Doug Geisler

A search for old star clusters in the large magellanic cloud

We report the first results of a color-magnitude diagram survey of 25 candidate old LMC clusters. For almost all of the sample, it was possible to reach the turnoff region, and in many clusters we have several magnitudes of the main sequence. Age estimates based on the magnitude difference

Ages and Metallicities of Star Clusters and Surrounding Fields in the Outer Disk of the Large Magellanic Cloud

delta T_1

Washington Photometry of the Globular Cluster System of NGC 4472. II. The Luminosity Function and Spatial Structure

between the giant branch clump and the turnoff revealed that no new old clusters were found. The candidates turned out to be of intermediate age (1-3 Gyr) We show that the apparently old ages as inferred from integrated UBV colors can be explained by a combination of stochastic effects produced by bright stars and by photometric errors for faint clusters lying in crowded fields. The relatively metal poor candidates from the CaII triplet spectroscopy also turned out to be of intermediate age. This, combined with the fact that they lie far out in the disk, yields interesting constraints regarding the formation and evolution of the LMC disk. We also study the age distribution of intermediate age and old clusters This homogeneous set of accurate relative ages allows us to make an improved study of the history of cluster formation/destruction for ages

Standard Giant Branches in the Washington Photometric System

>1

Hubble Space Telescope WFPC2 Color-Magnitude Diagrams of Halo Globular Clusters in M33: Implications for the Early Formation History of the Local Group*

Gyr. We confirm previous indications that there was apparently no cluster formation in the LMC during the period from 3-8 Gyr ago, and that there was a pronounced epoch of cluster formation beginning 3 Gyrs ago that peaked at about 1.5 Gyrs ago. Our results suggest that there are few, if any, genuine old clusters in the LMC left to be found.

A Photometric and Spectroscopic Study of the Southern Open Clusters Pismis 18, Pismis 19, NGC 6005, and NGC 6253

We present Washington system CT1 color-magnitude diagrams of 13 star clusters and their surrounding fields that lie in the outer parts of the LMC disk (r > 4°), as well as a comparison inner cluster. The total area covered is large ( deg2), allowing us to study the clusters and their fields individually and in the context of the entire Galaxy. Ages are determined by means of the magnitude difference δT1 between the giant branch clump and the turnoff, while metallicities are derived from the location of the giant and subgiant branches as compared with fiducial star clusters. This yields a unique data set in which ages and metallicities for both a significant sample of clusters and their fields are determined homogeneously. We find that in most cases the stellar population of each star cluster is quite similar to that of the field where it is embedded, sharing its mean age and metallicity. The old population (t ≥ 10 Gyr) is detected in most fields as a small concentration of stars on the horizontal branch blueward and faintward of the prominent clump. Three particular fields present remarkable properties: (1) The thus-far unique cluster ESO 121-SC03 at ≈9 Gyr has a surrounding field that shares the same properties (which, in turn, is also unique, in that such a dominant old-field component is not present elsewhere—at least not significantly in the fields as yet studied). (2) The field surrounding the far eastern intermediate-age cluster OHSC 37 is noteworthy in that we do not detect any evidence of LMC stars: it is essentially a Galactic foreground field. We can thus detect the LMC field out to greater than 11° (the deprojected distance of ESO 121-SC03), or ~11 kpc, but not to 13° (~13 kpc), despite the presence of clusters at this distance. (3) In the northern part of the LMC disk, the fields of SL 388 and SL 509 present color-magnitude diagrams with a secondary clump ≈0.45 mag fainter than the dominant intermediate-age clump, suggesting a stellar population component located behind the LMC disk at a distance comparable to that of the SMC. Possibly we are witnessing a depth effect in the LMC, and the size of the corresponding structure is comparable to the size of a dwarf galaxy. The unusual spatial location of the cluster OHSC 37 and the anomalous properties of the SL 388 and SL 509 fields might be explained as debris from previous LMC interactions with the Galaxy and/or the SMC. The mean metallicity derived for the intermediate-age outer disk clusters is [Fe/H] = -0.66, and for their surrounding fields [Fe/H] = -0.56. These values are significantly lower than those found by Olszewski et al. for a sample of clusters of similar age but are in good agreement with several recent studies. A few clusters stand out in the age-metallicity relation, in that they are intermediate-age clusters at relatively low metallicity ([Fe/H] ≈ -1).

