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The Post-Infall Evolution of a Satellite Galaxy⋆ A&A 582, A23 (2015) Astronomy DOI: 10.1051/0004-6361/201526113 & c ESO 2015 Astrophysics The post-infall evolution of a satellite galaxy? Matthew Nichols1, Yves Revaz1, and Pascale Jablonka1;2 1 Laboratoire d’Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), 1290 Sauverny, Switzerland e-mail: [email protected] 2 GEPI, Observatoire de Paris, CNRS UMR 8111, Université Paris Diderot, 92125 Meudon Cedex, France Received 17 March 2015 / Accepted 11 July 2015 ABSTRACT We describe a new method that only focuses on the local region surrounding an infalling dwarf in an effort to understand how the hot baryonic halo alters the chemodynamical evolution of dwarf galaxies. Using this method, we examine how a dwarf, similar to Sextans dwarf spheroidal, evolves in the corona of a Milky Way-like galaxy. We find that even at high perigalacticons the synergistic interaction between ram pressure and tidal forces transform a dwarf into a stream, suggesting that Sextans was much more massive in the past to have survived its perigalacticon passage. In addition, the large confining pressure of the hot corona allows gas that was originally at the outskirts to begin forming stars, initially forming stars of low metallicity compared to the dwarf evolved in isolation. This increase in star formation eventually allows a dwarf galaxy to form more metal rich stars compared to a dwarf in isolation, but only if the dwarf retains gas for a sufficiently long period of time. In addition, dwarfs that formed substantial numbers of stars post-infall have a slightly elevated [Mg=Fe] at high metallicity ([Fe=H] ∼ −1:5). Key words. galaxies: dwarf – galaxies: interactions – galaxies: individual: Sextans dwarf spheroidal – methods: numerical 1. Introduction Previous work has shown that the interaction of the hot corona and satellite galaxies is likely to remove all gas from The concordant cosmological model of dark energy and cold them (Mayer et al. 2006; Nichols & Bland-Hawthorn 2011, dark matter, ΛCDM, predicts that galactic halos contain numer- 2013; Zolotov et al. 2012; Gatto et al. 2013), however, prob- ous subhalos that have accreted over cosmic time (e.g. Klypin lems with Kelvin-Helmholtz instabilities (Mayer et al. 2006; et al. 1999; Moore et al. 1999). A portion of these accreted sub- Zolotov et al. 2012), or the treatment of star formation (Nichols halos are thought to host the dwarf galaxies that are visible today & Bland-Hawthorn 2011, 2013; Gatto et al. 2013) justify new as satellite galaxies, although exactly which subhalos would host simulations. galaxies is an open question (Boylan-Kolchin et al. 2011). Regardless of the details of early galaxy formation, only the The stripping of galaxies close in (<50 kpc) is likely to most massive satellite dwarf galaxies orbiting close in to the occur independent of any internal process within the galaxy host galaxy are able to retain gas to the present day (Grcevich (Mayer et al. 2006), but further out, internal feedback resulting & Putman 2009; Chiboucas et al. 2013), with the other dwarfs from star formation is needed to remove the gas (Gatto et al. being gas-poor dwarf spheroidals (dSphs). Further out, however, 2013). In previous work on the topic, the star formation has only dwarf galaxies of lower mass are able to retain a detectable been treated with simplistic models for analytic ease (Nichols & amount of gas, which is able to fuel ongoing star formation, sug- Bland-Hawthorn 2011, 2013) or assumed based on the observa- gesting that environmental effects are the cause of this gas-poor tionally derived star formation history (Gatto et al. 2013). These gas-rich dichotomy (Grcevich & Putman 2009). approaches gloss over the wide range of observational properties As galaxy simulations increase in resolution more attention that exist within the dSphs and what that might say about their is being paid towards the evolution of dwarf galaxies and how evolution post-infall. simulations compare to observations. Previously we have ex- Sawala et al.(2012) and Zolotov et al.(2012) simulated amined the chemodynamical evolution of dSphs as they inter- dwarf galaxies within a cosmological framework, running sim- act tidally with a larger galaxy (Nichols et al. 2014, hereafter ulations of Milky Way sized galaxies at high resolution. Here, NRJ14). Considering only tidal effects, dwarfs lose substantial using traditional smoothed particle hydrodynamic (SPH) sim- quantities of gas because of a synergistic effect of supernova in- ulations with self-consistent star formation and metallicity duced expulsion and tidal forces. However, all dwarfs were able injection, Zolotov et al. found that baryons can assist in the to retain quantities of gas that would be readily detectable today tidal stripping of dwarf galaxies and can dramatically change unless they were completely