A&A 405, 487–497 (2002) Astronomy DOI: 10.1051/0004-6361:20030598 & c ESO 2003 Astrophysics Extragalactic globular clusters in the near infrared III. NGC 5846 and NGC 7192?;?? Quantifying the age distribution of sub-populations M. Hempel1,M.Hilker2, M. Kissler-Patig1,T.H.Puzia3, D. Minniti4, and P. Goudfrooij5 1 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany e-mail: [email protected] 2 Sternwarte der Universit¨at Bonn, Auf dem H¨ugel 71, 53121 Bonn, Germany e-mail: [email protected] 3 Sternwarte der Ludwigs-Maximilians-Universit¨at, Scheinerstr. 1, 81679 M¨unchen, Germany e-mail: [email protected] 4 Departamento de Astronom´ıa y Astrof´ısica, P. Universidad Cat´olica, Casilla 306, Santiago 22, Chile e-mail: [email protected] 5 Space Telescope Science Institute, Instrument Division, 3700 San Martin Drive, Baltimore MD 21218, USA e-mail: [email protected] Received 10 February 2003 / Accepted 18 April 2003 Abstract. In this third paper of our series on near-IR and optical photometry of globular cluster systems in early-type galaxies we concentrate on the photometric results for NGC 5846 and NGC 7192, two group ellipticals, and on a first comparison between the globular cluster systems investigated so far. In NGC 5846 the colour-colour diagram shows clear bi-modality in (V K), which is confirmed by a KMM test. The mean colour of both peaks were estimated to be (V K)blue = 2:57 0:06 and− (V K) = 3:18 0:06. The situation in NGC 7192 is different, in that the colour-colour diagram− gives no evidence± for − red ± a distinct second population of globular clusters. Using simulated colour distributions of globular cluster systems, we make a first step in quantifying the cumulative age distribution in globular cluster systems. Also here the result for NGC 5846 leads us to the conclusion that its metal-rich globular cluster population contains two globular cluster populations which differ in age by several Gyr. The age structure for NGC 7192 shows instead strong similarity with a single-age population. Key words. galaxies: star clusters – galaxies: individual: NGC 5846, NGC 7192 1. Introduction Starting with Peebles & Dicke (1968) who assumed globular clusters to be the first objects formed in the early universe, As shown by many studies during the last decade (Zepf the number of possible formation scenarios increased drasti- & Ashman 1993; Ashman & Zepf 1998; Kundu & cally since then. One of the main issues is to explain the multi- Whitmore 2001a,b; Larsen et al. 2001; Kissler-Patig et al. modality found in colour-colour diagrams of globular cluster 2002) globular cluster systems are a very powerful tool in systems. In general it is agreed that different globular cluster galaxy formation and evolution studies. Although there is a populations are produced during strong star formation events, wide agreement about the existence of sub-populations in clus- but the nature of these events remains under debate. Besides ter systems regarding their metallicity and their age, the dis- the merger scenario, favoured by Ashman & Zepf (1993) (see cussion about the origin of those populations is still ongoing. also Ashman & Zepf 1992; Whitmore et al. 1993; Whitmore & Send offprint requests to: M. Hempel, e-mail: [email protected] Schweizer 1995; Schweizer et al. 1996; Kissler-Patig 2000), ? Based on observations at the Very Large Telescope of the there is a number of alternatives, i.e. the accretion scenario European Southern Observatory, Chile (Program 63.N-0287). (Cˆot´e et al. 1998; Cˆot´e et al. 2002; Hilker et al. 1999) or the ?? Based on observations made with the NASA/ESA Hubble monolithic collapse (Forbes et al. 1997a; Kissler-Patig et al. Space Telescope, obtained from the data archive at the Space 1998, and references therein). From the photometric point of Telescope Science Institute. STScI is operated by the association of view it has been shown that the multi-modality of the colour Universities for Research in Astronomy, Inc. under the NASA con- distribution is a common feature of globular cluster systems tract NAS 5-26555. 