ABSTRACT
Stellar ejecta gradually enrich the gas out of which subsequent stars form, making the least chemically enriched stellar systems direct fossils of structures formed in the early Universe1. Although a few hundred stars with metal content below 1,000th of the solar iron content are known in the Galaxy2-4, none of them inhabit globular clusters, some of the oldest known stellar structures. These show metal content of at least approximately 0.2% of the solar metallicity [Formula: see text]. This metallicity floor appears universal5,6, and it has been proposed that protogalaxies that merged into the galaxies we observe today were simply not massive enough to form clusters that survived to the present day7. Here we report observations of a stellar stream, C-19, whose metallicity is less than 0.05% of the solar metallicity [Formula: see text]. The low metallicity dispersion and the chemical abundances of the C-19 stars show that this stream is the tidal remnant of the most metal-poor globular cluster ever discovered, and is significantly below the purported metallicity floor: clusters with significantly lower metallicities than observed today existed in the past and contributed their stars to the Milky Way halo.
ABSTRACT
We analyze the total and baryonic acceleration profiles of a set of well-resolved galaxies identified in the eagle suite of hydrodynamic simulations. Our runs start from the same initial conditions but adopt different prescriptions for unresolved stellar and active galactic nuclei feedback, resulting in diverse populations of galaxies by the present day. Some of them reproduce observed galaxy scaling relations, while others do not. However, regardless of the feedback implementation, all of our galaxies follow closely a simple relationship between the total and baryonic acceleration profiles, consistent with recent observations of rotationally supported galaxies. The relation has small scatter: Different feedback implementations-which produce different galaxy populations-mainly shift galaxies along the relation rather than perpendicular to it. Furthermore, galaxies exhibit a characteristic acceleration g_{}, above which baryons dominate the mass budget, as observed. These observations, consistent with simple modified Newtonian dynamics, can be accommodated within the standard cold dark matter paradigm.
ABSTRACT
In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10-100 times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31's satellites brighter than M(v) = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.
ABSTRACT
I report on recent numerical simulations designed to study the inner structure of cold-dark-matter (CDM) haloes. This work confirms and extends earlier indications that the mass profiles of CDM haloes are roughly independent of mass and that the dark-matter density appears to diverge in a central "cusp". Our simulations also show that CDM halo-density profiles get increasingly shallow inwards down to the innermost radius (r(min)) resolved by the simulations, with no evidence for convergence to a well-defined asymptotic value (beta(0)) of the logarithmic slope. The dark mass contained within r(min) places strong constraints on beta(0); cusps as steep as beta(0) approximately -1.5 are clearly ruled out in our highest resolution simulations, although beta(0) less, similar -1 is consistent with the numerical data. The circular-velocity curves of the simulated CDM haloes are generally consistent with the rotation curves of low-surface-brightness (LSB) galaxies in the literature, but there are also some clearly discrepant cases. This disagreement has been interpreted as excluding the presence of cusps, but I argue that it may just reflect the difference between circular velocity and rotation speed likely to arise in gaseous discs embedded within realistic triaxial haloes. The perceived challenge to the CDM paradigm stemming from LSB rotation curves may thus plausibly result from the complex three-dimensional structure of CDM haloes, and might be resolved without appeal to a drastic revision of the paradigm.
