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-propagating 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, corresponding 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 determinations 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. In this respect it differs from the nuclei of-11°. of M 31 and M 32 (Tonry 1987; Kormendy 1988; Dressier 3. The Nucleus of M 33 & Richstone 1988). Spectroscopically (van den Bergh 1976) the nucleus of M 33 is clearly composite with a The Triangulum nebula consists of four principal com- late-A type being indicated by K/H + He, F2-F4 by ponents: (1) an exponential disk, (2) a halo containing 4226/H7 and F3-F4 by CH/H7. The observed spectra globular clusters, (3) a semistellar nucleus, and (4) proba- and integrated colors (see Table 3) of the semistellar TABLE 2 nucleus might be produced by either (1) a young metal- Recent Distance Determinations rich population or (2) an old population that is very metal poor. Van den Bergh's observations showed that λ4325 Method Reference (m - M)0 of Fe I was stronger in M 33 than in the spectra of very a Cepheids mpg + CCD Christian & Schommer 1987 24.36 ± 0.11 metal-poor globular clusters. The interpretation that the Cepheids I Christian & Schommer 1987 24.37a nucleus of M 33 consists of young metal-rich stars is Cepheids I Mould 1987 24.77 ± 0.15 a strongly supported by recent near-infrared spectra Cepheids H Madore et al. 1985 24.20 ± 0.14 (Schmidt, Bica & Alloin 1990). The fact that the dominant Cepheids BVRI Freedman et al. 1991 24.58 ±0.13 Red giants Wilson et al. 1990 24.60 ± 0.30 population in the nucleus of M 33 is both young (< 1 Gyr) Red giants Mould & Kristian 1986 24.80 ± 0.30 and metal rich rules out the possibility that this nucleus LP variables Kinman et al. 1987 24.55a ± 0.10 was formed by capture of (metal-poor) globular clusters LP variables Mould et al. 1990 24.52 ± 0.17 via tidal friction (cf. Tremaine, Ostriker & Spitzer 1975). RR Lyrae Pritchet 1988 24.60 ± 0.23 a This suggests that the nucleus of M 33 consists of stars Novae Deila Valle 1988 24.38 that were formed by one or more hursts of star formation a resulting from inflow of gas into the center of the Triangu- Following Freedman 1988, a value Av = 0.40 mag was adopted for the sum of the Galactic foreground and internal absorbtion. The corresponding lum nebula. O'Connell 1983 estimates a mean nuclear 4 absorptions at other wavelengths are AB = 0.52 mag, Aj· = 0.19 mag and star-formation rate over the last 1 Gyr of ~ 3 X 10~ 1 AK = 0.04 mag. Sftoyr" .

# ^

:$.·* : Êilm - è- -»&·· 'i -¾ »

ivi ïifcÉÎJ Im 9^

φφΛ^ i·;':

h3 * il ♦ ^ # : Λ

Fig. 2-100-sec blue exposure of M 33 obtained with the CFH telescope and a CCD detector in 0.4-arc-sec seeing by Kormendy and McClure. Area shown has dimensions of 70 X 113 arc sec. NE at top.

Ciani, DOdorico & Benvenuti 1984 have used Interna- M 33. These authors find that their data are well repre- tional Ultraviolet Explorer (IUE) spectra covering the sented by a model consisting of a young component with range λλ1200-3000, together with published UBV col- an age of ~ 107 years and an older component with an age ors, to constrain the stellar population in the nucleus of ~ 5 X 109 years. Cowley, Crampton & McClure 1982 find

