A rocket-borne observation of the far-infrared sky at high Galactic latitude

We have measured the surface brightness of the far-infrared sky at λ = 134, 154, and 186 μm at high Galactic latitude using a liquid-He-cooled, rocket-borne telescope. The telescope scanned over a 5° × 20° region which includes infrared cirrus, high-latitude molecular clouds, the starburst galaxy M82, and the H I Hole in Ursa Major, a region with uniquely low H I column density. The measured brightness at 134, 154, and 186 μm is well correlated with the 100 μm brightness measured by IRAS and, in regions excluding molecular clouds, with H I column density. The spectrum of the component correlated with H I is well fitted by a graybody spectrum with a temperature of 16.4(+2.3/-1.8) K, assuming an emissivity proportional to λ^-2. Assuming a constant far-infrared dust emissivity per hydrogen nucleus, the ratio of the H₂ column density to the velocity-integrated CO intensity in the high-latitude molecular cloud is N(H₂)/W_CO = (1.6 ± 0.3) × 10²⁰ cm^-2 (K km s^-1)^-1. The residual brightness after subtracting the emission correlated with H I column density is λI_λ(154 μm) = (1.4 ± 0.6) × 10^-12 W cm^-2 sr^-1, yielding an upper limit to the far-infrared extragalactic background radiation of λI_λ(154 μm) < 2.6 × 10^-12 W cm^-2 sr^-1.