![]() | Annu. Rev. Astron. Astrophys. 1991. 29:
543-79 Copyright © 1991 by Annual Reviews. All rights reserved |
In a galaxy more distant than D ~ 5-10 Mpc, the images of typical globular clusters (with core radii rc ~ 1 pc and half-mass radii rh ~ 5 pc) appear indistinguishable from stars under even the best ground-based seeing conditions. In such cases, the galaxy's GCS appears primarily as a statistical excess of faint starlike objects with a central concentration toward the galaxy, and in general, the only way to identify clusters individually is by time-consuming radial velocity measurement. For nearby galaxies, identifying clusters one by one is virtually a necessity.
2.1 Giant Elliptical Galaxies
Large ellipticals are the most natural environments for GCS work: Globular clusters may number in the thousands per galaxy, and no problems arise with internal reddening or confusion with disk objects. Unfortunately, E giants are the one major type of galaxy not represented in the Local Group, and much effort is still needed to connect their GCS properties with the more familiar ones in the M31 and Milky Way spirals.
VIRGO: This all-important center of the Local
Supercluster contains the nearest rich collection of gE-type galaxies.
A major starting point for all modern GCS work remains the
classic photometric study of Hanes (77, 78), who obtained
spatial distributions and luminosity functions to Blim
22.5 for several key
Virgo
members. During the next several years, additional photographic
imaging studies were carried out at
the CFHT, CTIO, and AAT observatories, which with limiting
magnitudes Blim
24 and one-degree fields of view were
well suited to GCS detection in galaxies closer than
D ~ 20 Mpc (cf 93). In Virgo, the supergiants
M87 (NGC 4486) and M49 (NGC 4472) were once
again the first targets of study (94, 107, 109).
Strom et al. (192) and Forte et al. (64) carried out
large-scale multicolor (UBR) surveys
of four Virgo Es - a formidable task of
plate measurement and data management during the era of photographic
photometry and low-speed computing.
Major new progress took place with the advent of CCD detectors. Photometry reaching to, and beyond, the turnover of the cluster luminosity function (Section 3) was carried out by van den Bergh et al. (217), Grillmair et al. (76), Cohen (38), and Harris et al. (103), along with multicolor photometry for samples of the brighter clusters (38, 39, 40). Low-dispersion spectra obtained for a few dozen clusters in M87 and M49 have yielded direct metallicity estimates, radial velocities, and dynamical analyses (81, 119, 173, and especially 151, 152). In total, the Virgo systems are the best understood GCSs outside the Local Group, and stand as the baseline for all comparisons of global GCS properties.
FORNAX: At nearly the same distance as Virgo, the southern Fornax Cluster also contains several large ellipticals. Dawe & Dickens (47) discovered GCSs in three of these, and more complete starcount studies for five galaxies were carried out by Hanes & Harris (83) and Harris & Hanes (105). Several groups have obtained deep CCD photometry for the very rich cluster system around the central cD, NGC 1399 (18, 71, 144a, 219).
SMALL GROUPS: Several E/S0 members of smaller groups within the Local Supercluster region have been surveyed - in some cases the groups include interacting or peculiar galaxies (17, 82, 88, 100, 104, 111, 113, 147, 169). In general, these objects exhibit much greater variety than do the rather homogeneous Virgo and Fornax members, but on average have less populous GCSs (see Section 4). None have yet been studied to the same level of detail as in the two rich clusters described above.
NGC 5128: Investigation of the cluster system in this, the nearest large E
galaxy, deserves special mention as an exemplary process of
ingenuity and persistence. Only a decade ago, this galaxy - essentially a
normal elliptical with accreted tidal debris - was thought to contain few, if
any, globular clusters (201) and thus to stand
as a strong counterexample to the Virgo Es. But the
serendipitous discovery of a single bright cluster
(74) proved to
be only the tip of a rapidly explored
iceberg. More clusters were soon found by their
nonstellar visual appearance and radial velocities, while starcounts over a
~ 1° field showed that
indeed NGC 5128 had a healthy GCS numbering many hundreds
of objects (117,
215).
