Deep HST Observations of 47 Tuc

Deep HST ACS Imaging of 47 Tuc and the SMC

Kalirai et al. 2012, AJ, 143, 11

Introduction

The footprint of the ACS observations is the darker, star-like pattern enclosed by the green circle. Modified from Fig. 3 of Kalirai et al. 2012, AJ, 143, 11.

Color magnitude diagrams showing giants, dwarfs, and white dwarfs from 47 Tuc, as well as a red giant branch, multiple main-sequence turn-offs, and a main sequence from the SMC down to 30th magnitude. The difference between the left and middle panel represents different selection cuts on photometric uncertainty, chi values, and sharp statistics.

Kalirai et al. have observed the globular cluster 47 Tucanae for 121 orbits using the Advanced Camera for Surveys. These extremely deep images were taken in two filters: F606W (broad V-band) and F814W (I-band). Using these observations, Kalirai et al. were able to construct one of the deepest and most complete color-magnitude diagrams of a stellar population, probing down to 30th magnitude and extending from the faintest end of the main sequence to the coolest white dwarf members. As an added bonus, members of the Small Magellanic Cloud represent background sources, and these observations are able to resolve SMC targets down to 0.2 solar masses.

The team have released their stacked ACS mosaics (FITS files), source catalog (ASCII text table), and artificial source lists (ASCII text table) used for testing photometry, astrometry, and completeness, as High Level Science Products. We summarize the creation of the stacked images and generation of the catalogs below, but refer you to the original publication for complete details.

Description of Data Products

The FITS mosaics in both bands were processed and stacked using Multidrizzle. Careful measurements of the shifts, rotations, and scales of the 121 individual images were required, along with parameter optimization, before creating the final, stacked mosaics. The mosaics are supersampled at 0.03 arcseconds per pixel. Please note that there is a small offset in position between the F606W and F814W stacked mosaic.

The source catalog contains all detections measured in a single pass of PSF photometry using DAOPHOT. The reported uncertainties in the catalog should only be considered in a relative sense. For absolute photometric uncertainty, use the extensive list of artificial star tests that are made available as separate catalogs. The magnitudes are all measured in the Vega system. Simple selection cuts based on flux uncertainty and chi/sharp morphology can be used to generate cleaner color-magnitude diagrams such as the ones presented to the right of this page.

The columns of the text table are as follows: Target Identifier, Detector Position (X), Detector Position (Y), F606W magnitude, F606W uncertainty, F814W magnitude, F814W uncertainty, chi goodness-of-fit statistic, and the "sharp" statistic (describes how much broader the object's profile appears compared to the PSF profile).

To assess absolute photometric uncertainties, and to test completeness, several catalogs of artificial sources have been provided. The artificial star tests are described in detail by Kalirai et al. (2012), AJ, 143, 11, and for the SMC tests, in Kalirai et al. (2013), ApJ, 763, 110. A separate set of tests were preformed for each population in the color-magnitude diagram: the 47 Tuc main sequence, the white dwarf cooling sequence in 47 Tuc, and the SMC background population. Each of these catalogs contain a target identifier and then, for both the F606W ("V") and F814W ("I") bands: the detector X and Y position, the input magnitude, the input flux uncertainty, the recovered magnitude, and the measured flux uncertainty. If a source was not recovered, then the magnitude is set to the value of 99.999 and the uncertainty is set to the value of 10.000.

Data Access

F606W

F814W

Source List

Artificial Source Lists

FITS (~400 MB)
Interactive Display

Catalog (~4 MB)

Main Sequence (~90 MB)

SMC (~465 MB)

White Dwarfs (~35 MB)