Star Clusters In M33 Research Articles

Ages and Masses of Star Clusters in M33: a Multiwavelength Study

Abstract We combine archival images for the nearby galaxy M33 (Triangulum Galaxy) from the ultraviolet (UV) to the infrared to derive ages, masses, and extinctions for the young star cluster population, and compare our physical parameters with published ones. Our goal is to test the robustness of clusters ages and masses, and possibly improve on existing ones both by expanding the wavelength range of the spectral-energy distribution (SED) fits and by using more recent population synthesis models. The rationale for this experiment is to verify the sensitivity of the clusters physical parameters to observational setups and model choices that span those commonly found in the literature. We derive the physical parameters of 137 clusters, using SEDs measured in eight UV-to-I bands, including Hα, from GALEX and ground-based images. We also add the 24 μm image from the Spitzer Space Telescope to help break some age degeneracies. We find that our derived cluster ages show significant differences with earlier determinations, while the masses remain relatively insensitive to the fitting approach adopted. We also highlight an already known difficulty in recovering old, low-extinction clusters, as SED-fitting codes tend to prefer younger, higher extinction solutions when the extinction is a free parameter. We publish updated ages, masses, and extinctions, with uncertainties for all sample star clusters, together with their photometry. Given the proximity of M33, this represents an important population to secure for the study of star formation and cluster evolution in spirals.

The massive M31 cluster G1: detailed chemical abundances from integrated light spectroscopy★

ABSTRACT G1, also known as Mayall II, is one of the most massive star clusters in M31. Its mass, ellipticity, and location in the outer halo make it a compelling candidate for a former nuclear star cluster. This paper presents an integrated light abundance analysis of G1, based on a moderately high-resolution (R = 15 000) spectrum obtained with the high-resolution spectrograph on the Hobby–Eberly Telescope in 2007 and 2008. To independently determine the metallicity, a moderate-resolution (R ∼ 4000) spectrum of the Ca ii triplet lines in the near-infrared was also obtained with the Astrophysical Research Consortium’s 3.5-m telescope at Apache Point Observatory. From the high-resolution spectrum, G1 is found to be a moderately metal-poor cluster, with $[\rm {Fe/H}]~=~-0.98\pm 0.05$. G1 also shows signs of α-enhancement (based on Mg, Ca, and Ti) and lacks the s-process enhancements seen in dwarf galaxies (based on comparisons of Y, Ba, and Eu), indicating that it originated in a fairly massive galaxy. Intriguingly, G1 also exhibits signs of Na and Al enhancement, a unique signature of GCs – which suggests that G1’s formation is intimately connected with GC formation. G1’s high [Na/Fe] also extends previous trends with cluster velocity dispersion to an even higher mass regime, implying that higher mass clusters are more able to retain Na-enhanced ejecta. The effects of intracluster abundance spreads are discussed in a subsequent paper. Ultimately, G1’s chemical properties are found to resemble other M31 GCs, though it also shares some similarities with extragalactic nuclear star clusters.

Gibbs point process model for young star clusters in M33

ABSTRACT We demonstrate the power of Gibbs point process models from the spatial statistics literature when applied to studies of resolved galaxies. We conduct a rigorous analysis of the spatial distributions of objects in the star formation complexes of M33, including giant molecular clouds (GMCs) and young stellar cluster candidates (YSCCs). We choose a hierarchical model structure from GMCs to YSCCs based on the natural formation hierarchy between them. This approach circumvents the limitations of the empirical two-point correlation function analysis by naturally accounting for the inhomogeneity present in the distribution of YSCCs. We also investigate the effects of GMCs’ properties on their spatial distributions. We confirm that the distribution of GMCs and YSCCs are highly correlated. We found that the spatial distributions of YSCCs reaches a peak of clustering pattern at ∼250 pc scale compared to a Poisson process. This clustering mainly occurs in regions where the galactocentric distance ≳4.5 kpc. Furthermore, the galactocentric distance of GMCs and their mass have strong positive effects on the correlation strength between GMCs and YSCCs. We outline some possible implications of these findings for our understanding of the cluster formation process.