The GCS had eluded notice for so long because, located at only ~ 4 Mpc
distance and at low latitude, the GCS is very
spread-out on the sky and thus almost invisibly
dilute against the Galactic field star population, yet
only a few of its biggest clusters stand out individually
as nonstellar at 1"
seeing. Additional photometry and multicolor starcounts
(86,
89)
refined the estimated size of the
system and the cluster luminosity function. Frogel's (67)
JHK photometry of a dozen of the brightest clusters
demonstrated that they were indeed old globulars, with
metallicities extending perhaps to above-solar levels.
G. Harris et al. (85) presented a more extensive analysis of
their metallicity distribution
with CMT1T2 photometry. Velocities
of many more objects have been obtained with various selection
approaches (116, 191), and by now a
remarkably complete understanding of the system has been
reached. The present overview of the system is nicely summarized
by H. Harris et al. (88).
DISTANT CLUSTERS: In galaxies progressively
further beyond the Local Supercluster region, only the
most luminous globular clusters
remain visible even under excellent ground-based imaging
conditions and very deep limiting magnitudes.
Harris (93)
argued that by careful attention to image classification and
selective identification of the faint nonstarlike images (that make up most of
the contaminating background field population at high latitude), one could
straightforwardly detect GCSs in Virgo-like
giant ellipticals as
distant as 100 Mpc.
The most distant system studied
with photographic methods was NGC 3311, the central cD in
Hydra I (A1060) at a redshift V0
3500 km/s
(110). Deeper
exploratory CCD
imaging has revealed GCSs in the Coma Cluster supergiants NGC 4874 and
4889 (V0
7000 km/s) (95,
194), NGC 3842
in A1367 (V0
6000 km/s) (26),
and the classic
giant cD NGC 6166 in
A2199 (V0
9000 km/s)
(168).
Several other distant galaxies
are currently under study, including the central giant
ellipticals in Hydra I, Centaurus, Pegasus I, A262, and other
clusters in the
2000-5000 km/s redshift range.
2.2 Giant Disk Galaxies
In large Sa/Sb galaxies the distinction between open (disk) and globular (halo) clusters is still relatively clear, but the GCS can readily be found only if the target galaxy is seen nearly edge-on to minimize confusion from the disk and spiral arm populations. (This requirement is stringent: even for M31, at i ~ 78° and at a distance of only 0.7 Mpc distant, roughly half its globulars are seen projected against the disk, with consequent severe reddening, crowding, and contamination effects.) In brief, the handful of such galaxies that have been surveyed to date amounts to a rather homogeneous group, appearing more or less as M31 or the Milky Way would be expected to look if placed at the same distance. Positive GCS detections include NGC 2683 (90); NGC 4565 (214); NGC 4594 (106); NGC 5170 (62); NGC 7814 (87); and the nearby M81 (21). Null or marginal detections have been reported for NGC 891 (214), 3717, 5866, and 5907 (87) - all of which are relatively isolated large galaxies or members of sparse groups. Hanes (77, 78) found slight excess counts around the Virgo spirals NGC 4216 and 4569, but these are barely significant within the statistical uncertainties and should probably also be regarded as marginal detections.
For M31 itself, several comprehensive
searches for globular clusters have now been carried out
with a variety of spectroscopic and imaging techniques
(8,
41,
183,
224; see
69 for a summary);
these now provide a nearly complete census of globular clusters
to V(lim) 18 (well
past the peak of the
cluster luminosity function) and out
to almost a 2° (25-kiloparsec) radius. Several
current observational programs are now leading a much-improved
understanding of the clusters' distributions over
metallicity, radial velocity, and luminosity, and even to
measurement of color-magnitude diagrams for selected clusters
(e.g. 20,
34a,
112,
122,
175; and
171a).
