Date: Wednesday 30-Aug-2023
Speaker: Niranjan Roy (University of Connecticut)
Title: Mock emission line observations of galaxies on FIRE
Abstract: Unveiling the drivers of galaxy growth is one of the key science goals of Astrophysics this decade. When did the first galaxies form? How did the clumpy early galaxies evolve to the diversity of morphologies that we see today? What is the impact of feedback from massive stars and black holes on galaxy evolution? We investigate these questions with detailed synthetic observations of FIRE (Feedback In Realistic Environments) simulated galaxies at a range of redshifts. The FIRE simulations resolve the multi-phase interstellar medium of galaxies while capturing their cosmological environment and self-consistently reproduce a range of observed galaxy properties, providing a unique platform to produce synthetic IFU data for one-to-one comparisons to observations by JWST and other IFU surveys. We leverage the non-equilibrium chemistry solver CHIMES and the 3D Monte-Carlo line radiative transfer code RADMC-3D to model the emission, propagation, and absorption of spectral lines from the FIR to the UV regimes along with stellar emission and absorption, scattering, and thermal emission from dust grains. Our goal is to recover intrinsic galaxy properties from mock IFU observations of FIRE galaxies and test observational inferences of the physical properties of galaxies. This comparison of observationally inferred versus intrinsic galaxy properties will greatly inform the use of emission line tracers across the FIR to UV range. Such data will be important to calibrate the reduction and analysis pipelines of IFU instruments that are already bringing a paradigm shift in our understanding of galaxy evolution.
Date: Wednesday 06-Sep-2023
Speaker: Saikat Das (Kyoto University)
Title: Hunting the origin of ultrahigh-energy cosmic rays through neutrinos and gamma-rays
Abstract: Ultrahigh-energy cosmic rays (UHECRs) are the highest-energy particles observed in the Universe, with their energy spectrum extending beyond a few times 1e20 eV. Their unknown sources can also contribute to the diffuse astrophysical neutrino flux measured by IceCube in the TeV-PeV energy range. By modelling their multi-wavelength energy spectrum, we probe cosmic ray acceleration in high-energy gamma-ray sources such as active galactic nuclei and gamma-ray bursts. In particular, I will discuss models explaining the correlation of a high-energy neutrino event from the direction of the Fermi-LAT gamma-ray blazar TXS 0506+056, and the >10 TeV afterglow emission in GRB 221009A. Using a luminosity-dependent density evolution, I will also present the PeV-EeV diffuse neutrino flux from the entire blazar population. For a generic source distribution, the detection of cosmogenic neutrinos beyond ~0.1 EeV is favourable by upcoming detectors, such as IceCube Gen-2, GRAND and POEMMA, depending on the UHECR mass composition. Lastly, I shall discuss the superheavy dark matter origin of the high-energy cosmic rays, gamma rays, and neutrinos.
Date: Wednesday 13-Sep-2023
Speaker: Arjun Savel (University of Maryland)
Title: Knowing when to know: bridging data-driven and physics-driven modeling for exoplanet atmospheres
The study of exoplanet atmospheres has long grappled with the difficulties inherent to low signal-to-noise data. Even as observations have become more constraining, ambitions for what constitutes "signal" have become proportionally larger. Recent high-resolution cross-correlation spectroscopy (HRCCS) studies exemplify this trend by aiming to precisely measure the abundance of multiple gasses in exoplanet atmospheres. Given the opaque nature of HRCCS data processing, it is unclear where biases in these inferred abundances may emerge. In this talk, I present my work done at the Flatiron Institute last fall in the Pre-Doctoral Program. While there, I investigated techniques for empirically diagnosing and empirically improving HRCCS. By forward-modeling the complex and non-linear data-processing of HRCCS, I demonstrated that two observing regimes — high photon noise and high telluric variability — can yield biased inferences of atmospheric properties. Beyond these results, I applied data-driven methods to improve atmospheric parameter inference, which is rapidly becoming a computationally intractable problem. Taking this approach produced an order of magnitude increase in memory efficiency and an order of magnitude decrease in compute hours. While describing this work, I’ll also provide an overview of useful data-driven approaches to Bayesian sampling, empirical modeling, and simulation optimization methods. By appropriately choosing when to model systems empirically in place of physically, it is possible to both comfortably push into lower signal-to-noise regimes and to infer more complex physics.
