The Colors of Globular ClustersAuthor(s): Sidney van den BerghSource: Publications of the Astronomical Society of the Pacific, Vol. 113, No. 780 (February2001), pp. 154-157Published by: The University of Chicago Press on behalf of the Astronomical Society of the PacificStable URL: http://www.jstor.org/stable/10.1086/318609 .

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154

Publications of the Astronomical Society of the Pacific, 113:154–157, 2001 Februaryq 2001. The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A.

The Colors of Globular Clusters

Sidney van den Bergh

Dominion Astrophysical Observatory, National Research Council of Canada, 5071 West Saanich Road,Victoria, BC V9E 2E7, Canada; sidney.vandenbergh@nrc.ca

Received 2000 August 24; accepted 2000 October 13

ABSTRACT. A compilation has been made of available data on the ratio of the number of metal-rich([Fe/H ) to metal-poor ([Fe/H ) clusters in various globular cluster systems. Among early-type] 1 21.0 ] ! 21.0galaxies of types E, E/S0, and S0, the ratio of blue to red globular clusters is found to vary by almost 2 ordersof magnitude. The data suggest that cD galaxies have the widest range of evolutionary histories. The fractionof metal-rich red clusters is largest among early-type galaxies and appears to decrease toward later Hubble types.

1. INTRODUCTION

In a recent review on extragalactic systems of globular clus-ters (van den Bergh 2000a), it was found that such systemsexhibit a wide range of evolutionary histories. Differences areobserved in the flattening and radial metallicity gradients ofsuch swarms of globular clusters. Furthermore, the metallicitydistributions of some of these systems are frequently complex,with two (or even more) maxima being observed in the me-tallicity distribution of some of these globular clusters systems(Ashman & Zepf 1998; Gebhardt & Kissler-Patig 1999; Neilsen& Tsvetanov 1999). Perhaps the simplest way of describingthese systems is in terms of the number of blue metal-poornb

clusters with [Fe/H and the number of red metal-] ! 21.0 nr

rich clusters with [Fe/H . In our own Milky Way system] 1 21.0such a separation at [Fe/H reveals a clear dichotomy] p 21.0between blue halo clusters with [Fe/H and red] ! 21.0bulge1thick disk clusters having [Fe/H . Forbes &] 1 21.0Forte (2000) have found an intimate link between the red glob-ular clusters and the properties of their host galaxy. On theother hand, these authors find that the mean colors of the blueglobular clusters are unrelated to those of their host galaxies.

2. DATA ON GLOBULAR CLUSTER COLORS

The results of an exhausting (but perhaps not exhaustive)literature search for published data on and are compiledn nb r

in Table 1. In many cases the quoted numbers are somewhatuncertain because they had to be read off from small-scalefigures given in the original publications. In cases where onlycluster colors were given, the reddening-corrected colors wereconverted to [Fe/H] using the transformations given by Kissler-Patig et al. (1998). The adopted values of the specific globularcluster frequency are from the article cited in the table or, failingthat, from Harris (1991). The “local” values of the specificglobular cluster frequency S given in Kundu (1999) have beenplaced in square brackets. Parent galaxy types are from a variety

of sources, with preference given to those with classificationsbased on large-scale photographic plates that have been pub-lished by Sandage & Tammann (1981).

3. CAVEATS

The values of that are listed in Table 1 are subjectlog (n /n )r b

to two sources of bias. One of these arises from the fact thatthe limiting magnitudes may differ for the blue and red images.An example of this effect is seen in Figure 6 of Rhode & Zepf(2000), which shows that the relative shallowness of their Bimages results in the exclusion of the reddest globular clustersfrom their survey.

