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

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 $z_0 = 0.29^{+0.02}_{-0.019}$ 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 $\kappa$ = 0.45$\pm{0.04}$ and a power-law index $p$ = 2.4$\pm{0.07}$. We found the fraction of M-type brown dwarfs in the local density that belong to the halo to be $f_{h}$ = 0.0075$^{+0.0025}_{-0.0019}$. 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 $58.2^{+9.81}_{-6.70} \times10^{9}$. 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 $4.34^{+0.73}_{-0.5}\times10^{9}$ $M_{\odot}$, assuming all M-type Brown Dwarfs have a mass of $80 M_J$.