The Evolved Stellar Content of NGC 147, NGC 185, and NGC 205*

Abstract

Broadband near-infrared images are used to probe the photometricproperties of the brightest asymptotic giant branch (AGB) stars within2' of the centers of the dwarf elliptical galaxies NGC 147, NGC 185, andNGC 205. Sequences originating from oxygen-rich M giants and C stars areclearly distinguished on the (K, H-K) and (K, J-K) color-magnitudediagrams. Based on the peak brightness and color of the M giantsequences, ages of 1 and 0.1 Gyr are predicted for the most recentsignificant star-forming events in NGC 185 and NGC 205, respectively.The bolometric luminosity function of M giants in NGC 205 is flatterthan in NGC 185, in agreement with studies at wavelengths shortward of 1{μ}m. The most luminous AGB stars in NGC 147 are well mixed withfainter stars throughout the area surveyed in this galaxy, and the peakbrightness of the M giant sequence indicates that the most recentsignificant star-forming activity occurred ~3 Gyr in the past.The C star contents of the three galaxies are compared in two ways.First, the notion of a specific frequency measurement for C stars, inwhich C star counts per unit magnitude are normalized to a commonintegrated K-band brightness using published surface photometry, isintroduced. The specific frequencies of C stars outside the areas ofmost recent star formation agree in all three galaxies. Second,comparisons are made using the integrated brightness from C stars, whichis normalized to the integrated light from M giants and the integratedlight from all stars. The fractional contribution made by C stars to thetotal AGB light in the K-band is found to be highest in NGC 147 andlowest in the central regions of NGC 205, which is qualitativelyconsistent with model predictions. The fractional contribution that Cstars make to the total K-band light is found to be constant both withinand between galaxies, with C stars contributing 2% of the total K-bandlight. We conclude that, when averaged over timescales of a fewgigayears, these galaxies have turned similar fractions of gas and dust,normalized according to total galaxy mass, into stars. We argue that thematerial for star formation likely originated in well-regulatedreservoirs and demonstrate that the mass of gas needed to fuel starformation during intermediate epochs could have been supplied by windsfrom evolved stars. Finally, multiepoch data are used to investigate theincidence of long-period variables (LPVs) in NGC 185 and NGC 205. Whiletight constraints cannot be placed on the LPV content of NGC 205,roughly 70% of the stars with M_{K} between -7.5 and -8.0 in thecentral regions of NGC 185 appear to be LPVs with amplitudes similar tothose of LPVs in the LMC.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center, California Institute ofTechnology, funded by NASA and the NSF.