Abstract

Binary star populations play a pivotal role in nearly all subfields of astronomy and cosmology, yet the quantitative details of how they evolve are still poorly described. This is due both to a wide array of uncertain physical interaction processes like mass exchange and supernova explosions that can work in compounding ways and a lack of large data sets that contain binary populations across different evolutionary phases. In this talk, I'll review how binary-star interactions shape stellar populations hosting black holes and introduce recent and upcoming gravitational wave and electromagnetic survey data releases that can be used to constrain models for binary evolution. I'll finish with a discussion of the discovery of a new population of compact objects in binaries with stellar companions made by the Gaia satellite that offer a unique window into compact object formation.

Abstract

Compared to filled-aperture telescopes, astronomical optical long-baseline interferometry typically provides higher angular resolution and higher-precision spatial measurements at the cost of fidelity, dynamic range, and consequently contrast of reconstructed images. I use the term precision interferometry to refer to efforts of overcoming this limitation through more precise measurements of the classical interferometric observables (visibilities and closure phases) or though circumventing them altogether using nulling or Fizeau (imaging) interferometry. Over the past almost two decades, these efforts have enabled the spatially resolved observations of the habitable zones and closer in of planetary systems with a sensitivity to detect circumstellar, exozodiacal dust. This dust poses both an opportunity to study the architectures and dynamics of planetary systems near their habitable zones and a potential obstacle to directly imaging rocky, habitable-zone planets. It's study is thus crucial for enabling and preparing for a future exo-Earth imaging mission such as the Habitable Worlds Observatory. I will review our work on exozodiacal dust with the Large Binocular Telescope Interferometer (LBTI) and the Very Large Telescope Interferometer (VLTI). I will further present our efforts to enable high-fidelity imaging at the angular resolution of a 23m telescope with the LBTI for general astronomical observations. This work makes the LBTI a critical pathfinder for future 30m-class telescopes. Both exo-Earth imaging and the completion and exploitation of 30m-class telescopes are major priorities of the US community as outlined in the National Academy's Astro2020 Decadal Survey. I will then close the loop and conclude my talk by outlining our efforts to obtain the first direct-imaging detection of a rocky, habitable-zone planet around a nearby, Sun-like star with the LBTI.

Abstract

The Universe is thrumming with gravitational waves. June 2023 brought the first evidence for an all-sky background of nanohertz-frequency gravitational waves, discovered by collaborations including the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) and groups in Europe, Australia, India, and China. This was an endeavor decades in the making, requiring painstakingly precise timing observations of scores of millisecond pulsars across the Milky Way using flagship radio telescopes. While the results from separate groups are consistent with one another—and the leading interpretation of a cosmic population of supermassive black-hole binaries as the source—the observations provoke many new questions. Do the results imply a population of binaries more massive than expected? What are the observational milestones as the first resolvable massive black-hole binary signals come into focus? Can we link these signals to host galaxies and leverage electromagnetic counterparts to do multi-messenger astronomy with massive black holes? In this talk, I will chart the path to discovery, reflect on what we have learned during our year+ since our announcement, and explore the exciting opportunities ahead—including the role of next-generation instruments in expanding our pulsar network to explore the low-frequency gravitational-wave and supermassive binary black-hole landscape.

Abstract

The space-time distortion caused by supermassive black holes provides a unique laboratory for violent physical processes like stellar tidal disruption, highly relativistic jets, and turbulent accretion flows. Synthesizing observations across many wavelengths and studying their time variability at slow and rapid timescales promises a new, dynamic, thorough understanding of how black holes form and grow, how they consume material, how they construct powerful relativistic jets, and how they bend the evolution of galaxies to a path that matches our observations of the Universe. I will discuss how both high-resolution radio imaging surveys of outflows and star formation and timing observations with breakthrough instruments like the NICER instrument on the ISS and the TESS exoplanet-hunting missions have provided new intersectional insights and promise a fertile new temporal phase space for exploring the detailed phenomenology and cosmological implications of active galactic nuclei. In my talk, I will discuss results from a 22 GHz radio imaging survey of hundreds of BAT AGN and implications for both the origin of radio emission their cores and AGN feedback on galaxy-wide scales. I will also discuss time domain investigations of accretion and jets with the Transiting Exoplanet Survey Satellite, and the future of high-cadence optical timing for both advances in accretion physics and detection of multimessenger systems of binary supermassive black holes.

