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

TL;DR

This study uses M-type brown dwarfs observed in Hubble Space Telescope data to model the Milky Way's disk and halo, providing new estimates of their structure, composition, and total brown dwarf population.

Contribution

First to analyze M-type brown dwarfs from the BoRG survey to constrain the Milky Way's disk and halo parameters using MCMC methods.

Findings

Disk scale height: 0.29 kpc

Halo flattening parameter: 0.45

Total M-type brown dwarfs in Galaxy: ~58 billion

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 $80 M_J$.