Benedikt Diemer
Graduate Student, Department of Astronomy and Astrophysics

Phone: (773) 702-0162
Location: LASR 215

Scientific Advisors: Andrey V. Kravtsov, Donald Q. Lamb

Affiliations: Kavli Institute for Cosmological Physics

Publications: ADS | arXiv

Member of Research Groups:

Ph.D. Thesis Defense
Defense date: June 11, 2015
Ph.D. Thesis: "On the (non-)universality of halo density profiles"

Ph.D. Committee members: Scott Dodelson, Joshua A. Frieman, Donald Q. Lamb

"In his PhD thesis Benedikt Diemer has shown that radial density profiles of dark matter halos cannot be characterized only as a function of halo mass, as was thought previously, but also depend on the mass accretion rate of halos. The work has resulted in a new model that accurately describes halo profiles in simulations from small radii out to 10 virial radii. Likewise, Benedikt has shown that halo concentrations depend not only on the halo mass (or more precisely on halo peak height), but also on the local slope of the power spectrum. Overall, this thesis showed that previously believed "universality" of the halo profiles is limited. Beyond just criticizing previous models, new models were developed that take into account the extra dependencies of halo profile parameters on the mass accretion rate and power law slope."
- Andrey V. Kravtsov, Ph.D. advisor

Thesis Abstract: We present a systematic study of the density profiles of dark matter halos in LCDM cosmologies, focusing on the question whether these profiles are "universal", i.e., whether they follow the same functional form regardless of halo mass, redshift, cosmology, and other parameters. The inner profile can be described as a function of mass and concentration, and we thus begin by investigating the universality of the concentration-mass relation. We propose a universal model in which concentration is a function only of a halo's peak height and the local slope of the matter power spectrum. This model matches the concentrations in LCDM and scale-free simulations, correctly extrapolates over 16 orders of magnitude in halo mass, and differs significantly from all previously proposed models at high masses and redshifts. Testing the universality of the outer regions, we find that the profiles are remarkably universal across redshift when radii are rescaled by R200m, whereas the inner profiles are most universal in units of R200c, highlighting that universality may depend upon the definition of the halo boundary. Furthermore, we discover that the profiles exhibit significant deviations from the supposedly universal analytic formulae previously suggested in the literature, such as the NFW and Einasto forms. In particular, the logarithmic slope of the profiles of massive or rapidly accreting halos steepens more sharply than predicted around ~R200m, where the steepness increases with increasing peak height or mass accretion rate. We propose a new, accurate fitting formula that takes these dependencies into account. Finally, we demonstrate that the profile steepening corresponds to the caustic at the apocenter of infalling matter on its first orbit. We call the location of the caustic the splashback radius, Rsp, and propose this radius as a new, physically motivated definition of the halo boundary. We discuss potential observational signatures of Rsp that would allow us to estimate the mass accretion rate of halos.

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