Abstract

What's an axion and why do people keep talking about Bose-Einstein condensates in space? In this talk, I will describe the axion as a popular solution for open problems in particle physics, most notably dark matter. I will discuss the possibility that neutron stars are astrophysical axion laboratories and what we may learn in future endeavors, including proposed X-ray and Gamma-ray missions such as STROBE-X, eXTP, and AMEGO.

Abstract

Black hole systems, composed of a black hole, accretion disk, and collimated outflow, will be discussed. Three active galactic nucleus (AGN) samples including 753 AGNs and 102 measurements of four stellar-mass galactic black holes (GBHs) will be considered. General expressions for black hole spin functions and accretion disk magnetic field strengths will be derived and applied to obtain the black hole spin function, spin, and accretion disk magnetic field strength in dimensionless and physical units for each source. Relatively high spin values of about (0.6 - 1) are obtained for the sources. The distributions of accretion disk magnetic field strengths for the three AGN samples are quite broad and have mean values of about 104 G, while those for stellar-mass GBHs have mean values of about 108 G. Good agreement is found between spin values obtained here and published values obtained with well-established methods; comparisons for one GBH and six AGNs indicate that similar spin values are obtained with independent methods. Black hole spin and disk magnetic field strength demographics indicate that black hole spin functions and spins are similar for all of the source types studied, including GBHs and different categories of AGNs. The method applied here does not depend on any specific accretion disk emission model and does not depend on a specific model that relates jet beam power to compact radio luminosity; hence, the results obtained here can be used to constrain and study these models.

Abstract

Brown dwarfs act as powerful analogs to the directly-imaged exoplanets, with similar temperatures, masses and compositions. Photometric variability monitoring of brown dwarfs is a unique probe of their atmospheres, as it is sensitive to condensate clouds as they rotate in and out of view. Variability has now been robustly observed in a range of L,T and Y spectral type brown dwarfs and more recently in planetary-mass companions and free-floating exoplanet analogs. In this talk I will discuss some of the key takeaways from our recent variability studies of brown dwarfs, as well as prospects for extending this work to directly-imaged exoplanets in the future.

Abstract

Nascent massive planets are surrounded by their own disk, the so-called circumplanetary disk. This channels material to the forming planet, serves as a birthplace for moons to grow, and affects the observational signatures of forming planets. The circumplanetary disk composition and chemistry will naturally affect that of the forming planet and of the moons. So understanding its role and characteristics is bringing us closer to understand planet- and moon-formation as a whole.

Our knowledge is still very limited on circumplanetary disks, as they are hard to resolve in computer simulations. We are just entering an era when the observations of these disks are possible, as the first observational evidence for their existence just came in May 2019.

I am carrying out sub-planet resolution thermo-hydrodynamical simulations of planet formation, trying to understand what are the characteristics of the circumplanetary environment, how we can detect forming planets and their circumplanetary disks in near-infrared, sub-millimeter and radio wavelengths or with hydrogen recombination lines, such as H-alpha. In my talk I will show mock observations in order to discuss which wavelength-range is the best to detect forming planets and what H-alpha fluxes we can expect from the circumplanetary environment. Finally, I will discuss how the circumplanetary disk alters the accretion rate and what does that mean for the timescales of planet-formation.

Abstract

The formation of stars in galaxies in the early Universe happened under different physical conditions from those commonly found at low redshift. The interstellar medium was turbulent and metal-poor, and gas densities were much higher on average. Nevertheless, detailed observations of such galaxies represent a considerable challenge due to the large distances involved. In this talk, I will give a review of the work currently being done by our research group focusing on the sample of Lyman break analogs, living fossils at low redshift that are selected based on their remarkable similarities to typical distant star-forming galaxies. I will show what properties appear to generate the high star formation rates observed and the perspectives for future work on this sample in upcoming observatories.

Abstract

In this presentation, I will discuss the science goals and design of the Galaxy Evolution Probe concept, development of enabling detector technology for GEP, and other potential far-infrared opportunities, such as a MIDEX or a balloon.

Mid- and far-infrared observations of galaxies yield star formation rates and measurements of physical conditions of interstellar medium associated with star formation and AGN. GEP is a concept for a mid-infrared and far-infrared dedicated survey mission whose goals are: (1) to map the history of galaxy growth by star formation and accretion by supermassive black holes and to characterize the relation between those processes, and (2) to measure the buildup of heavy elements, such as carbon, nitrogen, and oxygen, in the hearts of galaxies over cosmic time. GEP surveys will come in two forms. The first will be a low-resolution multispectral imaging survey with coverage from 10 to 400 microns. Photometric redshifts will be obtained using bright polycyclic aromatic hydrocarbon emission features. The second will be deep, moderate-resolution spectroscopy from 24 to 193 microns that will detect atomic fine-structure lines over a range of ionization states to measure the impact of star formation and AGN on the interstellar medium and the history of the growth of metal content in galaxies. GEP will be enabled by development of arrays of kinetic inductance detectors -- superconducting microresonators -- whose principles of operation I will describe. The GEP has a target launch date in early 2029. After describing GEP and its new technology, I will describe other exciting nearer-term potential opportunities.

