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THE H I ENVIRONMENT of the M101 GROUP Faculty Scholarship 2012 The H I nE vironment Of The M101 Group J. Christopher Mihos Katie M. Keating Kelly Holley-Bockelmann D. J. Pisano Namir E. Kassim Follow this and additional works at: https://researchrepository.wvu.edu/faculty_publications Digital Commons Citation Mihos, J. Christopher; Keating, Katie M.; Holley-Bockelmann, Kelly; Pisano, D. J.; and Kassim, Namir E., "The H I nE vironment Of The M101 Group" (2012). Faculty Scholarship. 700. https://researchrepository.wvu.edu/faculty_publications/700 This Article is brought to you for free and open access by The Research Repository @ WVU. It has been accepted for inclusion in Faculty Scholarship by an authorized administrator of The Research Repository @ WVU. For more information, please contact [email protected]. The Astrophysical Journal, 761:186 (11pp), 2012 December 20 doi:10.1088/0004-637X/761/2/186 C 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A. THE H i ENVIRONMENT OF THE M101 GROUP J. Christopher Mihos1, Katie M. Keating2, Kelly Holley-Bockelmann3,4,D.J.Pisano5,7, and Namir E. Kassim6 1 Department of Astronomy, Case Western Reserve University, Cleveland, OH 44106, USA; [email protected] 2 Rincon Research Corporation, Tucson, AZ 85711, USA; [email protected] 3 Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA; [email protected] 4 Department of Physics, Fisk University, Nashville, TN 37208, USA 5 Department of Physics, West Virginia University, Morgantown, WV 26506, USA; [email protected] 6 Naval Research Laboratory, Washington, DC 20375, USA; [email protected] Received 2012 September 21; accepted 2012 October 30; published 2012 December 6 ABSTRACT ◦ × ◦ × = 16.8 17.5 −2 We present a wide (8.5 6.7, 1050 825 kpc), deep (σNH i 10 –10 cm ) neutral hydrogen (H i)map of the M101 galaxy group. We identify two new H i sources in the group environment, one an extremely low surface brightness (and hitherto unknown) dwarf galaxy, and the other a starless H i cloud, possibly primordial in origin. Our data show that M101’s extended H i envelope takes the form of a ∼100 kpc long tidal loop or plume of 8 H i extending to the southwest of the galaxy. The plume has an H i mass of ∼10 M and a peak column density of 17 −2 NH i = 5 × 10 cm , and while it rotates with the main body of M101, it shows kinematic peculiarities suggestive of a warp or flaring out of the rotation plane of the galaxy. We also find two new H i clouds near the plume with 7 masses ∼10 M, similar to H i clouds seen in the M81/M82 group, and likely also tidal in nature. Comparing to deep optical imaging of the M101 group, neither the plume nor the clouds have any extended optical counterparts down to a limiting surface brightness of μB = 29.5. We also trace H i at intermediate velocities between M101 and NGC 5474, strengthening the case for a recent interaction between the two galaxies. The kinematically complex H i structure in the M101 group, coupled with the optical morphology of M101 and its companions, suggests that the group is in a dynamically active state that is likely common for galaxies in group environments. Key words: galaxies: dwarf – galaxies: evolution – galaxies: individual (M101) – galaxies: interactions – galaxies: ISM Online-only material: color figures 1. INTRODUCTION accretion of gas onto massive galaxies may happen either via “hot mode” accretion, where gas cools from a surrounding The gaseous ecosystems around nearby galaxies are thought hot halo, or by the accretion of smaller gas-rich companion to be rich and dynamic, reflecting a wide variety of processes galaxies. Of course, these scenarios are not mutually exclusive, associated with galaxy formation and evolution. Theoretical as several accretion processes may be operating simultaneously. arguments predict that galaxies can continually accrete gas from The exact mechanism driving gas accretion onto galaxies at their surroundings, as material falls in from the more diffuse low redshift—whether it be hot mode, vestigial cold mode, or intragroup medium (e.g., Larson 1972; Maller & Bullock 2004; satellite accretion—remains unclear. Keresetal.ˇ 2005; Sommer-Larsen 2006). Massive galaxies can Searches for signatures of gas accretion have shown a very host a significant population of gas-rich dwarf companions, complex H i environment around nearby galaxies. On large and interactions with these companions and with larger nearby scales, H i tails, plumes, warps, and lopsidedness certainly argue galaxies can imprint kinematic and morphological signatures that ongoing accretion is triggered by interactions and merg- in the extended H i disks, such as disk warps, H i plumes, tidal ers (see Sancisi et al. 2008 for a review). In many cases, the tails, and high-velocity cloud complexes (see compilations in clear culprit is a companion galaxy tidally stripping, and/or Hibbard et al. 2001; Sancisi et al. 2008). Radially extended gas being stripped by, the host galaxy. In contrast, searches for is responsive to even low-level perturbations, and this makes pure “starless” H i clouds in the intragroup medium that might deep H i imaging an excellent tool to pinpoint subtle dynamical drive gaseous accretion have yielded few detections (e.g., de effects that are missed in the optical and that help drive galaxy Blok et al. 2002;Kovacetal.