Bibcode
Stephens, A. W.; Frogel, J. A.; Freedman, W.; Gallart, C.; Jablonka, P.; Ortolani, S.; Renzini, A.; Rich, R. M.; Davies, R.
Bibliographical reference
American Astronomical Society, 197th AAS Meeting, #41.24; Bulletin of the American Astronomical Society, Vol. 32, p.1465
Advertised on:
12
2000
Citations
0
Refereed citations
0
Description
We have obtained HST-NICMOS observations of five of M31's most metal
rich globular clusters: G1, G170, G174, G177 & G280. In order to
disentangle the true properties of the population from the observational
effects associated with measurements made in very crowded fields, we
first present a careful analysis of photometry in crowded regions. Here
we show how image blending affects the scientific conclusions which may
be drawn from such data. We use three different techniques to analyze
the effects of crowding, including traditional completeness tests and
the creation of completely artificial clusters. These computer
simulations are used to derive quantitative limits determining where
reliable photometry can be achieved. We also use these results to
estimate the limits blending will place on future space-based
observations such as with NGST. For the two clusters farthest from the
nucleus of M31 we estimate metallicities of of -1.2 for G1 and -0.2 for
G280 based on the slopes of their infrared giant branches. On the other
hand, the location of G1's giant branch in the K-(V-K) color-magnitude
diagram is very similar to that of 47 Tucanae ([Fe/H] ~ -0.76). The
K-band luminosity functions of the upper few magnitudes of G1 and G280,
as well as for the fields surrounding all clusters, are all nearly
indistinguishable from the LF measured in the bulge of our Galaxy. For
the field surrounding G280, we estimate the metallicity to be -1.3 with
a spread of σ [Fe/H] ~ 0.5 from the slope and width of
the giant branch. Based on the luminosities of the brightest giants, we
conclude that a small fraction of the stars in this field could be as
young as 2Gyr. Support for this work was provided by NASA through grant
number GO-7826 and GO-7876 from the Space Telescope Science Institute.