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Shell galaxies make a class of tidally distorted galaxies, characterised by wide concentric arc(s), extending out to large galactocentric distances with sharp outer edges. Recent observations of young massive star clusters in the prominent outer shell of NGC 474 suggest that such systems host extreme conditions of star formation. In this paper, we present a hydrodynamic simulation of a galaxy merger and its transformation into a shell galaxy. We analyse how the star formation activity evolves with time, location-wise within the system, and what are the physical conditions for star formation. During the interaction, an excess of dense gas appears, triggering a starburst, i.e. an enhanced star formation rate and a reduced depletion time. Star formation coincides with regions of high molecular gas fraction, such as the galactic nucleus, spiral arms, and occasionally the tidal debris during the early stages of the merger. Tidal interactions scatter stars into a stellar spheroid, while the gas cools down and reforms a disc. The morphological transformation after coalescence stabilises the gas and thus quenches star formation, without the need for feedback from an active galactic nucleus. This evolution shows similarities with a compaction scenario for compact quenched spheroids at high-redshift, yet without a long red nugget phase. Shells appear after coalescence, during the quenched phase, implying that they do not host the conditions necessary for in situ star formation. The results suggest that shell-forming mergers might be part of the process of turning blue late-type galaxies into red and dead early-types., Accepted for publication by MNRAS, 13 pages

We introduce the Southern Stellar Stream Spectroscopy Survey (${S}^5$), an on-going program to map the kinematics and chemistry of stellar streams in the Southern Hemisphere. The initial focus of ${S}^5$ has been spectroscopic observations of recently identified streams within the footprint of the Dark Energy Survey (DES), with the eventual goal of surveying streams across the entire southern sky. Stellar streams are composed of material that has been tidally striped from dwarf galaxies and globular clusters and hence are excellent dynamical probes of the gravitational potential of the Milky Way, as well as providing a detailed snapshot of its accretion history. Observing with the 3.9-m Anglo-Australian Telescope's 2-degree-Field fibre positioner and AAOmega spectrograph, and combining the precise photometry of DES DR1 with the superb proper motions from $Gaia$ DR2, allows us to conduct an efficient spectroscopic survey to map these stellar streams. So far ${S}^5$ has mapped 9 DES streams and 3 streams outside of DES; the former are the first spectroscopic observations of these recently discovered streams. In addition to the stream survey, we use spare fibres to undertake a Milky Way halo survey and a low-redshift galaxy survey. This paper presents an overview of the ${S}^5$ program, describing the scientific motivation for the survey, target selection, observation strategy, data reduction and survey validation. Finally, we describe early science results on stellar streams and Milky Way halo stars drawn from the survey. Updates on ${S}^5$, including future public data release, can be found at \url{http://s5collab.github.io}., Comment: 25 pages, 14 figures (1 in appendix), 3 tables (1 in appendix). Published on MNRAS. See also paper from Shipp et al. 2019, which measures the proper motion of the DES streams

This paper is based on the multi-band (ugri) Fornax Deep Survey (FDS) with the VLT Survey Telescope (VST). We study bright early-type galaxies (m_B 28 mag/arcsec^2). Therefore, the new FDS data allow us to explore in great detail the morphology and structure of cluster galaxies out to the region of the stellar halo. The observations suggest that the Fornax cluster is not completely relaxed inside the virial radius. The bulk of the gravitational interactions between galaxies happens in the W-NW core region of the cluster, where most of the bright early-type galaxies are located and where the intra-cluster baryons (diffuse light and GCs) are found. We suggest that the W-NW sub-clump of galaxies results from an infalling group onto the cluster, which has modified the structure of the galaxy outskirts (making asymmetric stellar halos) and has produced the intra-cluster baryons (ICL and GCs), concentrated in this region of the cluster., Accepted for publication in Astronomy & Astrophysics. High-resolution version of paper is available at the following link: https://www.dropbox.com/s/ybe4tg2hbqayteh/FDS_V.ETGs.pdf?dl=0