Date: Wednesday 20-Sep-2023
Speaker: Paul Draghis (University of Michigan)
Title: Exploring the Spin Distribution of Stellar Mass Black Holes
Abstract: The launch of NuSTAR and the increasing number of binary black hole (BBH) mergers detected through gravitational wave (GW) observations have exponentially advanced our understanding of black holes. Despite the simplicity owed to being fully described by their mass and angular momentum, black holes have remained mysterious laboratories that probe the most extreme environments in the Universe. While significant progress has been made in the recent decade, the distribution of spin in black holes has not yet been understood. Exploring the spin distribution across stellar-mass black holes provides an insight into the formation of black holes, supernova events, collapsar models, gamma-ray bursts, the formation and evolution of X-ray binary systems and binary black hole systems, and the physics of accretion. The preferred spin measurement techniques are “continuum fitting” (see e.g., Gou et al. 2009) and “relativistic reflection” (see e.g., Miller 2007). Relativistic reflection is independent of black hole mass, accretion rate, and distance to the system, making it a more versatile technique. Up until now, spin measurements for the same black hole using the two methods did not always agree, and even measurements using the same method did not always adopt the same sets of assumptions and theoretical prescriptions. Our work provides a pipeline that uses state of the art relativistic reflection models to fully explore the entire physical parameter space and to provide a uniform treatment of a large sample of black holes. Using our pipeline on NuSTAR data, we measure more than a dozen new black hole spins in X-ray binary systems, significantly expanding the measured sample size, and remeasure existing black hole spins in order to compile a distribution of measurements made using entirely consistent methods and systematic uncertainties. Additionally, we analyze possible observational and modeling biases of the sample and compare the distribution to that of spins measured in mergers of binary black hole systems observed through gravitational waves, in order to offer a unified view of black hole spin evolution. We find that the observed spin distribution of black holes in X-ray binaries is sharply peaked at high values, incompatible with the spin distribution inferred based on GW signals from merging BBH, suggesting that the observed black hole distributions are inherently different. Understanding the distribution of black hole spins in X-ray binaries paves the way for future Gravitational Wave efforts and for future X-ray missions such as XRISM, HEX-P, AXIS, or ATHENA.
Date: Wednesday 27-Sep-2023
Speaker: Ari Brill (NASA GSFC)
Title: Investigating the Variability of Flaring Gamma-ray Blazars with Stochastic Processes and Deep Learning
Blazars exhibit stochastic flux variability at virtually all observed wavelengths and timescales, from minutes to years, but the physical origin of this variability is poorly understood. For 15 years, the Fermi Large Area Telescope (Fermi-LAT) has conducted regular space-based monitoring of thousands of blazars in the high-energy gamma-ray band. Measurements with Fermi-LAT of several bright flat spectrum radio quasars (FSRQs) have revealed extremely heavy-tailed flux distributions described well by the inverse gamma distribution, as well as complex short-timescale (days or less) variability patterns involving multiple bursts. We explain these observations using a model in which gamma rays are emitted in discrete bursts that are individually unresolved and averaged over within time bins. The statistical model replicates the typical gamma-ray variability properties of FSRQs and BL Lac objects. Often, however, the light curves of flaring blazars exhibit a range of variability patterns that high-level summary statistics or parameterizations may not be able to fully describe. Measurement errors, upper limits, and missing data further complicate the analysis. To interpret complex light curves more effectively, I demonstrate a novel method based on self-supervised deep learning. In this approach, a deep neural network is trained without external supervision to encode a model-independent representation of the stochastic variability structure in the data. The output of the self-supervised model can then be analyzed to extract scientifically relevant information.
