The size and shape of the Milky Way disc and halo from M-type brown dwarfs in the BoRG survey

We have identified 274 M-type brown dwarfs in the Hubble Space Telescope's Wide Field Camera 3 pure parallel fields from the Brightest of Reionizing Galaxies (BoRG) survey for high-redshift galaxies. These are near-infrared observations with multiple lines of sight out of our Milky Way. Using these observed M-type brown dwarfs, we fitted a Galactic disc and halo model with a Markov chain Monte Carlo analysis. This model worked best with the scalelength of the disc fixed at h= 2.6 kpc. For the scaleheight of the disc, we found zo = 0.291 ?,1, kpc and for the central number density, rho(0) = 0.291(-0.13)(+0.20) # pc(-3). For the halo, we derived a flattening parameter kappa = 0.45 +/- .04 and a power-law index p = 2.4 +/- 0.07. We found the fraction of M-type brown dwarfs in the local density that belong to the halo to be f(h) = 0.0075(-0.0019)(+0.0025). We found no correlation between subtype of M-dwarf and any model parameters. The total number of M-type brown dwarfs in the disc and halo was determined to be 58.2(-6.70)(+9.81) x 10(9). We found an upper limit for the fraction of M-type brown dwarfs in the halo of 7(-4)(+5) per cent. The upper limit for the total Galactic disc mass in M-dwarfs is 4.34(-0.5)(+0.73) x 10(9) M-circle dot, assuming all M-type brown dwarfs have a mass of 80 M-J.