The motions of Local Group dwarf galaxies
The dwarf galaxies orbiting the Milky Way and M31 are the smallest, most
dark-matter-dominated systems we can still resolve into individual stars.
How they move tells us about the mass and internal structure of the dark
matter halos they live in, which is exactly where competing cosmological
models make different predictions.
The catch is that these motions are tiny — tens of microarcseconds per year
— so the measurement is as much about controlling systematics as it is
about the data. My recent work measures the proper motion of the ultra-faint
dwarf Draco II from HST imaging using several independent reference frames
and methods, so that the reference frame itself can be tested rather than
assumed.
HubPUG: proper motions from HST anchored to Gaia
HubPUG measures proper motions for resolved stellar systems — dwarf
galaxies, globular clusters, stellar streams — that have two epochs of HST
imaging, using Gaia stars in the foreground as the reference frame instead
of distant background galaxies.
That choice does two things. It improves on Gaia-only proper motions for
Milky Way satellites, and it opens up systems that have few or no Gaia stars
of their own, such as ultra-faint dwarfs and the distant satellites of M31.
Cross-observatory astrometry with HST and JWST
HST has been imaging the Local Group for more than thirty years. JWST has
been at it for three, and Roman has not started. Tying those archives
together gives time baselines no single observatory can match — but only if
we understand what happens to an astrometric measurement when the two epochs
come from different telescopes, different detectors, and different
distortion solutions.
Using the JWST ERS data together with my own HST Cycle 31 and JWST Cycle 2
programs, I am characterising those cross-observatory systematics directly.
Getting this right is a prerequisite for using the HST archive as the first
epoch for anything JWST and Roman observe.
JWST Early Release Science — Resolved Stellar Populations
I am a member of the JWST Early
Release Science Resolved Stellar Populations team, which set out to
build the tools and best practices for reducing, analysing, and
interpreting JWST data of resolved stars. Our Cycle 1 data covered the
Milky Way ultra-faint dwarf Draco II, the isolated Local Group dwarf WLM,
and the globular cluster M92.
My contribution was a set of photometric criteria for telling stars from
background galaxies in NIRCam catalogues produced with
DOLPHOT — an
unglamorous but load-bearing step for any faint-end science with these data.
Stellar ages across the Milky Way
Before I worked on dwarf galaxies, I worked on the ages of red giants.
Combining APOGEE spectroscopy with K2 asteroseismology yields masses, and
therefore ages, for thousands of giants spread across the Galaxy — a much
wider footprint than Kepler’s single field.
In the APO-K2 catalog I derived ages for the red giant branch sample, and in
earlier work identified an intermediate-age, alpha-rich population in K2
that is difficult to produce in standard chemical evolution models. The age
tables are public.