Mon. Not. R. astr. Soc. (1992) 256, 500-514 .50OM Face-on disc galaxies* 2 1 2 92MNRAS.256. Giorgio Magrelli, Daniela Bettoni and Giuseppe Galletta 1 19 Osservatorio Astronómico, Vicolo Osservatorio 5, 35122 Padova, Italy 2Dipartimento di Astronomía, Università di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy Accepted 1991 December 20. Received 1991 December 18; in original form 1991 July 25 SUMMARY Bidimensional photometry is presented for 40 bright SO galaxies with apparent 2 flattening è0/ö0 > 0.9 at the 25 mag arcsec ~ level. They represent all of the bright SOs in the RC2 catalogue which are seen almost face-on and are sufficiently extended for detailed investigations to be possible. Ellipticity and PA profiles of each system have been analysed in order to test for the presence of triaxiality in their bulges and in their discs. Kinematical observations are also presented for one of the more circular systems in the sample and for a ‘template’ galaxy at higher inclination, in order to discuss the possible presence of stellar motions which are highly inclined with respect to the equatorial plane of the galaxy. A short discussion of the fitting errors for these fairly circular systems is also presented. From our data we find that (i) many of these systems are quite far from face-on orientation, at inclination i > 20°. (ii) About 45 per cent are barred, (iii) 67 per cent of bulges of the unbarred SOs considered show twisting of the isophotes and appear to be triaxial, or dominated by a triaxial component following the r1/4 law. (iv) About half of the discs show the same behaviour, suggesting a slight triaxiality. Also, some oblate discs are found. Discs of barred and unbarred systems do not appear to differ much, (v) A fit of the observed flattenings show that the triaxiality of the discs, if present, should be lower than 5 per cent (intrinsic bja > 0.95). (vi) Two galaxies of our sample, and possibly two more from the literature, show no indications of stellar streaming which is not coplanar with the galaxy plane, (vii) The observed velocity dispersion slightly decreases as we go from the bulge to the outer regions of the disc. Key words: galaxies: elliptical and lenticular, cD - galaxies: kinematics and dynamics - galaxies: photometry. apparent axis ratio is ¿ /ö >0.9. In reality, if these systems 1 INTRODUCTION 0 0 are oblate and have an intrinsic flattening cja = 0.25 (Sand- Face-on disc galaxies are good subjects for analysing the age, Freeman & Stokes 1970), a sample with &0/fl0>0.9 structure of the stellar systems in the equatorial plane: on one includes galaxies whose inclination of the equatorial plane hand, the photometry reveals the degree of oblateness of with respect to the sky plane is /<28°. An oblate disc really their discs and bulges; on the other hand, the study of their seen face-on (/< 5°) would present an isophote flattening of kinematics gives useful information on the ‘vertical’ compon- 0.997, which is impossible to measure with present ent of the velocity ellipsoids and on the percentage of orbits techniques, especially in the faint outer regions of the highly inclined with respect to the equatorial plane. Both galaxies where discs predominate (see Bender & Möllenhoff points reflect the processes by which the galaxies were 1987; Fasano & Bonoli 1990). However, the probability of formed and the way in which they acquired and redistributed observing this orientation is very low, just 0.38 per cent of a the angular momentum within them. sample of galaxies randomly orientated in space, or fewer In the literature, few systematic studies have been devoted than 10 spirals in the whole RC2 catalogue (de Vaucouleurs, to galaxies seen at this particular orientation, since it is diffi- de Vaucouleurs & Corwin 1976). cult to distinguish the true cases of face-on orientation. In If the disc of the galaxy is not really oblate, but slightly tri- fact, disc galaxies are considered as being face-on only if the axial, the problem of defining a face-on orientation becomes more complex. Here, the observed flattening does not corre- * Based on observations collected at the European Southern Obser- vatory, La Silla, Chile and on data obtained using the Guide Stars spond to a particular value of i but to a wide range of solu- Selection Ssytem Astrometric Support Program developed at the tions, as is also the case for elliptical galaxies (see Williams Space Telescope Science Institute. 