Dark matter and brown dwarfs: Prospects for the direct detection of a brown dwarf halo

Very low mass objects, referred to as brown dwarfs, could comprise the halo dark matter of galaxies. The possibility of directly detecting infrared emission from brown dwarfs is discussed here. The direct detection of the emission from brown dwarfs that could comprise the halo dark matter of a nearby galaxy, dark matter in a nearby cluster of galaxies and in distant galaxies and clusters of galaxies, and from individual brown dwarfs within our own Galaxy are discussed. The integrated infrared emission of the halo of a nearby galaxy could be detected or constrained by pointed observations if the halo is comprised of brown dwarfs with a mass in the range from ∼10^-1 to 10^-3 M_⊙ and an age of ∼ 10¹⁰ yr. The position and density profile of the halo are known. The expected infrared surface brightness is low, but could be detected with ISO or SIRTF. Extended sources, such as a nearby galaxy or cluster of galaxies, can best be observed at short wavelengths, since at longer wavelengths it is difficult to correct for galactic cirrus and zodiacal emission, so higher mass brown dwarfs (M ≳ 5 × 10^-3 M_⊙) will be most seriously constrained as a dark matter candidate. Similarly, the integrated infrared emission from clusters of galaxies will provide an interesting constrast on, or detection of, brown dwarfs as a cluster dark matter candidate. Individual brown dwarfs in the solar vicinity with an age of ∼10¹⁰ yr, and mass in the range from ∼10^-1 to 10^-3 M_⊙, could be detected or constrained as a halo dark matter candidate with a survey which covers a substantial fraction of the sky to 10-0.1 mJy levels, or which covers an area that is a few square degrees to 100-1 μJy levels, over the wavelength range from ∼4 to 150 μm. Since individual brown dwarfs in the solar vicinity are point sources, they can be observed or constrained at both long and short wavelengths. Observations with SIRTF or ISO could either detect individual brown dwarfs in the solar vicinity or significantly constrain them as a halo dark matter candidate.