Pulsar Timing Arrays (PTA) are galactic-scale low-frequency gravitational wave observatories. The primary source of gravitational radiation in this band is expected to come from a cosmic population of binary supermassive black holes (SMBHs) that form following galactic mergers. However, there are many open questions regarding the merger dynamics of binary SMBHs, and the relationship between a central black hole and its host galaxy. Using a new simulation, which incorporates the latest observational estimates of galaxy parameters including an updated empirical measurement of the galaxy merger rate, I investigate open questions around the spectral shape and amplitude of the gravitational wave background, and the stochastic nature of the signal, including the resolvability of discrete binaries in the PTA band.
The Cosmology Large Angular Scale Surveyor (CLASS) is a highly sensitive ground-based observational cosmology telescope array located in the Atacama Desert, Chile. The CLASS instrument is designed to measure the polarization of the Cosmic Microwave Background (CMB) at unprecedentedly large angular scales to detect evidence of primordial gravitational waves. With three currently operational telescopes on-site in Chile, CLASS surveys 75% of the sky at millimeter wavelengths in frequency bands near 40 GHz, 90 GHz, 150 GHz and 220 GHz. The CLASS telescope focal planes contain closely packed feedhorn-coupled arrays of superconducting Transition Edge Sensor (TES) bolometers designed to detect and measure CMB photons with extremely high sensitivity. CLASS also employs a unique variable-delay polarization modulator (VPM) to enable unparalleled polarization measurement precision. I will present on a recent focal plane upgrade that took place in Summer 2022 to improve the sensitivity of the 90 GHz telescope. I will also discuss plans for an upcoming deployment of the fourth CLASS array telescope containing a second 90 GHz TES array. Both upgrades promise to significantly advance the scientific reach of the CLASS experiment.
The CCAT-prime Project’s wide-field, six-meter aperture Fred Young Submillimeter Telescope (FYST) is currently under construction at 5600 m on Cerro Chajnantor in the Chilean Atacama Desert. Prime-Cam, a first-generation science instrument for FYST, will deliver over ten times greater sub-mm mapping speed than current and near-term facilities for unprecedented mm and sub-mm broadband and spectroscopic measurements with kinetic inductance detectors (KIDs). Mod-Cam, a first light and commissioning instrument for FYST, is now being integrated with the first 280 GHz instrument module and KID arrays for first light in 2024. I'll present the design and status of the Fred Young Submillimeter Telescope and the Prime-Cam and Mod-Cam instruments, and touch on the exciting array of cosmology and astrophysics measurements that will be advanced with these instruments. I will also give an update on the development of Cosmology Day, a half-day workshop for high school students to elucidate paths into the space sciences.
Mentorship is critical to student academic success and persistence, especially for students from historically underrepresented (HU) groups. In a program designed to support the academic success of HU undergraduates in STEM who wish to pursue a PhD in those fields, students experience comprehensive support including highly-engaged mentoring, dual faculty mentorship, financial aid, professional development workshops, and summer research experiences. Scholars in this program, the Cal-Bridge program, consistently report that faculty mentorship is the most impactful feature. While mentorship was rated highly, preliminary evaluation indicated an early deficit in a sense of community among scholars. In response, faculty professional development and support for peer networking were implemented to expand and enhance the relationships that support scholar success. Here I will present a promising multifaceted model of mentorship that can support the academic success of HU undergraduates, as well as discuss the positive impacts of peer mentorship and new work to deepen faculty mentor–scholar relationships.
Massive protostars attain high luminosities as they are actively accreting and the radiation pressure exerted on the gas in the star's atmosphere may launch isotropic high-velocity winds. These winds collide with the surrounding gas producing shock-heated (T ~10^7 K) tenuous gas that adiabatically expands and pushes on the dense gas that may otherwise be accreted. In this talk, I will present a series of 3D radiation-magnetohydrodynamic simulations of the collapse of massive prestellar cores and include radiative feedback from the stellar and dust-reprocessed radiation fields, collimated outflows, and, for the first time, isotropic stellar winds to model how these processes affect the formation of massive stars. I find that winds are initially launched when the massive protostar is still accreting and that the stellar wind properties evolve as it contracts to the main sequence. Wind feedback drives asymmetric adiabatic wind bubbles that have a bipolar morphology because the dense circumstellar accretion disk pinches the expansion of the hot shock-heated gas that preferentially expands along low-density channels, which I term as the “wind tunnel effect”. Eventually, the shock-heated gas produced by stellar winds quenches accretion onto massive stars that form from isolated cores when they reach ~30 Msol. Hence, stars more massive than this likely form from regions that supply mass via large-scale dynamical inflows within collapsing GMCs. Additionally, I will discuss the implications of observing adiabatic wind bubbles with Chandra while the massive protostars are still highly embedded.
