A Survey of Molecular Hydrogen in the Crab Nebula
  We have carried out a near-infrared, narrow-band imaging survey of the Crab
Nebula, in the H2 2.12 micron and Br-gamma 2.17 micron lines, using the Spartan
Infrared camera on the SOAR Telescope. Over a 2.8' x 5.1' area that encompasses
about 2/3 of the full visible extent of the Crab, we detect 55 knots that emit
strongly in the H2 line. We catalog the observed properties of these knots. We
show that they are in or next to the filaments that are seen in
optical-passband emission lines. Comparison to HST [S II] and [O III] images
shows that the H2 knots are strongly associated with compact regions of
low-ionization gas. We also find evidence of many additional, fainter H2
features, both discrete knots and long streamers following gas that emits
strongly in [S II]. A pixel-by-pixel analysis shows that about 6 percent of the
Crab's projected surface area has significant H2 emission that correlates with
[S II] emission. We measured radial velocities of the [S II] lambda6716
emission lines from 47 of the cataloged knots and find that most are on the far
(receding) side of the nebula. We also detect Br-gamma emission. It is right at
the limit of our survey, and our Br-gamma filter cuts off part of the expected
velocity range. But clearly the Br-gamma emission has a quite different
morphology than the H2 knots, following the long linear filaments that are seen
in H-alpha and in [O III] optical emission lines.
