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«Uncloaking globular clusters in the inner Galaxy1 Javier Alonso-Garc´ ıa Departamento de Astronom´ y Astrof´ ıa ısica, Pontificia Universidad ...»

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Uncloaking globular clusters in the inner Galaxy1

Javier Alonso-Garc´


Departamento de Astronom´ y Astrof´

ıa ısica, Pontificia Universidad Cat´lica de Chile,


782-0436 Macul, Santiago, Chile

Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1090


Mario Mateo

Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1090


Bodhisattva Sen

Department of Statistics, Columbia University, New York, NY 10027 bodhi@stat.columbia.edu Moulinath Banerjee Department of Statistics, University of Michigan, Ann Arbor, MI 48109-1107 moulib@umich.edu M´rcio Catelan a Departamento de Astronom´ y Astrof´ ıa ısica, Pontificia Universidad Cat´lica de Chile, o 782-0436 Macul, Santiago, Chile Dante Minniti Departamento de Astronom´ y Astrof´ ıa ısica, Pontificia Universidad Cat´lica de Chile, o 782-0436 Macul, Santiago, Chile Kaspar von Braun –2– NASA Exoplanet Science Institute, California Institute of Technology, Pasadena, CA 91125-2200 kaspar@caltech.edu Received ; accepted –3–


Extensive photometric studies of the globular clusters located towards the cen- ter of the Milky Way have been historically neglected. The presence of patchy differential reddening in front of these clusters has proven to be a significant obstacle to their detailed study. We present here a well-defined and reasonably homogeneous photometric database for 25 of the brightest Galactic globular clus- ters located in the direction of the inner Galaxy. These data were obtained in the B, V, and I bands using the Magellan 6.5m telescope and the Hubble Space Telescope. A new technique is extensively used in this paper to map the differen- tial reddening in the individual cluster fields, and to produce cleaner, dereddened color-magnitude diagrams for all the clusters in the database. Subsequent papers will detail the astrophysical analysis of the cluster populations, and the properties of the obscuring material along the clusters’ lines of sight.

Subject headings: Globular clusters: general – catalogs – Galaxy: bulge – Galaxy:

evolution – HertzsprungRussell (HR) diagram – Globular Clusters: individual (NGC 6121 - M 4, NGC 6144, NGC 6218 - M 12, NGC 6235, NGC 6254 - M 10, NGC 6266

- M 62, NGC 6273 - M 19, NGC 6287, NGC 6304, NGC 6333 - M 9, NGC 6342, NGC 6352, NGC 6355, NGC 6397, NGC 6522, NGC 6541, NGC 6553, NGC 6558, NGC 6624, NGC 6626 - M 28, NGC 6637 - M 69, NGC 6642, NGC 6656 - M 22, NGC 6681

- M 70, NGC 6809 - M 55) –4–

1. Introduction

The globular cluster system of the Milky Way has long been used to learn about the evolution of the Galaxy. As the Galactic globular clusters (GGCs) constitute some of the oldest systems in the Milky Way, the study of the stellar populations of these fossils can give us important clues of the early stages of the Galaxy’s formation. The tool most extensively used in this task has been the analysis of the color-magnitude diagrams (CMDs) (de Angeli et al. 2005; Mar´ ın-Franch et al. 2009). However, the presence of significant differential extinction in low-latitude fields, particularly near the Galactic center, greatly complicates traditional CMD analyses. As a result, the study of many GGCs located towards the inner Galaxy has been historically neglected.

Various recent studies have tried to overcome the difficulties associated with the study of inner GGCs to better exploit them as probes of the stellar populations near the Galactic Center. One obvious approach is to use near-infrared photometry to study these clusters (e.g.,Davidge (2000); Valenti et al. (2007)) to take advantage of the smaller extinction in these bands. But to extract precise information about the ages of the GGCs, an accurate location of the main sequence turn-off (MSTO) point is crucial in most methods of cluster dating (Stetson et al. 1996; Sarajedini et al. 1997; Gratton et al. 2003). Infrared photometry sufficiently deep to reach the main sequence (MS) with enough precision to accurately locate the turn-off (TO) point is difficult to achieve, and only now are we starting to see the first results after the careful application of new adaptive optics techniques on big 1 Based partly on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. This paper also includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile.

