Low-mass stars in cooling-flow galaxies

Abstract

Many cooling-flow models of X-ray data predict a very large population of low-mass stars in otherwise normal elliptical galaxies. To date, no 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 is to investigate the feasibility of near-infrared photometry and spectroscopy as a potential diagnostic for the existence of a substantial population of low-mass stars in cooling-flow ellipticals. The technique discussed here, based on the CO absorption band at 2.3 microns, appears to be very promising for this purpose because the 2.3-μm CO absorption band is a sensitive luminosity class indicator. We assume that a cooling-flow galaxy is a normal elliptical galaxy with a low-mass star accretion population. By simulating the K-band spectrum of the cooling-flow 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 accretion population. We find that the CO index is a much more sensitive indicator of the accretion population in the CO–MV plane than the V – K colour is in the (V – K)–MV plane. An accretion population with a mass of approximately 3 × 1012 M⊙ can be detected with high confidence in the CO–MV plane. If the upper mass cut-off of the accretion population 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 cooling-flow galaxy lies in the range –0.05 < CO < 00.15. We also perform differential spectral analysis by simulating the K-band spectrum (2–2.5 μm) of a cooling-flow galaxy under our above assumption. By varying the mass accretion rate and the signal-to-noise ratio, we study the sensitivity of spectral fitting, in order to constrain the mass function parameters of the accretion population. We find that a signal-to-noise ratio of about 100 allows useful constraints of mass function parameters, provided that the mass of the accretion population is more than about 3 × 1012 M⊙. Smaller accretion masses require larger signal-to-noise data for useful spectral fitting. Accretion rates of low-mass stars 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 galaxies. A cooling-flow galaxy with such an accretion rate is entirely composed of the accretion population and is physically inconsistent with the existence of a ‘parent’ elliptical galaxy.