Annu. Rev. Astron. Astrophys. 1994. 32: 531-590 Copyright © 1994 by Annual Reviews. All rights reserved

9.3. M-Dwarfs vs Brown Dwarfs

In determining how small LMOs would need to be to provide the disk or halo dark matter, important information comes from red and infrared observations. From searches for sources in our own halo, Richstone et al (1992) find that halo mass-to-light ratio from stars between 0.5 M and 0.8 M exceeds 400, while Bahcall & Soneira (1984) find that the ratio from stars down to 0.15 M must exceed 650. This implies that stars in these mass ranges can only contribute a small fraction to the halo density. Even stronger constraints come from Gilmore & Hewett (1983), who find that the local number density of stars in the mass range 0.08-0.1 M can be at most 0.01 pc^-3. This is a hundred times too small to explain the local dark matter problem and ten times to explain the halo problem.

A similar conclusion is indicated by infrared observations of other spiral galaxies. For example, the K-band mass-to-light ratio exceeds 50 for NGC 4565 (Boughn et al 1981), 100 for M87 (Boughn & Saulson 1983), 64 for NGC 5907 (Skrutskie et al 1985), and 140 for NGC 100 (Casali & James 1994). Since the mass-to-light ratio is less than 60 for stars bigger than 0.08 M, the lower limit for hydrogen-burning (D' Antona & Mazzitelli 1985), this suggests that any hydrogen-burning stars are excluded. Lake (1992) has criticized some of these limits on the grounds that they involve attributing all the dynamical mass to the halo objects but the correction to the mass-to-light ratio for M87 and NGC 100 could hardly get it below 60. These observations therefore suggest that the halo dark matter must be in the form of brown dwarfs.