New Clusters in M33

Newly Identified Star Clusters in M33. II.
Radial HST/ACS Fields 111Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.

Izaskun San Roman and Ata Sarajedini Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 32611-2055 izaskun@astro.ufl.edu, ata@astro.ufl.edu Donald R. Garnett 801 W. Wheatridge Drive, Tucson AZ 85704 drgarnett@msn.com Jon A. Holtzman New Mexico State University, Las Cruces, NM holtz@nmsu.edu
Abstract

We present integrated photometry and color-magnitude diagrams for 161 star clusters in M33, of which 115 were previously uncataloged, using the Advanced Camera For Surveys Wide Field Channel onboard the Hubble Space Telescope. The integrated V-band magnitudes of these clusters range from –9 to as faint as –4, extending the depth of the existing M33 cluster catalogs by 1 mag. Comparisons of theoretical isochrones to the color-magnitude diagrams using the Padova models yield ages for 148 of these star clusters. The ages range from Log (t)7.0 to Log (t)9.0. Our color-magnitude diagrams are not sensitive to clusters older than 1 Gyr. We find that the variation of the clusters’ integrated colors and absolute magnitudes with age is consistent with the predictions of simple stellar population models. These same models suggest that the masses of the clusters in our sample range from to M.

galaxies: individual (M33) – galaxies: spiral – galaxies: star clusters – galaxies: stellar content
slugcomment: Accepted by The Astrophysical Journal

1 Introduction

Star clusters provide a unique and powerful probe useful for studying the star formation histories of galaxies. In particular, the ages, metallicities and kinematics of star clusters bear the imprint of the galaxy formation process. These correlations underscore the importance of compiling complete catalogs of clusters in nearby galaxies. At a distance of  870kpc (distance modulus=24.69; Galleti et al. (2004)), M33 is the only nearby late-type spiral galaxy and it provides a notable connection between the cluster populations of earlier-type spirals and the numerous, nearby later-type dwarf galaxies.

There have been a number of M33 cluster catalogs published since the pioneering work of Hiltner (1960). The reader is referred to the work of Sarajedini & Mancone (2007) (hereafter referred to as SM), which merged all of the modern catalogs compiled before 2007, for a summary of the properties of all of these catalogs. Even the results of Park & Lee (2007), which appeared after the publication of SM have been incorporated into the web-based version of the SM catalog 222http://www.astro.ufl.edu/ata/cgi-bin/m33_cluster_catalog/index.cgi. This catalog contains 479 candidates of which 264 are confirmed clusters based on HST and high resolution ground-based imaging. The most recent work in this field is that of Zloczewski et al. (2008)(hereafter ZKH), which presents a catalog of 4780 extended sources in a 1 square degree region around M33 including 3554 new candidate stellar clusters. For the purpose of the present paper, it is important to take special note of the two cluster catalogs by Chandar et al. (1999, hereafter CBF99; 2001, hereafter CBF01, or collectively as CBF) and the one by Sarajedini et al (2006) because we will adopt similar reduction and analysis techniques as these previous studies.

In general, ground-based imaging cannot clearly distinguish a star cluster from another type of extended source (e.g. nebula, galaxy), but Hubble Space Telescope (HST) imaging with the Wide-Field Planetary Camera 2 (WFPC2) or the Advanced Camera for Surveys (ACS) provides the spatial resolution necessary to provide an unequivocal determination. It is for this reason that we have undertaken the present study. In addition, and just as importantly, the HST observations allow us to construct color-magnitude diagrams (CMDs) of the star clusters, which can be used to estimate ages and investigate any correlations that might exist between cluster ages and integrated photometric properties as well as allowing us to determine star cluster masses. Section 2 describes the observations and data reduction, and §3 discusses the identification and integrated magnitudes of the clusters. §4 includes a statistical analysis of the most recent previous survey of M33 clusters by Zloczewski et al. (2008). The analysis of the CMDs is presented in §5 while an analysis of the star cluster properties is in §6. Lastly, §7 presents our conclusions.

2 Observations and Data Reduction

The observations for the present study were obtained with the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) onboard the Hubble Space Telescope (HST). With a pixel scale of and a field of view of 3.3’ x 3.3’, ACS/WFC is able to resolve individual stars at the distance of M33. Twelve HST/ACS fields from the GO-10190 program (PI: D. Garnett) have been analyzed. Four primary fields were obtained along the major axis of M33. Eight coordinated parallel fields were also secured along both sides of the primary-field axis. Figure 1 shows the locations of these fields. We have three filters for the primary observations (F475W, F606W, F814W) and two filters for the parallel images (F606W, F814W). Table 1 presents a summary of the observations.

