The SRG/eROSITA All-Sky Survey
R. SeppiJohan ComparatV. GhirardiniC. GarrelE. ArtisAriel G. SánchezAng LiuN. ClercEsra BülbülS. GrandisM. KlugeT. H. ReiprichA. MerloniX. ZhangY. E. BaharSoumya ShreeramJ. S. SandersM. E. Ramos-CejaM. Krumpe
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Context. The spatial distribution of galaxy clusters provides a reliable tracer of the large-scale distribution of matter in the Universe. The clustering signal depends on intrinsic cluster properties and cosmological parameters. Aims. The ability of eROSITA on board Spectrum-Roentgen-Gamma (SRG) to discover galaxy clusters allows the association of extended X-ray emission with dark matter haloes to be probed. We measured the projected two-point correlation function to study the occupation of dark matter haloes by clusters and groups detected by the first eROSITA all-sky survey (eRASS1). Methods. We created five volume-limited samples probing clusters with different redshifts and X-ray luminosity values. We interpreted the correlation function with halo occupation distribution (HOD) and halo abundance matching (HAM) models. We simultaneously fit the cosmological parameters and halo bias of a flux-limited sample of 6493 clusters with purity > 96%. Results. We obtained a detailed view of the halo occupation for eRASS1 clusters. The fainter population at low redshift (S0: L̄ X = 4.63 × 10 43 erg s −1 , 0.1 < z < 0.2) is the least biased compared to dark matter, with b = 2.95 ± 0.21. The brightest clusters up to higher redshift (S4: L̄ X = 1.77 × 10 44 erg s −1 , 0.1 < z < 0.6) exhibit a higher bias b = 4.34 ± 0.62. Satellite groups are rare, with a satellite fraction < 14.9% (8.1) for the S0 (S4) sample. We combined the HOD prediction with a HAM procedure to constrain the scaling relation between L X and mass in a new way, and find a scatter of ⟨ σ Lx ⟩ = 0.36. We obtain cosmological constraints for the physical cold dark matter density ω c = 0.12 −0.02 +0.03 and an average halo bias b = 3.63 −0.85 +1.02 . Conclusions. We modelled the clustering of galaxy clusters with a HOD approach for the first time, paving the way for future studies combining eROSITA with 4MOST, SDSS, Euclid , Rubin , and DESI to unravel the cluster distribution in the Universe.The observations of microlensing events in the Large Magellanic Cloud suggest that a sizable fraction ($\sim$ 50%) of the galactic halo is in the form of MACHOs (Massive Astrophysical Compact Halo Objects) with an average mass $\sim 0.27 M_{\odot}$, assuming a standard spherical halo model. We describe a scenario in which dark clusters of MACHOs and cold molecular clouds (mainly of $H_2$) naturally form in the halo at galactocentric distances larger than 10--20 kpc.
Gravitational microlensing
Large Magellanic Cloud
Baryonic dark matter
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Cuspy halo problem
Shearing (physics)
Dynamical friction
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Self-consistent bars that form in galaxies embedded within cuspy halos are unable to flatten the cusp. Short bars form in models with quasi-flat rotation curves. They lose angular momentum to the halo through dynamical friction, but the continuous concentration of mass within the disk as the bar grows actually compresses the halo further, overwhelming any density reduction due to the modest angular momentum transfer to the halo. Thus, the Weinberg-Katz proposed solution to the nonexistence of the predicted cuspy halos from cold dark matter simulations would seem to be unworkable. I also find that the concerns over the performance of N-body codes raised by these authors do not apply to the methods used here.
Cuspy halo problem
Bar (unit)
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We study the growth of galactic disks in live triaxial DM halos. The halos have been assembled through constrained realizations method and evolved from the linear regime using cosmological simulations. The `seed' disks have been inserted at redshift z=3 and increased in mass tenfold over various time periods, ~1-3 Gyr, with the halo responding quasi-adiabatically to this process. We follow the dynamical and secular evolution of the disk-halo system and analyze changes in the most important parameters, like 3-D DM shapes, stellar and DM radial density profiles, stellar bar development, etc. We find that a growing disk is responsible for washing out the halo prolateness and for diluting its flatness over a period of time comparable to the disk growth. Moreover, we find that a disk which contributes more to the overall rotation curve in the system is also more efficient in axisymmetrizing the halo, without accelerating the halo figure rotation. The observational corollary is that the maximal disks probably reside in nearly axisymmetric halos, while disks whose rotation is dominated by the halo at all radii are expected to reside in more prolate halos. The halo shape is sensitive to the final disk mass, but is independent of how the seed disk is introduced into the system. We also expect that the massive disks are subject to a bar instability, while light disks have this instability damped by the halo triaxiality. Implications to the cosmological evolution of disks embedded in asymmetric halos are discussed and so are the corollaries for the observed fraction of stellar bars. Finally, the halo responds to the stellar bar by developing a gravitational wake -- a `ghost' bar of its own which is almost in-phase with that in the disk.