A second possible source of bias arises from the fact that, inmany cluster systems, the blue cluster subsystem is larger thanthe red cluster system. A sample based on a small-diameter imagewill therefore be biased against blue clusters, which mainly occurat large radii. A good place to check the possible effect of thisbias is the cluster NGC 4472, for which Rhode & Zepf give Band R photometry of images over a wide range of radii. Fromthe color transformations given by Forbes & Forte (2000) andthe color-metallicity relation of Couture, Harris, & Allwright(1990), it is found that globular clusters with [Fe/H] p 21.0have (B2 . From the plot of B2R color versus radiusR) p 1.260

shown in Figure 8 of Rhode & Zepf (2000), it is found that thelogarithm of the ratio between the number of blue and red clustersis 20.15 for , 20.31 for , 20.29 for , and′ ′ ′R ! 5 R ! 10 R ! 1520.29 for all clusters with . This suggests that the influ-′R ! 20ence of field size on is probably not much larger thanlog (n /n )r b

the statistical scatter that results from the small number statisticsof globular cluster colors and the systematic differences thatoccur between different observers. In particular, it is noted thatthe value of obtained from the data bylog (n /n ) p 20.29r b

Rhode & Zepf is smaller than the value 10.04 obtained fromthe data of Gebhardt & Kissler-Patig (1999). A final caveat isthat the colors of globular clusters, and hence the values of

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COLORS OF GLOBULAR CLUSTERS 155

