In the standard cold dark matter (CDM) theory for understanding the formation of structure in the Universe, there exists a tight connection between the properties of dark matter (DM) haloes, and ...their formation epochs. Such relation can be expressed in terms of a single key parameter, namely the halo concentration. In this work, we examine the median concentration–mass relation, c(M), at present time, over more than 20 orders of magnitude in halo mass, i.e. from tiny Earth-mass microhaloes up to galaxy clusters. The c(M) model proposed by Prada et al. (2012), which links the halo concentration with the rms amplitude of matter linear fluctuations, describes remarkably well all the available N-body simulation data down to ∼10−6
h
−1 M⊙ microhaloes. A clear fattening of the halo concentration–mass relation towards smaller masses is observed, that excludes the commonly adopted power-law c(M) models, and stands as a natural prediction for the CDM paradigm. We provide a parametrization for the c(M) relation that works accurately for all halo masses. This feature in the c(M) relation at low masses has decisive consequences e.g. for γ-ray DM searches, as it implies more modest boosts of the DM annihilation flux due to substructure, i.e. ∼35 for galaxy clusters and ∼15 for galaxies like our own, as compared to those huge values adopted in the literature that rely on such power-law c(M) extrapolations. We provide a parametrization of the boosts that can be safely used for dwarfs to galaxy cluster-size haloes.
A prediction of the standard Lambda cold dark matter cosmology is that dark matter (DM) haloes are teeming with numerous self-bound substructure or subhaloes. The precise properties of these ...subhaloes represent important probes of the underlying cosmological model. We use data from Via Lactea II and Exploring the Local Volume in Simulations N-body simulations to learn about the structure of subhaloes with masses 10(6)-10(11) h(-1) M circle dot. Thanks to a superb subhalo statistics, we study subhalo properties as a function of distance to host halo centre and subhalo mass, and provide a set of fits that accurately describe the subhalo structure. We also investigate the role of subhaloes on the search for DM annihilation. Previous work has shown that subhaloes are expected to boost the DM signal of their host haloes significantly. Yet, these works traditionally assumed that subhaloes exhibit similar structural properties than those of field haloes, while it is known that subhaloes are more concentrated. Building upon our N-body data analysis, we refine the substructure boost model of Sanchez-Conde & Prada (2014), and find boosts that are a factor 2-3 higher. We further refine the model to include unavoidable tidal stripping effects on the subhalo population. For field haloes, this introduces a moderate (similar to 20-30 per cent) suppression. Yet, for subhaloes like those hosting dwarf galaxy satellites, tidal stripping plays a critical role, the boost being at the level of a few tens of percent at most. We provide a parametrization of the boost for field haloes that can be safely applied over a wide halo mass range.
We review the current understanding of the Diffuse Gamma-Ray Background (DGRB). The DGRB is what remains of the total measured gamma-ray emission after the subtraction of the resolved sources and of ...the diffuse Galactic foregrounds. It is interpreted as the cumulative emission of sources that are not bright enough to be detected individually. Yet, its exact composition remains unveiled. Well-established astrophysical source populations (e.g. blazars, misaligned AGNs, star-forming galaxies and millisecond pulsars) all represent guaranteed contributors to the DGRB. More exotic scenarios, such as Dark Matter annihilation or decay, may contribute as well. In this review, we describe how these components have been modeled in the literature and how the DGRB can be used to provide valuable information on each of them. We summarize the observational information currently available on the DGRB, paying particular attention to the most recent measurement of its intensity energy spectrum by the Fermi LAT Collaboration. We also discuss the novel analyses of the auto-correlation angular power spectrum of the DGRB and of its cross-correlation with tracers of the large-scale structure of the Universe. New data sets already (or soon) available are expected to provide further insight on the nature of this emission. By summarizing where we stand on the current knowledge of the DGRB, this review is intended both as a useful reference for those interested in the topic and as a means to trigger new ideas for further research.
