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Register a new userHermeian dark matter haloes of the Local Group
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Authors
Oliver Newton
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The Local Group is a unique environment in which to study the astrophysics of galaxy formation. The proximity of the Milky Way and M31 causes a large fraction of the low-mass halo population to interact with more massive dark matter haloes, which increases their concentrations and strips them of gas and other material. Some low-mass haloes pass through the haloes of the Milky Way or M31 and are either ejected into the field or exchanged between the two primary hosts. We use high resolution gas-dynamical simulations to describe a new class of field halo that passed through the haloes of both the Milky Way and M31 at early times and is almost twice as concentrated as isolated field haloes. These Hermeian haloes are distributed anisotropically at greater distances from the Local Group barycentre than the primary haloes and appear to cluster close to the Milky Way and M31 in projection. We show that some Hermeian haloes can host galaxies that are promising targets for indirect dark matter searches and are competitive with signals from other dwarf galaxies. Hermeian galaxies in the Local Group should be detectable by forthcoming wide-field imaging surveys.
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We use the halo occupation distribution (HOD) framework to characterise the predictions from two independent galaxy formation models for the galactic content of dark matter haloes and its evolution with redshift. Our galaxy samples correspond to a range of fixed number densities defined by stellar mass and span $0 le z le 3$. We find remarkable similarities between the model predictions. Differences arise at low galaxy number densities which are sensitive to the treatment of heating of the hot halo by active galactic nuclei. The evolution of the form of the HOD can be described in a relatively simple way, and we model each HOD parameter using its value at $z=0$ and an additional evolutionary parameter. In particular, we find that the ratio between the characteristic halo masses for hosting central and satellite galaxies can serve as a sensitive diagnostic for galaxy evolution models. Our results can be used to test and develop empirical studies of galaxy evolution and can facilitate the construction of mock galaxy catalogues for future surveys.
Photoheating of the gas in low-mass dark matter (DM) haloes prevents baryons from cooling, leaving the haloes free of stars. Gas in these dark haloes remains exposed to the ultraviolet background (UVB), and so is expected to emit via fluorescent recombination lines. We present a set of radiative transfer simulations, which model dark haloes as spherical gas clouds in hydrostatic equilibrium with a DM halo potential, and in thermal equilibrium with the UVB at redshift z = 0. We use these simulations to predict surface brightnesses in H-alpha, which we show to have a characteristic ring-shaped morphology for haloes in a narrow mass range between 10^9.5 and 10^9.6 M_sun. We explore how this emission depends on physical parameters such as the DM density profile and the UVB spectrum. We predict the abundance of fluorescent haloes on the sky, and discuss possible strategies for their detection. We demonstrate how detailed observations of fluorescent rings can be used to infer the properties of the haloes which host them, such as their density profiles and the mass-concentration relation, as well as to directly measure the UVB amplitude.
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Small distortions in the images of Einstein rings or giant arcs offer the exciting prospect of detecting dark matter haloes or subhaloes of mass below $10^9$M$_{odot}$, most of which are too small to have made a visible galaxy. A very large number of such haloes are predicted to exist in the cold dark matter model of cosmogony; in contrast other models, such as warm dark matter, predict no haloes below a mass of this order which depends on the properties of the warm dark matter particle. Attempting to detect these small perturbers could therefore discriminate between different kinds of dark matter particles, and even rule out the cold dark matter model altogether. Globular clusters in the lens galaxy also induce distortions in the image which could, in principle, contaminate the test. Here, we investigate the population of globular clusters in six early type galaxies in the Virgo cluster. We find that the number density of globular clusters of mass $sim10^6$M$_{odot}$ is comparable to that of the dark matter perturbers (including subhaloes in the lens and haloes along the line-of-sight). We show that the very different degrees of mass concentration in globular clusters and dark matter haloes result in different lensing distortions. These are detectable with milli-arcsecond resolution imaging which can distinguish between globular cluster and dark matter halo signals.
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