View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server Departures From Axisymmetric Morphology and Dynamics in Spiral Galaxies David A. Kornreich1 Center for Radiophysics and Space Research Cornell University Space Sciences Building, Ithaca NY 14853 [email protected] Martha P. Haynes Center for Radiophysics and Space Research and National Astronomy and Ionosphere Center2 Cornell University Space Sciences Building, Ithaca NY 14853 [email protected] R. V. E. Lovelace Cornell University Department of Astronomy Cornell University Space Sciences Building, Ithaca NY 14853 [email protected] and Liese van Zee3 National Radio Astronomy Observatory4 [email protected] ABSTRACT New H I synthesis data have been obtained for six face{on galaxies with the Very Large Array. These data and reanalyses of three additional data sets make up a sam- ple of nine face{on galaxies analyzed for deviations from axisymmetry in morphology and dynamics. This sample represents a subsample of galaxies already analyzed for morphological symmetry properties in the R-band. Four quantitative measures of dy- namical nonaxisymmetry are compared to one another and to the quantitative measures of morphological asymmetry in H I and R-band to investigate the relationships between nonaxisymmetric morphology and dynamics. We find no significant relationship between 1NASA Space Grant Graduate Fellow. 2The National Astronomy and Ionosphere Center is operated by Cornell University under a cooperative agreement with the National Science Foundation. 3Present Address: Herzberg Institute of Astrophysics, 5071 W. Saanich Rd, Victoria BC, V8X 4M6, Canada. 4The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc. 1 asymmetric morphology and most of the dynamical measures in our sample. A possi- ble relationship is found, however, between morphology and dynamical position angle differences between approaching and receding sides of the galaxy. Subject headings: galaxies: kinematics and dynamics | galaxies: structure | methods: observational 2 1. Introduction non–axisymmetric morphology, and that many field spirals exhibit asymmetric dynamics, the question of Despite the fact that most studies of spiral galaxy whether the two phenomena are correlated, or even dynamics concentrate on understanding the proper- represent two pictures of a single underlying effect, ties of axisymmetric disks, evidence is accumulating remains open. that many galaxies lack such overall symmetry. Bald- Both optical morphology and gas dynamics pro- win et al. (1980) were first to seriously examine the vide clues as to the overall structure of a galaxy. For asymmetries in galactic disks, pointing out that lop- instance, Zaritsky & Rix (1997) proposed that op- sided galaxies were a common phenomenon not local- tical lopsidedness arises from tidal interactions, mi- ized to interacting pairs. nor mergers, or possibly gradual accretion. Conselice More recently, frequency of morphological asym- et al. (2000), on the other hand, find a correla- metry in galactic disks has been quantitatively stud- tion between optical asymmetry and B−V color, and ied at optical wavelengths. Based on optical mor- are able to use morphological asymmetry to identify phology, approximately 30% of disk galaxies exhibit whether starbursts in a given galaxy are likely caused significant “lopsidedness” (Rix & Zaritsky 1995; Ko- by interactions and mergers. Similarly, the neutral rnreich et al. 1998, hereafter KHL). These studies are hydrogen dynamics of asymmetric galaxies should be based on data sets containing some 30 targets each. able to distinguish between tidally deformed galaxies Dynamical asymmetry, too, has been examined in and those which are dynamically isolated. For in- large samples of single–dish H I line profiles. Richter stance, while the strongly optically–lopsided galaxy & Sancisi (1994), followed by the 104 galaxy sample of NGC 5474 is well–known to be under the tidal influ- Haynes et al. (1998, hereafter HHMRvZ), examined ence of its neighbor M101, the disturbed morpholo- the symmetry properties of single–dish H I line pro- gies of the other relatively isolated objects in the KHL files and determined that as many as ∼ 50% of spiral sample are not well–explained by standard tidal inter- galaxies show departures from the expected symmet- action models, which require particular dynamics as ric two–horned profile. Nevertheless, asymmetries in well as interactions with (unobserved) nearby com- line profiles are ambiguous evidence at best for dis- panions. turbed dynamics, since they combine both dynamic Alternatively, asymmetries may arise from the ex- and spatial information. citation of unstable, one–armed spiral modes possibly Until recently, however, little work had been done triggered by a past interaction or minor merger (Taga to examine the symmetry properties of neutral hy- & Iye 1998a, 1998b; Lovelace et al. 1999). Librations drogen in synthesis data. As a