All of the images were calibrated through the standard pipeline-process and downloaded from the HST archive. The ‘FLT’ images (Table 1), which were used for the point source photometry, were first multiplied by the geometric correction image to correct for the fact that each ACS pixel subtends a different angle on the sky. Then, the data quality files were applied by setting the values of bad pixels to a large negative number so that the photometry software will ignore them. The photometry was performed using the DAOPHOT/ALLSTAR and ALLFRAME routines (Stetson 1994) following the same procedure as that used by Sarajedini et al. (2000). A detailed description of how the point spread functions (PSFs) were constructed has been presented in Sarajedini et al. (2006). The corresponding frames in the F606W and F814W filters were matched to obtain mean instrumental magnitudes of common stars which were then matched to form colors. The photometry has been corrected for the charge transfer efficiency (CTE) problem that ACS suffers using the prescription of Riess & Mack (2004). In addition, the theoretical transformation of Sirianni et al. (2005) was used in order to convert the magnitudes to the ground-based Johnson-Cousins system.

The pipeline-processed drizzled (‘DRZ’) images were used for the integrated cluster photometry and were similarly obtained from the HST archive. In fields where multiple drizzled images were produced by the pipeline, we derived positional offsets between these frames using the imshift and imcombine tasks in IRAF to allow us to produce one master image per filter per field. The drizzling process removes the sky background and corrects the counts to an exposure time of one second. As a result, to make the calculation of photometric errors more straightforward, each DRZ image was multiplied by the exposure time and the background sky value was added back before performing photometry on these frames. The resultant images were then used in the derivation of the integrated cluster photometry.

3 Cluster Identification and Photometry

Star clusters are easily resolved on the HST/ACS images so the selection of objects has been done by visual inspection of each image. This is the technique employed in several previous papers where HST imaging was used (CBF, Sarajedini et al. 2006). Figure 2 shows sample images of some of the 161 clusters identified in this study. The cluster positions have been determined by convolving each image with an elliptical gaussian of =10 pixels. This kernel size was chosen so that the convolution process would yield a smooth cluster profile that is conducive to the next step, which is the application of the IRAF imexamine task. This routine was applied to the smoothed cluster profiles to determine the cluster centers. The optimum pixel coordinate positions were transformed to right ascension and declination using the World Coordinate System in the image headers. Based on the work of SM, which compared the positional accuracy of several catalogs with those determined using the Local Group Survey images of Massey et al. (2006), we estimate an internal precision of approximately 0.1 arcsec and an absolute accuracy of about 0.4 arcsec for these quoted cluster positions.

The integrated magnitudes and colors of each cluster have been calculated using the aperture photometry routines in DAOPHOT (Stetson 1987). To be consistent with the previous work of Sarajedini et al. (2006), we have adopted an aperture radius of 2.2” for the magnitude measurements and 1.5” for the colors. Note that CBF used the same aperture size for their magnitudes but a variable aperture ranging from 1.0” to 2.2” within which to measure cluster colors. Like in the CBF study, the background sky is always determined in an annulus with an inner radius of 3.5” and an outer radius of 5.0”. Once again, these magnitudes have been corrected for CTE based on Riess & Mack (2004) and calibrated to the ground-based system using the synthetic transformations of Sirianni et al. (2005).

Table 2 details the position of each cluster as well as its V magnitude, B–V color, V–I color, reddening, age, mass and also, if applicable, the alternative identification in Sarajedini & Mancone (2007). We could not determine the integrated magnitudes for two of the clusters because of their location near the edge of the field. The formal random errors on the magnitudes and colors are all less than 0.01 mag because of the high signal-to-noise ratio of these clusters. Cluster candidates number 139 and 59 in the catalog of Sarajedini & Mancone (2007) have been rejected as clusters in this study based on visual inspection.

3.1 Comparison with Previous Photometry

Comparing our results with the CBF study, we find a mean difference of V(Us-CBF)= 0.09 0.06 and (V-I)= 0.07 0.03 while comparing with Ma et al. (2001; 2002a; 2002b), we arrive at a difference of V(Us-Ma)= 0.04 0.05, where the uncertainties are standard errors of the mean. As we shall see below, these photometric differences are not unexpected for integrated photometry of extended objects such as star clusters (see also Table 2 of Sarajedini & Mancone 2007). Fig. 3 shows the offset in magnitude and color in both cases. The integrated B magnitudes are not plotted due to a lack of significant numbers of clusters with which to compare.

In order to examine these results in more detail, we analyzed the magnitude and color offsets as a function of field and position. Not surprisingly, the mean difference is larger in fields and positions near the center of the galaxy as a result of the higher degree of crowding. Analysis of the distance distribution of our sample reveals that 50% of the CBF clusters with measured V magnitudes are inside a distance of 1.7 kpc from the center of M33. The mean magnitude difference for the clusters inside this region is V(Us-CBF)= 0.17 0.07 while for those outside this region, the difference is V(Us-CBF)= 0.02 0.09. Examining different distance ranges, moving progressively outward to include more CBF clusters, we find the same tendency where clusters in the inner (more crowded) regions display a larger mean magnitude difference as compared with the outer (less crowded) regions.

