Abstract

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 $$z_0 = 0.29^{+0.02}_{-0.019}$$ kpc and for the central number density, $$\rho _0 = 0.29^{+0.20}_{-0.13}$$ # pc−3. For the halo, we derived a flattening parameter κ = 0.45 ± 0.04 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 fh = 0.0075$$^{+0.0025}_{-0.0019}$$. 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 $$58.2^{+9.81}_{-6.70} \times 10^{9}$$. We found an upper limit for the fraction of M-type brown dwarfs in the halo of 7$$^{+5}_{-4}$$ per cent. The upper limit for the total Galactic disc mass in M-dwarfs is $$4.34^{+0.73}_{-0.5}\times 10^{9}$$ M, assuming all M-type brown dwarfs have a mass of 80 MJ.

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