Comparisons of Different Fitting Methods for the Physical Parameters of a Star Cluster Sample of M33 with Spectroscopy and Photometry

Star clusters are good tracers for formation and evolution of galaxies. We compared different fitting methods by using spectra (or by combining photometry) to determine the physical parameters. We choose a sample of 17 star clusters in M33, which previously lacked spectroscopic observations. The low-resolution spectra were taken with the Xinglong 2.16-m reflector of NAOC. The photometry used in the fitting includes $\rm u_{SC}$ and $\rm v_{SAGE}$ bands from the SAGE survey, as well as the published $UBVRI$ and $ugriz$ photometry. We firstly derived ages and metallicities with the {\sc ULySS} (Vazdekis et al. and {\sc pegase-hr}) SSP model and the Bruzual \& Charlot (2003) (BC03) stellar population synthesis models for the full-spectrum fitting. The fitting results of both the BC03 and {\sc ULySS} models seem consistent with those of previous works as well. Then we add the SAGE $\rm u_{SC}$ and $\rm v_{SAGE}$ photometry in the spectroscopic fitting with the BC03 models. It seems the results become much better, especially for the Padova 2000+Chabrier IMF set. Finally we add more photometry data, $UBVRI$ and $ugriz$, in the fitting and we found that the results do not improve significantly. Therefore, we conclude that the photometry is useful for improving the fitting results, especially for the blue bands ($\lambda <4000$ {\AA}), e.g., $\rm u_{SC}$ and $\rm v_{SAGE}$ band. At last, we discuss the "UV-excess" for the star clusters and we find five star clusters have UV-excess, based on the $GALEX$ FUV, NUV photometry.

PHAT XX. AGB Stars and Other Cool Giants in M31 Star Clusters

Abstract The presence of asymptotic giant branch (AGB) stars in clusters provides key constraints for stellar models, as has been demonstrated with historical data from the Magellanic Clouds. In this work, we look for candidate AGB stars in M31 star clusters from the Panchromatic Hubble Andromeda Treasury survey. Our photometric criteria selects stars brighter than the tip of the red giant branch, which includes the bulk of the thermally pulsing AGB stars as well as early-AGB stars and other luminous cool giants expected in young stellar populations (e.g., massive red supergiants, and intermediate-mass red helium-burning stars). The AGB stars can be differentiated, a posteriori, using the ages already estimated for our cluster sample. 937 candidates are found within the cluster aperture radii, half (∼450) of which are very likely cluster members. Cross-matching with additional databases reveals two carbon stars and 10 secure variables among them. The field-corrected age distribution reveals the presence of young supergiants peaking at ages smaller than 108 yr, followed by a long tail of AGB stars extending up to the oldest possible ages. This long tail reveals the general decrease in the numbers of AGB stars from initial values of at 108 yr down to at 1010 yr. Theoretical models of near-solar metallicity reproduce this general trend, although with localized discrepancies over some age intervals, whose origin is not yet identified. The entire catalog is released together with finding charts to facilitate follow-up studies.

The Ages of M31 Star Clusters: Spectral Energy Distribution versus Color–Magnitude Diagram

