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We analyze circular velocity profiles of seven ultradiffuse galaxies (UDGs) that are isolated and gas-rich. Assuming that the dark matter halos of these UDGs have a Navarro–Frenk–White (NFW) density profile or a Read density profile (which allows for constant-density cores), the inferred halo concentrations are systematically lower than the cosmological median, even as low as −0.6 dex (about 5σaway) in some cases. Alternatively, similar fits can be obtained with a density profile that scales roughly as 1/r²for radii larger than a few kiloparsecs. Both solutions require the radius where the halo circular velocity peaks ($R_{\max }$) to be much larger than the median expectation. Surprisingly, we find an overabundance of such large-$R_{\max }$halos in the IllustrisTNG dark-matter-only simulations compared to the Gaussian expectation. These halos form late and have higher spins compared to median halos of similar masses. The inner densities of the most extreme among these late-forming halos are higher than their NFW counterparts, leading to a ∼1/r²density profile. However, the two well-resolved UDGs in our sample strongly prefer lower dark matter densities in the center than the simulated ones. Comparing to IllustrisTNG hydrodynamical simulations, we also find a tension in getting both lowmore »enough circular velocities and high enough halo mass to accommodate the measurements. Our results indicate that the gas-rich UDGs present a significant challenge for galaxy formation models.

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ABSTRACT Self-interacting dark matter (SIDM) models have received great attention over the past decade as solutions to the small-scale puzzles of astrophysics. Though there are different implementations of dark matter (DM) self-interactions in N-body codes of structure formation, there has not been a systematic study to compare the predictions of these different implementations. We investigate the implementation of dark matter self-interactions in two simulation codes:gizmo and arepo. We begin with identical initial conditions for an isolated 1010 M⊙ dark matter halo and investigate the evolution of the density and velocity dispersion profiles in gizmo and arepo for SIDM cross-section over mass of 1, 5, and 50 $\rm cm^2\, g^{-1}$. Our tests are restricted to the core expansion phase, where the core density decreases and core radius increases with time. We find better than 30 per cent agreement between the codes for the density profile in this phase of evolution, with the agreement improving at higher resolution. We find that varying code-specific SIDM parameters changes the central halo density by less than 10 per cent outside of the convergence radius. We argue that SIDM core formation is robust across the two different schemes and conclude that these codes can reliably differentiate between cross-sections of 1, 5, and 50 $\rm cm^2\,more »g^{-1}$, but finer distinctions would require further investigation.« less

ABSTRACT The presence of an invisible substructure has previously been detected in the gravitational lens galaxy SDSSJ0946+1006 through its perturbation of the lensed images. Using flexible models for the main halo and the subhalo perturbation, we demonstrate that the subhalo has an extraordinarily high central density and steep density slope. We robustly infer the subhalo’s projected mass within 1 kpc to be ∼2–3.7 × 109 M⊙ at >95 per cent CL for all our lens models, while the average log-slope of the subhalo’s projected density profile over the radial range 0.75–1.25 kpc is constrained to be steeper than isothermal (γ2D ≲ −1). By modeling the subhalo light, we infer a conservative upper bound on its luminosity LV < 1.2 × 108L⊙ at 95 per cent CL that shows that the perturber is dark matter dominated. We analyse lensing galaxy analogues in the Illustris TNG100-1 simulation over many lines of sight, and find hundreds of subhalos that achieve a mass within 1 kpc ≳ 2 × 109M⊙. However, less than 1 per cent of the mock observations yield a log-slope steep enough to be consistent with our lensing models, and they all have stellar masses exceeding that allowed by observations by an order of magnitude or more. We conclude that the presence of such a darkmore »highly concentrated subhalo is unexpected in a Lambda cold dark matter universe. While it remains to be determined whether this tension can be reduced by adding more complexity to the primary lens model, it is not significantly alleviated if the perturber is assumed to be a LOS structure, rather than a subhalo.« less

