investigation deals with the thousands of not being too sensitive to gaps in the better knowledge of the period-Iuminos­ of variable stars discovered by Terzan data. The periodogramme yields an esti­ ity relations for various kinds of objects, (Terzan, 1982, Terzan, 1990 and ref. mate of the probability of false periodici­ and understanding of the relations be­ therein) on ESO Schmidt plates. We ty detection. Both methods lead to simi­ tween the shape of the light curves, have scanned 22 ESO Red Schmidt lar results in most cases, as exemplified mass loss and other properties of vari­ plates centred on the star 45 Oph., with by Figure 1. As a result, we have ob­ able objects. the MAMA measuring machine (e.g. tained light curves for 118 variables Guibert and Moreau, 1991). In this field, (80 % of the stars under investigation). we have selected 150 variables with 6.mR > 2. The plates were calibrated Developments in Progress Referen ces with 55 photometrie standards. This programme is being continued A. Vassiliadis, P.R. Wood, 1993, Astrophys. by a systematic search for variables in J. in press. Light Curves A. Terzan, 1982, Astron. Astrophys. Suppt. the whole 100-square-degree field. It 49,715. Two methods have been imple­ will require additional plates to cover the A. Terzan, 1990, The Messenger, 59, 41. mented to derive the light curves from whole range of periods, as weil as near J. Guibert, O. Moreau, 1991, The Messen­ the time sampling series. The first one infrared and radio measurements. In­ ger, 64, 69. (Renson, 1978), presents the advantage terpretation of these data will lead to a P. Renson, 1978, Astron. Astrophys. 63, 125.

NGC4636 - a Rich Globular Cluster System in aNormal Elliptical Galaxy 1 1 2 4 M. KISSLER , T RICHTLER , E. V HEL0 , E.K. GREBEL 3, S. WAGNER , M. CAPACCIOL/5 lSternwat1e der Universität Bonn, Germany; 20sservatorio Astronomico di Bologna, Italy; 3ESO, La Silla, Chile; 4Landessternwat1e Heidelberg, Germany; 50ipat1imento di Astronomia, Universita di Padova, Italy