Stars form in the hearts of molecular clouds, collapsing over 10 orders in spatial magnitude. This collapse is dominated by gravity, turbulence, magnetic fields, and stellar feedback. While gravity ultimately wins, these other dynamical effects can either hinder or aid the collapse at various scales. The initial conditions of the earliest evolutionary phase of a protostar are essential in constraining the architecture of the stellar system (e.g., the number of stars that will form or the mass budget for planets). Arguably the magnetic field is one of the more difficult dynamical effects to constrain observationally. In this talk, I will present new research showing multi-wavelength polarization observations that probe the multi-scale magnetic field morphology of a young stellar system in an isolated Bok globule where we believe the collapse is magnetically regulated. I will show how the observed magnetic field signature of the protostellar envelope may provide a new way to probe binary star formation. Since most stars form in multiple systems, understanding the system's formation is crucial in understanding how the protostar and its planets evolve.
Episodes of widespread asteroid bombardment in the solar system are thought to have been excited by giant planet migration. While recent work constrains giant planet migrations to the first ~100 Myr of solar system history, the precise timing remains unresolved. Temperature-sensitive mineral systems of meteorites record the thermal imprint of energetic impact events, which we aim to use to constrain the timescales of giant planet migration and widespread excitement in the asteroid belt. We present a simple asteroid-scale code that simulates conductive cooling of a radiogenically heated spherical body through closure of the thermochronologic 40K-40Ar system and resetting of near-surface cooling ages by decaying impactor fluxes. The thermal code is coupled to a Markov chain Monte Carlo inversion that stochastically varies model parameters and calculates the likelihood that the model age distribution is drawn from the distribution of cooling ages in a database of chondrite K-Ar and Ar-Ar ages.
When we do not incorporate impact resetting of cooling ages, our simulations fail to converge on the observed distribution of cooling ages in chondrites. We therefore model two impact fluxes – a primordial flux anchored to the solar age (4.567 Gyr ago) and a “post-accretion” flux that is a free parameter explored by the model. We find that the post-accretion flux begins at a median age of ~60 Myr after the formation of the Sun. We assess the sensitivity of this estimate to various model assumptions.
The internal rotation rate of the Sun has been determined to great precision by helioseismology. One of the main observational surprises is the presence of a “tachocline:” A region of strong radial shear, in which the convection zone’s latitudinal differential rotation above transitions to nearly solid-body rotation in the radiative interior below. The tachocline’s shear makes it a primary feature in dynamo theories of the Sun’s 22-year cycle. It remains poorly understood dynamically, however, because of its tendency to spread via radiative diffusion. In this talk, I briefly review the nature of the tachocline and the problem of its confinement. I present one of my recent global MHD simulations, in which a tachocline appears to be confined by a self-consistently-generated dynamo magnetic field.
Our understanding of the origin of the elements is grounded in observational abundance measurements and theoretical models of stellar explosions. Used in tandem, empirical and theoretical nucleosynthesis results can constrain important aspects of enrichment events, such as the landscape of black hole formation--which massive stars explode as core-collapse supernovae and which implode to black holes. The true Galactic explosion landscape will impact the IMF-averaged nucleosynthetic yields of a stellar population and have important implications for stellar mass black hole astrophysics. In this talk, I will present the range of elemental yields achieved by varying degrees of explodability. I will discuss which abundance ratios may be useful in observationally constraining the black hole landscape. While no landscape choice achieves across-the-board agreement with observed abundance ratios, the discrepancies offer empirical clues to aspects of massive star evolution or explosion physics still missing from the models. Finally, I will discuss the potential extension of this methodology to understand other aspects of core-collapse supernovae enrichment, such as metallicity dependence and the impact of binary mass transfer.
High mass X-ray binaries (HMXBS) are systems that contain a compact object (neutron star or black hole) that accretes mass from a massive stellar companion. HMXBs are highly observable due to their bright X-ray luminosities, making them a key observational window into the complex process of massive binary stellar evolution. In Local Group galaxies we can combine high resolution, multiwavelength observations to characterize both the compact object and companion stars in these systems in the context of their host galaxies. In this talk I will discuss the population demographics of HMXBs in M31 and M33 measured with observations from the Hubble Space Telescope (HST), the Chandra X-ray Observatory, and the Nuclear Spectroscopic Telescope Array (NuSTAR), and the DEIMOS optical spectrograph on Keck. With these observations we can identify and characterize the HMXB companion stars, measure the overall HMXB age distribution and production rate for these galaxies, and constrain compact object types,. HMXB production rates of galaxies scale with their star formation rate and metallicity, and I will discuss what we can learn by comparing the HMXB populations of these two galaxies.
