The Circumgalactic Medium
--- Drummond Fielding, Eliot Quataert, Michael McCourt, and Todd Thompson
The Impact of Star Formation Feedback on the
Circumgalactic Medium. MNRAS, 466, 3810 (2017).
In the modern paradigm for the formation of structure in our universe,
matter over-densities imprinted during the big bang collapse to form
‘halos’ of dark matter. These dark matter halos are initially filled
with hot gas heated to the virial temperature by an accretion shock.
Though the gravitational process of structure formation is self-similar,
pioneering work in the 1970’s recognized that the cooling of halo
gas ultimately differentiates galaxies from more massive systems such as
galaxy groups or clusters. In galaxies with sufficiently low mass, halo
gas can in principle
cool, accrete, and play a role in star formation. In galaxy clusters, on
the other hand, halo gas cannot cool and instead persists for the age of
the universe, essentially independent of the galaxy formation taking
place at smaller radii. The transition between these two outcomes for halo
gas occurs around a total mass of ∼1012 M⊙,
relatively independent of redshift. Thus, a seemingly small distinction
related to atomic physics ultimately drives all of the marked observational
differences between galaxies and galaxy clusters. The basic physics of halo
gas therefore plays a primary, but relatively unexplored, role in our
history of the universe and the types of structures that form within it.
Until recently, we had only indirect measurements of halo gas in galaxies.
As a result, current cosmological simulations have been tuned to reproduce
the stellar properties of galaxies, which are much better constrained from
observations. As new observational techniques begin to probe the halo gas
in galaxies, however, it will become increasingly important to improve the
prescriptions for gas physics in our models. Many of the new
observations—particularly the HST COS quasar absorption spectra observations
(e.g., Werk et al. 2014)—are at odds with the predictions from state-of-the-art
cosmological simulations. Predicting, observing, and understanding the properties
of halo gas represents the new frontier in galaxy formation.
In our first paper on this topic, we use idealized three-dimensional hydrodynamic
simulations to study the dynamics and thermal structure of the circumgalactic
medium (CGM) resulting from the interplay of cooling and stellar feedback in the
presence of cosmological accretion of gas. As in the pioneering calculations
without stellar feedback (e.g., Birnboim & Dekel 2003; Keres et al 2005), we find
that above a critical halo mass (∼1012 M⊙) halo gas is
supported by thermal pressure created in the halo virial shock. The thermal
properties of the halo gas at small radii (near any central galaxy) are regulated
by feedback triggered when tcool/tff ~ 1−10 in the hot halo
gas. Below a critical halo mass, ∼1012 M⊙, however, there is
no thermally supported halo. Instead, the halo gas properties are determined by the
interaction between inflowing gas and galactic winds. The halo gas is not in
thermal pressure balance, but is instead effectively supported by random motions
and outflow ram pressure. This may explain the failure of pressure equilibrium to
account for the properties of cool halo gas inferred from COS observations.
In future work on this subject we will use a similar numerical setup to study
the effect of filamentary accretion, magnetic fields, anisotropic conduction and
viscosity, and halo growth rate.
high mass loading
low mass loading
Oblique Protoplanetary Disks
In addition to my work on the CGM and galactic winds,
with Chris McKee I made
contributions to the field of protostellar disk and
planet formation using a similar idealized simulation
approach. The Kepler mission found many Jupiter mass
planets with very short orbital periods
(P<10 days). How these "hot Jupiters" migrated to
such close-in orbits is debated. Observations
of non-zero stellar obliquity---the angle between the host star's spin axis and the hot Jupiter's orbital axis---seemed to
favor orbital angular momentum loss due to
tidal dissipation from the star-planet
interaction during close periastron passages
induced by torques from a distant third body
(Fabrycky and Tremain 2007). However, using
hydrodynamic and magnetohydrodynamic
simulations of protostar formation, I
demonstrated that a wide range of stellar
obliquities may be produced as a byproduct of
forming a star within a turbulent environment
(Fielding+15). I developed a simple
semi-analytic model that reveals this
connection between the turbulent motions and
the orientation of a star and its disk. Our
results are consistent with the observed
obliquity distribution of hot Jupiters implying
that the misaligned hot Jupiters may have
instead migration due to tidal dissipation in
the disk (Goldreich and Tremaine 1980). We have
also applied this same concept to explain the
observed misalignment of protostellar binaries
outflows (Offner et al. 2016).
--- Drummond Fielding, Christopher McKee, Aristotle Socrates, Andrew J. Cunningham, and Richard I. Klein, The Turbulent Origin of Spin-Orbit Misalignment in Planetary Systems, MNRAS, 450, 3306 (2015).
--- Stella Offner, Michael Dunham, Katherine Lee, --
--Héctor Arce, and Drummond Fielding, The Turbulent Origin of Outflow and Spin Misalignment in Multiple Star Systems
, ApJL, 827, 110 (2016).
The Yarkovsky (YORP) Effect
As an undergraduate I did a fun summer project with
Ned Wright at UCLA.
Using Ned's rotating, cratered, thermophysical astroid model I modeled the light
curves of WISE observed asteroids to
determine the asteroids' thermal properties. With an asteroid's specs (diameter,
albedo, rotation rate, and so on) in hand I then modeled how they will evolve
due to the Yarkovsky (or YORP) effect. We focused on finding out which
asteroids will make it into mean-motion resonances with Jupiter—catapulting them
chaotically into other parts of the solar system, maybe even onto a collison course
with the earth! We also made some cool estimates about the parent asteroid family
that may have spawed the dinosaur killing asteroid.
The Yarkovsky effect is an elegant phenomenon in which the non-heliocentric
component of the radiation pressure of an asteroid can torque the asteroid causing
it to slowly drift. If an asteroid isn't rotating then the hottest part of the
asteroid, the part emitting most of the radiation, is point back to the sun, so
there is no torque. But, if an asteroid is rotating then due to thermal inertia the
hottest part is offset from the sub-solar point (this is the same reason why the hottest time of day isn't noon but 1 or 2 pm), which means the asteroid is
radiating away from the sun, and so there is a torque. The direction of the torque depends on the direction the asteroid is rotating.
I am a finishing graduate student
in the astronomy department at UC Berkeley.
I am an NSF graduate research fellow and a
Berkeley fellow. At Berkeley, with my advisor
I use idealized simulations to study the
circumgalactic medium and galactic winds.
In short, we look at how galaxies get their gas
and how they expel it.
I did my undergraduate studies at Johns Hopkins
University, where I majored in Physics and in Math.
I was born in New York City and grew up a few miles
away in Montclair, New Jersey where I attended
Montclair High School.
I have more hobbies than I can really maintain. Right now I mostly focus on playing squash, making pottery, backpacking, bike touring, and keeping my far too many succulents thriving.
dfielding ‘at’ berkeley ‘dot’ edu
407D Campbell Hall
Department of Astronomy
University of California, Berkeley
Berkeley, CA 94720
A copy of my CV can be found here.