1. What Drives the Investigations clusters, colour, and spatial distribution) for ellipticals in the rich Virgo cluster it is of Globular Cluster Systems of to those of their host galaxies, the study about 6. The interpretation of this ten­ Elliptical Galaxies? of extragalactic GCSs leads to a new dency is far from being unambiguous, level of problems, which could not be but the dependence on the environment In 1918 Shapley started to use the foreseen by the study of the galactic suggests that interaction between galactic globular clusters to explore the GCS alone. One of the systematic galaxies may playa role. spatial dimensions of our Milky Way. patterns emerging from all previous Other morphological properties, for Three quarters of a century later, we still studies is that the richness of a GCS example the relation between the spatial cannot claim to have understood the depends somehow on the environment distribution of clusters and the light pro­ properties of our globular cluster system of the host galaxy. M87 became the first file of the host galaxy, or the luminosity (GCS) in the context of the early evo­ example of a giant elliptical galaxy, 10­ function of globular clusters, also bear lution of the Galaxy. This is mainly cated in the dynamical centre of a rich potential information wh ich may finally due to the fact that there are severe galaxy cluster, found to be surrounded lead us to an understanding of the for­ problems in disentangling age and by a huge number of globular clusters mation history of GCSs and thus to gain metallicity, wh ich are the most signifi­ (Baum 1955, Sandage 1961). Later on, deeper insight in the formation and cant parameters in theories of galaxy very rich GCSs have been also discov­ structure of the host galaxies them­ evolution. ered in the central giant ellipticals NGC selves. This field is still in the phase Given these problems with weil ob­ 1399 in the Fornax cluster (e.g. Wagner where the investigation of individual gal­ servable clusters, what can we hope to et al. 1991), NGC 3311 in the Hydra axies provides interesting and useful learn from the study of faint images of cluster (Harris et al. 1983), and NGC contributions to the general knowledge. globular clusters around distant galax­ 4874 in the Centaurus cluster (Harris The elliptical galaxy NGC 4636 appears ies, where individual clusters appear 1987). Harris and van den Bergh (1981) to violate these general properties. NGC star-like and only integrated magnitudes introduced the "specific frequency" S 4636 is supposed to be a member of the can be determined? as a quantitative measure: S = N. Virgo cluster of galaxies though it is Research on GCSs over the past 20 1Q0.4(Mv+15), where N is the total number Iying weil outside the main body of the years showed that there is an amazing of globular clusters and Mv the absolute cluster, in a region where the density of variety in the morphologies of GCSs visual brightness of the host galaxy. The galaxies is quite low. Using photo­ (see Harris 1991 for a review). Most above mentioned galaxies have S val­ graphie plates, Hanes (1977) deter­ studies refer to elliptical galaxies, be­ ues between 12 and 18. mined a specific frequency of 9.9 ± 3 cause globular clusters in spirals are All galaxies that do not occupy central for the GCS of this galaxy, wh ich is a normally much less numerous and they positions in galaxy clusters possess surprisingly high value considering loca­ are difficult to identify due to the in­ poorer GCSs, but a relation with the tion and environment of NGC4636. We homogeneous background of bright environment persists: e.g., the normal reobserved NGC4636 using CCO imag­ stars or H I1 regions. Relating the proper­ elliptical galaxies in the sparse Fornax ing to investigate the properties of its ties of GCSs (such as total number of cluster have a mean S value of 3, while GCS with higher accuracy. 32 Figure 1: Six N7T frames combined to a single image of one hour Figure 2: The image shown above results from subtracting a model of effective exposure time. The frame shows a 7.5' x 7.5' field north­ the galaxy light, which has been obtained by a median filtering east of NGC 4636. technique. This is the frame on which we performed our search and photometry of globular cluster candidates. The crowding of stellar­ like images near the galaxy centre is striking.