ABSTRACT In this work, we investigate the discovery potential of low-mass Galactic dark matter (DM) subhaloes for indirect searches of DM. We use data from the Via Lactea II (VL-II) N-body ...cosmological simulation, which resolves subhaloes down to $\mathcal {O}(10^4)$ solar masses and it is thus ideal for this purpose. First, we characterize the abundance, distribution and structural properties of the VL-II subhalo population in terms of both subhalo masses and maximum circular velocities. Then, we repopulate the original simulation with millions of subhaloes of masses down to about five orders of magnitude below the minimum VL-II subhalo mass (more than one order of magnitude in velocities). We compute subhalo DM annihilation astrophysical ‘J-factors’ and angular sizes for the entire subhalo population, by placing the Earth at a random position but at the right Galactocentric distance in the simulation. Thousands of these realizations are generated in order to obtain statistically meaningful results. We find that some nearby low-mass Galactic subhaloes, not massive enough to retain stars or gas, may indeed yield DM annihilation fluxes comparable to those expected from other, more massive and acknowledgeable DM targets like dwarf satellite galaxies. Typical angular sizes are of the order of the degree, thus subhaloes potentially appearing as extended sources in gamma-ray telescopes, depending on instrument angular resolution and sensitivity. Our work shows that low-mass Galactic subhaloes with no visible counterparts are expected to play a relevant role in current and future indirect DM search searches and should indeed be considered as excellent DM targets.
The isotropic gamma-ray background arises from the contribution of unresolved sources, including members of confirmed source classes and proposed gamma-ray emitters such as the radiation induced by ...dark matter annihilation and decay. Clues about the properties of the contributing sources are imprinted in the anisotropy characteristics of the gamma-ray background. We use 81 months of Pass 7 Reprocessed data from the Fermi Large Area Telescope to perform a measurement of the anisotropy angular power spectrum of the gamma-ray background. We analyze energies between 0.5 and 500 GeV, extending the range considered in the previous measurement based on 22 months of data. We also compute, for the first time, the cross-correlation angular power spectrum between different energy bins. We find that the derived angular spectra are compatible with being Poissonian, i.e. constant in multipole. Moreover, the energy dependence of the anisotropy suggests that the signal is due to two populations of sources, contributing, respectively, below and above ∼2 GeV. Finally, using data from state-of-the-art numerical simulations to model the dark matter distribution, we constrain the contribution from dark matter annihilation and decay in Galactic and extra-Galactic structures to the measured anisotropy. These constraints are competitive with those that can be derived from the average intensity of the isotropic gamma-ray background.
Abstract
Gamma-ray observations have long been used to constrain the properties of dark matter
(DM), with a strong focus on weakly interacting massive particles annihilating through
...velocity-independent processes. However, in the absence of clear-cut observational evidence for
the simplest candidates, the interest of the community in more complex DM scenarios involving a
velocity-dependent cross-section has been growing steadily over the past few years. We present
the first systematic study of velocity-dependent DM annihilation (in particular
p
-wave
annihilation and Sommerfeld enhancement) in a variety of astrophysical objects, not only including
the well-studied Milky Way dwarf satellite galaxies, but nearby dwarf irregular galaxies and local
galaxy clusters as well. Particular attention is given to the interplay between velocity
dependence and DM halo substructure. Uncertainties related to halo mass, phase-space and
substructure modelling are also discussed in this velocity-dependent context. We show that, for
s-wave annihilation, extremely large subhalo boost factors are to be expected, up to 10
11
in clusters and up to 10
6
–10
7
in dwarf galaxies where subhalos are usually assumed not to play
an important role. Boost factors for
p
-wave annihilation are smaller but can still reach 10
3
in clusters. The angular extension of the DM signal is also significantly impacted, with e.g. the
cluster typical emission radius increasing by a factor of order 10 in the
s
-wave case. We also
compute the signal contrast of the objects in our sample with respect to annihilation happening in
the Milky Way halo. Overall, we find that the hierarchy between the brightest considered targets
depends on the specific details of the assumed particle-physics model.
Tidal stripping in the adiabatic limit Stücker, Jens; Ogiya, Go; Angulo, Raul E ...
