relationship between the AGN and host galaxy. Water maser emission at 22 GHz provides a unique tracer, resolvable in position and velocity, of warm, dense molecular gas ~0.1-1.0 pc from the central engine. Furthermore, water masers exist in the narrow temperature range of ~400-1000K and can be used to probe the temperature and temperature gradient inside the AGN disk. I will discuss i) a test of disk heating in accretion models, using maser spectra and VLBI maps, ii) a study of the flow of material in NGC 4945, combining maser VLBI maps and multi-wavelength data, and iii) a search for new maser systems in the Southern Hemisphere, the Tidbinbilla AGN Maser Survey (TAMS).

Naoki Bessho

Particle acceleration during magnetic reconnection in ultrarelativistic electron-positron plasmas
In pulsar winds and jets from AGNs, plasma is considered to be composed of ultrarelativistic electrons and positrons with their Lorentz factors 10^3 to 10^6. How these high energy particles are produced is an open question, and magnetic reconnection is one of mechanisms to accelerate particles. We study magnetic reconnection in ultrarelativistic electron-positron plasmas by means of 2-D simulations that include kinetics of particle motion, and investigate particle acceleration mechanisms and energy spectra of accelerated particles.

02 Feb 2015

Marius Millea

Planck 2015 Constraints on the Cosmic Neutrino(-like) Background
The Planck 2015 results include the tightest measurements to-date of cosmic microwave background (CMB) temperature and polarization anisotropies up to few-arcminute angular scales. I will give a broad overview of the cosmology results from these data, with particular focus on what we have learned about the cosmic neutrino background (CNB). The new data allow more precise answers to questions such as 1) how much energy, parameterized by N_eff, is contained in the CNB? 2) what is the sum of the masses of the particles making up the CNB? and 3) are these particles really neutrinos, i.e. do they free-stream like neutrinos? One possibility I will explore is if some component of the CNB actually comes from axions or axion-like particles. Recent improvements in CMB and BBN data are shedding new light on this scenario. I will also discuss the status of agreement between Planck results and other cosmological probes such as BAO, H0, and low redshift structure measurements, and how the CNB may play a role in resolving tensions between some of them.

Colin Hill

Cosmology from the One-Point Function
Cosmological measurements have traditionally focused on the two-point correlation function or power spectrum. However, due to the non-gaussianity of the late-time density field, a vast amount of information potentially lies in the one-point probability distribution function (PDF) of various cosmological observables, such as the weak lensing (WL) convergence or thermal Sunyaev-Zel’dovich (tSZ) effect. We present analytic methods that allow for straightforward and efficient computations of these signals. Using data from the Atacama Cosmology Telescope (ACT), we explicitly demonstrate the power of the tSZ PDF, constraining the amplitude of density fluctuations with an error bar nearly twice as small as that obtained from ACT's earlier analysis of the tSZ skewness alone (with the same data). We extend these methods to the WL convergence field, for both CMB lensing and galaxy lensing, and verify their accuracy by comparing to ray-traced N-body simulations. Combining the WL PDF and power spectrum will increase the cosmological constraining power of upcoming surveys by at least a factor of two.

09 Feb 2015

Katherine Lee

CARMA Large Area Star Formation Survey (CLASSy)
I will present recent results from the CARMA Large Area Star Formation Survey (CLASSy) with a focus on the structures and kinematics of dense gas in Serpens Main. The survey mapped 150 square arcminutes of Serpens Main with an angular resolution of 7 arcsecs using N2H+(1-0), HCO+(1-0), and HCN(1-0) as dense has tracers. The gas emission is concentrated in two subclusters (the NW and SE subclusters). The SE subcluster has more prominent filamentary structures and more complicated kinematics compared to the NW subcluster. I will talk about the properties of the filaments, and their implications to the formation of the SE subcluster. Also, I will compare the properties of the filaments with the distribution of YSOs. The comparison suggests that the YSOs are formed on gravitationally unstable filaments. Finally, I will show velocity gradients perpendicular to the filaments at 0.03 pc scale across CLASSy regions. Such velocity gradients can be a natural consequence of converging flows.