2. Our NTT Observations 3. The Specific Frequency 1992), it may indicate a large total mass ofNGC4636 ofNGC4636 which in combination with a rich envi­ ronment causes effective accretion of NGC 4636 was observed at the ND To calculate the total number of matter. This matter could be for exam­ using EMMI and a 1k Thomson chip in globular clusters, we had to correct for pie in the form of dwarf galaxies. We July 1992, in the context of a collabora­ the incomplete spatial coverage of the recall in this respect that the dwarf tion between German and Italian groups galaxy and for the fact that we only see spheroidal in the constellation Fornax interested in the GCS problem. The see­ the brightest part of the luminosity func­ possesses 5 globular clusters. Adding ing during our run was only moderate tion (see below). After all corrections, we 500 dwarf spheroidals of the Fornax (1.5") and this prevented an investiga­ obtained a total number of 3500 ± 200 type to NGC4636 could account for tion of the luminosity function beyond its globular clusters around NGC 4636, 2500 clusters, but would increase its turnover (see section 4). but the data wh ich corresponds to a specific fre­ totalluminosity by only 0.1 mag. quality turned out to suffice for a study quency of $ = 7.3 ± 2.0, if (m - M) of the main properties of the GCS of = 31.2 and Mv = -21.7 ± 0.3 are NGC 4636. We combined six frames in adopted. As can be noticed, the error in 4. The Luminosity Function of the the V filter of 10 minutes each to a frame $ is quite substantial. This is the conse­ GCS: Gaussian or Power Law? of an effective exposure time of one quence of the fact that $ is very sensi­ hour (Figure 1). We modelIed the galaxy tive to the absolute brightness of the Apparently, globular cluster luminosi­ light by a median filtering technique and galaxy, and an error of 0.1 in (m - M) ty functions (GCLFs) exhibit a nearly uni­ subtracted it from the original frame to implies an error of 10% in $. versal shape, wh ich in the past has been obtain a homogeneous background. We This $ value of 7.3 ± 2.0 for NGC represented by Gaussians with disper­ then ran DAOPHOT on the resulting im­ 4636 is marginally below the one found sions of 1.3 mag and a peak or "turn­ age (Figure 2). About 1500 point sour­ by Hanes (1977), and slightly higher over" of Mv = -7.1 ± 0.3 mag, as de­ ces down to a magnitude of 24.5 were than the mean value for elliptical galax­ rived for several ellipticals in the Virgo identified. At the faint magnitude limit ies in the Virgo cluster. It is thus at the cluster (Secker & Harris 1993). For we reached a completeness in finding upper limit of what can be considered a NGC 4636, we find the turnover to be at point sources of 60%. "normal" specific frequency. However, Mv = 24.1. This corresponds to a dis­ We calibrated our data by simulating the question remains what distinguishes tance modulus of (m - M) = 31.2 ± 0.3, aperture photometry on a frame centred NGC 4636 for example from NGC 4697, which is in good agreement with previ­ on NGC 4636. The zeropoint has been another Virgo elliptical with $ = 3.5, but ously determined distances of NGC obtained from the photometry published almost twice as bright. There is much 4636 (31.1 ± 0.4 using the supernova by Poulain (1988). and we estimate a room for speculation. NGC 4636 has an 1993a, Capaccioli et al. 1990; zeropoint error of 0.05 mag. X-ray luminosity ten times as high as 30.96 ± 0.08 by surface brightness fluc­ To account for contamination with NGC 4697 (Roberts et al. 1991) and tuation, Tonry et al. 1990). background objects we statistically sub­ presumably a high dark matter content We believe that the choice of a Gaus­ tracted the objects found on a frame 7' (Saglia et al. 1992). While the X-ray flux sian for a GCLF does not bear any phys­ away from the galaxy centre, and con­ is not generally correlated with $, as the ical significance. In particular, the possi­ sidered the remaining objects as globu­ pair NGC 1399 and NGC 1404 in the ble existence of a universal turnover lar clusters belonging to NGC4636. Fornax cluster shows (Richtler et al. magnitude does not mean that there is a

33 2 NGC 4365 and 4649, Harris et al. 1991). while in poorer systems the GCSs seem to follow the light of the host galaxy cD (NGC 3379, Pritchet & van den Bergh, ., 1985; NGC 1404, Richtler et al. 1992). " Again, this can be understood as an 1.5 indication that rich cluster systems " might be partially formed through ", merger er accretion processes. -."

1 6. Concluding Remarks o 1 g This investigation of GCS of NGC 4636 demonstrates that more questions are open than answered. The . fundamental problem of what deter­ . 5 .. mines the total number of globular clus­ .. ters in a galaxy is still open, but the first .. ' steps to an answer point in directions that are closely connected with general problems of galaxy structure and evolu­ tion. Moreover, the use of globular clus­ ter luminosity functions as distance indi­ 1.5 1. 75 2 2,25 2.5 cators will perhaps play an important log(rad1us["]) role in the forthcoming VLT era. A huge potential of information about the phys­ Figure 3: This plot compares the profile of the sur1ace density ofglobular clusters with the light ics of galaxies and their stellar popula­ of NGC 4636. The abscissa is the logarithm of the galactocentric radius (arcsec) along the major axis. The ordinate is the logarithm of the number ofglobular clusters per square arcmin. tions awaits its exploitation. Open circles represent the globular cluster profile. The small dots are measurements of the galaxy light. The density profile ofglobular clusters is flatter than the galaxy light. This suggests that the globular clusters do not share the dynamical history of the field population.

