The Unusual Barred SO Galaxy NGC 4546G. GALLETTA, Institute of Astronomy, University of Padua, Italy

Figure 2: The same as Figure 1, but for thespectrum of the major axis (centre) and twooffset spectra at the same P. A. but at 5"(430 pe) SE and NW from the nucieus. Theextension of the phenomenon of counter ro­tation between stars and gas appears withsimilar behaviour.

Serego Alighieri on the night of May 6 atlower dispersion (5.16 Apixel-I, g,rating16) in the interval n 4730-7300 A.

In addition to these spectroscopicdata, V, Ha and Gunn I frames wererecorded with the CCO of the 1.5-mOanish telescope on the nights of May 6and 7, 1986. The image of the galaxy isshown in Figure 3, from a 1m (top) and a15m (bottom) V exposure. From theseimages, it appears that the galaxyshows a disk strongly inclined to theplane of the sky, as stated by the pre­sence in the shorter exposure of Fi­gure 3 of two spindies in the outerisophotes. The bar, whose presence hasbeen mentioned by de Vaucouleurs etal. (1976), appears just like an irregulari­ty of the isophotes roughly at 45° NE ofthe major axis and is completely em­bedded in the galaxy body. A faint ab­sorption, probably a dust lane, appearsat P.A. = 248°, ten degrees SW of themajor axis. In agreement with its relativecloseness, 17.8 Mpc according to thegroup distance (Virgo V, de Vau­couleurs, 1975), in the deeper exposure,NGC 4546 reveals in its halo the pre­sence of a number of faint, almost stellarimages, probably globular clusters.Some of them are visible in Figure 3(bottom).

In the spectra of Figure 1, to make thelines more evident, the continuum hasbeen partially subtracted in all the im­ages, showing the complex texture ofthe spectral lines. The [0111] emissionlines are visible, but the faintest one,A4959, is too faint to give a measurablesignal, also because of the many ab-

NGC 4546P.A.= 258 0

Observations and Data Analysis

NGC 4546 has been studied in thenights of May 4 and 5, 1986, with theeco and the Boiler and Chivens spec­trograph of the 2.2-m ESO-MPI tele­scope at six different position angles.The exposure times were ranging from90m for minor axis and intermediate an­gles spectra to 120m for the major axisspectrum, at P.A. = 258° (Figure 1).The dispersion used was 1.77 Npixel(grating 10, second order) while thescale perpendicular to the dispersionwas 1.808 arcsec pix-1 or about 59 arc­sec mm- 1

• The spectral interval exploredwas ranging from 4900 to 5700 A. Inorder to study the extension of the gasand its motions outside the main galac­tic plane, two 60m spectra were securedoffsetting the centre of the slit by 5/1 onboth sides of the nucleus (NW and SE,Figure 2) and perpendicularly to the ma­jor axis. An additional major axis spec­trum of 15m exposure was kindly takenfor us with the same instrument by S. di

spectroscoplc and photometric data forfour galaxies. The goals of this pro­gramme are to reach a good knowledgeof the kinematical properties of diskgalaxies in the presence of a bar and anappropriate analysis of the stellar orbitswithin the bars, considered as typicaltriaxial structures. As a first result, wefound the existence of somepeculiarities in the velocity field of thefirst SBO studied, NGC 6684: an ovaldistortion of the stellar disk and the pre­sence of elongated stellar orbits withinthe bar. They confirm the existence ofan oval distortion suggested by thestudy of some barred disk galaxies (Kor­mendy, 1979, 1983) and invoked also toexplain and stabilize the structure of"spindie" galaxies, a type of 80's with aluminous and extended gas disk per­pendicular to its symmetry plane(Schweizer, 1983).

The selection of NGC 4546 as a can­didate for spectroscopic and photomet­ric studies derives from several charac­teristics: (i) it is quite a luminous galaxy,with B magnitude 11.3; (ii) its ellipticity is0.52, indicating an almost edge-on sys­tem where it is possible to study the baralong one of its principal planes ; (iii) itshows the presence of the [Oll] lines atn 3727-29 A (Humason et al. 1956);(iv) it possesses an HI rotating disk witha velocity width for the line of 350 km/s(Bieging, 1978). All these propertiesmake this galaxy an interesting subjectof study.