Spectroscopy of Globular Clusters in NGC 4472

We present a comprehensive study of the luminosity function and spatial structure of the globular cluster system of NGC 4472, the brightest galaxy in Virgo, based on deep wide-field Washington CT1 CCD images. The globular cluster luminosity function shows a peak at T1 = 23.3 ± 0.1 mag, about 1.5 mag brighter than our 50% completeness limit. Comparing this value with that of the Galactic globular clusters, we estimate the true distance modulus to NGC 4472 to be (m - M)0 = 31.2 ± 0.2 (corresponding to a distance of 17.4 ± 1.6 Mpc). With our large sample (≈2000) of bright globular clusters over a wide field, we make a definitive investigation of the spatial structures of the metal-poor and metal-rich cluster populations and find that they are systematically different: (1) the metal-rich clusters are more centrally concentrated than the metal-poor clusters; and (2) the metal-rich clusters are elongated roughly along the major axis of the parent galaxy, while the metal-poor clusters are essentially spherically distributed. In general, the metal-rich clusters closely follow the underlying halo starlight of NGC 4472 in terms of spatial structure and metallicity, while the metal-poor clusters do not. The global value of the specific frequency of the globular clusters in NGC 4472 is estimated to be SN = 4.7 ± 0.6. The local specific frequency increases linearly outward from the center of NGC 4472 until ~55, beyond which it levels off at SN ~ 8.5 until the limit of our data at 7. The specific frequency of both the metal-rich and metal-poor populations shows similar behavior. However, SN of the metal-poor clusters is about a factor of 2 greater than that of the metal-rich clusters in the outer regions. Implications of these results for the origin of the globular clusters in NGC 4472 are discussed. These results are consistent with many of the predictions of both the model of episodic in situ formation plus tidal stripping of globular clusters given by Forbes et al. and the Ashman & Zepf merger formation model, but each of the models also has some problems.

Metallicity and Luminosity Functions of the Globular Clusters in NGC 4472

We have obtained CCD photometry in the Washington system C, T1 filters for some 850,000 objects associated with 10 Galactic globular clusters and two old open clusters. These clusters have well-known metal abundances, spanning a metallicity range of 2.5 dex from [Fe/H] ~ -2.25 to +0.25 at a spacing of ~0.2 dex. Two independent observations were obtained for each cluster, and internal checks, as well as external comparisons with existing photoelectric photometry, indicate that the final colors and magnitudes have overall uncertainties of 0.03 mag. Analogous to the method employed by Da Costa & Armandroff for V, I photometry, we then proceed to construct standard [MT1, (C-T1)0] giant branches for these clusters adopting the Lee et al. distance scale, using some 350 stars per globular cluster to define the giant branch. We then determine the metallicity sensitivity of the (C-T1)0 color at a given MT1 value. The Washington system technique is found to have 3 times the metallicity sensitivity of the V, I technique. At MT1 = -2 (about a magnitude below the tip of the giant branch, roughly equivalent to MI = -3), the giant branches of 47 Tuc and M15 are separated by 1.16 mag in (C-T1)0 and only 0.38 mag in (V-I)0. Thus, for a given photometric accuracy, metallicities can be determined 3 times more precisely with the Washington technique. We find a linear relationship between (C-T1)0 (at MT1 = -2) and metallicity (on the Zinn scale) exists over the full metallicity range, with an rms of only 0.04 dex. We also derive metallicity calibrations for MT1 = -2.5 and -1.5, as well as for two other metallicity scales. The Washington technique retains almost the same metallicity sensitivity at faint magnitudes, and indeed the standard giant branches are still well separated even below the horizontal branch. The photometry is used to set upper limits in the range 0.03?0.09 dex for any intrinsic metallicity dispersion in the calibrating clusters. The calibrations are applicable to objects with ages 5 Gyr?any age effects are small or negligible for such objects. This new technique is found to have many advantages over the previous two-color diagram technique for deriving metallicities from Washington photometry. In addition to requiring only two filters instead of three or four, the new technique is generally much less sensitive to reddening and photometric errors, and the metallicity sensitivity is many times higher. The new technique is especially advantageous for metal-poor objects. The five metal-poor clusters determined by Geisler et al., using the old technique, to be much more metal-poor than previous indications, yield metallicities using the new technique that are in excellent agreement with the Zinn scale. The anomalously low metallicities derived previously are undoubtedly a result of the reduced metallicity sensitivity of the old technique at low abundance. However, the old technique is still competitive for metal-rich objects ([Fe/H] -1). We have extended the method developed by Sarajedini to derive simultaneous reddening and metallicity determinations from the shape of the red giant branch (RGB), the T1 magnitude of the horizontal branch, and the apparent (C-T1) color of the RGB at the level of the horizontal branch. This technique allows us to measure reddening to 0.025 magnitudes in E(B-V) and metallicity to 0.15 dex. Reddenings can also be derived from the blue edge of the instability strip, with a similar error. We measure the apparent T1 magnitude of the red giant branch bump in each of the calibrating clusters and find that the difference in magnitude between the bump and the horizontal branch is tightly and sensitively correlated with metallicity, with an rms dispersion of 0.1 dex. This feature can therefore also be used to derive metallicity in suitable objects. Metallicity can be determined as well from the slope of the RGB, to a similar accuracy. Our very populous color-magnitude diagrams reveal the asymptotic giant branch bump in several clusters. Although MT1 of the RGB tip is not as constant with metallicity and age as MI, it is still found to be a useful distance indicator for objects with [Fe/H] -1.2. For the six standard clusters in this regime, = -3.22 ? 0.11(?), with only a small metallicity dependence. This result is found to be in very good agreement with the predictions of the Bertelli et al. isochrones. We also note that the Washington system holds great potential for deriving accurate ages as well as metallicities.

