Showing papers by "Justin I. Read published in 2018"

Cosmology and Fundamental Physics with the Euclid Satellite

Abstract: Euclid is a European Space Agency medium class mission selected for launch in 2020 within the Cosmic Vision 2015 2025 program. The main goal of Euclid is to understand the origin of the accelerated expansion of the universe. Euclid will explore the expansion history of the universe and the evolution of cosmic structures by measuring shapes and redshifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid's Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.

The case for a cold dark matter cusp in Draco

Abstract: We use a new mass modelling method, GRAVSPHERE, to measure the central dark matter density profile of the Draco dwarf spheroidal galaxy. Draco's star formation shut down long ago, making it a prime candidate for hosting a `pristine' dark matter cusp, unaffected by stellar feedback during galaxy formation. We first test GRAVSPHERE on a suite of tidally stripped mock `Draco'-like dwarfs. We show that we are able to correctly infer the dark matter density profile of both cusped and cored mocks within our 95 per cent confidence intervals. While we obtain only a weak inference on the logarithmic slope of these density profiles, we are able to obtain a robust inference of the amplitude of the inner dark matter density at 150 pc, ρ _DM(150 pc). We show that, combined with constraints on the density profile at larger radii, this is sufficient to distinguish a Λ Cold Dark Matter (ΛCDM) cusp - that has ρ _DM(150 pc) ≳ 1.8 × 10^8 M_☉ kpc^{-3} - from alternative dark matter models that have lower inner densities. We then apply GRAVSPHERE to the real Draco data. We find that Draco has an inner dark matter density of ρ _DM(150 pc) = 2.4_{-0.6}^{+0.5} × 10^8 M_☉ kpc^{-3}, consistent with a ΛCDM cusp. Using a velocity-independent SIDM model, calibrated on ΛSIDM cosmological simulations, we show that Draco's high central density gives an upper bound on the SIDM cross-section of σ/m < 0.57 cm2 g-1 at 99 per cent confidence. We conclude that the inner density of nearby dwarf galaxies like Draco provides a new and competitive probe of dark matter models.

Extending the globular cluster system–halo mass relation to the lowest galaxy masses

Abstract: High-mass galaxies, with halo masses M200 ≥ 1010M⊙ , reveal a remarkable near-linear relation between their globular cluster (GC) system mass and their host galaxy halo mass. Extending this relation to the mass range of dwarf galaxies has been problematic due to the difficulty in measuring independent halo masses. Here we derive new halo masses based on stellar and H i gas kinematics for a sample of nearby dwarf galaxies with GC systems. We find that the GC system mass–halo mass relation for galaxies populated by GCs holds from halo masses of M200 ∼ 1014 M ⊙ ⊙ down to below M200 ∼109 M ⊙ ⊙ , although there is a substantial increase in scatter towards low masses. In particular, three well-studied ultradiffuse galaxies, with dwarf-like stellar masses, reveal a wide range in their GC-to-halo mass ratios. We compare our GC system–halo mass relation to the recent model of El Badry et al., finding that their fiducial model does not reproduce our data in the low-mass regime. This may suggest that GC formation needs to be more efficient than assumed in their model, or it may be due to the onset of stochastic GC occupation in low-mass haloes. Finally, we briefly discuss the stellar mass–halo mass relation for our low-mass galaxies with GCs, and we suggest some nearby dwarf galaxies for which searches for GCs may be fruitful.

The Local Dark Matter Density from SDSS-SEGUE G-dwarfs

Abstract: We derive the local dark matter density by applying the integrated Jeans equation method from Silverwood et al. to SDSS-SEGUE G-dwarf data processed and presented by Budenbender et al. We use the MULTINEST Bayesian nested sampling software to fit a model for the baryon distribution, dark matter, and tracer stars, including a model for the `tilt term' that couples the vertical and radial motions, to the data. The α-young population from Budenbender et al. yields the most reliable result of ρ_dm= 0.46^{+0.07}_{-0.08} {GeV cm}^{-3}= 0.012^{+0.002}_{-0.002} M_{☉} pc^{-3}. Our analyses yield inconsistent results for the α-young and α-old data, pointing to problems in the tilt term and its modelling, the data itself, the assumption of a flat rotation curve, or the effects of disequilibria.

