An INFRARED SPACE OBSERVATORY Upper Limit to the Low-Mass Star Halo in the Edge-on Galaxy NGC 4565

We present deep ISOCAM observations taken at 4.5 μm (LW1) in search of a faint halo surrounding the edge-on spiral galaxy NGC 4565. Such a halo might exist if the massive halo that is needed to explain the flat rotation curve of this galaxy were attributable to a population of faint, red objects. The upper limit, 5.4 kJy sr^-1 (3 σ) at 20 kpc, reported here excludes a halo consisting of late-M dwarfs distributed in a spherical halo but not of lower luminosity brown dwarf stars.