MUSE sneaks a peek at extreme ram-pressure stripping events - II. The physical properties of the gas tail of ESO137-001

We present a study of the physical properties of the disc and tail of ESO137-001, a galaxy suffering from extreme ram-pressure stripping during its infall into the Norma cluster. With sensitive and spatially resolved MUSE (Multi Unit Spectroscopic Explorer) spectroscopy, we analyse the emission line diagnostics in the tail of ESO137-001, finding high values of [N II]/H alpha and [O I]/H alpha that are suggestive of the presence of shocks in turbulent gas. However, the observed line ratios are not as strong as commonly seen in pure shock heating models, suggesting that other emission mechanisms may contribute to the observed emission. Indeed, part of the observed emission, particularly at close separations from the galaxy disc, may originate from recombination of photoionized gas stripped from the main body of ESO137-001. We also identify a large number of bright compact knots within in the tail, with line ratios characteristic of H II regions. These H II regions, despite residing in a stripped gas tail, have quite typical line ratios, densities, temperatures, and metallicity (similar to 0.7 solar). The majority of these H II regions are embedded within diffuse gas from the tail, which is dynamically cool (sigma similar to 25-50 km s(-1)). This fact, together with a lack of appreciable gradients in age and metallicity, suggests that these H II regions formed in situ. While our analysis represents a first attempt to characterize the rich physics of the ESO137-001 tail, future work is needed to address the importance of other mechanisms, such as thermal conduction and magnetohydrodynamic waves, in powering the emission in the tail.