Mon. Not. R. astr. Soc. (1990) 242, Short Communication, 24p-27p .24C 42P. Further evidence of continuity between elliptical and disc galaxies* 90MNRAS.2 19 Massimo Capaccioli Osservatorio Astronómico, Vicolo Osservatorio 5,1-35122 Padova, Italy Nicola Caon Dipartimento di Astronomía, Università di Padova, Vicolo Osservatorio 5,1-35122, Italy Roberto Rampazzo Osservatorio Astronómico di Brera, Via Brera 28,1-20121 Milano, Italy Accepted 1989 October 23. Received 1989 October 20; in original form 1989 July 28 SUMMARY The behaviour of the isophotes of the E galaxies in a new photometric sample of 33 early-type galaxies belonging to the Virgo cluster (Caon) is compared to that of the SO galaxies. Almost all objects flatter than b/a - 0.3 are found to exhibit ‘pointed’ (or ‘lemon-shaped’) isophotes irrespective of their Hubble class, while almost none do if rounder than the same limit. This observation is used to support the incipient scenario (Bender et al.) in which a large fraction ( - 1/2) of E galaxies - the so-called ‘disky ellipticals’ - populates one end of a disc-to-bulge sequence including SO and spiral galaxies. 1 INTRODUCTION 1987,1989a,b; Capaccioli etal. 1989; Carter 1987). Indirect About 15 years ago the first rotation curves for E galaxies evidence that a significant fraction of E galaxies may share (Bertola & Capaccioli 1975; Illingworth 1977) broke down the same structural properties of SO bulges has been already the classical picture in which all galaxies (and galaxy com- contained in Michard’s (1985) observation of the similarity ponents) were seen as rotationally supported oblate between the ellipticity profiles of flat Es and edge-on lenticu- spheroids. Since then, elliptical galaxies have been conven- lars (cf. also Capaccioli, Held & Nieto 1987; Capaccioli, tionally described as a homogeneous class of objects charac- Piotto & Rampazzo 1988; Nieto 1989). In view of these and terized photometrically by r1/4-like light profile^ (de other arguments, Capaccioli (1987) speculated that several E Vaucouleurs 1948) and kinematically by a low ratio of the galaxies could be in fact misclassified SOs with vanishing maximum rotational velocity V over the central velocity discs. Dynamical support for this speculation came from a m critical revision of the value of the apparent flattening dispersion oc. This generic description is equivalent to the statement that E galaxies populate a ‘fundamental plane’ in £=l-bla entering the VJoc versus £ = l-b/a test (Nieto, the ‘global parameters’ space (Djorgovski & Davis 1987), i.e. Capaccioli & Held 1988). that elliptical galaxies are a two-parameter family. ' A decisive contribution to consolidate a new understand- ing of the Hubble class E as a composite family has resulted In the meantime, it has been realized that several features which are characteristic of ‘classical’ Es are also common to from the analysis of the deviations of the isophotes from pure standard disc (SO) galaxies, and vice versa: isophotal twisting ellipses. The amplitude of the cos(40) term in a Fourier (Barbon, Benacchio & Capaccioli 1976; Williams & expansion of the isophotal radius in a polar reference frame (typical deviations from pure ellipses are of the order of 1 per Schwarzschild 1979), dust-lanes (Bertola & Galletta 1978; Sadler & Gerhard 1985), shells (Malin & Carter 1983; cent of the semimajor axis) has allowed Bender et al. (1989a) to classify ~ 80 per cent of all E galaxies either as ‘boxy- Schweizer & Seitzer 1988), kinematical manifold (Davies et al. 1983), neutral and ionized gas (Wardle & Knapp 1986; shaped’ or as ‘pointed-shaped’. Most importantly, Bender van Driel 1987; Burstein, Krumm & Salpeter 1987; Bettoni and collaborators have shown that each one of these two new & Buson 1987), and counter rotation of stars and gas (Franx classes is much more homogeneous with respect to a number & Illingworth 1988; Bender 1988). Furthermore, positive of physical properties than the total ensemble of E galaxies. evidence for the existence of hidden faint discs has been This is the landscape in which the growing idea that ellipti- shown recently for a handful of E galaxies (e.g. Capaccioli cals constitute a composite, non-homogeneous family - in which ‘boxy’ and ‘pointed’ E galaxies represent independent * Based on observations made at the European Southern ‘physical’ classes - finds its nourishment (Bender et al. Observatory, La Silla, Chile. 