We have studied the behaviour of stellar streams in the Aquarius fully cosmological N-body simulations of the formation of Milky Way halos. In particular, we have characterised the streams in angle/frequency spaces derived using an approximate but generally well-fitting spherical potential. We have also run several test-particle simulations to understand and guide our interpretation of the different features we see in the Aquarius streams. Our goal is both to establish which deviations of the expected action-angle behaviour of streams exist because of the approximations made on the potential, but also to derive to what degree we can use these coordinates to model streams reliably. We have found that many of the Aquarius streams wrap in angle space along relatively straight lines, and also in frequency space. On the other hand, from our controlled simulations we have been able to establish that deviations from spherical symmetry, the use of incorrect potentials and the inclusion of self-gravity lead to streams in angle space to still be along relatively straight lines but also to depict wiggly behaviour whose amplitude increases as the approximation to the true potential becomes worse. In frequency space streams typically become thicker and somewhat distorted. Therefore, our analysis explains most of the features seen in the approximate angle and frequency spaces for the Aquarius streams with the exception of their somewhat `noisy' and `patchy' morphologies. These are likely due to the interactions with the large number of dark matter subhalos present in the cosmological simulations. Since the measured angle-frequency misalignments of the Aquarius streams can largely be attributed to using the wrong (spherical) potential, determining the mass growth history of these halos will only be feasible once the true potential has been determined robustly., 21 pages, 18 figures. Accepted for publication in A&A

In this work, a series of Smoothed Particle Hydrodynamics simulations of the coalescence between two He and CO white dwarfs is presented. The simulations discussed here encompass a wide range of stellar masses that are representative of the Galactic white dwarf field population. Both the chemical and the hydrodynamical evolution of the mergers simulated here have been analyzed in detail. This has allowed us to characterize possible different outcomes for the merger and to calculate its potential observational signatures, such as the associated gravitational waveforms, the expected number of neutrinos emitted, the fallback luminosities, and the emitted light curves. Special attention has been paid to the formation and evolution of the tidal stream formed during the process of disruption of the secondary white dwarf. Unlike any previous study in the subject, we have been found that the tidal stream is fragmented. Its potential consequences have been explored and possible different explanations for this have been considered. En este proyecto, una serie de simulaciones realizadas con el código "Smoothed Particle Hydrodynamics" de la fusión entre dos enanas blancas de He y CO han sido realizadas. Estas simulaciones abarcan un amplio rango de masas estelares que son representativas de la población galáctica de enanas blancas. Tanto la evolución química como la hidrodinámica han sido analizadas en detalle, permitiéndonos así caracterizar los posibles resultados de la fusión y calcular sus potenciales características observacionales como las ondas gravitatorias asociadas, el numero esperado de neutrinos emitidos, las luminosidades del material de vuelve a caer tras la fusión y las curvas de luz. Se ha prestado especial atención a la formación y evolución de la corriente de marea formada durante el proceso de ruptura de la enana blanca secundaria. Al contrario que en anteriores trabajos en este campo, hemos encontrado que esta corriente de marea se fragmenta. Sus consecuencias potenciales han sido exploradas y diferentes posibles explicaciones para este hecho han sido consideradas. En aquest projecte, una sèrie de simulacions realitzades amb el codi "Smoothed Particle Hydrodynamics" sobre la fusió de dues nanes blanques de He i CO han estat realitzades. Aquestes simulacions comprenen un ampli rang de masses estel·lars, que són representatives de la població galàctica de nanes blanques. Tant la evolució química com la hidrodinàmica han sigut analitzades al detall, permetent així caracteritzar els possibles resultats de la fusió i calcular les potencials característiques observacionals com les ones gravitatòries associades, el número esperat de neutrinos emesos, les lluminositats del material que torna a caure després de la fusió i les corbes de llum. S'ha prestat especial atenció a la formació i evolució de la corrent de marea formada durant el procés de ruptura de la nana blanca secundaria. Al contrari que en anteriors treballs en aquest camp, hem trobat que aquesta corrent de marea es fragmenta. Les seves conseqüències potencials han sigut explorades i diferents possibles explicacions sobre aquest fet han sigut considerades.

