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A Globular Cluster Toward M87 with a Radial Velocity < Zurich Open Repository and Archive University of Zurich Main Library Strickhofstrasse 39 CH-8057 Zurich www.zora.uzh.ch Year: 2013 A Globular Cluster toward M87 with a Radial Velocity < - 1000 km s-1: The First Hypervelocity Cluster Caldwell, N ; Strader, J ; Romanowsky, A J ; Brodie, J P ; Moore, B ; Diemand, J ; Martizzi, D Abstract: We report the discovery of an object near M87 in the Virgo Cluster with an extraordinary blueshift of -1025 km s-1, offset from the systemic velocity by >2300 km s-1. Evaluation of photometric and spectroscopic data provides strong evidence that this object is a distant massive globular cluster, which we call HVGC-1 in analogy to Galactic hypervelocity stars. We consider but disfavor more exotic interpretations, such as a system of stars bound to a recoiling black hole. The odds of observing an outlier as extreme as HVGC-1 in a virialized distribution of intracluster objects are small; it appears more likely that the cluster was (or is being) ejected from Virgo following a three-body interaction. The nature of the interaction is unclear, and could involve either a subhalo or a binary supermassive black hole at the center of M87. DOI: https://doi.org/10.1088/2041-8205/787/1/L11 Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-98126 Journal Article Published Version Originally published at: Caldwell, N; Strader, J; Romanowsky, A J; Brodie, J P; Moore, B; Diemand, J; Martizzi, D (2013). A Globular Cluster toward M87 with a Radial Velocity < - 1000 km s-1: The First Hypervelocity Cluster. Astrophysical Journal Letters, 787:1-5. DOI: https://doi.org/10.1088/2041-8205/787/1/L11 The Astrophysical Journal Letters, 787:L11 (5pp), 2014 May 20 doi:10.1088/2041-8205/787/1/L11 C 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. A GLOBULAR CLUSTER TOWARD M87 WITH A RADIAL VELOCITY <−1000 km s−1: THE FIRST HYPERVELOCITY CLUSTER Nelson Caldwell1, Jay Strader2, Aaron J. Romanowsky3,4, Jean P. Brodie4, Ben Moore5, Jurg Diemand5, and Davide Martizzi6 1 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA; [email protected] 2 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA 3 Department of Physics and Astronomy, San Jose´ State University, San Jose, CA 95192, USA 4 University of California Observatories, Santa Cruz, CA 95064, USA 5 Institute for Theoretical Physics, University of Zurich, CH-8057 Zurich, Switzerland 6 Department of Astronomy, University of California, Berkeley, CA 94720, USA Received 2014 February 25; accepted 2014 April 4; published 2014 May 6 ABSTRACT We report the discovery of an object near M87 in the Virgo Cluster with an extraordinary blueshift of −1025 km s−1, offset from the systemic velocity by >2300 km s−1. Evaluation of photometric and spectroscopic data provides strong evidence that this object is a distant massive globular cluster, which we call HVGC-1 in analogy to Galactic hypervelocity stars. We consider but disfavor more exotic interpretations, such as a system of stars bound to a recoiling black hole. The odds of observing an outlier as extreme as HVGC-1 in a virialized distribution of intracluster objects are small; it appears more likely that the cluster was (or is being) ejected from Virgo following a three-body interaction. The nature of the interaction is unclear, and could involve either a subhalo or a binary supermassive black hole at the center of M87. Key words: galaxies: clusters: individual (Virgo) – galaxies: individual (M87) – galaxies: kinematics and dynamics – galaxies: star clusters: general – globular clusters: general Online-only material: color figures 1. INTRODUCTION images, and sky was subtracted using dedicated fibers. Multiple observations were coadded; about half of the objects were Extreme astrophysical objects are occasionally found in large observed more than once. Velocities were measured through samples of data. While classifying a million galaxy images, the cross-correlation as described in Strader et al. (2011b). We Galaxy Zoo project found Hanny’s Voorwerp, an unusual cloud estimate that ∼1800 objects have secure velocities, though this ionized by an active galactic nucleus (Lintott et al. 2009). In number is not yet final. a study of blue horizontal branch stars in the Galactic halo se- Of these 1800, more than 1000 objects have measured lected from the Sloan Digital Sky Survey (SDSS), Brown et al. velocities between 500 and 3000 km s−1, with a clear median (2005) found hypervelocity stars that are thought to originate in ∼1300 km s−1. These are all likely Virgo Cluster members. three-body interactions with the supermassive black hole The remainder are Galactic stars (∼600 objects) or