Abstract

We use Spitzer/IRAC deep-exposure data covering two significantly larger than before sky areas to construct maps suitable for evaluating source-subtracted fluctuations in the cosmic infrared background (CIB). The maps are constructed using the self-calibration methodology eliminating artifacts to sufficient accuracy, and subset maps are selected in each area containing approximately uniform exposures. These maps are clipped and removed of known sources and then Fourier transformed to probe the CIB anisotropies to new larger scales. The power spectrum of the resultant CIB anisotropies is measured from the data to >1°, revealing the component well above that from remaining known galaxies on scales >1′. The fluctuations are demonstrated to be free of Galactic and solar system foreground contributions out to the largest scales measured. We discuss the proposed theories for the origin of the excess CIB anisotropies in light of the new data. Out of these, the model where the CIB fluctuation excess originates from the granulation power due to LIGO-observed primordial black holes as dark matter appears most successful in accounting for all observations related to the measured CIB power amplitude and spatial structure, including the measured coherence between the CIB and unresolved cosmic X-ray background (CXB). Finally we point out the use of the data to probe the CIB-CXB cross power to new scales and higher accuracy. We also discuss the synergy of these data with future CIB programs at shorter near-IR wavelengths with deep wide surveys and subarcsecond angular resolution as provided by Euclid and Roman space missions.

Abstract

Neutron star (NS) low-mass X-ray binaries (LMXBs) accrete via Roche-lobe overflow from a stellar companion of roughly 1 solar mass. The accretion disk surrounding the NS in these systems can be externally illuminated by X-rays that are reprocessed by the accreting material and re-emitted as the reflection spectrum, which is comprised of emission lines superimposed onto a reprocessed continuum. Due to proximity of the inner region of the disk to the compact accretor, strong gravity and relativistic effects are imparted to the reflection spectrum. Modeling of these effects allows us to infer properties of the NS itself (such as magnetic field strength and limits on the radial extent), as well as other aspects of the system like accretion and illumination source geometry, ionization state and composition of the material, and density of the disk. Much has been learned about these systems using advanced X-ray missions like NICER and NuSTAR, which can obtain spectra from bright sources without distortion. The recent successful operation of a microcalorimeter in space aboard XRISM provides high-energy resolution observations of several persistently accreting NSs. These datasets reveal undeniable structure within the prominent Fe K line region of the reflection spectrum and provide further insights into the accretion geometry of these systems. I will highlight what has been learned about NS LMXBs via reflection modeling over the last decade and prospects for further insights.

Abstract

This presentation explores questions related to the emergence of habitable conditions, and to the origins of the architecture and chemistry of planets in general. Planets form from materials transported from the interstellar medium into and through protoplanetary disks. Hence, to understand the ubiquity, or rarity, of elements critical for life as we know it, we must understand the chemically complicated conditions in planet-forming regions. Initial observations of the chemical composition of exoplanetary atmospheres - their complement of particularly carbon and oxygen - are often interpreted in the context of protoplanetary disks. I will review our current knowledge of the chemistry of the primary planet-forming region around young stars, show how JWST is revolutionizing our understanding, and look forward to the future in the context of the Probe Infrared Mission for Astrophysics (PRIMA).

Abstract

The nature of the recently discovered class of object known as long period radio transients is uncertain. New evidence suggests two distinct classes are emerging: white dwarf binary systems and isolated, slowly rotating neutron stars. In the neutron star interpretation, long periods strongly challenge all current rotation-powered models of pulsar radio emission. In this talk I will discuss the observational status of these novel sources, and present a new model for radio emission from highly magnetized neutron stars, in which crustal stress driven mild magnetospheric twists produce coherent radio emission. I will introduce the likely driving forces behind emission including the magnetically driven plastic crustal flow, and show how inverse-Compton scattered or curvature photons can naturally produce coherent radio emission which matches observed luminosities, variability, pulse profiles and polarimetric properties. I will further detail our predictions of multi-wavelength counterparts and new 'active-zones' in which further long period sources may soon be discovered. I will conclude with an observational and theoretical outlook for these fascinating sources.

Abstract

Optical transient surveys, enabled by their large grasp, can now probe the phase space of fast and rare transients. In my talk, I will show that systematic rapid discovery, with targeted multiwavelength follow-up, has enabled us to answer decades-old questions in the field of gamma-ray bursts and discover unexpected types of cosmic explosions that are prime targets for millimeter-band observatories such as ALMA. Significant progress on these questions will be enabled by upcoming high-cadence surveys in other bands, particularly X-ray, UV, and submillimeter.

