A z=2.5 protocluster associated with the radio galaxy MRC 2104-242: star formation and differing mass functions in dense environments

We present results from a narrow-band survey of the field around the high-redshift radio galaxy MRC 2104-242. We have selected H alpha emitters in a 7 arcmin(2) field and compared the measured number density with that of a field sample at similar redshift. We find that MRC 2104-242 lies in an overdensity of galaxies that is 8.0 +/- 0.8 times the average density of a blank field, suggesting it resides in a large-scale structure that may eventually collapse to form a massive cluster. We find that there is more dust obscured star formation in the protocluster galaxies than in similarly selected control field galaxies and there is tentative evidence of a higher fraction of starbursting galaxies in the denser environment. However, on average we do not find a difference between the star formation rate (SFR)-mass relations of the protocluster and field galaxies and so conclude that the SFR of these galaxies at z similar to 2.5 is governed predominantly by galaxy mass and not the host environment. We also find that the stellar mass distribution of the protocluster galaxies is skewed towards higher masses and there is a significant lack of galaxies at M < 10(10) M-circle dot within our small field of view. Based on the level of overdensity we expect to find similar to 22 star-forming galaxies below 10(10) M-circle dot in the protocluster and do not detect any. This lack of low-mass galaxies affects the level of overdensity which we detect. If we only consider high-mass (M > 10(10.5) M-circle dot) galaxies, the density of the protocluster field increases to similar to 55 times the control field density.