As part the Broader Impacts program of my fellowship, I worked with Tumble Media, an established science podcast group for children, to develop and record podcast based audio-courses for elementary students designed to train the next generation of STEM-literate leaders using inquiry-based learning and citizen science. Our first audio courses pairs real scientific research from the Galaxy Zoo project on the citizen science platform, Zooniverse, with short podcasts and audio guided experiences to be used in science classrooms nationwide. Tumble Media, in collaboration with myself and Galaxy Zoo PI, Karen Masters, produced these podcasts and helped develop enrichment activities to accompany existing Zooniverse resources, tailored to meet the needs of teachers across the US following the Next Generation Science Standards. This first audio course will be distributed widely using the Tumble website, Zooniverse blogs, and other connections in mid-late January, 2023.
In this talk, I will discuss the effects of supermassive black hole (SMBH) feedback on the circumgalactic medium (CGM) using the cosmological hydrodynamic simulation, Romulus25 (Tremmel et al. 2017). Within our simulated galaxies, we trace the motion of metals from their formation in the disk out into the CGM and beyond. We explore the mechanisms that drive metal flow during the evolution of ~L* galaxies and what properties constrain the amount of metals left within the disk and CGM. We find that the retention of metals in the central 0.1Rvir of these galaxies trends with the scatter of SMBH mass on the empirical M-sigma relation. Our results link the accretion history of the SMBH and its efficiency at impacting the gas and metal enrichment of the CGM to its host galaxy’s gravitational potential.
The intersection between the atomic and molecular interstellar medium (ISM) is still relatively mysterious. In the past two decades, indirect gas tracers such as gamma-ray and dust emission have implied the existence of abundant molecular hydrogen (H2) not traced by our canonical molecular tracer, the CO molecule. This H2 likely lies in diffuse clouds where CO will be not sufficiently collisionally excited or even photodissociated. I will discuss recent efforts in using the OH molecule in emission at 18cm to trace the large-scale Galactic dark H2, and what we have learned about this previously invisible phase of the diffuse molecular ISM through ultra-sensitive (RMS ~ 1mK) 18cm OH emission surveys with the 100m Green Bank Telescope (GBT). Sensitive OH observations serendipitously revealed an immense amount of dark H2 in the Outer Galaxy in the form of a diffuse disk, co-spatial with the atomic phase as traced by the HI 21cm line. I will also discuss upcoming projects using the OH lines to investigate galactic structure in our galaxy and others.
The reservoir of baryons and metals surrounding galaxies is known as the circumgalactic medium (CGM). The CGM plays a crucial role in the growth of galaxies; galaxies require a continuous gas supply to sustain star formation, and this gas accretion process is regulated by feedback from newly formed stars. However, observational analysis of the CGM is challenging. Not only because the low gas density makes the CGM difficult to image directly, but the CGM is also multiphase, i.e., the CGM consists of gas structures of different temperatures and densities. We will present our ongoing projects of studying the multiphase circumgalactic gas flow. In particular, we will discuss our recent results on analyzing the kinematics of the circumgalactic gas traced by the highly ionized OVI absorption detected in background quasar sightlines of low-redshift, star-forming galaxies. We will contrast the results with that of the cool CGM traced by low-ionization-state ion, which is typically found to be corotating with the galaxy disks. Finally, we will also present analyses from cosmological simulations and compared that with the observed OVI gas kinematics.
The structure of our Galaxy, the Milky Way, remains uncertain despite almost a century of research. Dust obscuration hinders optical and near-IR efforts, the complex velocity field of the Galactic disk complicates interpretation of position-velocity data, and obtaining accurate distances to Galactic structure tracers remains an on-going challenge. The vast amount of new data and modern analysis tools, however, provide a new potentially breakthrough avenue in our understanding of the Galaxy. I will discuss two ongoing projects that leverage these new datasets and tools. First, we use the complete catalog of Milky Way HII regions, the sites of recent massive star formation, and simulation-based inference to constrain the morphological structure of the Galaxy. By training a neural network on the relationship between a complicated model of Galactic structure and
simulated HII region position-velocity data, we determine the morphological parameters that best represent the observed HII region data. Second, we develop a data-driven model to reproduce the all-sky emission of neutral hydrogen. The three-dimensional model captures the physical conditions of the gas as well as the full phase-space kinematics of the Galactic disk. We incorporate both HI absorption data as well as the positions and 3-D velocities of masers associated with high-mass star forming regions to further constrain the model. In the end, we obtain a complete three-dimensional model of Galactic kinematics and the neutral hydrogen distribution, which benefits not only our understanding of Galactic structure but also many other areas of Galactic (and extragalactic) astrophysics.