background (SMBH) at the center of the Galaxy. In an extensive study galaxies (∼100 objects). Figure 1 shows a preliminary histogram of star clusters, planetary nebulae, and H ii regions in M31, of velocities in our survey, plus GCs from Strader et al. (2011b), Caldwell et al. (2010) found one star with the most negative showing clear peaks associated with the foreground star and velocity known (−780 km s−1), a probable member of the An- Virgo GC populations. The distribution of Virgo galaxies is also dromeda giant stream. plotted. The survey target with a velocity <−1000 km s−1 is the Here, we report on an object found in a different large subject of this Letter. survey that has an even more extreme negative velocity: an The J2000 decimal coordinates are (R.A., decl.) = apparent globular cluster (GC) toward the central Virgo Cluster (187.72791, +12.68295). It is located 17.6 north of M87, a galaxy M87. projected distance of ∼84 kpc if the object has the same dis- tance as M87 (we adopt 16.5 Mpc for consistency with Strader 2. OBSERVATIONS et al. 2011b). For reasons explained below, we dub the object HVGC-1 (for the first “hypervelocity globular cluster”); in the We have been collecting spectra of GC candidates in the nomenclature of Strader et al. (2011b) for M87 GCs, this object Virgo Cluster for several years, using Keck/DEIMOS and LRIS has the catalog designation H70848. and MMT/Hectospec (Romanowsky et al. 2012; Strader et al. We observed the object on three separate occasions, finding 2011b). Those papers reported roughly 500 new confirmed GCs. consistent results in each case. The final combined radial Our more recent data set, taken mostly with MMT/Hectospec velocity is −1026 ± 13 km s−1. This is the most negative, bulk from 2010–2013, contains more than 5000 separate observations velocity ever measured for an astronomical object not orbiting of 2500 candidate GCs and ultra-compact dwarfs, covering a another object. non-uniform area within 1◦ of M87 and 0◦.5 of M60. Details of the survey will be presented elsewhere, but in brief, − Hectospec with the 270 l mm 1 grating provided spectra with a 3. WHAT IS THIS OBJECT? resolution of 5 Å over the range 3700–9200 Å (Fabricant et al. 2005). The total exposure times were 1–4 hr per field. Data were The only reasonable possibilities are that this object is a GC extracted and wavelength calibrated from the two-dimensional in or near Virgo or an individual Galactic star. The extreme 1 The Astrophysical Journal Letters, 787:L11 (5pp), 2014 May 20 Caldwell et al. Figure 2. Two-color uiK diagram, derived for objects with Hectospec velocity classification. Objects with velocities between 500 and 2300 km s−1 are considered GCs; those <250 km s−1 stars. HVGC-1 clearly falls in the GC sequence. A CFHT g-band cutout (40) is inset. (A color version of this figure is available in the online journal.) Figure 1. Velocity distribution of objects toward Virgo, including all confirmed GCs, all Hectospec velocities, and galaxies (from Rines & Geller 2008). The (Larsen 1999) we fit King models with fixed c = 30 con- distinct stellar and GC distributions are clear, as is the broader galaxy distribution volved with a point-spread function made from bright stars near (dotted and shaded magenta). HVGC-1 is the marked extreme left outlier. HVGC-1. The r ∼ 20 pc objects are clearly resolved and we (A color version of this figure is available in the online journal.) h reproduce the published rh estimates to within ∼15%–20%. HVGC-1 shows modest evidence for being resolved with negative radial velocity is very difficult to explain if the source rh ∼ 6 pc. While we believe this measurement is too small is a star, and only somewhat easier to explain if not. to be reliable, the fitting—and the comparison to known objects—suggests an upper limit of rh ∼ 10–15 pc. This is 3.1. A Star? consistent with a GC, but rules out a larger, more distant galaxy infalling to Virgo. The known Galactic hypervelocity stars all have positive We have obtained photometry of HVGC-1 using ground- velocities (Brown et al. 2012), as expected if they have been based CFHT images taken for the Next Generation Virgo ejected by the central SMBH into the halo. A highly negative Cluster Survey (NGVS; Ferrarese et al. 2012) and the associated velocity for an ejected star would be observed only in the NGVS-IR K-band survey, but separately processed by CADC unlikely event that a star was ejected toward the Sun, and quite (Gwyn 2008). Foreground extinction corrections were taken recently (8 Myr, given a distance of ∼8 kpc). Our object has ◦ from Schlafly & Finkbeiner (2011). a galactic latitude of 74 , and thus is not in the direction of Munoz˜ et al.
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