It is important to note that we also performed additional tests of the photometry to investigate the effects of spatial resolution and errors in cluster centering. The CBF photometry comes mostly from the Wide Field (WF) CCDs that are part of the WFPC2 onboard HST. The spatial resolution of the WF CCDs is roughly 4 times coarser as compared with ACS/WFC. We performed photometry of a subset of our clusters using ACS/WFC images that were resampled to replicate the resolution of the WF CCDs. We found no significant difference between these results and the magnitudes as measured on the original ACS/WFC frames. Additionally, we analyzed the sensitivity of our photometry to the adopted cluster center by using the values from the Sarajedini & Mancone (2007) catalog, which are measured from ground-based images taken from the Local Group Survey using the MOSAIC instrument as well as from the work of CBF. Again, we find no significant difference in the magnitudes and colors of the clusters we have measured.

A further check of our photometry is provided by the realization that, as shown in Fig. 1, several of our fields exhibit significant overlap. There are 10 clusters that have multiple measurements of their magnitudes and colors: one common cluster between d2 and f4, five between f1 and f2 and four between d3 and f7. Comparing the photometry of these common clusters, we obtain an offset in V magnitude of V=0.042 0.026 and an offset in color of (V–I)=0.060 0.012.

At first glance, this level of disagreement between measurements of the same clusters in different fields seems to be a potential cause for concern. However, an examination of a similar situation encountered by CBF99 and CBF01 reveals that their studies show offsets of the same order. In particular, as pointed out by SM, there are 3 clusters in common between CBF99 and CBF01, and these objects appear on two different WFPC2 fields. The mean difference in the V mags of these clusters is V(CBF99-CBF01) = 0.106 0.048.

3.2 Comparison with Previous Catalogs

As noted in §1, the SM catalog is the pre-eminent compilation containing all of the existing information for M33 star clusters up to 2007. Of the 161 clusters identified in the present work, 46 have been previously cataloged in Sarajedini & Mancone (2007). Fig. 4 shows the integrated magnitude CMD for all of the genuine clusters in the SM catalog. We see that the clusters span a magnitude range of 16 V20, which corresponds to –9–5, using our adopted distance modulus of =24.69 (Galleti et al., 2004). Fig. 4 also illustrates the locations of the clusters identified in the present study. This new sample extends to –4, which is 1 mag fainter than the least luminous clusters in the SM catalog, closer to the faintest clusters in the Large Magellanic Cloud (Sarajedini & Mancone, 2007).

We can also compare our cluster catalog with the most recent catalog produced by Zloczewski et al. (2008)(hereafter ZKH); they present a photometric survey for stellar clusters in M33 based on deep ground-based images obtained with the MegaCam instrument on the Canada-France-Hawaii telescope, and their classifications are based on visual inspection of the images. Their catalog contains 4780 extended sources in a region approximately 1 x 1 degree. Among these, there are 3554 new candidate stellar clusters of which 122 are relatively bright, likely globular clusters.

Sixty of the 3554 objects are present in our HST/ACS fields. The ZKH catalog suggests that 51 of these are new candidate stellar clusters; however our study shows that only 21 objects resolve into stars and therefore appear to be clusters. Some of the other objects that ZKH consider to be extended turn out to be close groupings of two or three stars. Others appear to be background galaxies or diffuse nebulae. Table 3 provides a cross-identification of the 21 common objects that are genuine clusters. If we extrapolate the ratio of true clusters to total cluster candidates in the ZKH catalog, this suggests that only around 40 of the 3554 proposed candidates from ZKH will be actual stellar clusters. Nonetheless, even if there are 1400 stellar clusters in M33, it would have the highest number of known star clusters per unit luminosity of any spiral galaxy in the Local Group.

4 Color-Magnitude Diagrams

Intrinsic properties such as age, metallicity and reddening govern the integrated magnitudes and colors of clusters. Using the PSF-photometry of each star in each field we are able to examine these parameters by constructing CMDs of each cluster. Figs. 5 - 11 show radial CMDs and the best fit isochrone for the region around each cluster for seven clusters in our list as an example of the procedure we followed and the range of ages found. The top-left to the bottom-right in the left panel correspond to stars within 1” of the cluster center, between 1” and 2”, 2” and 3” and finally between 3” and 4”. The solid lines represent theoretical isochrones from Girardi et al. (2000) for ages of , and yrs with a metallicity of Z=0.004. This metal abundance has been chosen as a representative mean of the disk abundance gradient based on the work of Kim et al. (2002) (see also Sarajedini et al. 2006). However, we point out that for ages younger than 1 Gyr, there is very little sensitivity of the isochrone-derived age on the assumed metal abundance. The isochrones have been shifted by a distance modulus of =24.69 (Galleti et al., 2004) and a line-of-sight reddening value of E(V-I)=0.06 (Sarajedini et al., 2000). Then, we overplot the isochrones in the observed CMDs looking for the best fit to the main sequence turnoff (MSTO) region. Based on Cardelli et al. (1989), we adopt the following relation between the extinction and the reddening: .