1989b). Furthermore, since ‘pointed’ isophotes in SO galaxies © Royal Astronomical Society • Provided by the NASA Astrophysics Data System Continuity between elliptical and disc galaxies 25p .24C result from the presence of a disc, a fundamental question upon the disc-to-bulge and scalelength ratios, an underlying 42P. arises: may we interpret the ‘pointed’ Es and the SO galaxies disc stretches the isophotes of an elliptical bulge along their in terms of a continuous stuctural/physical sequence charac- major axes - at least if the disc and the spheroidal com- terized by the variation of some morphological/genetic para- ponents are not too far from being co-axial. Hence, each meter (for instance, the disc-to-bulge ratio, D/B, as tracer of isophote is interpolated twice: 90MNRAS.2 the average specific angular momentum)? 19 (i) using the entire contour, and A first hint comes from the study of the two ‘standards’ for (ii) removing from it the two sections intercepted by the the E and the SO classes, namely NGC 3379 and NGC 3115. major axis, which might be influenced by the presence of a Capaccioli et al (1989) have shown that the luminosity hidden disc (for details and for a test of this procedure, see distribution of the El NGC 3379 is very well reproduced by 1/4 Caon 1989). the sum of a r bulge and a non-negligible exponential disc: in other words, the E galaxy appears to differ from the edge- It is clear that, if a disc is present and seen close enough to on SO NGC 3115 only because of its smaller inclination of edge-on, the ellipticity curve £{r) corresponding to case (i) is the line-of-sight. expected to lie above that of case (ii). Hereafter this be- A more general way to investigate the link between haviour of e{r), illustrated by the results of Capaccioli et al ‘pointed’ E galaxies and lenticulars requires a comparative (1987; their fig. 13) relative to the standard SO NGC 3315, analysis of the isophotal properties of a large, homogeneous will be conventionally indicáted with the word ‘disky’. On the and unbiased sample of E and SO galaxies. A substantial contrary, if one galaxy consists of just one component, the allocation of telescope time has been made by the European ‘spheroid’, or if the ratio D/B is sub-threshold, and/or if the Southern Observatory in the framework of key-program- inclination of the disc is unfavourable, then the ellipticity ming (van der Laan 1988, 1989) to carry on this exploration curves obtained with by the two interpolation methods are for some 300 galaxies (see Bender et al 1989b). In this paper expected to coincide or to produce an ‘inverse effect’ [e(r) in we analyse a small but statistically significant sample of early- (i) staying below the ellipticity curve of case (ii) if the bulge is type galaxies, all belonging to the Virgo cluster (Caon 1989). ‘boxy’]. Table 1 summarizes the global trend of the ellipticity and 2 THE DATA Our sample consists of 33 E and non-barred SO galaxies Table 1. (a) Global geometrical properties: E galaxies. belonging to the Virgo cluster (classification according to Object Morph.Type BtBf <t<r £ (e) Shape AP.A. c 1 max Binggeli, Sandage & Tammann 1985). This is practically the [mag] [km s" ] [deg.] whole set of objects brighter than Z?T — 14 mag and contained IC 3653 E3 14.45 0.13 0.08 28 within the 60x6° fields of two deep UK Schmidt plates NGC 4434 E0/S0(0) 13.10 114 0.14 0.07 29 centred at Dec. = 11° 30', and RA=12h22m and NGC 4458 El 12.93 100 0.14 0.08 16 RA=12h36m, respectively. Only two objects were not NGC 4415 d:El,N 13.60 0.16 0.12 14 NGC 4374 El 9.65 296 0.18 0.09 49 included: NGC 4486 A, a dwarf galaxy whose image is NGC 4478 E2 12.30 144 0.20 0.16 41 seriously contaminated by an overlapping foreground star NGC 4168 E2 12.10 182 0.24 0.15 14 (Nieto & Prugniel 1987), and NGC 4482, for which no CCD IC 3468 E1,N 13.67 0.27 0.19 23 material was available to us. NGC 4464 E3 13.46 124 0.27 0.19 22 NGC 4551 E2 13.03 121 0.29 0.26 10 The inner galaxian regions (//B^22.5) were mapped NGC 4486 E0 9.45 335 0.35 0.15 48 photometrically using CCD material collected with the 2.2-m NGC 4621 E4 10.25 225 0.38 0.27 disky 3 ESO-MPI and 1.5-m ESO-Danish telescopes at La Silla; the NGC 4387 E5 13.03 111 0.42 0.35 boxy 14 outskirts (22.5 ^¡u ^28) rely instead upon the UK Schmidt NGC 4660 E3/S0(3) 12.08 196 0.46 0.30 disky 27 B NGC 4473 E5 11.03 197 0.47 0.40 disky 4 photographic material. The reduction procedure was the NGC 4436 dE6/dS0,N 14.04 0.58 0.51 disky ? 6 same as in Capaccioli et al (1988). In particular, the NGC 4564 E6 12.02 165 0.58 0.48 disky 17 modelling of the isophotes with ellipses - and the consequent NGC 4550 E7/S0(7) 12.66 84 0.70 0.66 disky 5 investigation on the occurrence of disc’s signatures - rested on the fitting algorithm of the Interactive Numerical Mapping Table 1.