LOW-MASS STARS IN COOLING-FLOW GALAXIES

Many cooling-flow models of X-ray data predict a very large population of low-mass stars in otherwise normal elliptical galaxies. To date, n o conclusive evidence either in favour of or against the formation of this significant population of low-mass stars has been found, largely because of the lack of appropriate techniques. The aim of this paper i s to investigate the feasibility of near-infrared photometry and spect roscopy as a potential diagnostic for the existence of a substantial p opulation of low-mass stars in cooling-flow ellipticals. The technique discussed here, based on the CO absorption band at 2.3 mum, appears t o be very promising for this purpose because the 2.3-mum CO absorption band is a sensitive luminosity class indicator. We assume that a cool ing-flow galaxy is a normal elliptical galaxy with a low-mass star acc retion population. By simulating the K-band spectrum of the cooling-fl ow galaxy, we investigate the sensitivity of the broad-band V-K colour and the CO index on the mass function and accretion rate of the accre tion population. We find that the CO index is a much more sensitive in dicator of the accretion population in the CO-M(V) plane than the V-K colour is in the (V-K)-M(V) plane. An accretion population with a mass of approximately 3 x 10(12) M., can be detected with high confidence in the CO-M(V) plane. If the upper mass cut-off of the accretion popul ation is less than 0.5 M. then its CO index is approximately -0.4 and is independent of the stellar mass function. The CO index of any cooli ng-flow galaxy lies in the range -0.05 < CO < 0. 15. We also perform d ifferential spectral analysis by simulating the K-band spectrum (2-2.5 mum) of a cooling-flow galaxy under our above assumption. By varying the mass accretion rate and the signal-to-noise ratio, we study the se nsitivity of spectral fitting, in order to constrain the mass function parameters of the accretion population. We find that a signal-to-nois e ratio of about 100 allows useful constraints of mass function parame ters, provided that the mass of the accretion population is more than about 3 x 10(12) M.. Smaller accretion masses require larger signal-to -noise data for useful spectral fitting: Accretion rates of low-mass s tars on to the central galaxy that are larger than about 500 M. yr-1, with a duration of a Hubble time, result in a cooling-flow galaxy that is brighter than the average brightest galaxy of a large cluster of g alaxies. A cooling-flow galaxy with such an accretion rate is entirely composed of the accretion population and is physically inconsistent w ith the existence of a 'parent' elliptical galaxy.