Phase transitions and equations of state of the alkaline earth fluorides CaF2, SrF2, and BaF2 were examined by static compression to pressures as high as 146 GPa. Angle-dispersive x-ray diffraction experiments were performed on polycrystalline samples in the laser-heated diamond-anvil cell. We confirmed that at pressures less than 10 GPa all three materials undergo a phase transition from the cubic (Fm (3) over barm) fluorite structure to the orthorhombic (Pnam) cotunnite-type structure. This work has characterized an additional phase transition in CaF2 and SrF2: these materials were observed to transform to a hexagonal (P6(3)/mmc) Ni2In-type structure between 63-79 GPa and 28-29 GPa, respectively, upon laser heating. For SrF2, the Ni2In-type phase was confirmed by Rietveld refinement. Volumes were determined as a function of pressure for all high-pressure phases and fit to the third-order Birch-Murnaghan equation of state. For CaF2 and SrF2, the fluorite-cotunnite transition results in a volume decrease of 8-10 %, while the bulk modulus of the cotunnite-type phase is the same or less than that of the fluorite phase within uncertainty. For all three fluorides, the volume reduction associated with the further transition to the Ni2In-type phase is similar to 5%. The percentage increase in the bulk modulus (Delta K) across the transition is greater when the cation is smaller. While for BaF2, Delta K is 10-30 %, Delta K values for SrF2 and CaF2 are 45-65 % and 20-40 %. Although shock data for CaF2 have been interpreted to show a transition to a highly incompressible phase above 100 GPa, this is not consistent with our static equation of state data.

We present a search for "hypercompact" star clusters in the Milky Way using a combination of Gaia and the Dark Energy Camera Legacy Survey (DECaLS). Such putative clusters, with sizes of similar to 1 pc and containing 500-5000 stars, are expected to remain bound to intermediate-mass black holes (M (BH) approximate to 10(3)-10(5) M (circle dot)) that may be accreted into the Milky Way halo within dwarf satellites. Using the semianalytic model SatGen, we find an expected similar to 100 wandering intermediate-mass black holes if every infalling satellite hosts a black hole. We do not find any such clusters in our search. Our upper limits rule out 100% occupancy but do not put stringent constraints on the occupation fraction. Of course, we need stronger constraints on the properties of the putative star clusters, including their assumed sizes and the fraction of stars that would be compact remnants.

We present a new Hubble Space Telescope Cosmic Origins Spectrograph (COS) absorption-line survey to study halo gas around 16 luminous red galaxies (LRGs) at z = 0.21-0.55. The LRGs are selected uniformly with stellar mass M-star > 10(11) M-circle dot and no prior knowledge of the presence/absence of any absorption features. Based on observations of the full Lyman series, we obtain accurate measurements of neutral hydrogen column density N(H I) and find that high-N(H I) gas is common in these massive quiescent haloes with a median of < log N(H I)> = 16.6 at projected distances d less than or similar to 160 kpc. We measure a mean covering fraction of optically thick gas with log N(H I) greater than or similar to 17.2 of (LLS) = 0.44(-0.11)(+0.12) at d less than or similar to 160 kpc and (LLS) = 0.71(-0.20)(+0.11) at d less than or similar to 100 kpc. The line-of-sight velocity separations between the HI absorbing gas and LRGs are characterized by a mean and dispersion of < v(gas-gal)> = 29 km s(-1) and sigma (< vgas-gal >) = 171 km s(-1). Combining COS far-ultraviolet and ground-based echelle spectra provides an expanded spectral coverage for multiple ionic transitions, from low-ionization MgII and Si II, to intermediate-ionization SiIII and C III, and to high-ionization OVI absorption lines. We find that intermediate ions probed by C III and Si III are the most prominent UV metal lines in LRG haloes with a mean covering fraction of (0.1) = 0.75(-0.13)(+0.08) for W-r(977) >= 0.1 angstrom at d < 160 kpc, comparable to what is seen for CIII in L-* and sub-L-* star-forming and red galaxies but exceeding MgII or OVI in quiescent halos. The COS-LRG survey shows that massive quiescent haloes contain widespread chemically enriched cool gas and that little distinction between LRG and star-forming haloes is found in their HI and C III content.

This paper presents a study of the galactic environment of a chemically pristine (<0.6 per cent solar metallicity) Lyman Limit system (LLS) discovered along the sightline towards QSO SDSSJ 135726.27+043541.4 (Z(QSO) = 1.233) at projected distance d = 126 physical kpc (pkpc) from a luminous red galaxy (LRG) at z = 0.33. Combining deep Hubble Space Telescope images, MUSE integral field spectroscopic data, and wide-field redshift survey data has enabled an unprecedented, ultradeep view of the environment around this LRG-LLS pair. A total of 12 galaxies, including the LRG, are found at d less than or similar to 400 pkpc and line-of-sight velocity Delta v < 600 km S-1 of the LLS, with intrinsic luminosity ranging from 0.001 L-* to 2 L-* and a corresponding stellar mass range of M-star approximate to 10(7-11) M-circle dot. All 12 galaxies contribute to a total mass of M-star = 1.6 x 10(11) M-circle dot with approximate to 80 per cent contained in the LRG. The line-of-sight velocity dispersion of these galaxies is found to be sigma (group) = 230 km s(-1) with the centre of mass at d(group) = 118 pkpc and line-of-sight velocity offset of Delta v(group) = 181 km s(-1) from the LLS. Three of these are located at d less than or similar to 100 pkpc from the LLS, and they are all faint with intrinsic luminosity less than or similar to 0.02 L-* and gas-phase metallicity of approximate to 10 per cent solar in their interstellar medium. The disparity in the chemical enrichment level between the LLS and the group members suggests that the LLS originates in infalling intergalactic medium and that parts of the intergalactic gas near old and massive galaxies can still remain chemically pristine through the not too distant past.