• Einstein
• Fellowship

The Curious Case of NGC4342, an Optically Faint but Gas-Rich Early-Type Galaxy

We investigate NGC4342, an optically faint early-type galaxy in the outskirts of the Virgo cluster. This galaxy hosts an unusually bright, hot X-ray corona. However, the low stellar mass of NGC4342 should not be sufficient to gravitationally bind such a significant amount of hot gas. Therefore, NGC4342 is a truly unique and curious object. Our goal is to understand the origin and nature of the unusually large gas mass surrounding NGC4342, and measure the properties of the external medium producing the observed gas distortions.

Can a Satellite Galaxy Merger Explain the Active Past of the Galactic Center?

Observations of the Galactic Center (GC) have accumulated a multitude of "forensic" evidence indicating that several million years ago the center of the Milky Way galaxy was teaming with starforming and accretion-powered activity - this paints a rather different picture from the GC as we understand it today. I will examine a possibility that this epoch of activity could have been triggered by the infall of a satellite galaxy into the Milky Way which began at the redshift of 10 and ended few million years ago with a merger of the Galactic supermassive black hole with an intermediate mass black hole brought in by the inspiralling satellite.

Transition to Explosion in Core-Collapse Supernovae

The explosion mechanism of core-collapse supernovae remains an unsolved problem in theoretical astrophysics. The key difficulty is finding a robust mechanism that taps a small fraction of the gravitational binding energy of the forming neutron star and converts it into heat. Numerical simulations find that multi-dimensional hydrodynamic instabilities of the stalled shock wave facilitate this conversion, with increasing efficiency for higher dimensionality. The details of this process remain murky, however, because local and global non-linear processes are involved. I'll report on parametric numerical simulations of stalled supernova shocks that probe the hydrodynamic processes responsible for the onset of explosion. Results provide a framework to systematically analyze the effects of dimensionality on the explosion mechanism.

Diffuse Radio Sources in Clusters of Galaxies

A growing number of galaxy clusters exhibit diffuse radio emission from the intracluster gas in the form of cluster-wide synchrotron radio halos, relics and smaller minihalos. This emission comes from ultrarelativistic electrons coexisting with the cluster thermal plasma, and the origin of these electrons is one of the intriguing problems of the physics of the intracluster medium. I will present results of the 610 MHz GMRT Radio Halo Survey -- an ongoing observational campaign to survey massive galaxy clusters for the presence of diffuse radio sources and to follow up at lower frequencies any new sources found. Emphasis will be placed on the discovery of a new population of halos -- the ultra-steep spectrum radio halos. I will also discuss the connection between the presence of diffuse radio emission and the cluster dynamical state.

Foundations for Gravitational Self-force Calculations

The inspiral of a compact object into a supermassive black hole is a promising source of gravitational radiation for future space-based gravitational-wave interferometers. Detection and parameter estimation will require matched filtering techniques using a bank of waveform templates. To obtain a template sufficiently accurate to track the phase of an orbit over an observation time will require going beyond the geodesic approximation to include first- and second-order gravitational self-force effects on the motion of the compact object. I will describe recent progress in the theoretical and computational foundations for this ambitious project.

How do Massive Black Holes Get their Gas?

Super-massive black holes (SMBHs) and AGN are of fundamental interest both in their own right and to cosmology and galaxy formation. However, the physics of angular momentum transport from galactic scales to an accretion disk is one of the outstanding problems in our understanding of the formation and evolution of SMBHs. New, multi-scale hydrodynamic simulations can probe these scales and show for the first time that, when the proper physics is included, gravitational instabilities can drive inflow rates down to a viscous accretion disk sufficient to power the most luminous quasars. The last stage of this instability takes the form of a lopsided eccentric nuclear disk. Seemingly unrelated observations have discovered that there is a lopsided stellar disk of unknown origin orbiting the BH in M31, and possibly many other systems. I'll discuss how these nominally independent puzzles are in fact closely related -- the disk in M31 and other nearby galaxies may in fact be the 'smoking gun' for these instabilities and tell us how and when SMBHs were grown. The same disk dominates the AGN obscuration, and may be the putative ``torus'' invoked to explain obscured active galactic nuclei and the cosmic X-ray background. I'll discuss the implications of these simulations for BH fueling, quasar activity and obscuration, and the evolution of BH spin. Finally, I'll discuss what this means for models of AGN feedback and its effects on galaxy formation.