Abstract It is well-known that fitting Color–Magnitude Diagrams (CMDs) to theoretical isochrones is the main method to determine star cluster ages. However, when CMDs are not available, the Spectral Energy Distribution (SED)-fitting technique is the only other approach, although it suffers the age–metallicity–reddening degeneracy. In this work, we gather the ages, metallicities, and masses of dozens of M31 star clusters from the CMD-fitting with Hubble Space Telescope images from the literature for comparison. We check the reliability of the SED-fitting results with different models, i.e., Bruzual &amp; Charlot model (BC03), Galaxy Evolutionary Synthesis Models (GALEV), and Advanced Stellar Population Synthesis (ASPS) for the simple stellar populations (SSPs) with single stars (ss)-SSP/binary-star (bs)-SSPs models. The photometry bands includes the Galaxy Evolution Explorer GALEX FUV/NUV bands, optical/near-infrared UBVRIJHK bands, as well as the Wide-field Infrared Survey Explorer (WISE) W1/W2 bands. The comparisons show that the SED-fitting ages agree well with the CMD-fitting ages, either with the fixed metallicity or with the free metallicity for both the BC03 and the GALEV model. However, for the ASPS models, it seems that SED-fitting results are systematically older than the CMD ages, especially for the ages log t &lt; 9.0 (year). The fitting also shows that the GALEX FUV/NUV-band are more important than the WISE W1/W2 for constraining the ages, which confirms the previous findings. We also derived the masses of our sample star clusters from the BC03 and GALEV models and it is found that the values agree well with those in the literature.

The effects of the WISE/GALEX photometry for the SED-fitting with M31 star clusters and candidates

Spectral energy distribution (SED) fitting of stellar population synthesis models is an important and popular way to constrain the physical parameters ---e.g., the ages, metallicities, masses for stellar population analysis. The previous works suggest that both blue-bands and red-bands photometry works for the SED-fitting. Either blue-domained or red-domained SED-fitting usually lead to the unreliable or biased results. Meanwhile, it seems that extending the wavelength coverage could be helpful. Since the Galaxy Evolution Explorer ({\it GALEX}) and Wide-field Infrared Survey Explorer (WISE) provide the FUV/NUV and mid-infrared $W1$/$W2$ band data, we extend the SED-fitting to a wider wavelength coverage. In our work, we analyzed the effect of adding the FUV/NUV and $W1$/$W2$ band to the optical and near-infrared $UBVRIJHK$ bands for the fitting with the Bruzual \& Charlot 2003 (BC03) models and {\sc galev} models. It is found that the FUV/NUV bands data affect the fitting results of both ages and metallicities much more significantly than that of the WISE $W1$/$W2$ band with the BC03 models. While for the {\sc galev} models, the effect of the WISE $W1$/$W2$ band for the metallicity fitting seems comparable to that of {\it GALEX} FUV/NUV bands, but for age the effect of the $W1$/$W2$ band seems less crucial than that of the FUV/NUV bands. Thus we conclude that the {\it GALEX} FUV/NUV bands are more crucial for the SED-fitting of ages and metallicities, than the other bands, and the high-quality UV data (with high photometry precision) are required.

Deriving physical parameters of unresolved star clusters

This study is the third of a series that investigates the degeneracy and stochasticity problems present in the determination of physical parameters such as age, mass, extinction, and metallicity of partially resolved or unresolved star cluster populations situated in external galaxies when using broad-band photometry. This work tests the derivation of parameters of artificial star clusters using models with fixed and free metallicity for the WFC3+ACS photometric system. Then the method is applied to derive parameters of a sample of 203 star clusters in the Andromeda galaxy observed with the HST. Following Papers I \& II, the star cluster parameters are derived using a large grid of stochastic models that are compared to the observed cluster broad-band integrated WFC3+ACS magnitudes. We derive the age, mass, and extinction of the sample of M31 star clusters with one fixed metallicity in agreement with previous studies. Using artificial tests we demonstrate the ability of the WFC3+ACS photometric system to derive the metallicity of star clusters. We show that the metallicity derived using photometry of 36 massive M31 star clusters is in a good agreement with the metallicity previously derived using spectroscopy taken from literature.

THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY. V. AGES AND MASSES OF THE YEAR 1 STELLAR CLUSTERS

We present ages and masses for 601 star clusters in M31 from the analysis of the six filter integrated light measurements from near ultraviolet to near infrared wavelengths, made as part of the Panchromatic Hubble Andromeda Treasury (PHAT). We derive the ages and masses using a probabilistic technique, which accounts for the effects of stochastic sampling of the stellar initial mass function. Tests on synthetic data show that this method, in conjunction with the exquisite sensitivity of the PHAT observations and their broad wavelength baseline, provides robust age and mass recovery for clusters ranging from $\sim 10^2 - 2 \times 10^6 M_\odot$. We find that the cluster age distribution is consistent with being uniform over the past $100$ Myr, which suggests a weak effect of cluster disruption within M31. The age distribution of older ($>100$ Myr) clusters fall towards old ages, consistent with a power-law decline of index $-1$, likely from a combination of fading and disruption of the clusters. We find that the mass distribution of the whole sample can be well-described by a single power-law with a spectral index of $-1.9 \pm 0.1$ over the range of $10^3-3 \times 10^5 M_\odot$. However, if we subdivide the sample by galactocentric radius, we find that the age distributions remain unchanged. However, the mass spectral index varies significantly, showing best fit values between $-2.2$ and $-1.8$, with the shallower slope in the highest star formation intensity regions. We explore the robustness of our study to potential systematics and conclude that the cluster mass function may vary with respect to environment.

STAR CLUSTERS IN M33: UPDATED UBVRI PHOTOMETRY, AGES, METALLICITIES, AND MASSES

The photometric characterization of M33 star clusters is far from complete. In this paper, we present homogeneous $UBVRI$ photometry of 708 star clusters and cluster candidates in M33 based on archival images from the Local Group Galaxies Survey, which covers 0.8 deg$^2$ along the galaxy's major axis. Our photometry includes 387, 563, 616, 580, and 478 objects in the $UBVRI$ bands, respectively, of which 276, 405, 430, 457, and 363 do not have previously published $UBVRI$ photometry. Our photometry is consistent with previous measurements (where available) in all filters. We adopted Sloan Digital Sky Survey $ugriz$ photometry for complementary purposes, as well as Two Micron All-Sky Survey near-infrared $JHK$ photometry where available. We fitted the spectral-energy distributions of 671 star clusters and candidates to derive their ages, metallicities, and masses based on the updated {\sc parsec} simple stellar populations synthesis models. The results of our $\chi^2$ minimization routines show that only 205 of the 671 clusters ($31\%$) are older than 2 Gyr, which represents a much smaller fraction of the cluster population than that in M31 ($56\%$), suggesting that M33 is dominated by young star clusters ($<1$ Gyr). We investigate the mass distributions of the star clusters---both open and globular clusters---in M33, M31, the Milky Way, and the Large Magellanic Cloud. Their mean values are $\log(M_{\rm cl}/M_{\odot})=4.25$, 5.43, 2.72, and 4.18, respectively. The fraction of open to globular clusters is highest in the Milky Way and lowest in M31. Our comparisons of the cluster ages, masses, and metallicities show that our results are basically in agreement with previous studies (where objects in common are available); differences can be traced back to differences in the models adopted, the fitting methods used, and stochastic sampling effects.

Additional Newly Identified Star Clusters in M31

ABSTRACTUsing the HST archives, we have searched portions of the southern half and northwest quarter of the Andromeda Galaxy (M 31, NGC 224) for previously unrecognized star clusters. The result is a table of the characteristics of 71 newly identified star clusters. Most are moderately faint, low-mass clusters. Absolute magnitudes range from M(V) = -6.3 to -2.5.

NEWUBVRIPHOTOMETRY OF 234 M33 STAR CLUSTERS

This is the second paper of our series. In this paper, we present $UBVRI$ photometry for 234 star clusters in the field of M33. For most of these star clusters, there is photometry in only two bands in previous studies. The photometry of these star clusters is performed using archival images from the Local Group Galaxies Survey, which covers 0.8 deg$^2$ along the major axis of M33. Detailed comparisons show that, in general, our photometry is consistent with previous measurements, especially, our photometry is in good agreement with Zloczewski & Kaluzny. Combined with the star clusters' photometry in previous studies, we present some results: none of the M33 youngest clusters ($\sim 10^7$ yr) have masses approaching $10^5$ $M_{\odot}$; comparisons with models of simple stellar populations suggest a large range of ages of M33 star clusters, and some as old as the Galactic globular clusters.

