HELP*: star formation as a function of galaxy environment with Herschel†
Scott, D.; Sargent, M. T.; Buat, V.; Wang, L.; Farrah, D.; Ibar, E.; Symeonidis, M.; Vaccari, M.; Griffin, M.; Efstathiou, A.; Oliver, S.; Jarvis, M.; Hurley, P. D.; Darvish, B.; Viero, M. P.; Duivenvoorden, S.; Papadopoulos, A.; Scudder, J. M.
United Kingdom, France, United States, Cyprus, Chile, South Africa, Canada, Italy, Netherlands
Abstract
The Herschel Extragalactic Legacy Project (HELP) brings together a vast range of data from many astronomical observatories. Its main focus is on the Herschel data, which maps dust-obscured 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 star-forming 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.