destroyed by tides. A potential solu- the inner region of a dwarf through supernova feedback (see tion to the removal of gas, without tidally destroying the dwarfs, also Governato et al. 2010; Pontzen & Governato 2012). Known lies in ram pressure stripping. problems with the traditional SPH models (Agertz et al. 2007) however, allowed many dwarf spheroidal analogues to retain gas ? Appendices are available in electronic form at to the present day and potentially underestimated the impact of http://www.aanda.org ram pressure stripping. Article published by EDP Sciences A23, page 1 of 26 A&A 582, A23 (2015) Sawala et al. found that many dwarfs are likely to enter the The dwarf is placed inside a gas-filled periodic box, where par- halo already gas-free, and tidal stripping combined with UV ticles are injected from one side and deleted when crossing the heating and supernova feedback can remove gas from many of opposite side. The use of the pressure-entropy SPH formulation the remaining dwarfs. With the use of traditional SPH models allows us to account for Kelvin-Helmholtz instabilities (Hopkins and at low resolution, however, ram pressure stripping may have 2013) a known problem with the traditional SPH models (Agertz been overestimated for some dwarfs, which may accelerate the et al. 2007). To properly describe the tidal forces acting upon the stripping and cause some dwarfs to be tidally destroyed through dwarf, we rotate this periodic box along the orbit of the dwarf. this baryon-tide synergy. This kind of approach allows us to capture any synergy between Ideally large cosmological simulations, such as found in the tidal and ram pressure forces upon the dwarf. The moving Sawala et al.(2012), Zolotov et al.(2012), could be run for any of the periodic box is conceptually similar to the method used change in the initial conditions or numerical methods (e.g. mesh- by Ploeckinger et al.(2014) to study the evolution of tidal dwarf less or updated SPH methods). Because of the large computa- galaxies, but with an evolving halo density. We describe this ro- tional cost, however, more efficient methods must be considered tation scheme in Sect. 2.1. for the bulk of simulations. In addition to the tidal forces changing over the orbit of the The need for star formation, metallicity evolution, and tidal dwarf, the changing density of the galactic corona should be re- forces to be handled simultaneously can be understood by con- flected in the density of gas within the periodic box. In addition, sidering the wide range of properties impacted by internal star the wake behind a dwarf should not be able to cross the periodic formation and tidal or ram-pressure stripping. For example, boundary to prevent it from interacting with the dwarf. These present day gas-rich dwarfs show constant density cores (Oh issues can be addressed through the deletion of particles at the et al. 2015), a process thought to be caused by supernova feed- boundary behind the dwarf, and an insertion of new gas in front back (Governato et al. 2010; Pontzen & Governato 2012). On the of the dwarf. This kind of approach has already been used suc- scales of the dwarf spheroidals the existence of cores is slightly cessfully in grid codes to study the hot halo density (Gatto et al. less clear (see e.g. de Blok et al. 2008; Walker & Peñarrubia 2013) and the wake produced by galaxies infalling into a cluster 2011; Breddels & Helmi 2013) and, as the inner profile can al- (Roediger et al. 2015). We describe a similar adaption for the ter the tidal evolution (Kazantzidis et al. 2013), simulating the SPH code GEAR here, described in Sects. 2.2 and 2.3, for the cre- internal star formation is vitally important to understanding the ation and deletion, respectively. A schematic illustration of both evolution of a dwarf. In addition to the internal star formation processes is shown in Fig.1 in which we show how the periodic changing its profile, the removal of gas also impacts the inner box moves along the orbit and the effect of the rotation scheme density (Arraki et al. 2014) and the inclusion of this baryon loss within the periodic box. may lead to a lower fraction of surviving dwarfs than expected from simulations that retain gas. In addition the metallicity evo- lution is intricately linked with the star formation history (and 2.1. Rotation scheme and the reference frame contributes towards its regulation through cooling), and with To simulate the tidal forces upon the dwarf galaxy, we apply the high-resolution studies of dwarf spheroidals now available (e.g. standard fictitious forces throughout the periodic box to simulate Shetrone et al. 2001; Aoki et al. 2009; Tafelmeyer et al. 2010; a solid rotation of the box (a non-solid rotation is also possible, Kirby et al. 2011, just for Sextans dwarf spheroidal); Theler et al. although this introduces further issues, e.g.
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