References Baum, WA., 1955, PASp, 67, 328. Capaccioli, M., Cappellaro, E., Della Valle, characteristic mass among globular clusters decreases with increasing M., D'Onofrio, M., Rosino, L. and Turatto, clusters connected with this charac­ galactocentric distance in the same way M., 1990, ApJ 350, 110. Cohen, J., 1988, AJ 95, 682. teristic magnitude. Richtler (1993) and as the galaxy light itself. If it does, this Fischer, P., Hesser, J.E., Harris, H.C., Harris & Pudritz (1993) show that the would indicate that globular clusters Bothun, G.D., 1990, PASP 102, 5. mass distribution of globular clusters in and field population behave dynamically Hanes, DA, 1977, MemRAS 84,45. the Milky Way, M31, and some Virgo in the same way and probably date from Harris, WE., van den Bergh, S., 1981, AJ 86, galaxies is weil described by apower the same epoch of formation. For many 1627. law ~~ - m-ü where a is about -1.7. In galaxies this is not easy to investigate Harris, WE., Smith, M.G., Myra, E.S., 1983, the Milky Way and M31, i.e. in those due to the large scatter in the globular ApJ 272, 456. galaxies where the luminosity function cluster counts in small number statis­ Harris, WE., 1986, AJ 91, 822. can be followed weil beyond the turn­ tics. In case of rich systems like the one Harris, WE., 1987, ApJ Lett. 315, L29. Harris, WE., 1991, ARM 29,543. over magnitude, this power law con­ discussed, the statistics are more trust­ Harris, WE., Allwright, J.WB., Pritchet, C.J., tinues beyond the turnover magnitude worthy. We find the density profile of the van den Bergh, S., 1991, ApJS 76, 115. and has a cut-off at very low mas­ GCS of NGC 4636 to be flatter than the Harris, WE., Pudritz, A.E., 1993, AJ, in press. ses, probably due to the dissolution of galaxy halo light itself. If we represent Mandushev, G., Spassova, N., Staneva, A., less massive clusters. Concerning NGC both pofiles by apower law Q - r-(t., 1991, A & A 252, 94. 4636, we transformed the luminosities where Q is the surface density of globu­ Poulain, P., 1988, A& AS 72, 215. of globular cluster candidates to lar clusters and r the projected ga- Pritchet, C.J., van den Bergh, S., 1985, AJ masses using the relation given by lactocentric distance, we get 90, 2027. Mandushev et al. (1991), and calculated a=-1.0±0.1 for the GCS and Richtler, 1., Grebel, E.K., Domgörgen, H., Hilker, M., Kissler, M., 1992, A&A 264,25. the mass distribution of the globular a = -1.5 ± 0.1 for the galaxy light. The Richtler, 1., 1993, 11 '11 Santa Cruz Sommer cluster in NGC 4636. We found b = -1.9 density profile is plotted in Figure 3, Workshop proceedings, The globular clus­ which fits weil to the Milky Way system. where the ordinate is the logarithm of ter-galaxy connection, in press. The existence of a power-Iaw (wh ich is the number of globular clusters per Sandage, A., 1961 The Hubble Atlas of equivalent to the absence of a charac­ square arcminute. The abscissa is the Galaxies, Carnegie Inst. Washington Publ. teristic mass scale) and its apparently logarithm of the galactocentric radius in N.618. universal shape are remarkable fea­ arcseconds along the major axis. The Saglia, R.P., Bertini, G., Stiavelli, M. 1992, tures. A satisfactory explanation has yet small dots are measurements of the ApJ, 384, 433. to be found. galaxy light with an arbitrary offset. The Shapley, H., 1918, ApJ 48,89. Secker, J., Harris, WE., 1993, AJ 105, 1358. four open circles represent the globular Tonry, J.L., Ajhar, E.A., Luppino, GA, 1990, cluster density profile. Interestingly, this 5. The Density Profile AJ 100, 1416. has also been found in other rich globu­ Tu 11 y, R.B., Shaya, E.J., 1984, ApJ 281, 31. A further interesting question is lar cluster systems (NGC 1399, Wagner Wagner, S., Richtler, T., Hoop, U., 1991, whether the surface density of globular et al. 1991; NGC 4472, Cohen 1988; A&A241,399.

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