Date: Wednesday 05-Feb-2025
Speaker: Hayley Beltz (University of Maryland)
Title: 3D Modeling of Eccentric Hot Jupiters with a Side of Magnetic Effects
Abstract: Eccentric Hot Jupiters experience variable irradiation throughout their orbit and provide insight into formation mechanisms of hot Jupiters. In this talk, I discuss my 3D atmospheric modeling work of TOI-150b: an eccentric (e=0.26) hot Jupiter whose equilibrium temperature varies by over 400K throughout its orbit. I will discuss the atmospheric effects of an eccentric orbit, and how they manifest observationally. Additionally, I will present kinematic MHD models of the planet and discuss how time varying irradiation and spatially varying drag change the atmospheric structure of the planet.
Date: Wednesday 12-Feb-2025
Speaker: Brian Clark (University of Maryland)
Title: POSTPONED
Date: Wednesday 19-Feb-2025
Speaker: Tatsuya Akiba (University of Colorado Boulder)
Title: On the Hunt for Recoiling Supermassive Black Holes Using Tidal Disruption Events
Abstract: Following the merger of two supermassive black holes (SMBHs), a gravitational wave recoil kick is imparted on the merger remnant due to the anisotropic emission of gravitational waves. These recoil kicks can be as high as ~10^3 km/s which exceeds the escape velocity of most galaxies, and these super-kicks can lead to ejected or rogue SMBHs. While there are several candidates, recoiling SMBHs are difficult to uniquely identify since observations so far cannot rule out alternate possibilities such as dual active galactic nuclei. Here, we present a novel observational signature: off-nuclear or extragalactic tidal disruption events of stars by recoiling SMBHs. When a super-kick is imparted on a SMBH, there is a tightly bound cluster of stars recoiling with it. We show that these bound stars should be in an eccentric, apse-aligned disk where stars are strongly torqued to extremely high eccentricities. The corresponding rate of tidal disruption events in an eccentric disk is expected to be of order ~0.1 yr^-1 gal^-1, several orders of magnitude higher than in an isotropic cluster. We show that this high expected rate of tidal disruption events in an eccentric disk implies off-nuclear or extragalactic tidal disruption events as viable and likely observables of a recoiling or rogue SMBH.
Date: Wednesday 26-Feb-2025
Speaker: Michael Moss (NASA GSFC)
Title: Understanding the Instrumental Bias of Durations and Fluences for High-z GRBs
Abstract: GRBs are the most powerful explosions in the universe. These catastrophic events are the signatures of highly relativistic material being jetted away from a compact object newly formed by either the core-collapse of a massive star or the merger of a compact binary system. Due to their incredible luminosities, GRBs can be detected across the entire observable universe and can be used as cosmological probes. We investigate how measurements of Gamma-Ray Burst (GRB) prompt emission duration and fluences are affected by increasing distance to the source. We selected a sample of 26 bright GRBs with measured redshifts š§ < 1 observed by the Burst Alert Telescope (BAT) on board the Neil Gehrels Swift Observatory (Swift). We simulated what BAT would have observed if the GRB were at larger redshifts up until the point the burst signal was no longer detectable above background and measured the duration and fluence of the simulated count light curves. As expected, we found that almost all durations (fluences) measured for simulated high-š§ GRBs were shorter (less) than their true durations (energies). The underestimations are due to low signal-to-noise emission being lost into the background, i.e., the so-called ātip-of-the-iceberg" effect. The amount of signal lost depends on the profile and intensity of the synthetic light curve, making the true signal unrecoverable without prior knowledge of the underlying light curve and, due to the uniqueness of GRB light curve profiles, means that there is no common behavior in the evolution of the measured durations with redshift. We then compared our simulated high-š§ sample (i.e., š§ > 3) to a sample of observed high-š§ bursts and found that the two samples were consistent with being drawn from the same underlying population. This implies that (i) prompt emission durations and fluences of high-z GRBs observed by Swift/BAT may all be underestimations, sometimes by up to an order of magnitude or a factor of ā¼ 2, respectively, and (ii) GRBs do not require any parameter evolution with increasing redshift.
Date: Wednesday 05-Mar-2025
Speaker: Brian Welch (NASA GSFC)
Title: Exploring the Smallest Scales of Galaxy Formation with JWST
Abstract: Galaxies are not monolithic; they are composed of substructures on many scales. In order to fully understand galaxy formation and evolution, it is important to study the structures that make up galaxies, and to understand how they interact to form the larger system. Gravitationally lensed galaxies, which appear magnified by the mass of a foreground object, provide an opportunity to study these substructures directly in the distant universe. The high spatial resolution and sensitivity of JWST coupled with the magnification of gravitational lensing is particularly adept at characterizing the substructures of distant galaxies. In this talk I will discuss how JWST observations are shaping our understanding of the star clusters and HII regions that comprise distant galaxies. I will particularly focus on recent results from the Sunburst Arc at z=2.4, where we have measured the chemical abundance pattern of an individual HII region along with the stellar population of the star cluster at the center of this region. We find that a population of Wolf-Rayet stars in this cluster are driving an enhanced nitrogen abundance in the surrounding nebula, directly connecting the massive stellar populations to their environmental impacts. Additional detailed spectroscopic measurements of lensed star clusters and HII regions can shed more light on how massive stellar populations drive the chemical evolution of galaxies across cosmic time.