TABLE 3 the exponential, suggesting the presence of a bulge com- UBV Photometry of the Nucleus of M33 ponent. However, close examination of the galaxy images Diaphragm B-V Reference shows that this region is still dominated by the spiral structure of the disk." Since the presence of such struc- 6'.4. 0.65 Sandage 1963b 10.4 14.54 0.68 + 0.05 Walker3 1964 ture can render the traditional decomposition into bulge 13.9 14.16 0.62 -0.06 Walkera 1964 and disk components uncertain, the reality of the nuclear 24 13 .68 0.56 + 0.01 Walker3 1964 bulge in M 33 remains to be established with certainty. From I-band CCD frames and IRAS 12-μιη data Bothun aAll of Walker's measurements are referred to the average brightness of the nebula one diaphragm diameter Ε and W of the nucleus. 1991 finds an upper limit of Mv = —14 for the integrated luminosity of the bulge of M 33. that their scanner observations of the nucleus of M 33 can According to de Vaucouleurs 1959a the outer regions of be represented by young starlight plus a small contribu- the M 33 disk are slightly bluer ((Β — V) = 0.50, {U —B) = tion from old metal-poor stars. —0.17) than are its inner regions {{Β —V) = 0.59, ((7-ß) Wilson et al. 1988 have observed three molecular = —0.04). Comparison of the colors of the M 33 disk with clouds near the nucleus of M 33. For the cloud closest those of the nucleus of M 33 (see Table 3) shows that the to the nucleus to be stable against tidal disruption the nucleus is redder than the disk in{B —V) even though the mass of the inner 100 pc of M 33 must be < 3 X 1073K . o nucleus has an earlier integrated spectral type than the This value is comfortably above the ~ 106 SJÍq mass ob- inner disk/bulge on which it is superimposed (Mayall & tained for the semistellar nucleus of M 33 by Gallagher Aller 1942). This result, which needs to be confirmed et al. 1982. by more modern spectroscopy, suggests that the nucleus According to Rubin & Ford 1986 the nuclear spectrum of M 33 is more heavily reddened than is the disk of of M 33 is unique because it shows [Ν π] λ6548 and λ6583 this galaxy. in emission but Ha in absorption. This suggests that the nitrogen-to-hydrogen ratio is particularly high for the gas For an assumed distance of795 kpc the inner 2.5 kpc of situated in the nucleus of M 33. Rubin and Ford point out the disk of the Triangulum nebula has a mass-to-light ratio that nuclear Ha absorption is likely to be overwhelmed in the range 2.0 to 2.5 (Zaritsky et al. 1989). Boulesteix & by disk Ha emission in more distant spiral galaxies. Monnet 1970 showed that the mass-to-light ratio in M 33 The nucleus of M 33 (Trinchieri, Fabbiano & Peres increases by a factor of eight between 5' and 40' (1-9 kpc) 1988; Peres et al. 1989) contains an X-ray source with an from the nucleus. This observation constitutes the first 0.1-6 keV luminosity of ~ 1.5 X 1039 ergs-1, which makes clear detection of dark matter in the halo of the Triangu- this object the brightest steady X-ray source in the Local lum nebula. Group. In many ways the semistellar core of M 33 resem- In the regions between the spiral arms of M 33 Sandage bles an active galactic nucleus in a low state of activity. 1956 and Walker 1964 have detected a centrally concen- trated sheet of red stars. According to Sandage 1971 4. Disk and Bulge of M 33 Dixon obtained V = 21.5 ± 0.2 for the brightest Popula- Neutral hydrogen observations (Corbelli, Schneider & tion II stars on the Palomar 5-m plates taken by Sandage. Salpeter 1989) show that the gaseous envelope of M 33 This result should, however, be viewed with some cau- has dimensions of 74' X 135', which are significantly tion because old stars of Population I might provide a larger than the optical dimensions of 53' X 83' (Holmberg significant contribution to the interarm population. The 1958). The sharp cutoff of this neutral hydrogen envelope spiral arms themselves are most clearly outlined by Η π of the Triangulum nebula (Kenney 1990) is believed to be regions (Courtès & Cruvellier 1965) and less clearly by due to ionization by the intergalactic radiation field. Em- OB associations but, perhaps surprisingly (see Fig. 3), not bedded within this hydrogen envelope is an exponential by dust clouds (Humphreys & Sandage 1980) or by neu- stellar disk which, at optical wavelengths, has a scale tral hydrogen gas (Wright, Warner & Baldwin 1972). length of 7.'8 (1.8 kpc) according to de Vaucouleurs 1959a As is the case for most other spiral galaxies, the Η π or 9.'6 (2.2 kpc) according to Kent 1987. regions in M 33 exhibit a radial abundance gradient The existence of a tiny nuclear bulge within this disk (Pagel & Edmunds 1981; Vilchez et al. 1988; Zaritsky remains controversial. Such a bulge was first detected by et al. 1989). The low heavy-element abundance in the Patterson 1940 (see also de Vaucouleurs 1959b) and con- outer part of the Triangulum nebula may account for the firmed by Boulesteix et al. 1979, who found it to fit a de smaller dust absorption that is found in the outer disk of 1/4 Vaucouleurs r law with an effective radius re = 2.'75 and this galaxy by Israel & Kennicutt 1980 and by Wilson, a luminosity of ~ 1% of that of the M 33 exponential disk. Freedman & Madore 1990. It is not yet known whether Possible evidence for such a bulge is also seen in the VRI abundance differences between M 31 and M 33 are re- photometry of Guidoni, Messi & Natali 1981. More re- sponsible for the fact that the Η π regions in the An- cently Kent 1987 writes: "Inside 3' the profile rises above dromeda nebula tend to form associated clumps whereas

