The largest Herschel extragalactic surveys , H-ATLAS and HerMES , have selected a sample of “ ultrared ” dusty , star-forming galaxies ( DSFGs ) with rising SPIRE flux densities ( S _ { 500 } > S _ { 350 } > S _ { 250 } ; so-called “ 500 \mu m-risers ” ) as an efficient way for identifying DSFGs at higher redshift ( z > 4 ) .
In this paper , we present a large Spitzer follow-up program of 300 Herschel ultrared DSFGs .
We have obtained high-resolution ALMA , NOEMA , and SMA data for 63 of them , which allow us to securely identify the Spitzer /IRAC counterparts and classify them as gravitationally lensed or unlensed .
Within the 63 ultrared sources with high-resolution data , \sim 65 % appear to be unlensed , and \sim 27 % are resolved into multiple components .
We focus on analyzing the unlensed sample by directly performing multi-wavelength spectral energy distribution ( SED ) modeling to derive their physical properties and compare with the more numerous z \sim 2 DSFG population .
The ultrared sample has a median redshift of 3.3 , stellar mass of 3.7 \times 10 ^ { 11 } M _ { \odot } , star formation rate ( SFR ) of 730 M _ { \odot } yr ^ { -1 } , total dust luminosity of 9.0 \times 10 ^ { 12 } L _ { \odot } , dust mass of 2.8 \times 10 ^ { 9 } M _ { \odot } , and V-band extinction of 4.0 , which are all higher than those of the ALESS DSFGs .
Based on the space density , SFR density , and stellar mass density estimates , we conclude that our ultrared sample can not account for the majority of the star-forming progenitors of the massive , quiescent galaxies found in infrared surveys .
Our sample contains the rarer , intrinsically most dusty , luminous and massive galaxies in the early universe that will help us understand the physical drivers of extreme star formation .