Bibcode
DOI
Stephens, Andrew W.; Frogel, Jay A.; Freedman, Wendy; Gallart, Carme; Jablonka, Pascale; Ortolani, Sergio; Renzini, Alvio; Rich, R. Michael; Davies, Roger
Referencia bibliográfica
The Astronomical Journal, Volume 121, Issue 5, pp. 2584-2596.
Fecha de publicación:
5
2001
Número de citas
12
Número de citas referidas
10
Descripción
Astronomers are always anxious to push their observations to the
limit-basing results on objects at the detection threshold, spectral
features barely stronger than the noise, or photometry in very crowded
regions. In this paper we present a careful analysis of photometry in
crowded regions and show how image blending affects the results and
interpretation of such data. Although this analysis is specifically for
our NICMOS observations in M31, the techniques we develop can be applied
to any imaging data taken in crowded fields; we show how the effects of
image blending will limit even the Next Generation Space Telescope. We
have obtained HST-NICMOS observations of five of M31's most metal-rich
globular clusters. These data allow photometry of individual stars in
the clusters and their surrounding fields. However, to achieve our
goals-obtain accurate luminosity functions to compare with their
Galactic counterparts, determine metallicities from the slope of the
giant branch, identify long-period variables, and estimate ages from the
AGB tip luminosity-we must be able to disentangle the true properties of
the population from the observational effects associated with
measurements made in very crowded fields. We thus use three different
techniques to analyze the effects of crowding on our data, including the
insertion of artificial stars (traditional completeness tests) and the
creation of completely artificial clusters. These computer simulations
are used to derive threshold- and critical-blending radii for each
cluster, which determine how close to the cluster center reliable
photometry can be achieved. The simulations also allow us to quantify
and correct for the effects of blending on the slope and width of the
RGB at different surface brightness levels. We then use these results to
estimate the limits blending will place on future space-based
observations. Based on observations with the NASA/ESA Hubble Space
Telescope obtained at the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in Astronomy
(AURA), Inc., for NASA under contract NAS 5-26555.