My group is involved in a number of observational and theoretical collaborations.  I list the major efforts here, but know that we have many other irons in the fire!  You can get a sense of what I think are problems and opportunities in dark matter astrophysics in this review paper by Prof. Matt Buckley (Rutgers) and me. [Last updated Nov. 2020…]

Projects related to dark matter + dwarf galaxies

Large Binocular Telescope Satellites Of Nearby Galaxies (LBT-SONG)

Three of these are real galaxies, one is an artificial galaxy inserted into an image to test our photometry methods. Can you tell which one is not like the others?

The premier optical galaxy search for our group, we are recycling data from Chris Kochanek’s failed supernova search program, taken with the Large Binocular Telescope Large Binocular Camera (LBT LBC) over the course of the past decade.  This is the project that inspired me to liken our group to a bunch of “data raccoons”!  We are able to find satellite galaxies down to ultrafaint scales close in projection to a variety of star-forming hosts in the Local Volume (up to ~10 Mpc from us).  Because we are able to find satellite galaxies so close to their hosts, we have a long lever arm to explore satellite quenching as a function of environment, and the central concentration of satellites.  Among other key results, we find the first evidence for quenching of star formation in satellite galaxies by the cool gas outflow from low-mass host galaxies.

First results from this survey + theory interpretation:

This survey was funded through NSF grant AST-1615838.  The key graduate students leading this work are Kirsten Casey, Bianca Davis, and Chris Garling.  We also worked with several undergraduates on the project.  Important aspects of this project would not have been possible without the involvement and guidance of former CCAPP fellow and current UC Merced assistant professor Anna Nierenberg, and former NSF and CCAPP fellow Dr. Johnny Greco.  This grant also enabled our development of a semi-empirical approach to making theoretical predictions for satellite populations and their quenching mechanisms (former students Drs. Greg Dooley, Chris Garling, and Stacy Kim), of observational studies of satellites of other systems (lead by former Princeton grad student Dr. Scott Carlsten), and the development of a new LBT/LBC data reduction and surface brightness fluctuation distance measurement pipeline (lead by grad student Kirsten Casey).

Magellanic Analog Dwarf Companions And Stellar Halos (MADCASH)

HSC image of Local Volume Magellanic Cloud analog NGC 2403 and the small dwarf satellite discovered by the MADCASH collaboration. From Carlin et al. (2016).

A survey with DECam and the Hyper Suprime-Cam to measure the satellite luminosity function and quenching mechanisms for isolated analogs of the Magellanic Clouds.  Our goal is to determine if the satellite luminosity function follows LambdaCDM predictions, and if Magellanic-Cloud-mass systems environmentally influence their satellites in different ways than do more massive hosts.

Selected results from the survey:

This program is funded by NSF grant AST-1813628.  The overall grant PI is Prof. Denija Crnojevic (Tampa), and our fellow Co-PI is Dr. Jeff Carlin (LSST).  I am so excited to collaborate with our amazing MADCASH team (including my former student Dr. Chris Garling), and am forever grateful to Dr. Beth Willman (NOIRLab) for inviting me onto the project.  Chris and I are building on our theory work from LBT-SONG to interpret the satellite properties of these companions of low-mass hosts.

In a related project, former student Dr. Daniella Roberts, Prof. Anna Nierenberg (UC Merced), and I measured the satellite luminosity function of Large-Magellanic-Cloud mass systems out to half the age of the Universe in the COSMOS field as imaged by the Hubble Space Telescope.

DES Local Volume galaxies

The ESO540-032 Local Volume dwarf galaxy as imaged in the Dark Energy Survey. We will find and characterize galaxies like this and smaller!