Rongmon Bordoloi

Investigating the Milky Way’s Nuclear Outflow Kinematics
Recent observations with gamma ray emission to microwaves and polarized radio waves have detected giant lobes of plasma (Fermi Bubbles) extending above and below the Galactic plane of the Milky Way. These are possible signs of a Nuclear wind powered by either the central black hole or high-surface-density star formation, but our understanding is hampered by a lack of kinematic information. I will report the first results of a HST/COS survey to constrain the velocity of the outflowing gas within these regions, using ultraviolet absorption-line spectra.We perform a comprehensive spectroscopic program to survey the nuclear outflow in both the northern and southern Galactic hemispheres.We combine high-resolution STIS E140M observations of distant halo stars at low latitude with medium-resolution COS observations of AGNs at higher latitude. These sightline pass through a clear biconical structure seen in hard X-ray and gamma-ray emission of the Fermi Bubble. I will report detections of high velocity metal absorption lines, which cannot be explained by co-rotating gas in the Galactic disk or halo. Their velocities are suggestive of an origin on the front and back side of an expanding biconical outflow emanating from the Galactic center. We develop simple kinematic biconical outflow models that can explain the observed profiles with an outflow velocity of ~900/1000 km/s and a full opening angle of ≈110° (matching the X-ray bicone). This indicates Galactic center activity over the last ≈2.5-5.0 Myr, in line with age estimates of the Fermi Bubbles. The observations illustrate the use of UV spectroscopy to probe the properties of swept-up gas venting into the Fermi Bubbles.

16 Feb 2015

Yacine Ali-Haïmoud

Perturbative interaction approach to cosmological structure formation

The statistical properties of cold dark matter (CDM) in the non-linear regime make for a technically challenging problem, and their study has been the bread-and-butter of several generations of cosmologists. Standard analytical methods improve upon linear perturbation theory on quasi-linear scales, but usually fail dramatically at non-linear scales. A new and promising method was recently introduced by researchers in the field, relying on an expansion in the gravitational interaction, and using mathematical tools inspired by those of quantum field theory. This method seemed to produce results in good agreement with numerical simulations, deep inside the non-linear regime. In this talk, after reviewing standard perturbation schemes, I will lay out a simpler formalism for the perturbative interaction approach, using implicit forms for particle trajectories. I will show that this approach fails at recovering the linear growth factor on large scales, and that the apparent agreement on non-linear scales results from unjustified approximations. The problem of finding an analytical description of non-linear scales therefore remains open.

Nao Suzuki

Future SNIa surveys and Blackbody Spectra

Hyper Suprime-Cam (HSC) is a new wide-field camera on Subaru Telescope in Hawaii. HSC has a 1.5-degree field-of-view (FOV) in diameter with 104 CCD chips and 5 broad-band filters (g,r,i,z,y). Started from March 2014, a five-year survey program has been running, and I will introduce the survey plan and current status with emphasis on Type Ia supernova (SNIa) survey. Also, I will introduce a potential mid-size IR satellite mission, WISH (http://wishmission.org/en/index.html). In SNIa cosmology, the reduction of the systematic error is an urgent task, and I will propose how to reduce the calibration error by using white dwarfs with nearly a perfect blackbody spectrum.

23 Feb 2015

Rubab Khan

Massive Star Geriatrics

The evolution of the most massive stars such as Eta Carinae is controlled by the effects of mass-loss. Understanding these stars is challenging because no true analogs of Eta Car have been clearly identified in the Milky Way or other galaxies. Copious mass-loss leads to circumstellar dust formation, obscuring the star in the optical. But as the light is re-emitted by the dust, these objects become very luminous in the mid-IR. We have carried out a systematic search for Eta Car analogs in 7 galaxies, utilizing data from Spitzer, Herschel, HST and other sources. Our search detected no true analogs of Eta Car, however, we do identify a significant population of 18 lower luminosity (log(L/L_sun)=~5.5-6.0) dusty stars. This is consistent with all 25 < M < 60 M_sun stars undergoing an obscured phase at most lasting a few thousand years once or twice. The mass of the obscuring material is of order ~M_sun, and we simply do not find enough heavily obscured stars for these phases to represent more than a modest fraction (~10% not ~50%) of the total mass lost by these stars. While this search has been feasible using archival Spitzer data, JWST will be a far more powerful probe of these stars. The HST-like resolution of JWST will greatly reduce the problem of confusion and expand the possible survey volume.