Fig. 3-Distribution of dust absorption features in M 33 according to Humphreys & Sandage 1980. Surprisingly, dust clouds and patches do not appear to outline spiral arms. those in the Triangulum nebula often occur as single loops stars and the surface density of neutral hydrogen. Reasons and rings (Arp & Brueckel 1973). It appears unlikely for this are probably that (1) a significant fraction of the gas that these differences between the morphology of Η π in the central regions of M 33 is in molecular form (Wilson regions in M 31 and M 33 are due to the influence of et al. 1988), (2) the thickness of the M 33 gas layer may magnetic fields. This is so because both M 31 and M 33 increase with distance from the nucleus (as it does on the have typical measured total magnetic-field strengths of Galaxy), so that there is no one-to-one correspondence ~ 4 μΟ (Buczilowski & Beck 1991). between the surface density of young luminous stars and the space density of H I, and (3) a minimum gas density 5. Stars in the Disk of M 33 may be required to trigger star formation (Kennicutt Resolution of M 33 into stars was first achieved by the 1989) (but see Wilson, Scoville & Rice 1991 for observa- Earl of Rosse 1850 using his 72-inch reflector. Lundmark tions of M 33 that appear to challenge the notion that such 1921 found that the brightest stars in M 33 have Β — 15.7. a minimum density really exists). Counts of the number of early-type stars with U ^20 over Freedman 1985 has studied the upper ends of the the face of M 33 have been published by Madore, van den stellar luminosity functions in a number of galaxies in- Bergh & Rogstad 1974. Their data show that there is no cluding M 31, M 33, and the LMC. She concludes that simple relationship between the surface density of OB the slope of the stellar luminosity function shows little