The left panel illustrates the gradual decrease in cluster stars at increasing distance from the cluster center. Consequently, CMDs within 1” of the cluster center display a significant fraction of stars belonging to the cluster revealing features to estimate ages, while the lower panels have been used to monitor non-cluster stars. Right panel in each figure shows the best fit isochrone for stars within 1” of the cluster center. In this way, the ages of 148 star clusters have been estimated from a comparison of the main-sequence turnoff photometry with theoretical isochrones. In some cases, an additional reddening adjustment was needed in order to align the main sequence of the isochrones with the data. In several instances, the main sequence and the turnoff are not satisfactorily defined to permit a comparison with isochrones. We estimate a precision in the isochrone-fitting ages of 0.05 dex based on neighboring isochrones (in age) that could also potentially fit the data. In the case of clusters 12, 15, 18 and 111, the isochrone fit is not as well established and the precision of the age reaches 0.1 dex. The precision of the reddenings is approximately 0.05mag. The ages and reddenings obtained during this process are listed in Table 2.

5 Analysis

The reddening-corrected colors of the clusters should correlate with their ages and metallicities. The upper panel of Fig. 12 shows the variation of integrated cluster V-I color with the estimated isochrone age. These have been reddening corrected using the reddening listed in Table 2. The lines are the expected relations according to simple stellar populations Girardi et al. (2002) with Z=0.004 (solid line), Z=0.001 (dotted line) and Z=0.019 (dashed line). As expected, this plot reveals a positive correlation between cluster age and V–I color in much the same manner as the models predict.

Our data allow us to compare the luminosities of the clusters with their ages in order to derive the cluster masses. The bottom panel of Fig. 12 shows the integrated absolute magnitudes as a function of the estimated isochrone ages. These have been corrected using an extinction based on the reddening listed in Table 2. The dashed lines are the expected relations for simple stellar populations with Z=0.004 taken from Girardi et al. (2002) for masses of , , and M and assuming a Salpeter initial mass function (IMF). This diagram indicates that the clusters in our sample are consistent with the theoretical predictions of simple stellar population fading models. Additionally, the majority of the clusters have masses between and M. These estimated masses would not change significantly if we were to assume other power-law IMFs in constructing the fading lines (Tantalo, 2005) although we should consider them as upper limits in other cases like exponential IMFs (Chabrier, 2001) binary-corrected IMFs (Kroupa, 1998). It should be noted that the dearth of clusters older than years is attributable to the fact that our CMDs are generally not deep enough to detect the main sequence turnoffs of clusters older than this limit.

6 Summary

This paper presents HST/ACS integrated photometry of 161 star clusters in M33 as well as individual stellar photometry in twelve fields. Forty six clusters of the sample have been previously cataloged by Sarajedini & Mancone (2007). Color-magnitude diagrams of each cluster have been constructed in order to determine ages via isochrone fitting. Simple stellar population models reproduce the behavior of the cluster ages with dereddened V–I color as well as with absolute magnitude. We have incorporated these new clusters into the existing catalog of M33 clusters established by Sarajedini & Mancone (2007) increasing by more than 40% the set of confirmed star clusters in this galaxy.

We thank an anonymous referee for comments that greatly improved this paper. We are grateful for support from NASA through grant GO-10190 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract NAS5-26555.

