Jesus Zavala

Constraining self-interacting dark matter with the Milky Way’s dwarf spheroidals

Self-Interacting Dark Matter is an attractive alternative to the Cold Dark Matter paradigm only if it is able to substantially reduce the central densities of dwarf-size haloes while keeping the densities and shapes of cluster-size haloes within current constraints. Given the seemingly stringent nature of the latter, it was thought for nearly a decade that Self-Interacting Dark Matter would be viable only if the cross section for self-scattering was strongly velocitydependent. However, it has recently been suggested that a constant cross section per unit mass of T=m 0:1cm 2 g 1 is sufficient to accomplish the desired effect. We explicitly investigate this claim using high resolution cosmological simulations of a Milky-Way size halo and find that, similarly to the Cold Dark Matter case, such cross section produces a population of massive subhaloes that is inconsistent with the kinematics of the classical dwarf spheroidals, in particular with the inferred slopes of the mass profiles of Fornax and Sculptor. This problem is resolved if T=m 1cm 2 g 1 at the dwarf spheroidal scales. Since this value is likely inconsistent with the halo shapes of several clusters, our results leave only a small window open for a velocity-independent Self-Interacting Dark Matter model to work as a distinct alternative to Cold Dark Matter.

The abundance of (not just) dark matter haloes

We study the effect of baryons on the abundance of structures and substructures in a_cold dark matter (CDM) cosmology, using a pair of high-resolution cosmological simulations from the Galaxies-Intergalactic Medium Interaction Calculation project. Both simulations use identical initial conditions, but while one contains only dark matter, the other also includes baryons.We find that gas pressure, reionization, supernova feedback, stripping and truncated accretion systematically reduce the total mass and the abundance of structures below ∼1012M_ compared to the pure dark matter simulation. Taking this into account and adopting an appropriate detection threshold lower the abundance of observed galaxies with maximum circular velocities vmax < 100 km s−1, significantly reducing the reported discrepancy between _CDM and the measured H I velocity function of the Arecibo Legacy Fast ALFA survey. We also show that the stellar-to-total mass ratios of galaxies with stellar masses of ∼105–107M_ inferred from abundance matching of the (sub) halo mass function to the observed galaxy mass function increase by a factor of ∼2. In addition, we find that an important fraction of low-mass subhaloes are completely devoid of stars. Accounting for the presence of dark subhaloes below 1010M_ further reduces the abundance of observable objects and leads to an additional increase in the inferred stellar-to-total mass ratio by factors of 2–10 for galaxies in haloes of 109–1010M_. This largely reconciles the abundance matching results with the kinematics of individual dwarf galaxies in_CDM.We propose approximate corrections to the masses of objects derived from pure dark matter calculations to account for baryonic effects.

Bulges versus discs: the evolution of angular momentum in cosmological simulations of galaxy formation

We investigate the evolution of angular momentum in simulations of galaxy formation in a cold dark matter universe. We analyse two model galaxies generated in the N-body/hydrodynamic simulations of Okamoto et al. Starting from identical initial conditions, but using different assumptions for the baryonic physics, one of the simulations produced a bulge-dominated galaxy and the other one a disc-dominated galaxy. The main difference is the treatment of star formation and feedback, both of which were designed to be more efficient in the disc-dominated object. We find that the specific angular momentum of the disc-dominated galaxy tracks the evolution of the angular momentum of the dark matter halo very closely: the angular momentum grows as predicted by linear theory until the epoch of maximum expansion and remains constant thereafter. By contrast, the evolution of the angular momentum of the bulge-dominated galaxy resembles that of the central, most bound halo material: it also grows at first according to linear theory, but 90 per cent of it is rapidly lost as pre-galactic fragments, into which gas had cooled efficiently, merge, transferring their orbital angular momentum to the outer halo by tidal effects. The disc-dominated galaxy avoids this fate because the strong feedback reheats the gas, which accumulates in an extended hot reservoir and only begins to cool once the merging activity has subsided. Our analysis lends strong support to the classical theory of disc formation whereby tidally torqued gas is accreted into the centre of the halo conserving its angular momentum.

