A survey of molecular hydrogen in the Crab Nebula

We have carried out a near-infrared, narrowband imaging survey of the Crab Nebula, in the H₂ 2.12 μm and Brγ 2.17 μm lines, using the Spartan Infrared camera on the SOAR Telescope. Over a 28 × 51 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 Hubble Space Telescope [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% 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] λ6716 emission lines from 47 of the cataloged knots and find that most are on the far (receding) side of the nebula. We also detect Brγ emission. It is right at the limit of our survey, and our Brγ filter cuts off part of the expected velocity range. But clearly the Brγ emission has a quite different morphology than the H₂ knots, following the long linear filaments that are seen in Hα and in [O III] optical emission lines.