TABLE 1Data on the Number of Metal-rich and Metal-poor Globular Clusters

Galaxy Type nr nb log (n /n )r b Sa Reference

NGC 205 . . . . . . . . . . . . . S0/E5 0 6 !20.8: 1.6 5 0.7 1NGC 224 . . . . . . . . . . . . . Sb 54 122 20.35 1.6 5 0.4 2NGC 253 . . . . . . . . . . . . . Sc 7 5 10.15 0.6 3NGC 474 . . . . . . . . . . . . . S0/Sa 10 24 20.38 [0.5 5 0.2] 4NGC 524 . . . . . . . . . . . . . S0/Sa 74 37 10.30 5.7 5 2.0 4NGC 584 . . . . . . . . . . . . . S0 36 31 10.06 1.3 5 0.5 5NGC 596 . . . . . . . . . . . . . E0 21 10 10.32 … 5NGC 821 . . . . . . . . . . . . . E6 19 36 20.28 1.9 5 0.9 5NGC 1023 . . . . . . . . . . . SB0 66 82 20.09 … 5NGC 1201 . . . . . . . . . . . S0 34 43 20.10 [0.9 5 0.4] 4NGC 1316 . . . . . . . . . . . Sap 33 153 20.67 … 5NGC 1332 . . . . . . . . . . . S0 113 90 10.10 [1.7 5 0.4] 4NGC 1375 . . . . . . . . . . . S0p 3 8 20.43 [0.5 5 0.4] 4NGC 1380 . . . . . . . . . . . S0/Sa 69 164 20.38 … 6NGC 1389 . . . . . . . . . . . S0/SB0 7 12 20.23 [0.4 5 0.2] 4NGC 1399 . . . . . . . . . . . E1/cD 183 227 20.09 12 5 4 5NGC 1400 . . . . . . . . . . . E1/S0 77 80 20.02 5.4 5 2.0 4NGC 1404 . . . . . . . . . . . E2 53 85 20.21 0.9 5 0.4 5NGC 1426 . . . . . . . . . . . E4 37 37 0.00 … 5NGC 1427 . . . . . . . . . . . E5 57 95 20.22 [3.9 5 2.2] 4NGC 1439 . . . . . . . . . . . E1 31 65 20.32 [2.1 5 1.7] 4NGC 1553 . . . . . . . . . . . S0p 30 40 20.12 1.8 5 0.6 4NGC 1581 . . . . . . . . . . . S0 6 2 10.48 [0.2 5 0.8] 4NGC 1700 . . . . . . . . . . . E3 17 10 10.23 … 5NGC 2300 . . . . . . . . . . . E3 38 44 20.06 … 5NGC 2434 . . . . . . . . . . . E0 28 114 20.61 … 5NGC 2768 . . . . . . . . . . . S0 43 74 20.24 [1.0 5 0.2] 4NGC 2778 . . . . . . . . . . . E 8 9 20.05 … 5NGC 2902 . . . . . . . . . . . S0 12 3 10.60 [0.2 5 0.3] 4NGC 3056 . . . . . . . . . . . S0 11 13 20.07 [0.6 5 0.6] 4NGC 3115 . . . . . . . . . . . S0 84 54 10.19 2.0 5 1.0 4NGC 3115 DW . . . . . . dE1 10 27 20.43 4.9 5 1.9 7NGC 3156 . . . . . . . . . . . E5: 4 8 20.30 [0.3 5 0.3] 4NGC 3311 . . . . . . . . . . . cD 241 594 20.38 15 5 6 8NGC 3377 . . . . . . . . . . . E5 44 42 10.02 2.6 5 0.6 5NGC 3379 . . . . . . . . . . . E1 31 21 10.17 1.3 5 0.7 5NGC 3384 . . . . . . . . . . . SB0 15 24 20.20 1.1 5 0.5 5NGC 3414 . . . . . . . . . . . S0 68 49 10.14 [1.3 5 0.4] 4NGC 3489 . . . . . . . . . . . S0/Sa 10 35 20.54 [1.4 5 0.5] 4NGC 3585 . . . . . . . . . . . E7/S0 43 27 10.20 … 5NGC 3599 . . . . . . . . . . . S0 5 16 20.51 [1.2 5 0.5] 4NGC 3608 . . . . . . . . . . . E1 25 78 20.49 [2.3 5 1.3] 4NGC 3610 . . . . . . . . . . . E5 18 11 10.21 1.1 5 0.5 5NGC 3640 . . . . . . . . . . . E2 24 24 0.00 … 5NGC 3870 . . . . . . . . . . . S0? 2 6 20.48 [0.8 5 0.9] 4NGC 3923 . . . . . . . . . . . E4/S0 111 37 10.48 … 9NGC 4125 . . . . . . . . . . . E6 24 44 20.26 … 5NGC 4150 . . . . . . . . . . . S0/Sa 2 8 20.60 [1.7 5 1.4] 4NGC 4192 . . . . . . . . . . . Sb 14 34 20.39 … 5NGC 4203 . . . . . . . . . . . S0 32 71 20.35 [1.4 5 0.4] 4NGC 4278 . . . . . . . . . . . E1 105 167 20.20 6.1 5 1.5 4NGC 4291 . . . . . . . . . . . E3 21 50 20.38 … 5NGC 4343 . . . . . . . . . . . … 9 16 20.25 … 5NGC 4365 . . . . . . . . . . . E3 224 185 10.08 2.5 5 0.9 5NGC 4374 . . . . . . . . . . . E1 39 83 20.33 5.6 5 1.3 5NGC 4379 . . . . . . . . . . . S0 6 16 20.43 [0.5 5 0.3] 4NGC 4406 . . . . . . . . . . . S0/E3 51 80 20.20 5.4 5 1.3 5NGC 4431 . . . . . . . . . . . S0 2 8 20.60 [0.5 5 0.5] 4NGC 4450 . . . . . . . . . . . Sab p 8 21 20.42 1.6 5 0.8 5NGC 4458 . . . . . . . . . . . E0 16 17 20.03 2.3 5 2.7 5NGC 4459 . . . . . . . . . . . S0 30 44 20.17 [0.8 5 0.2] 4NGC 4472 . . . . . . . . . . . E1/S0 254 232 10.04 5.0 5 1.4 5NGC 4473 . . . . . . . . . . . E5 42 70 20.22 1.5 5 0.5 5

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156 VAN DEN BERGH

2001 PASP, 113:154–157

TABLE 1 (Continued)

