Abstract

Theoretical astrophysicists are building models that predict the formation of the first generations of stars and their effect on the intergalactic medium. Experimental astrophysicists are building large radio telescopes that in principle are sensitive enough to detect the 21cm signals emitted by neutral hydrogen during this important epoch in the universe's history. The Hydrogen Epoch of Reionization Array (HERA) is a second-generation low-frequency radio array designed to detect these signals. HERA is under phased construction in the Karoo Highlands of South Africa, and we are collecting data as the array grows. The status of measurements with HERA will be reported, including 21cm power spectrum limits and their implications for the heating of the intergalactic medium.

Abstract

Accreting compact objects such as neutron stars and black holes in X-ray Binaries (XRBs) and the supermassive black holes at the center of galaxies, called Active Galactic Nuclei (AGN), exhibit complex variability in their emission. The variability is ubiquitous and operates over a wide range of timescales from seconds up to years. Traditional studies of XRBs and AGN in large time domain surveys aim to connect stochastic characterizations of the variability to the underlying physical processes responsible for accretion onto the compact object, but frequently fail to capture nonlinear or higher-order modes of variability. Methods from topology and nonlinear dynamics distinguish between deterministic, chaotic, and stochastic behavior, and can be used to identify modes of variability that relate to dominant accretion processes. I will review several applications of nonlinear dynamics and chaos theory to the study of both XRBs and AGN that show a connection between nonlinear behavior and the underlying accretion processes. I will outline an approach from nonlinear time series analysis - called recurrence analysis - applied to the systematic and ensemble study of variable sources that can inform current and future time domain missions.

Abstract

To understand our origins, we must understand the role that interstellar dust plays in delivering heavy elements to planetary systems. This requires an understanding of the chemical composition, sizes and shapes of interstellar dust: astromineralogy. X-ray observations provide the most direct means for astromineralogy through the absorption signatures of dust imprinted in the spectra of bright Galactic X-ray sources. I'll review the latest results in astromineralogy and our work on connecting interstellar dust with suspected extra-solar grains collected in our own Solar System. I'll also describe how the recently launched XRISM space mission is poised for major breakthroughs in astromineralogy. Finally, I'll show how I am adapting this work to study aerosols in exoplanet atmospheres, which have the power to alter planetary climate and habitability. By investigating exoplanet transits at short wavelengths, we are opening new windows on aerosols, atmospheric escape, and the role that stars play in sculpting planets.

Abstract

We measured the dipole of the diffuse γ-ray background (DGB), identifying a highly significant time- independent signal coincidental with that of the Pierre Auger UHECR. The DGB dipole is determined from flux maps in narrow energy bands constructed from 13 yr of observations by the Large Area Telescope (LAT) of the Fermi satellite. The γ-ray maps were clipped iteratively of sources and foregrounds similar to that done for the cosmic infrared background. The clipped narrow energy band maps were then assembled into one broad energy map out to the given energy starting at E = 2.74 GeV, where the LAT beam falls below the sky's pixel resolution. Next we consider cuts in Galactic latitude and longitude to probe residual foreground contaminations from the Galactic plane and center. In the broad energy range 2.74 < E < 115.5 GeV, the measured dipoles are stable with respect to the various Galactic cuts, consistent with an extragalactic origin. The γ-ray sky's dipole/monopole ratio is much greater than that expected from the DGB clustering component and the Compton-Getting effect origin with reasonable velocities. At (6.5-7)% it is similar to the Pierre Auger UHECRs with EUHECR >8 EeV, pointing to a common origin of the two dipoles. However, the DGB flux associated with the found DGB dipole reaches parity with that of the UHECR around EUHECR < 1 EeV, perhaps arguing for a non-cascading mechanism if the DGB dipole were to come from the higher-energy UHECRs. The signal-to-noise ratio of the DGB dipole is largest in the 5-30 GeV range, possibly suggesting the γ-photons at these energies are the ones related to cosmic rays.

Abstract

Observations of polarized dust emission from the Planck satellite challenged our classical models of interstellar dust grains. In this talk, I will argue that initially distinct populations of carbonaceous and silicate stardust get rapidly homogenized in the ISM into a composite material ("astrodust"). I will show that the astrodust+PAH model is compatible with current observational constraints on dust extinction and emission in the diffuse ISM, and that it provides a more natural explanation for the observed polarized emission than do two-component models. I will discuss implications for the lifecycle of dust in galaxies and highlight the power of future polarimetric observations on platforms like the PRIMA far-infrared probe to illuminate the formation and evolution of dust.