Abstract

Although the carbon is one of the most abundant elements in the Universe and of essential importance to life, the pathway followed by carbon from the interstellar medium to the planet-forming dust in protoplanetary disk environments, the analogues of our early Solar System, is poorly understood. As a puzzling fact, the inner Solar System is depleted in refractory carbon compared to the interstellar medium. This depletion most likely took place during the protoplanetary disk phase.

In this context, one of our challenges is to understand whether a solid carbon depletion is a common situation in the very inner region of protoplanetary disk at the stage when planets are forming. We use interferometric techniques involving the MATISSE instrument at the VLTI to reach the required angular resolution of the astronomical unit (au) scale. I will present our results and their interpretation focusing on two solid carbon tracers and two sources presenting their spectral signatures in the mid-infrared: HD169142 with its PAHs emission and HD97048 with its nano-diamonds.

Abstract

Dwarf galaxies are the most common systems in the Universe and therefore crucial to a holistic understanding of galaxy evolution; however, current observations remain strongly biased towards giant galaxies. This inhibits the ability to address fundamental questions spanning many fields in astronomy. In particular, the question of how giant galaxies form supermassive black holes is one that dwarfs are well suited to answer. I will present state-of-the-art photoionization models tailored to finding accreting intermediate-mass black holes in dwarf AGN, which are metal-poor and highly star-forming, while accounting for XRB populations. I will highlight the recent application of these models to identify dwarf AGN throughout cosmic time, notably increasing the local dwarf AGN fraction up to 15x previous estimates. I will show that while X-ray binaries can masquerade as dwarf AGN, emission lines diagnostics detectable with JWST can effectively separate the two excitation mechanisms.

Abstract

Feedback from low-metallicity massive stars regulates the evolution of both dwarf and high-redshift galaxies. To understand those processes requires robust models of the metal-poor ISM in which stars form and of massive stars' winds and ionizing spectra. Yet, these models remain entirely theoretical and uncertain due to a lack of observational constraints in the extremely low-metallicity regime. In this talk, I will present a suite of JWST, HST, and Keck observations of the nearby 3% Solar metallicity galaxy Leo P. First, I will describe novel constraints on the wind properties and ionizing spectrum of the galaxy's only O-type star from modeling HST/COS and Keck/KCWI spectroscopy. This star is part of the Treasury of Extremely Metal-Poor O Stars (TEMPOS), a Large Treasury HST program that will extend this initial analysis to a much larger sample at ~5-10% Solar metallicity. I will then present new JWST/MIRI-MRS observations of Leo P, which enabled the first detection of cold molecular hydrogen at such low metallicity via rotationally excited emission from the photodissociation region illuminated by the O star. Our detailed understanding of that star's UV radiation from the HST data constrains the temperature and mass of the detected molecular hydrogen, providing a benchmark for models of the ISM at the very low metallicities typical at high redshift. Leo P showcases the potential of the metal-poor dwarf galaxies in our backyard to inform models of early galaxies.

Abstract

Variations in the abundance of interstellar dust have important implications for our ability to trace the chemical enrichment of the universe, stellar mass assembly, and profoundly affect galaxy evolution. I will present results from three observational efforts to characterize the variations of the dust content and properties within and between galaxies, in particular as a function of metallicity and gas density. The dust and gas contents of nearby galaxies were measured using far-infrared, HI 21 cm, and CO emission on the one hand, and UV absorption spectroscopy with Hubble on the other hand. Both approaches demonstrate a significant increase of the dust abundance with gas density, even in very low metallicity systems such as Sextans A (7% solar metallicity). Furthermore, the fraction of metals locked in dust decreases with decreasing metallicity, by a factor of 2 from the Milky Way to the SMC, and a factor 4 from the Milky Way to Sextans A. This results in the dust-to-gas ratio (D/G) decreasing faster than metallicity, consistent with chemical evolution models and with measurements in Damped Lyman-alpha systems at redshift <4. However, this decrease in D/G is not as dramatic as suggested from previous FIR measurements in nearby galaxies. Furthermore, ongoing efforts to measure the properties and abundance of the smallest dust grains, polycyclic aromatic hydrocarbons (PAHs), in very low metallicity systems with JWST are revealing that PAHs do exist in those environments, but were previously missed owing to the very small size (pc-scale) of the dense regions in which they can form and survive.