Monthly Notices of the Royal Astronomical Society,
05/2023, Letnik:
521, Številka:
3
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
We present a model for the remnants of haloes that have gone through an adiabatic tidal stripping process. We show that this model exactly reproduces the remnant of an NFW halo that is ...exposed to a slowly increasing isotropic tidal field and approximately for an anisotropic tidal field. The model can be used to predict the asymptotic mass loss limit for orbiting subhaloes, solely as a function of the initial structure of the subhalo and the value of the tidal field at pericentre. Predictions can easily be made for differently concentrated host-haloes with and without baryonic components, which differ most notably in their relation between pericentre radius and tidal field. The model correctly predicts several empirically measured relations such as the ‘tidal track’ and the ‘orbital frequency relation’ that was reported by Errani & Navarro (2021) for the case of an isothermal sphere. Further, we propose applications of the ‘structure–tide’ degeneracy, which implies that increasing the concentration of a subhalo has exactly the same impact on tidal stripping as reducing the amplitude of the tidal field. Beyond this, we find that simple relations hold for the bound mass, truncation radius, WIMP annihilation luminosity, and tidal ratio of tidally stripped NFW haloes in relation to quantities measured at the radius of maximum circular velocity. Finally, we note that NFW haloes cannot be completely disrupted when exposed adiabatically to tidal fields of arbitrary magnitudes. We provide an open-source implementation of our model and suggest that it can be used to improve predictions of dark matter annihilation.
Abstract
As searches for thermal and self-annihilating dark matter (DM) intensify, it becomes
crucial to include as many relevant physical processes and ingredients as possible to refine
signal ...predictions, in particular those which directly relate to the intimate properties of DM. We
investigate the combined impact of DM subhalos and of the (velocity-dependent) Sommerfeld
enhancement of the annihilation cross section. Both features are expected to play an important
role in searches for thermal DM particle candidates with masses around or beyond TeV, or in
scenarios with a light dark sector. We provide a detailed analytical description of the phenomena
at play, and show how they scale with the subhalo masses and the main Sommerfeld parameters. We
derive approximate analytical expressions that can be used to estimate the overall boost factors
resulting from these combined effects, from which the intricate phenomenology can be better
understood. DM subhalos lead to an increase of the Sommerfeld effect by several orders of
magnitude (for both
s
- and
p
-wave annihilation processes), especially on resonances, which
makes them critical to get sensible gamma-ray signal predictions for typical targets of different
masses (from dwarf galaxies to galaxy clusters).
ABSTRACT
In this work, we carry out a suite of specially designed numerical simulations to shed light on dark matter (DM) subhalo survival at mass scales relevant for gamma-ray DM searches, a topic ...subject to intense debate nowadays. We have employed an improved version of DASH, a GPU N-body code, to study the evolution of low-mass subhaloes inside a Milky-Way-like halo with unprecedented accuracy, reaching solar-mass and sub-parsec resolution. We simulate subhaloes with varying mass, concentration, and orbital properties, and consider the effect of baryons in the host. We analyse the evolution of the bound mass fraction and annihilation luminosity, finding that most subhaloes survive until present, yet losing in some cases more than 99 per cent of their initial mass. Baryons induce a much greater mass-loss, especially when the subhalo orbit is more parallel to the Galactic disc. Many of these subhaloes cross the solar Galactocentric radius, making it easier to detect their annihilation fluxes from Earth. We find subhaloes orbiting a DM-only halo with a pericentre in the solar vicinity to lose 70–90 per cent of their initial annihilation luminosity at present, which increases up to 99 per cent when including baryons. We find a strong relation between subhalo’s mass-loss and the effective tidal field at pericentre. Indeed, much of the dependence on all considered parameters can be explained through this single parameter. In addition to shedding light on the survival of low-mass Galactic subhaloes, our results can provide detailed predictions that will aid current and future quests for the nature of DM.
Imaging atmospheric Cherenkov telescopes (IACTs) that are sensitive to potential γ-ray signals from dark matter (DM) annihilation above ∼50 GeV will soon be superseded by the Cherenkov Telescope ...Array (CTA). CTA will have a point source sensitivity an order of magnitude better than currently operating IACTs and will cover a broad energy range between 20 GeV and 300 TeV. Using effective field theory and simplified models to calculate γ-ray spectra resulting from DM annihilation, we compare the prospects to constrain such models with CTA observations of the Galactic center with current and near-future measurements at the Large Hadron Collider (LHC) and direct detection experiments. For DM annihilations via vector or pseudoscalar couplings, CTA observations will be able to probe DM models out of reach of the LHC, and, if DM is coupled to standard fermions by a pseudoscalar particle, beyond the limits of current direct detection experiments.
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