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Publications of the Astronomical Society of the Pacific 98:403-422, April 1986 SYSTEMATIC REINVESTIGATION of the RADIAL VELOCITI

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Publications of the Astronomical Society of the Pacific 98:403-422, April 1986 SYSTEMATIC REINVESTIGATION of the RADIAL VELOCITI Publications of the Astronomical Society of the Pacific 98:403-422, April 1986 SYSTEMATIC REINVESTIGATION OF THE RADIAL VELOCITIES OF THE GALACTIC GLOBULAR CLUSTERS: IMAGE-TUBE RESULTS JAMES E. HESSER* Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V8X 4M6, Canada STEPHEN J. SHAWL*t AND JAMES E. MEYERti Clyde W. Tombaugh Observatory, University of Kansas, Lawrence, Kansas 66045 Received 1985 ABSTRACT Radial velocities measured from ~ 260 image-tube spectrograms at 120 A mm-1 of 90 Galactic globular clusters and NGC 121 in the Small Magellanic Cloud are presented and compared with previous major surveys. Velocities are also deduced from 23 evolved stars in four globular clusters. The data base is the largest homogeneous one currently available. Taken in conjunction with our earlier work, as well as that of Zinn and West (1984), Table III provides velocities for 22 Galactic globular clusters previously lacking them. Furthermore, the precision with which an additional ~ 18 cluster velocities are known should be substantially improved by virtue of our second, independent measurements. Rest wavelengths are given for the spectral features analyzed, and approximate wavelengths are tabulated for other features measured but not used in the velocity analysis. Solid-body, solar-motion solutions are used to probe the impact of our homogeneous set of image-tube velocities on earlier studies; in spite of a larger data base and more uniform sky coverage, net differences are small. From our data alone the solar apex is found to lie at α = 21h10m ± 52m and δ = 44?9 ± 9?8 or € = 88?1 and b = — 2?2. Evidence for the G-type clusters rotating more rapidly than the F-type ones persists. The availability of new spectral classifications reveals more distinct differences in velocity dispersions from the solar motion solutions for F- and G-type clusters, ~ 128 km s-1 and ~ 86 km s-1, respectively, than heretofore recognized. The velocity dispersion for the cluster system as a whole is ~ 120 km s_1. Similar results were found in an extensive analysis by Zinn (1985) based on separating the clusters into two metallicity groups at [Fe/H] = -0.8. Key words: catalogs-clusters of stars: globular-galaxies: structure-galaxies: Magellanic Clouds-Milky Way system-radial velocities-spectroscopy I. Introduction velocities of clusters close to the Galactic center are not Precise radial velocities are required for analyses of consistent with a simple steady-state model of the Galac- global properties—spatial, kinematical, and chemical-of tic halo; Rodgers and Paltoglou (1984) infer from their the Galactic globular clusters. For example, Hartwick derived Galactic rotation that the Galactic globular clus- and Sargent (1978), Frenk and White (1980), Lynden- ters originated from the coalescence of a small number of Bell, Cannon, and Godwin (1983), and Peterson (1985) galaxies; and Zinn and West (1984) and Zinn (1985) have analyze the velocities of outlying clusters and dwarf inferred abundance and spatial distribution patterns spheroidal galaxies for their implications about the mass within the Galactic globular-cluster system. Accurate ra- of the Galaxy; Clube and Watson (1979) suggest that radial dial velocities also find application to the search for gas in globular clusters (e.g., Klein 1976; Smith, Hesser, and Shawl 1976; Faulkner and Freeman 1977; Hesser and Shawl 1977; VandenBerg 1978; Troland, Hesser, and *Visiting Astronomer, Cerro Tololo Inter-American Observatory, Heiles 1978; Bowers et al. 1979) and for discriminating National Optical Astronomy Observatories, which is operated by AURA, Inc., under contract to the National Science Foundation. cluster members from field stars (e.g., Zinn, Newell, and tVisiting Astronomer, Dominion Astrophysical Observatory. Gibson 1972; Nemec 1978; Smith and Perkins 1982; Har- tPresently at San Diego State University. ris, Nemec, and Hesser 1983; Peterson 1984). 403 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 404 HESSER, SHAWL, AND MEYER At the genesis of this project in 1975, radial velocities was installed on the CTIO/Yale 1-meter telescope, we were available for 76 clusters, 70 of which arose princi- decided to obtain spectrograms of as many clusters as pally from the work of Mayall (1946) and Kinman (1959a). possible. In addition to the previously mentioned need The monumental, pioneering work of Mayall (1946) for initial velocity values to interpret the Fabry-Perot largely relied on the 0.9-m Crossley reflector at Lick data, we felt that a general consistency check with the Observatory (latitude +37°) and its two-prism spec- velocities to be deduced from Fabry-Perot observations trograph, which gave a linear reciprocal