Extragalactic stellar photometry and the blending problem

TL;DR

This paper investigates how star blending in crowded extragalactic fields observed with HST affects the accuracy of color magnitude diagrams, luminosity functions, and reddening estimates, using simulations and real data.

Contribution

It provides a detailed analysis of the blending problem in extragalactic stellar photometry and assesses its impact on key observational diagnostics.

Findings

Blending can significantly distort CMDs and LFs in crowded fields.

Some blends are detectable via PSF features, others are not.

The study offers methods to evaluate and mitigate blending effects.

Abstract

The images provided by the Advanced Camera for Surveys at the Hubble Space Telescope (ACS/HST) have the amazing spacial resolution of 0".05/pixel. Therefore, it is possible to resolve individual stars in nearby galaxies and, in particular, young blue stars in associations and open clusters of the recent starburst. These data are useful for studies of the extragalactic young population using color magnitude diagrams (CMD) of the stellar groups. However, even with the excellent indicated spatial resolution, the blending of several stars in crowded fields can change the shape of the CMDs. Some of the blendings could be handled in the cases they produce particular features on the stellar PSF profile (e.g. abnormal sharpness, roundness, etc). But in some cases, the blend could be difficult to detect, this is the case, were a pair or several stars are in the same line of sight (e.g. observed in the same pixel). In this work, we investigated the importance of the blending effect in several crowded regions, using both numerical simulations and real ACS/HST data. In particular, we evaluated the influence of this effect over the CMDs, luminosity functions (LFs) and reddening estimations obtained from the observations.