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System STELLAR POPULATIONS OF M 33 615 variation from galaxy to galaxy and no dependence on nitrogen-rich layers resulting in an increased WC-to-WN metallicity. ratio. Since M 33 has a radial abundance gradient one According to Golev, Ivanov & Kunchev 1987 many of would therefore expect the WC-to-WN ratio to decrease the brightest objects in M 33, which have 15 < V < 16, with increasing galactocentric distance. This expectation are multiple stars resembling dense stellar groups such as is confirmed by the observations of Massey & Conti 1983. R136a in the Large Magellanic Cloud (LMC). This is According to Schild, Smith and Willis 1990 linewidths of consistent with the work of Massey & Hutchings 1983, early WC stars in M 33 also appear to correlate with who found that many of the brightest starlike objects in galactocentric distance and are loosely connected with H il regions on the Triangulum nebula have UV spectra, ambient metallicity. obtained with IUE, resembling that of R136a in the 30 According to Massey et al. 1987 the total number of Doradus nebula. These authors also note that the ultravi- confirmed WR stars in M 33 is now 80, while 35 good olet extinction curve of M 33 resembles that of the LMC, candidates remain to be observed spectroscopically. rather than that of the Galaxy. Humphreys & Sandage 1980 show that the brightest blue supergiant in M 33 has 6. The Halo of M 33 Mv (1) = —9.4 and that the mean luminosity of the three In a disk field, at a projected distance of 3 kpc from the brightest blue stars is ( Mv (3) ) = —9.3. Subsequently, nucleus of M 33, Wilson et al. 1990 have observed the Humphreys 1980 was able to confirm membership of red-giant branch of Population II stars. It is not yet clear these stars in M 33 spectroscopically. Furthermore, whether most of these objects belong to the old disk or to Humphreys, Massey & Freedman 1990 were able to the halo of the Triangulum nebula. Fitting the I versus show that Β 342, which is the brightest star in M 33, (V —/) diagram of these objects to that of Galactic globular appears to be a single star with spectral type A5e la. For clusters yields a distance modulus (m — M)0 = 24.6 ± 0.3, the red supergiants Humphreys and Sandage find Mv (1) which is consistent with other distance estimates given in = —8.15 and ( Mv (3) ) = —7.95. These results for the Table 2. A halo field, at a projected distance of 5 kpc from M-type supergiants in M 33 have subsequently been con- the nucleus of M 33, has been studied by Mould & Kris- firmed by Humphreys, Jones & Sitko 1984, who used IJK tian 1986. Their I versus (V—Ζ) color-magnitude diagram photometry to show that the reddening of the M super- shows a rather small dispersion and is reasonably well giants in the Triangulum nebula is highly variable and fitted by the color-magnitude diagrams of metal-poor ranges from Av = 0.3 mag to Av = 1.4 mag. From CCD Galactic globular clusters. This result is consistent with photometry of OB stars Wilson 1990 finds ( EB_V ) = 0.3 the discovery of six RR Lyrae suspects in this same field for young luminous stars. by Pritchet and van den Bergh (Pritchet 1988). A more Walker 1964 found that the ratio of the number of blue detailed discussion of these variables will be given in to red supergiants in M 33 varied with distance from the Pritchet & van den Bergh 1992. nucleus of that galaxy. This conclusion subsequently ap- peared to be confirmed by Humphreys & Sandage 1980, 7. Star Clusters in M 33 who also noted that the ratio of blue to red supergiants Christian & Schommer 1982 have published a catalog was greatest near the nucleus of M 33. A similar radial of 250 nonstellar objects in or near M 33. These authors variation in the ratio of blue to red (B/R) supergiants may also provide BVR photometry for some 60 possible clus- exist in the Milky Way system (Humphreys 1978). How- ters and references to previous work on star clusters in the ever, a recent study by Freedman 1985 does not confirm Triangulum nebula. Additional cluster photometry is the radial B/R variation found earlier by Walker 1964 and given by Cohen, Persson & Searle 1984 and by Christian Humphreys & Sandage 1980. Recent CCD observations & Schommer 1988. The color-magnitude diagrams for the (Wilson 1990) show no gradient in the ratio of red to blue integrated light of clusters in M 33 (Christian & Schom- supergiants over the inner 2 kpc of M 33. Data on OB mer 1988) and in the LMC (van den Bergh 1981b) are associations at larger radii (and lower metallicities) should shown in Figure 4. This figure shows two striking differ- be obtained to strengthen and confirm this conclusion. It ences between these two galaxies. In the first place the is particularly important to carry out such observations brightest blue clusters in the LMC are significantly more over a large enough field so that good statistical subtrac- luminous than those in M 33, i.e., the Large Cloud con- tion of foreground M dwarfs can be obtained. tains more "populous clusters" (van den Bergh 1991). The rate at which massive stars lose matter in radiation- (The actual difference is larger than suggested by the driven winds is expected to increase with metallicity figure because the LMC data are based on photoelectric (Smith 1988). A star will become a WR star when most of aperture photometry (which excludes the outer parts of its hydrogen atmosphere has been removed by such large populous clusters), whereas the observations of winds. A metal-rich O star will therefore become a WR M 33 clusters give true integrated cluster magnitudes.) star sooner (and longer) than a more metal-poor O star. Only S 135 (Mv = -8.8, {Β-V) = 0.11), and C 39 (Mv Rapid mass loss will also speed up the removal of the outer = —8.8, (B—V) = 0.56) seem to rival LMC populous