Fourteen candidate RR Lyrae star streams in the inner Galaxy

Abstract: We apply the GC3 stream-finding method to RR Lyrae stars (RRLSs) in the Catalina survey. We find 2 RRLS stream candidates at >4σ confidence and another 12 at >3.5σ confidence over the Galactocentric distance range 4 < D/kpc < 26. Of these, only two are associated with known globular clusters (NGC 1261 and Arp2). The remainder are candidate `orphan' streams, consistent with the idea that globular cluster streams are most visible close to dissolution. Our detections are likely a lower bound on the total number of dissolving globulars in the inner galaxy, since many globulars have few RRLSs, while only the brightest streams are visible over the Galactic RRLS background, particularly given the current lack of kinematical information. We make all of our candidate streams publicly available and provide a new galstreamsPYTHON library for the footprints of all known streams and overdensities in the Milky Way.

Probing dark matter with star clusters: a dark matter core in the ultra-faint dwarf Eridanus II

Abstract: We present a new technique to probe the central dark matter (DM) density profile of galaxies that harnesses both the survival and observed properties of star clusters. As a first application, we apply our method to the `ultra-faint' dwarf Eridanus II (Eri II) that has a lone star cluster ~45 pc from its centre. Using a grid of collisional N-body simulations, incorporating the effects of stellar evolution, external tides and dynamical friction, we show that a DM core for Eri II naturally reproduces the size and the projected position of its star cluster. By contrast, a dense cusped galaxy requires the cluster to lie implausibly far from the centre of Eri II (>1 kpc), with a high inclination orbit that must be observed at a particular orbital phase. Our results imply that either a cold DM cusp was `heated up' at the centre of Eri II by bursty star formation, or we are seeing an evidence for physics beyond cold DM.

On the early evolution of Local Group dwarf galaxy types: star formation and supernova feedback

Abstract: According to star formation histories (SFHs), Local Group dwarf galaxies can be broadly classified in two types: those forming most of their stars before z = 2 (fast) and those with more extended SFHs (slow). The most precise SFHs are usually derived from deep but not very spatially extended photometric data; this might alter the ratio of old to young stars when age gradients are present. Here, we correct for this effect and derive the mass formed in stars by z = 2 for a sample of 16 Local Group dwarf galaxies. We explore early differences between fast and slow dwarfs, and evaluate the impact of internal feedback by supernovae (SNe) on the baryonic and dark matter (DM) component of the dwarfs. Fast dwarfs assembled more stellar mass at early times and have larger amounts of DM within the half-light radius than slow dwarfs. By imposing that slow dwarfs cannot have lost their gas by z = 2, we constrain the maximum coupling efficiency of SN feedback to the gas and to the DM to be 10 per cent. We find that internal feedback alone appears insufficient to quench the SFH of fast dwarfs by gas deprivation, in particular for the fainter systems. Nonetheless, SN feedback can core the DM halo density profiles relatively easily, producing cores of the sizes of the half-light radius in fast dwarfs by z = 2 with very low efficiencies. Amongst the `classical' Milky Way satellites, we predict that the smallest cores should be found in Draco and Ursa Minor, while Sculptor and Fornax should host the largest ones.

The influence of Massive Black Hole Binaries on the Morphology of Merger Remnants

