Production of Sterile Neutrino Dark Matter and the 3.5 keV line

The recent observation of an X-ray line at an energy of 3.5 keV mainly from galaxy clusters has initiated a discussion about whether we may have seen a possible dark matter signal. If confirmed, this signal could stem from a decaying sterile neutrino of a mass of 7.1 keV. Such a particle could make up all the dark matter, but it is not clear how it was produced in the early Universe. In this letter we show that it is possible to discriminate between different production mechanisms with present-day astronomical data. The most stringent constraint comes from the Lyman-{\alpha} forest and seems to disfavor all but one of the main production mechanisms proposed in the literature, which is the production via decay of heavy scalar singlets. Pinning down the production mechanism will help to decide whether the X-ray signal indeed comprises an indirect detection of dark matter.Comment: Appendix added; slight change in distribution function, main results not affecte

On the Stability of Tidal Streams

We explore the stability of tidal streams to perturbations, motivated by recent claims that the clumpy structure of the stellar streams surrounding the globular cluster Palomar 5 are the result of gravitational instability. We calculate the Jeans length of tidal streams by treating them as a thin expanding cylinder of collisionless matter. We also find a general relation between the density and the velocity dispersion inside a stream, which is used to determine the longitudinal Jeans criterion. Our analytic results are checked by following the time evolution of the phase space density within streams using numerical simulations. We conclude that tidal streams within our galactic halo are stable on all length scales and over all timescales.Comment: Accepted for publication in MNRA

Structure formation with suppressed small-scale perturbations

All commonly considered dark matter scenarios are based on hypothetical particles with small but non-zero thermal velocities and tiny interaction cross-sections. A generic consequence of these attributes is the suppression of small-scale matter perturbations either due to free-streaming or due to interactions with the primordial plasma. The suppression scale can vary over many orders of magnitude depending on particle candidate and production mechanism in the early Universe. While nonlinear structure formation has been explored in great detail well above the suppression scale, the range around suppressed perturbations is still poorly understood. In this paper we study structure formation in the regime of suppressed perturbations using both analytical techniques and numerical simulations. We develop simple and theoretically motivated recipes for the halo mass function, the expected number of satellites, and the halo concentrations, which are designed to work for power spectra with suppression at arbitrary scale and of arbitrary shape. As case studies, we explore warm and mixed dark matter scenarios where effects are most distinctive. Additionally, we examine the standard dark matter scenario based on weakly interacting massive particles (WIMP) and compare it to pure cold dark matter with zero primordial temperature. We find that our analytically motivated recipes are in good agreement with simulations for all investigated dark matter scenarios, and we therefore conclude that they can be used for generic cases with arbitrarily suppressed small-scale perturbations.Comment: Improvement of the method to remove halo artefacts. Results are unaffecte

Halo Mass Function and the Free Streaming Scale

The nature of structure formation around the particle free streaming scale is still far from understood. Many attempts to simulate hot, warm, and cold dark matter cosmologies with a free streaming cutoff have been performed with cosmological particle-based simulations, but they all suffer from spurious structure formation at scales below their respective free streaming scales -- i.e. where the physics of halo formation is most affected by free streaming. We perform a series of high resolution numerical simulations of different WDM models, and develop an approximate method to subtract artificial structures in the measured halo mass function. The corrected measurements are then used to construct and calibrate an extended Press-Schechter (EPS) model with sharp-$k$ window function and adequate mass assignment. The EPS model gives accurate predictions for the low redshift halo mass function of CDM and WDM models, but it significantly under-predicts the halo abundance at high redshifts. By taking into account the ellipticity of the initial patches and connecting the characteristic filter scale to the smallest ellipsoidal axis, we are able to eliminate this inconsistency and obtain an accurate mass function over all redshifts and all dark matter particle masses covered by the simulations. As an additional application we use our model to predict the microhalo abundance of the standard neutralino-CDM scenario and we give the first quantitative prediction of the mass function over the full range of scales of CDM structure formation.Comment: 16 pages, 10 figures, published in MNRA

On the stability of tidal streams

We explore the stability of tidal streams to perturbations, motivated by recent claims that the clumpy structure of the stellar streams surrounding the globular cluster Palomar 5 are the result of gravitational instability. We calculate the Jeans length of tidal streams by treating them as a thin expanding cylinder of collisionless matter. We also find a general relation between the density and the velocity dispersion inside a stream, which is used to determine the longitudinal Jeans criterion. Our analytic results are checked by following the time evolution of the phase space density within streams using numerical simulations. We conclude that tidal streams within our Galactic halo are stable on all length scales and over all time-scale

Dark acoustic oscillations: imprints on the matter power spectrum and the halo mass function

Many non-minimal dark matter scenarios lead to oscillatory features in the matter power spectrum induced by interactions either within the dark sector or with particles from the standard model. Observing such dark acoustic oscillations would therefore be a major step towards understanding dark matter. We investigate what happens to oscillatory features during the process of non-linear structure formation. We show that at the level of the power spectrum, oscillations are smoothed out by non-linear mode coupling, gradually disappearing towards lower redshifts. In the halo mass function, however, the oscillatory features remain visible until the present epoch. As a consequence, dark acoustic oscillations could be detectable in observations that are either based on the halo mass function or on the high-redshift power spectrum. We investigate the effect of such features on different observables, namely the cluster mass function, the stellar-to-halo mass relation, and the Lyman α flux power spectrum. We find that oscillatory features remain visible in all of these observables, but they are very extended and of low amplitude, making it challenging to detect them as distinct features in the data

Impact of Dark Matter Microhalos on Signatures for Direct and Indirect Detection

Detecting dark matter as it streams through detectors on Earth relies on knowledge of its phase space density on a scale comparable to the size of our solar system. Numerical simulations predict that our Galactic halo contains an enormous hierarchy of substructures, streams and caustics, the remnants of the merging hierarchy that began with tiny Earth mass microhalos. If these bound or coherent structures persist until the present time, they could dramatically alter signatures for the detection of weakly interacting elementary particle dark matter (WIMP). Using numerical simulations that follow the coarse grained tidal disruption within the Galactic potential and fine grained heating from stellar encounters, we find that microhalos, streams and caustics have a negligible likelihood of impacting direct detection signatures implying that dark matter constraints derived using simple smooth halo models are relatively robust. We also find that many dense central cusps survive, yielding a small enhancement in the signal for indirect detection experiments.Comment: 6 pages, revision in response to referees report. Now accepted by Phys. Rev D., in pres