AGE AND MASS STUDIES FOR YOUNG STAR CLUSTERS IN M31 FROM SEDS-FIT

In this paper, we present photometry for young star clusters in M31, which are selected from Caldwell et al. These star clusters have been observed as part of the Beijing--Arizona--Taiwan--Connecticut (BATC) Multicolor Sky Survey from 1995 February to 2008 March. The BATC images including these star clusters are taken with 15 intermediate-band filters covering 3000--10000 \AA. Combined with photometry in the {\sl GALEX} far- and near-ultraviolet, broad-band $UBVRI$, SDSS $ugriz$, and infrared $JHK_{\rm s}$ of Two Micron All Sky Survey, we obtain their accurate spectral energy distributions (SEDs) from 1538-20000 \AA. We derive these star clusters' ages and masses by comparing their SEDs with stellar population synthesis models. Our results are in good agreement with previous determinations. The mean value of age and mass of young clusters ($<2$ Gyr) is about 385 Myr and $2\times 10^4 {M_\odot}$, respectively. There are two distinct peaks in the age distribution, a highest peak at age $\sim$ 60 Myr and a secondary peak around 250 Myr, while the mass distribution shows a single peak around $10^4 {M_\odot}$. A few young star clusters have two-body relaxation times greater than their ages, indicating that those clusters have not been well dynamically relaxed and therefore have not established the thermal equilibrium. There are several regions showing aggregations of young star clusters around the 10 kpc ring and the outer ring, indicating that the distribution of the young star clusters is well correlated with M31's star-forming regions. The young massive star clusters (age $\leq 100$ Myr and mass $\geq 10^4 {M_\odot}$) show apparent concentration around the ring splitting region, suggesting a recent passage of a satellite galaxy (M32) through M31 disk.

Newly identified star clusters in M33 - III. Structural parameters

We present the morphological properties of 161 star clusters in M33 using the Advanced Camera For Surveys Wide Field Channel onboard the Hubble Space Telescope using observations with the F606W and F814W filters. We obtain, for the first time, ellipticities, position angles, and surface brightness profiles for a significant number of clusters. On average, M33 clusters are more flattened than those of the Milky Way and M31, and more similar to clusters in the Small Magellanic Cloud. The ellipticities do not show any correlation with age or mass, suggesting that rotation is not the main cause of elongation in the M33 clusters. The position angles of the clusters show a bimodality with a strong peak perpendicular to the position angle of the galaxy major axis. These results support the notion that tidal forces are the reason for the cluster flattening. We fit King and EFF models to the surface brightness profiles and derive structural parameters including core radii, concentration, half-light radii and central surface brightness for both filters. The surface brightness profiles of a significant number of clusters show irregularities such as bumps and dips. Young clusters (Log age < 8) are notably better fitted by models with no radial truncation (EFF models), while older clusters show no significant differences between King or EFF fits. M33 star clusters seem to have smaller sizes, smaller concentrations, and smaller central surface brightness as compared to clusters in the MW, M31, LMC and SMC. Analysis of the structural parameters presents a age-radius relation also detected in other star cluster systems. The overall analysis shows differences in the structural evolution between the M33 cluster system and cluster systems in nearby galaxies. These differences could have been caused by the strong differences in these various environments.

