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Abstract Globular clusters (GCs) provide statistically significant coeval populations of stars spanning various evolutionary stages, allowing robust constraints on stellar evolution model parameters and ages. We analyze eight old Milky Way GCs with metallicities between [Fe/H] = −2.31 and −0.77 by comparing theoretical isochrone sets from the Dartmouth Stellar Evolution Program to Hubble Space Telescope (HST) observations. The theoretical isochrones include uncertainties introduced by 21 stellar evolution parameters such as convective mixing, opacity, diffusion, and nuclear reactions, capturing much of the quantifiable physics used in our code. For each isochrone, we construct simulated color–magnitude diagrams (CMDs) near the main-sequence turnoff region and apply two full-CMD-fitting methods to fit HST Advanced Camera for Surveys data across a range of distances and reddening and measure the absolute age of each GC from the resulting posterior distribution, which accounts for uncertainties in the stellar models, observations, and fitting method. The resulting best-fitting absolute ages range from ≈11.5 to 13.5 Gyr, with a typical error of 0.5–0.75 Gyr; the data show a clear trend toward older ages at lower metallicities. Notably, distance and reddening account for over 50% of the uncertainty in age determination in each case, with metallicity,αabundance, mixing length, and helium diffusion being the most important stellar physics parameters for the error budget. We also provide an absolute age–metallicity relation for Milky Way GCs. 

Abstract We investigate the impact of massive primordial black holes (PBHs;m_BH ∼ 10⁶M_⊙) on the star formation and first galaxy assembly process using high-resolution hydrodynamical simulations fromz= 1100 toz ∼ 9. We find that PBH accretion is self-regulated by feedback, suppressing mass growth unless feedback is weak. PBHs accelerate structure formation by seeding dark matter (DM) halos and gravitationally attracting gas, but strong feedback can delay cooling and suppress star formation. In addition, the presence of baryon-DM streaming creates an offset between the PBH location and the peaks induced in gas density, promoting earlier and more efficient star formation compared to standard ΛCDM. Byz ∼ 10, PBH-seeded galaxies form dense star clusters, with PBH-to-stellar mass ratios comparable to observed high-zactive galactic nuclei like UHZ-1. Our results support PBHs as viable supermassive black hole (SMBH) seeds but do not exclude alternative scenarios. We emphasize that PBH-seeding provides a natural explanation for some of the newly discovered overmassive SMBHs at high redshift, in particular those with extreme ratios of BH-to-dynamical (virial) mass that challenge standard formation channels. Future studies with ultra-deep JWST surveys, the Roman Space Telescope, and radio surveys with facilities such as the Square Kilometre Array and Hydrogen Epoch of Reionization Array will be critical in distinguishing PBH-driven SMBH growth from other pathways. 

Abstract From >1000 orbits of HST imaging, we present deep homogeneous resolved star color–magnitude diagrams that reach the oldest main-sequence turnoff and uniformly measured star formation histories (SFHs) of 36 dwarf galaxies (−6 ≥M_V≥ −17) associated with the M31 halo, and for 10 additional fields in M31, M33, and the Giant Stellar Stream. From our SFHs, we find: (i) The median stellar age and quenching epoch of M31 satellites correlate with galaxy luminosity and galactocentric distance. Satellite luminosity and present-day distance from M31 predict the satellite quenching epoch to within 1.8 Gyr at all epochs. This tight relationship highlights the fundamental connection between satellite halo mass, environmental history, and star formation duration. (ii) There is no difference between the median SFH of galaxies on and off the great plane of Andromeda satellites. (iii) ~50% of our M31 satellites show prominent ancient star formation (>12 Gyr ago) followed by delayed quenching (8–10 Gyr ago), which is not commonly observed among the MW satellites. (iv) A comparison with TNG50 and FIRE-2 simulated satellite dwarfs around M31-like hosts shows that some of these trends (dependence of SFH on satellite luminosity) are reproduced in the simulations while others (dependence of SFH on galactocentric distance, presence of the delayed-quenching population) are weaker or absent. We provide all photometric catalogs and SFHs as High-Level Science Products on MAST.