Research Interests
I am interested in a wide range of astrophysical topics, from improving our understanding of fundamental stellar evolution to studying galaxy structure, formation, and evolution. I conduct observations from a number of ground and space based observatories, including the CFHT, Subaru, Gemini, Keck, MMT, KPNO, HST, GALEX, and XMM telescopes. This research has led to about 20 scientific publications in peer-reviewed journals (as first author) as well as many additional papers written by my collaborators. A brief summary of a few of the current projects I am involved in is given below. A complete list of these publications can be found here.
1.) The "SPLASH" Survey - M31's Stellar Halo
I am a member of a group (PI: Raja Guhathakurta) leading a large international imaging/spectroscopic project called SPLASH (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo). The Andromeda Galaxy (M31) represents the nearest large, spiral system that is similar to the Milky Way. Given our vantage point, studies of M31 are in many ways superior to those in the Milky Way for contributing to our understanding of the processes that shape the formation of galaxies and their evolution. Recently, our team has published a series of papers addressing the detailed properties of M31, studied for the first time using individual spectroscopically confirmed member stars. Our analysis demonstrates that, contrary to previous findings, M31 does contain an extended, metal-poor, power-law, sparse stellar halo, and that this halo dominates the more metal-rich inner spheroid of M31 at a distance beyond 20-30 kpc. We are now studying the detailed properties of the true halo of M31 and confronting improved galaxy formation simulations (e.g., Via Lactea) to gain significant insights into this tenuous and elusive stellar population.
2.) The "SPLASH" Survey - M31's Dwarf Satellites
In the currently accepted picture of galaxy formation,
dwarf satellites are participants in the most recent, ongoing, and future accretion
events. Our research on M31's stellar halo demonstrates that it's past assembly
history was likely more violent than in the Milky Way. Such a different
merging process may have imprinted a signature on the seeds of the accretion
process that survived, the present day dwarf galaxies. I am leading an effort (with
James Bullock) to obtain
accurate velocities and metallicities for hundreds of individual stars in each of
M31's dwarf satellites. Our first results on six satellites (one of which we discovered)
have established their mean velocity, velocity dispersion, and mass-to-light ratio
for the first time. We are in the process of searching for kinematical signatures of
tidal interaction (e.g., stripping), measuring the nature of the satellite dark matter
halos, and estimating the abundance distributions within each satellite in relation to
the global M31 stellar halo. Our results, published in two recent ApJ papers, suggest
both similarities and important differences between the Milky Way dwarf spheroid
population and the M31 population.
As our wide-field photometric surveys continue to target relatively unexplored
regions in M31's halo, new dwarf galaxies will be found. A simple integration
of the current sample over the area that has not been surveyed indicates that 50-60
such systems reside in M31. As our group, and others, discover these new
galaxies, we will continue to follow them up with spectroscopy. This is essential
both for relating the satellites to specific substructures in the halo and for
confirming the general relationship between the halo and satellite systems. Finally,
this work provides an important sample to test cold dark matter galaxy formation
models on small scales (e.g., addressing the "missing satellite problem").
3.) Open Star Clusters and Stellar Evolution
Star clusters in the Milky Way have served as the most important laboratory for our
understanding of fundamental stellar evolution. By virtue of their coeval, cospatial,
and isometallic nature, each individual star cluster evolved in a unique environment
and therefore represents a controlled test-bed for theoretical models. In spite of the
homogeneity of stars within a cluster, the sample of nearby open clusters spans a
diverse range of properties (e.g., ages, chemical abundances, and interaction histories)
and therefore the comparison of stellar populations in different star clusters leads to
a thorough understanding of the processes that shape the evolution of stars and the
formation of the Galactic disk.
I am the principal
investigator of a large survey with the primary goal of establishing accurate, deep
color-magnitude diagrams for several dozen of these Galactic disk clusters as a part
of the Canada-France-Hawaii Telescope (CFHT)
Open Cluster Survey. For many clusters, this photometry represents the first probe
of the low mass hydrogen burning stellar populations in the system. The ensemble
of clusters has provided a homogenous test of stellar evolution theory over a large mass
(e.g., 0.3-6.0 Msun) and metallicity range, and helped better define the fundamental
parameters of each cluster.