–5– telescopes (Moretti et al. 2009). Deep photometry for non-highly reddened GGCs is much more easily obtained in the optical, but at the cost of increased extinction. A number of techniques have been developed to produce extinction maps for individual clusters as a means of dealing with this issue (e.g., Melbourne & Guhathakurta (2004); von Braun & Mateo (2001)). In general, the resolution and accuracy of optical extinction maps are not adequate to fully eliminate the effects of differential reddening at a level of precision to produce deep optical CMDs of similar high quality now typical for high-latitude clusters with little or no differential extinction (e.g.,Rosenberg et al. (2000a); Piotto et al. (2002)).

In this paper we present a new optical photometric database consisting of a sample of 25 GGCs located towards the Galactic Center. We use a new dereddening technique (Alonso-Garc´ et al. (2011), from now on referred to as Paper I) to map the differential ıa extinction along their fields, and produce new, cleaner, differentially dereddened CMDs of the clusters. In section 2, we give the criteria used to define our sample of GGCs. Section 3 summarizes the steps that we followed to obtain precise astrometry and optical photometry of the stars in these clusters suitable for our analyses. In section 4 we apply the dereddening technique described in Paper I, and provide an overview of the differentially dereddened CMDs of the clusters in our sample, along with extinction maps along their fields. We also describe the characteristic features of the environments in which they are located, and briefly summarize previous optical and infrared photometric studies where appropriate.

Finally, in section 5, we provide a summary of our results.

–  –  –

The cluster sample presented in this study consists of 25 GGCs in the direction of the inner Galaxy, all located within 30 deg of the Galactic Center. Because the precision of our dereddening technique (Paper I) requires good sampling and photometry down to a –6– few magnitudes below the MSTO, and also depends on the density of stars and the spatial variations in the extinction, the clusters in our sample were chosen to also satisfy the

following criteria:

• Exhibit moderate mean extinction, implying that they may suffer from extinction variations. We explicitly restricted our sample to clusters with a mean reddening of E(B − V ) ≥ 0.07 mag.

• Be sufficiently luminous to possess a well-defined MS. Our analysis requires a significant number of stars (at least a few hundreds) to calculate the extinction in a region. We therefore chose clusters with a luminosity satisfying MV ≤ −6.

• Be relatively nearby. With our dereddening technique, the stars that provide most of the information about the differential extinction are those located in the CMD sequences most nearly orthogonal to the reddening vector (subgiant branch (SGB) and upper MS). Since one of the goals of this project is to accurately calculate the relative ages of these clusters, we must also reach the MS with good photometric precision at the TO in order to carry out a reliable age/metallicity analysis. Hence, we chose clusters with an apparent distance modulus of (m − M )V ≤ 16.6.

• Be sufficiently extended so that we can define maps that cover a significant solid angle around the clusters. We therefore chose clusters with a tidal radius rt ≥ 7.5 arcmin.

We were able to observe 25 of the 31 GGCs that fulfill these requirements, according to the 2003 version of the Harris (1996) catalog. Their positions are shown in Figure 1 along with the rest of the clusters located in the inner Galaxy. Their characteristics, according to –7– the most recent (December 2010) version2 of the Harris catalog, are given in Table 1.

–  –  –

We obtained optical photometric data for our sample of GGCs using the Magellan

6.5m Baade Telescope located at the Las Campanas Observatory (LCO) in Chile, and the Hubble Space Telescope (HST). In this section we provide a complete summary of these observations and explain in detail the steps followed for the reduction and calibration of the data. The final photometry has an absolute precision with respect to the calibrating stars of σ ∼ 0.02 − 0.03 magnitudes in most cases. The internal precision among the stars observed in each field is around σ ∼ 0.01 − 0.02 magnitudes, sufficient to achieve the precise extinction maps that will allow us to produce clean CMDs. From these we will aim to derive the cluster parameters with an accuracy similar to those derived from high-Galactic latitude clusters CMDs that suffer little or no differential extinction.

