The total mass of dark matter haloes

The simple, conventional dark matter halo mass definitions commonly used in cosmological simulations (‘virial' mass, FoF mass, M50, 100, 200, ...) only capture part of the collapsed material and are therefore inconsistent with the halo mass concept used in analytical treatments of structure formation. Simulations have demonstrated that typical dark matter particle orbits extend out to about 90 per cent of their turnaround radius, which results in apocentre passages outside of the current ‘virial' radius on the first and also on the second orbit. Here we describe how the formation history of haloes can be used to identify those particles which took part in the halo collapse, but are missed by conventional group finders because of their remote present location. These particles are added to the part of the halo already identified by FoF. The corrected masses of dark haloes are significantly higher (the median mass increase is 25 per cent) and there is a considerable shift of the halo mass function towards the Press and Schechter form. We conclude that meaningful quantitative comparisons between (semi-)analytic predictions of halo properties (e.g. mass functions, mass accretion rates, merger rates, spatial clustering, etc.) and simulation results will require using the same halo definition in both approache

Cores in warm dark matter haloes: a Catch 22 problem

The free streaming of warm dark matter particles dampens the fluctuation spectrum, flattens the mass function of haloes and sets a fine-grained phase density limit for dark matter structures. The phase-space density limit is expected to imprint a constant-density core at the halo centre in contrast to what happens for cold dark matter. We explore these effects using high-resolution simulations of structure formation in different warm dark matter scenarios. We find that the size of the core we obtain in simulated haloes is in good agreement with theoretical expectations based on Liouville's theorem. However, our simulations show that in order to create a significant core (rc ∼ kpc) in a dwarf galaxy (M∼ 1010 M⊙), a thermal candidate with mass as low as 0.1 keV is required. This would fully prevent the formation of the dwarf galaxy in the first place. For candidates satisfying large-scale structure constraints (mν larger than ≈1-2 keV), the expected size of the core is of the order of 10 (20) pc for a dark matter halo with a mass of 1010 (108) M⊙. We conclude that ‘standard' warm dark matter is not a viable solution for explaining the presence of cored density profiles in low-mass galaxie

Cores in warm dark matter haloes: a Catch 22 problem

The free streaming of warm dark matter particles dampens the fluctuation spectrum, flattens the mass function of haloes and sets a fine-grained phase density limit for dark matter structures. The phase-space density limit is expected to imprint a constant-density core at the halo centre in contrast to what happens for cold dark matter. We explore these effects using high-resolution simulations of structure formation in different warm dark matter scenarios. We find that the size of the core we obtain in simulated haloes is in good agreement with theoretical expectations based on Liouville's theorem. However, our simulations show that in order to create a significant core (? kpc) in a dwarf galaxy (M˜ 1010 Msun), a thermal candidate with mass as low as 0.1 keV is required. This would fully prevent the formation of the dwarf galaxy in the first place. For candidates satisfying large-scale structure constraints (mν larger than ≈1-2 keV), the expected size of the core is of the order of 10 (20) pc for a dark matter halo with a mass of 1010 (108) Msun. We conclude that 'standard' warm dark matter is not a viable solution for explaining the presence of cored density profiles in low-mass galaxies

The Total Mass of Dark Matter Haloes

The simple, conventional dark matter halo mass definitions commonly used in cosmological simulations ("virial" mass, FoF mass, $M_{50,100,200,...}$) only capture part of the collapsed material and are therefore inconsistent with the halo mass concept used in analytical treatments of structure formation. Simulations have demonstrated that typical dark matter particle orbits extend out to about 90 per cent of their turnaround radius, which results in apocenter passages outside of the current "virial" radius on the first and also on the second orbit. Here we describe how the formation history of haloes can be used to identify those particles which took part in the halo collapse, but are missed by conventional group-finders because of their remote present location. These particles are added to the part of the halo already identified by FoF. The corrected masses of dark haloes are significantly higher (the median mass increase is 25 per cent) and there is a considerable shift of the halo mass function towards the Press & Schechter form. We conclude that meaningful quantitative comparisons between (semi-)analytic predictions of halo properties (e.g. mass functions, mass accretion rates, merger rates, spatial clustering, etc.) and simulation results will require using the same halo definition in both approaches.Comment: 7 pages, 6 figures. Accepted for publication in MNRA

Cores in warm dark matter haloes: a Catch 22 problem

The free streaming of warm dark matter particles dampens the fluctuation spectrum, flattens the mass function of haloes and imprints a fine grained phase density limit for dark matter structures. The phase space density limit is expected to imprint a constant density core at the halo center on the contrary to what happens for cold dark matter. We explore these effects using high resolution simulations of structure formation in different warm dark matter scenarios. We find that the size of the core we obtain in simulated haloes is in good agreement with theoretical expectations based on Liouville's theorem. However, our simulations show that in order to create a significant core, (r_c~1 kpc), in a dwarf galaxy (M~1e10 Msun), a thermal candidate with a mass as low as 0.1 keV is required. This would fully prevent the formation of the dwarf galaxy in the first place. For candidates satisfying large scale structure constrains (m_wdm larger than 1-2 keV) the expected size of the core is of the order of 10 (20) pc for a dark matter halo with a mass of 1e10 (1e8) Msun. We conclude that "standard" warm dark matter is not viable solution for explaining the presence of cored density profiles in low mass galaxies.Comment: 9 pages, 8 figures, new theory section, fig 8 updated, conclusions unchanged, accepted for publication on MNRA

Understanding Dwarf Galaxies in order to Understand Dark Matter

Much progress has been made in recent years by the galaxy simulation community in making realistic galaxies, mostly by more accurately capturing the effects of baryons on the structural evolution of dark matter halos at high resolutions. This progress has altered theoretical expectations for galaxy evolution within a Cold Dark Matter (CDM) model, reconciling many earlier discrepancies between theory and observations. Despite this reconciliation, CDM may not be an accurate model for our Universe. Much more work must be done to understand the predictions for galaxy formation within alternative dark matter models.Comment: Refereed contribution to the Proceedings of the Simons Symposium on Illuminating Dark Matter, to be published by Springe

The internal structure of halos and the nature of dark matter

This thesis studies the formation and the internal structure of dark matter halos in standard and non-standard cosmological models. In a first part, we present a group finding algorithm that accounts for the total collapsed mass of a halo in the sense of classical Press-Schechter theory. We show that such an approach is indispensable in order to make meaningful comparisons between results from numerical simulations and theoretical frameworks. In a second part, that consists of three individual papers, we investiga- te the formation, evolution and internal structure of halos in various non- standard cosmologies. The first paper examines how thermal velocities of warm dark matter particles influence the density and phase-space density profiles of dark matter halos. We show that warm dark matter alone, alt- hough still an attractive candidate, is not enough to solve the core / cusp problem. The second paper studies the internal structure of galaxy-sized objects in generic cold + warm dark matter cosmologies. We quantify how the presence of a warm dark matter component leads to an imbalance between the warm to cold ratio locally and on average in the Universe; a fact that needs to be considered in dark matter searches. In the last paper of the second part, we show that a multi-dimensional small scale approach is a powerful method to set strong limits on the nature of dark matter. In the third and last part of the thesis, we again return to a standard cold dark matter cosmology and investigate the smallest structures that form in a hierarchical bottom-up scenario: Earth-mass microhalos. A detailed study of the very inner density profile reveals the slope to be considerably steeper than their larger counterparts. Based on this result, we compute the change in the total annihilation luminosity boost factor, a quantity of great importance for indirect dark matter detection