Jon Bird

Clues to Galaxy Formation from the Milky Way's Cosmological Context

Very different data sets guide galaxy formation theory across cosmic history: from the global properties of >10^7 galaxies at high redshift (z>0.5) to the kinematics and chemistry of >10^6 stars here in the Milky Way. Traditional observational and computational limitations have dictated independent study of these two regimes. I will discuss how this picture is changing rapidly and how viewing the MW as important boundary condition on galaxy evolution puts unprecedented demands on galaxy formation theory. In particular, I will discuss a novel disk formation mechanism and its signature in current observations of the Milky Way and the resolved kinematics of high redshift galaxies. Modern, high-resolution, cosmological galaxy formation simulations reveal that disks can grow ``upside-down" in the sense that progressively younger stellar populations are born with increasingly smaller vertical velocity dispersion, tracing the kinematics of the collapsing gas disk from which they form. We find that the upside-down model matches the most stringent observational constraints here in the MW, including the steep stellar age-velocity relationship measured in the solar neighborhood. I will argue that traditional interpretations of the MW stellar AVR contradicts evidence from IFU observations of high-redshift disk galaxies and must be revised. Our findings suggest that the "upside-down" model is currently the only self-consistent formation mechanism able to match kinematic constraints from z~2 to z~0. I will conclude with preliminary, yet tantalizing, evidence connecting the star formation history of simulated galaxies with their detailed morphology.

02 Mar 2015

Alexie Leauthaud

Evolving Galaxies in a Dark Universe

A fundamental goal in observational cosmology is to understand the link between the luminous properties of galaxies and the dark matter halos in which they reside. Because this link is fundamental to processes that determine the growth, evolution, and global properties of galaxies, key insight can be gained by mapping how the distributions of dark and luminous matter vary across different scales and over cosmic time. In this talk I will discuss new methods to probe the galaxy-halo connection from galaxy scales (tens of kpc to 100 kpc) out to the scale of dark matter halos, themselves (hundreds of kpc to a Mpc). On the smallest scales, I will show that novel weak lensing techniques applied to upcoming surveys such as WFIRST and Euclid can map the inner density profiles of galaxies and provide strong constraints on the inner slope of dark matter as well as the stellar IMF. In the second half of the talk I will shift to the largest scales where a combination of probes provides insight about how galaxies grow (or do not grow) in relation to their global reservoirs of fuel. In particular, I will present a new, comprehensive framework that describes how the most massive galaxies populate dark matter halos and how their colors may be determined by their halo assembly history. Even before more powerful constraints from future surveys, I will show how such models combined with state-of the art measurements of weak gravitational lensing and galaxy clustering from the CS82 and BOSS surveys are already yielding surprising discoveries.

09 Mar 2015

Nicole Czakon

Scaling Sunyaev-Zel'dovich Observables to Dark Matter Halos for Cluster Cosmology
The Sunyaev-Zel’dovich effect (SZE) is a powerful tool to study galaxy clusters out to large radii and to detect clusters at high redshifts. To first order, clusters are self-similar and one can link the SZE signal to a cluster’s physical properties by assuming a spherical distribution of matter in hydrostatic equilibrium. The SZE signal, however, will be affected by any astrophysical process that contributes non-thermal pressure support or if the cluster has non-spherical morphology. We have measured the SZE signal of 45 massive clusters using Bolocam at 140 GHz. After measuring the scaling relations of the SZE signal with total cluster mass, we find our clusters to be approximately 5-sigma shallower than the self-similar HSE prediction--a result that is in tension with most other SZE scaling relations studies. To confirm our measurements, we have implemented a series of tests to see whether, among others, sample selection, redshift, degree of disturbance, or alternative mass proxies might affect our measurements. We believe our results to be robust to the extent to which we are able to constrain the cluster properties with current observations. If confirmed, this would have a major impact on our understanding of galaxy clusters and cluster cosmology.

Kate Daniel

Constraints on the Efficiency of Radial Migration in Spiral Galaxies
A transient spiral pattern can permanently rearrange the orbital angular momentum distribution of a stellar disk without inducing kinematic heating. This redistribution happens around the radius of corotation, where the circular orbital frequency equals the spiral pattern speed, and leads to what is now called “radial migration”. Should radial migration be an efficient process it could cause a large fraction of disk stars to experience significant changes in their individual orbital angular momenta over the lifetime of the disk. Such scenarios have strong implications for the chemical, structural and kinematic evolution of disk galaxies. I present some results from an investigation into the physical dependencies of the efficiency of radial migration on stellar kinematics and spiral structure.