Systematic differences in simple stellar population model results: application to the M31 globular-like cluster system

Simple stellar population (SSP) synthesis models are useful tools for studying the nature of unresolved star clusters in external galaxies. However, the plethora of currently available SSP models gives rise to significant and poorly documented systematic differences. Here we consider the outputs of the commonly used Bruzual & Charlot and GALEV models, as well as a recently updated SSP model suite which attempts to include the contributions of binary merger products in the form of blue straggler stars (BS-SSP). We rederive the ages, metallicities, extinction values and masses of 445 previously observed globular-like clusters in M31 based on chi-square minimisation of their spectral energy distributions with respect to these three different SSP models and adopting a Chabrier-like stellar initial mass function. A comparison between our new results and previous estimates of the same parameters shows that the Bruzual & Charlot models yield the youngest ages and lowest masses, while adoption of the BS-SSP models results in the oldest ages and highest mass estimates. Similarly, the GALEV SSP models produce the lowest metallicities, with the highest values resulting from the BS-SSP model suite. These trends are caused by intrinsic differences associated with the models, and are not significantly affected by the well-known age-metallicity degeneracy. Finally, we note that the mass function of the massive M31 star clusters is similar to that of the Milky Way's globular clusters, which implies that the two star cluster systems likely formed under similar environmental conditions.

AN UPDATED CATALOG OF M33 CLUSTERS AND CANDIDATES:UBVRIPHOTOMETRY AND SOME STATISTICAL RESULTS

We present $UBVRI$ photometry for 392 star clusters and candidates in the field of M33, which are selected from the most recent star cluster catalog. In this catalog, the authors listed star clusters' parameters such as cluster positions, magnitudes and colors in the $UBVRIJHK_s$ filters, and so on. However, a large fraction of objects in this catalog do not have previously published photometry. Photometry is performed using archival images from the Local Group Galaxies Survey, which covers 0.8 deg$^2$ along the major axis of M33. Detailed comparisons show that, in general, our photometry is consistent with previous measurements. Positions (right ascension and declination) for some clusters are corrected here. Combined with previous literature, we constitute a large sample of M33 star clusters. Based on this cluster sample, we present some statistical results: none of the M33 youngest clusters ($\sim 10^7$ yr) have masses approaching $10^5$ $M_{\odot}$; roughly half the star clusters are consistent with the $10^4$ to $10^5$ $M_{\odot}$ mass models; the continuous distribution of star clusters along the model line indicates that M33 star clusters have been formed continuously from the epoch of the first star cluster formation until recent times; there are $\sim 50$ star clusters which being overlapped with the Galactic globular clusters on the color-color diagram, and these clusters are old globular clusters candidates in M33.

A COMPREHENSIVE GALEX ULTRAVIOLET CATALOG OF STAR CLUSTERS IN M31 AND A STUDY OF THE YOUNG CLUSTERS

We present a comprehensive catalog of 700 confirmed star clusters in the field of M31 compiled from three major existing catalogs. We detect 418 and 257 star clusters in Galaxy Evolution Explorer (GALEX) near-ultraviolet (NUV) and far-ultraviolet (FUV) imaging, respectively. Our final catalog includes photometry of star clusters in up to 16 passbands ranging from FUV to NIR as well as ancillary information such as reddening, metallicity, and radial velocities. In particular, this is the most extensive and updated catalog of UV integrated photometry for M31 star clusters. Ages and masses of star clusters are derived by fitting the multi-band photometry with model spectral energy distribution (SED); UV photometry enables more accurate age estimation of young clusters. Our catalog includes 182 young clusters with ages less than 1 Gyr. Our estimated ages and masses of young clusters are in good agreement with previously determined values in the literature. The mean age and mass of young clusters are about 300 Myr and 10^4 M_sun, respectively. We found that the compiled [Fe/H] values of young clusters included in our catalog are systematically lower (by more than 1 dex) than those from recent high-quality spectroscopic data and our SED fitting result. We confirm that most of the young clusters kinematics show systematic rotation around the minor axis and association with the thin disk of M31. The young clusters distribution exhibits a distinct peak in the M31 disk around 10 - 12 kpc from the center and follow a spatial distributions similar to other tracers of disk structure such as OB stars, UV star-forming regions, and dust. Some young clusters also show concentration around the ring splitting regions found in the southern part of the M31 disk and most of them have systematically younger (< 100 Myr) ages.

