Coral Wheeler


About Me

I am now a DuBridge Postdoctoral Fellow at TAPIR at Caltech, working closely with Phil Hopkins' research group. Please stay tuned for a new website. You can email me at coral at caltech dot edu.

My research primarily focuses on dwarf and satellite galaxies - their morphology, kinematics, stellar populations and the details of their star formation histories. How is star formation triggered and then quenched in low-mass galaxies? What effects — environmental or otherwise — are related to this quenching? Do hundreds, or even thousands of ultra-faint satellite galaxies, red and dead due to reionization-related quenching, orbit the Milky Way (lying just outside of current detection limits)? Why do dwarf spheroidal galaxies exhibit little to no rotation? What is the source of the inverted age gradients found in many dwarfs (with younger populations in the center and older populations more extended)? I search for the answers to these questions by running N-body and hydrodynamic simulations of galaxy formation, and by analyzing these and existing simulations and comparing to observations.

I am a strong advocate of social justice and increasing diversity in astronomy. I serve as an elected Physical Sciences representative to the Associated Graduate Students council and a head steward for UAW 2865, our union for TAs, Readers and Tutors at UC. If you are a graduate student at the UC, I encourage you to get involved in both your grad student government as well as your union (undergrads who work as TAs, Readers or Tutors should also get involved)! I am also on the Organizational Committee for Women in Physics at UC Irvine, and the creator of our Grad-Undergrad Mentoring Match-up -- a program that pairs female graduate students with undergrad women in the Physics department to improve retention.

"The true sign of intelligence is not knowledge but imagination."
albert einstein



Structure of Dwarf Galaxies Why do dwarf galaxies tend to be puffier, exhibit little to no rotation, and have proportionally more gas than their more massive counterparts? Using simple analytic arguments, I show that the imposition of a simple temperature floor leads to low mass galaxies forming as thicker disks with higher gas fractions, that are dispersion rather than rotationally supported. I follow up that work by running fully hydrodynamic zoom-in simulations of isolated dwarfs (Mvir ~ 10^9 - 10^10 Msun). Without an ad-hoc temperature floor, these dwarfs naturally form as puffy, gas-dominated systems with little to no rotation.

Satellites of Isolated Dwarfs In the currently favored cosmological paradigm, dark matter halos have substructure, and that substructure extends to the lowest masses probed by current simulations. But how often do these low mass dwarf galaxies have subhalos that actually form stars? At what mass do we expect all subhalos to be dark because of the ionizing background released from the first galaxies? We use some of the highest-resolution hydrodynamic simulations to date to predict the existence of star-forming ultrafaint (M* ~ 3000 Msun) satellites around isolated dwarf galaxies (Mvir ~10^10 Msun). These low mass satellite galaxies were able to form almost all of their stars in the first couple of Gyr after the Big Bang, which is how they were able to form in the presence of the ionizing background.

Quenching in Dwarf Galaxies What shuts down star formation is low mass galaxies? Observations done with the NASA Sloan Atlas (NSA) show that even within close projected distance to a more luminous neighboring galaxy, only about 30% of dwarf galaxies with stellar masses ~10^9 Msun have had their star formation shut down. By comparing these observations to the cosmological Millennium II simulation (MSII), we show that whatever is quenching star formation in these low mass galaxies is inefficient, requiring a long timescale and suggesting a slow-acting quenching mechanism such as strangulation. This is in surprising contrast to Local Group (LG) dwarfs at even lower mass. Observations of classical dwarfs of the Milky Way and Andromeda suggest a critical mass for quenching, below which the quenching timescale takes a drastic plunge to ~ 1-2 Gyr. This suggests that an alternative mechanism, such as ram-pressure stripping, for shutting down star formation operates in galaxies below this critical mass.

