Mid-Infrared Photometry of Cold Brown Dwarfs: Diversity in Age, Mass, and Metallicity

We present new Spitzer Infrared Array Camera (IRAC) photometry of 12 very late-type T dwarfs: nine have [3.6], [4.5], [5.8], and [8.0] photometry and three have [3.6] and [4.5] photometry only. Combining this with previously published photometry, we investigate trends with type and color that are useful for both the planning and interpretation of infrared surveys designed to discover the coldest T or Y dwarfs. The online appendix provides a collation of MKO-system YJHKL'M' and IRAC photometry for a sample of M, L, and T dwarfs. Brown dwarfs with effective temperature (T{sub eff}) below 700 K emit more than half their flux at wavelengths longer than 3 {mu}m, and the ratio of the mid-infrared flux to the near-infrared flux becomes very sensitive to T{sub eff} at these low temperatures. We confirm that the color H (1.6 {mu}m) - [4.5] is a good indicator of T{sub eff} with a relatively weak dependence on metallicity and gravity. Conversely, the colors H - K (2.2 {mu}m) and [4.5] - [5.8] are sensitive to metallicity and gravity. Thus, near- and mid-infrared photometry provide useful indicators of the fundamental properties of brown dwarfs, and if temperature and gravity are known, then mass and age can be reliably determined from evolutionary models. There are 12 dwarfs currently known with H- [4.5] >3.0, and 500 K {approx}< T{sub eff} {approx}