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Stellar Tidal Streams in Spiral Galaxies of the Local Volume: a Pilot Survey with Modest Aperture Telescopes

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Stellar Tidal Streams in Spiral Galaxies of the Local Volume: a Pilot Survey with Modest Aperture Telescopes The Astronomical Journal, 140:962–967, 2010 October doi:10.1088/0004-6256/140/4/962 C 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A. STELLAR TIDAL STREAMS IN SPIRAL GALAXIES OF THE LOCAL VOLUME: A PILOT SURVEY WITH MODEST APERTURE TELESCOPES David Mart´ınez-Delgado1,2, R. Jay Gabany3, Ken Crawford4, Stefano Zibetti1, Steven R. Majewski5, Hans-Walter Rix1,Jurgen¨ Fliri2,6, Julio A. Carballo-Bello2, Daniella C. Bardalez-Gagliuffi2,7, Jorge Penarrubia˜ 8, Taylor S. Chonis9, Barry Madore10, Ignacio Trujillo2, Mischa Schirmer11, and David A. McDavid5 1 Max Planck Institut fur¨ Astronomie, Heidelberg, Germany 2 Instituto de Astrof´ısica de Canarias, La Laguna, Spain 3 Black Bird Observatory, Mayhill, NM, USA 4 Rancho del Sol Observatory, Modesto, CA, USA 5 Department of Astronomy, University of Virginia, Charlottesville, VA 22904-4325, USA 6 GEPI, Observatoire de Paris, Meudon Cedex, France 7 Massachussetts Institute of Technology, Cambridge, MA, USA 8 Institute of Astronomy, University of Cambridge, UK 9 Department of Astronomy, University of Texas, Austin, TX 78712-0259, USA 10 The Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA 11 Argelander Institut fur¨ Astronomie, Universitat¨ Bonn, Germany Received 2010 April 11; accepted 2010 July 21; published 2010 September 7 ABSTRACT Within the hierarchical framework for galaxy formation, minor merging and tidal interactions are expected to shape all large galaxies to the present day. As a consequence, most seemingly normal disk galaxies should be surrounded by spatially extended stellar “tidal features” of low surface brightness. As part of a pilot survey for such interaction signatures, we have carried out ultra deep, wide field imaging of eight isolated spiral galaxies in the Local Volume, with data taken at small (D = 0.1–0.5 m) robotic telescopes that provide exquisite surface −2 brightness sensitivity (μlim(V ) ∼ 28.5 mag arcsec ). This initial observational effort has led to the discovery of six previously undetected extensive (to ∼30 kpc) stellar structures in the halos surrounding these galaxies, likely debris from tidally disrupted satellites. In addition, we confirm and clarify several enormous stellar over-densities previously reported in the literature, but never before interpreted as tidal streams. Even this pilot sample of galaxies exhibits strikingly diverse morphological characteristics of these extended stellar features: great circle-like features that resemble the Sagittarius stream surrounding the Milky Way, remote shells and giant clouds of presumed tidal debris far beyond the main stellar body, as well as jet-like features emerging from galactic disks. Together with presumed remains of already disrupted companions, our observations also capture surviving satellites caught in the act of tidal disruption. A qualitative comparison with available simulations set in a ΛCold Dark Matter cosmology (that model the stellar halo as the result of satellite disruption evolution) shows that the extraordinary variety of stellar morphologies detected in this pilot survey matches that seen in those simulations. The common existence of these tidal features around “normal” disk galaxies and the morphological match to the simulations constitutes new evidence that these theoretical models also apply to a large number of other Milky Way-mass disk galaxies in the Local Volume. Key words: dark matter – galaxies: dwarf – galaxies: evolution – galaxies: halos – galaxies: interactions – galaxies: structure Online-only material: color figure 1. INTRODUCTION However, minor mergers (i.e., the coalescence of a satellite galaxy and its halo with a much more luminous and massive Galactic mergers have long been recognized as crucial agents companion) are expected to be significantly more common (e.g., in shaping and evolving galaxies (Toomre & Toomre 1972). Cole et al. 2000). Indeed, such minor mergers should remain Within the hierarchical galaxy formation framework (e.g., White frequent to the present epoch in a ΛCDM cosmogony. As minor & Frenk 1991), dark matter halo mergers are a dominant mergers do not destroy pre-existing stellar disks (e.g., Robertson evolutionary driver on the scale of galaxies. For all mass scales et al. 2006), signs of recent or ongoing minor mergers should be lower than entire galaxy clusters, the merger of two DM halos is apparent around spirals, the most common type of large galaxy. followed quickly by the merger of the (stellar) galaxies that had If the satellite galaxies become tidally disrupted while still in an been sitting at the halo’s centers (e.g., Kauffmann et al. 1993). orbit that extends beyond the stellar body of the larger galaxy The most spectacular manifestations of this process may be companion, then they should form stellar tidal “features,” which major mergers (i.e., the coalescence of galaxies with comparable extend into the halo of the central galaxy. mass) that usually entails the destruction of any pre-existing The observational consequences of this scenario, where the stellar disk and may lead to star bursts. Such events have been stellar halo of spiral galaxies is essentially comprised of tidal relatively rare at least since z ∼ 1, with only a few percent of stellar debris from merged satellite galaxies, have been explored luminous galaxies being involved in an ongoing major merger by Bullock & Johnston (2005, BJ05) and others (e.g., Tumlinson at any point in time (e.g., Robaina et al. 2010). 