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Field R.A. (J2000.0) Decl. (J2000.0) Filter Exp. Time (s)
D1 01 33 49.89 +30 35 48.12 F606W 2 2480, 1 1300, 1 120
F814W 2 2480, 1 1522, 1 142
F475W 3 700
D2 01 33 39.69 +30 28 59.98 F606W 8 2480, 1 1300, 1 120
F814W 10 2480, 1 1500, 1 120
F475W 3 700
D3 01 33 20.49 +30 22 14.99 F606W 8 2480, 1 1300, 1 120
F814W 10 2480, 1 1500, 1 120
F475W 3 700
D4 01 33 07.89 +30 15 06.98 F606W 8 2480, 1 1300, 1 120
F814W 10 2480, 1 1500, 1 120
F475W 3 700
F1 01 33 40.32 +30 38 39.89 F606W 1 2160
F814W 1 2160
F2 01 33 27.52 +30 39 15.42 F606W 1 2400
F814W 1 2500
F3 01 34 00.23 +30 32 56.43 F606W 1 2400
F814W 1 2500
F4 01 33 26.04 +30 30 37.64 F606W 1 2160
F814W 1 2160
F5 01 34 11.78 +30 27 21.94 F606W 1 2160
F814W 1 2160
F6 01 33 05.39 +30 27 11.88 F606W 1 2400
F814W 1 2500
F7 01 33 13.43 +30 23 29.64 F606W 1 2160
F814W 1 2160
F8 01 33 32.92 +30 17 32.37 F606W 1 2400
F814W 1 2400
Table 1: Observation Summary
ID RA (J2000) Dec (J2000) V (V-I) (B-V) E(V-I) Log AgeaaUnits of age are in years. MassbbUnits of mass are Notes Alter. Id ddIdentification Number in Sarajedini & Mancone (2007)
1 1 32 59.38 30 26 54.12 19.960 0.909 0.10 8.90 6.0
2 1 32 59.95 30 27 19.57 19.736 1.750 0.10 8.55 4.0
3 1 33 0.37 30 26 47.25 19.530 0.744 0.10 8.40 4.0 Small 31
4 1 33 1.50 30 27 47.84 19.896 0.457 0.06 8.20 2.0 Small
5 1 33 2.39 30 26 55.71 19.768 0.472 0.10 8.30 2.5
6 1 33 2.98 30 26 34.56 19.499 1.031 0.10 8.70 6.5
7 1 33 3.29 30 16 2.26 20.348 0.816 0.403 0.10 8.75 3.0
8 1 33 4.35 30 27 13.24 19.798 0.689 0.10 8.45 3.0 Small
9 1 33 5.62 30 14 23.44 19.361 0.789 0.501 0.10 8.50 5.0
10 1 33 5.87 30 28 49.46 19.198 0.493 0.10 8.25 4.0
11 1 33 8.11 30 27 59.77 18.941 0.859 0.10 8.65 10.0 40
12 1 33 8.93 30 16 51.53 20.771 0.876 0.637 0.10 8.85eefootnotemark: 2.5
13 1 33 9.27 30 26 6.78 19.460 0.588 0.10 8.40 4.0
14 1 33 9.77 30 22 35.46 17.806 0.718 0.06 8.15 10.0
15 1 33 10.33 30 23 4.26 17.530 -0.066 0.06 7.60eefootnotemark: 7.5 Small
16 1 33 10.38 30 15 45.93 20.352 1.159 1.043 0.15 8.70 3.0 Small
17 1 33 12.14 30 22 36.97 19.818 1.181 0.06 8.70 4.5
18 1 33 12.95 30 23 7.14 18.984 0.004 0.06 7.70eefootnotemark: 2.5 Small
19 1 33 13.93 30 27 59.99 19.171 0.791
20 1 33 13.96 30 14 31.85 20.218 0.944 0.702 0.15 9.00 5.5
21 1 33 14.04 30 15 16.47 20.075 0.770 0.511 0.20 8.65 4.0 Small
22 1 33 14.33 30 28 22.42 18.252 1.117 0.06 8.80 25.0 49
23 1 33 14.71 30 23 19.05 19.328 0.908 0.15 8.80 9.0
24 1 33 15.22 30 21 14.07 19.355 0.148 -0.092 0.06 8.20 3.0
25 1 33 16.10 30 20 56.55 18.245 0.566 0.244 0.10 8.30 10.0 53
26 1 33 19.09 30 30 10.68 18.917 0.833 0.06 8.50 7.5
27 1 33 19.16 30 23 22.62 18.470 0.886 0.06 9.00 25.0 57
28 1 33 19.91 30 30 20.22 19.359 1.180 0.15 8.80 9.0
29 1 33 21.09 30 37 55.77 18.943 0.804 0.10 8.55 8.5 61
30 1 33 21.31 30 20 31.88 18.184 0.429 0.236 0.06 8.20 8.5
31 1 33 21.34 30 31 0.89 19.351 1.091 0.15 8.80 9.0
32 1 33 21.35 30 31 31.31 18.906 0.871
33 1 33 21.38 30 31 12.56 18.585 0.551 0.06 8.20 6.0
34 1 33 21.57 30 31 50.96 18.636 1.276 62
35 1 33 21.59 30 37 48.69 19.027 0.675 0.15 8.40 6.5 63
36 1 33 22.09 30 40 26.20 18.339 0.382 Small 66
37 1 33 22.14 30 38 28.06 18.999 0.932 0.06 8.50 7.0 Small
38 1 33 22.37 30 30 14.12 17.426 0.484 0.06 8.00 15.0 69
39 1 33 22.67 30 38 0.06 19.235 1.049 0.06 9.20 20.0 Small
40 1 33 22.76 30 38 19.87 19.050 1.050 0.10 8.35 5.5 Small
41 1 33 23.10 30 32 22.69 18.946 0.909 0.15 8.35 6.5 71
42 1 33 23.83 30 40 26.26 19.004 0.753 0.06 8.50 7.0 Small 74
43 1 33 23.84 30 39 36.58 18.882 0.636 0.15 8.30 6.0
44 1 33 24.61 30 37 49.88 18.605 0.784 0.10 8.45 9.5
45 1 33 25.16 30 32 18.06 20.064 1.031 0.10 8.70 4.0
46 1 33 25.33 30 23 38.97 20.091 0.744 0.446 0.10 8.45 2.5 Small
47 1 33 25.73 30 18 1.12 19.719 0.157 0.06 8.25 2.5
48 1 33 25.76 30 31 19.80 19.610 0.927 0.10 8.60 5.0
49 1 33 26.50 30 30 1.71 19.448 0.909 0.10 8.45 4.5 Small
50 1 33 26.52 30 37 55.19 19.530 0.612 0.06 8.45 4.0 Small
51 1 33 26.63 30 31 30.09 18.114 0.546 0.10 8.15 9.0
52 1 33 27.43 30 30 3.38 18.574 0.900 0.15 8.50 12.0
53 1 33 27.86 30 38 49.00 18.004 0.773
54 1 33 29.26 30 29 13.67 18.918 0.681 0.06 8.25 5.0 Small
55 1 33 29.47 30 30 1.88 17.932 0.544 0.10 8.00 9.5 98
56 1 33 29.55 30 29 34.38 19.206 0.796 0.06 8.25 3.5
57 1 33 29.87 30 31 16.14 18.944 0.529 0.06 8.35 5.5 Small
58 1 33 30.18 30 29 33.65 19.514 0.798 0.06 8.25 3.0
59 1 33 30.34 30 37 43.37 18.834 0.771 0.10 8.40 7.0
60 1 33 30.83 30 29 14.10 18.218 0.668 0.10 7.65 4.5 Small
61 1 33 31.31 30 40 20.54 17.966 0.844 0.20 8.45 20.0 110
62 1 33 31.44 30 28 42.00 18.398 0.896 0.119 0.20 8.10 7.0
63 1 33 32.09 30 40 32.02 18.494 0.344 0.10 7.80 4.5 114
64 1 33 32.18 30 30 17.21 18.158 0.459 0.06 8.15 8.0
65 1 33 32.35 30 38 24.70 18.376 0.828 0.20 8.30 10.0 117
66 1 33 32.51 30 39 24.66 18.835 1.028 0.10 8.45 8.0 118
67 1 33 32.75 30 31 44.91 19.395 1.221 0.15 8.65 7.0 Small 120
68 1 33 32.87 30 39 31.82 18.940 0.626 0.10 8.40 6.5
69 1 33 32.88 30 15 46.76 19.893 0.287 0.10 8.15 2.0
70 1 33 33.37 30 29 45.73 19.027 0.938 -0.107 0.06 7.85 3.0 Small
71 1 33 33.63 30 28 9.53 18.167 0.326 -0.047 0.10 8.10 8.0
72 1 33 33.64 30 40 3.15 18.279 0.747 0.10 8.35 10.0 124
