MHOs toward HMOs: A Search for Molecular Hydrogen emission-line Objects
  toward High-Mass Outflows
  We present the results of a narrow-band near-infrared imaging survey for
Molecular Hydrogen emission-line Objects (MHOs) toward 26 regions containing
high-mass protostellar candidates and massive molecular outflows. We have
detected a total of 236 MHOs, 156 of which are new detections, in 22 out of the
26 regions. We use H$_2$ 2.12-$\mu$m/H$_2$ 2.25-$\mu$m flux ratios, together
with morphology, to separate the signatures of fluorescence associated with
photo-dissociation regions (PDRs) from shocks associated with outflows in order
to identify the MHOs. PDRs have typical low flux ratios of ~ 1.5 - 3, while the
vast majority of MHOs display flux ratios typical of C-type shocks (~ 6-20). A
few MHOs exhibit flux ratios consistent with expected values for J-type shocks
(~ 3-4), but these are located in regions that may be contaminated with
fluorescent emission. Some previously reported MHOs have low flux ratios, and
are likely parts of PDRs rather than shocks indicative of outflows. We identify
a total of 36 outflows across the 22 target regions where MHOs were detected.
In over half these regions, MHO arrangements and fluorescent structures trace
features present in CO outflow maps, suggesting the CO emission traces a
combination of dynamical effects, which may include gas entrained in expanding
PDRs as well as bipolar outflows. Where possible, we link MHO complexes to
distinct outflows and identify candidate driving sources.