2.3 Sc/Irr Systems
Detecting and measuring globular clusters in late-type galaxies is a truly difficult task. In these, clusters of all ages are found throughout the body of the host galaxy in a rather uniform mixture, and even in the biggest Sc spirals the globular (old-halo) clusters are likely to number only in the dozens. Experience has shown that the globulars in late-type galaxies must be isolated one by one through an exhaustive process of wide-field multicolor photometry and image analysis, thorough removal of field objects, and (if possible) spectra and velocities (e.g. 11, 34). These requirements in turn limit the possible targets to systems in or not far beyond the Local Group. Even so, only in the Magellanic Clouds have researchers been possible to select the old clusters definitively (through the use of color-magnitude diagrams and RR Lyrae variables.) Nevertheless, these systems help establish just how active the earliest epoch of star and cluster formation was along the full continuum of the Hubble sequence.
LOCAL GROUP IRREGULARS: In the Magellanic Clouds, dozens of recent studies have reduced the list of old globular clusters to just 9 in both the LMC and SMC combined [for literature compilations, see (178, 213, 220); I add the Reticulum cluster (75) to the objects listed by Westerlund (220) and van den Bergh (213)]. The dwarf irregulars NGC 6822 (118, 216) and WLM (1, 182) each contain one cluster of the right luminosity and color range to be a possible globular, but both could as well be intermediate-age objects. IC 1613 contains no known globular cluster candidates (cf 118).
M33: Christian & Schommer and their collaborators (35, 36, 37, 184a) have extensively studied the clusters in this nearest Sc spiral. They find a rather uniform age distribution from young clusters to very old, with a sample of about 20 probable old globulars now identified. The difficulty of separating the oldest clusters unambiguously from intermediate-age objects here is particularly severe; as Christian (34) discusses, integrated color indices or spectra are not sensitive discriminants in the ~ 5-15 Gy age range. For a sample of the old clusters, Brodie & Huchra (21) find a range of metallicities similar to those in the Milky Way halo clusters.
NGC 3109: Demers et al. (49) and Blecha (12) have reported finding a small number (~ 10-20) of globular-like clusters in the halo of this Magellanic-type galaxy, which lies marginally beyond the Local Group.
SCULPTOR GROUP: In NGC 55, Da Costa &
Graham (44)
found two clusters projected near the disk
whose spectra suggest they are old, metal-poor globulars.
A more systematic halo search conducted by Liller & Alcaino
(141)
yielded no significant result.
The bigger and somewhat more distant spiral NGC 253
has a more noticeable GCS, with perhaps ~ 20 halo clusters
brighter than B 21
(11,
142).
See the latter paper particularly for a careful discussion of
methodology.
M81 GROUP: For the Sc spiral NGC 2403, Battistini et al. (9) find 8 halo globular cluster candidates. More work is reportedly in progress (145).
2.4 Dwarf Ellipticals
Four dwarf elliptical members of the Local Group (NGC 147, 185, 205, and Fornax) all have handfuls of metal-poor globulars (45, 68, 108, 118). For the Fornax clusters, color-magnitude diagrams have also been published which confirm their generic similarity to Milky Way halo clusters (22, 48). The lack of any known clusters in M32 (the high-surface-brightness companion to M31) remains a minor puzzle: If it had a normal specific frequency it would be expected to carry ~ 15-20 globulars, and no one has quantitatively demonstrated that any original population of clusters could have been totally removed by a combination of tidal stripping from M31 and dynamical friction and tidal shocking from the dense nucleus of M32 itself.
Half a dozen GCSs have been more-or-less accidentally discovered in dwarf ellipticals beyond the Local Group (102): One is a companion to NGC 3115 (82); four are in the Virgo Cluster (38, 181); and one in the Fornax Cluster (60). Notably, all six of these GCS dwarfs are nucleated dEs. This connection may support the evidence of Ferguson & Sandage (61) that the dE, N galaxies are spatially and generically related to the true big ellipticals and not the spirals or irregulars. More systematic deep CCD imaging of these interesting small systems could well be rewarding, as they may resemble the fundamental units from which the galaxies are hypothesized to form (cf 126, 127).
In Table 1, the properties of 60 galaxies with GCSs are cataloged.