Thick disk
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Halo white dwarfs can provide important information about the properties and evolution of the Galactic halo. In this paper we compute, assuming a standard initial mass function (IMF) and updated models of white dwarf cooling, the expected luminosity function, both in luminosity and in visual magnitude, for different star formation rates. We show that a deep enough survey (limiting magnitude ≳20) could provide important information about the halo age and the duration of the formation stage. We also show that the number of white dwarfs produced using the recently proposed biased IMFs cannot represent a large fraction of the halo dark matter if they are constrained by the presently observed luminosity function. Furthermore, we show that a robust determination of the bright portion of the luminosity function can provide strong constraints on the allowable IMF shapes.
Absolute magnitude
Limiting
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The central region of the dark matter halo of the Milky Way is a promising target for a search for a particle dark matter annihilation signal. The H.E.S.S. Collaboration has published a search for a photon flux originating from dark matter particles annihilating in the galactic center region. No significant excess was observed and upper limits on the velocity averaged dark matter self annihilation cross section were derived. The limits exclude the self annihilation of dark matter particles in the $\sim 1$ TeV to $\sim 4$ TeV mass range with a velocity averaged annihilation cross section larger than $\sim 3\cdot 10^{-25}\:\mathrm{cm^3/s}$, i.e. about one order of magnitude above the prediction for a thermal relic dark matter particle. A detailed and realistic Monte Carlo study of new strategies for the search for a particle dark matter annihilation signal from the Milky Way dark matter halo with the High Energy Stereoscopic System is presented and the sensitivity of different experimental approaches is compared.
Annihilation
Light dark matter
Warm dark matter
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I discuss the dynamical interaction of galactic disks with the surrounding dark matter halos. In particular it is demonstrated that if the self-gravitating shearing sheet, a model of a patch of a galactic disk, is embedded in a live dark halo, this has a strong effect on the dynamics of density waves in the sheet. I describe how the density waves and the halo interact via halo particles either on orbits in resonance with the wave or on non-resonant orbits. Contrary to expectation the presence of the halo leads to a very considerable enhancement of the amplitudes of the density waves in the shearing sheet. This effect appears to be the equivalent of the recently reported enhanced growth of bars in numerically simulated stellar disks embedded in live dark halos. Finally I discuss the counterparts of the perturbations of the disk in the dark halo.
Cuspy halo problem
Shearing (physics)
Dynamical friction
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Seven cosmological hydrodynamic simulations of disk galaxy formation are analyzed to determine the alignment of the disk within the dark matter halo and the internal structure of the halo. We find that the orientation of the outer halo, beyond ~0.1rvir, is unaffected by the presence of the disk. In contrast, the inner halo is aligned such that the halo minor axis aligns with the disk axis. The relative orientations of these two regions of the halo are uncorrelated. The alignment of the disk and inner halo appears to take place simultaneously through their joint evolution. The lack of connection between these two regions of the halo should be taken into account when modeling tidal streams in the halos of disk galaxies and when calculating intrinsic alignments of disk galaxies based on the properties of dark matter halos.
Cuspy halo problem
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We show that the mass of a dark matter halo can be inferred from the dynamical status of its satellite galaxies. Using 9 dark-matter simulations of halos like the Milky Way (MW), we find that the present-day substructures in each halo follow a characteristic distribution in the phase space of orbital binding energy and angular momentum, and that this distribution is similar from halo to halo but has an intrinsic dependence on the halo formation history. We construct this distribution directly from the simulations for a specific halo and extend the result to halos of similar formation history but different masses by scaling. The mass of an observed halo can then be estimated by maximizing the likelihood in comparing the measured kinematic parameters of its satellite galaxies with these distributions. We test the validity and accuracy of this method with mock samples taken from the simulations. Using the positions, radial velocities, and proper motions of 9 tracers and assuming observational uncertainties comparable to those of MW satellite galaxies, we find that the halo mass can be recovered to within $\sim$40%. The accuracy can be improved to within $\sim$25% if 30 tracers are used. However, the dependence of the phase-space distribution on the halo formation history sets a minimum uncertainty of $\sim$20% that cannot be reduced by using more tracers. We believe that this minimum uncertainty also applies to any mass determination for a halo when the phase space information of other kinematic tracers is used.
Satellite galaxy
Cuspy halo problem
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I discuss the dynamical interaction of galactic disks with the surrounding dark matter halos. In particular it is demonstrated that if the self-gravitating shearing sheet, a model of a patch of a galactic disk, is embedded in a live dark halo, this has a strong effect on the dynamics of density waves in the sheet. I describe how the density waves and the halo interact via halo particles either on orbits in resonance with the wave or on non-resonant orbits. Contrary to expectation the presence of the halo leads to a very considerable enhancement of the amplitudes of the density waves in the shearing sheet. This effect appears to be the equivalent of the recently reported enhanced growth of bars in numerically simulated stellar disks embedded in live dark halos. Finally I discuss the counterparts of the perturbations of the disk in the dark halo.
Cuspy halo problem
Shearing (physics)
Dynamical friction
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