0.0 0.5 1.0 0.0 0.5 1.0 B-V Fig. 4-Comparison of the color-magnitude diagrams for the integrated colors of clusters in M 33 and the LMC. The Large Cloud is seen to contain more bright blue clusters than M 33. Furthermore, the color gap between blue and red clusters is more pronounced in the LMC than it is in the Triangulum nebula. clusters in luminosity. A second difference between the 1991. The results of these authors are shown in Figure 5. Large Cloud and the Triangulum nebula is that the color This figure shows that the velocity differences AV = gap between blue and red clusters in the LMC is much y (disk) — V(cluster) are quite small for (ß —V) < 0.6 and more pronounced than it is in M 33. Part of this difference much larger for {Β — V) > 0.6. According to Schommer is probably due to a more continuous history of cluster et al. 1991 the clusters with (Β — V) > 0.6 have a velocity 1 formation in M 33. However, larger average reddening dispersion of — 70 km s" and exhibit little or no systemic values in the Triangulum nebula might also contribute to rotation. This shows that the oldest red clusters in M 33 filling the color gap at (ß —V) — 0.5. Christian & Schom- belong to a halo population. In this respect the oldest mer 1988 conclude that the ages of M 33 clusters do not clusters in the Triangulum nebula appear to differ from their counterparts in the LMC (Freeman, Illingworth & exhibit a hiatus in their star-forming history like that Oemler 1983; Storm et al. 1991), which seem to be associ- which is observed in the LMC. ated with the (thick) disk of the Large Cloud. The inte- The M 33 clusters with (ß — V) > 0.6 appear to be true grated magnitude of the nuclear bulge of M 33 is > —14. globular clusters. Cohen et al. 1984 showed that four of From this it follows that the specific globular frequency these objects have integrated spectra which indicate that (Harris & van den Bergh 1981) would have to be unrea- they are more metal poor than the Galactic globular sonably high (S > 50) if the halo globular clusters were cluster 47 Tucanae. Christian & Schommer 1983a con- cluded from their integrated spectra that none of the reddest clusterlike objects in M 33 are as metal poor as the Galactic globular cluster M 15 or as metal rich as the strong-lined clusters in M 31. Recently Brodie & Huchra 1990, 1991 have obtained spectra for 22 clusters in M 33. These authors find a mean metallicity ( [Fe/H] ) = —1.55 ± 0.37 for the M 33 clusters, compared to ( [Fe/H] ) = — 1.21 ± 0.02 for M 31. The observed difference in the mean metallicity between these two cluster systems is in the sense expected (van den Bergh 1975; Brodie & Huchra 1991) from the luminosity difference of their parent galaxies, i.e., the most luminous galaxy (M 31) has the most metal-rich globular-cluster system. Christian & Schommer 1983b found that the velocities of the younger bluer clusters in M 33 closely match the 0.4 0.6 B-V disklike motion of the gas, while the older redder clus- Fig. 5-Velocity difference AV = V(cluster) — V(disk) in km s_1 versus ters have more discrepant velocities. This conclusion is cluster color (Schommer et al. 1991). The plot shows that old M 33 strongly confirmed by the recent work of Schommer et al. clusters with (B —V)> 0.6 belong to a halo population.

