High-energy Particle Astrophysics using the Radio Technique

Cosmic engines accelerate particles to the highest energies ever recorded. The origin of these particles, called cosmic rays, remains an unsolved mystery, largely because they are deflected by magnetic fields as they propagate away from their sources. Neutrinos trace the acceleration sites of cosmic rays, are undeflected by magnetic fields, and provide a unique view of the most extreme environments in the universe. In fact, there are two expected populations of high energy neutrinos: an astrophysical flux observed by IceCube (~10¹⁵ eV) and the cosmogenic flux due to the interactions of cosmic rays with background photons (~10¹⁸ eV). Balloon-borne experiments such as ANITA are optimized for detecting both the cosmic rays and the highest-energy cosmogenic neutrinos, while in-ice arrays are sensitive to both cosmogenic neutrinos and the high-energy cutoffs of astrophysical neutrino sources. I will discuss recent results from ANITA including a beam-line experiment aimed at understanding radio-frequency emission from cosmic rays as well as a new technique based in phased arrays that targets the astrophysical flux of neutrinos.

The Lyman alpha revolution (This talk will be held in the ITST Conference Room)

Half a century after Partridge & Peebles predicted that young galaxies should emit copious amounts of Lyα emission, the redshifted Lyα line has allowed us to both find and identify galaxies out to z~9. In the next years, the number of Lyα emitting sources is expected to increase by > 2 orders of magnitude at most redshifts. In addition, new integral field unit spectographs will enable us to obtain spatially resolved spectra, and map out Lyα emission at surface brightness levels at which the environment of galaxies 'glows' in Lyα. In this talk I will talk about how existing observations already have provided leading constraints on the ionization state of the Universe at z>6, and how Lyα emitting galaxies constrain the nature of the sources reionizing the Universe. I will discuss how future observations improve these constraints, and how these generally will help us get unique insights on gaseous flows in and around galaxies.

Tiny, Cold Clouds in Galaxy Halos

Modern models of structure formation predict that galaxy halos are filled with hot, ~million-degree gas. While this hot plasma is not yet directly observable, a number of measurement techniques are sensitive to much cooler gas, below about ten thousand degrees. Unexpectedly, these observations indicate that galaxy halos are also full of cold gas, in addition to the theoretically-predicted hot plasma. These observations typically indicate a relatively modest total fraction of cold gas (< 1% by volume), yet find it in essentially every sightline through the galaxy.

I will show that cold gas clouds in galaxies are prone to "shattering" into tiny fragments, and that the resulting small clouds naturally reproduce the large area-covering fractions and small volume-filling fractions inferred from observations. This same process effectively enhances the drag force coupling the dynamics of cold and hot gasses; I will also discuss potential applications to the measured entrainment of cold gas in galaxy winds.

Inside-Out Planet Formation

The Kepler-discovered systems with tightly-packed inner planets (STIPs), typically with several planets of Earth to super-Earth masses on well-aligned, sub-AU orbits may host the most common type of planets in the Galaxy. They pose a great challenge for planet formation theories, which fall into two broad classes: (1) formation further out followed by migration; (2) formation in situ from a disk of gas and planetesimals. I review the pros and cons of these classes, before focusing on a new theory of sequential in situ formation from the inside-out via creation of successive gravitationally unstable rings fed from a continuous stream of small (~cm-m size) "pebbles," drifting inward via gas drag. Pebbles first collect at the pressure trap associated with the transition from a magnetorotational instability (MRI)-inactive ("dead zone") region to an inner MRI-active zone. A pebble ring builds up until it either becomes gravitationally unstable to form an Earth to super-Earth-mass planet directly or induces gradual planet formation via core accretion. The planet continues to accrete until it becomes massive enough to isolate itself from the accretion flow via gap opening. The process repeats with a new pebble ring gathering at the new pressure maximum associated with the retreating dead-zone boundary. I discuss the theory's predictions for planetary masses, relative mass scalings with orbital radius, and minimum orbital separations, and their comparison with observed systems. Finally I speculate about potential causes of diversity of planetary system architectures, i.e. STIPs versus Solar System analogs.