Merger Signatures in the Dynamics of Star-forming Gas
  Spatially resolved kinematics have been used to determine the dynamical
status of star-forming galaxies with ambiguous morphologies, and constrain the
importance of galaxy interactions during the assembly of galaxies. However,
measuring the importance of interactions or galaxy merger rates requires
knowledge of the systematics in kinematic diagnostics and the visible time with
merger indicators. We analyze the dynamics of star-forming gas in a set of
binary merger hydrodynamic simulations with stellar mass ratios of 1:1 and 1:4.
We find that the evolution of kinematic asymmetries traced by star-forming gas
mirrors morphological asymmetries derived from mock optical images, in which
both merger indicators show the largest deviation from isolated disks during
strong interaction phases. Based on a series of simulations with various
initial disk orientations, orbital parameters, gas fractions, and mass ratios,
we find that the merger signatures are visible for ~0.2-0.4 Gyr with kinematic
merger indicators but can be approximately twice as long for equal-mass mergers
of massive gas-rich disk galaxies designed to be analogs of z~2-3 submillimeter
galaxies. Merger signatures are most apparent after the second passage and
before the black holes coalescence, but in some cases they persist up to
several hundred Myr after coalescence. About 20-60% of the simulated galaxies
are not identified as mergers during the strong interaction phase, implying
that galaxies undergoing violent merging process do not necessarily exhibit
highly asymmetric kinematics in their star-forming gas. The lack of
identifiable merger signatures in this population can lead to an
underestimation of merger abundances in star-forming galaxies, and including
them in samples of star-forming disks may bias the measurements of disk
properties such as intrinsic velocity dispersion.
