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NEUTRAL HYDROGEN CLOUDS in the M81/M82 GROUP KM Chynoweth

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NEUTRAL HYDROGEN CLOUDS in the M81/M82 GROUP KM Chynoweth University of Massachusetts Amherst ScholarWorks@UMass Amherst Astronomy Department Faculty Publication Series Astronomy 2008 NEUTRAL HYDROGEN CLOUDS IN THE M81/M82 GROUP KM Chynoweth GI Langston Min Yun University of Massachusetts - Amherst FJ Lockman KHR Rubin See next page for additional authors Follow this and additional works at: https://scholarworks.umass.edu/astro_faculty_pubs Part of the Astrophysics and Astronomy Commons Recommended Citation Chynoweth, KM; Langston, GI; Yun, Min; Lockman, FJ; Rubin, KHR; and Scoles, SA, "NEUTRAL HYDROGEN CLOUDS IN THE M81/M82 GROUP" (2008). The Astrophysical Journal. 1129. 10.1088/0004-6256/135/6/1983 This Article is brought to you for free and open access by the Astronomy at ScholarWorks@UMass Amherst. It has been accepted for inclusion in Astronomy Department Faculty Publication Series by an authorized administrator of ScholarWorks@UMass Amherst. For more information, please contact [email protected]. Authors KM Chynoweth, GI Langston, Min Yun, FJ Lockman, KHR Rubin, and SA Scoles This article is available at ScholarWorks@UMass Amherst: https://scholarworks.umass.edu/astro_faculty_pubs/1129 Neutral Hydrogen Clouds in the M81/M82 Group Katie M. Chynoweth1 Vanderbilt University, Physics and Astronomy Department, 1807 Station B, Nashville, TN 37235 Glen I. Langston National Radio Astronomy Observatory, Green Bank, WV 24944 Min S. Yun University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01002 Felix J. Lockman, K.H.R. Rubin2 and Sarah A. Scoles3 National Radio Astronomy Observatory, Green Bank, WV 24944 ABSTRACT We have observed a 3◦ ×3◦ area centered on the M81/M82 group of galaxies using the Robert C. Byrd Green Bank Telescope (GBT) in a search for analogs to the High Velocity Clouds (HVCs) of neutral hydrogen found around our galaxy. The velocity range from -605 to -85 km s−1 and 25 to 1970 km s−1 was searched for H i clouds. Over the inner 2◦ × 2◦ the 7σ detection threshold 5 was 9:6 × 10 M . We detect 5 previously unknown H i clouds associated with the group, as well as numerous associated filamentary H i structures, all lying in the range −105 ≤ Vhelio ≤ +280 km s−1. From the small angular distance of the clouds to group members, and the small velocity difference between group members and clouds, we conclude that the clouds are most likely relics of ongoing interactions between galaxies in the group. Subject headings: galaxies: clusters: individual(M81) | galaxies: interactions | surveys | radio lines: galaxies 1. Introduction neutral found in large complexes and as smaller isolated clouds which together cover nearly 40% of H i clouds are found around some spiral galax- the sky (Wakker & van Woerden 1997; Lockman arXiv:0803.3631v1 [astro-ph] 25 Mar 2008 ies at velocities not simply related to the rotation et al. 2002). M31 has a similar system (Thilker of the galactic disk. In the Milky Way there are et al. 2004). These clouds most likely result from a the high velocity clouds (HVCs) both ionized and variety of phenomena. Interactions between galax- ies can produce filamentary structures of gas and 1Graduate Summer Research Assistantship Program at tidal streams like the Magellanic Stream (Putman NRAO. Present address: Vanderbilt University, Physics and Astronomy Department, 1807 Station B, Nashville, TN et al. 2003) and the tidal tails of the M81 group 37235 (Yun et al. 1993; Walter et al. 2002). Some HVCs 2Research Experience for Undergraduates at NRAO. may be the remnants of galaxy formation that Present address: UCO/Lick Observatory, University of are currently being accreted (Maller & Bullock California, Santa Cruz, CA 95064 2004), or clumps and filaments of associated 3 H i Research Experience for Undergraduates at NRAO. with faint dwarf galaxies (Thilker et al. 2004; Ben Present address: Cornell Space Sciences Building, Cornell University, Ithaca, NY 14853 Bekhti et al. 2006; Begum et al. 2006) . Some 1 clouds may also arise from a \galactic fountain", to the expected angular size of these objects, so wherein hot gas created by supernovae expands our study provides new insight into their struc- thermally, cools, and condenses and falls back into ture. The study of Boyce et al. (2001) using HI- the galaxy (see Wakker & van Woerden (1997) JASS data covered a larger angular area (8◦ × 8◦) and references therein). Studies of cloud prop- than our observations, but with a much smaller erties such as mass, morphology, and kinematics velocity range search (-500 to 500 km s−1) and a are necessary to distinguish between these possi- coarser (26 km s−1) velocity resolution. bilities. In addition to distinct clumps of hydrogen, Measurement of many physical quantities