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Register a new userMilky Way mass constraints from the Galactic satellite gap
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Marius Cautun
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We use the distribution of maximum circular velocities, $V_{max}$, of satellites in the Milky Way (MW) to constrain the virial mass, $M_{200}$, of the Galactic halo under an assumed prior of a $Lambda$CDM universe. This is done by analysing the subhalo populations of a large sample of halos found in the Millennium II cosmological simulation. The observation that the MW has at most three subhalos with $V_{max}ge30 km/s$ requires a halo mass $M_{200}le1.4times10^{12} M_odot$, while the existence of the Magellanic Clouds (assumed to have $V_{max}ge60 km/s$) requires $M_{200}ge1.0times10^{12} M_odot$. The first of these conditions is necessary to avoid the too-big-to-fail problem highlighted by Boylan-Kolchin et al., while the second stems from the observation that massive satellites like the Magellanic Clouds are rare. When combining both requirements, we find that the MW halo mass must lie in the range $0.25 le M_{200}/(10^{12} M_odot) le 1.4$ at $90%$ confidence. The gap in the abundance of Galactic satellites between $30 km/sle V_{max} le 60 km/s$ places our galaxy in the tail of the expected satellite distribution.
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We perform a comprehensive study of Milky Way (MW) satellite galaxies to constrain the fundamental properties of dark matter (DM). This analysis fully incorporates inhomogeneities in the spatial distribution and detectability of MW satellites and marginalizes over uncertainties in the mapping between galaxies and DM halos, the properties of the MW system, and the disruption of subhalos by the MW disk. Our results are consistent with the cold, collisionless DM paradigm and yield the strongest cosmological constraints to date on particle models of warm, interacting, and fuzzy dark matter. At $95%$ confidence, we report limits on (i) the mass of thermal relic warm DM, $m_{rm WDM} > 6.5 mathrm{keV}$ (free-streaming length, $lambda_{rm{fs}} lesssim 10,h^{-1} mathrm{kpc}$), (ii) the velocity-independent DM-proton scattering cross section, $sigma_{0} < 8.8times 10^{-29} mathrm{cm}^{2}$ for a $100 mathrm{MeV}$ DM particle mass (DM-proton coupling, $c_p lesssim (0.3 mathrm{GeV})^{-2}$), and (iii) the mass of fuzzy DM, $m_{phi}> 2.9 times 10^{-21} mathrm{eV}$ (de Broglie wavelength, $lambda_{rm{dB}} lesssim 0.5 mathrm{kpc}$). These constraints are complementary to other observational and laboratory constraints on DM properties.
The satellite galaxies of the Milky Way (MW) are effective probes of the underlying dark matter (DM) substructure, which is sensitive to the nature of the DM particle. In particular, a class of DM models have a power spectrum cut-off on the mass scale of dwarf galaxies and thus predict only small numbers of substructures below the cut-off mass. This makes the MW satellite system appealing to constrain the DM properties: feasible models must produce enough substructure to host the number of observed Galactic satellites. Here, we compare theoretical predictions of the abundance of DM substructure in thermal relic warm DM (WDM) models with estimates of the total satellite population of the MW. This produces conservative robust lower limits on the allowed mass, $m_mathrm{th}$, of the thermal relic WDM particle. As the abundance of satellite galaxies depends on the MW halo mass, we marginalize over the corresponding uncertainties and rule out $m_mathrm{th} leq 2.02, mathrm{keV}$ at 95 per cent confidence independently of assumptions about galaxy formation processes. Modelling some of these - in particular, the effect of reionization, which suppresses the formation of dwarf galaxies - strengthens our constraints on the DM properties and excludes models with $m_mathrm{th} leq 3.99, mathrm{keV}$ in our fiducial model. We also find that thermal relic models cannot produce enough satellites if the MW halo mass is $M_{200}leq 0.6times 10^{12}, mathrm{M_odot}$, which imposes a lower limit on the MW halo mass in CDM. We address several observational and theoretical uncertainties and discuss how improvements in these will strengthen the DM mass constraints.
We use new kinematic data from the ultra-faint Milky Way satellite Segue 1 to model its dark matter distribution and derive upper limits on the dark matter annihilation cross-section. Using gamma-ray flux upper limits from the Fermi satellite and MAGIC, we determine cross-section exclusion regions for dark matter annihilation into a variety of different particles including charged leptons. We show that these exclusion regions are beginning to probe the regions of interest for a dark matter interpretation of the electron and positron fluxes from PAMELA, Fermi, and HESS, and that future observations of Segue 1 have strong prospects for testing such an interpretation. We additionally discuss prospects for detecting annihilation with neutrinos using the IceCube detector, finding that in an optimistic scenario a few neutrino events may be detected. Finally we use the kinematic data to model the Segue 1 dark matter velocity dispersion and constrain Sommerfeld enhanced models.
A small fraction of thermalized dark radiation that transitions into cold dark matter (CDM) between big bang nucleosynthesis and matter-radiation equality can account for the entire dark matter relic density. Because of its transition from dark radiation, late-forming dark matter (LFDM) suppresses the growth of linear matter perturbations and imprints the oscillatory signatures of dark radiation perturbations on small scales. The cutoff scale in the linear matter power spectrum is set by the redshift $z_T$ of the phase transition; tracers of small-scale structure can therefore be used to infer the LFDM formation epoch. Here, we use a forward model of the Milky Way (MW) satellite galaxy population to address the question: How late can dark matter form? For dark radiation with strong self-interactions, which arises in theories of neutrinolike LFDM, we report $z_{T}>5.5times 10^6$ at $95%$ confidence based on the abundance of known MW satellite galaxies. This limit rigorously accounts for observational incompleteness corrections, marginalizes over uncertainties in the connection between dwarf galaxies and dark matter halos, and improves upon galaxy clustering and Lyman-$alpha$ forest constraints by nearly an order of magnitude. We show that this limit can also be interpreted as a lower bound on $z_T$ for LFDM that free-streams prior to its phase transition, although dedicated simulations will be needed to analyze this case in detail. Thus, dark matter created by a transition from dark radiation must form no later than one week after the big bang.
We calculate the probability that a Milky-Way-like halo in the standard cosmological model has the observed number of Magellanic Clouds (MCs). The statistics of the number of MCs in the LCDM model are in good agreement with observations of a large sample of SDSS galaxies. Under the sub-halo abundance matching assumption of a relationship with small scatter between galaxy r-band luminosities and halo internal velocities v_max, we make detailed comparisons to similar measurements using SDSS DR7 data by Liu et al. (2010). Models and observational data give very similar probabilities for having zero, one, and two MC-like satellites. In both cases, Milky Way-luminosity hosts have just a sim 10% chance of hosting two satellites similar to the Magellanic Clouds. In addition, we present a prediction for the probability for a host galaxy to have Nsats satellite galaxies as a function of the magnitudes of both the host and satellite. This probability and its scaling with host properties is significantly different from that of mass-selected objects because of scatter in the mass- luminosity relation and because of variations in the star formation efficiency with halo mass.
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