The structure of star clusters in the outer halo of M31

We present a structural analysis of halo star clusters in M31 based on deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging. The clusters in our sample span a range in galactocentric projected distance from 13 to 100 kpc and thus reside in rather remote environments. Ten of the clusters are classical globulars, while four are from the Huxor et al. (2005, 2008) population of extended, old clusters. For most clusters, contamination by M31 halo stars is slight, and so the profiles can be mapped reliably to large radial distances from their centres. We find that the extended clusters are well fit by analytic King (1962) profiles with ~20 parsec core radii and ~100 parsec photometric tidal radii, or by Sersic profiles of index ~1 (i.e. approximately exponential). Most of the classical globulars also have large photometric tidal radii in the range 50-100 parsec, however the King profile is a less good fit in some cases, particularly at small radii. We find 60% of the classical globular clusters exhibit cuspy cores which are reasonably well described by Sersic profiles of index ~2-6. Our analysis also reinforces the finding that luminous classical globulars, with half-light radii <10 parsec, are present out to radii of at least 100 kpc in M31, which is in contrast to the situation in the Milky Way where such clusters (other than the unusual object NGC 2419) are absent beyond 40 kpc.

THE M33 GLOBULAR CLUSTER SYSTEM WITH PAndAS DATA: THE LAST OUTER HALO CLUSTER?

We use CFHT/MegaCam data to search for outer halo star clusters in M33 as part of the Pan-Andromeda Archaeological Survey (PAndAS). This work extends previous studies out to a projected radius of 50 kpc and covers over 40 square degrees. We find only one new unambiguous star cluster in addition to the five previously known in the M33 outer halo (10 kpc <= r <= 50 kpc). Although we identify 2440 cluster candidates of various degrees of confidence from our objective image search procedure, almost all of these are likely background contaminants, mostly faint unresolved galaxies. We measure the luminosity, color and structural parameters of the new cluster in addition to the five previously-known outer halo clusters. At a projected radius of 22 kpc, the new cluster is slightly smaller, fainter and redder than all but one of the other outer halo clusters, and has g' ~ 19.9, (g'-i') ~ 0.6, concentration parameter c ~ 1.0, a core radius r_c ~ 3.5 pc, and a half-light radius r_h ~ 5.5 pc. For M33 to have so few outer halo clusters compared to M31 suggests either tidal stripping of M33's outer halo clusters by M31, or a very different, much calmer accretion history of M33.

A Photometric Catalog of 77 Newly Recognized Star Clusters in M31

ABSTRACTThis paper describes the results of a Hubble Space Telescope (HST) WFPC2 search for star clusters in active star-formation regions of M31. Nine of the clusters were previously cataloged and 77 are new. Our 23 fields cover key areas of the galaxy’s recent star-formation activity. We provide a catalog of positions and integrated magnitudes in four colors, taken with the 336W, 439W, 555W, and 814W filters of the HST. A future article will discuss the results of stellar photometry in some of the clusters in six colors, including two additional uv colors. The integrated magnitudes and colors of the clusters show a range of characteristics, but the mean color is bluer than for previous surveys, reflecting the concentration of our sample on active star-forming regions. Absolute magnitudes range from M555 = -10.3 to -3.5. The observed luminosity function shows a nearly Gaussian distribution with a peak value at M555 = -5.4 and a shoulder of unusually bright clusters. We look in detail at two of these unusually bright examples, cluster 45 (C410) and cluster 10 (BH05). C410 lies at the core of a bright H II region. Its absolute magnitude is M555 = -10.3. BH05 is a similar object, with an absolute magnitude of M555 = -8.9. These two clusters are among the most luminous young clusters in M31.