dispersion of ~ was desirable, inasmuch as its previous use had been only 430 A mm1 at H7. As many clusters as possible were also on emission-line nebulae. This paper, then, reports the observed with the 0.9-m refractor at ~ 130 A mm"1 as a velocities deduced from — 260 image-tube spectrograms check on the velocity system of the lower-dispersion of 91 globular clusters, while the next paper in this series Crossley spectra. Although the southerly declinations of (Shawl and Hesser 1986) will present the Fabry-Perot the bulk of the Galactic globular clusters posed enormous velocity results. difficulties,1 Mayall nonetheless succeeded in obtaining four, and sometimes more, spectrograms of each cluster. II. Observations and Analysis Kinman (1959a) took advantage of the southern lati- Details of the observations are reported in our previous tude (—26°) of the Radcliffe Observatory at Pretoria to papers on spectral classifications (Hesser and Shawl measure clusters which could not be studied from Lick. 1985-Paper I) and on the establishment of the velocity He used the two-prism Cassegrain spectrograph with a system for the image-tube spectrograph as applied to dispersion of ~ 86 A mm-1 at H7 to obtain 69 spectro- ~ F0-~ K4 Population I stars (Shawl et al. 1985-Paper grams from 4000 Â-4400 A of 18 clusters and 64 spectra of II). Briefly, the spectrograms were obtained with the 41 giant stars in 13 clusters. Kinman's Table VIII compila- Boiler and Chivens Cassegrain spectrograph and RCA tion served as the standard reference on cluster velocities 33033, two-stage, magnetically-focused image tube on until Webbink s (1981) compilation appeared. the CTIO/Yale 1-meter telescope on 27 nights between While searching for ionized hydrogen in globular clus- June 1975 and May 1978. The important spectrograph ters using the AURA single-etalon Fabry-Perot interfer- parameters were as follows: 3.63 mm slit length (71 arc sec ometer (Smith, Hesser, and Lasker 1978), the potential of at the focal plane); 150 microns (2.9 arc sec) slit width (21 this instrument to provide precision, integrated-light ra- microns projected at the plate); 120 A mm-1 nominal dial velocities became clear (Smith et al. 1976; Hesser and linear reciprocal dispersion; 0.53 mm projected spectrum Shawl 1977). However, its ~ 150 km s-1 free spectral width. Exposure times ranged from a few minutes to range requires prior knowledge of the cluster velocity to more than three hours. Sampling of the composite light ± 50 km s1 to determine to which order a given line (described in Paper I) was achieved (with few exceptions) profile belongs. This poses problems when clusters lack- by trailing the in-focus image on the slit. Care was taken ing a previous velocity determination are observed, or to avoid very bright stars, or dwelling on obvious clumps when previous measurements are so uncertain that the of stars that might have dominated the resultant spec- order choice becomes problematical. Our initial experi- trum. (That the spectra obtained are, with rare excep- ences suggested that some velocities in the literature may tions, uniformly exposed (see Fig. 1 of Paper I) suggests be substantially in error. that our sampling process was successful.) All told, some Thus, when in 1975 a new image-tube spectrograph ~ 260 spectra of 91 globular clusters were obtained; 72% of the clusters were observed more than once (Fig. 1). In addition to the spectral observations of the integrated or a June 1985 letter to us Dr. Mayall writes: "My chief recollection of composite light of Galactic globular star clusters, individ- using the Crossley on the globulars is how often I had to reverse the tube ual giant stars were also measured in four clusters.2 and rotate the upper section with the spectrograph. I would disable the platform limit switch by manually keeping it closed until the lead-screw turned to the end of the thread, thereby getting several degrees more of minus declination. I would set the tube on the west side and wait for the 2As discussed in Paper I (see particularly Appendices 1 and 2), the lack cluster to drift in, and started the exposure as soon as it reached the slit. of sky subtraction capability in the equipment inevitably signifies in- When it came to the meridian, I put in the dark slide, paralleled the tube creased uncertainties for results derived from data for a few clusters of and polar axle for reversal, and started the exposure with the tube down low central surface brightness and/or projected against dense star fields. to the object. All this took only a few minutes, and I did it so much that I With the exception of those clusters noted in Paper I, we do not believe had to keep my eye on the oil and grease in the bearings of the polar and our integrated spectrograms (and velocities therefrom) are likely to be declination axles! systematically in error because we took particular care when sampling "Getting NGC 1851 was a risky business with the 36-inch refractor. the more difficult objects.
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