Introduction

At the beginning of May 1986, duringan observing run concerning the stellarkinematics in barred SO galaxies, weobserved the SBO NGC 4546, an almostedge-on disk system, finding the pre­sence of a very peculiar phenomenon:within the galaxy, the gas clouds aremoving with similar velocities but inopposite direction with respect to thestars. This fact, present also at differentposition angles, makes this galaxy aunique case of disk system with large­scale, retrograde gaseous motions withrespect to the stars.

The observations of NGC 4546 arepart of a larger programme of study forthe structure and the kinematics ofSBO's, programme started at ESO in1984 and including at the present time

NGC 4546

Figure 1: Composition of the spectra ofNGC 4546 at six different position angles. Tomake the lines more evident, the continuumhas been partially subtracted in all the im­ages. The emission lines are represented by[OIllJ features (Ieft of the spectra), with onlyA5007 clearly visible. The intense absorptionlines visible in the portion of the spectrashown here are Mg I A 5175.4, near the5200.0 [NIJ sky lines, and Ca+Fe A5269.0.Note that nearer to the major axis (P. A. =

258°), the emission and the absorption linestend to be inclined because of the rotation inopposite directions. This fact characterizesthe exceptionality of the kinematics of thisstellar system.

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NGC4546 attempt has been made to add togetherscan lines, in order to increase the sig­nal-to-noise ratio. This process was re­peated for each spectrum and for eachscan line using a batch IHAP procedure.A sampie of the results is shown inFigures 4 and 5, where the rotationcurves measured from emission and ab­sorption lines along the major axis of thegalaxy are plotted. A more completereduction of the velocity field will beperformed during the next months usingthe technique of the Fourier Quotientwhich produces also the velocity disper­sion of the stars.

10"

Figure 3: Two images of the galaxy, from a 1m (top) and a 15m (bottom) Vexposure. The barappears just like an irregularity of the isophotes roughly at 45° NE of the major axis and iscompletely embedded in the galaxy body. A faint absorption, probably a dust lane, is visibleten degrees SW of the major axis. In the deeper exposure, NGC 4546 reveals in its halo thepresence of a number of faint, almost stellar images, probably globular clusters.

sorption features that are intersecting itwith opposite inclination. The spectralfeatures of MG I A5175.4, near the5200.0 [N I] sky lines, Ca+ Fe A5269.0and Fe A5331.5 are visible in absorptionand are the predominant lines in thespectra. From the low dispersion spec­trum it is possible to see also the pre-

sence of the emission lines Ha, [N 11]n6548-84 and [SII] n6717-31.

All the spectra were preliminarily re­duced with the IHAP package at thePadova HP computer centre and ana­Iysed kinematically by interpolatingGaussian functions to the more intenseemission and absorption lines. No

Discussion

In most of the spectra (Figure 1). thegas lines appear inclined because of therotation in a direction opposite to that ofthe stellar lines, indicating opposite di­rections of motion. This fact charac­terizes the exceptionality of the kinema­tics of this stellar system. In fact, at thepresent time, no SO or spiral galaxy isknown to possess this characteristic.The same behaviour is present in thespectra taken parallel to the major axisbut with a ± 5" offset with respect to thenucleus (Figure 2). At the assumed dis­tance of 17.8 Mpc, these offsets corre­spond to about ± 430 pc, showing thatthe phenomenon is quite extended.

A look at Figure 1 shows that alongthe minor axis (P. A. = 168°) the starsexhibit almost no rotation, as indicatedby the lack of inclination of the absorp­tion lines. At the same position angle, onthe contrary, the gas presents residualmotions that decrease until, at 135°,about along the minor axis of the bar,the emission lines tend to straighten.This fact indicates that the lines ofnodes of the planes where the gas andthe stars are moving are not at the sameposition angle. This kinematical differ­ence is confirmed by the analysis of thevelocity gradients. Their values for thestellar motions show a cosine decreasewith the P.A. reaching a maximum ofabout 12 km S-1 arcsec-1 at P.A. = 80°,very close to the major axis of thegalaxy, in agreement with the expecta­tion for circular orbits on the galacticplane. Peculiar orbits appear in the innerpart of the system. The study of the gasrises more complex problems: in fact,along the major axis, when the slit cros­ses the bar, a dip of the rotation curvestowards lower velocity is visible. Thischaracteristic disappears in the offsetspectrum at 5" SE, indicating gas flow orelongated orbits within the bar. Thesame behaviour is presented by theanalysis of the velocity gradients ofemission lines, which follow an approxi­mate cosine law with line of the nodes at225° (near the bar major axis) and a

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Figure 5: The same as Figure 4, but for the stars. The stellar fines used are labelled withdifferent symbols.

