Using the 2-MASS photometric redshift survey to optimize LIGO follow-up observations
The initial discovery of Laser Interferometer Gravitational-WaveObservatory (LIGO) on 2015 September 14 was the inspiral merger andring-down of the black hole binary at a distance of about 500 Mpc or aredshift of about 0.1. The search for electromagnetic counterparts forthe inspiral of binary black holes is impeded by coarse initial sourcelocalizations and a lack of a compelling model for the counterpart;therefore, rapid electromagnetic follow-up is required to understand theastrophysical context of these sources. Because astrophysical sources ofgravitational radiation are likely to reside in galaxies, it would makesense to search first in regions where the LIGO-Virgo probability islarge and where the density of galaxies is large as well. Under theassumption that the probability of a gravitational-wave event from agiven region of space is proportional to the density of galaxies withinthe probed volume, one can calculate an improved localization of theposition of the source simply by multiplying the LIGO-Virgo skymap bythe density of galaxies in the range of redshifts. We propose using the2-MASS photometric redshift galaxy catalogue for this purpose anddemonstrate that using it can dramatically reduce the search region forelectromagnetic counterparts.