4.) Globular Star Clusters and the Age of the Galactic Halo
With a team led by Harvey
Richer, we have successfully obtained two of the largest allocations of
HST telescope time ever awarded (123 orbits
in Cycle 9 and 126 orbits in Cycle 13). We performed ultra-deep observations of the
entire stellar population in two nearby globular star clusters, M4 and NGC 6397.
By matching the positions of stars in these data sets to older, shallower exposures,
we can astrometrically select (via proper motion) the cluster stars and eliminate
foreground/background contamination. The resulting color-magnitude
diagrams for each cluster allow us to track the stellar mass function of each system
from the brightest giants to the hydrogen burning limit.
As these clusters are among the first objects to have formed in the Galactic halo,
all stars above 0.8 Msun have now evolved off the main-sequence and formed (mostly)
low mass white dwarfs (stellar remnants with no fuel). We characterize the complete
white dwarf cooling sequence in each cluster and model it to independently derive the
age of M4 and NGC 6397. Our findings suggest that the globular cluster population
(and therefore the stellar halo of the Milky Way) formed roughly 12 billion years
ago, about 1.7 billion years after the Big Bang.
In addition to the primary goals described above, I have been involved in over a
dozen papers from these data sets related to secondary science. These include
measuring the space motion of each cluster to unprecedented accuracy, measuring
the binary fraction and radial distribution of different species of stars, investigating
the cluster dynamics and mass segregation effects, studying the field distribution
of stars along the line of sight to set constraints on Galactic structure models,
looking for exotic stellar populations such as cataclysmic variables, and more.
In HST Cycle 17, our team has been granted another 121 orbits of observing time to target
a third globular cluster in this program, 47 Tuc. The first papers from this project are
now published.
5.) The Initial-Final Mass Relation
I have been leading an effort to follow up the white dwarf detections in our
imaging campaigns with multiobject spectroscopic observations (largely from
Keck). These remnant stars
represent our only link to understand the progenitor population that has now
exhausted its hydrogen supply. Through the modeling of Balmer lines in the
spectrum of each white dwarf (in collaboration with
Pierre Bergeron),
we can establish the star's temperature, gravity,
mass, and cooling age. These remnant masses can be linked to their
progenitor masses (i.e., the progenitor lifetime is the difference between the
cluster age and the white dwarf cooling age) and therefore the amount of stellar
mass loss in different environments can be directly probed. This research
therefore establishes constraints on one of the most fundamental relations in
stellar astrophysics, the initial-final mass relation. The relation is vital
to our understanding of chemical evolution in galaxies, enrichment of the
interstellar medium (and therefore the efficiency of star formation), and the
birth rates of neutron stars and type II supernovae.
In our recent study of the super-solar metallicity cluster NGC 6791, we found that
the white dwarfs are significantly undermassive. This suggests that
post main-sequence mass loss rates are enhanced in higher metallicity environments,
as predicted theoretically. The result provides a natural explanation for the production
of extreme horizontal branch stars in this cluster, and resolves the anomalously low
white dwarf cooling age recently measured by another group. Further, the
result may be critical in interpreting the UV upturn in old metal-rich systems, such as
elliptical galaxies.
In addition to our study of NGC 6791, white dwarf measurements in the
old solar metallicity clusters NGC 7789 and NGC 6819 have established the first direct
constraints on the low mass end of the initial-final mass relation and measurements in M4
have provided the first direct estimates of mass loss in population II (metal-poor) stars
through this technique. These data allow us to reconstruct the stellar mass function of
the now evolved progenitor population in our Galaxy's past 12 billion year lifetime.
Additionally, we've now used this relation to measure the age of the Galactic disk and
halo, directly from white dwarf studies.