Date: Wednesday 04-Oct-2023
Speaker: Hayley Beltz (University of Maryland)
Title: Ultrahot Jupiters and the Magnetic Circulation Regime
Abstract With orbital periods of only a few days, ultrahot Jupiters (Teq>2200) are the hottest class of exoplanets known. Due to their proximity to their host star, these planets are expected to have tidally synchronous orbits, resulting in a permanent dayside and nightside and complex temperature structures around the planet. Using 3D numerical atmospheric modeling, I simulate these atmospheres with our state-of-the-art kinematic MHD approach. When applied, the planetary circulation changes to a magnetic circulation regime. In this talk I describe this regime as well as observational signatures present in high-resolution spectroscopy.
Date: Wednesday 11-Oct-2023
Speaker: Viraj Pandya (Flatiron CCA)
Title: Galaxies Going Bananas: The Surprising 3D Geometry of High Redshift Dwarf Galaxies from JWST
Abstract I will present surprising observational results on the 3D geometry of high-redshift star-forming galaxies based on JWST-CEERS Early Release Science data. Using a novel differentiable Bayesian model with Hamiltonian Monte Carlo, I will show that there are many more flat, elongated dwarf galaxies than there are round, circular dwarf galaxies seen in projection in the early Universe. This puzzle can only be explained if ~50-80% of high-redshift dwarfs are not axisymmetric (oblate) disks or spheroids. Instead, they must be significantly flattened in two directions either as prolate ellipsoids (i.e., cigars) or as oval (triaxial, i.e., surfboard-shaped) disks. These findings imply that galactic disks settle not only in thickness but also from oval to more circular shapes with time. Both prolate and triaxial ellipsoids trace out a "banana" on the projected b/a-log(a) diagram with an excess of low b/a (edge-on) and deficit of high b/a (round) objects as observed. The stellar masses of these objects are such that they would be the progenitors of galaxies like the Milky Way, implying that our own Galaxy may have gone through a prolate/triaxial phase in its past. Exploratory comparisons of high-probability prolate and oblate candidates reveal that the two are remarkably similar in terms of many properties except for dust and kinematics, which may be the key to distinguishing between these two types of 3D geometry. I will rule out selection effects, consider follow-up prospects and discuss theoretical implications for both galaxy formation and cosmology.
Date: Wednesday 18-Oct-2023
Speaker: Eltha Teng (UC San Diego)
Title: Revealing the Drivers of CO-to-H2 Conversion Factor Variation and its Impact on Star Formation Efficiency
Abstract Star formation in galaxies is governed by the amount of molecular gas and the efficiency that gas is converted into stars. However, assessing the amount of molecular gas relies on the CO-to-H2 conversion factor (α_CO), which is known to vary with molecular gas conditions like density, temperature, and dynamical state – the same conditions that also alter star formation efficiency. The variation of α_CO, particularly in galaxy centers where α_CO can drop by nearly an order of magnitude, thus causes major uncertainties in current molecular gas and star formation efficiency measurements. Using ALMA observations of multiple 12CO, 13CO, and C18O lines in several barred galaxy centers, we found that α_CO is primarily driven by CO opacity changes and therefore shows strong correlations with observables like velocity dispersion and 12CO/13CO line ratio. Motivated by these results, we have constructed a new α_CO prescription which accounts for emissivity effects in galaxy centers and verified it on a set of barred and non-barred galaxies with measured α_CO values from dust. Applying our new prescription to 65 galaxies from the PHANGS survey, we found an overall 3x higher star formation efficiency in barred galaxy centers than in non-barred centers, and such a trend is obscured when using a MW α_CO or other existing prescriptions. Our results suggest that the high star formation rates observed in barred centers are not simply due to an increased amount of molecular gas but also an enhanced star formation efficiency compared to non-barred centers or disk regions.
