Nomads of the Galaxy

TL;DR

This paper estimates the abundance of unbound compact objects, called nomads, in the Galaxy and discusses their potential detection through microlensing surveys like WFIRST, GAIA, and LSST.

Contribution

It provides the first comprehensive estimate of Galactic nomads' numbers and explores detection strategies using current and future microlensing surveys.

Findings

Up to ~10^5 nomads per main sequence star in the Galaxy.

WFIRST can measure the number of nomads > Jupiter mass with 13% accuracy.

All-sky surveys can identify nomads > Jupiter mass.

Abstract

We estimate that there may be up to ~10^5 compact objects in the mass range 10^{-8} -10^{-2} solar mass per main sequence star that are unbound to a host star in the Galaxy. We refer to these objects as nomads; in the literature a subset of these are sometimes called free-floating or rogue planets. Our estimate for the number of Galactic nomads is consistent with a smooth extrapolation of the mass function of unbound objects above the Jupiter-mass scale, the stellar mass density limit, and the metallicity of the interstellar medium. We analyze the prospects for detecting nomads via Galactic microlensing. The Wide-Field Infrared Survey Telescope (WFIRST) will measure the number of nomads per main sequence star greater than the mass of Jupiter to ~ 13%, and the corresponding number greater than the mass of Mars to ~25%. All-sky surveys such as GAIA and LSST can identify nomads greater than about the mass of Jupiter. We suggest a dedicated drift scanning telescope that covers approximately 100 square degrees in the Southern hemisphere could identify nomads as small as 10^{-8} solar mass via microlensing of bright stars with characteristic lightcurve timescales of a few seconds.