About me

I am a sixth and final year 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 Eliot Quataert, I design and run idealized simulations to study the launching of galactic winds and the structure of the circumgalactic medium.

Launching galactic winds

Galactic winds play a critically important role in galaxy evolution—they help limit galactic star formation efficiencies by expelling material from the interstellar medium and by halting gas inflow into galaxies, and they enrich and heat the circumgalactic medium. We use idealized global and local simulations to study how SNe drive winds.

Circumgalactic medium

Galaxies are surrounded by diffuse gas that fills their dark matter halos. The structure and kinematics of the CGM depend sensitively on halo mass (and possibly feedback properties). In recent years CGM observations have presented rich but surprising constraints that we use controlled simulations to try to shed light on.

Supernova driven galactic winds

Drummond Fielding, Eliot Quataert, Davide Martizzi, and Claude-André Faucher-Giguère How Supernovae Launch Galactic Winds MNRAS letters submitted (2017). arXiv:1704.01579


We use idealized three-dimensional hydrodynamic simulations of global galactic discs to study the launching of galactic winds by supernovae (SNe). The simulations resolve the cooling radii of the majority of supernova remnants (SNRs) and thus self-consistently capture how SNe drive galactic winds. We find that SNe launch highly supersonic winds with properties that agree reasonably well with expectations from analytic models. The energy loading (ηE = Ėwind / ĖSN) of the winds in our simulations are well converged with spatial resolution while the wind mass loading (ηM = Ṁwind / Ṁ∗) decreases with resolution at the resolutions we achieve. We present a simple analytic model based on the concept that SNRs with cooling radii greater than the local scale height breakout of the disc and power the wind. This model successfully explains the dependence (or lack thereof) of ηE (and by extension ηM) on the gas surface density, star formation efficiency, disc radius, and the clustering of SNe. The winds in the majority of our simulations are weaker than expected in reality, likely due to the fact that we seed SNe preferentially at density peaks. Clustering SNe in time and space substantially increases the wind power.



Davide Martizzi, Drummond Fielding, Claude-André Faucher-Giguère, and Eliot Quataert, Supernova Feedback in a Vertically Stratified Medium: Interstellar Turbulence and Galactic Wind Launching, MNRAS, 459, 2311 (2016).

We use local Cartesian simulations with a vertical gravitational potential to study how supernova (SN) feedback in stratified galactic discs drives turbulence and launches galactic winds. Our analysis includes three disc models with gas surface densities ranging from Milky Way-like galaxies to gas-rich ultra-luminous infrared galaxies (ULIRGs), and two different SN driving schemes (random and correlated with local gas density). In order to isolate the physics of SN feedback, we do not include additional feedback processes. We find that, in these local box calculations, SN feedback excites relatively low mass-weighted gas turbulent velocity dispersions ~3-7 km/s and low wind mass loading factors, less than 1, in all the cases we study. The low turbulent velocities and wind mass loading factors predicted by our local box calculations are significantly below those suggested by observations of gas-rich and rapidly star-forming galaxies; they are also in tension with global simulations of disc galaxies regulated by stellar feedback. Using a combination of numerical tests and analytic arguments, we argue that local Cartesian boxes cannot predict the properties of galactic winds because they do not capture the correct global geometry and gravitational potential of galaxies. The wind mass loading factors are in fact not well-defined in local simulations because they decline significantly with increasing box height. More physically realistic calculations (e.g., including a global galactic potential and disc rotation) will likely be needed to fully understand disc turbulence and galactic outflows, even for the idealized case of feedback by SNe alone.

Supernova driven galactic winds

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


I did a project with Chris McKee when I first arrived at Berkeley. We studied how the turbulent environment from which stars form may lead to misalignment between the stellar spin and the remnant protoplanetary disc. By using hydrodynamic and magnetohydrodynamic simulations, we demonstrated that a wide range of stellar obliquities may be produced as a by-product of forming a star within a turbulent environment. We presented a simple semi-analytic model that revealed this connection between the turbulent motions and the orientation of a star and its disk. Our results were consistent with the observed obliquity distribution of hot Jupiter, which indicates that the migration of misaligned hot Jupiters may be due to tidal dissipation in the disk, rather than tidal dissipation of the star-planet interaction.

--- 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).

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.

About Me

I am a fifth year 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, Eliot Quataert, 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.


Contact


dfielding ‘at’ berkeley ‘dot’ edu

407D Campbell Hall
Department of Astronomy
University of California, Berkeley
Berkeley, CA 94720

CV

A copy of my CV can be found here.

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