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Lingyu Wang
SRON and University of Groningen

Hyperluminous infrared galaxies (HLIRGs) - the greatest challenge in ultramassive galaxy formation

We have identified a population of highly star-forming ultra-massive galaxies by exploiting the deepest-ever low-frequency data from LOFAR and the Herschel deep surveys in the best-studied northern deep fields. The depth of the LOFAR data is comparable to the deepest existing radio continuum surveys (e.g. VLA-COSMOS) but with over 10 times larger areal coverage. The high sensitivity and angular resolution of the LOFAR data allow us to identify the multi-wavelength counterparts of Herschel sources. This powerful combination of LOFAR and Herschel results in an unprecedented large and highly complete sample of HLIRGs with IR luminosities LIR > 1013L⊙. We exploit our multi-wavelength catalogue to derive photometric redshifts and other physical properties. Most of the HLIRGs are extreme starburst factories with SFR above 1000 M_solar / yr and are ultra massive with stellar mass > 10^11.5 M_solar. We find strong evidence that the abundance of the ultra-massive galaxies may have been severely underestimated in past studies. If confirmed, it would imply that our current understanding of massive galaxy formation is very incomplete and significant modifications to models are required. In addition, as massive galaxies sample the high-mass end of the halo mass function, the abundance of the most massive and therefore rarest galaxies also provide stringent tests on the ΛCDM paradigm. The HLIRGs emit over 90% of their energy in the rest-frame IR. Therefore the wavelength range of IR and submm is of critical importance to understand the nature of these extreme systems. Large-area imaging surveys are needed to find large samples of these galaxies while spectroscopic observations are crucial for confirming their extreme nature and for understanding their physical conditions.