73 1 33 33.88 30 39 54.12 18.524 0.665 0.06 8.10 5.5 Small
74 1 33 35.54 30 38 36.90 17.190 0.261 132
75 1 33 35.86 30 27 44.54 19.428 0.622 0.307 0.06 8.40 4.0
76 1 33 36.24 30 27 56.79 19.212 0.960 0.472 0.10 8.60 7.0
77 1 33 36.38 30 15 32.61 20.602 0.904 0.10 8.70 2.5
78 1 33 36.66 30 27 8.04 18.468 0.970 0.278 0.15 8.45 12.0 136
79 1 33 37.33 30 38 38.67 18.805 0.815 0.10 8.20 5.0 Small
80 1 33 37.51 30 28 4.58 17.685 0.809 0.250 0.10 8.25 15.0 141
81 1 33 37.90 30 38 2.42 17.465 1.251 0.10 8.60 35.0 143
82 1 33 37.94 30 18 51.74 19.889 0.646 0.10 8.65 4.0
83 1 33 38.29 30 17 35.99 19.927 1.482 0.06 8.50 3.0
84 1 33 39.08 30 38 25.21 17.501 1.322 0.06 7.70 9.0
85 1 33 39.47 30 28 7.29 19.405 1.361 0.761 0.10 8.90 9.5 Small
86 1 33 39.63 30 31 9.16 16.575 0.331 -0.162 0.06 7.90 30.0 152
87 1 33 39.84 30 38 26.43 16.013 0.879 0.06 7.25 20.0 154
88 1 33 40.12 30 37 45.82 18.035 0.793 0.10 8.10 9.0
89 1 33 41.13 30 29 53.76 18.961 0.854 0.458 0.15 8.35 6.0 159
90 1 33 41.43 30 31 13.39 0.247 -0.136 0.10 7.85 Edge
91 1 33 41.49 30 30 24.11 18.975 0.621 0.130 0.06 8.15 4.0 Small 162
92 1 33 41.53 30 28 9.24 19.005 0.277 -0.046 0.06 8.25 4.5 163
93 1 33 41.69 30 34 57.49 19.446 0.636 -0.267 0.10 8.05 2.5 Small
94 1 33 41.77 30 29 32.49 18.146 0.600 0.161 0.10 8.05 8.0
95 1 33 41.98 30 38 20.55 18.342 0.933 0.15 8.35 10.0
96 1 33 42.62 30 34 58.93 19.579 1.350 0.324 0.06 8.85 7.0
97 1 33 42.62 30 38 22.11 18.350 0.747 0.10 8.05 7.0 Small
98 1 33 42.89 30 27 46.46 19.226 1.155 0.334 0.10 8.60 7.0 Small
99 1 33 42.90 30 38 30.91 18.313 0.889 Small
100 1 33 43.47 30 28 8.20 18.996 1.104 0.600 0.06 8.25 4.5 Small
101 1 33 43.53 30 27 58.28 18.118 0.693 -0.086 0.10 8.15 9.0
102 1 33 43.93 30 36 13.09 19.049 1.029 -0.127 0.06 8.45 6.0
103 1 33 44.00 30 30 0.79 18.164 0.669 0.169 0.10 8.20 9.0 174
104 1 33 44.15 30 35 25.44 19.062 0.928 -0.032 0.20 8.30 5.5
105 1 33 44.35 30 38 5.36 18.194 0.376 0.06 8.20 8.5
106 1 33 44.42 30 37 52.96 17.314 0.681 0.15 8.10 20.0 176
107 1 33 44.49 30 37 6.90 19.723 0.909 -0.102 0.06 8.50 3.5
108 1 33 44.49 30 36 43.50 19.463 1.298 -0.033 0.10 8.20 3.0 Small
109 1 33 44.52 30 39 19.62 18.349 1.053 0.15 8.30 10.0
110 1 33 44.56 30 37 34.16 18.613 0.817 0.15 8.15 6.0
111 1 33 44.64 30 36 35.65 18.303 0.558 -0.799 0.06 7.50eefootnotemark: 3.0 Small
112 1 33 44.84 30 34 38.87 18.808 0.564 -0.333 0.06 7.70 2.5 Small
113 1 33 45.96 30 36 49.35 18.561 0.562 -0.397 0.10 8.20 6.5
114 1 33 46.18 30 35 27.75 18.323 0.764 -0.287 0.06 7.85 5.5 Small
115 1 33 46.75 30 35 59.00 19.829 1.218 -0.057 0.15 8.35 3.0 Small
116 1 33 47.35 30 39 59.41 18.405 0.967 0.10 8.45 12.0
117 1 33 47.44 30 39 3.71 17.975 0.740 0.06 8.05 9.0
118 1 33 48.05 30 39 29.01 18.151 1.196
119 1 33 48.55 30 37 5.08 18.547 1.121 -0.318 0.25 8.00 6.5
120 1 33 48.58 30 35 23.25 19.093 0.901 -0.156 0.15 8.50 7.0
121 1 33 49.53 30 28 42.42 19.198 0.630 0.312 0.20 8.55 7.5
122 1 33 49.58 30 34 25.55 18.361 0.960 -0.213 0.20 8.15 8.0 191
123 1 33 49.87 30 36 34.28 18.022 0.949 -0.346 0.06 8.00 8.5
124 1 33 50.15 30 34 18.67 19.098 0.922 -0.026 0.20 8.45 7.0 192
125 1 33 50.98 30 35 24.43 19.688 1.042 -0.026 Small
126 1 33 51.20 30 34 12.97 18.479 0.747 -0.234 0.10 8.10 6.0 201
127 1 33 51.78 30 34 18.44 19.630 0.959 0.023 Small
128 1 33 52.34 30 35 0.56 18.937 0.743 -0.186 0.20 8.10 4.5 207
129 1 33 52.35 30 34 20.92 18.967 0.866 -0.124 0.20 8.35 6.5 208
130 1 33 53.30 30 33 2.89 18.938 1.007 0.15 8.40 7.0 212
131 1 33 54.16 30 36 6.73 19.437 0.849 -0.083 0.20 8.30 4.0
132 1 33 54.39 30 32 23.90 18.611 0.997 0.15 8.60 14.0
133 1 33 54.59 30 34 48.08 19.167 1.237 -0.153 0.20 8.40 6.0 217
134 1 33 54.66 30 32 15.71 17.644 0.392 0.06 8.05 12.0 220
135 1 33 54.91 30 32 14.46 18.454 0.596 0.10 8.30 8.5 221
136 1 33 55.64 30 33 44.62 18.553 0.934 0.10 8.35 8.5 Small
137 1 33 56.42 30 36 10.42 18.702 1.012 -0.167 0.20 8.10 5.5 231
138 1 33 57.08 30 34 15.09 18.286 0.375 0.06 7.85 5.5 Small
139 1 33 57.75 30 33 25.67 17.033 0.406 0.10 7.90 20.0 243
140 1 33 57.80 30 35 31.57 18.221 1.072 0.072 0.15 8.45 15.0 241
141 1 33 57.84 30 32 21.86 18.749 1.940 0.15 8.60 12.0
142 1 33 58.87 30 33 29.30 18.948 0.640 0.10 8.35 6.0 Small
143 1 33 59.19 30 33 45.56 18.126 0.483 0.15 8.25 12.0 Small
144 1 33 59.53 30 36 24.30 18.555 0.581 -0.304 0.10 8.10 5.5
145 1 33 59.70 30 32 0.31 19.271 1.333 Small
146 1 33 59.88 30 33 54.27 16.273 0.960 0.06 7.50 20.0 260
147 1 34 1.63 30 32 25.60 18.493 0.948 0.10 8.50 12.0 271
148 1 34 1.74 30 34 6.17 18.995 1.020 0.12 8.90 15.0 Small
149 1 34 2.25 30 32 37.75 18.659 1.187
150 1 34 3.25 30 27 56.15 17.842 0.500 0.06 8.10 10.0
151 1 34 3.44 30 33 41.47 18.943 0.511 0.10 8.30 5.5
152 1 34 4.60 30 27 21.66 19.722 0.814 0.06 8.80 6.0
153 1 34 6.68 30 28 4.43 19.187 0.674 0.06 8.20 3.5
154 1 34 6.85 30 32 0.08 18.314 0.798 0.10 8.55 15.0 306
155 1 34 7.96 30 31 18.36 Edge
156 1 34 9.64 30 25 5.03 19.626 0.335 0.06 8.50 4.0
157 1 34 12.88 30 28 46.31 18.037 0.619 0.10 7.80 6.5
158 1 34 13.48 30 28 44.11 19.721 0.873 0.10 8.75 5.5
159 1 34 13.93 30 27 59.02 18.342 1.767 0.06 7.95eefootnotemark: 6.0 346
160 1 34 17.72 30 27 8.99 20.037 0.657 0.15 8.40 2.5
161 1 34 18.62 30 27 19.31 19.729 0.945 0.10 8.85 6.5

