On the smoothness of static multi-black hole solutions of higher-dimensional Einstein-Maxwell theory

Previous work has shown that static multi-black hole solutions of higher-dimensional Einstein-Maxwell theory do not possess smooth horizons. We show that the lack of smoothness is worse than previously demonstrated. We consider solutions describing multiple black holes on a common axis. In five dimensions, the metric is generically twice, but not three times, continuously differentiable at the horizon. The Maxwell field is generically continuous, but not differentiable, at the horizon. In more than five dimensions, the metric is once, but not twice, continuously differentiable, and there is a parallely-propagated curvature singularity at the horizon. The Maxwell field strength is again continuous, but not differentiable, at the horizon.Comment: 19 pages; minor correction

Cosmological simulations of a momentum coupling between dark matter and quintessence

Dark energy is frequently modelled as an additional dynamical scalar field component in the Universe, referred to as "quintessence", which drives the late-time acceleration. Furthermore, the quintessence field may be coupled to dark matter and/or baryons, leading to a fifth force. In this paper we explore the consequences for non-linear cosmological structure formation arising from a momentum coupling between the quintessence field and dark matter only. The coupling leads to a modified Euler equation, which we implement in an N-body cosmological simulation. We then analyse the effects of the coupling on the non-linear power spectrum and the properties of the dark matter halos. We find that, for certain quintessence potentials, a positive coupling can lead to significantly reduced structure on small scales and somewhat enhanced structure on large scales, as well as reduced halo density profiles and increased velocity dispersions.Comment: 19 pages, 20 figures, accepted by MNRA

On the smoothness of the multi-BMPV black hole spacetime

We demonstrate that, in a multi-BMPV black hole spacetime, the event horizon is not smooth. We explicitly show that for a simpler configuration comprising a line of static extremal black holes and a single BMPV black hole, the metric at the horizon of the BMPV black hole is once, but not twice, continuously differentiable. We argue that this result is also valid when all the black holes are rotating. The Maxwell field strength is shown to be continuous, but not differentiable at the horizon. We also briefly demonstrate that previous work done to show lack of smoothness of static multi-centre solutions in five dimensions is not significantly modified by the inclusion of a higher derivative term in the action for five dimensional supergravity.Comment: 17 pages; reference adde

Could Segue 1 be a destroyed star cluster? - a dynamical perspective

We attempt to find a progenitor for the ultra-faint object Segue 1 under the assumption that it formed as a dark matter free star cluster in the past. We look for orbits, using the elongation of Segue 1 on the sky as a tracer of its path. Those orbits are followed backwards in time to find the starting points of our N-body simulations. The successful orbit, with which we can reproduce Segue 1 has a proper motion of mu_alpha = -0.19 mas/yr and mu_delta = -1.9 mas/yr, placing Segue 1 near its apo-galacticon today. Our best fitting model has an initial mass of 6224 Msun and an initial scale-length of 5.75 pc.Comment: 9 pages, 5 figures, 3 tables, accepted by MNRA

Leavers and remainers:Galaxies split by group-exit

The disruption of substructure in galaxy clusters likely plays an important role in shaping the cluster population as a significant fraction of cluster galaxies today have spent time in a previous host system, and thus may have been pre-processed. Once inside the cluster, group galaxies face the combined environmental effects from group and cluster - so called 'post-processing'. We investigate these concepts, by tracking the evolution of satellites and their hosts after entering the cluster and find that tidal forces during their first pericentric passage are very efficient at breaking up groups, preferentially removing satellites at larger distances from their hosts. 92.2% of satellites whose host has passed pericentre will leave their host by $z=0$, typically no later than half a Gyr after pericentric passage. We find satellites leave with high velocities, and quickly separate to large distances from their hosts, making their identification within the cluster population challenging. Those few satellites ($\sim$7.8%) that remain bound to their hosts after a pericentric passage are typically found close to their host centres. This implies that substructure seen in clusters today is very likely on first infall into the cluster, and yet to pass pericentre. This is even more likely if the substructure is extended, with satellites beyond R$_{200}$ of their host. We find the cluster dominates the tidal mass loss and destruction of satellites, and is responsible for rapidly halting the accretion of new satellites onto hosts once they reach 0.6-0.8 R$_{200}$ radii from the cluster.Comment: 14 pages, 14 figures, accepted for publication in MNRAS on 04.10.1