23 Mar 2015

Kendrick Smith

Primordial non-Gaussianity in the CMB and Large-Scale Structure
I'll give a pedagogical review of inflation and explain how its physics can be constrained by searching for "primordial non-Gaussianity", i.e., differences between the statistics of the initial curvature field in our universe and the statistics of an ideal Gaussian field. Then I'll talk about observational CMB constraints, including some new results from Planck. Finally I'll discuss future prospects for improving Planck constraints with large-scale surveys such as Euclid and LSST.

Sanch Borthakur

Probing the Connection Between the Circumgalactic Medium and the Interstellar Medium of Galaxies

We present the first statistical study probing the connection between the circumgalactic medium (CGM) and the atomic hydrogen content within galaxies. The survey utilizes Hubble Space Telescope ultraviolet spectroscopy to probe the hidden baryonic content in the CGM for 47 galaxies from the GALEX Arecibo SDSS Survey (GASS).

We find strong correlations between the amount of H I gas in the ISM of the galaxies and the neutral gas content in the CGM. These are stronger than the analogous correlations between the star-formation rates and the CGM content. Additionally, the velocity spread of the circumgalactic gas is consistent with that seen in the atomic gas in the interstellar medium. These results imply a physical connection between the H I disk and the CGM on scales an order-of-magnitude larger. This is consistent with the picture in which the H I disk is nourished by accretion of gas from the CGM.

30 Mar 2015

Katie Harrington

CLASS: The Cosmology Large Angular Scale Surveyor
The detection and characterization of the primordial B-modes in the polarization of the cosmic microwave background (CMB) is one of the next major steps in developing our understanding of the early universe. Primordial B-modes, a divergence-free polarization pattern in the CMB, are solely sourced by gravitational waves created during the epoch of inflation. Detecting the primordial B-modes would represent a “smoking gun” for inflation and provide evidence of a period of exponential expansion in the early universe. The detection of the primordial B-modes requires a precise measurement of the CMB polarization on large angular scales with multiple frequencies for galactic foreground removal. The Cosmology Large Scale Angular Surveyor (CLASS) is a four frequency telescope array with a rapid front-end polarization modulator sited in the Chilean Atacama desert, making it uniquely suited for detecting the primordial B-mode signal.

Peter Polko

From Accretion Flow to Particle Acceleration: New Relativistic Magnetohydrodynamic Jet Solutions

Observations of jets show us that the electrons throughout the jet have a power-law energy distribution, even though the electron cooling time due to radiation is much shorter than the lifetime of the jet. This result implies that the electrons are continuously accelerated in the jet, and we associate the onset of this process with a shock at the outermost magnetohydrodynamic (MHD) singular point in the outflow.

I will give a short overview of MHD jet physics, and show how we developed a semi-analytical model that describes the jet from very close to the black hole to this first shock, allowing us to better model the expected emission from a black hole system.

06 April 2015

Tomohiro Nakama

On whether supermassive black holes can be explained by primordial black holes
Supermassive black holes and intermediate mass black holes are believed to exist in the Universe. There is no established astrophysical explanation for their origin and considerations have been made in the literature that those massive black holes (MBHs) may be primordial black holes (PBHs), black holes which are formed in the early universe (well before the matter-radiation equality) due to the direct collapse of primordial overdensities. I will discuss the possibility of excluding the PBH scenario as the origin of the MBHs. I first revisit the constraints on PBHs obtained from the CMB distortion that the seed density perturbation causes. I also discuss a new method which can potentially exclude small PBHs as well. We first observe that large density perturbations required to create PBHs also result in the copious production of ultracompact minihalos (UCMHs), compact dark matter halos formed at around the recombination. From this observation, we show that weakly interacting massive particles (WIMPs) as dark matter annihilate efficiently inside UCMHs to yield cosmic rays far exceeding the observed flux. Our bound gives severe restriction on the compatibility between the particle physics models for WIMPs and the PBH scenario as the explanation of MBHs.

Brooks Kinch

Fe K-alpha Emission Lines from Simulations of Black Hole Accretion
The Fe K-alpha fluorescent emission line is a ubiquitous spectral feature in observations of both stellar-mass black holes and active galactic nuclei. It originates from X-ray irradiation of the accretion disk, which extends deep into the gravitational well---thus the Fe K-alpha line provides an excellent diagnostic of the physical conditions and spacetime geometry in the vicinity of the black hole. I will describe how, starting from genuine physical principles and introducing as few assumptions as possible, we produce theoretical predictions for the Fe K-alpha line shape and intensity. In addition, I will give a brief comparison to and discussion of some current Fe K-alpha observations. Finally, I will discuss some frontier applications of this work, e.g., the use of X-ray reflection spectroscopy in the measurement of black hole spin.