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System STELLAR POPULATIONS OF M 33 617 associated with the M 33 bulge. Apparently a galaxy Such holes might have been produced by supernova- can be embedded in a major globular-cluster system with- driven winds, or they could be due to the fact that hydro- out having a significant bulge population. This indicates gen in star-forming regions is mainly in molecular, rather that galactic halos are not simply a continuation of the than atomic, form. bulges to larger radii, i.e., the nuclear bulges and halos Neutral hydrogen in M 33 has been mapped by Deul & of galaxies are distinct population components. On the van der Hülst 1987. Furthermore, Wilson & Scoville other hand, observations of Galactic globular clusters 1989 have studied CO emission in the central part of the (Armandroff 1989) and of globulars in the LMC (Freeman Triangulum nebula. From these observations the central et al. 1983; Storm et al. 1991) suggest that some globular 3.9 kpc2 of M 33 is found to contain 3.4 X 107 SKq 0f clusters can also be associated with an old thick-disk molecular hydrogen in addition to 1.3 X 107 STÍq 0f atomic population. In the Large Cloud these disk globular clus- hydrogen (Wilson et al. 1991). Assuming that 2/3 of all gas ters are quite metal poor, whereas the thick-disk clusters (by mass) is in the form of hydrogen, it then follows that in the Galaxy are metal rich. the total amount of gas in the central 3.9 kpc2 of M 33 is 7 According to Harris 1991 the globular clusters in M 33 7.5 X 10 SÍq- Assuming a mean reddening EB_V = 0.3 2 have ( Mv )= —7.0 ± 0.2, with a dispersion σ = 1.2 mag. one then finds that the central 3.9 kpc of the Triangulum This indicates that the luminosity function of the M 33 nebula emits 4.8 X 1039 erg s1 in Ha. Adopting a modi- globulars does not differ significantly from that of the fied Miller-Scalo luminosity function (Kennicutt 1983), globular clusters in M 31 and the Milky Way system (van this yields a total rate of star formation, for stars with SKq den Bergh 1985). Accurate cluster reddening data will, > 0.1 SKq' 0f 0.043 SKq yr1. (The corresponding rate for however, be required to substantiate this conclusion. stars with SJÍq ^ 10 Ϊ01Θ is 0.0068 SKq yr M The time scale for exhaustion of all gas in the central 3.9 kpc-2 of M 33 is 8. OB Association and Spiral Structure therefore 1.7 X 109 yr for stars with SKq > 0-1 3Wq· This Photographs of M 33 show that the disk of this galaxy is time scale would be approximately doubled (Wilson 1991) peppered with a clumpy distribution of early-type stars. A if recycling of gas by evolving stars is taken into account. catalog containing positions and CCD UBV magnitudes The evolutionary time scale would, of course, be longer for 2499 bright blue stars and for 396 red stars in M 33 will for a "top-heavy" mass spectrum of star formation (Scalo soon be published by Freedman & Ivanov 1992. Because 1990). If only stars with SJÍq ^ are formed the time of the high density of stellar images, the OB associations scale for gas depletion becomes 11 X 109 yr. These results in M 33 are much more difficult to identify than they are indicate that the gas-depletion time scale in the central in M 31. Humphreys & Sandage 1980 were able to isolate region of M 33 is significantly shorter than the Hubble 143 OB associations in the Triangulum nebula. The distri- time. A similar problem in the Galactic disk near the Sun bution of these associations (see Fig. 6) clearly outlines was first noticed by van den Bergh 1957 and is discussed the two major spiral arms of M 33. Additional spiral-like in detail by Sandage 1986. interarm features may also be identified with a lower degree of confidence. From UBV photometry of individ- 9. Cepheids in M 33 ual stars, Ivanov 1987 was able to identify 460 OB associa- As has been previously noted, the first variable stars tions in M 33. The mean diameter of these associations in M 33 were discovered by Duncan 1922 and indepen- is ~ 100 pc. Many of the associations appear to form larger dently by Wolf 1923. This work opened the way for dumpings with dimensions of — 0.5 kpc, which are prob- Hubbie s 1926 definitive study which showed that 35 ably similar to the "constellations" (McKibben Nail & of the variable stars in M 33 are classical Cepheids. Due Shapley 1953; van den Bergh 1981b) in the Large Magel- to systematic errors of photometric scales at faint lanic Cloud. magnitudes (Sandage & Carlson 1983; Christian & Twenty associations have recently been discovered by Schommer 1987), and problems with internal absorption Ivanov, Georgiev & Kunchev 1989 in the outermost re- within the Triangulum nebula. Hubbie s pioneering work gions of the M 33 disk. This brings the total number of remained unsurpassed for over half a century. These associations known in the Triangulum nebula to 480. problems have now been largely overcome by employing From the distribution of associations in M 33 (see Fig. 7) CCD detectors working at long wavelengths, or by using the scale length of the OB association distribution is 9!9 multicolor photometry of Cepheids (Freedman, Wilson (2.3 kpc) for the region with ß < 8 kpc. This value is very & M adore 1991). Presently available data, which are similar to the 9^6 disk scale length determined by Kent compiled in Table 2 (which, as noted previously, is based 1987 but longer than the 7.^ disk scale length found by de on an assumed distance modulus (m—M)0 = 18.45 for Vaucouleurs 1959a. the LMC), show reasonably good agreement between Deul & den Hartog 1990 have observed that the posi- distance determinations obtained independently by dif- tions of small (D < 200 pc) holes in the M 33 Η I distribu- ferent observers. The distances based on the Cepheid tion correlate well with the positions of OB associations. period-luminosity relation yield an unweighted mean

141 • o1 .45 o0 ^09 (] •14,

o© Ûœ

CP,« . is , ^ . ra Ql!l ■ cS C? .. • ■. α, . . 0'"·

10 Λ * ' **

Fig. 6-Distribution of OB associations in M 33 according to Humphreys & Sandage 1980. Note that the two major spiral arms are clearly outlined by associations.