Date: Wednesday 25-Oct-2023
Speaker: Nhut Truong (NASA GSFC)
Title: Probing the effects of SMBH feedback on the CGM with Line Emission Mapper (LEM)
Abstract: The circumgalactic medium (CGM) plays an essential role in the formation and evolution of galaxies. It serves as the interface where inflows of cosmic gas and outflows driven by galactic feedback occur. Supermassive black holes (SMBH) are the power engine of AGN feedback, generating large-scale outflows, which subsequently deposit mass, energy, and metals into the CGM significantly altering its physical state. However, probing these SMBH-CGM interactions is proven challenging with current CCD-based X-ray telescopes. In this talk, I’ll present a forecast study on the prospects of the “Line Emission Mapper” (LEM) mission, which is currently planned as a proposal for a NASA Probe mission. LEM is a microcalorimeter-based X-ray telescope with large field of view (30’x30’) and advanced spectral resolution (~2 eV). Based on analysing data from current cosmological simulations, including IllustrisTNG, EAGLE, and SIMBA, I’ll highlight LEM capabilities in probing key effects of SMBH feedback on the CGM through the observation of prominent emission lines like OVIIf, OVIII, and FeXVII.
Date: Wednesday 01-Nov-2023
Speaker: Joanna Berteaud (University of Maryland)
Title: Millisecond pulsars in the Galactic bulge: An adventure across the electromagnetic spectrum
Abstract: About fifteen years ago, the Fermi Large Area Telescope discovered a mysterious gamma-ray signal strongly peaked towards the Galactic center, the so-called Galactic center or GeV excess. Should this emission be caused by millisecond pulsars (MSPs) in the Galactic bulge, their discovery would greatly benefit many domains of physics and astrophysics. Thanks to a simulation of the population of MSPs in the Milky Way, we have shown that some bulge MSPs have likely been detected in past X-ray observations. Motivated by this result, we have selected bulge MSP candidates among yet unidentified X-ray sources according to their spectral shape and their lack of strong UV, optical and IR counterparts. We are now conducting deep targeted pulsation searches at radio wavelengths, aiming to unveil the first direct evidence of a population of bulge MSPs.
Date: Wednesday 08-Nov-2023
Speaker: Robert Caddy (University of Pittsburgh)
Title: Exascale MHD Simulations with Cholla
Abstract: The impact of magnetic fields on galactic outflows and winds is not well understood. Modeling these effects has proven challenging in large part due to the cost of MHD simulations: across the field, different compromises between simulation cost, resolution, and robustness lead to substantially different results. This tension is particularly noticeable in galactic outflows where the effects of magnetic fields in simulations range from non-existent to dramatic. To resolve this tension and accurately model the galactic dynamo and magnetic fields in the circumgalactic medium (CGM) we need high resolution and robust MHD simulations. In this talk, I will describe my work to add magnetohydrodynamics to Cholla, a massively parallel, GPU accelerated, code for modeling astrophysical fluid dynamics. With Cholla MHD, and the new Frontier supercomputer, we can robustly simulate a Milky Way like galaxy with resolution elements of a few parsecs in size throughout the entire galaxy and CGM and resolve this tension. As scientific codes, like Cholla, grow scientific software best practices become especially important. I will also discuss some of the best practices that we have implemented, the challenges in doing so, and their benefits.
Date: Wednesday 15-Nov-2023
Speaker: Lisa Dang (University of Montreal)
Title: Exploring the Diversity of Highly Irradiated Exoplanets by Revealing their Multidimensional Nature
Abstract: Although we will never get the same level of details for exoplanets as we do for Solar System bodies, the large diversity of exoplanets revealed by exoplanet hunting missions, e.g. Kepler and TESS, provide thousands of study cases to refine formation and evolution pathways as well as theories of how their climate is shaped by their environment. Particularly amenable for atmospheric characterization, short-period exoplanets with dayside blasted with stellar radiation are some of the best-characterized exoplanets to this day. Due to their synchronous rotation, they exhibit large day-to-night differences and their observation can be difficult to interpret without a full understanding of their “3D-ness”. Fortunately, phase-resolved observations can reveal the inhomogeneous nature of these exoplanets and provide a more comprehensive view into their atmosphere. In this talk, I will present what we have learned from Spitzer phase curve observations of a variety of close-in planets from large hot Jupiters to small lava planets. More excitingly, I will also discuss the continuation of Spitzer’s legacy in the era of JWST and ground-based high-resolution spectroscopy.