2010; Cooper et al. 2010). Satellites that merged on compact 962 No. 4, 2010 STELLAR TIDAL STREAMS IN THE LOCAL VOLUME 963 Table 1 Observatories and Other facilities Observatory Location Telescope Focal Ratio CCD Scale (arcsec pixel−1) FOV (arcmin) BBO NM, USA RCO 0.508 m f/8.1 STL-11000 0.46 20.4 × 30.6 RdS CA, USA RCO 0.508 m f/8.1 Alta KAF09000 0.58 29.3 × 29.3 Mrk South Australia RC0 0.368 m f/9 Alta 16803 0.56 38.2 × 38.2 New Mexico Skies (NMS) NM, USA APS 0.160 m f/7 STL-11000 1.66 73.7 × 110.6 orbits or a long time ago have phase-mixed into a seemingly During the past few years, we have initiated a pilot survey smooth component by now. In contrast, merger remnants that of stellar tidal streams in a selected number of spiral systems are only a few dynamical periods old, either because they using modest telescopes operating at very dark sites. Ultimately, occurred recently or on orbits with torbit 1 Gyr, should leave the most basic question we will seek to answer concerns the stellar streams, rings, or plumes as the “fossil record” of their frequency of stellar streams in the Local Volume. Our aim interactions. BJ05 showed that such tidal debris can exhibit a is to test theoretical predictions by comparing substructure wide range of morphologies and that such distinctive structural counts from our galaxy sample to cosmological simulations. features should be common, perhaps ubiquitous around normal But the models also make predictions about a number of direct disk galaxies. They also showed that most of the features occur observational characteristics (such as the colors, morphologies, −2 at very low surface brightness (μV 28.5 mag arcsec ) and spatial coherence, and extent of halo substructures) that can be would therefore not be recognizable in traditional images of tested with the results of our survey. nearby galaxies. This paper describes the initial results of our pilot study on The Milky Way and the Andromeda galaxy, both resolvable eight nearby spiral galaxies. These systems were selected for into individual stars so that low surface brightness streams can the study because they were already suspected of being sur- more readily be seen, show a wealth of (sub-)structure in the rounded by diffuse-light over-densities based on data collected stellar distribution of their outskirts. The most spectacular cases from available surveys (e.g., POSS-II; SDSS-I) and previously are the Sagittarius tidal stream surrounding the Milky Way (e.g., published deep images posted on the Internet by amateur as- Majewski et al. 2003) and the Great Southern stream around tronomers. While based on a biased sample of systems pre- the Andromeda galaxy (e.g., Ibata et al. 2001), which have selected for substructures, our pilot study serves as a proof of become archetype fossil records of satellite galaxy mergers. concept for the intended, more systematic survey of halo sub- But overall, the stellar halo structure of both galaxies is complex structure around spiral galaxies. It also enabled us to resolve the (e.g., Majewski et al. 1996; Belokurov et al. 2006;Belletal. required observing strategies and data reduction methodologies. 2008; McConnachie et al. 2009). The results presented here come from a productive collaboration Both simulations and empirical evidence suggest that there between amateur and professional astronomers, dedicated to ex- is a great deal of galaxy-to-galaxy variation in the level and ploiting the scientific potential of modest aperture telescopes. the epoch of merging and hence variation in the amount and morphology of tidal debris. Therefore, a more than qualitative 2. OBSERVATIONS comparison between the predicted and observed prevalence of stellar debris around disk galaxies requires a much larger sample This pilot survey was conducted with three privately owned that necessarily must include galaxies well beyond the Local observatories equipped with modest-sized telescopes located in Group. The current models predict that a survey of between the USA and Australia (see Table 1). Each observing site fea- tures very dark, clear skies with seeing that is routinely below 50 and 100 parent galaxies reaching to a surface brightness of ∼30 mag arcsec−2 should reveal many tens of tidal features, 1.5. These telescopes are manufactured by RC Optical Systems perhaps nearly one detectable stream per galaxy (Bullock & and follow a classic Ritchey–Chretien design.
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