10. Long-Period Variables Infrared JHK photometry of 53 long-period variable stars in M 33 has been reported by Mould et al. 1990. From a comparison of the distribution of stars in the K-band period-luminosity diagram these authors find that the difference in the distance moduli of M 33 and the LMC in the K- band is δ (m — M )K = 6.09 ± 0.14 mag. The difference in the true distance moduli of these two galaxies may actually be ~ 0.02 mag smaller than this (Feast 1988) because of the larger average extinction in M 33. With a LMC distance modulus (m—M)0 = 18.45 ±0.1 (van den Bergh 1989) one therefore obtains {m—M)0 = 24.52 ± 0.17. Mould et al. 1990 found that the long-period variables in M 33 scatter more about the adopted period-luminosity relation than do those in the LMC. The reasons for this could be that M 33, which is metal richer and more luminous than the LMC, may exhibit a larger range of absorption values than the Large Cloud. Alternatively, supergiant variables in M 33 might exhibit a larger intrin- sic range in metallicities than do those in the Large Cloud. Such a larger range is expected because the interstellar gas in the Triangulum nebula exhibits a metallicity gradient (Pagel & Edmunds 1981), while the Bar of the LMC has presumably kept the gas in the Large Cloud stirred up. As a consequence of the resulting streaming in the interstellar gas no abundance gradient is expected (or observed) in the LMC. Fig. 7-Normalized and rectified surface density of associations in M 33 according to Ivanov 1987. For fí < 8 kpc the observed density distribu- According to van den Bergh, Herbst & Kowal 1975 the tion has a scale length of 9.'9 (2.3 kpc). brightest red supergiant variable in M 33 has V (max) — 17.0. These authors also find that the brightest red vari- true distance modulus ( (m—M)0 ) = 24.46. This com- ables preferentially occur in the outer regions of M 33. pares favorably with an unweighted mean of five determi- The light curves of the brightest blue variables in the nations based on nova, LP variables, RR Lyrae stars, Triangulum nebula are discussed by Sharov 1990. and color-magnitude diagrams which yields ( (m—M)0 ) = 24.58. 11. Nova Frequency in M 33 Unfortunately, presently available data on variables are Delia Valle 1988 has used systematic observations of not yet numerous or (homogeneous) enough to study the M 33 obtained at the Asiago Observatory to derive a nova distribution of Cepheids with differing periods relative to rate of 3.5 per year. After making allowance for the fact the spiral arms of M 33. that some novae may have occurred outside the survey Wilson & Scoville 1991 show that the relative offset of area, Delia Valle estimates a total nova rate of 4 ± 2 per the H I and CO peaks in the southern spiral arm of M 33 year for the Triangulum nebula. This value may be com- is not consistent with a picture in which atomic gas con- pared to a rate of 29 ± 4 novae per year in M 31 (Capacci- denses in the arms to form molecular clouds. According to olietal. 1989). Since the absorption-corrected blue lumi- Wilson 1990 the ordering of the ages of eight associations nosity of M 31 is 10.4 times greater than that of M 33, one provides weak evidence for the presence of a spiral den- might have predicted an M 33 nova rate of 2.8 ± 0.4 per sity wave in the southern spiral arm of M 33. However, year. The data are therefore consistent with the hypothe- two other associations run counter to this trend. The ages sis that the nova rate is approximately proportional to the and positions of eight associations in the main northern blue luminosity of a galaxy. This conclusion is, however, spiral arm of M 33 are more consistent with a spiral arm at variance with that of Ciardullo et al. 1987, who con- generated by self-propagating star formation. It is disap- cluded that novae in M 31 are mainly associated with the pointing that these detailed observations of the spiral central bulge of that galaxy. structure in M 33 do not provide us with a conclusive An independent estimate of the nova rate in M 33 may choice between the density wave and the self-propagating be obtained from observations of novae in the LMC, star-formation interpretations of spiral arms. which has a stellar population which is rather similar to