Dwarf Galaxies' Stellar Populations While massive spiral galaxies have stellar age gradients that go from old to young as you move outward in the disk, dwarfs have the opposite trend. In dwarf galaxies, it is the younger populations that inhabit the central regions of the galaxy, while the older stars are more extended in space. I show, using fully hydrodynamic simulations of dwarf galaxies, that these gradients can arise naturally due to strong feedback in dwarfs and are tied to their star formation histories.

Isolation and Selection Criteria Due to projection effects and velocity-space distortion, observations of ``hosts and satellite pairs” tend to contain many falsely identified background contaminants, while observations of ``isolated” galaxies tend to identify objects in groups or clusters. Imposing simple projected distance and velocity-offset limits does little to remove these interlopers. We use the Millennium II simulation (MSII) to develop and test criteria for selecting and isolating Milky Way sized galaxies and their massive dwarf-sized satellites, showing that very strict isolation criteria are required to ensure that a high fraction of the pairs are actual host-satellite pairs and do not reside in large groups and clusters.


Graduate Publications

Wheeler, Coral; Pace, Andrew B.; Bullock, James S.; Boylan-Kolchin, Michael; Onorbe, Jose; Fitts, Alex; Hopkins, Phil; Keres, Dusan; “The no-spin zone: rotation vs dispersion support in observed and simulated dwarf galaxies”, 2015, preprint (arxiv:1511.01095), submitted to MNRAS

Wheeler, Coral; Onorbe, Jose; Bullock, James S.; Boylan-Kolchin, Michael; Elbert, Oliver; Garrison-Kimmel, Shea; Hopkins, Phil; Keres, Dusan; “Sweating the small stuff: simulating dwarf galaxies, ultra-faint dwarf galaxies, and their own tiny satellites ”, MNRAS, 2015, Volume 453, Issue 2, p.1305-1316

Wheeler, Coral; Phillips, John I.; Cooper, Michael C.; Boylan-Kolchin, Michael; Bullock, James S., “The surprising inefficiency of dwarf satellite quenching”, MNRAS, 2014, Volume 442, Issue 2, p.1396-1404

Fillingham, Sean; Cooper, Michael C.; Wheeler, Coral; Garrison-Kimmel, Shea; Boylan-Kolchin, Michael; Bullock, James S., “Taking care of business in a flash: Constraining the timescale for low-mass satellite quenching with ELVIS”, MNRAS, 2015, Volume 454, Issue 2, p.2039-2049

Phillips, John I.; Wheeler, Coral; Cooper, Michael C.; Boylan-Kolchin, Michael; Bullock, James S.; Tollerud, Erik J., “The mass dependance of satellite quenching in Milky Way-like halos”, MNRAS, 2015, Volume 447, Issue 1, p.698-710

Phillips, John I.; Wheeler, Coral; Boylan-Kolchin, Michael; Bullock, James S.; Cooper, Michael C.; Tollerud, Erik J., “ A dichotomy in satellite quenching around L* galaxies”, MNRAS, 2014, Volume 437, Issue 2, p.1930-1941

Kaufmann, Tobias; Wheeler, Coral; Bullock, James S., “On the morphologies, gas fractions, and star formation rates of small galaxies&rdquo, MNRAS, 2007, Volume 382, Issue 3, pp. 1187-1195

Undergraduate Publications

C R Wheeler, R D Ramsier and P N Henriksen, “Visibility of thin-film interference fringes, Amer. J. Phys. 72, 279, 2004

C R Wheeler, R D Ramsier and P N Henriksen, “Observing thin-film interference effects, Phys. Educ. 38, No 6, 495-496, Nov 2003

C R Wheeler, R D Ramsier and P N Henriksen, “An investigation of the temporal coherence length of light, Eur. J. Phys. 24 No 4, 443-450, July 2003

Brian Cheyne, Vishal Gupta, Coral Wheeler, “Hamilton cycles in addition graphs, Rose Hullman Undergraduate Mathematics Journal, Volume 4(1), 2003

"I love the smell of the universe in the morning."

Neil deGrasse Tyson

Curriculum vitae

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Email: crwheele at uci dot edu

Twitter @coralrosew