NGC4636 - a Rich Globular Cluster System in Anormal Elliptical Galaxy 1 1 2 4 M
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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.