Abstract

Primordial black holes are black holes that may have formed in the early Universe. Their masses potentially span a range from as low as the Planck mass up to many orders of magnitude above the solar mass. This, in particular, includes those black holes recently discovered by LIGO/Virgo, and these may conceivably be primordial in origin. Furthermore, there are now numerous recent hints for compact bodies constituting (part of) the dark matter. After a general introduction of the topic, I will talk about those, their connection and future detection possibilities.

Abstract

Launched from Cape Canaveral on July 23, 1999, Chandra is the NASA Great Observatory for the X-ray band. Now in its 25th year of operations, Chandra has revolutionized our view of the X-ray sky, thanks to its sub-arcsecond imaging and spectral capabilities. This talk will introduce Chandra and will then concentrate on Chandra's contributions to our understanding of one of the open mysteries of Astrophysics: "Feedback" - How is it that supermassive black holes (SMBH, with mass ~10 millions to billions of that of our Sun) in their Active Galactic Nucleus (AGN) phase interact with the billion times larger host galaxy and shape its evolution? Some form of "feedback" is needed to explain universe survey data and simulations, but observationally constraining its physical mechanisms is still an open field of research. Observing nearby AGN host galaxies with Chandra we have studied in detail the spatial and spectral properties of the AGN-host interaction, from spatial scales ranging from a few kiloparsecs (~1/10 of the radius of the Milky Way) to ~30 parsec (~3 times the size of the Orion Nebula). In these galaxies, we have found evidence for all four vectors of feedback: the photons emitted by the AGN (radiation), relativistic jets embedded in the host galaxy disk, and both nuclear winds and, most recently, very fast nuclear winds. These observations are pointing to a revision of the AGN standard model, one where interactions with the host interstellar medium and molecular clouds are important parts of the AGN phenomenon.

Abstract

Dynamical resonances are essential to the evolution of disk galaxies. These resonances arise in the presence of massive structures, like Giant Molecular Clouds, spiral patterns and satellites, and reshape a galaxy as they diffuse, shepherd and rearrange stellar populations. Until recently, many of the signatures that would reveal the past dynamical evolution of the Milky Way have been obscured. We are now entering a new era, rich with high precision chrono-chemodynamic (kinematic, chemical, and age) data from current and upcoming surveys. These data are revealing complex post-resonant structures in this multidimensional space and highlight a critical need for new theoretical tools to disentangle these signatures. In this talk I will demystify the physics that governs various resonant phenomena and then discuss, using demonstrative examples, how chrono-chemodynamic space and novel orbital analysis techniques are opening exciting pathways to reconstruct the history of the Milky Way. At the end of this talk I will touch on the status of the next generation gravitational wave experiment in the US, Cosmic Explorer. In particular, I will describe how our approach to site evaluation and selection integrates local and Indigenous community partnerships and why this approach is critically important to how we do science.

Abstract

Massive black hole binaries (MBHBs) with masses of 0.1-10 million Solar masses in low-redshift galaxies will be among the loudest sources of gravitational radiation at milli-Hz frequencies observable with the future Laser Interferometer Space Antenna (LISA). While the detection of such systems with LISA will be groundbreaking, we can learn a great deal more if we can also detect their electromagnetic (EM) counterparts. To help identify the counterpart, early warning from LISA on-the-fly parameter estimation will yield time-evolving constraints on sky localization, luminosity distance, chirp mass, and mass ratio. But developing strategies to pick out the EM counterpart from all the candidates in the multi-dimensional error volume of the gravitational wave source requires a detailed inventory of this volume and a systematic evaluation of the credentials of the astrophysical sources within it. With this in mind, I will review EM methods used to search for supermassive black holes, with an emphasis on some of the challenges these methods may face when translated down to lower masses relevant for LISA. I will also discuss efforts to incorporate large-area astronomical surveys in this context in order to inform joint multi-messenger detections.

Abstract

The Allen Telescope Array (ATA) is a radio interferometer hosted on the Hat Creek Radio Observatory, a facility owned and operated by the SETI Institute. Comprised of 42 elements, each 6.1m in diameter, the ATA is the first instrument built from the ground up to perform the search for extraterrestrial intelligence (or SETI). Operating within the 1 to 12 GHz frequency spectrum, the ATA captures electromagnetic radiation, with its digitized data processed centrally in a dedicated signal processing cluster onsite. Observers can then access this data to perform a plethora of scientific investigations. This spans the study of the enigmatic Fast Radio Bursts, brief bursts of radio energy with elusive origins, to the exploration of slow transients like supernovae and tidal disruption events.

