274 M-type Brown Dwarfs IDed

The Size and Shape of the Milky Way Disk and Halo from M-type Brown Dwarfs in the BoRG Survey

Authors:

van Vledder et al

Abstract:

We have identified 274 M-type Brown Dwarfs in the Hubble Space Telescope’s Wide Field Camera 3 (WFC3) 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 disk and halo model with a Markov chain Monte Carlo (MCMC) analysis. This model worked best with the scale length of the disk fixed at h = 2.6 kpc. For the scale height of the disk, we found z0=0.29+0.02−0.019 kpc and for the central number density ρ0=0.29+0.20−0.13 #/pc3. 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 disk and halo was determined to be 58.2+9.81−6.70×109. We found an upper limit for the fraction of M-type Brown Dwarfs in the halo of 7+5−4%. The upper limit for the total Galactic Disk mass in M-dwarfs is 4.34+0.73−0.5×109 M⊙, assuming all M-type Brown Dwarfs have a mass of 80MJ.