Alfred Rosenberg

The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters without Previous Hubble Space Telescope Photometry

We present the first results of a large Advanced Camera for Surveys (ACS) survey of Galactic globular clusters. This Hubble Space Telescope (HST) Treasury project is designed to obtain photometry with S/N (signal-to-noise ratio) 10 for main-sequence stars with masses 0.2 M⊙ in a sample of globulars using the ACS Wide Field Channel. Here we focus on clusters without previous HST imaging data. These include NGC 5466, NGC 6779, NGC 5053, NGC 6144, Palomar 2, E3, Lynga 7, Palomar 1, and NGC 6366. Our color-magnitude diagrams (CMDs) extend reliably from the horizontal branch to as much as 7 mag fainter than the main-sequence turnoff and represent the deepest CMDs published to date for these clusters. Using fiducial sequences for three standard clusters (M92, NGC 6752, and 47 Tuc) with well-known metallicities and distances, we perform main-sequence fitting on the target clusters in order to obtain estimates of their distances and reddenings. These comparisons, along with fitting the cluster main sequences to theoretical isochrones, yield ages for the target clusters. We find that the majority of the clusters have ages that are consistent with the standard clusters at their metallicities. The exceptions are E3, which appears ~2 Gyr younger than 47 Tuc, and Pal 1, which could be as much as 8 Gyr younger than 47 Tuc.

The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851

Photometry with the Hubble Space Telescope Advanced Camera for Surveys (HST ACS) reveals that the subgiant branch (SGB) of the globular cluster NGC 1851 splits into two well-defined branches. If the split is due only to an age effect, the two SGBs would imply two star formation episodes separated by ~1 Gyr. We discuss other anomalies in NGC 1851 that could be interpreted in terms of a double stellar population. Finally, we compare the case of NGC 1851 with the other two globulars known to host multiple stellar populations, and show that all three clusters differ in several important respects.

The ACS survey of globular clusters. V. Generating a comprehensive star catalog for each cluster

The ACS Survey of Globular Clusters has used Hubble Space Telescopes Wide-Field Channel to obtain uniform imaging of 65 of the nearest globular clusters to provide an extensive homogeneous data set for a broad range of scientific investigations. The survey goals required not only a uniform observing strategy, but also a uniform reduction strategy. To this end, we designed a sophisticated software program to process the cluster data in an automated way. The program identifies stars simultaneously in the multiple dithered exposures for each cluster and measures them using the best available point-spread function models. We describe here in detail the programs rationale, algorithms, and output. The routine was also designed to perform artificial-star tests, and we ran a standard set of ~105 tests for each cluster in the survey. The catalog described here will be exploited in a number of upcoming papers and will eventually be made available to the public via the World Wide Web.The ACS Survey of Globular Clusters has used HSTs Wide-Field Channel to obtain uniform imaging of 65 of the nearest globular clusters to provide an extensive homogeneous dataset for a broad range of scientific investigations. The survey goals required not only a uniform observing strategy, but also a uniform reduction strategy. To this end, we designed a sophisticated software program to process the cluster data in an automated way. The program identifies stars simultaneously in the multiple dithered exposures for each cluster and measures them using the best available PSF models. We describe here in detail the programs rationale, algorithms, and output. The routine was also designed to perform artificial-star tests, and we run a standard set of ~10^5 tests for each cluster in the survey. The catalog described here will be exploited in a number of upcoming papers and will eventually be made available to the public via the world-wide web.

GALACTIC GLOBULAR CLUSTER RELATIVE AGES

We present accurate relative ages for a sample of 55 Galactic globular clusters. The ages have been obtained by measuring the difference between the horizontal branch and the turnoff in two internally photometrically homogeneous databases. The mutual consistency of the two data sets has been assessed by comparing the ages of 16 globular clusters in common between the two databases. We have also investigated the consistency of our relative age determination within the recent stellar model framework. All clusters with [Fe/H] -0.8 are ~1 Gyr younger than the most metal-poor ones, with a relatively small age dispersion, although the metal-rich sample is still too small to allow firmer conclusions. There is no correlation of the cluster age with the galactocentric distance. We briefly discuss the implication of these observational results for the formation history of the Galaxy.

The ACS Survey of Galactic Globular Clusters: M54 and Young Populations in the Sagittarius Dwarf Spheroidal Galaxy

As part of the ACS Survey of Galactic Globular Clusters, we present new Hubble Space Telescope photometry of the massive globular cluster M54 (NGC 6715) and the superposed core of the tidally disrupted Sagittarius (Sgr) dSph galaxy. Our deep (F606W ~ 26.5), high-precision photometry yields an unprecedentedly detailed color-magnitude diagram showing the extended blue horizontal branch and multiple main sequences of the M54+Sgr system. The distance and reddening to M54 are revised using both isochrone and main-sequence fitting to (m - M)0 = 17.27 and E(B - V) = 0.15. Preliminary assessment finds the M54+Sgr field to be dominated by the old metal-poor populations of Sgr and the globular cluster. Multiple turnoffs indicate the presence of at least two intermediate-aged star formation epochs with 4 and 6 Gyr ages and [Fe/H] = -0.4 to -0.6. We also clearly show, for the first time, a prominent, ~2.3 Gyr old Sgr population of near-solar abundance. A trace population of even younger (~0.1-0.8 Gyr old), more metal-rich ([Fe/H] ~ 0.6) stars is also indicated. The Sgr age-metallicity relation is consistent with a closed-box model and multiple (4-5) star formation bursts over the entire life of the satellite, including the time since Sgr began disrupting.