488 M. Hempel et al.: Extragalactic globular clusters in the near infrared. III. (Gebhardt & Kissler-Patig 1999). In particular, the existence of Table 1. General information about the host galaxies NGC 5846 a blue, old and metal-poor cluster population (Ashman & Bird and NGC 7192. The references are (1): de Vaucouleurs et al. 1991, 1993; Burgarella et al. 2001) is a general feature. Regarding (2): Schlegel et al. 1998, (3): Buta & Williams 1995, (4): Frogel et al. the red sub-population the almost only consensus which has 1978, (5): Tonry et al. 2001. been reached so far, is about the large varieties between dif- ferent cluster systems. This includes the possible existence of Property NGC 5846 NGC 7192 Reference sub-populations within the red population which is interpreted RA (J2000) 15h06m29s 22h06m50s (1) as a sign of different star formation events in later stages of the o o Dec (J2000) +01 3602500 64 1805700 (1) − galaxy evolution. BT;0 10.87 12.19 (1) The main problem in specifying and dating different star EB V 0.055 0.034 (2) formation events arises from the age-metallicity degeneracy of (B− V) 0.96 0.92 (1) − o the most commonly used optical colours. So far the bulk of (V I)eff;o 1:28 0:01 1:24 0:01 (3) − ± ± high-quality photometric investigations have been performed (V K)eff;o 3:51 0:01 (4) (m − M) 31:98 ± 0:20 32:89 0:32 (5) using Hubble Space Telescope (HST) in the optical wavelength − V ± ± M 22:07 0:20 21:62 0:35 (1), (5) regime (Forbes et al. 1998; Kundu & Whitmore 1998; Gebhardt V − ± − ± & Kissler-Patig 1999; Larsen et al. 2001). It has been shown (Minniti et al. 1996; Kissler-Patig 2000; Kissler-Patig et al. 2002 (hereafter cited as Paper I); Puzia et al. 2002 (hereafter the determination of their age structure and a first comparison Paper II)) that combining optical and near-infrared data is a of various globular cluster systems. The present paper is or- more promising method to separate age and metallicity effects ganised as follows. In Sect. 2 the observations and the data re- and to access the relative ages of the globular cluster popula- duction procedures are described. Section 3 contains the main tions. The method relies on a sampling effect, where the V-band results of the observations and Sect. 4 describes our approach is dominated by stars near the turn-off (TO) region, whereas the towards quantifying the age structure in globular cluster sys- main contribution to the K-band is from giant branch stars (Yi tems and the results for both systems. In Sect. 5 we will give et al. 2001). Whereas the TO is dominated by age effects, the an outlook on the upcoming work. giant branch shows a high sensitivity to metallicity (Saviane et al. 2000). This results in a similar dependence of (V I) and (V K) on the age of the clusters, but a higher sensitivity− 2. Observations and data reduction − of (V K) to the metallicity. 2.1. VLT/ISAAC near–infrared data In− the previous two papers of this series (Papers I and II), a systematic survey of globular cluster systems of E and The Ks band exposures for both galaxies have been taken in ser- S0 galaxies in the combined optical and near-infrared wave- vice mode (ESO program 63.N–0287) with the Near-Infrared length range (using the V, I,andKs bands) has been started. In Spectrometer And Array Camera (ISAAC) attached to the Unit Paper I, we compared the globular cluster systems of two ellip- Telescope 1 (Antu) of the European Southern Observatory’s ticals in the Virgo cluster, namely the giant central galaxy M 87 Very Large Telescope (VLT). The field-of-view of ISAAC’s and an intermediate-luminosity galaxy NGC 4478. We found Rockwell infrared array is 2:50 2:50, with a pixel scale × that in those cases, the (V K) colour distribution yielded of 0.14700/pixel. All data were obtained in April and June 1999 roughly consistent conclusions− relative to those derived from in different nights under varying photometric and seeing con- the (V I) colours measured by HST. In Paper II however, the ditions. Standard star observations revealed that the conditions (V K)− colours of globular clusters in NGC 4365 led us to pos- were not photometric during all nights and an adjustment (de- tulate− the existence of a significant population of intermediate- scribed below) was necessary to align all nights onto a photo- age ( 2–6 Gyr old), metal-rich globular clusters, which was metric system. As in Papers I and II we will always refer to the ∼ not revealed by the (V I) colours. This important result has Ks filter as K. recently been confirmed− by deep spectroscopy (Larsen et al. The NGC 5846 data were obtained during the nights of 2003), adding credibility to the results derived from our opti- April 6th, 8th, and 9th 1999 while the NGC 7192 data were cal + near-infrared imaging program. taken during the nights of June 1st, 2nd, 21st, and 22nd. The One of the aims of this series is to study the the globular observing strategy for NGC 5846 data was the following: cluster systems of galaxies in different environments, e.g. gi- 10 10 s object + 5 (2 10 s) sky+10 10 s object. The one for ant ellipticals in centres of clusters as well as rather isolated NGC× 7192: 10 10 s× object× + 2 (6 10× s) sky + 10 10 s object.