Dwarf spheroidal galaxies (dSphs) are the most compact dark matter-dominated objects observed so far. The Pauli exclusion principle limits the number of fermionic dark matter particles that can compose a dSph halo. This results in a well-known lower bound on their particle mass. So far, such bounds were obtained from the analysis of individual dSphs. In this paper, we model dark matter halo density profiles via the semi-analytical approach and analyse the data from eight 'classical' dSphs assuming the same mass of dark matter fermion in each object. First, we find out that modelling of Carina dSph results in a much worse fitting quality compared to the other seven objects. From the combined analysis of the kinematic data of the remaining seven 'classical' dSphs, we obtain a new 2σ lower bound of m 190 eV on the dark matter fermion mass. In addition, by combining a sub-sample of four dSphs -Draco, Fornax, Leo I and Sculptor -we conclude that 220 eV fermionic dark matter appears to be preferred over the standard CDM at about 2σ level. However, this result becomes insignificant if all seven objects are included in the analysis. Future improvement of the obtained bound requires more detailed data, both from 'classical' and ultra-faint dSphs.

Observations of the redshifted 21-cm signal (in absorption or emission) allow us to peek into the epoch of the "Dark Ages" and the onset of reionization. These data can provide a novel way to learn about the nature of dark matter, in particular about the formation of small-size dark matter halos. However, the connection between the formation of structures and the 21-cm signal requires knowledge of a stellar to total mass relation, an escape fraction of UV photons, and other parameters that describe star formation and radiation at early times. This baryonic physics depends on the properties of dark matter and in particular, in warm-dark-matter (WDM) models, star formation may follow a completely different scenario, as compared to the cold-dark-matter case. We use the recent measurements by EDGES [An absorption profile centred at 78 megahertz in the sky-averaged spectrum, Nature (London) 555, 67 (2018).] to demonstrate that when taking the above considerations into account, the robust WDM bounds are in fact weaker than those given by the Lyman-α forest method and other structure formation bounds. In particular, we show that a resonantly produced 7-keV sterile neutrino dark matter model is consistent with these data. However, a holistic approach to modeling of the WDM universe holds great potential and may, in the future, make 21-cm data our main tool to learn about DM clustering properties.

Abstract. Recent reports of a weak unidentified emission line at ∼3.5 keV found in spectra of several matter-dominated objects may give a clue to resolve the long-standing problem of dark matter. One of the best physically motivated particle candidate able to produce such an extra line is sterile neutrino with the mass of ∼7 keV. Previous works show that sterile neutrino dark matter with parameters consistent with the new line measurement modestly affects structure formation compared to conventional cold dark matter scenario. In this work, we concentrate for the first time on contribution of the sterile neutrino dark matter able to produce the observed line at ∼3.5 keV, to the process of reionization. By incorporating dark matter power spectra for ∼7 keV sterile neutrinos into extended semi-analytical 'bubble' model of reionization we obtain that such sterile neutrino dark matter would produce significantly sharper reionization compared to widely used cold dark matter models, impossible to 'imitate' within the cold dark matter scenario under any reasonable choice of our model parameters, and would have a clear tendency of lowering both the redshift of reionization and the electron scattering optical depth (although the difference is still below the existing model uncertainties). Further dedicated studies of reionization (such as 21 cm measurements or studies of kinetic Sunyaev-Zeldovich effect) will thus be essential for reconstruction of particle candidate responsible the ∼3.5 keV line.

The number density of small dark matter (DM) halos hosting faint high-redshift galaxies is sensitive to the DM free-streaming properties. However, constraining these DM properties is complicated by degeneracies with the uncertain baryonic physics governing star formation. In this work, we use a flexible astrophysical model and a Bayesian inference framework to analyse ultra-violet (UV) luminosity functions (LFs) at z = 6 − 8. We vary the complexity of the astrophysical galaxy model (single vs double power law for the stellar – halo mass relation) as well as the matter power spectrum (cold DM vs thermal relic warm DM), comparing their Bayesian evidences. Adopting a conservatively wide prior range for the WDM particle mass, we show that the UV LFs at z = 6 − 8 only weakly favour CDM over WDM. We find that particle masses of ≲ 2 keV are rejected at a 95% credible level in all models that have a WDM-like power spectrum cutoff. This bound should increase to ∼2.5 keV with the James Webb Space Telescope (JWST).