High Eccentricity Scenarios of Hot Jupiter Formation

Various formation scenarios proposed for hot Jupiters involve the excitation of extremely high eccentricities, e ~ 0.99, of a Jupiter at few AU allowing for migration due to efficient tidal dissipation at the close pericenter. Some observable features of this class of models are discussed. An analytical treatment of the particular scenario of Kozai oscillations due to a distant perturber is presented. The long term modulation of the Kozai cycles due to the octupole contribution to the perturbing potential is analytically found and shown to significantly increase the phase space of initial conditions leading to very high eccentricities.

Modelling the Gamma-ray Emission of Young Pulsars

Geometric models of gamma-ray emission from pulsars - such as the outer gap (OG; assuming uniform emissivity) and two-pole caustic (TPC) models - provide predictions for light curves as a function of observer and magnetic inclination to the pulsar spin axis. We use a generalized geometric model of emission from a zone along the last open field lines - OG and TPC are special cases - to determine the shape of the emitting region that best fits the observed light curve of each of the young LAT pulsars. For a given emitting volume, simplifying assumptions about the global current / potential structure also allow a direct prediction of the gamma-ray luminosity, and we test the validity of these assumptions by comparison with the observed luminosities for pulsars with distance estimates.

Ultrahigh-Energy Cosmic Rays and the Nearby Universe

Measurements of ultrahigh-energy cosmic rays provide the opportunity to determine not only the origins of these particles, but other presently unknown properties of the nearby universe. The discovery of UHECR anisotropies by the Pierre Auger Observatory is an important step towards achieving these goals. I will address a few implications arising from these observations, focusing on the excess of ~10^20 eV events seen towards the nearby radio galaxy Cen A. These discussions will include the prospects for inferring properties of the local intergalactic magnetic field and the history of particle acceleration in Cen A.

Observable Signatures of Extreme Mass-Ratio Black Hole Binaries Embedded in Thin Accretion Disks

We examine the electromagnetic and gravitational wave (GW) signatures of stellar-mass compact objects (COs) spiraling into a supermassive black hole (extreme mass-ratio inspirals), embedded in a thin, radiation-pressure dominated, accretion disk. At large separations, the tidal effect of the secondary CO clears a gap. We derive the conditions necessary for gap opening in a radiation-pressure dominated disk and show that the gap refills during the late GW-driven phase of the inspiral, leading to a sudden electromagnetic brightening of the source. The accretion disk leaves an imprint on the GW through its angular momentum exchange with the binary, the mass increase of the binary members due to accretion, and its gravity. We compute the disk-modified GWs both in an analytical Newtonian approximation and in a numerical effective-one-body approach. We find that disk-induced migration provides the dominant perturbation to the inspiral, with weaker effects from the mass accretion onto the CO and hydrodynamic drag. Depending on whether a gap is present, the perturbation of the GW phase is between 10 and 1000 rad per year, detectable with the future Laser Interferometer Space Antenna at high significance. The perturbation is significant for disk models with an effective viscosity proportional to gas pressure but much less so if proportional to the total pressure. The Fourier transform of the disk-modified GW in the stationary phase approximation is sensitive to disk parameters with a frequency trend different from post-Newtonian vacuum corrections. Our results suggest that observations of extreme mass-ratio inspirals may place new sensitive constraints on the physics of accretion disks.

From Plasma Microphysics to Global Dynamics in Clusters of Galaxies

In weakly-collisional plasmas such as the intracluster medium (ICM), heat and momentum transport become anisotropic with respect to the local magnetic field direction. Anisotropic heat conduction causes the slow magnetosonic wave to become buoyantly unstable to the magnetothermal instability (MTI) when the temperature increases in the direction of gravity and to the heat-flux--driven buoyancy instability (HBI) when the temperature decreases in the direction of gravity. The local changes in magnetic field strength that attend these instabilities cause pressure anisotropies that viscously damp motions parallel to the magnetic field. In this talk, I will discuss two important effects of pressure anisotropy on the dynamical and thermal stability of the ICM. First, by stifling the convergence/divergence of magnetic field lines, pressure anisotropy significantly affects how the ICM interacts with the temperature gradient. Instabilities which depend upon the convergence/divergence of magnetic field lines to generate unstable buoyant motions (the HBI) are suppressed over much of the wavenumber space, whereas those which are otherwise impeded by field-line convergence/divergence (the MTI) are strengthened. Second, because the viscous heating of the ICM is regulated by the pressure anisotropy -- which itself is nonlinearly regulated by the plasma beta parameter via rapidly-growing microscale instabilities -- pressure anisotropy may play a crucial role in mitigating cooling flows and preventing cluster core collapse. I will discuss the physical interpretation of these effects in detail, placing them within the larger context of formulating a pragmatic analytical and numerical framework for modelling astrophysical multiscale plasma dynamics.

