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The Formation of Shell Galaxies Similar to NGC 7600 in the Cold Dark Matter

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The Formation of Shell Galaxies Similar to NGC 7600 in the Cold Dark Matter Draft version October 30, 2018 A Preprint typeset using LTEX style emulateapj v. 5/2/11 THE FORMATION OF SHELL GALAXIES SIMILAR TO NGC 7600 IN THE COLD DARK MATTER COSMOGONY Andrew P. Cooper Max Planck Institut f¨ur Astrophysik, Karl-Schwarzschild-Str. 1 85741 Garching, Germany David Mart´ınez-Delgado Max Planck Institut f¨ur Astronomie, K¨onigstuhl 17 D-69117, Heidelberg, Germany John Helly, Carlos Frenk, Shaun Cole Institute For Computational Cosmology, Department of Physics, University of Durham, South Road, DH1 3LE, Durham, UK Ken Crawford Rancho del Sol Observatory, Camino, CA 95709, USA Stefano Zibetti Dark Cosmology Centre, Niels Bohr Institute - University of Copenhagen Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark Julio A. Carballo-Bello Instituto de Astrof´ısica de Canarias, La Laguna, Spain and R. Jay Gabany Black Bird Observatory II, California, USA Draft version October 30, 2018 ABSTRACT We present new deep observations of ‘shell’ structures in the halo of the nearby ellipti- cal galaxy NGC 7600, alongside a movie of galaxy formation in a cold dark matter universe (http://www.virgo.dur.ac.uk/shell-galaxies). The movie, based on an ab initio cosmological simulation, shows how continuous accretion of clumps of dark matter and stars creates a swath of diffuse circumgalactic structures. The disruption of a massive clump on a near-radial orbit creates a complex system of transient concentric shells which bare a striking resemblance to those of NGC 7600. With the aid of the simulation we interpret NGC 7600 in the context of the CDM model. Subject headings: galaxies: elliptical and lenticular, cD — galaxies: halos — galaxies: individual (NGC 7600) — galaxies: kinematics and dynamics — galaxies: peculiar — galaxies: structure 1. INTRODUCTION of these haloes. In this way, they were able to model Deep observations of nearby galaxies have uncov- the build-up of galactic stellar haloes through the tidal disruption of satellite galaxies. ered a wealth of faint circumgalactic streams, shells 1 arXiv:1111.2864v1 [astro-ph.GA] 11 Nov 2011 and other structures (e.g. McConnachie et al. 2009; In this paper, we present a movie from one of the six Mart´ınez-Delgado et al. 2010). Such features are a nat- simulations of C10 (Aq-F-2). The movie is a compelling ural occurrence in the cold dark matter (CDM) cos- illustration of the complexity and dynamism of structure mogony, in which the dark haloes hosting massive galax- formation in CDM. Because of this complexity, full cos- ies continually accrete and disrupt their smaller compan- mological modeling is essential – a conclusion emphasized ions. Cooper et al. (2010, hereafter C10) have carried by our movie. out ultra-high resolution simulations of this process us- The stellar halo of Aq-F-2 contains an extensive system ing six N-body models of Milky Way-mass dark matter of interleaved ‘shells’. Examples of stellar haloes with haloes from the Aquarius project (Springel et al. 2008). this distinctive morphology have been known for decades Using the galform semi-analytic model of galaxy for- (e.g. Arp 1966; Malin & Carter 1983) and their fainter mation to calculate the epoch and location of star forma- analogs seem to be common in the local universe (e.g. tion in the simulation, C10 tagged dark matter particles Mart´ınez-Delgado et al. 2010; Tal et al. 2009). In this in appropriate regions of phase-space to follow the dy- paper we present a new deep panoramic image of the namical evolution of stars stripped from the progenitors diffuse light around one such galaxy, NGC 7600, showing 1 [email protected] See the online edition of the journal and [email protected] http://www.virgo.dur.ac.uk/shell-galaxies 2 A.P. Cooper et al. 2. THE MOVIE Three still frames from our movie are shown in Fig 1. The galform model follows the formation of the entire galaxy (disk, bulge, halo; see C10 for details), but the movie shows only the evolution of the stars formed in (and stripped out of) all progenitors of the final galaxy except for the main progenitor. Stars formed in situ – those that would make up all of the disk and part of the bulge – are not included in our particle-tagging procedure and are not shown. The movie begins with the close encounter of two dark matter haloes at z = 4. These haloes merge after ∼ 2.5 Gyr (z = 2.7). Both are surrounded by a num- ber of dwarf galaxies and the debris of earlier mergers. The halo entering the picture from below is the more massive. As the haloes coalesce, their cores oscillate ra- dially about the