1981). The triaxiahty of disc galaxies is not just an abstract © Royal Astronomical Society • Provided by the NASA Astrophysics Data System Face-on disc galaxies 501 .50OM possibility: Grosböl (1985) deduced slight deviations from a 2.1 Photometric and geometric measurements regularly oblate shape in the outer regions of a sample of 605 spiral galaxies. In addition, Binney & de Vaucouleurs (1981) For all of these galaxies, we analysed PDS scans of Schmidt suspected the presence of similar deviations from the statist- plates, in order to measure the ellipticity and PA profiles, as a ical analysis of disc galaxies from RC2. function of the distance from the centre. The files containing 92MNRAS.256. 19 This difficulty in the determination of i has direct conse- the images of these fields have been extracted from the data- quences for the interpretation of observed galaxy kinematics. base on optical discs of the Guide Star Selection System An oblate galaxy with a rotational velocity of 300 km s-1 Astrometric Support Program developed at the Space Tele- observed at an axis ratio of 0.9 and considered face-on, will scope Science Institute in Baltimore (STScIf ). These data show projected velocities as great as 150 km s~1. If observed allowed us to investigate the intrinsic structure of the with long-slit spectroscopy, these motions could be misinter- selected systems and the occurrence of substructures such as preted as being due to orbits which are very inclined with bars or rings. respect to the equatorial plane of the galaxy; if studied at 21 In addition, for all of the galaxies of the sample present in cm and at low spatial resolution, they produce a double- the fields of the ESO/SRC Atlases, we extracted from the horned line profile. Possible examples of this situation are the ESO-LV catalogue (Lauberts & Valentijn 1989) calibrated velocity gradients observed in NGC 1574 and 4477 (two SOs images of the field in the B and R wavebands, stored at ESO with ¿>0/tfo>Ö.9 in RC2) by Jarvis et al (1988), and the sys- on optical discs. Since only six galaxies of our sample are tems of the Lewis (1987) H i survey of face-on galaxies included in this southern sky survey, we extended the photo- which show half-power widths of around 100 km s "1. metric material by collecting calibrated photographic plates On the other hand, we know that vertical gas motions (on from the ESO-Schmidt telescope for two more galaxies. orbits which are very inclined with respect to the equatorial Table 1 describes the type of data available for each galaxy. plane of the galaxy) actually exist in SO galaxies with polar These images have been reduced by means of the rings (Schweizer, Whitmore & Rubin 1983; Whitmore et al Eso-IHAP software package to convert the counts into inten- 1990) and in some spirals (Lewis 1987). Vertical stellar sity values. The images from the ESO-LV Catalogue have motions, if they exist, should be observable in at least one already been calibrated in surface brightness but have not face-on case. been sky-subtracted, so an appropriate subtraction of the sky The purpose of this study is to investigate the properties has been carried out using the sky level reported for each of face-on disc galaxies and, if possible, to deduce the intrin- image in the Catalogue itself. The CCD images have been sic structure of the galaxy components for a selected sample calibrated using the aperture photometry values listed by of systems. To do this, we analysed the isophotes of all the Longo & de Vaucouleurs (1983). For the uncalibrated nearly circular SO galaxies included in the RC2 catalogue. images from STScI we assumed an arbitrary sky level -2 The sample selection and its analysis is presented in Section pv = 2\.5 mag arcsec . Finally, we made an elliptical inter- 2 and a discussion of the results is presented in Section 3. polation of the galaxy isophotes, using the Numerical Map- The presence of vertical motions and the shape of the ping Package available in Padova (Barbon, Benacchio & velocity dispersion field have been analysed by means of Capaccioli 1975; Fasano 1990). long-slit spectra in two galaxies of our sample and, for com- The resulting ellipticity and PA profiles for each galaxy are parison, in a flatter SB0 (NGC 4643) with similar absolute plotted in Fig. 1; luminosity profiles are also presented in magnitude. These results are also discussed in Section 3. the same figure. When equivalent ellipticity and PA measurements were available from different sources, they have been plotted with different symbols. The data for the flatter systems NGC 4643 and 5820, which have been 2 A SAMPLE OF FACE-ON GALAXIES excluded from the sample, are plotted in Fig. 2. Despite the difficulty in determining whether the orientation The comparison between data coming from the ESO-LV of a single galaxy is truly face-on, the existence of triaxiality Catalogue and the STScI images allows us to estimate the or of vertical streams can be studied by observing a sample errors made in determining the geometrical parameters.