X-ray Number Counts in the 4 Ms Chandra Deep Field-South: Characterizing New X-ray Source Populations

Recently, the cumulative Chandra Deep Field-South (CDF-S) exposure has been doubled and now reaches an integrated depth of ~4 Ms. This exposure makes the CDF-S the deepest X-ray observation of the extragalactic Universe ever undertaken. In this talk, I will present the latest estimates of the extragalactic X-ray source number counts, emphasizing the nature of the sources that contribute at the faintest flux levels. We find that the majority of the 740 X-ray detected sources in the 4 Ms CDF-S are active galactic nuclei (AGNs); however, we estimate that normal galaxies, shining primarily by emission from X-ray binaries and hot gas, contribute ~40% of the total number counts above 0.5-2 keV fluxes of 10^-17 ergs/cm^2/s. I will present a detailed break-down of the X-ray number count contributions based on a variety of source properties (e.g., AGN intrinsic column density and galaxy optical morphology) and discuss their implications for semi-analytic models that aim to parameterize the SMBH accretion history and X-ray binary evolution in the Universe.

The Search for TZOs

Thorne-Zytkow objects (TZOs) are a theoretical class of star in which a compact neutron star is surrounded by a large, diffuse envelope. Supergiant TZOs are predicted to be almost identical in appearance to red supergiants (RSGs), with their very red colors and cool temperatures placing them at the Hayashi limit on the H-R diagram. The only features that can be used to distinguished TZOs from the general RSG population are the unusual heavy-element abundance profiles present in their atmospheres. These elements are the unique products of the star's fully convective envelope linking the photosphere with the extraordinarily hot burning region in the vicinity of the neutron star core. The positive detection of a TZO would provide the first direct evidence of a completely new model of stellar interiors, as well a never-before-seen nucleosynthetic process that would offer a new channel for heavy-element production in our universe. I will present the latest results of our recent high-resolution spectroscopic search for TZOs within our previously-studied sample of RSGs in the Milky Way and Magellanic Clouds.

The Frequency and Demographics of Dual Active Galactic Nuclei

Dual active galactic nuclei are actively accreting massive black hole pairs co-rotating in merging galaxies with separations of less than a few kpc. The frequency and demographics of dual AGNs as a function of separation offer an important probe of the hierarchical merger paradigm and the accretion and evolution of MBH pairs in merging galaxies. I will discuss our recent results of systematic studies of dual AGNs using the SDSS combined with multi-wavelength followup observations, highlighting Chandra and HST imaging as well as IFU spectroscopy of optically identified dual AGNs.

Dissecting Supernova Remnants Observed by Chandra

Supernova remnants (SNRs) are a complex class of sources, and their heterogeneous nature has hindered the characterization of their general observational properties. To overcome this challenge, we have developed several statistical tools to facilitate morphological comparison between sources. In this talk, I will highlight results from applying these techniques to archival Chandra observations of a large sample of Milky Way and Magellanic Cloud SNRs. As the first systematic study of the X-ray properties of SNRs, this work has uncovered crucial insights regarding the nature of explosions, the effects of heating and dense environments, and particle acceleration properties.

21cm Tomography of the High-redshift Universe

I will compare and contrast the multitude of efforts that aim to detect redshifted 21cm emission from the high-redshift IGM. I will discuss the science these efforts aim to probe as well as the severe challenges which they must overcome.

What is the Most Promising Electromagnetic Counterpart of a Neutron Star Merger?

The inspiral and merger of double neutron star (NS) and NS-black hole binaries are among the most promising gravitational wave (GW) sources for ground-based interferometers. However, maximizing the science from such a discovery will require identifying an electromagnetic (EM) counterpart. In this talk I will critically compare several proposed counterparts, including short duration gamma-ray bursts (GRBs); `orphan' optical afterglows; off-axis radio afterglows; and ~day long optical transients (`kilonova') powered by the radioactive decay of heavy nuclei synthesized in the merger ejecta. I will argue that GRBs are most useful to confirm the cosmic origin of a few events, but for the more ambitious goal of localizing and obtaining redshifts for a large sample of GW events, then kilonova or radio afterglows are instead more promising. I will conclude with specific recommendations for future GW-triggered followup to maximize the chance of successfully identifying the NS merger counterpart.

A Comparison of MUSTANG High-Resolution Sunyaev-Zel'dovich Effect Imaging with X-ray and Lensing Cluster Properties

I present recent high angular resolution (9'') Sunyaev-Zel'dovich effect (SZE) observations with MUSTANG, a 90-GHz bolometric receiver on the 100-meter Green Bank Telescope. MUSTANG is now imaging a sample of clusters with complementary Chandra X-ray observations, HST optical observations for strong and weak lensing mass determinations, and lower resolution SZE observations that can recover larger scales (>1 arcminute). The MUSTANG observations, which will be used to assess the impact of substructure on SZE scaling relations, are some of the highest resolution SZE images to date, and are revealing complex pressure substructures in intermediate redshift clusters.