Date: Wednesday 29-Nov-2023
Speaker: Sagnick Mukherjee (UC Santa Cruz)
Title: Constraints on Atmospheric Mixing in Brown Dwarf and Exoplanet Atmospheres in the JWST Era
Abstract: JWST has brought on a new dawn of discovery and has completely revolutionized our understanding of exoplanet atmospheres. It has brought the opportunity to understand exoplanetary atmospheres in unprecedented detail. One of the least constrained atmospheric processes in exoplanet atmospheres is atmospheric vertical dynamics, often parametrized with the eddy diffusion parameter - Kzz. The vertical mixing process significantly impacts atmospheric chemistry and clouds, but it's currently uncertain by 6-8 orders of magnitude. I will briefly introduce atmospheric mixing and show how uncertainty in the Kzz parameter leads to uncertainty in our overall understanding of exoplanet atmospheres. I will show how the similarities between brown dwarf and exoplanet atmospheres can help us to constrain and understand the vertical mixing process better. I will present results where, using a new generation of state-of-the-art Sonora Elf Owl atmospheric models and spectroscopic data from JWST, Spitzer, and AKARI telescopes, we have constrained the vigor of atmospheric vertical mixing in the deep atmospheres of brown dwarfs. We find that the observed Kzz constraints from our work are several orders of magnitude lower than theoretical predictions. I will present what this theory-observation mismatch teaches us about the deep atmospheres of directly imaged planets and brown dwarfs. I will also discuss how we are trying to measure Kzz in transiting exoplanet atmospheres using the PICASO model and observations from our MANATEE JWST collaboration.
Date: Wednesday 06-Dec-2023
Speaker: Lia Hankla (University of Maryland)
Title: Kinetic and Two-Temperature Physics of Black Hole Accretion Disks and X-ray Coronae
Abstract: Understanding the plasma physics of accretion disks and coronae around black holes is crucial for interpreting the radiation observed from these systems. However, these plasmas span several different physical regimes. They can be highly collisional and well-described by a single temperature, or collisionless with nonthermal particles that have been accelerated to high energies. My work brings small-scale kinetic and two-temperature physics into the global setting of the accretion disk/corona system. I first use particle-in-cell (PIC) simulations to understand turbulence and particle acceleration in a collisionless, magnetized plasma. Next, I build an analytic model using prescriptions from PIC simulations to demonstrate an observable power-law from within the plunging region of a black hole. Finally, I use general relativistic magnetohydrodynamic (GRMHD) simulations to examine the impact of two-temperature physics on the radial structure of the full accretion disk.
Date: Wednesday 13-Dec-2023
Speaker: Corey Spohn (NASA GSFC)
Title: Scheduling direct imaging observations and surveys of exoplanets already detected with radial velocity
Abstract: NASA is in the early planning phase of a flagship, space-based telescope called the Habitable Worlds Observatory (HWO), which has a preliminary goal of directly imaging 25 Earth-like exoplanets to search for biosignatures. Earth-like exoplanets will be among the faintest objects ever observed, possibly requiring weeks of observation time per target. This makes the efficiency of the observations incredibly important for both the success of the survey and maximizing the time available for other science programs. Obviously, it is easier to image an Earth-like exoplanet if it has already been detected and the expectation is that radial velocity (RV) data will be used to inform HWO's survey. This presentation demonstrates, with many fun animations, what RV data can and cannot provide for imaging, how to estimate when a planet detected with RV will be detectable with an imaging instrument, and how to solve the Travelling Salesman-like problem of generating efficient imaging schedules with constraint programming. Full simulations show that an HWO-like telescope can directly image 32 Earth-like exoplanets in 375 days of observation time if a 5-year extreme precision RV survey takes place before the launch of HWO.