Abstract

Gravitational waves near the Earth affect the light that reaches us from distant objects. One consequence of this is a periodic red/blue-shifting of the radio pulses from millisecond pulsars. A closely related effect is the period change in the apparent astrometric position of a star. Both of these effects can, in principle, be used to detect ultra-low frequency gravitational waves. In this talk I will describe the different ways in which astrometric measurements can be used to detect gravitational waves and compare this with the ongoing efforts using pulsar timing.

Abstract

Reionization was the last major phase transition of the universe, and both the time evolution and spatial variation of this process encode key information about the onset of luminous objects in the universe. While we think that massive stars within star-forming galaxies provide the needed ionizing photons, the observed escape fractions of these photons may be too low to complete reionization when combined with typical assumptions. We have devised a new semi-empirical model which resolves this tension by using simulation-motivated escape fractions, where the smallest halos have the highest escape fractions, leading to a successful completion of reionization by z=5.5 with low (<5%) average ionizing photon escape fractions. Our model makes a number of testable predictions, including: 1) AGNs contribute non-negligibly to the end of reionization, 2) the neutral fraction at z~7 is only 20%, and 3) significant star-formation must be occurring at z~9-10. I will show observational results from my group at UT Austin testing all of these assumptions, including results from ultra-deep Keck spectroscopy for Lyman-alpha emission at z~7-10, and the discovery of several remarkably bright galaxy candidates at z > 9. The abundance of bright galaxies at early times implies that the first quenched galaxies may exist earlier than predicted. I will finish by discussing a new moderate-depth (K~23) imaging survey over the ~20 square degree SHELA (the Spitzer/HETDEX Exploratory Large Area) survey, which we have used to discover ~5600 massive (log M/Msol > 11) galaxies at 3 < z < 5. Our analysis of the stellar populations in these objects shows that ~10% are quiescent (sSFR < 10^-11 yr^-1), nearly 20X the largest previous sample of massive quiescent galaxies in this epoch. Simulations have struggled to replicate this, finding that increasing AGN feedback to hasten quenching slows down galaxy growth enough such that massive galaxies form too late. I will discuss ongoing followup efforts with DECam, ALMA and Keck to confirm the nature of a significant fraction our sample, and accurately quantify the contamination rate.

Abstract

The idea is old but the time is now: orbiting guide stars can enable adaptive optics (AO) on giant ground-based telescopes at visible wavelengths, with angular resolution and sensitivity comparable to space telescopes of the same size. The current AO technology with sodium layer guide stars does not reach the visible band, and may never. But a CubeSat or SmallSat in the right orbit can provide a V=0 artificial laser guide star for 20 min to 2 hours, depending on declination, even without active propulsion. With active propulsion, it could stay exactly aligned with a target star for hours at a time, enabling laser-assisted extreme AO and observations of exoplanet systems in reflected light, with both imaging and spectroscopy. Simulated spectra show that terrestrial molecules would not prevent observation of oxygen and water in an exo-Earth.

The case of an orbiting starshade has also been developed. It would also enable direct observations of exoplanet systems in reflected light, with contrast > 1E10, sufficient to see an Earth around a G star. This concept was documented in a Decadal white paper, and a manuscript has been submitted to Nature Astronomy.

Abstract

The James Webb Space Telescope is the scientific successor to the Hubble and Spitzer Space Telescopes. It is a large (6.6m) cold (50K) telescope to be launched into orbit around the second Earth-Sun Lagrange point. It is a partnership of NASA with the European and Canadian Space Agencies. Webb’s science goals include the formation of the first stars and galaxies in the early universe; the chemical, morphological and dynamical buildup of galaxies, the formation of stars and planetary systems and understanding exoplanets and our Solar System. Webb has four instruments that will do both imaging and spectroscopy from 0.6 to 28.5 microns. Recent progress includes the completion of the assembly of the observatory. The project is currently preparing for the final environmental and deployment testing, which will be followed by launch, commissioning and scientific operations. The Guaranteed Time Observation and Early Release Science programs are final, and the call for proposals for Cycle 1 General Observer science is currently open.