is de- galaxies are found to have filamentary structures pendent on an accurate value for distance. The ex- of H i gas and tidal streams associated with in- act distance of the clouds in our galaxy is very dif- teractions (Yun et al. 1993; Walter et al. 2002). ficult to measure, so determination of their proper- Begum et al. (2006) also found clumps and fila- ties has been difficult. Many attempts have been ments of H i associated with faint dwarf galaxies. made to measure the distances of clouds in the The M81 group is notoriously rich in large-scale H Milky Way (see, e.g., Schwarz et al. (1995); van i filaments, a clear signature of interaction between Woerden et al. (1999); Ben Bekhti et al. (2006)). the galaxies. We include in our study a search for However, these efforts have succeeded only in plac- such filamentary structures and smaller scale fila- ing limits on cloud distances. ments that might provide additional information Other galaxies may also have these high veloc- about interactions. ity clouds. For extragalactic sources, distances H i clouds are also related to dwarf galaxies. from a H i cloud to its associated galaxy can In a study of nine Local Group dwarf galaxies, be determined to a high degree of accuracy, so Bouchard et al. (2006) found that the concentra- their properties may be studied more readily. We tion of HVCs was markedly enhanced near dwarfs, observed the M81/M82 cluster with the GBT in and that there were H i clouds near (in projection) order to map its neutral hydrogen cloud popula- most of the dSphs and dIrr/dSphs of the LG, al- tion. The group contains optically bright galaxies though they were typically offset from the optical M81, M82, NGC 3077, and NGC 2976, which show center. Blitz & Robishaw (2000) found that many the remnants of strong interactions (Walter et al. H i envelopes of dwarf galaxies in the local group 2002), as well as over 40 dwarf galaxies. See Ta- were very similar to extragalactic H i clouds in ble 1 for a summary of galaxies within our field of mass and size, and although most neutral hydro- observation. The M81 group has been the subject gen clouds are thought to contain no stars (Siegel of many studies including, e.g., Roberts (1972), et al. 2005), they found that two previously cat- Appleton et al. (1981), Lo & Sargent (1979), Yun aloged clouds actually harbored dwarf galaxies. et al. (1993), and Boyce et al. (2001). Lockman These results suggest a strong relationship be- et al. (2002) also found diffuse neutral hydrogen tween H i clouds and dwarf galaxies, although emission in the direction of the M81 group. They the nature of the connection has yet to be fully find locations where the emission is likely due to elucidated. Perhaps some H i clouds enshroud the interactions of M81/M82 but also find lower low surface-brightness galaxies, or are ejected from level emission away from known galaxies. dwarf galaxies through other mechanisms. In addition to our velocity range search of -605 Bouchard et al. (2006) concluded that ram pres- to -85 km s−1 and 25 to 1970 km s−1, we have sure or tidal stripping are not likely to be the cause achieved a lower mass detection threshold than of the observed offset of H i clouds from their as- previous studies. Our observations have a mass sociated galaxies. An alternative explanation is sensitivity 50-100% lower than the studies by Ap- that the gas could be blown out by star forma- pleton et al. (1981) and Boyce et al. (2001), due to tion. Recently, Walter et al. (2002) have shown our high velocity resolution and the excellent sen- that episodes of star formation in the dwarf galaxy sitivity of the GBT. Our observations cover a large NGC 3077 have disrupted and ejected the gas out- angular area in order to make a complete study side of the region where the stars are optically of the H i cloud population and properties. The identified. Our observations cover a large angu- beam size of the GBT at L band is well matched lar area around this galaxy (as well as others in 2 the group) to search for these blown out compo- 3. Data Reduction nents. In addition, these larger angular regions enable us to distinguish foreground components in The archived GBT data were reduced in the 2 our galaxy and smaller angular size components of standard manner using the GBTIDL data reduc- dwarf galaxies in the group. tion package. In sections 2 and 3 we summarize the observa- In order to match our velocity resolution to tions and data reduction. In section 4 we describe the expected linewidths of H i clouds in the methods used to interpret our observations. In group, we smoothed the data to contain 2048 section 5, mass calculations for known objects in channels with a channel spacing of 24.4 kHz, cor- −1 the group are presented. In section 6 we detail new responding to a velocity resolution of 5.2 km s .
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