THE GHOSTS SURVEY. I. HUBBLE SPACE TELESCOPE ADVANCED CAMERA FOR SURVEYS DATA

Abstract

We present an overview of the GHOSTS survey, the largest study to date of the resolved stellar populations in the outskirts of disk galaxies. The sample consists of 14 disk galaxies within 17 Mpc, whose outer disks and halos are imaged with the Hubble Space Telescope Advanced Camera for Surveys (ACS). In the first paper of this series, we describe the sample, explore the benefits of using resolved stellar populations, and discuss our ACS F606W and F814W photometry. We use artificial star tests to assess completeness and use overlapping regions to estimate photometric uncertainties. The median depth of the survey at 50% completeness is 2.7 mag below the tip of the red giant branch (TRGB). We comprehensively explore and parameterize contamination from unresolved background galaxies and foreground stars using archival fields of high-redshift ACS observations. Left uncorrected, these would account for 10 0.65×F814W−19.0 detections per mag per arcsec 2. We therefore identify several selection criteria that typically remove 95% of the contaminants. Even with these culls, background galaxies are a significant limitation to the surface brightness detection limit which, for this survey, is typically V ∼ 30 mag arcsec −2. The resulting photometric catalogs are publicly available and contain some 3.1 million stars across 76 ACS fields, predominantly of low extinction. The uniform magnitudes of TRGB stars in these fields enable galaxy distance estimates with 2%-7% accuracy.

A tale of two galaxies: light and mass in NGC 891 and NGC 7814

“…In order to make the fit with the standard interpolation function compatible with the data of NGC 7814, the distance to this galaxy should be increased by a factor ∼1.5. This is not compatible with the new error determination that allows, at 3σ, an increase of at most 15% (Radburn-Smith et al 2011). To improve significatly the fit of NGC 891's rotation curve the distance should be decreased by a factor >2, incompatible with the new determinations.…”

Extragalactic archeology with the GHOSTS Survey

“…The galaxies in the GHOSTS sample span a wide range of morphologies, from Sab to Sd, and masses (with V rot ranging from 80 km s −1 to 230 km s −1 ). A detailed description of the survey is given in Radburn-Smith et al (2011) and the data are publicly available at the Mikulski Archive for Space Telescopes (MAST) 4 .…”

“…All details of the observations and the data reduction can be found in Radburn-Smith et al (2011, for the ACS data) and Monachesi et al (2015, for the WFC3 data). Here, we only summarize the main points of the data reduction.…”

“…Since the typical number of contaminants is very small (about a few dozen per ACS field) compared to the number of stars in crowded regions (a few thousand per ACS field), the larger number of real detections outweighs the increased number of contaminants. For more details on determining optimal culls we refer to the appendix of Radburn-Smith et al (2011). In the following we always use the crowded field culls, if not stated otherwise.…”

“…(1) Right ascension, declination, and scale heights from K-Band fits by Seth et al (2005a) ; (2) position angle, inclination, morphology index, and rotational velocity from HyperLEDA (Paturel et al 2003) ; (3) distance modulus and distance from Radburn-Smith et al (2011) . (a) While we list the inclination value from HyperLEDA here, our selection of edge-on galaxies was made based on a visual inspection of the images.…”