13 April 2015

Xavier Dumusque

Pushing the radial-velocity precision to unveil Earth-mass exoplanets
At the meter per second precision reached by the best nowadays radial-velocity (RV) spectrographs, very subtle signatures of astrophysical noises start to be revealed. From a nearly continuous timescale ranging from several minutes to several years, stellar oscillations, granulation phenomena, short-term and long-term activity induce a RV signal that mask the RV signature induced by small-mass planets orbiting far from their host star. Some of these astrophysical noises are now well understood and strategies to average them out have been found, however still an important work needs to be done for other sources of noise. During my talk, I will give an overview of the work that have been carried on these last years and will try to focus on the main nowadays limitations.

Samantha Hoffmann

Mega-SH0ES: Searching for Cepheid Variables in Type Ia Supernova Host Galaxies
The Mega-SH0ES project aims to measure the Hubble constant to percent-level uncertainty. As part of this ongoing effort, we determine accurate and precise Cepheid distances to host galaxies of type Ia supernovae within 50 Mpc. I will present preliminary results from our latest HST/WFC3 observations, in which time-series data is obtained using a long pass filter (F350LP) to detect variability while conserving orbits and infrared images (F160W) are used to construct a precise P-L relation with low systematic uncertainty. We combine the new targets with the ones previously analyzed in Riess et al. (2011) and will calibrate the Extragalactic Distance Scale with a record 17 SNeIa host galaxies.

20 April 2015

Liang Dai

Relativistic clustering and separate universes
The separate universe conjecture states that in General Relativity a long-wavelength density perturbation affects physics locally such that short-scale gravitational clustering takes place in a separate universe with different background density and curvature. We construct Conformal Fermi Coordinates to verify this conjecture for small cosmological scalar perturbations on arbitrary scales. In this case, the isotropic part of the perturbation is entirely absorbed into a modified local expansion history and spatial geometry. The anisotropic part, on the other hand, is exactly captured by a tidal field in the Newtonian form. The separate universe picture is restricted to scales larger than the sound horizons of relevant cosmic fluids. Using this formalism, an expression can be derived for the locally measured matter bispectrum induced by a long-wavelength mode of arbitrary wavelength, a new result which in standard perturbation theory is equivalent to a relativistic second-order calculation. It can be shown that nonlinear gravitational dynamics does not generate nonlinear clustering that scales like local-type primordial non-Gaussianity f_NL. Rather, contamination to f_NL type non-Gaussianity only arises from relativistic projection effects on photon propagation, which depend on the specific large-scale structure tracer and observable considered, and are in principle distinguishable from nonlinear gravitational clustering.

Jennifer Sobeck

APOGEE I/O: Efforts to Effectively Harness the Large Scale Data Set of the APOGEE 1+2 Survey
The first phase of the Apache Point Observatory Galactic Evolution Experiment (APOGEE-1), a cornerstone project of the Sloan Digital Sky Survey III (SDSS-III), performed a large-scale, spectroscopic survey of the Milky Way conducted from the Northern Hemisphere, aimed at tracing the chemodynamical evolution of the Milky Way. Now, the SDSS-IV APOGEE-2 Survey is underway, with observations being carried out at both Northern and Southern Hemisphere locations. APOGEE acquires high-resolution, high signal-to-noise spectra in the H-band (1.51-1.69 microns) and produces stellar atmospheric parameters, individual element abundance ratios for 15 species, and radial velocity measurements for its stars. By 2020, data will have been collected for several hundred thousand stars from all components of the Galaxy (all disk quadrants, the inner and outer halo, the full expanse of the bulge, and local satellites). Accordingly, the combined APOGEE 1+2 data set is tailor-made for the comprehensive and systematic examination of Galactic formation and evolution. I will give an updated overview of the APOGEE 1+2 Survey, briefly discuss endeavors to improve the data product determination (such as enhancing the atomic physics inputs), and describe initial data mining efforts and the use of APOGEE data products to assemble composite chemical and kinematic profiles for target stellar populations.

27 April 2015

Johan Mazoyer

Extrasolar Planetary Systems Imaging
Johan will discuss the development of innovative instruments for imaging circumstellar environments and in the analysis of high contrast images to detect and analyze planets and/or dust at close separations around these stars.