Date: Wednesday 12-Mar-2025
Speaker: Ryan Golant (Columbia University)
Title: To B or not to B: The Creation and Consequences of Astrophysical Magnetic Fields
Abstract: Magnetic fields play a key role in a wide array of astrophysical systems, yet, in many cases, the origins of these magnetic fields remain unclear. In this talk, I will describe one possible channel for generating and amplifying magnetic fields in the low-density Universe: the fully-kinetic, collisionless dynamo. In particular, I will discuss the potential link between the collisionless dynamo and the magnetic fields in cosmic voids, the emptiest regions of the cosmos. I will also show some preliminary analysis of the properties of the tangled field lines produced by the collisionless dynamo and by large-amplitude kinetic turbulence, which may be relevant to the confinement of Galactic cosmic rays.
Date: Wednesday 19-Mar-2025
Speaker: NO SEMINAR
Title: SPRING BREAK
Date: Wednesday 26-Mar-2025
Speaker: Philip Mocz (Flatiron Institute)
Title: Differentiable Astrophysics Simulations
Abstract: Reverse-mode auto-differentiable simulations offer a powerful framework for solving inverse problems, optimizing design, and conducting sensitivity analysis across scientific disciplines. In this talk we present work in progress on building a differentiable astrophysics code using JAX, supporting compressible hydrodynamics, magnetohydrodynamics (MHD), scalar fields, and self-gravity. We demonstrate its applications in solving inverse problems on some toy problems and showcase the code's scalability on distributed GPU clusters. These early findings highlight the potential of differentiable programming in accelerating scientific discovery in astrophysics through AI-driven optimization and inverse problem-solving.
Date: Wednesday 02-Apr-2025
Speaker: Johannes U. Lange (American University)
Title: Cosmic tensions in the DESI era: Is lensing still low?
Abstract: Recent cosmology experiments have uncovered several potential tensions in our standard model of the Universe. These findings, including the well-known Hubble tension and new evidence for time-evolving dark energy, indicate that we may be at the cusp of transforming our view of the Universe. In this talk, I will discuss the so-called lensing-is-low problem, the finding that the measured gravitational lensing amplitude around massive galaxies is smaller than expected from dark matter simulations. Since its first discovery around a decade ago, we have made significant advances in our understanding of cosmic large-scale structure and galaxy formation. Similarly, we now have access to much larger data sets such as new measurements from the Dark Energy Spectroscopic Instrument (DESI) and stage-III weak lensing surveys such as the Dark Energy Survey (DES) and the Kilo-Degree Survey (KiDS). In this talk, I will present new results from recent DESI, DES, and KiDS data on the potential origin of the lensing-is-low problem. By examining this new data, I will discuss whether we are closer to resolving this issue or whether it continues to challenge our standard model of the Universe.
Date: Wednesday 09-Apr-2025
Speaker: Cosima Eibensteiner (NRAO)
Title: POSTPONED
Date: Wednesday 16-Apr-2025
Speaker: Cuncheng(Mandy) Chen (Caltech/Carnegie Observatory)
Title: Unveiling Turbulence Drivers in Quasar Halos: Insights from IFS Observations
Abstract: The recent advent of high-throughput integral-field-unit spectrographs (IFS) has enabled unprecedented sensitivity in mapping low-density gas, opening new avenues for probing the dynamical states of the interstellar and circumgalactic medium (ISM and CGM). In this talk, I will present an investigation into turbulence and energy injection mechanisms in luminous quasar halos across redshifts 0.5
Date: Wednesday 23-Apr-2025
Speaker: Nico Garavito-Camargo (University of Arizona)
Title: Galaxies in disequilibrium and the nature of Dark Matter
Abstract: Galaxies in the local universe exhibit signatures of disequilibrium at various scalesāfor example, disk perturbations such as warps, phase-space spirals, or lopsided stellar distributions. At even larger scales, the distribution of satellite galaxies is highly asymmetric in systems like the Milky Way and Andromeda.
In this talk, I will discuss how the presence of disequilibrium encodes valuable information about the dynamical state of dark matter halos and, consequently, the nature of dark matter itself. I will show how Basis Function Expansions (BFE) are an ideal tool to characterize the interplay between dark matter and baryonic matter in cosmological simulations. As a result, BFE enable the quantification of the dynamical state of galaxies across different dark matter models, allowing for direct comparison with the signatures of disequilibrium observed in the local universe.
Date: Wednesday 30-Apr-2025
Speaker: Mark Avara (Georgia Institute of Technology)
Title: A āLeave No Traceā Approach to Simulations of Black Hole Accretion
Abstract: GRMHD simulations of accretion disks provide the bedrock for interpreting many modern observations of black hole accretion, especially direct radio imaging, which relies on large simulations libraries. Unfortunately, even with the most careful choices for initial conditions, simulations can retain an imprint of those biasing choices even at late times. Reconciling this intrinsic bias, which may not well represent actual astrophysical behavior, remains an outstanding and, I will argue, under-addressed theoretical problem. By making new, but not radically different choices in these initial conditions, I will demonstrate accretion disk simulations we have performed that represent a cost-saving approach to reaching long evolution timescales, and which demonstrate significant differences in comparison to the traditional approach. I will explain the fundamentals of our approach, a suite of new simulations of hot accretion flows in full 3D-GRMHD, and how these simulations may shift our interpretation of observations.
Date: Wednesday 07-May-2025
Speaker: Guochao Sun (Northwestern University)
Title: Timing Cosmic Reionization with the 21 cm and Near-infrared Backgrounds
Abstract: The timeline of cosmic reionization remains uncertain despite sustained efforts to study how the ionizing output of early star-forming galaxies shaped the intergalactic medium. Intensity mapping (IM) offers a promising way to reliably constrain the reionization timeline by measuring the spatial fluctuations in various types of cosmic background radiation. I will introduce our recent investigation into the opportunities, challenges, and prospects for timing the reionization process using the cross-correlation between the HI 21 cm line and the cosmic near-infrared background (NIRB) ā two interesting cosmological signals that are now being pursued by the Square Kilometre Array and the SPHEREx experiments, respectively.