Our group has an external collaborator agreement to use the Dark Energy Survey to find and characterize satellite galaxies of Local Volume host galaxies targeted by the deep IMAGINE 21-cm survey.  Our goal is to further develop and apply the distance method of surface brightness fluctuations to dwarf galaxies in DES; understand the connection between the stellar content of dwarf galaxies and their cold gas fuel (especially after infall to a more massive host); and use globular cluster counts, atomic gas velocity measurements, and semi-empirical galaxy-halo matching methods to understand the connection between the stellar and dark-matter content of dwarf galaxies.  We will apply the tools we develop for this project more broadly to Local Volume dwarf galaxies.  This project is newly funded by the NSF AST-2008110.  We are delighted to collaborate with Prof. Alex Drlica-Wagner’s group at the University of Chicago on this project.  Key OSU members of this project are grad students Kirsten Casey, Dr. Chris Garling, and Dr. Daniella Roberts; NSF postdoctoral fellows Dr. Johnny Greco (Mr. Dwarf Galaxy Surface Brightness Fluctations) and Dr. Amy Sardone (queen of radio measurement of low-column gas).  Dr. Roberts’ paper is almost done, and an expansion of her work is being led by OSU astronomy student Joy Bhattacharyya, in collaboration with my astronomy colleague Prof. Paul Martini.


The Merian Survey. Images by Erin Kado-Fong and Alexie Leauthaud.

A complication in using dwarf galaxies to trace dark matter halos is that there are few direct tracers of halo mass for small galaxies.  The Merian survey, lead by Prof. Alexie Leauthaud (UCSC) and Prof. Jenny Greene (Princeton), will yield halo masses and density profiles for Magellanic-Cloud-mass galaxies via weak lensing.  We have been awarded 62 nights on the Blanco telescope in Chile to perform narrow-band imaging to select a pure sample of Magellanic-Cloud-mass lenses in the Hyper Suprime-Cam Subaru Strategic Program field.

Projects related to tiny dark matter halos

A grism and broad-band data and model for the strong gravitational lens HE0435-1223. We found no evidence for small halos near the QSO images either in the lens or along the line of sight.  From Nierenberg et al. (2017).  With many more systems followed up by HST, JWST, and the Nancy Grace Roman Space Telescope, we will be able to constrain novel models for dark matter physics.

Dark matter halos below about 100 million solar masses are largely devoid of luminous baryons.  As it turns out, though, these halos are of particular interest to dark matter physicists, since different theories of dark matter predict different abundance of these small halos.  Of particular interest are models of dark matter in which it is but one (or several) species of particle in a “hidden sector”.  Models such as these have rich phenomenology in traditional particle searches for dark matter, as well as interesting cosmological behavior.  These models allow for large, velocity-dependent self-scattering probabilities, which can lead to features in the matter power spectrum and unique late-time evolution of halos.  In a project funded by NASA, we are using a combination of simulations (lead by my students Carton Zeng and Charlie Mace, and with former student Dr. Stacy Kim) and semi-analytic modeling (a huge effort by Carnegie Observatories collaborators Drs. Andrew Benson, Xiaolong Du, and Fangzhou Jiang, and NYU collaborators Prof. Anthony Pullen and graduate student Shengqi Yang) to predict optimal strategies for constraining these models using substructure lensing.  We have an awesome crew of substructure lensing observers and modelers (Dr. Leonidas Moustakas at JPL, Prof. Anna Nierenberg at UC Merced, Prof. Francis-Yan Cyr-Racine and grad student Birendra Dhanasingham at University of New Mexico, and Prof. Anthony Pullen and grad student Ekapob Kulchoakrungsun).  Like Captain Planet, when our powers combine, we’ll be able to constrain hidden-sector models with next-generation NASA facilities.

Selected publications:

Astroparticle physics

The GeV-TeV Sun

Gamma rays from the Sun at solar minimum (left column) and non-minimum (right column) as a function of gamma-ray energy. Note how the morphology of solar gamma-ray emission shifts with the solar cycle, and that the equatorial flux changes dramatically while the polar flux stays relatively constant. From Linden et al. (2018).