Daan Meerburg

Optimal estimator for resonant bispectra in the Cosmic Microwave Background
With a cosmic variance limited measurement of the CMB power spectrum we have almost exhausted the information available in the almost Gaussian CMB. Within the next years we hope to further explore the polarization and measure higher order correlation functions in the CMB that improve our understanding of the early Universe. In this talk I will focus on a particular class of CMB bispectra known as resonant bispectra. These spectra have rapidly varying features on top of the late-time BOA features. Only recently, and only for a limited number of frequencies, have these bispectra been constrained using the Planck data. I will discuss a newly proposed estimator of such bispectra and show that we should be able to probe any frequency that one expect from an EFT argument (and beyond), because the estimator relies on the fact that most of the theoretical shapes are at most 1 dimensional (while the full bispectrum is 3 dimensional).

04 May 2015

Agnieszka Cieplak

Theoretical Predictions of Large Scale Clustering in the Lyman-alpha Forest
With the recent progress of Lyman-alpha forest power spectrum measurements, understanding of the bias between the measured flux and the underlying matter power spectrum is becoming crucial to the percent level cosmological interpretation of these measurements. Whereas previous theoretical studies of this bias have used N-body and hydro-PM simulations, we have run hydrodynamic simulations to study the response of the Lyman-alpha forest clustering to large wavelength modes of the underlying matter large-scale structure. Our results demonstrate that this can be simulated by evolving smaller, curved universe cosmologies, representing the same universe with different overdense patches. We use these to study the assumptions of the analytical bias formula derived by Seljak (2012), and compare these results with previous numerical methods of determining bias. With several forthcoming large data sets, such theoretical predictions are important to fully understand the clustering of the Lyman-alpha forest and its cosmological implications.

Lixin Dai

X-ray Temperature Tidal Disruption Events from Stars on Deep Plunging Orbits
When a star approaches a supermassive black hole closer than its tidal radius, the star can be deformed and disrupted by the black hole tidal field. Such events can be used to probe quiescent supermassive black holes and help us understand accretion/jet physics around black holes. The canonical picture by Rees 1988 predicts that in such events a small accretion disk forms from stellar debris and the light curve follows the mass fallback rate. However, recent hydrodynamical simulations have shown that this picture, especially the fast circularization of debris, cannot be easily realized. It is then quite puzzling why some observed TDEs have light curves following the fallback rate and have spectra peaked at soft X-ray temperature. In this talk we will employ first-order calculations to show that such X-ray temperature TDEs can be produced from stars disrupted on orbits with high penetration parameters.

11 May 2015

Tim Brandt

Extended Main Sequence Turnoffs in ~1-1.5 Gyr-old Clusters: Age Ranges or Stellar Rotation?
Stellar evolution models have failed to reproduce the main sequence turnoffs both in the nearby Hyades and Praesepe clusters and, more dramatically, in ~1-1.5 Gyr-old clusters in the LMC. In order to fit the clusters, a combination of isochrones spanning hundreds of Myr has been necessary. I will show that stellar models with rotation, through a combination of extended main sequence lifetimes and orientation effects, can also reproduce these turnoffs. The Hyades and Praesepe become perfectly consistent with a single age and composition, though the best-fit age (nearly 800 Myr) is significantly older than the consensus Hyades age of 600-650 Myr. A range of initial rotation rates and viewing angles also spans the dramatically extended main sequence turnoffs seen in intermediate-aged LMC clusters, and accounts for their appearance at <~1 Gyr and disappearance at ~1.5-2 Gyr. If rotation does explain the extended turnoffs in the LMC, it provides a way to probe the angular momentum distribution and evolution of stars at low metallicity.

Alvise Raccanelli

Testing cosmological models with galaxy surveys
The strangest feature of our current cosmological model is the observation that the expansion rate of the universe is accelerating. Our ignorance is summarized by the simple name for the cause of the observed phenomenon: dark energy. Alternatively, it could be explained by the break down of Einstein's gravitation theory on cosmological scales. Observations of large-scale structure have played an important role in developing our standard cosmological model and will play an essential role in our investigations of the origin of cosmic acceleration. However, the theoretical models currently used to interpret the data often rely on simplifications that make them not accurate enough for precise measurements at very large scales. I will show improvements to the theoretical modeling that arise when using a proper general relativistic formalism, and illustrate how it is possible to test different models of gravity, dark matter, dark energy and inflation via galaxy clustering using forthcoming cosmological galaxy surveys.