Abstract

The Galactic magnetic field (GMF) plays an important role in a variety of astrophysical processes but is not well understood. Magnetic fields in the interstellar medium are difficult to study directly but affect phenomena as diverse as: the propagation of cosmic rays; the formation of stars; the morphology of supernova remnants; the deflections in the arrival directions of extragalactic ultra-high energy cosmic rays (UHECRs); the cosmic microwave background (CMB). I will review how data across the electromagnetic spectrum are giving us new and different views of the fields (for example, polarized dust emission from Planck, or anisotropies in the arrival directions of UHECRs seen with Auger or IceCube) and how these data are difficult to interpret because of the complexity of the different contexts and the degeneracies in the parameter space. The Interstellar MAGnetic field INference Engine (IMAGINE) is our new framework for combining all available observables as well as theoretical knowledge into a statistically rigorous Bayesian analysis. This will allow us to incorporate *all* available information to break some of these degeneracies as well as to explicitly quantify how well different models reproduce the same observables with the Bayesian evidence. I will summarize the project, the infrastructure we have made publicly available, and our plans to build more realistic GMF models based on magneto-hydrodynamical dynamo equations and to attempt both parametric and non-parametric reconstructions of the GMF.

Abstract

Reionization of the intergalactic medium (IGM) marks the time in the early universe when the first stars and galaxies began to affect the universe around them. As during this last major phase transition high-energy ultraviolet photons from these objects ionized the gas in the IGM, studying reionization can provide key insights into the formation and evolution of galaxies in the early universe. As Lyman-alpha photons are resonantly scattered by neutral hydrogen in the IGM, an analysis of this line can be used to trace the existence of neutral hydrogen in the IGM at different points in the history of the universe (i.e., when the IGM becomes neutral, we should stop seeing these photons, as they are likely scattered out of our line-of-sight). Spectroscopic data of galaxies in the early universe, obtained with the DEIMOS (optical) and MOSFIRE (near-infrared; NIR) spectrographs on the Keck telescopes, is utilized for investigating the evolution of the IGM during reionization by measuring the Lyman-alpha equivalent width (EW) distribution. I will present the results of the spectroscopic analysis and discuss what can be done with current telescopes and JWST in future on investigating time and spatial evolution of reionization.

Abstract

I Zw 18 is a blue compact dwarf galaxy with the lowest known metallicity, log O/H+12=7.2, hence, the closest (expected) match to galaxies in the early universe. This galaxy is expected to form a baseline of comparison for observations of z>6 galaxies by JWST and future extremely large telescopes. New theoretical models and observational analyses of I Zw 18 indicate that a very low metallicity can produce markedly different evolution paths from those of less metal-poor stars. The differences include a tendency among rapidly rotating massive stars to evolve chemically homogenously (i.e. same elemental abundances at the stellar surface as at the nuclear-burning core) and/or a tendency among very massive stars to end up as massive black holes without undergoing a supernova explosion. These findings will provide a guide to understanding high-redshift galaxies and will inform cosmological simulations such as the FIRE simulations (Feedback In Realistic Environments).

Abstract

A comprehensive understanding of Gamma Ray Bursts (GRBs) has been elusive due to the variety of questions surrounding the radiation mechanism at play in these events. Polarization measurements of GRBs can heavily constrain the relevant radiation mechanisms and the structure of the GRB jet; however, there is a limited number of theoretical predictions that observed GRB polarizations can be compared against. Here, we conduct radiative transfer calculations of a set of two dimensional relativistic hydrodynamic long GRB (LGRB) jet simulations, of a constant and a variable jet, using the Monte Carlo Radiation Transport (MCRaT) code. MCRaT has been enhanced by the inclusion of polarization; it has been first verified by reproducing a variety of results in the literature and then used to obtain the time integrated and resolved polarization degrees and angles of the synthetic LGRBs. While the obtained time-integrated polarization degrees are consistent with the constraints from the POLAR experiment, they are lower than other theoretical studies due to the lack of strong gradients in the model jet profiles that we use. The time resolved results suggests that GRBs with wide jets observed on axis will have small polarization degrees and constant polarization angles, during the brightest portion of the light curve. GRBs observed off axis will have larger polarization degrees and polarization angles that change with the temporal structure of radiating shells in the outflow. We then place our results in the context of GRB prompt emission models and future LEAP and POLAR-2 GRB polarimetry detections.

Abstract

Eclectic? Never. Star formation (and its modulation) in galaxies is at the heart of some of the most exciting and confusing work in understanding the evolution of our universe. I will present an odd sample of nearby star forming galaxies that are forming stars in their bulge - but appear in many different environments. I'll present a recent result where we find similar galaxies in the IllustrisTNG simulations as well. Science is done by people - and I will talk briefly about the systemic challenges and discrimination many scientists face, some exacerbated by our current pandemic. Lastly, I'll highlight some instruments both on the ground and space based that I'm working on to understand galaxy evolution - and especially the gas nearby that may drive their evolution.