New Galactic star clusters in VVV survey (Borissova+, 2011)

We have constructed color-magnitude diagrams for 10 globular clusters in the halo of the nearby spiral galaxy M33 based on Hubble Space Telescope Wide Field Planetary Camera 2 observations in the F555W (~V) and F814W (~I) filters. These data reveal the morphology of the horizontal branch (HB) and allow us to estimate the cluster metallicity using the shape and color of the red giant branch. The principal result we report herein is that eight of the 10 clusters possess exclusively red HB morphologies and yet their metallicities are as metal poor as [Fe/H]=-1.6. Indeed, these eight clusters basically present only giant branch clumps reminiscent of intermediate-age star clusters in the Magellanic Clouds. In addition, two of the clusters form a second parameter pair that have similar metallicities but very disparate HB types. Under the assumption that cluster age is the global second parameter, the average age of halo globular clusters in M33 appears to be a few gigayears younger than halo clusters in the Milky Way. Using the observed properties of HB stars in M31 and M33, along with published main-sequence turnoff ages for the globular clusters in the Milky Way, LMC, SMC, and the Sagittarius dwarf spheroidal (Sgr), we attempt to sketch the early formation history of these galaxies. This indicates that the Milky Way, M31, M33, the LMC, and Sgr all experienced their first epoch of cluster formation soon after the big bang. And 3-4 Gyr later, the SMC began to form its first generation of clusters; the bulk of the M33 clusters formed later still. We note that the halo clusters in M33 formed over a much larger time period than those in the Milky Way and M31.

Estimacin de edades de 15 cmulos estelares de la Nube Menor de Magallanes

CCD observations in the B, V, and I passbands have been used to generate color-magnitude diagrams (CMDs) for the southern open cluster candidates Pismis 18, Pismis 19, and NGC 6005, as well as for the old open cluster NGC 6253. The sample consists of about 1550 stars reaching down to V ~ 19 mag. From analysis of the CMDs, the physical reality of the three cluster candidates is confirmed and their reddening, distance, and age are derived, as well as those of NGC 6253. In addition, integrated spectra for Pismis 18, Pismis 19, and NGC 6253 covering a range from 3500 to 9200 A were obtained. The reddening, age, and metallicity of these three clusters were derived from Balmer and Ca II triplet equivalent widths by comparing the observed spectra with those of template clusters. The photometric and spectroscopic results allow us to conclude that Pismis 18, Pismis 19, and NGC 6005 are intermediate-age (t ≈ 1 Gyr), genuine open clusters, while NGC 6253 has an age t ≈ 5 Gyr. All these clusters are located approximately toward the Galactic center, between 1.5 and 2.7 kpc from the Sun. Their metallicities range over values that are typical of moderately metal-poor to moderately metal-rich open clusters.