  Gravitational wave transients are caused by some of the most energetic events
in the Universe, and a precise location would allow deep examination of the
counterpart by electromagnetic waves (telescopes collecting light), the
combination of GW and EM resulting in very much improved science return
(multi-messenger Astronomy). Since the GW detectors do not provide good
localization on the sky, the faint counterpart will be very difficult to find.
One strategy to help the search is to look first where mass is concentrated and
thus the prior probability of GW events is highest. In the first part of this
paper, we present methods used to estimate stellar masses and metallicities of
galaxies and galaxy clusters in different catalogs. In the second part of the
paper, we test our estimation accuracy by comparing our results with stellar
masses given in Stripe 82 Massive Galaxy Catalogue (S82-MGC). The relation
between stellar mass we found and that from S82-MGC is provided for GWGC,
2MASS-GLADE, and WISExSCOS catalog in the last part of the paper. Our results
are used in an interactive web-based tool (Skymap Viewer) for astronomers to
decide where to look first in EM follow-up observations of GW events in the
future.