Figure 4: Rotation curve for the gas along the major axis, from a Gaussian interpolation of thespectralline [0111} ;" 5007.

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Applications forObserving Time at La SillaPeriod 39 (ApriI1-0ctober 1,1987)

Please do not forget that your propo­sais should reach the Section VisitingAstronomers before October 15,1986.

axis of the galaxy NGC 7097, classifiedE4, Caldwell et al. (1986) have mea­sured a gas rotation of 200 km/ssuperimposed on a slow stellar rotationof a few tens of km/s in opposite direc­tion.

ReferencesBieging, J. H., 1978, Astron. and Astrophys.

64,26.Caldwell, N., Kirshner, R. P., Richstone, 0.,

1986, Astrophys. J., 305, 136.Contopoulos, G. and Papayannopoulos, Th.,

1980, Astron. and Astrophys., 92, 33.de Vaucouleurs, G., 1975, in Stars and Stellar

Systems, The University of Chicago Press,Vol. IX, p. 584.

de Vaucouleurs, G., de Vaucouleurs, A. andCorwin, H.G.Jr. 1976, RC2, Second Re­ference Cat. ofbright galaxies, Austin, Uni­vers. 01 Texas Press.

de Zeeuw, T. and Merrit, 0., 1983, Astrophys.J., 267, 571.

Freeman, K.C., 1966, Mon. Not. of RoyalAstr. Soc., 134, 1.

Humason, M. L., MayalI, N. V. and Sandage,A. R., 1956, Astron. J. 61, 101.

Kormendy, J., 1979, Astrophys.J., 227, 714.Kormendy, J., 1983, Astrophys. J., 275, 529.Oort, J. H., 1965, in Stars and Stellar Sys-

tems, The University of Chicago Press,Vol. V, p. 486.

Schweizer, F., 1983, lAU Symp. nO.100,p.319.

radius

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gas and stars with common origin, driv­ing it in two opposite directions of rota­tion. A third and alternative hypothesis isthat the gas has not the same origin asthe stars but is the result of a more orless recent acquisition, and comes fromaretrograde collision with a dust cloudor a gas-rich dwarf galaxy. The discov­ery of hot stars or H I bridges with otherelose stellar systems would give an in­teresting check of this hypothesis.

Concluding this short note, we wouldlike to draw the attention to anothercase of counter-rotation known in theliterature, although the nature of thissystem is quite different fram the diskgalaxy considered here: along the major

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maximum of 12.5 km S-1 arcsec- 1 gra­dient, with the exception of the rotationcurve measured at P.A. = 213°, whoseinner slope of about 5 km S-1 arcsec- 1 isweil down the expected value off15 km S-1 arcsec-1.

Then, if all these motions are evolvingalong a plane, the plane of the gas is notcoplanar with that of the stars and thegas existing within the galaxy shows aradial flow along the bar which couldexplain the asymmetries observed. Inaddition, since the maximum extensionof the gas is observed near the majoraxis, about 30° from its direction ofmaximum velocity gradient, the gas it­self appears confined within a more orless asymmetrie disk, of projected di­mension 1.7 x 8.2 kpc, not aligned withthe stellar disk of the galaxy neither withthe bar.

Where did this gas come from? Onepossible hypothesis is that the gas iscirculating in one of the retrogradefamilies of orbits possible in triaxial sys­tems (de Zeeuw and Merrit 1983) or thebars (Freeman 1966, Contopoulos andPapayannopoulos 1980). It is expectedin this case that, since the gas cloudsare moving together with the stars in anarrow tube, this prevents their collapseby collision or dynamical friction. Butthis phenomenon involves confined re­gions of space, contrary to that ob­served. The same problem is present fora second possibility: that we are observ­ing gas confined in a retrograde portionof a "hot" velocity field, similar to thatfound for some globular clusters in ourgalactic halo (Oort 1965). With the ex­ception of the two above-mentionedcases, we find it difficult to imagine amechanism that discriminates between

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