PACSIn this paper, we formulate a new model of density distribution for halos made of warm dark matter (WDM) particles. The model is described by a single microphysics parameter the mass (or, equivalently, the maximal value of the initial phase-space density distribution) of dark matter particles. Given the WDM particle mass and the parameters of a dark matter density profile at the halo periphery, this model predicts the inner density profile. In case of initial Fermi-Dirac distribution, we successfully reproduce cored dark matter profiles from N -body simulations. Also, we calculate the core radii of warm dark matter halos of dwarf spheroidal galaxies for particle masses mfd = 100, 200, 300 and 400 eV. K e y w o r d s: Dark matter: warm, cold; dark matter halo profile; cores; Navarro-Frenk-White profileThe nature of dark matter the largest gravitating substance in the Universe is not yet identified. Usual (left-handed) neutrinos the only natural dark matter candidate within the Standard Model of particle physics are too light to form the observed large-scale structure of the Universe [1] and the densest dark matter-dominated objects, dwarf spheroidals (dSphs) [2]. So far, many extensions of the Standard Model containing a viable dark matter candidate have been proposed; see, e.g., reviews [3][4][5][6]. In terms of their initial velocities, valid dark matter candidates can be split in two groups 1 (see, e.g., [9]):• cold dark matter (CDM), composed of particles with small (non-relativistic) initial velocities [10,11]; c A.V. RUDAKOVSKYI, D.O. SAVCHENKO, 2018 1 Note that, for some specific dark matter particle candidates, their initial velocity spectrum can be approximated by a mixture of 'cold ' and 'warm' components [7, 8].• warm dark matter (WDM), composed of particles with large (relativistic) initial velocities [12,13].Density distribution of CDM haloes is often described by the Navarro-Frenk-White (NFW) profile [14,15] ρ nfw (r) = ρ s r s r 1 + r rs 2 . (1) Its parameters ρ s and r s are connected with the halo mass M 200 (the mass within the sphere of radius R 200 , within which the average density is 200 times larger than the critical density ρ crit of the Universe) and halo concentration parameter c 200 = R 200 /r s .The phase-space density for CDM haloes becomes infinite towards the halo centre; see, e.g., [16]. For WDM, this is not true: its maximal phase-space density f max is finite at early times and does not increase during halo formation [17]. Usually, density distri-

The recent detection of the 21-cm absorption signal by the EDGES collaboration has been widely used to constrain the basic properties of dark matter particles. However, extracting the parameters of the 21-cm absorption signal relies on a chosen parametrisation of the foreground radio emission. Recently, the new parametrisations of the foreground and systematics have been proposed, showing significant deviations of the 21-cm signal parameters from those assumed by the original EDGES paper. In this paper, we consider this new uncertainty, comparing the observed signal with the predictions of several dark matter models, including the widely-used cold dark matter (CDM) model, 1–3 keV warm dark matter models (WDM), and 7 keV sterile neutrino (SN7) model, capable of producing the reported 3.5 keV line. We show that all these dark matter models cannot be statistically distinguished using the available EDGES data.

One of possible explanations of a faint narrow emission line at 3.5 keV reported in our Galaxy, Andromeda galaxy and a number of galaxy clusters is the dark matter made of 7 keV sterile neutrinos. Another signature of such sterile neutrino dark matter could be fewer ionizing sources in the early Universe (compared to the standard 'cold dark matter' (CDM) scenario), which should affect the reionization of the Universe. By using a semi-analytical model of reionization, we compare the model predictions for CDM and two different models of 7 keV sterile neutrino dark matter (consistent with the 3.5 keV line interpretation as decaying dark matter line) with available observations of epoch of reionization (including the final measurements of electron scattering optical depth made by Planck observatory). We found that both CDM and 7 keV sterile neutrino dark matter well describe the data. The overall fit quality for sterile neutrino dark matter is slightly (with ∆ χ 2 ≃ 2 − 3) better than for CDM, although it is not possible to make a robust distinction between these models on the basis of the given observations.

Stellar and gas kinematics of galaxies are a sensitive probe of the dark matter distribution in the halo. The popular fuzzy dark matter models predict the peculiar shape of density distribution in galaxies: specific dense core with sharp transition to the halo. Moreover, fuzzy dark matter predicts scaling relations between the dark matter particle mass and density parameters. In this work, we use a Bayesian framework and several dark matter halo models to analyse the stellar kinematics of galaxies using the Spitzer Photometry & Accurate Rotation Curves database. We then employ a Bayesian model comparison to select the best halo density model. We find that more than half of the galaxies prefer the fuzzy dark model against standard dark matter profiles (NFW, Burkert, and cored NFW). While this seems like a success for fuzzy dark matter, we also find that there is no single value for the particle mass that provides a good fit for all galaxies.