result, the connection of the optical galaxy about the minimum of the grav- between disturbed morphology and disturbed dynam- itational potential might also be set in motion by a ics in field disk galaxies is only now beginning to be previous interaction. As density is dependent on ra- seriously studied. Schoenmakers et al. (1997, here- dius in a galaxy, one might expect that the natural after SFdZ) outline a method for measuring small de- modes of a galaxy also depend on radius. The result- viations from axisymmetry of the potential of a filled ing differential oscillation might result in both lop- gas disk by breaking down the observed velocity field sided appearance and kinematic decoupling from the into its harmonic components. Recently, Swaters et optical light distribution. An understanding of these al. (1999, hereafter S3vA) have applied this method modes, if they exist, could provide a direct measure to the H I synthesis data of two galaxies, DDO 9 and of the dark matter distribution (Jog 1997; SFdZ). NGC 4395, where the hallmark of dynamical asymme- Another type of nonaxisymmetry, warping of galac- try is found to be asymmetry in the rotation curve, tic disks due to non-planar motions, has been pro- where one side of the curve rises more steeply than posed as an indicator of inclined flattened halo po- I the other. H synthesis observations of several other tentials (e.g. Dekel & Shlosman 1983; Toomre 1983), galaxies, e.g. NGC 3631 (Knapen 1997), NGC 5474 which are required to stabilize the warp against differ- (Rownd et al. 1994, hereafter RDH), and NGC 7217 ential precession, as well as observational indicators of (Buta et al. 1995), have also curiously revealed asym- massive dark halos (Tubbs & Sanders 1979). Warps metries or offsets between the optical centers of light may also be due to tidal interactions and accretion, and the kinematic centers of the neutral gas. While and have been reported to be related to non–circular much evidence exists that many field spirals exhibit motions, particularly m = 1 modes (Weinberg 1998). 3 Although “sloshing” librations in the plane of the UGC 6420, and NGC 4688 were selected for H I 21-cm galaxy would not imply a correlation between warps synthesis observations based on their apparent degree and lopsided dynamics, “flapping” librations normal of asymmetry and position in the sky such that they to the galactic plane could contribute to warping. could be observed at night during the time sched- In this paper, we present HI synthesis data for nine uled for the CS array. As the photometric analysis galaxies whose optical asymmetry properties were conducted in KHL had not yet been completed at quantified by KHL. Data obtained from the archives the time of the target selection, estimates of asym- at the Very Large Array (VLA)5 have been reana- metry were made by eye. Night–time observations lyzed for the galaxies NGC 5474, NGC 5701, and were preferred in order to minimize solar interference. UGC 12732, and new VLA data are presented for the The KHL sample was an optically selected sample of galaxies NGC 991, NGC 1024, UGC 3685, NGC 3596, disk galaxies chosen on the basis of face–on appear- UGC 6420, and NGC 4688. These galaxies represent ance to permit estimation of the optical morphological a sample of face–on galaxies selected on the basis of asymmetry unencumbered by factors such as absorp- their optical properties. tion by dust and inclination errors. The KHL selec- In §2, we discuss the acquisition and reduction of tion criteria were: small axial ratio, narrow (typically W < −1 the new and archived data for our sample of nine 50 ∼ 100 km s ) velocity width obtained from galaxies. In §3, we analyze the data obtained for devi- the private database of Giovanelli & Haynes known ations from morphological and dynamical axisymme- as the Arecibo General Catalog (AGC), and (except try and describe methods of determining the magni- for NGC 5474 and NGC 1042) isolation from obvious tudes of such deviations. These quantitative measures companions. See KHL for details of the optical target are useful for quantifying warps in face–on galaxies selection. and for non–circular motions in more inclined galax- H I observations of these targets were conducted in ies. We then use the sample to draw correlations be- August 1997, December 1998, and January 1999 with tween the symmetry parameters of the galaxies to one the VLA in its CS configuration. The CS configu- another and to global H I properties. Finally, in §4, ration is a modification of the C configuration which we present our conclusions in the context of other re- provides the resolution of the C array while main- cent work on galaxy asymmetry. taining a sufficient number of short spacings to pro- vide sampling of large–scale structure by positioning 2. Observations and Data Reduction one or more antennas at D configuration locations. In 1997, the CS configuration consisted of the stan- 2.1.