–  –  –

The GGCs in our sample were observed over four nights, May, 30th to June, 2nd 2005, with the LCO 6.5 m Magellan Baade Telescope, using the Inamori Magellan Areal Camera and Spectrograph (IMACS) in imaging mode. We used the f/4 camera to image a 2 According to this updated version of the catalog, two of the clusters in our sample, NGC 6287 and NGC 6355, do not fulfill anymore the apparent distance modulus requirement to belong to our sample. However, we have still kept these clusters in our studied sample, since previous studies of these objects have been very scarce, and their published parameters are accordingly quite uncertain.

–8– field of view (FOV) of 15.46′ × 15.46′, with a pixel size of 0.11′′. All fields were observed using standard Johnson-Cousins B,V, and I filters (Bessell 1979). Two sets of observations with different exposure times (short and long) were taken in the B and V filters, and three (extra-short, short, and long) in I, for every cluster. Although the nights during our observing run were not all completely photometric, the seeing conditions were very good, with average values of ∼ 0.6′′ in V for the whole run. Table 2 lists the details of these ground-based photometry observations.

In order to obtain useful photometry of the inner regions of the more centrally crowded clusters, we supplemented our Magellan images with images taken with the Advanced Camera for Surveys (ACS) aboard HST. These data were obtained in by our group’s Snapshot program 10573. The ACS has a FOV of 3.37′ × 3.37′, with a pixel size of 0.05′′. Five clusters of our sample were observed using the f 435w(B435 ), f 555w(V555 ), and f 814w(I814 ) filters. Table 3 lists the details of these new HST observations.

To better calibrate our photometry, we also used images available through the HST data archive for all the clusters in our sample. The data taken from the HST data archive are comprised of f 439w(B439 ), f 555w(V555 ), f 606w(V606 ), and f 814w(I814 ) images obtained with the Wide Field Planetary Camera 2 (WFC2), and of f 435w(B435 ), f 606w(V606 ), and f 814w(I814 ) images taken with the ACS. Table 4 lists the different HST programs that we use data from.

–  –  –

For the ground-based data, the initial processing of the raw CCD images was done with the routines in the ccdred package of the Image Reduction and Analysis Facility (IRAF).

The images were first corrected for bias, then flatfielded using a combination of dome and –9– twilight flats. Afterward, since different frames for every cluster were taken with different exposure times in the B, V, and I filters (see Table 2), and they had small spatial offsets between them, the frames of every individual cluster were aligned, using the IRAF task imalign, and average-combined for every exposure time and filter, with the IRAF task imcombine.

Stellar photometry was carried out on the ground-based processed images using an updated version of DoPHOT (Schechter et al. 1993). This version works on any platform and accepts images consisting of real data values. It also provides better aperture corrections than previous versions, allowing for variations in aperture corrections as a function of field position and stellar magnitude (see Appendix A).

We also carried out an astrometric analysis of the cluster fields in our sample. We derived coordinates (Right Ascension α and Declination δ) for all stars detected in our photometric study by comparison with bright stars obtained in each field from the Two Micron All Sky Survey (2MASS) catalog stars available through the Infrared Processing and Analysis Center (IPAC) website. In practice, more than 100 stars were available as astrometric references within the fields of every chip of our CCD camera. A third order polynomial fit, done with the IRAF task mscpeak, produced dispersions of σ ∼ 0.25′′, consistent with the catalog precision. Using the astrometric information of the images we could also calculate if there were variations in the pixel area coverage accross the images, which could lead to miscalculations in the measured fluxes. But the pixel area changed by less than 1% across any of the chips of the camera, and therefore we did not need to apply any corrections to the measured photometry.

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