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 $z_0=0.29_{-0.019}^{+0.02}$ kpc and for the central number density $\ddot{I}{}_0=0.29_{-0.13}^{+0.20}$ \#/pc${}^3$. For the halo we derived a flattening parameter $\hat{I}º$ = 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${}_{-0.0019}^{+0.0025}$. We found no correlation between subtype of M-dwarf and any model parameters. The total number of M-type Brown Dwarfs in the disk and halo was determined to be $58.2_{-6.70}^{+9.81}\times 10^9$. We found an upper limit for the fraction of M-type Brown Dwarfs in the halo of 7${}_{-4}^{+5}$\%. The upper limit for the total Galactic Disk mass in M-dwarfs is $4.34_{-0.5}^{+0.73}\times 10^9$ $M_{\odot }$, assuming all M-type Brown Dwarfs have a mass of $80M_J$.

9 Figures, 8 Tables, MNRAS