We present a measure of the inclination of the velocity ellipsoid at 1 kpc below the Galactic plane using a sample of red clump giants from the RAVE DR2 release. We find that the velocity ellipsoid is tilted towards the Galactic plane with an inclination of 7.3 +/-1.8 degree. We compare this value to computed inclinations for two mass models of the Milky Way. We find that our measurement is consistent with a short scale length of the stellar disc (Rd ~2 kpc) if the dark halo is oblate or with a long scale length (Rd~3 kpc) if the dark halo is prolate. Once combined with independent constraints on the flattening of the halo, our measurement suggests that the scale length is approximately halfway between these two extreme values, with a preferred range [2.5-2.7] kpc for a nearly spherical halo. Nevertheless, no model can be clearly ruled out. With the continuation of the RAVE survey, it will be possible to provide a strong constraint on the mass distribution of the Milky Way using refined measurements of the orientation of the velocity ellipsoid., Comment: Accepted for publication in MNRAS, 10 pages, 9 figures, 2 tables

Website about the SeaCurrent Project: harvesting energy from tidal streams.

Stellar streams result from the tidal disruption of satellites and star clusters as they orbit a host galaxy, and can be very sensitive probes of the gravitational potential of the host system. We select and study narrow stellar streams formed in a Milky-Way-like dark matter halo of the Aquarius suite of cosmological simulations, to determine if these streams can be used to constrain the present day characteristic parameters of the halo's gravitational potential. We find that orbits integrated in static spherical and triaxial NFW potentials both reproduce the locations and kinematics of the various streams reasonably well. To quantify this further, we determine the best-fit potential parameters by maximizing the amount of clustering of the stream stars in the space of their actions. We show that using our set of Aquarius streams, we recover a mass profile that is consistent with the spherically-averaged dark matter profile of the host halo, although we ignored both triaxiality and time evolution in the fit. This gives us confidence that such methods can be applied to the many streams that will be discovered by the Gaia mission to determine the gravitational potential of our Galaxy., Comment: ApJ subm

We propose a new model for the dark matter halo of the Milky Way that fits the properties of the stellar stream associated with the Sagittarius dwarf galaxy. Our dark halo is oblate with q_z = 0.9 for r < 10 kpc, and can be made to follow the Law & Majewski model at larger radii. However, we find that the dynamical perturbations induced by the Large Magellanic Cloud on the orbit of Sgr cannot be neglected when modeling its streams. When taken into account, this leads us to constrain the Galaxy's outer halo shape to have minor-to-major axis ratio (c/a)_��= 0.8 and intermediate-to-major axis ratio (b/a)_��= 0.9, in good agreement with cosmological expectations., 5 pages, 5 figures. Accepted for publication in ApJ Letters. Minor changes to match published version

We have measured the amount of kinematic substructure in the Galactic halo using the final data set from the Spaghetti project, a pencil-beam high latitude sky survey. Our sample contains 101 photometrically selected and spectroscopically confirmed giants with accurate distance, radial velocity and metallicity information. We have developed a new clustering estimator: the "4distance" measure, which when applied to our data set leads to the identification of 1 group and 7 pairs of clumped stars. The group, with 6 members, can confidently be matched to tidal debris of the Sagittarius dwarf galaxy. Two pairs match the properties of known Virgo structures. Using models of the disruption of Sagittarius in Galactic potentials with different degrees of dark halo flattening, we show that this favors a spherical or prolate halo shape, as demonstrated by Newberg et al. (2007) using SDSS data. One additional pair can be linked to older Sagittarius debris. We find that 20% of the stars in the Spaghetti data set are in substructures. From comparison with random data sets we derive a very conservative lower limit of 10% to the amount of substructure in the halo. However, comparison to numerical simulations shows that our results are also consistent with a halo entirely built up from disrupted satellites, provided the dominating features are relatively broad due to early merging or relatively heavy progenitor satellites., Comment: 15 pages, 14 figures. Replaced to match the published version in ApJ; very minor corrections

We study the evolution of satellite debris to establish the most suitable space to identify past merger events. We confirm that the space of orbital frequencies is very promising in this respect. In frequency space individual streams can be easily identified, and their separation provides a direct measurement of the time of accretion. We are able to show for a few idealised gravitational potentials that these features are preserved also in systems that have evolved strongly in time. Furthermore, this time evolution is imprinted in the distribution of streams in frequency space. We have also tested the power of the orbital frequencies in a fully self-consistent (live) N-body simulation of the merger between a disk galaxy and a massive satellite. Even in this case streams can be easily identified and the time of accretion of the satellite can be accurately estimated., Comment: 17 pages. MNRAS accepted. Revised to reflect final version