During the last ten years, observing extra-galactic globular clusters has become a popular deep-sky challenge. Many amateurs have seen globular clusters associated with the Fornax System, M31 and even M 110. Detailed notes and charts can be found in Luginbuhl and Skiff (1989), Deep Sky and in Webb Society journal and section reports. However, for M33 very few references can be found. Brian Skiff's (1983) article mentions an unsuccessful observing attempt of M33's brightest globular cluster, Mayall "C" ( = C 39). After several queries on the internet, and exchanging email with Brian Skiff, I decided to conduct research on the M33 cluster system.
The M33 Cluster System
Over the past 35 years, less than one dozen articles have been written on the globular clusters associated with M33. This is in direct contrast to the dozens of articles written on the M31 system during the past ten years. This is in part a reflection of the smaller size of the globular population and the difficulty in identifying suitable candidates in M33. Perhaps the most useful reference for serious observer is Christian and Schommer's (1982) article The Cluster System of M33. Using high resolution plates, they compiled a catalogue of over 250 non-stellar objects, and obtained BVR photometry on 60 of these objects. Most of the "60" are probable cluster candidates. Also included in this reference are detailed plates, positions (1950 coordinates), and photometrically derived magnitudes.
For this study, I selected clusters with a photometrically derived magnitude brighter than ~ 17.7. The B-V for most of these is a reddish value of 0.6, which is considered the "cutoff" for most "true" globular clusters (Christian and Schommer, 1982). Eight of the twelve objects in Table 1 are probably "true" globular clusters, while the other four are significantly bluer and resemble similar objects in the Large Magellenic Cloud. One of the brightest clusters in M33 is U62, which is a blue (B-V = 0.12) object that rivals the brightest clusters of the LMC.
Less than 20 "true" globular clusters (B-V ( 0.6) have been found associated with M33. This is a somewhat larger population than found associated with the LMC, and far short of the ~300 globular clusters surrounding M31. The average absolute magnitude (Mv ) of the M33 clusters is -7.0 ( 0.2 (Van Den Bergh 1991), somewhat lower than the average Mv of - 7.9 obtained for the M31 system (Hodge, 1992). Likewise, the brightest M33 cluster ( C 39; Mv = -8.8 ) is nearly two magnitudes fainter than brightest associated with M31. Visually, C 39 (Mayall's cluster "C") has a magnitude of 15.9, while most of the globular cluster candidates have magnitudes > 17.5. The small population size, combined with the extreme faintness of the objects and the complexity of M33 region make for an extremely challenging project for the deep-sky observer.
I made a series of observations of the brighter globular cluster candidates using the Atlanta Astronomy Club's 0.5-meter, f/4.5 reflector. The observatory is located about 60 kilometers west of Atlanta near the small city of Villa Rica, Georgia. The limiting magnitude ( Lm) of the site is generally between 6.0 to 6.2 during very good or better conditions. Using magnifications of 260 x, telescopic Lm was ( 17.0 during periods of good seeing.
To prepare for a "night" of globular cluster observing, I generated a series of charts using MegaStar and plotted positions of the objects. Since MegaStar uses the Guide Star Catalogue (GSC), the effective magnitude cutoff is only ~15th magnitude. To compensate for this shortfall, I hand plotted stars to ~ 18th magnitude using plates from Christian and Schommer around selected globulars. Most of the brighter globulars are concentrated in the northern portion of the galaxy, within 10 arcminutes of the giant HII region NGC 604.
Observed "Clusters": December 20 and 24, 1995. 50-cm telescope, f/4.5 at 260x. Telescope limiting magnitude at this site was 17.2.
C 39: This is the brightest globular cluster, located ~ 22 minutes southeast of M33's nucleus. It is easily visible in most deep-sky photographs and is plotted in the GSC as a "star". In the 50-cm, it was visible with direct vision as a slightly fuzzy "star" of 16th magnitude. The extended halo or envelope was < 2" in diameter. It was the only globular I observed that had a non-stellar appearance.
C 27: Located about 2.5' east of NGC 604, this cluster was very faint and difficult to observe. It was definitely fainter than the older published magnitude of 16.48, appearing to be close to the photometric value of 17.19. It was distinctly stellar, with no evidence of an extended envelope visible.
C 13: A difficult object, though not as challenging as C 27. The cluster appeared stellar, and close to the published value of 16.76 magnitude. It is located about 7' NNE of NGC 604.
U49: A very faint cluster appearing as a "star" just northeast of a small triangle of 16th to 17th magnitude stars. This was visible > 75% of the time using adverted vision.
M 9: No position was given in Christian and Schommer, instead this cluster was plotted using their detailed plates of M33. The M9 image on the plates was quite bright, and was alternatively plotted as a "star" in the GSC. The cluster itself was visible as a faint 16th magnitude star. I also hunted for nearby U 101, but had no success.
Other Possible Candidates
The observations made do not exhaust the "list" of observable clusters, but they do represent the "easier" objects. One of the brightest clusters, U 62, lies only 1' south of the relatively bright Association 75. Even more challenging is U 77 which is within the envelope of the giant H II region, NGC 595. Christian and Schommer listed several other "M" clusters with magnitudes between 16.5 to 17.2. However, though positions were not given these objects are listed as globular clusters in Melnick and D'Odorico's catalogue.
The observation of M 33's globular cluster system should challenge even the most seasoned observers. Most objects require apertures > 17.5", though the brightest cluster (C 39) may be visible in a 12.5-inch scope. Since these objects are smaller and fainter than brighter globular clusters associated with M33, most will appear stellar in all but the largest amateur telescopes. The identification of the fainter cluster members would make a worthy project for anyone with a good CCD setup.
I would like to thank Brian Skiff for suggesting good starter references and giving supportive email for this project. I would also like to thank Steve Gottlieb for his support and direction.
Christain, C. A. and Schommer, R.A. 1982. ApJS. v49, 405
Hodge, P. 1992. The Andromeda Galaxy, Kluwer Academic Publishers, The Netherlands
Lughinbuhl, C and Skiff, B. 1989. Observing Handbook and Catalogue of Deep Sky Objects, Cambridge University Press, New York
Skiff, B. 1983. Deep Sky, v4, 18
Van Den Bergh, S. 1991.PASP, v103, 609