An example E+A galaxy (bottom right panel) dissected into a spatially contiguous grid of 20 spectra (remaining panels).

For astronomers, spectroscopy is one of their major tools for measuring and deciphering the physical characteristics of the extra-terrestrial objects they observe. Stellar spectroscopy has matured to the point where the surface temperature, gravity, abundance, motions, and magnetic field strength of stars can be measured as a matter of course. For galaxies, these same properties can be measured, albeit in an average sense for the ensembles of stars and gas clouds that are contained within these vast and generally much more distant systems. In addition, the age of and thus the time at which stars in galaxies formed, and hence the levels of current and past star formation activity, are also vital pieces of information that spectroscopy can provide, since they directly address the most fundamental of questions – how do galaxies form and evolve?

In trying to reconstruct the formation and evolutionary history of galaxies, astronomers also have another important factor on their side and that is their ability to look back in time and see galaxies as they were when the universe was significantly younger than its present age. However, they pay a price for this in that the galaxies are so distant that they are both very faint and barely resolved above the ‘natural’ resolution of ground-based telescopes. Hence spectroscopy of these ‘faint fuzzy blobs’ is very challenging.

The new generation of 8-10m class telescopes, with their much superior light-grasp and image quality, has opened up a completely new era, making it feasible to spatially resolve distant galaxies and obtain spectra at different locations across their face. Such observations have been greatly facilitated through the development of spectrographs with an ‘integral field unit’ (IFU) which, using mirror and/or lenslet/optical fibre technology, provides a means of subdividing their field of view into a series of contiguous spatial pixels and producing a spectrum for each. Using such IFU spectrographs on the 8m Gemini and VLT telescopes, we have employed this ‘spectroscopic dissection’ approach to understand the physical mechanisms that are driving the rapid evolution in what are known as ‘E+A’ galaxies. These enigmatic galaxies, which are found in significant numbers in high redshift clusters, have a spectral signature which indicate they have gone through a recent burst of star formation activity, which for some mysterious reason has been suddenly truncated.

Our observations of these galaxies (a typical example is shown in the accompanying figure) have yielded some surprising results. Most notably, there appears to be a dichotomy in star formation geography. In some E+A galaxies, the recently formed population of new stars is very much concentrated towards their centre, which point to galaxy merging and tidal interactions most likely triggering and then halting the recent star formation activity. In the other E+A galaxies, the distribution of newly formed stars is more widespread, and in some cases more prominent in the outer (disk) regions, suggestive of an abrupt truncation of the star formation right across the disk of an active, spiral galaxy. Such a truncation event is more likely due to the interaction of the galaxy with the hot intracluster gas in which it is embedded. Hence it would appear that at least two mechanisms are responsible for the rapid evolution associated with E+A galaxies. The challenge now is to further elucidate the circumstances under which they operate, a task we are undertaking through further observations with the Gemini telescopes.

Michael Pracy, Warrick Couch,
Chris Blake and Kenji Bekki