Research
I'm studying the evolution of galaxies and quasars (also known as QSOs) at the peak epochs of star formation and black hole accretion (2 < z < 3). I am particularly interested in the properties of dwarf galaxies as well as the environments around the most powerful quasars in the Universe. Most of my work uses optical and infrared images and spectra from the LRIS, MOSFIRE, and KCWI instruments on the 10-m Keck 1 telescope at the W.M. Keck Observatory, as well as data from the Hubble Space Telescope. I was also part of the team that built and commissioned MOSFIRE.
My current research falls under several related topics, which I describe below (on the left). On the right, you can find a glossary of terms involved in my research, just in case you have not spent the better part of your life neck-deep in extragalactic astronomy.
An up-to-date list of my publications can be found at the ADS.
Understanding Lyman-alpha Emission
Several of my projects focus on understanding a specific type of light called Lyman-alpha (Lya) emission. As described in the right sidebar, Lya emission is emitted by excited hydrogen gas, and excited hydrogen tends to sit near newly-formed stars and "active" black holes in galaxies. Using this light is therefore a good way to see where stars and black holes are forming in the Universe.
This Lya light is also complicated, however, because it is scattered by the hydrogen in galaxies as it tries to get out. In one of our recent papers, we conducted a careful analysis of different methods for estimating how many Lya photons were created in galaxies and how many of them escape in order to predict the total efficiency with which these photons are detected in our measurements of galaxies. (see paper)
Properties of "baby" galaxies
I am currently studying young galaxies called Lyman-alpha emitters (LAEs). These galaxies have a lot of gas that they are beginning to turn into stars. Using images and spectra from Keck and the Hubble Space Telescope, I am studying the properties of stars in these galaxies and how stellar feedback produces heavy elements within the galaxies as well as driving gas and ultraviolet photons out of the galaxies.
We recently published a paper showing that these faint, young LAE galaxies are able to produce outflows that drive gas (and heavy elements) out of the galaxies, but that these outflows are much weaker than the outflows produced by brighter, more massive galaxies. These observations show that galaxy feedback works differently at different galaxy mass scales. (see paper)
More recently, we published another paper demonstrating that LAEs have extreme properties that suggest the presence of very pristine gas (without much heavy element contamination) and very hot populations of massive stars. These stars may be produced by the evolution of binary systems, as described by new models of stellar evolution. This work would not have been possible without the new instrument MOSFIRE described below. (see paper)
Host Halos of QSOs
Hyperluminous QSOs are extremely bright and exceedingly rare; these objects are brighter than 10^14 solar luminosities, and there are likely only a few dozen of them in the observable universe.
By studying the spatial and velocity distribution of galaxies near the QSOs, we were able to determine that these QSOs sit in dark-matter halos of similar masses to those hosting lower-luminosity AGN and typical star-forming galaxies at these redshifts. In addition, we found that the QSOs are associated with high local densities of galaxies, which suggests that recent galaxy mergers are more important than halo mass in producing efficient (ie. hyperluminous) black-hole accretion. (see paper)
The IGM Near QSOs
The brighest QSOs produce intense ionizing fields that can exceed the UVB by factors of ~1000x over scales of an Mpc or more. This makes the fields around QSOs excellent places to look for fluorescent emission, where ionizing photons are reprocessed by dense Hydrogen gas and reemitted as Lya photons. Using narrow-band filters tuned to the Lya line at the redshift of the QSOs, we have identified ~1000 Lyman-alpha emitters (LAEs) that may be exhibiting fluorescent emission, and we have obtained ~400 spectra of the Lya lines.
The Lya luminosity function and distribution of Lya equivalent widths (far exceeding those seen in star-forming galaxies in many cases) show strong evidence for fluorescent emission in many of our candidates, and the distribution of these candidates (in redshift and the plane of the sky) implies that the hyperluminous QSOs in our sample have lifetimes between 1 and 20 Myr. (see paper)
Furthermore, our Lya spectra reveal rich properties in their line profiles (multi-peaked emission, red- and blue-dominant peaks, etc.) that may tell us about their kinematics and the effects of radiative transfer. With continuing MOSFIRE observations, we will further probe the physical properties of these objects.
MOSFIRE
As a member of the MOSFIRE
instrument team, I helped
calibrate and commission the
instrument, focusing in
particular in the modeling of
the instrument flexure (as it
lies at the Cassegrain focus
of the Keck 1 telescope) and
calibrating the flexure
compensation system.
MOSFIRE is now fully operational and taking beautiful NIR spectra and images (with minimal residual flexure). Check astro-ph or the ADS for recent results!
Glossary
Redshift - Astronomers use the letter z to denote the "redshift" of a galaxy, which is a measure of how much the light from the galaxy has been stretched on its way to us. This stretching occurs because the Universe has been expanding ever since the Big Bang! I study galaxies around z ~ 2.5, which are so far away that the light we see was emitted about 11 billion years ago, when the Universe was very young (only 2-3 billion years old).
Feedback - Galaxy "feedback" describes the fact that galaxies tend to grow in self-destructive ways. As new stars form (and as black holes grow), the energy produced tends to blow gas (the fuel for star-formation) back out of galaxies, which keeps them from growing further. This process is important for understanding how many stars form in a galaxy, how they are arranged, and how long it takes to form them.
AGN - An AGN is an Active Galactic Nucleus, which is a very bright component at the center of a galaxy that we can identify from its spectrum. AGN are powered by the accretion of material onto a supermassive black hole. Seeing an AGN thus means that its black hole is in the process of growing (and potentially affecting its galaxy with the light and material it emits).
QSO - Also known as a quasar, a QSO is the brightest type of AGN when viewed in optical light. QSOs are among the brightest objects in the universe, and can outshine their host galaxies by 10-1000x! Because QSOs are rare and powerful, it is important to know how they form and how they affect their surroundings.
Dark Matter Halo - A dark matter halo is a particularly dense clump of dark matter that has formed through gravitational attraction. These clumps of dark matter are able to capture large amounts of gas that form stars and galaxies (and feed QSOs). While we can't measure the amount of dark matter in a galaxy's halo directly, we know that more massive dark matter halos cluster together more than less massive halos. By measuring how galaxies and QSOs cluster, we can then infer the mass of their dark matter halos.
IGM - While some gas falls into dark matter halos to form stars, much of the gas in the universe lies in the seemingly empty spaces between galaxies (particularly when the universe was young); this gas is called the Intergalactic Medium (IGM). The IGM helps us understand how gas gets into and out of galaxies, but studying it requires special techniques because the gas in the IGM does not emit much light on its own.
Lya - Lyman-alpha emission is light emitted by a Hydrogen atom (the most abundant element in the universe) when it undergoes a specific transition from its first excited state to its ground state. Hydrogen atoms in this excited state are especially common when an energetic object is nearby (such as a newborn star or a QSO), so measuring Lya emission can reveal regions that are being affected by QSOs or where stars are being formed.