In this talk, I will give an overview of the historical significance of radio astronomy emphasizing its pivotal role in astrophysics, and then delve into the ongoing upgrades of the ATA, elucidating the telescope's potential to advance both slow and fast transient science and the exciting prospects it holds for future discoveries.

Abstract

Due to galactic mergers, massive black hole binaries are thought to reside in the cores of numerous galaxies. As the massive black holes migrate inwards, they will eventually emit gravitational waves, which are expected to be detected by LISA. A critical component to understanding where and how black holes merge and how they shape galactic evolution is host galaxy identification, which relies on electromagnetic (EM) observations. In my talk, I will show two novel tell-tale EM signatures that would provide strong evidence for a black hole binary before or during a merger. The first is when the binary is seen close to edge on; in that case, the binary produces self-lensing flares (SLFs) when one of the black holes moves in front of the background black hole. This causes the emission to be gravitationally lensed. This signal can additionally hold information on the size and shape of the emission morphology of the lensed black hole. Secondly, I will argue that when the binary is close to merger, the accretion is disrupted, turning the binary X-ray dark. I will argue that the upcoming time-domain surveys and X-ray mission might be able to observe these signatures, and that they could be crucial for LISA source identification.

Abstract

Between the discovery of gravitational waves from over a hundred merging compact objects and the advent of micro-arcsecond astrometry realized by the Gaia space telescope, the study of the complexities of binary stellar evolution - including mass transfer, tides, and r-process nucleosynthesis - has taken on a new urgency. In this talk, I will describe the current status of modeling binary star populations as well as several critical shortcomings that multiple groups throughout the field are working to address. In particular, I will introduce our binary population synthesis code, POSYDON, which combines large grids of MESA simulations with machine learning algorithms to produce the next generation of binary population models. In addition to their implications for the origin of gravitational wave sources, these models promise to improve our interpretation of a wide range of stellar population observations, ranging from nearby open clusters to galaxies at high redshift. Finally, I will speculate on the importance of forthcoming observations to constrain key binary evolution processes.

Abstract

The advent of the James Webb Space Telescope has opened our first window onto detailed abundance studies of high redshift (z > 6) extreme emission line galaxies. The coming years will provide the first spectroscopic samples of these galaxies in the epoch of reionization with which we can constrain their chemical compositions and histories and resulting conditions that shaped the escape of ionizing photons. As we build our samples of high-ionization nebular emission lines and absorption profiles of outflowing gas, we can begin to understand how the time-dependent nature of elemental production affects our interpretation of different epochs of galaxies and the resulting strung-together evolutionary story. I will present recent HST and JWST observations of the dynamic ISM across multiple epochs and the lessons we are learning about chemical evolution, star formation histories, and feedback prescriptions.

Abstract

The circumgalactic medium (CGM) is an important component of a galaxy, at the interface between the intergalactic medium and the galactic disk, playing a critical role in galaxy formation and evolution. The X-ray missions Chandra, XMM-Newton, and Suzaku opened a new window on CGM studies, allowing us to probe the warm-hot gas where most of the galactic baryons reside. In over two decades since the launch of the great X-ray observatories, we have made great strides in understanding the CGM, but significant challenges remain. I'll review our progress so for, highlight new discoveries, outline the open questions, and discuss paths for future progress.

Abstract

Metallicities of galaxies at all redshifts are critical for deciphering a plethora of physical and evolutionary processes taking place among and inside galaxies, including star formation, stellar feedback and interstellar/intergalactic chemical enrichment. Studies of local star-forming galaxies (SFGs) can be performed at exquisite signal-to-noise and spatial resolution, which are not achievable at higher redshift. Therefore, these studies establish a baseline in understanding how gas and stellar properties evolve through cosmic time. I take advantage of the unique FUV spectroscopic capabilities of the Hubble Space Telescope, along with the high sensitivity in the MIR of the James Webb Space Telescope (JWST), and complement our space observations with those from several ground-based telescopes to investigate a sample of local galaxies, which span a wide range in morphological types, metallicities and star formation rates. The results from my metallicity studies of nearby (<100 Mpc) galaxies will be discussed with a dedicated focus on their stellar, neutral-gas, and ionized-gas components. I will show how using spectroscopic observations of star clusters and their nearby gas allows us to observe the chemical evolution of galaxies through a much larger window in both space and time. Lastly, I will present some of the most recent results from our JWST program dedicated to dissect a key ingredient in the recipe for star formation: molecular gas. I will close with a brief discussion on the implications these data have in the understanding of the processes that drive the evolution of galaxies.