The ACS Survey Of Galactic Globular Clusters. VIII. Effects Of Environment On Globular Cluster Global Mass Functions

We have used observations obtained as part of the Hubble Space Telescope/ACS Survey of Galactic Globular Clusters to construct global present-day mass functions for 17 globular clusters utilizing multi-mass King models to extrapolate from our observations to the global cluster behavior. The global present-day mass functions for these clusters are well matched by power laws from the turnoff, 0.8 M ☉, to 0.2-0.3 M ☉ on the lower main sequence. The slopes of those power-law fits, α, have been correlated with an extensive set of intrinsic and extrinsic cluster properties to investigate which parameters may influence the form of the present-day mass function. We do not confirm previous suggestions of correlations between α and either metallicity or Galactic location. However, we do find a strong statistical correlation with the related parameters central surface brightness, μ V , and inferred central density, ρ0. The correlation is such that clusters with denser cores (stronger binding energy) tend to have steeper mass functions (a higher proportion of low-mass stars), suggesting that dynamical evolution due to external interactions may have played a key role in determining α. Thus, the present-day mass function may owe more to nurture than to nature. Detailed modeling of external dynamical effects is therefore a requisite for determining the initial mass function for Galactic globular clusters.

Photometric catalog of nearby globular clusters - II. A large homogeneous

In this paper we present the second and nal part of a large and photometrically homogeneous CCD color-magnitude diagram (CMD) data base, comprising 52 nearby Galactic globular clusters (GGC) imaged in the V and I bands. The catalog has been collected using only two tele- scopes (one for each hemisphere). The observed clus- ters represent 75% of the known Galactic globulars with (m M)V 16:15 mag, cover most of the globular clus- ter metallicity range ( 2:2 (Fe=H) 0:4), and span Galactocentric distances from 1: 2t o 18: 5k pc. In particular, here we present the CMDs for the 13 GGCs observed in the Northern hemisphere. The remaining 39 Southern hemisphere clusters of the catalog have been presented in a companion paper (Rosenberg et al. 2000). We present the rst CCD color magnitude diagram for NGC 6779 (M 56). All the CMDs extend from at least 2 magnitudes below the turn-o (i.e. Vlim 22) to the tip of the red giant branch. The calibration has been done using a large number of standard stars, and the absolute calibration is reliable to a 0:02 mag level in both lters. This catalog, because of its homogeneity, is expected to represent a useful data base for the measurement of the main absolute and relative parameters characterizing the CMD of GGCs.

(V,I)

FastCam is an instrument jointly developed by the Spanish Instituto de Astrofísica de Canarias and the Universidad Politécnica de Cartagena designed to obtain high spatial resolution images in the optical wavelength range from ground-based telescopes. The instrument consists of a very low noise and very fast readout speed EMCCD camera capable of reaching the diffraction limit of medium-sized telescopes from 500 to 850 nm. FastCam incorporates a FPGAs-based device to save and evaluate those images minimally disturbed by atmospheric turbulence in real time. The undisturbed images represent a small fraction of the observations. Therefore, a special software package has been developed to extract, from cubes of tens of thousands of images, those with better quality than a given level. This is done in parallel with the data acquisition at the telescope. After the first tests in the laboratory, FastCam has been successfully tested in three telescopes: the 1.52-meter TCS (Teide Observatory), the 2.5-meter NOT, and the 4.2-meter WHT (Roque de los Muchachos Observatory). The theoretical diffraction limit of each telescope has been reached in the I band (850 nm) -0.15, 0.08 and 0.05 arcsec, respectively-, and similar resolutions have been also obtained in the V and R bands. Future work will include the development of a new instrument for the 10.4-meter GTC telescope on La Palma.

color-magnitude diagram data-base

?????Deep V and I CCD images of the loosely populated Galactic globular cluster Pal 1 and the surrounding field have been obtained with the Isaac Newton Telescope. A color-magnitude diagram (CMD) down to Vlim ~ 24 and a luminosity function down to Vlim = 23.25 have been constructed. Adopting a reddening E(V - I) = 0.20 ? 0.04 and comparing the CMD of Pal 1 with that of 47 Tucanae, we obtain a distance modulus (m - M)0 = 15.25 ? 0.25, indicating that Pal 1 is located 3.7 ? 0.4 kpc above the Galactic disc and 17.3 ? 1.6 kpc from the Galactic center. Comparison with 47 Tuc and M71 shows that Pal 1 must be significantly younger. The best-fitting isochrones, as listed by Bertelli et al., yield an age between 6.3 and 8 Gyr, which would make Pal 1 the youngest Galactic globular cluster identified thus far, casting some doubt on the real nature of this object. The luminosity function shows some evidence of mass segregation, consistent with the very short relaxation time. The global mass function can be fitted with a power law of slope x = -1.4 ? 0.7. This mass function is anomalously flat, suggesting either a strong dynamical evolution or an initial mass function significantly different from most other halo globular clusters. A discussion of the possible nature of Pal 1 is presented.

FastCam: a new lucky imaging instrument for medium-sized telescopes

We have used observations obtained as part of the Hubble Space Telescope/ACS Survey of Galactic globular clusters (GCs) to construct a color-magnitude diagram for the bulge cluster, NGC 6366. The luminosity function derived from those data extends to M F606W ~ 9, or masses of ~0.3 M ☉. Unlike most GCs, the mass function peaks near the main-sequence turnoff with significantly fewer low-mass stars even after correction for completeness and mass segregation. Using a multimass King model, we extrapolate the global cluster behavior and find the global mass function to be poorly matched by a power law, with a particular deficit of stars with masses between 0.5 and 0.7 M ☉. We briefly discuss this interesting anomaly within the context of tidal stripping.