Note. – Units of RA are hours, minutes, and seconds, and units of Dec are degrees, arcminutes, and arcseconds. ccfootnotetext: Error in Log Age of 0.1.

Table 2: Cluster Properties
ID(ZKH) ID(us) R.A. (J2000.0) Decl. (J2000.0) Notes Type aaProposed classification in ZKH: -1 galaxy, 0 unclassified, 1 likely stellar cluster and 2 an already known high confidence cluster included in Sarajedini & Mancone (2007)
25-1-009 1 1 32 59.38 30 26 54.12 0
25-1-008 2 1 32 59.95 30 27 19.57 0
25-1-003 5 1 33 02.39 30 26 55.71 0
25-1-001 6 1 33 02.98 30 26 34.56 0
34-2-001 7 1 33 03.29 30 16 02.26 1
33-4-018 9 1 33 05.62 30 14 23.44 1
33-5-022 12 1 33 08.93 30 16 51.53 0
33-6-016 14 1 33 09.77 30 22 35.46 1
33-5-019 16 1 33 10.38 30 15 45.93 0
33-6-014 17 1 33 12.14 30 22 36.97 1
33-6-010 18 1 33 12.95 30 23 07.14 Small 0
33-5-014 20 1 33 13.96 30 14 31.85 0
33-5-013 21 1 33 14.04 30 15 16.47 Small 0
33-6-009 23 1 33 14.71 30 23 19.05 1
33-6-008 24 1 33 15.22 30 21 14.07 0
33-6-006 25 1 33 16.10 30 20 56.55 2
33-3-021 27 1 33 19.16 30 23 22.62 1
33-3-020 30 1 33 21.31 30 20 31.88 0
33-2-010 47 1 33 25.73 30 18 01.12 0
33-2-003 69 1 33 32.88 30 15 46.76 0
32-5-024 77 1 33 36.38 30 15 32.61 0

