GMOS SV program 56

The cores of rich clusters at medium redshifts (z = 0.2 - 0.6) contain higher numbers of blue galaxies seen during or just after a major bursts of star formation (eg., Butcher & Oemler, 1978, Dressler & Gunn, 1982, Couch & Sharples, 1987). Although several mechanism have been suggested for the origin of these episodes of intense star formation, studies based on HST imaging (eg., Dressler et al., 1997, Couch et al., 1998) have been unable to single out a particular cause. The most likely mechanisms are ram pressure stripping, galaxy harassment and galaxy mergers. Spatially resolved spectroscopy of starburst galaxies in these clusters can distinguish between these possibilities by allowing us to chart the spatial distribution of star formation and the velocity structure of the ionised gas. Ram pressure induced star fromation be concentrated on the leading edge of the galaxy disk (Abadi, Moore & Bower, 1998), while mergers and harrassment will disrupt the smooth rotation of the disk leading to orthogonal or counter rotating gas. Mergers and harrassment can be distinguished by the relative velocity of the galaxies involved in the encounter.

These observations will clearly demonstrate the capabilities of the Gemini GMOS-IFU. High spatial resolution is required as the galaxies have typical sizes of 3-5 arcsec. This is will matched to the dimensions of the GMOS IFU. Measurements of a few such galaxies, chosing representitiative systems from HST imaging will yield conclusive results and establish the driving mechanism of the starburst phenomenon in intermediate redshift galaxy clusters. Understanding this effect is crucial to explaining rapid evolution in the properties of galaxies in clusters.

We have selected a small sample of the brightest and most spectacular objects in well-studied clusters at intermediate redshift (all have HST images). The phenemonon of "down-sizing", in which activity passes from luminous galaxies at high-z to less-luminous ones at low-z, actually makes it better to observe the more luminous starbursting galaxies at higher z rather than less luminous ones at lower z. This also allows the galaxy to fit comfortably within the IFU field of view (mu_r less than 22 mag/arcsec^2 at a major-axis diameter of 4"). For SV, we have selected the best H-delta-strong example.

SNR > 10 per 5A, will allow the H-delta line to be measured to an accuracy of 0.7A which allow regions of enhanced H-delta to be distinguished from "normal" H-delta strengths of 1-2A. We assume that the continuum can be accurately modelled using the brighter parts of the galaxy to create a template. We have modelled the accuracy with which dynamical information can be recovered from such spectra. A 5A equivalent width line can be centroided to better than 15% (1-sigma) of the line width, easily allowing us to compare the rotation motion with the velocity dispersion. Background subtraction will be done using the background field via a careful choice of PA to align the (halved) object field optimally with the galaxy and to avoid contamination of the background field.