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A.W. Stephens, J.A. Frogel (Ohio State University), W. Freedman, C. Gallart (Carnegie Observatories), P. Jablonka (Observatoire de Paris-Meudon), S. Ortolani (Universita di Padova), A. Renzini (ESO), R.M. Rich (UCLA), R. Davies (University of Durham)
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 \sigma[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.
The author(s) of this abstract have provided an email address for comments about the abstract: stephens@astronomy.ohio-state.edu