Abstract

The formation of supermassive black holes (SMBHs) in the first billion years after the Big Bang remains a fundamental question in astrophysics. I will review current theoretical models of SMBH seeding, including both light and heavy seed scenarios, and explore how the redshift evolution of AGN obscuration can inform our understanding of early black hole growth. I will discuss observational constraints from the AGN luminosity function and emphasize the potential of future X-ray missions—particularly the proposed AXIS probe—to resolve faint AGN populations and extend measurements to higher redshifts. I will also present recent results on the cross-power spectrum between the cosmic infrared background (CIB) and the cosmic X-ray background (CXB), based on data from Spitzer, JWST, and Chandra, which offer a direct evidence of early accreting black holes. Finally, I will consider the possibility that primordial black holes (PBHs) may play a role in seeding SMBHs, providing an alternative pathway for their rapid growth.

Abstract

In this talk, I will talk about galaxy clusters and their X-ray properties. I will introduce the CLUS model, which has been implemented in the X-ray spectral fitting software package SPEX. This talk is based on my paper that has been recently published in A&A. We show a few applications of this new model on sources like the massive elliptical galaxy NGC 4636; galaxy clusters A383, A2029, A1795, and A262; and the Perseus cluster. In the paper, we quantify the effect of projection as well as the resonant scattering on inferred profiles of the iron abundance and temperature, assuming a resolution similar to Chandra ACIS-S and XRISM Resolve. The results show that depending on the mass of the object as well as the projected distance from its core, neither a single-temperature or double-temperature model nor the Gaussian-shaped differential emission measure model can accurately describe the input emission measure distribution of these massive objects. The largest effect of projection as well as resonant scattering was observed for projected profiles of iron abundance of NGC 4636, which is where we could reproduce the observed iron abundance drop in its innermost few kiloparsecs. Furthermore, we find that projection effects also influence the best-fit temperature, and the magnitude of this effect varies depending on the underlying hydrodynamical profiles of individual objects.

Abstract

Galaxy mergers are a standard aspect of galaxy formation and evolution, and most (likely all) large galaxies contain supermassive black holes. As part of the merging process, the supermassive black holes in-spiral together and eventually merge, generating a background of gravitational radiation in the nanohertz regime. An array of precisely timed pulsars spread across the sky can form a galactic-scale gravitational wave detector in the nanohertz band. I describe the current efforts to develop and extend the pulsar timing array concept, together with recent evidence for a gravitational wave background, and what the immediate future holds.

Abstract

Multiwavelength observations of populations of evolved massive stellar binaries are crucial for understanding the complex process of massive binary stellar evolution. High mass X-ray binaries (HMXBs) are evolved massive stellar binaries in which the primary star has become a compact object and accretes from a massive secondary star. In this talk I will discuss ongoing work to characterize the population of HMXBs in massive spiral and low mass, dwarf galaxies in the Local Group. Combining deep optical and X-ray observations allows us to characterize each galaxy’s HMXB population including measuring X-ray luminosities, physical properties of the secondary stars, and the HMXB age distribution and production rate for each galaxy. These measured population demographics can be used to constrain theoretical binary stellar evolution models.

Abstract

Extraordinary advances in quantum control of matter and light have revolutionized the development of quantum sensors, enabling precision tests of the fundamental laws of Nature and deeper insights into the physical Universe. The unique conditions of the space environment, free from many terrestrial limitations, open new frontiers for physics beyond what is accessible on Earth. In this talk, I will explore the scientific opportunities enabled by deploying quantum sensors in space, with a particular emphasis on atomic clocks. These include precision tests of general relativity, searches for dark matter, the detection of gravitational waves, and others.

Abstract

One of the more surprising results from JWST has been the discovery of a large population of broad-line AGN at z > 5 that are 2-3 dex fainter that bright quasars identified from the ground. These sources are powered by black holes (BHs) with masses of order 10^7 Msol, making them the least-massive BHs known in the early universe. Studies in this low-mass regime are key to constraining models of BH seeding and the early growth history of SMBHs. I will discuss recent AGN-related results from the CEERS survey and what they tell us about galaxy-BH co-evolution out to the epoch of reionization. This includes the discovery of an actively accreting SMBH at z=9.3, as well as our work constraining the BH-galaxy mass relationship in the lowest mass range yet probed in the early universe. Finally, I will discuss the discovery of a large population of obscured AGN candidates at z>5 known as Little Red Dots (LRDs). While the nature of these sources is still heavily debated, we find that roughly 80% exhibit broad emission lines in their spectra and our X-ray spectral analysis confirms that they are moderately obscured, with column densities of log (nH/cm^-2) > 23. In addition, we find a quarter of LRDs exhibit narrow blue-shifted Balmer absorption features in their spectra, suggesting an outflow of high-density, low ionization gas that may also help obscure these sources. I’ll discuss the properties of these enigmatic sources and the current evidence for and against their AGN nature.