For sure the weirdest and most confounding thing I have ever worked on is the emission of the Sun at GeV to TeV energy scales.  I got interested in this topic because of a referee report I got on one of my thesis papers, on neutrinos from dark matter annihilation in the Sun.  A referee suggested that cosmic rays hitting the Sun might produce a comparable and completely spatially coincident foreground of neutrinos.  The cosmic-ray interactions that produce neutrinos also produce gamma rays.  Once I got to OSU, I teamed up with Prof. Carsten Rott (formerly a CCAPP fellow, now a professor at SKKU), Prof. John Beacom, and Prof. Kenny Ng (then one of John’s graduate students, now a professor at the Chinese University of Hong Kong).  Kenny did the first time-dependent analysis of Fermi Gamma-Ray Space Telescope data on the Sun, finding that the solar gamma-ray emission is brighter, spectrally harder, and more time-variable than we or anyone else expected (you should also check out his work on trying to model this flux using a couple different models of coronal magnetic fields).  Since then, supported by a number of Fermi Guest Investigator grants, various of us including Dr. Bei Zhou and Guanying Zhu (former and current Beacom students; Bei is now a postdoc at Johns Hopkins), Prof. Tim Linden (now at Stockholm University, then an Einstein and CCAPP fellow), and Prof. Qingwen Tang (now at Nanchang University) found a number of other very surprising things about the Sun’s gamma-ray emission.  Most startlingly, data ninja Tim discovered that the Sun is especially variable at energies above 50 GeV that anti-tracks the solar cycle, and that the variability is driven by the equatorial region of the Sun (see above).  We teamed up with Dr. Rebecca Leane (SLAC) and Dr. Mehr Un Nisa (Michigan State) to investigate the emission at even higher energies with HAWC.  With a new NASA H-SR grant, we are teaming up with bona fide space physicists Prof. Ofer Cohen (UMass Lowell) and Dr. Igor Sokolov (Michigan) to comprehensively model cosmic-ray paths and interactions through the solar system.  We’re now collaborating with the cosmic-ray transport group at University of Arizona, and welcomed new postdoc Dr. Jung-Tsung Li to the project.  Stay tuned!  If the past is a guide, we are going to find even more and stranger things in the near future!

Selected publications:

Dark matter direct detection

Measuring the local dark matter speed distribution with direct detection experiments. The halo + dark disk input model is denoted in black, and the red points show the projected constraints on the speed distribution assuming a benchmark WIMP model and G2-type direct-detection experiments. From Peter (2011).

I work on this less now than I used to, but I spent years fascinated by how we could use dark matter direct detection experiments as probes of the local dark matter phase-space density.  We usually trace the Milky Way’s assembly history with stars (especially exciting in the Gaia era!), but the dark matter is also a fossil record of the Milky Way’s evolution.  Moreover, it is not 100% clear how stars trace dark matter.  Since the signal of dark matter in direct detection depends on its phase-space density, I spent some time thinking about how best to pull out the dark matter phase-space density from the data in addition to the standard dark matter properties we care about–particle mass, cross section(s).  I had many delightful collaborators along the way.

Most recently, I worked with some outstanding collaborators, former grad student and current CCA postdoc Dr. Nico Garavito-Camargo and the one and only Prof. Gurtina Besla (U of A), to reveal the imprint of the biggest merger event of the Milky Way, the Large Magellanic Cloud (LMC), on direct detection measurements.  Because of the unusual geometry of the LMC’s orbit with respect to the Sun’s about the Galactic Center, the debris from the galaxy populates the high-speed tail of the dark matter velocity distribution.  And it’s not just that the tidal debris that matters, it matters that the LMC accelerates Milky Way dark matter particles to high speed.  Because the stellar part of the LMC is buried deep in its halo, and because that halo has not been too heavily stripped, there is no stellar counterpart to the high-speed dark matter streaming through our solar system.  The implication for direct detection is, experiments are sensitive to lower-mass dark matter particles than previously expected.  Good times!

The local dark matter density as a function of speed relative to the Earth. Black lines denote the speed distribution at the Sun’s position at two times of the year in the absence of the Magellanic Clouds. Because the Large Magellanic Cloud’s orbit is almost completely opposite the Sun’s motion, it drops high-speed particles near the Sun, and accelerates local Milky Way dark matter particles. Colored lines show the total dark matter speed distribution, and the histograms show particles originating in the Large Magellanic Cloud. From Besla, Peter, and Garavito-Camargo (2019).


Selected publications: