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609-622, July 1991 PUBLICATIONS of the ASTRONOMICAL Publications OFTHE Astronomical SociE-noFTHK Pacific 103: 609-622, July 1991 PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC Vol. 103 July 1991 No. 665 THE STELLAR POPULATIONS OF M 331 SIDNEY VAN DEN BERGH Dominion Astrophysical Observatory, National Research Council of Canada 5071 West Saanich Road, Victoria, BC V8X 4M6, Canada Received 199 ABSTRACT A review is given of present ideas on the evolution and stellar content of the Triangulum nebula = M 33 = NGC 598. A distance modulus of (m — M)() = 24.5 ± 0.2 (D = 795 ± 75 kpc) and a Galactic foreground reddening £ß_v = 0.07, from which Mv = —18.87, are adopted throughout this paper. The disk of M 33 is embedded in a halo of globular clusters, metal-poor red giants, and RR Lyrae stars. Its nuclear bulge component is weak (Mv > -14). This suggests that the halos of galaxies are not extensions of their bulges to large radii. The ages of M 33 clusters do not appear to exhibit a hiatus in their star-forming history like that which is observed in the Large Magellanic Cloud. Young and intermediate-age clusters with luminosities rivaling the populous clusters in the LMC are rare in M 33. The integrated light of the semistellar nucleus of M 33, which contains the strongest X-ray source in the Local Group, is dominated by a young metal-rich population. At optical wavelengths the disk scale length of M 33 is 9.'6, which is similar to the 9.'9 scale length of OB associations. The ratio of the nova rate in M 33 to that in M 31 is approximately equal to the ratio of their luminosities. This suggests that the nova rate in a galaxy is not determined entirely by the integrated luminosity of old bulge stars. The gas-depletion time scale in the central region of M 33 is found to be ~ 1.7 X 109 yr, which is significantly shorter than a Hubble time. Available data do not yet allow an unambiguous choice between the density wave and self-prop- agating star-formation models for the two main spiral arms of M 33. Key words: M 33-galaxies-stellar populations 1. Introduction and independently by Wolf 1923. Neither in Duncan's M 33, which is the third-brightest member of the Local short paper nor in Wolf's note is there any indication that Group, is a spiral type Se II-III. The integrated magni- these authors had any inkling of the revolutionary impli- tude and colors of this galaxy are V = 5.85, {Β — V) = 0.65, cations of the discovery of variable stars in spiral nebulae. and (U — B) = 0.00 (Jacobsson 1970 and references Duncan's work was published in October 1922. Wolf's therein). On the sky M 33 covers an area of 53' X 83' note was dated December 20, 1922, and published Janu- (Holmberg 1958). Its large angular extent and intermedi- ary 15, 1923. Wolf does not mention Duncan's earlier ate inclination i = 56° (Zaritsky, Elston & Hill 1989) make announcement. Hence, while there is no question of it eminently suitable for studies of spiral structure and Duncan's priority. Wolf's discovery may be accepted as stellar content. having been independent. The discovery of variable stars The modern era of exploration of M 33 started with the in M 33 by Duncan and by Wolf opened the way for discovery of variable stars in this galaxy by Duncan 1922 Hubbie s 1926 definitive study which showed that 35 of ^ne in a series of invited review papers currently appearing in these the variable stars in M 33 are classical Cepheids. This Publications. work finally established M 33 as an extragalactic stellar 609 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 610 SIDNEY VAN DEN BERGH system and ended the great debate that iad previously of M 33. This value is in good agreement with the value raged (Heatherington 1972; Hoskin 1976; van den Bergh Eb_v = 0.08 ± 0.03 that Schmidt-Kaler 1967 obtained 1988a) about the nature of spiral nebulae. In particular, from a comparison of the colors of young clusters in M 33 Hubble was able to show that the observations of novae with the integrated colors of bright open clusters in the and star clusters in M 33 were entirely consistent with the Galaxy. It should, however, be emphasized (Dixon, Ford conclusion that this object was a huge extragalactic stellar & Robertson 1972) that reddening determinations from system. the integrated colors of open clusters may be affected by An excellent review of