198 9Apj. . .344. .613F the Astrophysical Journal, 344:613-636
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.613F The Astrophysical Journal, 344:613-636,1989 September 15 © 1989. The American Astronomical Society. All rights reserved. Printed in U.S.A. .344. 9ApJ. MAJOR AND MINOR AXIS KINEMATICS OF 22 ELLIPTICALS1 198 Marijn Franx,2, 3’4 Garth Illingworth,3, 4'5 and Timothy Heckman4, 6 Received 1988 Decembers; accepted 1989 February 24 ABSTRACT Rotation curves and velocity dispersion profiles have been determined for the major and the minor axes of 22 elliptical galaxies. The uncertainty in the mean rotational velocity is typically only 5 km s-1 for either axis. Rotation (<1^> >20 km s-1) was detected in all but one galaxy (NGC 5846), even though the sample was biased toward round ellipticals. Minor axis rotation larger than the major axis rotation was measured in two galaxies, NGC 4406 and NGC 7507. While the sample of such galaxies is still small (now three in total), it appears plausible that « 10% of ellipticals may show large minor axis velocities relative to those on the major axis. Two more ellipticals, NGC 1549 and NGC 7145, show yminor « rmajor. In total six galaxies, or 27% of the sample, show significant minor axis rotation. A simple model is used to derive a rotational axis from the observed minor and major axis velocities to a typical accuracy of 6°. The rotational and photometric minor axes align to better than 10° for 60% of the sample. This implies that the direction of the angular momentum is related to the orientation of the figure of the galaxy. This is surprising, since ellipticals are not supported by rotation, and considerable freedom in the direction of the angular momentum is allowed if galaxies are triaxial and have insignificant figure rotation. This suggests that figure rotation could play an important role in ellip- tical galaxies, or that these galaxies are near to oblate, and/or that events during the formation of elliptical galaxies conspire to orient the figure such that the angular momentum vector aligns approximately with the shortest of the principal axes. The galaxies with large photometric twists generally show significant kinematic misalignment. In addition, the centers of these galaxies are better aligned photometrically with their kinematic axes than are the outer parts. At least two galaxies have rapidly rotating, kinematically skew components in their cores. IC 1459 has a kinematically distinct core with its angular momentum opposite to the angular momentum of the outer parts, and NGC 4406 (a minor-axis rotator) has a core with its angular momentum perpendicular to that of the outer parts. The outer part of the latter galaxy may well rotate around the long axis. An additional four galaxies show weaker effects that need confirmation. The minor axis rotation and the misaligned cores support the view that ellipticals as a class are triaxial. Subject headings: galaxies: evolution — galaxies: formation — galaxies: internal motions — galaxies: nuclei — galaxies: structure I. INTRODUCTION 1989) indicate that these galaxies are triaxial. The fraction of Our present-day understanding of bright ellipticals is that ellipticals with regular disks of ionized gas is too low, however, they have a triaxial shape and are supported by anisotropies in to use this method for the determination of the general dis- their velocity dispersion. The observational evidence for this is tribution of the shapes of ellipticals. the slow rotation of bright ellipticals, too slow to account for Binney (1985) has shown how the stellar kinematics of ellip- their flattening (see e.g., Davies et al. 1983). Theoretical studies ticals can be used to constrain their intrinsic shapes sta- have shown that triaxial galaxies can exist in equilibrium (for a tistically. The rotation along the major and minor axis of review, see de Zeeuw 1987), and that they form easily in ellipticals provides valuable information about their intrinsic iV-body experiments (e.g., Wilkinson and James 1982). shapes. In the first place, detection of minor axis rotation rules Unfortunately, not very much more is known about their out the possibility (1) that ellipticals are oblate spheroids rotat- intrinsic shapes. The deprojection of the two-dimensional ing about their short axis (“ oblate rotators”). The assumption surface brightness distribution into the three-dimensional that they are (2) strongly prolate tumbling bars can also be luminosity distribution is highly degenerate. This prohibits the tested, as can the view (3) that ellipticals rotate about their long determination of the intrinsic shapes of galaxies from surface axes. The limited data available to Binney were inconsistent photometry alone. For a few individual galaxies, the shapes are with hypotheses (1), (2), and (3). Binney also showed how better constrained by the kinematics of the stars and gas. various hypotheses concerning the “ triaxiality ” of ellipticals Studies of NGC 5128 (e.g., Wilkinson et al 1986), NGC 1052 could be tested against the observed ratios of minor axis-to- (Davies and Illingworth 1986), and NGC 5077 (Bertola et al major axis rotation. He noted that the available data suggested that ellipticals were “ optimally triaxial,” i.e., that they could be 1 Partly based on observations made at the European Southern Observa- characterized as triaxial figures whose intermediate axis was tory, La Silla, Chile. truly intermediate between the longest and shortest axes. This 2 Sterrewacht Leiden, Leiden University. result, however, is very uncertain, either because the rotation 3 Space Telescope Science Institute, which is operated by AURA, Inc., for curves are poorly determined or because the galaxies them- the National Aeronautics and Space Administration. selves are unusual in their photometric and kinematical 4 Visiting Astronomer at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory, operated by AURA, Inc., under properties. Examples of such galaxies with multiaxis velocity contract to the National Science Foundation. profiles would be NGC 596 (Schechter and Gunn 1979; Wil- 5 liams 1981) and NGC 4125 (Bertola et al 1984). 6 Lick Observatory. Astronomy Program, University of Maryland. We have started a program to measure the major and minor 613 © American Astronomical Society • Provided by the NASA Astrophysics Data System .613F 614 FRANX, ILLINGWORTH, AND HECKMAN Vol. 344 .344. highest priority. We assume here, and in the following, a value . axis rotation curves of a large sample of ellipticals. This paper -1 -1 is the second in a series of papers. Paper I (Franx, Illingworth, of H0 of 50 km s Mpc . Round galaxies were given high and Heckman 1989) described two-dimensional multicolor priority because Binney’s (1985) models predict that appar- 9ApJ. surface photometry on a sample of galaxies. Here we present ently round galaxies have the highest chance of showing minor 198 kinematical observations on 22 elliptical galaxies in the north- axis rotation that is a measurable fraction of the major axis ern and southern hemisphere. The results will be analyzed in a rotation. Furthermore, there is a scarcity of kinematical obser- third paper (Franx, Illingworth and de Zeeuw 1989, hereafter vations of round galaxies. FIZ). A preliminary discussion of the results is given by Franx The observed galaxies are listed in Table 1, with classi- (1988). fications, total magnitudes, length scales, mean ellipticities, The paper is organized as follows. The sample is discussed in position angles, group velocities, central velocity dispersions, § II. The observations are described in § III. Section IV deals visible luminosities, and IRAS 100 /mi, X-ray, and radio fluxes. with the data reduction, and the derivation of the rotational The sample is by no means complete in any parameter, partly velocities and velocity dispersions. The resulting rotation because there is no complete all-sky sample of ellipticals with curves and velocity dispersion profiles are given in § V. The accurate two-dimensional surface photometry, and partly implications of these results are discussed in § VI. because of the bias toward round galaxies noted above. The Those readers interested in the results would be advised to galaxies were selected irrespective of their radio fluxes, X-ray skip initially to § V from § II or § III. Section IV is lengthy, properties and of their 100 /mi IRAS fluxes, and thus should because of the variety of detectors and spectrographs used and have radio, X-ray, and far-IR properties characteristic of the difficulties encountered in reducing some of the data. nearby elliptical galaxies, except for those ellipticals that were known to be very dusty or quite unusual. These were excluded II. SAMPLE SELECTION from our sample because of the difficulty of deriving photo- A sample of galaxies with accurately known position angles metric and kinematic properties. Some of the galaxies were and ellipticities was required for the spectroscopic observa- subsequently found to have (weak) shells or dust features. tions. We compiled a list of galaxies from Paper I and the photometric studies of Davis et al. (1985), Djorgovski (1985), III. OBSERVATIONS Lauer (1985), Jedrzejewski (1987), and Peletier et al (1989). Our long-slit spectroscopic observations were taken with the Using these data and galaxy classifications taken from the RC2 4 m KPNO and CTIO telescopes, and the 2.2 m and 3.6 m (de Vaucouleurs, de Vaucouleurs, and Corwin 1976), a set of ESO telescopes on nine usable nights between 1984 and 1987. elliptical galaxies was selected based on apparent size, absolute A wide