ETHOS—an effective theory of structure formation: From dark particle physics to the matter distribution of the Universe

We formulate an effective theory of structure formation (ETHOS) that enables cosmological structure formation to be computed in almost any microphysical model of dark matter physics. This framework maps the detailed microphysical theories of particle dark matter interactions into the physical effective parameters that shape the linear matter power spectrum and the self-interaction transfer cross section of nonrelativistic dark matter. These are the input to structure formation simulations, which follow the evolution of the cosmological and galactic dark matter distributions. Models with similar effective parameters in ETHOS but with different dark particle physics would nevertheless result in similar dark matter distributions. We present a general method to map an ultraviolet complete or effective field theory of low-energy dark matter physics into parameters that affect the linear matter power spectrum and carry out this mapping for several representative particle models. We further propose a simple but useful choice for characterizing the dark matter self-interaction transfer cross section that parametrizes self-scattering in structure formation simulations. Taken together, these effective parameters in ETHOS allow the classification of dark matter theories according to their structure formation properties rather than their intrinsic particle properties, paving the way for future simulations to span the space of viable dark matter physics relevant for structure formation.

Scattering, damping, and acoustic oscillations: Simulating the structure of dark matter halos with relativistic force carriers

We demonstrate that self-interacting dark matter models with interactions mediated by light particles can have significant deviations in the matter power spectrum and detailed structure of galactic halos when compared to a standard cold dark matter scenario. While these deviations can take the form of suppression of small-scale structure that are in some ways similar to that of warm dark matter, the self-interacting models have a much wider range of possible phenomenology. A long-range force in the dark matter can introduce multiple scales to the initial power spectrum, in the form of dark acoustic oscillations and an exponential cutoff in the power spectrum. Using simulations we show that the impact of these scales can remain observationally relevant up to the present day. Furthermore, the self-interaction can continue to modify the small-scale structure of the dark matter halos, reducing their central densities and creating a dark matter core. The resulting phenomenology is unique to these type of models.

Relic density and CMB constraints on dark matter annihilation with Sommerfeld enhancement

We calculate how the relic density of dark matter particles is altered when their annihilation is enhanced by the Sommerfeld mechanism due to a Yukawa interaction between the annihilating particles. Maintaining a dark matter abundance consistent with current observational bounds requires the normalization of the s-wave annihilation cross section to be decreased compared to a model without enhancement. The level of suppression depends on the specific parameters of the particle model, with the kinetic decoupling temperature having the most effect. We find that the cross section can be reduced by as much as an order of magnitude for extreme cases. We also compute the {mu}-type distortion of the CMB energy spectrum caused by energy injection from such Sommerfeld-enhanced annihilation. Our results indicate that in the vicinity of resonances, associated with bound states, distortions can be large enough to be excluded by the upper limit |{mu}|{<=}9.0x10{sup -5} found by the FIRAS (Far Infrared Absolute Spectrophotometer) instrument on the COBE (Cosmic Background Explorer) satellite.

Extragalactic gamma-ray background radiation from dark matter annihilation

If dark matter is composed of neutralinos, one of the most exciting prospects for its detection lies in observations of the gamma-ray radiation created in pair annihilations between neutralinos, a process that may contribute significantly to the extragalactic gamma-ray background (EGB) radiation. We here use the high-resolution Millennium-II simulation of cosmic structure formation to produce the first full sky maps of the expected radiation coming from extragalactic dark matter structures. Our map-making procedure takes into account the total gamma-ray luminosity from all haloes and their subhaloes, and includes corrections for unresolved components of the emission as well as an extrapolation to the damping scale limit of neutralinos. Our analysis also includes a proper normalization of the signal according to a specific supersymmetric model based on minimal supergravity. The new simulated maps allow a study of the angular power spectrum of the gamma-ray background from dark matter annihilation, which has distinctive features associated with the nature of the annihilation process and may be detectable in forthcoming observations by the recently launched Fermi satellite. Our results are in broad agreement with analytic models for the gamma-ray background, but they also include higher order correlations not readily accessible in analytic calculations and, in addition, provide detailed spectral information for each pixel. In particular, we find that difference maps at different energies can reveal cosmic large-scale structure at low and intermediate redshifts. If the intrinsic emission spectrum is characterized by an emission peak, cosmological tomography with gamma-ray annihilation radiation is in principle possible.