Galaxy Type nr nb log (n /n )r b Sa Reference

NGC 4478 . . . . . . . . . . . E2 24 26 20.03 … 5NGC 4486 . . . . . . . . . . . E0 613 230 10.43 9.6 5 2.3 10NGC 4486B . . . . . . . . . E0 124 94 10.12 … 5NGC 4494 . . . . . . . . . . . E1 42 85 20.31 [1.0 5 0.4] 4NGC 4526 . . . . . . . . . . . S0 35 40 20.06 6.6 5 1.6 5NGC 4536 . . . . . . . . . . . Sc 21 65b 20.49 … 5NGC 4550 . . . . . . . . . . . E7/S0 17 20 20.07 … 5NGC 4552 . . . . . . . . . . . S0 80 91 20.06 1.3 5 0.4 5NGC 4565 . . . . . . . . . . . Sb 2 38 21.28 0.6 11NGC 4569 . . . . . . . . . . . Sab 14 53a 20.58 1.5 5 0.6 5NGC 4589 . . . . . . . . . . . E2 75 106 20.15 [3.7 5 2.6] 4NGC 4594 . . . . . . . . . . . Sab 58 62 20.03 1.7 5 0.4 5NGC 4621 . . . . . . . . . . . E5 69 57 10.08 5.4 5 1.1 5NGC 4649 . . . . . . . . . . . S0 198 180 10.04 5.9 5 1.3 5NGC 4660 . . . . . . . . . . . E5 17 57 20.53 4.0 5 1.8 5NGC 4874 . . . . . . . . . . . cD 8 138 21.24 3.7 5 0.5 12NGC 4881 . . . . . . . . . . . E 22 61 20.44 … 5NGC 5018 . . . . . . . . . . . E4 2 8 20.60 … 5NGC 5061 . . . . . . . . . . . E0 49 34 10.16 … 5NGC 5128 . . . . . . . . . . . Ep 34 26 10.12 2.6 5 0.6 13NGC 5322 . . . . . . . . . . . E4 81 90 20.05 [1.2 5 0.5] 4NGC 5813 . . . . . . . . . . . E1 71 141 20.30 5.2 5 1.5 4NGC 5845 . . . . . . . . . . . E 15 13 10.06 2.2 5 1.4 5NGC 5846 . . . . . . . . . . . S0 275 482 20.24 2.6 5 1.9 5NGC 5907 . . . . . . . . . . . Sc 0 25 !21.40 0.6 11NGC 5982 . . . . . . . . . . . E3 40 48 20.08 2.7 5 0.8 4NGC 6703 . . . . . . . . . . . S0 55 24 10.36 [0.7 5 0.4] 4NGC 6861 . . . . . . . . . . . S0 136 37 10.57 [2.0 5 0.8] 4NGC 7192 . . . . . . . . . . . S0 59 36 10.21 … 5NGC 7457 . . . . . . . . . . . S0 36 31 10.06 2.7 5 1.1 14NGC 7626 . . . . . . . . . . . E1 104 46 10.35 [2.2 5 1.4] 4IC 1459 . . . . . . . . . . . . . . E4 114 104 10.04 1.9 5 0.7 15IC 1919 . . . . . . . . . . . . . . S0 1 11 21.04 [1.0 5 1.2] 4IC 3540 . . . . . . . . . . . . . . SB0 2 11 20.74 [2.8 5 2.7] 4IC 4051 . . . . . . . . . . . . . . E2 650 89 10.86 11 5 2 16IC 4889 . . . . . . . . . . . . . . S0 30 23 10.12 … 5ESO 358 . . . . . . . . . . . . . S0 3 4 20.12 [0.5 5 1.0] 4Fornax . . . . . . . . . . . . . . . . dSph 0 5 !20.7 6 5 1 1Galaxy . . . . . . . . . . . . . . . Sbc 36 100 20.44 0.5 5 0.1 17LMC . . . . . . . . . . . . . . . . . Ir 0 13 !21.1 0.5 1Sgr . . . . . . . . . . . . . . . . . . . dSph 1 3 20.48 ? 1

a The “local” values of the specific globular cluster frequency S given in Kundu 1999are presented in square brackets.

b May contain some open clusters.References.—(1) van den Bergh 2000b; (2) Barmby et al. 2000; (3) Beasley & Sharples

2000; (4) Kundu 1999; (5) Gebhardt & Kissler-Patig 1999; (6) Kissler-Patig et al. 1997;(7) Puzia et al. 2000; (8) Brodie et al. 2000; (9) Zepf, Ashman, & Geisler 1995; (10) Kunduet al. 1999; (11) Kissler-Patig et al. 1999; (12) Harris et al. 2000; (13) Zepf & Ashman1993; (14) Chapelon et al. 1999; (15) Forbes et al. 1996; (16) Woodworth & Harris 2000;(17) Harris 1996 (the “Catalogue of Milky Way Globular Clusters” is updated at http://physun.physics.mcmaster.ca/Globular.html).

