Understanding How Galaxies Reionized the Universe
Sanchayeeta Borthakur, Arizona State University
Identifying the population of galaxies that was responsible for the reionization of the universe is a long-standing quest in astronomy. While young stars can produce large amounts of ionizing photons, the mechanism behind the escape of Lyman continuum photons (wavelength < 912 A) from star-forming regions has eluded us. To identify such galaxies and to understand the process of the escape of Lyman continuum, we present an indirect technique known as the residual flux technique. Using this technique, we identified (and later confirmed) the first low-redshift galaxy that has an escape fraction of ionizing flux of 21%. This leaky galaxy provides us with valuable insights into the physics of starburst-driven feedback. In addition, since direct detection of ionizing flux is impossible at the epoch of reionization, the residual flux technique presents a highly valuable tool for future studies to be conducted with the upcoming large telescopes such as the JWST.
Cold Gas and the Evolution of Early-type Galaxies
Lisa Young, New Mexico Tech
A major theme of galaxy evolution is understanding how today’s Hubble sequence was
established — what makes some galaxies red spheroidals and others blue disks, and what
drives their relative numbers and their spatial distributions. One way of addressing these
questions is that galaxies themselves hold clues to their formation in their internal
structures. Recent observations of early-type galaxies in particular (ellipticals and
lenticulars) have shown that their seemingly placid, nearly featureless optical images can
be deceptive. Kinematic data show that the early-type galaxies have a wide variety of
internal kinematic structures that are the relics of dramatic merging and accretion
events. A surprising number of the early-type galaxies also contain cold atomic and
molecular gas, which is significant because their transitions to the red sequence must
involve removing most of their cold gas (the raw material for star formation). We can now
also read clues to the evolution of early-type galaxies in the kinematics and the
metallicity of their gas, and possibly also in the rare isotope abundance patterns in the
cold gas. Numerical simulations are beginning to work on reproducing these cold gas
properties, so that we can place the early-type galaxies into their broader context.
Starless clumps and the earliest phases of high-mass star formation in the Milky Way
Brian Svoboda, NRAO Jansky Fellow
High-mass stars are key to regulating the interstellar medium, star formation activity, and overall evolution of galaxies, but their formation remains an open problem in astrophysics. In order to understand the physical conditions during the earliest phases of high-mass star formation, I will present observational studies we have carried out on dense starless clump candidates (SCCs) that show no signatures of star formation activity. We identify 2223 SCCs from the 1.1 mm Bolocam Galactic Plane Survey, systematically analyse their physical properties, and show that the starless phase is not represented by a single timescale, but evolves more rapidly with increasing clump mass. To investigate the sub-structure in SCCs at high spatial resolution, we investigate the 12 most high-mass SCCs within 5 kpc using ALMA. We find previously undetected low-luminosity protostars in 11 out of 12 SCCs, fragmentation equal to the thermal Jeans length of the clump, and no starless cores exceeding 30 solar masses. While uncertainties remain concerning the star formation efficiency in this sample, these observational facts are consistent with models where high-mass stars form from initially low- to intermediate-mass protostars that accrete most of their mass from the surrounding clump. I will also present on-going research studying gas inflow signatures with GBT/Argus and ALMA, and the dense core mass function with the JVLA.
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