ˇ 2009; Chynoweth et al. 2009; evolution. Pisano et al. 2007, 2011). While strongly interacting groups can Galaxies are believed to be embedded in a gaseous network host a population of massive free-floating H i clouds (such as that can feed galaxies through accretion. At high redshift, this those found in the M81/M82 group; Chynoweth et al. 2008), accretion is thought to be rapid, perhaps in a “cold” accretion the clouds are typically linked to the more diffuse tidal de- mode where gas flows smoothly onto galaxies along dense dark bris and have a phase space structure consistent with a tidal matter filaments, never shock heating to the virial temperature formation scenario, rather than being fresh material accreting of the host dark matter halo (e.g., Keresetal.ˇ 2005, 2009; from the intragroup medium or being “dark galaxies” hosted by Dekel & Birnboim 2006; Stewart et al. 2011). However, under their own dark matter halo (Chynoweth et al. 2011a). In short, these models, the fraction of cold gas accretion plummets despite solid theoretical evidence that galaxies should accrete 12 once the dark matter halo reaches a critical mass of ∼10 M gas from the intergalactic medium or draw from the warm gas (Dekel & Birnboim 2006). At the present epoch, therefore, the reservoir in the halo, most of the observed H i structures are con- sistent with interaction with or accretion from another gas-rich 7 Adjunct Assistant Astronomer at National Radio Astronomy Observatory. galaxy. 1 The Astrophysical Journal, 761:186 (11pp), 2012 December 20 Mihos et al. The ubiquity of tidal interaction signatures in the H i distribu- Table 1 tion around nearby galaxies is easily understood by the radially M101 Group Observations Summary extended nature of the neutral interstellar medium in galaxies. Area Because gas at large radius is less tightly bound to its host α range (J2000) 13:26:39.1–14:30:27.7 galaxy than is the more concentrated stellar component, it is δ range (J2000) 50:46:13–57:50:27 more responsive to tidal perturbations and can reveal interaction Observations signatures even in systems that show little or no direct evidence Center frequency (MHz) 1418 Bandwidth (MHz) 12.5 of tidal debris in the optical, such as the Leo Ring around the − M96 group (Schneider 1985; Michel-Dansac et al. 2010), the Velocity range (heliocentric, km s 1) −787–1855 Channel width (kHz) 24.4 tidal features connecting galaxies in the M81/M82 system (Yun −1) et al. 1994), or the tidal tail in the Leo Triplet, first detected in Velocity resolution (km s 5.2 Integration time (hr) 86 H i (Haynes et al. 1979). Beyond simply identifying interacting Sensitivity, inner region systems, the morphology and kinematics of the H i tidal debris rms noise (mJy beam−1)3.0 −2 16.8 can be used to develop detailed dynamical models of the inter- NH i (cm )10 action (e.g., Hibbard & Mihos 1995;Yun1999; Michel-Dansac Sensitivity, outer region et al. 2010; Barnes 2011). rms noise (mJy beam−1) 15.0 8 −2 17.5 The nearby M101 galaxy group (Tully 1988) provides an NH i (cm )10 i Beam size opportunity to study the connection between the group H environment and the properties of the individual galaxies. Angular ( )9.1 M101 itself is a well-studied, massive Sc galaxy, and its Physical (kpc) 18.2 highly asymmetric disk suggests ongoing interactions with −1 other galaxies within the group (Beale & Davies 1969; Waller Note. Sensitivities are 1σ per 5.2 km s velocity channel. et al. 1997). Previous 21 cm neutral atomic hydrogen (H i) imaging of M101, both single dish and synthesis mapping, sampling interval corresponds to a slightly better than Nyquist has shown a complex H i environment: a distorted H i disk rate at approximately 3 pixels per beam. Total integration time with several high-velocity cloud complexes (van der Hulst & was approximately 86 hr. Table 1 gives a summary of the Sancisi 1988, hereafter vdHS88; Walter et al. 2008), and an observations, including the rms noise figures. extended, asymmetric outer H i plume at lower column density We calibrated the GBT data in the standard manner using (Huchtmeier & Witzel 1979, hereafter HW79). The outer disk the GBTIDL and AIPS10 data reduction packages.
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