Note. – Units of RA are hours, minutes, and seconds, and units of Dec are degrees, arcminutes, and arcseconds.

Table 3: Cross identification with Zloczewski et al. (2008)
Figure 1: Location of our observed ACS/WFC fields overplotted on an image of M33. North is up and east is to the left. White squares are primary fields and black squares are parallel fields.
Figure 2: Representative sample of star clusters present in our fields in the F606W filter. Each image is shown with the same gray-scale intensity and 5” arsec on a side, with north up and east to the left.
Figure 3: Comparison of the integrated cluster photometry from the present study with that of CBF and Ma et al. (2001; 2002a; 2002b).
Figure 4: Color-magnitude diagram of the genuine M33 clusters from the SM catalog (open circles) as compared with those from the present study (filled circles). The distribution of the two types of points is largely similar except that the present catalog contains more faint star clusters than that of SM. The faintest clusters here have –4.0 rivaling the faintest globular clusters in the Milky Way and populous clusters in the LMC.
Figure 5: Left: CMD for the region around cluster 7. The top left panel shows the stars within 1” of the cluster center, the top right panel includes the stars between 1” and 2”, while the stars between 2” and 3” and 3” and 4” are displayed in the bottom right panel and the bottom left, respectively. The solid lines correspond to isochrones from Girardi et al. (2000) for ages of , and yr and a metallicity of Z=0.004. Right: The best fit isochrone for stars within 1” of the cluster center.
Figure 6: Same as Fig. 5, but for cluster 9.
Figure 7: Same as Fig. 5, but for cluster 24.
Figure 8: Same as Fig. 5, but for clusters 20.
Figure 9: Same as Fig. 5, but for cluster 30.
Figure 10: Same as Fig. 5, but for cluster 38.
Figure 11: Same as Fig. 5, but for cluster 63.
Figure 12: The top panel shows the variation of the clusters’ integrated (V–I) colors with age. The correction for reddening has been effected using the reddening values obtained in the isochrones fitting process. The lines represent the expected relations for a simple stellar population from Girardi et al. (2002) with Z=0.004 (solid line), Z=0.001(dotted line) and Z=0.019 (dashed line). The lower panel shows the extinction corrected absolute magnitude as a function of cluster age. The dashed lines are the expected relations for a simple stellar population from Girardi et al. (2002) with Z=0.004 and masses of ,, and .
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