Abstract: Massive black hole (MBH) binaries, formed as a result of galaxy mergers, are expected to harden by dynamical friction and three-body stellar scatterings, until emission of gravitational waves (GWs) leads to their final coalescence. According to recent simulations, MBH binaries can efficiently harden via stellar encounters only when the host geometry is triaxial, even if only modestly, as angular momentum diffusion allows an efficient repopulation of the binary loss cone. In this paper, we carry out a suite of N-body simulations of equal-mass galaxy collisions, varying the initial orbits and density profiles for the merging galaxies and running simulations both with and without central MBHs. We find that the presence of an MBH binary in the remnant makes the system nearly oblate, aligned with the galaxy merger plane, within a radius enclosing 100 MBH masses. We never find binary hosts to be prolate on any scale. The decaying MBHs slightly enhance the tangential anisotropy in the centre of the remnant due to angular momentum injection and the slingshot ejection of stars on nearly radial orbits. This latter effect results in about 1% of the remnant stars being expelled from the galactic nucleus. Finally, we do not find any strong connection between the remnant morphology and the binary hardening rate, which depends only on the inner density slope of the remnant galaxy. Our results suggest that MBH binaries are able to coalesce within a few Gyr, even if the binary is found to partially erase the merger-induced triaxiality from the remnant.

Abundance matching with the mean star formation rate: there is no missing satellites problem in the Milky Way

Abstract: We introduce a novel abundance matching technique that produces a more accurate estimate of the pre-infall halo mass, $M_{200}$, for satellite galaxies To achieve this, we abundance match with the mean star formation rate, averaged over the time when a galaxy was forming stars, $\langle {\rm SFR}\rangle$, instead of the stellar mass, $M_*$ Using data from the Sloan Digital Sky Survey, the GAMA survey and the Bolshoi simulation, we obtain a statistical $\langle {\rm SFR}\rangle-{\rm M}_{200}$ relation in $\Lambda{\rm CDM}$ We then compare the pre-infall halo mass, $M^{\rm abund}_{200}$, derived from this relation with the pre-infall dynamical mass, $M^{\rm dyn}_{200}$, for 21 nearby dSph and dIrr galaxies, finding a good agreement between the two As a first application, we use our new $\langle {\rm SFR}\rangle-{\rm M}_{200}$ relation to empirically measure the cumulative mass function of a volume-complete sample of bright Milky Way satellites within 280 kpc of the Galactic centre Comparing this with a suite of cosmological 'zoom' simulations of Milky Way-mass halos that account for subhalo depletion by the Milky Way disc, we find no missing satellites problem above $M_{200} \sim 10^9\,{\rm M}_\odot$ in the Milky Way We discuss how this empirical method can be applied to a larger sample of nearby spiral galaxies

The alignment is in their stars: on the spin-alignment of stars in star clusters

Abstract: We simulate star formation in two molecular clouds extracted from a larger disc-galaxy simulation with a spatial resolution of ∼0.1 pc, one exiting a spiral arm dominated by compression, and another in an inter-arm region more strongly affected by galactic shear. Treating the stars as ‘sink particles’, we track their birth angular momentum, and the later evolution of their angular momentum due to gas accretion. We find that in both clouds, the sinks have spin vectors that are aligned with one another, and with the global angular momentum vector of the star cluster. This alignment is present at birth, but enhanced by later gas accretion. In the compressive cloud, the sink-spins remain aligned with the gas for at least a free-fall time. By contrast, in the shear cloud, the increased turbulent mixing causes the sinks to rapidly misalign with their birth cloud on approximately a gas free-fall time. In spite of this, both clouds show a strong alignment of sink-spins at the end of our simulations, independently of environment.

A Gaia DR2 search for dwarf galaxies towards Fermi-LAT sources: implications for annihilating dark matter

Abstract: We make the first attempt to find dwarf galaxies in eight Fermi-LAT extended, unassociated, source fields using Gaia DR2. After probing previously unexplored heliocentric distances of d ˂ 20 kpc with an extreme-deconvolution (XD) technique, we find no sign of a dwarf galaxy in any of these fields despite Gaia's excellent astrometric accuracy. Our detection limits are estimated by applying the XD method to mock data, obtaining a conservative limit on the stellar mass of M* ˂ 104 M☉ for d ˂ 20 kpc. Such a low stellar mass implies either a low-mass subhalo or a massive stripped-down subhalo. We use an analytic model for stripped subhaloes to argue that, given the sizes and fluxes of the Fermi-LAT sources, we can reject the hypothesis that they owe to dark matter annihilation.