Formation of low-mass stars in elliptical galaxy cooling flows

Thermal X-ray emission from cooling flows in elliptical galaxies indicates that similar to 1 M-circle dot of hot (T similar to 10(7) K) interstellar g as cools each year, accumulating similar to 10(10) Mo over a Hubble time. P aradoxically, optical and radio frequency emission from the cooled gas is l acking, indicating that less than similar to 10(-3) of the cooled gas remai ns. Many have speculated that the cooled gas has formed into relatively inv isible low-mass stars, particularly in the context of massive cooling hows in galaxy clusters. We focus here on cooling flows in elliptical galaxies l ike NGC 4472 where the cooled gas is made visible in emission lines from H II regions ionized and heated (T-H II similar to 10(4) K) by stellar ultrav iolet radiation. The low filling factor of H II gas requires that the hot g as cools at similar to 10(6) cooling sites within several kpc of the galact ic center. H II mass slowly increases at each site at similar to 10(-6) Mo yr-l until a neutral core develops. Neutral cores are heated (T-H (I) simil ar to 15 K) and ionized (x similar to 10(-6)) by thermal X-rays from the en tire interstellar cooling flow. We show that the maximum mass of spherical H I cores that become gravitationally unstable is only similar to 2 M-circl e dot. No star can exceed this mass, and fragmentation of collapsing cores produces stars of even lower mass. By this means we establish with some con fidence that the hypothesis of low-mass star formation is indeed correct-th e initial mass function is bottom heavy, but its stars may be optically lum inous. Slightly more massive stars less than or similar to 4.5 M-circle dot can form near the effective radius (r = 8.57 kpc in NGC 4472) if sufficien t masses of interstellar gas cool there, producing a luminous population of intermediate-mass stars perhaps with radial orbits that may contribute to the stellar H beta index. The degree of ionization in gravitationally colla psing cores is sufficiently low to allow magnetic fields to disconnect by a mbipolar diffusion. Low-mass star formation is very efficient, involving si milar to 106 Mo of galactic cold gas at any time, in agreement with observe d upper limits on cold gas mass. We discuss the cooling region surrounding a typical cooling site and show that the total X-ray absorption in cold and cooling gas is much less than that indicated by recent X-ray observations. Using a mass dropout scheme consistent with X-ray observations and dynamic al mass-to-light ratios, we plot the global H beta surface brightness profi le in NGC 4472 and compare it with the smaller contribution from H II gas r ecently ejected from red giant stars. The lifetime of cooled gas at each co oling site, similar to 10(5) yr, is too short to permit dust formation and perhaps also gas phase formation of molecules.