Program: GN-2007B-Q-79

A large fraction of the highest mass galaxies known stopped forming stars prior to z = 2. Observations have shown that the most active sites of continuing star formation have shifted from high mass galaxies at z > 2 to progressively lower mass galaxies over time. Neither the cause of this mass-dependent evolution nor the significance of the contribution of low mass galaxies to the global star formation history between z = 1 and 2 is known. We propose to obtain deep narrow-band H-alpha imaging of the distant galaxy cluster, XMMU J2235.3-2557, using NIRI at Gemini-North with the H-continuum filter (central wavelength = 1.57 microns, corresponding to H-alpha at redshift z= 1.4). We also need NIRI H-band broad filter imaging to measure the continuum flux (and it's contribution to the narrow-band luminosities) of our targets. From the flux calibrated H-alpha luminosities, we will calculate star formation rates that we will combine with mass measurements we determine from publicly available multiwavelength data, to measure specific star formation rates in the cluster. These observations will allow us to directly measure to what extent low-mass galaxies at z=1.4 are actively forming stars compared to their lower redshift counterparts.