center of the potential, creating a series of compact shells (density caustics at the apocenters of approximately radial stellar orbits; Quinn 1984). This merger establishes the main halo, on which the movie is centered for the remainder of the simulation. The shells propagate rapidly outwards and by z = 2 have phase- mixed into a diffuse bow-tie-shaped cloud. In the next phase of the movie (z =2 − 0.5, spanning ∼ 5 Gyr – see the topmost panel of Fig. 1) the halo is bombarded by a number of smaller satellites on high- angular-momentum orbits. Tidal forces disrupt many of these satellites, leaving behind streams of debris that crisscross the halo. As with the shells seen earlier, the stars in these streams pile up at the apocenters of their orbits. Where the stream progenitor crosses the center of the halo perpendicular to the line of sight, it appears as an ‘umbrella’: a broad arc at the end of a thin stream (e.g. Mart´ınez-Delgado et al. 2010). These features are rapidly erased by phase-mixing, perturbations to the po- tential and the decay of satellite orbits through dynami- cal friction. The final and most spectacular stage of the movie be- gins at z =0.4. As shown in the central panel of Fig. 1, a bright satellite appears in the upper right of the frame. 6 Figure 1. The evolution of the stellar halo in the Aq-F-2 simu- The satellite (whose stellar mass of ∼ 7 × 10 M⊙ at lation (Cooper et al. 2010). These are still frames from the movie z = 0.4 is only ∼ 0.1% of the stellar mass of the main available in the electronic edition of the Journal, and show stages in the formation of the shell system at redshifts z = 0.68, 0.38 and galaxy at that time, although its dark matter halo is 0.17, corresponding to lookback times of 6, 4 and 2 Gyr respec- 1/3 of the main dark matter halo) seems much brighter tively. The final state is shown in Fig. 2. Brightness corresponds than the central galaxy because only stars accreted by the to projected stellar mass surface density. Colors, ranging from galform ∼ central galaxy are shown – predicts that the dark blue to yellow, correspond to velocity dispersion (from 50– majority of its stars form in situ, and these are not tagged 315 km s−1). All images are centered on the main halo and the field of view in each image is approximately 256 kpc by 160 kpc by dark matter particles in our model (see C10). The (comoving). Only accreted stars are shown; the majority of the satellite galaxy brings with it its own extensive stellar stars in the central galaxy do not appear in these images – see text halo and set of companions, one of which is already being and the caption of Fig. 2 for further details. disrupted into a wide stream when its host arrives in the its well-known shell system and previously undetected main halo. The bright satellite makes two pericentric faint features. Even though our ab initio simulation was passages as the angular momentum is drained from its not constrained to reproduce NGC 7600 in any way, the orbit. These early passages strip the satellite of its stellar observations and the simulation are strikingly similar, halo and its companions. Meanwhile, the stellar halo suggesting that such systems arise naturally in the CDM of the main galaxy is distorted and mixed by the new arrival, destroying pre-existing tidal features. model. ∼ The movie shows how the simulated system is formed What happens next (z 0.27, lowest panel of Fig 1.) 12 in a major (3 : 1) merger between a ∼ 10 M⊙ halo is crucial for understanding NGC 7600. The bright satel- and its brightest satellite at z ∼ 0.4. At z = 0, our semi- lite is now on an approximately radial orbit and will pass analytic model predicts that this halo hosts an ellipsoidal through the center of the potential at subsequent peri- galaxy (bulge-to-total mass ratio B/T = 0.85) of total centers. Material is stripped from the satellite on each of 10 these passages, just like the stellar streams. Here, how- stellar mass 1.3 × 10 M⊙. The formation of shell galaxies 3 Figure 2. Top: final (z = 0) frame of the movie. The brightness scale (surface mass density) was chosen to show the full extent of the structure in the simulation. Color (from blue to yellow) corresponds to velocity dispersion. Only accreted stars are shown so the central concentration of light corresponds to the small fraction of bulge stars that were stripped from satellites. Bottom left: a deep image of NGC 7600 obtained with the Rancho del Sol 0.5-meter telescope. North is up, East is left. The original image (see Fig. 3) has been cropped. The total field of view is 12′ × 12′ (∼ 175 × 175 kpc). Bottom right: the same field of view in Aq-F-2, with a brightness scale chosen to match the extent of the visible structure around NGC 7600.
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