Tracing ram-pressure stripping with warm molecular hydrogen emission

We use the Spitzer Infrared Spectrograph to study four infalling cluster galaxies with signatures of ongoing ram-pressure stripping. H₂ emission is detected in all four, and two show extraplanar H₂ emission. The emission usually has a warm (T 115-160 K) and a hot (T 400-600 K) component that is approximately two orders of magnitude less massive than the warm one. The warm component column densities are typically 10¹⁹ to 10²⁰ cm^-2 with masses of 10⁶ to 10⁸ M _o The warm H₂ is anomalously bright compared with normal star-forming galaxies and therefore may be excited by ram-pressure. In the case of CGCG 97-073, the H₂ is offset from the majority of star formation along the direction of the galaxy's motion in the cluster, suggesting that it is forming in the ram-pressure wake of the galaxy. Another galaxy, NGC 4522, exhibits a warm H₂ tail approximately 4 kpc in length. These results support the hypothesis that H₂ within these galaxies is shock-heated from the interaction with the intracluster medium. Stripping of dust is also a common feature of the galaxies. For NGC 4522, where the distribution of dust at 8 μm is well resolved, knots and ripples demonstrate the turbulent nature of the stripping process. The Hα and 24 μm luminosities show that most of the galaxies have star-formation rates comparable to similar mass counterparts in the field. Finally, we suggest a possible evolutionary sequence primarily related to the strength of ram-pressure that a galaxy experiences to explain the varied results observed in our sample.