NGC | Group | Type | V0 | (m-M)V | MTV | Nobs | Blim | Nel | SN | Ref |
Galaxy | Local | Sbc | -21.3 | 140 | 160 ± 20 | 0.5 ± 0.1 | 172,
219a | |||
LMC | Local | Sm | 12 | 18.64 | -18.6 | 8 | ![]() | 0.5: | 213,
220 | |
SMC | Local | Im | -30 | 18.92 | -16.9 | 1 | 2 ± 1 | 0.4 ± 0.2 | 213,
220 | |
Fornax | Local | dE0 | -51 | 20.70 | -12.3 | 5 | 6 ± 1 | 73 ± 12 | 22,
118 | |
147 | Local | dE5 | 89 | 24.45 | -15.0 | 4 | 19 (V) | 4 ± 1 | 4.0 ± 1.0 | 108,
118 |
185 | Local | dE3 | 39 | 24.40 | -15.2 | 6 | 19 (V) | 8 ± 2 | 6.5 ± 1.6 | 108,
118 |
205 | Local | dE5 | -1 | 24.49 | -16.5 | 8 | 19 (V) | 9 ± 1 | 2.3 ± 0.3 | 108,
118 |
221 | Local | E2 | 35 | 24.54 | -16.3 | 0 | 18.5 (V) | ![]() | ![]() | 108 |
M31 | Local | Sb | -59 | 24.54 | -21.7 | 300 ± 50 | 18.5 (V) | 350 ± 100 | 0.7 ± 0.2 | 8,
190 |
M33 | Local | Scd | 3 | 24.75 | -19.2 | 19 | 19.5 (V) | 30: | 0.6: | 21,
37 |
55 | Sculptor | Sm | 106 | 25.75 | -19.0 | 2 | 44 | |||
253 | Sculptor | Sc | 260 | 26.8 | -20.4 | 20: | 21 (V) | 22: | 0.2 ± 0.1 | 11 |
524 | CfA13 | S0 | 2600 | 32.7 | -22.1 | 700 ± 150 | 24.9 | 2830 ± 880 | 4.0 ± 1.2 | 104 |
1052 | Cetus | E4 | 1400 | 31.3: | -20.8 | 220 ± 25 | 24.4 | 460 ± 90 | 2.3 ± 0.4 | 104 |
1374 | Fornax | E0 | 1390 | 31.1 | -19.8 | 104 ± 47 | 23.6 | 330 ± 160 | 3.9 ± 1.9 | 83 |
1379 | Fornax | E0 | 1390 | 31.1 | -20.0 | 108 ± 29 | 23.8 | 290 ± 100 | 2.8 ± 0.9 | 83 |
1387 | Fornax | S0 | 1390 | 31.1 | -20.3 | 150 ± 30 | 24.2 | 320 ± 90 | 2.5 ± 0.7 | 83 |
1399 | Fornax | E1/cD | 1390 | 31.1 | -21.2 | 1690 ± 380 | 24.2 | 3600 ± 1100 | 12 ± 4 | 83 |
1404 | Fornax | E1 | 1390 | 31.1 | -20.8 | 97 ± 28 | 24.4 | 190 ± 80 | 0.9 ± 0.4 | 83 |
1549 | Doradus | E0 | 1010 | 30.7: | -20.8 | 110 ± 40 | 24.9 | 140 ± 60 | 0.7 ± 0.3 | 17 |
1553 | Doradus | S0 | 1010 | 30.7: | -21.2 | 450 ± 100 | 25.2 | 540 ± 160 | 1.8 ± 0.6 | 17 |
2403 | M81 | Scd | 260 | 27.64 | -19.5 | 8: | 19 (V) | 9 | ||
3031 | M81 | Sab | 260 | 27.75 | -21.2 | 8 | 21 | |||
2683 | Sb | 370 | 29.75 | -20.8 | 300 ± 93 | 25.0 | 310 ± 100 | 1.7 ± 0.5 | 90 | |
3109 | Im | 130 | 26.35 | -17.3 | 20: | 20 (V) | 12,
49 | |||
3115 | S0 | 460 | 30.2 | -21.1 | 520 ± 120 | 24.7 | 630 ± 150 | 2.3 ± 0.5 | 82 | |
3115B | dE1, N | 460 | 30.2 | -16.7 | 25 ± 10 | 24.7 | 30 ± 15 | 6.3 ± 3.1 | 82 | |
3226 | CfA58 | E2 | 1210 | 31.4: | -20.0 | 115 ± 40 | 23.6 | 470 ± 200 | 4.5 ± 2.0 | 111 |
3311 | A1060 | E0/cD | 3420 | 33.5 | -22.4 | 414 ± 31 | 24.7 | 17000 ± 6000 | 18 ± 6 | 94,
110 |
