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Photometric studies of composite stellar systems. IV - Infrared photometry of globular clusters in M31 and a comparison with early-type galaxies

Frogel, Jay A. and Persson, S. E. and Cohen, Judith G. (1980) Photometric studies of composite stellar systems. IV - Infrared photometry of globular clusters in M31 and a comparison with early-type galaxies. Astrophysical Journal, 240 . pp. 785-802. ISSN 0004-637X. doi:10.1086/158291.

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The results of an infrared photometric investigation of 40 globular clusters in and around M31 are presented. The (V - K)_0 colors of the M31 globulars are tightly correlated with other broadband colors and with reddening-free metallicity parameters derived from optical spectrophotometry by Searle. Over a range of ˜1.2 mag in (V - K)_0, the scatter is consistent with observational error. Thus the 0.3-2.2 μm energy distributions are uniquely predicted by the metallicity and vice versa. A comparison of the (V - K)0 colors with those of galactic globulars allows an independent derivation of the metallicities of individual M31 globulars. The broad-band infrared data are compared with predictions from integrated light models based on the Ciardullo and Demarque isochrones. The agreement is quite good for models with an initial mass function of slope ≲ the Salpeter value independent of metallicity, thus ruling out the possibility that a late-type dwarf component is making a significant contribution to the infrared light. CO and H_2O indices measured for eight and seven of the clusters, respectively, give the same result. Early-type galaxies are seen to have much redder broad-band colors and stronger CO and H_2O indices than the most metal-rich M31 or galactic globular observed. Compared to the reddest globular clusters, at a given (U - V)_0 early-type galaxies are on average 0.3 mag redder in (V - K)_0. Although the stellar synthesis models reproduce cluster broad-band colors reasonably well, they do not reproduce the U - V/ V - K distribution of early-type galaxies. We propose that the early-type galaxies contain a population of cool luminous stars present neither in the clusters nor in the stellar synthesis models. One candidate for this population is a giant branch of stars considerably more metal rich than the Sun. More interesting is the possibility that there is a contribution to the integrated infrared light from asymptotic giant-branch stars above the first red giant tip. Such stars could be of intermediate age. The luminosity functions for the M31 and the galactic globulars are examined with the aid of models to investigate the possibility that metal-enhanced star formation or variations in the initial mass function can be detected in integrated light. Two appendices present new infrared data for a faint dE galaxy in the Virgo cluster, and a recalibration of the integrated light models presented by Aaron son et al..

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Cohen, Judith G.0000-0002-8039-4673
Additional Information:© American Astronomical Society • Provided by the NASA Astrophysics Data System. Received 1980 January 25; accepted 1980 March 19. Guest Investigator at the Hale Observatories, 1976-1978, and at Kitt Peak National Observatory, which is supported by the National Science Foundation under contract AST 78-27879. Cerro Tololo Inter-American Observatory is supported by the National Science Foundation under contract AST 78-27879. J. A. F. is grateful to the Director of the Hale Observatories for Guest Investigator privileges without which this project could not have been carried out. We thank L. Searle for many fruitful conversations and his continuing interest in this program. M. Aaronson was kind enough to provide us with tabulations of his unpublished data. D. Rabin pointed out some ofthe numerical problems in the Yale tracks. We particularly appreciate the assistance of S. Beckwith in obtaining some of the observations discussed here. Several of our colleagues at CTIO and Hale have made useful comments on a draft of this paper. This work was supported in part by NSF grants AST 77-20516 and AST 76-22676 and NASA grant NGL 05-002-207.
Funding AgencyGrant Number
NSFAST 78-27879
NSFAST 77-20516
NSFAST 76-22676
NASANGL 05-002-207
Subject Keywords:clusters: globular — galaxies: individual — galaxies: photometry — galaxies : stellar content — infrared : sources
Record Number:CaltechAUTHORS:20190531-091630652
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:95969
Deposited By: George Porter
Deposited On:31 May 2019 20:53
Last Modified:16 Nov 2021 17:17

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