The Impact of Degraded Charge Transfer Efficiency on Extended Sources in ACS/WFC

TL;DR

This study assesses how degraded Charge Transfer Efficiency (CTE) affects photometric accuracy and shape measurements of extended sources in ACS/WFC images over 17 years, providing practical guidelines for data analysis.

Contribution

It offers a detailed analysis of CTE degradation effects on extended source photometry and morphology, and suggests improved data reduction practices for HST ACS/WFC data.

Findings

Global brightness measurements are reliable within 0.05 mag if background >20 e-/pixel.

Small-scale brightness can be off by >0.1 mag unless sources are near serial registers or very bright.

Degraded CTE can cause artificial asymmetries in galaxy light distributions.

Abstract

Using repeat imaging of a galaxy cluster taken over a seventeen-year baseline, we examine the impact that degraded Charge Transfer Efficiency (CTE) has on photometric measurements of extended sources using the ACS/WFC on HST. We examine how measured brightnesses depend on time since ACS installation, source location on the WFC detectors, source brightness, and local background level in individual exposures. We find that global brightness measurements using large apertures are generally reliable within $\sim$0.05 magnitudes across the WFC detectors if exposure backgrounds are above $20e^-/{pixel}$ and sources are brighter than $\sim300e^-$ in a single exposure. However, brightness measurements on smaller scales can suffer deficiencies in excess of 0.1 mags (sometimes, significantly more) in recent data unless sources are very close to the CCD serial registers ($\lesssim 512$ pixels), or brighter than $\sim3000\,e^-$ in a single exposure. We also show how degraded CTE can result in artificial asymmetries in galaxy light distributions, which are largely mitigated if backgrounds are $>20e^-/{pixel}$ and targets are not far ($>1536$ pixels) from the serial registers. As expected, brightness measurements in later epoch data are best when using CTE-corrected images (FLC/DRC), but our results imply that the pixel-based CTE correction algorithm employed by the ACS reduction pipeline does not necessarily place charge back into its original location within extended sources. Based on this study, users are advised to keep backgrounds above the already recommended $30e^-/\mathrm{pixel}$, ensure targets will have at least $\sim 300e^-$ in a single exposure, and place targets close to the serial registers if analysis of their spatially resolved properties is needed.