early work on M 33 has been whether or not the cluster is rich enough to contain red given by Gordon 1969, and a good bibliography, com- supergiants. A significantly lower Galactic foreground plete to May 1973, is provided by Brosche, Einasto & reddening Eß_v = 0.03 ± 0.02 was found by McGlure and Kümmel 1974. Nomenclature for objects in M 33 is re- Racine 1969. Their result was derived by combining in- viewed by Lortet 1986. Some observational and derived termediate-band photometry on the DDO system with parameters for the Triangulum nebula are summarized in UBV photometry. From intermediate-band photometry Table 1. of individual stars out to a distance of 1 kpc Johnson and Joner 1987 obtain a foreground reddening of E _ = 0.08. 2. Foreground Reddening and Distance B V Assuming the standard ratio of hydrogen column density According to Sandage 1963a UBV photometry by John- to absorption, Burstein & Heiles 1984 found AB = 0.18 son and Sandage shows that EB_V = 0.09 in the direction mag, corresponding to EB_V = 0.04. In the following TABLE 1 Summary of M33 Data Parameter Value Reference a(1950) 01h 31™ 01.s67 de Vaucouleurs & 0(1950) +30° 24' 15"0 Leach 1981 Radial velocity -172 ± 6 km s"1 Zaritsky et al. 1989 Type Sc II-III Foreground reddening Eg_v = 0.07 True distance modulus 24.5 ± 0.2 Table 2 Distance 795 ± 75 kpc Optical size 53 'χ 83 ' (12 χ 19 kpc) Holmberg 1958 Major axis 23° ± Io de Vaucouleurs 19 59a Neutral hydrogen size 74^ 135' (17 χ 31 kpc) Corbelli et al. 1989 Inclination 56° ± Io Zaritsky et al. 1989 Disk scale-length (B) 7 '. 8 (1.8 kpc) de Vaucouleurs 19 59a Vt 5.85 Jacobsson 197 0 (B-V) T 0.65 Jacobsson 197 0 (U-B)T 0.00 Jacobsson 197 0 My -18.87 9 Ly 3.1 χ 10 ^ 9 RAS 1.2 χ 10 Lq Rice et al. 1990 9 2 χ 10 Mq Gordon 1971 10 MT (disk) 0.8-1.4 χ 10 ΛΓθ Zaritsky et al. 1989 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System STELLAR POPULATIONS OF M 33 611 discussion a foreground reddening of EB_V = 0.07, corre- sponding to Av = 0.22 mag, will be adopted. With these So° values Vq = 6.28, {B-V)0 = 0.58, and {U-B)0 = -0.05 for M 33. 185« .147 Recent reviews of work on the distance to M 33 have been given by Feast 1988 and van den Bergh 1989. Table 2 summarizes the results of these investigations and more 40° M31_e205 recent distance determinations. All Cepheid distances M32·^ assume (m — M)0 = 18.45 for the Large Magellanic Cloud. An unweighted mean of the eleven distance determina- tions listed in this table yields (m—M)0 = 24.5, with an m estimated uncertainty of — 0.2 mag. The corresponding M33 • I distance to M 33 is 795 ± 75 kpc. This distance will be 30° 0 adopted throughout the present review. Eventually, it should also be possible to derive a distance to M 33 from a comparison of the radial velocities and proper motions (Greenhill et al. 1990) of water-vapor maser sources in 111 M 33. The fact that the true distance moduli of M 31 and Fig. 1-The Andromeda subgroup of the Local Group contains M 31, M 33 are 24.3 ±0.1 and 24.5 ± 0.2, respectively (van 1 M 32, and at least eight dwarf galaxies. LGS 3 (α^ = OI ' 01T2, 65(, = den Bergh 1989), and that these objects are separated by +21037') lies just outside the area shown in the figure. only 15° in the sky (see Fig. 1), supports the conclusion that both of these galaxies are members of the same bly a small nuclear bulge. A recent CFHT image of the compact subgroup within the Local Group. Other proba- central region and nucleus of M 33 is shown in Figure 2. ble members of this subgroup are NGC 147, NGC 185, The semistellar nucleus of M 33 has FWHM ~ 0'.'8, NGC 205, and the dwarf galaxies Andromeda I, An- corresponding to a diameter of ~ 3 pc (Gallagher, Goad & dromeda II, Andromeda III, and LGS 3. According to van Mould 1982). This nucleus has ß = 14.5 ± 0.1 (Nieto & den Bergh 1981a the mass of the Andromeda subgroup of Aurière 1982) which, with (rn—M)B = 24.8, yields MB = the Local Group is (7.5 ± 3.9) X 1011 SJÍq· With the — 10.3, i.e., it is more luminous than any Galactic globu- distance moduli adopted above, the linear separation of lar cluster. According to Nieto & Aurière 1982 the inter- M 31 and M 33 is ~ 210 kpc. As viewed from M 33, nal velocity dispersion in the nucleus of M 33 is small ( < 30 km s^), which indicates that the mass-to-light the Andromeda nebula would have an apparent diameter σ ratio must be low.
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