3377 | Leo | E5 | 630 | 30.00 | -19.8 | 140 ± 30 | 23.7 | 210 ± 50 | 2.6 ± 0.6 | 111 |
3379 | Leo | E1 | 630 | 30.00 | -20.7 | 170 ± 90 | 23.7 | 260 ± 140 | 1.3 ± 0.7 | 111 |
3384 | Leo | S0 | 630 | 30.00 | -20.0 | 75 ± 35 | 23.7 | 110 ± 60 | 1.1 ± 0.5 | 111 |
3557 | E3 | 2560 | 33.0 | -22.6 | 130 ± 90 | 25.5 | 400 ± 300 | 0.4 ± 0.3 | 147 | |
3607 | CfA77 | S0 | 1090 | 31.3 | -21.3 | 50 ± 35 | 22.3 | 800 ± 600 | 2.5 ± 1.8 | 111 |
4278 | Coma I | E1 | 910 | 31.0 | -20.8 | 470 ± 55 | 23.7 | 1300 ± 300 | 6.1 ± 1.5 | 111 |
4565 | Coma I | Sb | 910 | 31.0 | -22.2 | 90 ± 20 | 23.8 | 240 ± 70 | 0.3 ± 0.1 | 214 |
4216 | Virgo | Sb | 1080 | 31.3 | -22.0 | 20 ± 10 | 22.6 | 700 ± 380 | 1.1 ± 0.6 | 77 |
4340 | Virgo | SB0 | 1080 | 31.3 | -20.3 | 25 ± 10 | 22.6 | 880 ± 380 | 6.8 ± 2.9 | 77 |
4365 | Virgo | E2 | 1080 | 31.3 | -21.6 | 310 ± 30 | 26.2 | 3500 ± 1200 | 7.7 ± 2.7 | 103 |
4374 | Virgo | E1 | 1080 | 31.3 | -22.0 | 98 ± 13 | 22.6 | 3400 ± 800 | 5.6 ± 1.3 | 77 |
4406 | Virgo | E3 | 1080 | 31.3 | -22.1 | 108 ± 13 | 22.6 | 3800 ± 900 | 5.4 ± 1.3 | 77 |
4472 | Virgo | E2 | 1080 | 31.3 | -22.9 | 2300 ± 400 | 23.3 (V) | 7400 ± 2100 | 5.0 ± 1.4 | 94 |
4486 | Virgo | E0 | 1080 | 31.3 | -22.7 | 5700 ± 600 | 24.2 | 16000 ± 4000 | 14 ± 3 | 94 |
4526 | Virgo | S0 | 1080 | 31.3 | -21.7 | 87 ± 13 | 22.6 | 3000 ± 800 | 6.6 ± 1.6 | 77 |
4564 | Virgo | E6 | 1080 | 31.3 | -20.4 | 34 ± 10 | 22.6 | 1200 ± 400 | 8.5 ± 3.0 | 77 |
4569 | Virgo | Sab | 1080 | 31.3 | -22.1 | 30 ± 10 | 22.6 | 1000 ± 400 | 1.5 ± 0.6 | 77 |
4596 | Virgo | SB0 | 1080 | 31.3 | -20.8 | 82:: | 22.6 | 2800:: | 13:: | 77 |
4621 | Virgo | E5 | 1080 | 31.3 | -21.5 | 62 ± 12 | 22.6 | 2200 ± 440 | 5.4 ± 1.1 | 77 |
4636 | Virgo | E0 | 1080 | 31.3 | -21.7 | 150 ± 20 | 22.6 | 4900 ± 1200 | 9.9 ± 2.4 | 77 |
4649 | Virgo | E2 | 1080 | 31.3 | -22.5 | 165 ± 15 | 22.6 | 5800 ± 1300 | 5.9 ± 1.3 | 77 |
4697 | Virgo | E6 | 1080 | 31.3 | -22.3 | 80 ± 20 | 22.6 | 2800 ± 900 | 3.5 ± 1.1 | 77 |
4594 | Virgo SE | Sa | 840 | 30.5 | -22.9 | 1250 ± 120 | 23.1 (V) | 2500 ± 600 | 1.7 ± 0.4 | 106 |
4874 | A1656 | E0 | 6950 | 35.0 | -23.1 | 121 ± 14 | 25.5 | 21000: | 12: | 95 |
4889 | A1656 | E4 | 6950 | 35.0 | -23.6 | 60 ± 12 | 25.5 | 11000: | 4: | 95 |
5128 | Centaurus | E0 | 320 | 28.25 | -22.0 | 1400 ± 300 | 22.2 (V) | 1700 ± 400 | 2.6 ± 0.6 | 85 |
5170 | Virgo SE | Sb | 1350 | 31.7 | -21.2 | 310 ± 50 | 25.0 (V) | 390 ± 140 | 1.2 ± 0.4 | 62 |
5813 | CfA150 | E1 | 1810 | 32.3 | -21.6 | 450 ± 100 | 24.2 | 2400 ± 700 | 5.2 ± 1.5 | 111 |