Discovery of ram-pressure stripped gas around an elliptical galaxy in Abell 2670

Studies of cluster galaxies are increasingly finding galaxies with spectacular one-sided tails of gas and young stars, suggestive of intense ram-pressure stripping. These so-called "jellyfish" galaxies typically have late-type morphology. In this paper, we present MUSE observations of an elliptical galaxy in Abell 2670 with long tails of material visible in the optical spectra, as well as blobs with tadpole-like morphology. The spectra in the central part of the galaxy reveals a stellar component as well as ionized gas. The stellar component does not have significant rotation, while the ionized gas defines a clear star-forming gas disk. We argue, based on deep optical images of the galaxy, that the gas was most likely acquired during a past wet merger. It is possible that the star-forming blobs are also remnants of the merger. In addition, the direction and kinematics of the one-sided ionized tails, combined with the tadpole morphology of the star-forming blobs, strongly suggests that the system is undergoing ram pressure from the intracluster medium. In summary, this paper presents the discovery of a post-merger elliptical galaxy undergoing ram pressure stripping.Comment: 6 pages, 5 figures, accepted for publication in ApJ

3D hydrodynamic simulations for the formation of the local group satellite planes

Funding: IB is supported by Science and Technology Facilities Council grant ST/V000861/1. He acknowledges support from a ‘Pathways to Research’ fellowship from the University of Bonn in 2021 after an Alexander von Humboldt Foundation postdoctoral research fellowship (2018–2020). IT acknowledges support through the Stellar Populations and Dynamics research group at the University of Bonn. GC acknowledges support from Chile’s National Fund for Scientific and Technological Development (FONDECYT) Regular No. 1181708. BF and RI acknowledge funding from the Agence Nationale de la Recherche (ANR project ANR-18-CE31-0006 and ANR-19-CE31-0017) and from the European Research Council (ERC) under the European Union’s Horizon 2020 Framework Programme (grant agreement number 834148). MSP was supported by the Leibniz Junior Research Group grant J94/2020 via the Leibniz Competition, and a Klaus Tschira Boost Fund provided by the Klaus Tschira Stiftung and the German Scholars Organization.The existence of mutually correlated thin and rotating planes of satellite galaxies around both the Milky Way (MW) and Andromeda (M31) calls for an explanation. Previous work in Milgromian dynamics (MOND) indicated that a past MW–M31 encounter might have led to the formation of these satellite planes. We perform the first-ever hydrodynamical MOND simulation of the Local Group using PHANTOM OF RAMSES. We show that an MW–M31 encounter at z ≈ 1, with a perigalactic distance of about 80 kpc, can yield two disc galaxies at z = 0 oriented similarly to the observed galactic discs and separated similarly to the observed M31 distance. Importantly, the tidal debris are distributed in phase space similarly to the observed MW and M31 satellite planes, with the correct preferred orbital pole for both. The MW–M31 orbital geometry is consistent with the presently observed M31 proper motion despite this not being considered as a constraint when exploring the parameter space. The mass of the tidal debris around the MW and M31 at z = 0 compare well with the mass observed in their satellite systems. The remnant discs of the two galaxies have realistic radial scale lengths and velocity dispersions, and the simulation naturally produces a much hotter stellar disc in M31 than in the MW. However, reconciling this scenario with the ages of stellar populations in satellite galaxies would require that a higher fraction of stars previously formed in the outskirts of the progenitors ended up within the tidal debris, or that the MW–M31 interaction occurred at z > 1.Publisher PDFPeer reviewe