ABSTRACT We demonstrate that the perturbations of strongly lensed images by low-mass dark matter subhaloes are significantly impacted by the concentration of the perturbing subhalo. For subhalo concentrations expected in Lambda cold dark matter (ΛCDM), significant constraints on the concentration can be obtained at Hubble Space Telescope (HST) resolution for subhaloes with masses larger than about $10^{10}\, {\rm M}_\odot$. Constraints are also possible for lower mass subhaloes, if their concentrations are higher than the expected scatter in CDM. We also find that the concentration of lower mass perturbers down to $\sim 10^8\, {\rm M}_\odot$ can be well constrained with a resolution of ∼0.01 arcsec, which is achievable with long-baseline interferometry. Subhalo concentration also plays a critical role in the detectability of a perturbation, such that only high-concentration perturbers with mass $\lesssim 10^9\, {\rm M}_\odot$ are likely to be detected at HST resolution. If scatter in the ΛCDM mass–concentration relation is not accounted for during lens modelling, the inferred subhalo mass can be biased by up to a factor of 3 (6) for subhaloes of mass $10^9 \, {\rm M}_\odot \,(10^{10} \, {\rm M}_\odot$); this bias can be eliminated if one varies both mass and concentration during lens fitting. Alternatively, onemore »may robustly infer the projected mass within the subhalo’s perturbation radius, defined by its distance to the critical curve of the lens being perturbed. With a sufficient number of detections, these strategies will make it possible to constrain the halo mass–concentration relation at low masses in addition to the mass function, offering a probe of dark matter physics as well as the small-scale primordial power spectrum.« less

We analyse strongly lensed images in eight galaxy clusters to measure their dark matter density profiles in the radial region between 10 kpc and 150 kpc, and use this to constrain the self-interaction cross-section of dark matter (DM) particles. We infer the mass profiles of the central DM haloes, bright central galaxies, key member galaxies, and DM subhaloes for the member galaxies for all eight clusters using the qlens code. The inferred DM halo surface densities are fit to a self-interacting dark matter model, which allows us to constrain the self-interaction cross-section over mass σ/m. When our full method is applied to mock data generated from two clusters in the Illustris-TNG simulation, we find results consistent with no dark matter self-interactions as expected. For the eight observed clusters with average relative velocities of $1458_{-81}^{+80}$ km s−1, we infer $\sigma /m = 0.082_{-0.021}^{+0.027} \rm cm^2\, g^{ -1}$ and $\sigma /m \lt 0.13~ \rm cm^2\, g^{ -1}$ at the 95 per cent confidence level.

ABSTRACT We present a Bayesian method to identify multiple (chemodynamic) stellar populations in dwarf spheroidal galaxies (dSphs) using velocity, metallicity, and positional stellar data without the assumption of spherical symmetry. We apply this method to a new Keck/Deep Imaging Multi-Object Spectrograph (DEIMOS) spectroscopic survey of the Ursa Minor (UMi) dSph. We identify 892 likely members, making this the largest UMi sample with line-of-sight velocity and metallicity measurements. Our Bayesian method detects two distinct chemodynamic populations with high significance (in logarithmic Bayes factor, ln B ∼ 33). The metal-rich ([Fe/H] = −2.05 ± 0.03) population is kinematically colder (radial velocity dispersion of $\sigma _v=4.9_{-1.0}^{+0.8} \, \mathrm{km} \, \mathrm{s}^{-1}$) and more centrally concentrated than the metal-poor ($[{\rm Fe/H}]=-2.29_{-0.06}^{+0.05}$) and kinematically hotter population ($\sigma _v =11.5_{-0.8}^{+0.9}\, \mathrm{km} \, \mathrm{s}^{-1}$). Furthermore, we apply the same analysis to an independent Multiple Mirror Telescope (MMT)/Hectochelle data set and confirm the existence of two chemodynamic populations in UMi. In both data sets, the metal-rich population is significantly flattened (ϵ = 0.75 ± 0.03) and the metal-poor population is closer to spherical ($\epsilon =0.33_{-0.09}^{+0.12}$). Despite the presence of two populations, we are able to robustly estimate the slope of the dynamical mass profile. We found hints for prolate rotation of order ${\sim}2 \, \mathrm{km} \, \mathrm{s}^{-1}$more »in the MMT data set, but further observations are required to verify this. The flattened metal-rich population invalidates assumptions built into simple dynamical mass estimators, so we computed new astrophysical dark matter annihilation (J) and decay profiles based on the rounder, hotter metal-poor population and inferred $\log _{10}{(J(0{^{\circ}_{.}}5)/{\rm GeV^{2} \, cm^{-5}})}\approx 19.1$ for the Keck data set. Our results paint a more complex picture of the evolution of UMi than previously discussed.« less