5846 | CfA150 | E0 | 1810 | 32.3 | -22.1 | 135 ± 80 | 23.5 | 1800 ± 1300 | 2.6 ± 1.9 | 111 |
6166 | A2199 | E2/cD | 9080 | 35.6 | -23.6 | 156 ± 35 | 26.8 | 10000 ± 5000 | 4 ± 2 | 168 |
7814 | Sab | 1110 | 30.5 | -20.1 | 197 ± 28 | 24.5 (V) | 230 ± 80 | 2.2 ± 0.7 | 88 | |
a Column entries are as follows: 1. Galaxy
name or NGC number. 2. The group or cluster in which the galaxy is
found; CfA numbers are from Geller & Huchra (72). 3. Hubble
type. 4. Mean radial velocity of the group V0, in
km/s. (For the Local Group members, individual velocities are given.)
5. Apparent visual distance modulus. For the nearby galaxies
[(m-M)V 30.5], the values given are individually determined
from Cepheids, RR Lyraes, planetary nebula luminosity functions, and
other near-field standard candles. Data are taken from the review by
van den Bergh (209) with additional material for individual galaxies
from several other sources (46,
49,
65,
73,
121,
149,
150,
179,
180,
182a,
195). For
more distant systems, moduli are calculated assuming a
Hubble parameter H0 = 75 km s-1
Mpc-1 and a Local Group infall to Virgo of 250 km
s-1. The Fornax cluster is taken to be 0.2 mag closer than
Virgo
(15,
18,
167). 6. Total luminosity of the galaxy
MTV, in most cases from the
BT magnitudes and (B - V) colors in the de
Vaucouleurs RC2 catalog (50); other sources are Tully
(199) for the
Milky Way, NGC 1374, 1379, 1387, and 5170; Carignan (28) for NGC 3109;
Hanes & Harris (82) for NGC 3115B; Burkhead (23) for NGC 4594; and
Harris et al. (110) for NGC 3311. The values listed for spirals
and irregulars are face-on luminosities corrected for internal
absorption. 7., 8. Number of observed globular clusters, brighter than
the magnitude limit Blim. Reference sources are
listed in Column 11. If indicated, the limiting magnitude is in
V. 9. Estimated total number of globular clusters in the
galaxy, extrapolated from Nobs over all magnitudes
and radii. For E galaxies, the cluster luminosity distribution (see Section 3 below) is assumed to be Gaussian
with mean at MB = -6.84 and dispersion
= 1.4 mag following Harris
et al. (103). For spirals,
= 1.2 mag is assumed, although Nt is
insensitive to the choice of
as long as mlim is fairly close to the
peak of the cluster luminosity function (94,
111). 10. Specific
frequency SN (number of clusters per unit galaxy
luminosity; see Section 4 below). Here,
SN is calculated using the total light
MTV of the galaxy, not just the spheroid
light.