| Description |
Compact groups of galaxies represent density enhancements in the Universe comparable with clusters and are dominated by early type galaxies that lie in the Green Valley between the blue and red sequence. Our observations of a sample of such groups has revealed that 10 percent of our targeted group members show unusually powerful mid-IR H2 emission lines relative to PAH emission--suggesting shock-excitation. Most of these galaxies lie in the optical green valley, and exhibit specific star formation rates that lie between spirals and elliptical suggesting that they are a transition population in which shocks may play a role in their color evolution. We propose to map key far-IR cooling lines [OI], [CII] and CO with the PACS and SPIRE spectrometers to both map and quantify the strength and distribution of shocks to search for clues about how they might be affected by their environment. We will also use PACS and SPIRE imaging to constrain dust SEDs to estimate SFRs and dust-gas masses to explore whether the shocks may heat the gas above the threshold for star formation--hence shutting down new star formation. Alternatively, by comparison with VLA HI maps which show considerable tidal material in the groups, we will search for evidence that accretion from tidal streams is responsible for re-igniting new star formation-thus modifying their optical colors. This proposal will, for the first time, test whether shocks may play a transformative role in galaxy evolution. |