, will be (slightly) dependent on cluster age. This effectlog (n /n )r b

is expected to be almost negligible for elliptical galaxies, in whichmost clusters are probably very old. However, the degeneracybetween color and age might play a larger role for those clustersystems in which some spiral galaxies are embedded. For ex-ample, in the case of our own Milky Way system, up to ∼10%of all globular clusters may be a few Gyr younger than themajority of halo clusters.

4. DISCUSSION

Inspection of Table 1 shows that early-type galaxies of typesE, E/S0, S0, E/Sa, and S0/Sa constitute the overwhelming ma-jority of systems for which information is presently availableon the ratio of blue to red globular clusters. The main reasonsfor this are that (1) it is much more difficult to discover globularclusters in the clumpy and dusty disks of late-type spirals than

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COLORS OF GLOBULAR CLUSTERS 157

2001 PASP, 113:154–157

it is to find such objects in smooth (and often absorption free)early-type galaxies, and (2) the specific frequency of globularclusters is higher in early-type galaxies than it is in late-typesystems (Harris 1991). The study of early-type galaxies is there-fore more rewarding to students of globular cluster statisticsthan are investigations of galaxies of later Hubble type. Thesample of galaxies in which had been studied is magnituden /nr b

limited. Intrinsically faint galaxies, which tend to have metal-poor globular clusters (van den Bergh 1975), are thereforestrongly underrepresented in Table 1. As a result of the over-whelming preponderance of luminous early-type galaxies inthis table, it is not yet possible to make any definitive statementsabout the ratio of metal-rich to metal-poor globular clusters inintermediate-type and late-type galaxies.

Inspection of the data in Table 1 shows that the ratiosn /nr b

of E, E/S0, and S0 for individual early-type galaxies have arange of almost 2 orders of magnitude. However, about half ofthese early-type galaxies have logarithmic blue-to-red clusterratios that lie in the range . For 79 E,20.2 ! log (n /n ) ! 10.2r b

E/S0, and S0 galaxies , comparedAlog (n /n )S p 20.05 5 0.04r b

to for nine somewhat later typeAlog (n /n )S p 20.37 5 0.10r b

galaxies of Hubble classes S0/Sa, Sa, and Sab. Taken at facevalue, this result suggests that the globular clusters in Sa galaxiesmay, on average, be slightly more metal-poor (or younger) thanthose in E and S0 galaxies. This trend of bluer cluster systemstoward later Hubble types appears to be confirmed by the fourSb and Sbc galaxies in the present sample, which have

values of 20.35 (M31), 20.39 (NGC 4192), 21.28log (n /n )r b

(NGC 4565), and 20.44 (Milky Way). The valuefor the Sc galaxy NGC 4536 may havelog (n /n ) p 20.49r b

been affected by the inclusion of some blue intermediate-agepopulous disk clusters in the globular cluster sample. Finally theLarge Magellanic Cloud, which is a barred irregular galaxy,contains 13 blue metal-poor globular clusters and no red metal-rich ones. These data suggest that the fraction of blue globularclusters increases as one moves along the Hubble sequence fromtype E to Ir. It is noted in passing that there is no significantdifference between the values of E and S0 galaxies.log (n /n )r b

Such a difference might perhaps have been expected if S0 gal-axies are (on average) significantly younger than ellipticalgalaxies.

It is also noted that the small number of central and cD galaxiesin the present sample appear to have an unusually large spreadin values that ranges from 21.24 for NGC 4874log (n /n )r b

(Harris et al. 2000) to 10.43 for NGC 4486 (Kundu et al. 1999).The recent reobservation of NGC 3311 by Brodie, Larsen, &Kissler-Patig (2000), which had previously been observed bySecker et al. (1995), shows that the range of values forn /nr b

central and cD galaxies is not quite as large as had previouslybeen believed. Nevertheless, it still appears that cD galaxies mayhave experienced a somewhat larger range of evolutionary his-tories than typical elliptical galaxies.

Also included in Table 1 are values of the specific globularcluster frequency for each of the listed clusters. Inspection ofthese data yielded no correlations other than those that havealready been noted by Harris (1999) and van den Bergh(2000a).

It is a pleasure to thank a helpful referee for a number ofuseful suggestions.

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