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 620 SIDNEY VAN DEN BERGH that in the Triangulum nebula. Capaccioli et al. 1991 From = —19.07 and SKq (^) ^ +5.48 it follows estimate the Large Cloud nova rate to be 2 ± 1 per year. that M 33 has an absorption-corrected luminosity L0 = Since M 33 is ~ 2 times more luminous than the LMC 6.6 X 109 Lq (B ). With a supernova rate of 5.2 h2 SNu one (in blue light), the predicted nova rate in M 33 is ~ 4 ± 2, would therefore predict that the supernova frequency in which is in excellent agreement with the estimate of Delia M 33 should be 3.4 h2 per century. In fact, no supernova Valle 1988. has occurred in the Triangulum nebula for at least one The luminosity of the Milky Way system is only very century. From Poisson statistics the a priori probability poorly known (van den Bergh 1988b). Racine 1991 esti- that no supernova occurs, when the rate is 3.4 per cen- mates a Galaxy/M 31 population ratio of 0.56 from globu- tury, is only 3%. This suggests that either (1) the super- lar-cluster counts. Application of the same ratio to nova nova rate in Sc galaxies has been greatly overestimated by rates yields a Galactic nova rate of ~ 16 per year. This van den Bergh & Tammann 1991 or (2) the Hubble value is significantly lower than the value 73 ± 24 novae parameter is much smaller than 100 km s"1 Mpc-1. per year recently estimated by Liller & Meyer 1987. This From the Lyman continuum flux, a luminosity cor- suggests that Liller & Meyer probably overestimated the 0 9 rected for internal absorption L {B) = 6.4 X 10 Lg(B), incompleteness of Galactic nova discoveries. and the assumption that all stars with SKq > 8 SK© become One of the M 33 novae (Richter & Börngen 1981) supernova, Berkhuijsen 1984 derives a mean-time inter- appears to have been a halo object located 1?34 (19 kpc) + val of 286 *g8 years between core-collapse supernovae in from the nucleus of this galaxy. M 33. This value is marginally shorter than the values 12. Supernova Frequency calculated previously from supernova remnant diameters and E /n = 1051 erg cm3. The relation between the age and the diameter of a 0 It is puzzling that Reynolds & Fix 1987 have found no supernova remnant (SNR) is (neglecting the short free- flat-spectrum Grab-like objects among the eleven super- expansion phase) given by an adiabatic (Sedov 1959) solu- nova remnants which they have studied in M 33. tion of the form 11 115 215 D(pc) = 4.3 X 10" {E0ln) t (years) , (1) 13. X-Ray Sources The X-ray sources in M 33 have recently been dis- in which E0 is the initial explosion energy and η is the density of hydrogen atoms in the interstellar medium. cussed by Trinchieri et al. 1988 and by Peres et al. 1989. From this equation the times required to reach a SNR In addition to the nuclear source, which is ten times 4 51 brighter than any of the other point sources, there is diameter D = 30 pc are 1.3 X 10 yr for E0/n = 10 erg 3 4 50 3 diffuse X-ray emission in the plane of M 33. The hard cm and 4.0 X 10 yr for E0/n = 10 erg cm . According to Sabbadin 1979, M 33 contains between 54 (> 2 keV) component of this diffuse emission probably and 67 supernova remnants with D < 30 pc. A more represents the integrated light produced by accreting recent survey by Long et al. 1990 lists 25 supernova systems and young supernova remnants. On the other remnants with D ^ 30 pc. After allowing for incomplete- hand the soft (< 1 keV) component of the disk X-ray ness in the Long et al. survey, the total number of super- emission is likely due to the integrated emission of stellar nova remnants with D ^ 30 pc in M 33 is estimated to be coronae, old supernova remnants, and, perhaps, hot dif- ~ 40. The corresponding supernova frequencies are one fuse gas in the disk of M 33. 51 3 per 325 yr for E0/n = 10 erg cm and one per 1000 yr for 50 3 14. Desiderata E0/n = 10 erg cm . Adopting = -19.07 (Sandage & Tammann 1981) for the absorption-free total blue lumi- a. It would be of interest to carry out a complete search nosity of M 33 yields supernova rates of 0.45 S Nu and for Gepheids in M 33. This would allow one to study the 0.15 SNu, respectively, in which one SNu is defined to be distribution of Gepheids of differing periods (ages) rela- a rate of one supernova per century per 1010 L© (B) of tive to spiral arms. Such an investigation is particularly parent galaxy luminosity. These rates fall significantly attractive because the main southern spiral arm in M 33 is below the rates of 1.9 h2 SNu found by Evans, van den the best-defined spiral structure in the Local Group. Bergh & McGlure 1989 and 5.2 h2 SNu derived by van b. Photometry and spectroscopy of a complete sample den Bergh & Tammann 1991 for all supernovae in galaxies of old clusters in M 33 might allow one to determine the of types Sbc-Sd. Either (1) extragalactic supernova rates luminosity function of its globular clusters with more have been overestimated, (2) most supernova do not leave confidence. It would be particularly important to know if long-lasting remnants, (3) the parameter h = H/100 km all metal-poor clusters in M 33 belong to the halo subsys- 1 1 51 3 s Mpc" is <^1, or (4) E0/n > 1 X 10 erg cm for tem. supernova in M 33. (Dennefeld & Kunth 1981 advocate a c. A systematic discovery program for novae might 51 value E0/n = 5 X 10 , which would increase the super- allow one to establish if the progenitors of the majority of nova rate in M 33 by a factor of 2.2.) these objects belong to a pure disk population.

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