Finding the First Black Holes

I will discuss the prospects for finding remnants of the first black holes in the local universe. A central black hole may lurk in the center of some of the smallest star clusters in the Milky Way halo. These clusters could have formed in one of two ways. 1) The clusters may be the end state of tidally stripped star clusters that contained a central black hole or 2) the cluster may be the remnant nuclear stellar cluster that was carried off by the ejection of a merging black hole binary in the first galaxies in the universe. In both cases, the black holes will have encountered little gas and should be a representative sample of the first black holes in the universe.

Probing the Early Evolution of Massive Black Holes with Star-Forming Dwarf Galaxies

Supermassive black holes are now thought to inhabit essentially all modern massive galaxies with a bulge, yet the birth and early evolution of the first high-redshift seed black holes is poorly constrained. Reines et al. (2011) have recently identified a million-solar mass black hole in the vigorously star-forming, bulgeless dwarf galaxy Henize 2-10. This discovery offers the first opportunity to study a growing black hole in a nearby galaxy much like those in the earlier universe, and opens up an entirely new class of host galaxies in which to search for local analogues of primordial black hole growth. Moreover, this finding has important implications for our understanding of the co-evolution of galaxies and their central black holes. In particular, the lack of a discernible bulge in Henize 2-10 indicates that black hole growth can precede the build-up of galaxy spheroids, which has been a subject of debate in the community. It is clearly important to search for other examples of massive black holes in dwarf starburst galaxies to begin to characterize them as a population, and improve the current understanding of the early stages of supermassive black hole growth.

Stellar Mass Black Holes - Accretion Geometry and Black Hole Spin

An astrophysical black hole is fully defined by just two parameters: its mass and spin. With the advent of modern X-ray satellites such as XMM-Newton and Chandra came a leap in our understanding of the fundamental physics of these powerful objects. We are now at a stage where information on the mass and spin of black holes is available for nearly a dozen X-ray binaries (XRBs), and we are in the process of obtaining the spin for a similar number of supermassive black holes. X-ray observations of stellar-mass black holes provide three independent means from which we can estimate the black hole spin: i) thermal emission emanating from the accretion disk; ii) relativistic distortion to emission lines and other reflection features; and iii) quasi-periodic variations in the light originating in the system. In this brief talk, I will summarize the current status of the field, concentrating on the wealth of information already obtained from reflection features and briefly point towards the way we can use reflection to further our knowledge of the geometry of the accretion flow in X-ray binaries.

The SZ Signal of maxBCG Galaxy Clusters, and Multi-Wavelength Scaling Relations

We discuss the SZ signal of maxBCG galaxy clusters as probed by the CARMA array, WMAP, and the Planck satellite. We discuss the implications of these measurements for optical mass estimates, highlighting the importance of a multi-wavelength approach towards cluster cosmology for ensuring the robustness of the results to systematic uncertainties in mass estimation.

The Puzzle of Secular Black Hole Growth

Recent observations are showing that, while there is wide diversity in the nature of AGN host galaxies as a function of luminosity and redshift, a substantial fraction of cosmic black hole growth occurs in disky host galaxies, not in the remnants of major mergers. This challenges predictions from simulations and raises the question of the origin of the well-known galaxy-black hole scaling relations, which may have been in place as early as z~6.

Anisotropies in the Gamma-ray Background Measured by the Fermi-LAT

The contribution of unresolved sources to the diffuse gamma-ray background could induce anisotropies in this emission on small angular scales. Recent studies have considered the angular power spectrum and other anisotropy metrics as tools for identifying the contributions to diffuse emission from unresolved source classes, such as extragalactic and Galactic dark matter as well as various astrophysical gamma-ray source populations. I will present the results of an angular power spectrum analysis of the high-latitude diffuse emission measured by the Fermi-LAT, and discuss the implications of the measured angular power spectrum for gamma-ray source populations that may provide a contribution to the diffuse background.

Update on Suzaku Studies of the Outskirts of the Nearest, Brightest Galaxy Clusters

Thanks to the low instrumental background of the Suzaku satellite, we are only recently starting to get a detailed view of the faint edges of clusters of galaxies. I will present the latest results from a series of Suzaku Large and Key Projects aimed at extending our knowledge of the thermodynamic properties of cluster outskirts for several bright, nearby systems, spanning a range of masses and dynamical states. I will discuss the implications of these results both for cluster cosmology and for understanding the Physics of large scale structure formation.