tokens: mergers might be detectable for longer (e.g. through stellar streams, bright cores) than gas-poor galaxies. CO surveys of protocluster and identically selected field galaxies would allow one to compare the gas mass fractions of these two populations. Protocluster galaxies might be expected to have higher gas fractions due to efficient supply of pristine intergalactic gas through filamentary accretion, but this is a picture yet to be empirically tested. Lyα blobs are generally found in high- z protoclusters and indicate the presence of large volumes of cold gas around forming galaxies. Fakhouri Ma ( 2009 ) examined the role of environment on the halo merger rate in N -body simulations, using the friend-of-friends group finder (Davis et al. 1985 ) within the millennium simulation (Springel, Matteo Hernquist 2005 ) to trace the merger histories of dark matter sub-haloes out to z = 2. A key finding was a correlation between environment and merger rate, such that sub-haloes in the densest environments underwent merger rates 2–2.5 times higher than those in the lowest density regions. With the caveat that galaxies are biased tracers of the matter field, it is reasonable to assume that there will be a similar trend for the galaxies themselves. It is interesting that the level of enhancement of the merger rate found by Fakhouri Ma ( 2009 ) in the simulations is broadly in agreement with the enhancement we find for SSA22. Our analysis traces the rest-frame ultraviolet emission which is very sensitive to dust obscuration; for example, extended/clumpy star-forming regions could be hidden and the merger fraction understated, or a clumpy dust distribution could lead to very high extinction to certain stellar populations, giving the stellar emission overall a clumpy morphology, that could possibly be misinterpreted as a merger. We do not expect the use of rest-frame UV observations to lead to a significant bias in our interpretation unless there is a significant difference between the obscuration properties of galaxies in the protocluster and the field, since we have applied an identical analysis to both. Nevertheless, we are now obtaining near-infrared HST -WFC3 observations that will allow us to repeat our analysis based on the morphology of the rest-frame optical flux. Similarly, we have ALMA follow-up observations of LBGs along the ‘merger sequence’ in SSA22 that will allow us to directly detect the thermal emission from cold dust responsible for extinction in the ultraviolet/optical light. Our result is marginal mainly due to small number statistics, which dominate the size of the uncertainties on our merger fractions. This arises from the lack of a complete LBG catalogue covering the whole SSA22 field and sparse HST coverage of SSA22. It would be useful to extend our analysis to additional protoclusters and fields at a similar redshift to overcome the potential issues of small number statistics and cosmic variance, but high- z protoclusters are
ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O B-Model I-Model I-Model O O B-Model I-Model I-Model B-Citation I-Citation I-Citation I-Citation O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O B-Formula I-Formula I-Formula O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O B-CelestialObject O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Telescope B-Instrument O O O O O O O O O O O O O O O O O O O O O B-Observatory O O O O O O O O O B-CelestialObject O O O O O O O O O O O O O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject O O O B-Telescope O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O