HELP: Star formation as a function of galaxy environment with Herschel

The Herschel Extragalactic Legacy Project (HELP) brings together a vast range of data frommany astronomical observatories. Its main focus is on the Herschel data, which maps dustobscured star formation over 1300 deg2. With this unprecedented combination of data sets, it is possible to investigate how the star formation versus stellar mass relation (main sequence)of star-forming galaxies depends on environment. In this pilot study, we explore this question within 0.1<z<3.2 using data in the COSMOS field. We estimate the local environment from a smoothed galaxy density field using the full photometric redshift probability distribution. We estimate star formation rates by stacking the SPIRE data from the Herschel Multi-tiered Extragalactic Survey. Our analysis rules out the hypothesis that the main sequence for starforming systems is independent of environment at 1.5 < z <2, while a simple model in which the mean specific star formation rate declines with increasing environmental density gives a better description. However, we cannot exclude a simple hypothesis in which the main sequence for star-forming systems is independent of environment at z < 1.5 and z > 2. We also estimate the evolution of the star formation rate density in the COSMOS field, and our results are consistent with previous measurements at z < 1.5 and z > 2 but we find a 1.4+0.3 -0.2 times higher peak value of the star formation rate density at z ~ 1.9.