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