Tracing Ram-Pressure Stripping with Warm Molecular Hydrogen Emission

TL;DR

This study uses infrared spectroscopy to detect and analyze warm molecular hydrogen emission in galaxies undergoing ram-pressure stripping, revealing shock-heating effects and providing insights into galaxy evolution in clusters.

Contribution

First detection of warm H$_2$ emission in multiple ram-pressure stripped galaxies, linking shock-heating to the stripping process and galaxy evolution.

Findings

Warm H$_2$ emission is brighter than in normal galaxies.

H$_2$ is offset or tail-like, indicating shock excitation.

Star formation rates are comparable to field galaxies.

Abstract

We use the Spitzer Infrared Spectrograph (IRS) to study four infalling cluster galaxies with signatures of on-going ram-pressure stripping. H$_2$ emission is detected in all four; two show extraplanar H$_2$ emission. The emission usually has a warm (T $\sim$ $115 - 160$K) and a hot (T $\sim$ 400 $-$ 600K) component that is approximately two orders of magnitude less massive than the warm one. The warm component column densities are typically $10^{19} - 10^{20}$ cm$^{-2}$ with masses of $10^6 - 10^8 M_\odot$. The warm H$_2$ 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 H$_2$ is offset from the majority of star formation along the direction of the galaxy's motion in the cluster, suggesting it is forming in the ram-pressure wake of the galaxy. Another galaxy, NGC 4522, exhibits a warm H$_2$ tail approximately 4 kpc in length. These results support the hypothesis that H$_2$ 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 $\mu$m is well resolved, knots and ripples demonstrate the turbulent nature of the stripping process. The H$\alpha$ and 24 $\mu$m 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 a galaxy experiences to explain the varied results observed in our sample.