Evidence for Photometric Contamination in Key Observations of Cepheids in the Benchmark Galaxy IC 1613

TL;DR

This paper investigates how photometric contamination affects Cepheid distance measurements in galaxy IC 1613, revealing that crowding can cause systematic brightness offsets, which has implications for cosmic distance scale accuracy.

Contribution

It demonstrates the impact of unresolved stellar contamination on Cepheid distance estimates and assesses metallicity effects using multi-band calibrations in IC 1613 and NGC 6822.

Findings

Photometric contamination causes Cepheids near galaxy cores to appear brighter.

Contamination effects are less significant in NGC 6822 due to its proximity.

Period-magnitude relations are relatively insensitive to metallicity in the studied bands.

Abstract

This study aims to increase awareness concerning the pernicious effects of photometric contamination (crowding/blending), since it can propagate an undesirable systematic offset into the cosmic distance scale. The latest Galactic Cepheid W_VIc and Spitzer calibrations were employed to establish distances for classical Cepheids in IC 1613 and NGC 6822, thus enabling the impact of photometric contamination to be assessed in concert with metallicity. Distances (W_VIc, [3.6]) for Cepheids in IC 1613 exhibit a galactocentric dependence, whereby Cepheids near the core appear (spuriously) too bright (r_g < 2'). That effect is attributed to photometric contamination from neighboring (unresolved) stars, since the stellar density and surface brightness may increase with decreasing galactocentric distance. The impact is relatively indiscernible for a comparison sample of Cepheids occupying NGC 6822, a result which is partly attributable to that sample being nearer than the metal-poor galaxy IC 1613. W_VIc and [3.6] distances for relatively uncontaminated Cepheids in each galaxy are comparable, thus confirming that period-magnitude relations (Leavitt Law) in those bands are relatively insensitive to metallicity (d[Fe/H] ~ 1).