  We present F110W (~J) and F160W (~H) observations of ten submillimeter
galaxies (SMGs) obtained with the Hubble Space Telescope's (HST's) NICMOS
camera. Our targets have optical redshifts in the range 2.20<z<2.81 confirmed
by millimeter CO or mid-IR spectroscopy, guaranteeing that the two bands sample
the rest-frame optical with the Balmer break falling between them. Eight of ten
are detected in both bands, while two are detected in F160W only. We study
their F160W morphologies, applying a maximum-deblending detection algorithm to
distinguish multiple- from single-component configurations, leading to
reassessments for several objects. Based on our NICMOS imaging and/or previous
dynamical evidence we identify five SMGs as multiple sources, which we
interpret as merging systems. Additionally, we calculate morphological
parameters asymmetry (A) and Gini coefficient (G); thanks to our sample's
limited redshift range we recover the trend that multiple-component,
merger-like morphologies are reflected in higher asymmetries. We analyze the
stellar populations of nine objects with F110W/F160W photometry, using archival
HST optical data when available. For multiple systems, we are able to model the
individual components that build up an SMG. With the available data we cannot
discriminate among star formation histories, but we constrain stellar masses
and mass ratios for merger-like SMG systems, obtaining a mean
log(M_*/M_sun)=10.9+/-0.2 for our full sample, with individual values
log(M_*/M_sun)~9.6-11.8. The morphologies and mass ratios of the least and most
massive systems match the predictions of the major-merger and cold accretion
SMG formation scenarios, respectively, suggesting that both channels may have a
role in the population's origin.