ON THE BARYONIC, STELLAR, AND LUMINOUS SCALING RELATIONS OF DISK GALAXIES

We explore how the slopes and scatters of the scaling relations of disk galaxies (V m-L[-M], R-L[-M], and V m-R) change when moving from B- to K-bands and to stellar and baryonic quantities. For our compiled sample of 76 normal, non-interacting, high and low-surface brightness (SB) disk-galaxies, we find important changes, which evidence evolution effects, mainly related to the gas infall and star-formation (SF) processes. We also explore correlations among the (B – K) color, stellar mass fraction, f s, mass, M, luminosity, L, and surface density (or SB), as well as correlations among the residuals of the scaling relations, and among these residuals and those of the other relations studied here. Some of our findings are the following: (i) the scale length R bar is a third parameter in the baryonic Tully-Fisher relation (TFR) and the residuals of this relation follow a trend (slope ≈–0.15) with the residuals of the R bar-M bar relation; for the stellar and K-band cases, the scale length is no longer a third parameter and the mentioned trend disappears; (ii) among the TFRs, the B-band TFR is the most scattered; in this case, the color is a third parameter, in agreement with previous works; (iii) the low-SB (LSB) galaxies break some observed trends in diagrams that include SD, color, and f s, suggesting then a threshold in the gas SD, Σ g , below which the SF efficiency becomes independent of Σ g and of the gas infall rate. Our results are interpreted and discussed in the light of Λ Cold Dark Matter (ΛCDM)-based models of disk-galaxy formation and evolution. These models are able to explain not only the baryonic scaling correlations, but also most of the processes responsible for the observed changes in the slopes, scatters, and correlations among the residuals when changing to stellar and luminous quantities. The galaxy baryon fraction, f gal, is required to be smaller than 0.05 on average. We detect some potential difficulties for the models: the observed color-M and SD-M correlations are steeper, and the intrinsic scatter in the baryonic TFR is smaller than those predicted.

Characterization of dark-matter-induced anisotropies in the diffuse gamma-ray background

The Fermi-LAT collaboration has recently reported the detection of angular power above the photon noise level in the diffuse gamma-ray background between 1 and 50 GeV. Such signal can be used to constrain a possible contribution from dark matter (DM) induced photons. We estimate the intensity and features of the angular power spectrum (APS) of this potential DM signal, for both decaying and annihilating DM candidates, by constructing template all-sky gamma-ray maps for the emission produced in the galactic halo and its substructures, as well as in extragalactic (sub)haloes. The DM distribution is given by state-of-the-art N-body simulations of cosmic structure formation, namely Millennium-II for extragalactic (sub)haloes, and Aquarius for the galactic halo and its subhaloes. We use a hybrid method of extrapolation to account for (sub)structures that are below the resolution limit of the simulations, allowing us to estimate the total emission all the way down to the minimal self-bound halo mass. We describe in detail the features appearing in the APS of our template maps and we estimate the effect of various uncertainties such as the value of the minimal halo mass, the fraction of substructures hosted in a halo and the shape of the DM density profile. Our results indicate that the fluctuation APS of the DM-induced emission is of the same order as the Fermi-LAT APS, suggesting that one can constrain this hypothetical emission from the comparison with the measured anisotropy. We also quantify the uncertainties affecting our results, finding ‘theoretical error bands’ spanning more than two orders of magnitude and dominated (for a given particle physics model) by our lack of knowledge of the abundance of low-mass (sub)haloes.

Direct detection of self-interacting dark matter

Self-interacting dark matter offers an interesting alternative to collisionless dark matter because of its ability to preserve the large-scale success of the cold dark matter model, while seemingly solving its challenges on small scales. We present here the first study of the expected dark matter detection signal taking into account different self-scattering models. We demonstrate that models with constant and velocity dependent cross sections, which are consistent with observational constraints, lead to distinct signatures in the velocity distribution, because non-thermalised features found in the cold dark matter distribution are thermalised through particle scattering. Depending on the model, self-interaction can lead to a 10% reduction of the recoil rates at high energies, corresponding to a minimum speed that can cause recoil larger than 300 km/s, compared to the cold dark matter case. At lower energies these differences are smaller than 5% for all models. The amplitude of the annual modulation signal can increase by up to 25%, and the day of maximum amplitude can shift by about two weeks with respect to the cold dark matter expectation. Furthermore, the exact day of phase reversal of the modulation signal can also differ by about a week between the different models. In general, models with velocity dependent cross sections peaking at the typical velocities of dwarf galaxies lead only to minor changes in the detection signals, whereas allowed constant cross section models lead to significant changes. We conclude that different self-interacting dark matter scenarios might be distinguished from each other through the details of direct detection signals. Furthermore, detailed constraints on the intrinsic properties of dark matter based on null detections, should take into account the possibility of self-scattering and the resulting effects on the detector signal.