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Register a new userConstraining ultra light fermionic dark matter with Milky Way observations
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The equation of state for a degenerate gas of fermions at zero temperature in the non relativistic case is a polytrope, i.e. $p=gamma rho^{5/3}/m_F^{8/3}$. If dark matter is modelled by such non interacting fermion, this dependence in the mass of the fermion $m_F$ explains why if dark matter is very heavy the effective pressure of dark matter is negligible. Nevertheless, if the mass of the dark matter is very small, the effective pressure can be very large, and thus, a system of self-gravitating fermions can be formed. In this work we model the dark matter halo of the Milky-Way by solving the Tolman-Oppenheimer-Volkoff equations, with the equation of state for a partially degenerate ultralight non interacting fermion. It is found that in order to fit its rotational velocity curve of the Milky Way, the mass of the fermion should be in the range $29 ~mbox{eV} < m_F < 33~$eV. Moreover, the central density is constrained to be in the range of $46 < rho_0<61$ GeV/cm$^3$. The fermionic dark matter halo has a very different profile as compared with the standard Navarro-Frenk-White profile, thus, the possible indirect signals for annihilating dark matter may change by orders of magnitude. We found bounds for the annihilation cross section in this case by using the Saggitarius A* spectral energy distribution. Those limits are very strong confirming the idea that the lighter the dark matter particle is, the darkest it becomes.
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There are many well-known correlations between dark matter and baryons that exist on galactic scales. These correlations can essentially be encompassed by a simple scaling relation between observed and baryonic accelerations, historically known as the Mass Discrepancy Acceleration Relation (MDAR). The existence of such a relation has prompted many theories that attempt to explain the correlations by invoking additional fundamental forces on baryons. The standard lore has been that a theory that reduces to the MDAR on galaxy scales but behaves like cold dark matter (CDM) on larger scales provides an excellent fit to data, since CDM is desirable on scales of clusters and above. However, this statement should be revised in light of recent results showing that a fundamental force that reproduces the MDAR is challenged by Milky Way dynamics. In this study, we test this claim on the example of Superfluid Dark Matter. We find that a standard CDM model is strongly preferred over a static superfluid profile. This is due to the fact that the superfluid model over-predicts vertical accelerations, even while reproducing galactic rotation curves. Our results establish an important criterion that any dark matter model must satisfy within the Milky Way.
The unambiguous detection of Galactic dark matter annihilation would unravel one of the most outstanding puzzles in particle physics and cosmology. Recent observations have motivated models in which the annihilation rate is boosted by the Sommerfeld effect, a non-perturbative enhancement arising from a long range attractive force. Here we apply the Sommerfeld correction to Via Lactea II, a high resolution N-body simulation of a Milky-Way-size galaxy, to investigate the phase-space structure of the Galactic halo. We show that the annihilation luminosity from kinematically cold substructure can be enhanced by orders of magnitude relative to previous calculations, leading to the prediction of gamma-ray fluxes from up to hundreds of dark clumps that should be detectable by the Fermi satellite.
We employ data from the recently observed high-energy neutrino events at the IceCube Neutrino Observatory to constrain interactions between the dark matter (DM) in the Milky Way and the neutrino sector. We construct an extended un-binned likelihood in order to explore the parameter space of allowed interactions. We present results in the specific case of a scalar DM candidate interacting via a scalar mediator, and show that due to the energy dependence of the interaction cross section, this approach can constrain the coupling more strongly than traditional cosmological probes for some regions of the parameter space.
Even if Dark Matter (DM) is neutral under electromagnetism, it can still interact with the Standard Model (SM) via photon exchange from higher-dimensional operators. Here we classify the general effective operators coupling DM to photons, distinguishing between Dirac/Majorana fermion and complex/real scalar DM. We provide model-independent constraints on these operators from direct and indirect detection. We also constrain various DM-lepton operators, which induce DM-photon interactions via RG running or which typically arise in sensible UV-completions. This provides a simple way to quickly assess constraints on any DM model that interacts mainly via photon exchange or couples to SM leptons.
We use the Milky Ways nuclear star cluster (NSC) to test the existence of a dark matter soliton core, as predicted in ultra-light dark matter (ULDM) models. Since the soliton core size is proportional to mDM^{-1}, while the core density grows as mDM^{2}, the NSC (dominant stellar component within about 3 pc) is sensitive to a specific window in the dark matter particle mass, mDM. We apply a spherical isotropic Jeans model to fit the NSC line-of-sight velocity dispersion data, assuming priors on the Milky Ways supermassive black hole (SMBH) mass taken from the Gravity Collaboration et al. (2020) and stellar density profile taken from Gallego-Cano et al. (2018). We find that the current observational data reject the existence of a soliton core for a single ULDM particle with mass in the range 10^{-20.0} < mDM < 10^{-18.5} eV, assuming that the soliton core structure is not affected by the Milky Ways SMBH. We test our methodology on mock data, confirming that we are sensitive to the same range in ULDM mass as for the real data. Dynamical modelling of a larger region of the Galactic centre, including the nuclear stellar disc, promises tighter constraints over a broader range of mDM. We will consider this in future work.
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