The APEX submillimeter imaging survey of distant galaxies in the COSMOS field

Abstract

Submillimeter galaxies (SMGs) are far-infrared luminous, vigorously star forming galaxies in the early universe. They are major contributors to the extragalactic far-infrared background emission, and they trace the most intense phase of stellar mass build up in cosmic history. The star formation rates in SMGs is higher by a factor hundred to thousand compared to that of "normal" galaxies such as our Milky Way galaxy.<br /> In order to understand the nature of SMGs, numerous (sub)millimeter surveys have been carried out to collect large statistical samples of these galaxies. In particular, surveys at wavelengths in the range 800-2000 um have the unique advantage that, at a fixed wavelength, a galaxy with a given infrared luminosity is observed with the same flux density in the redshift range z ~ 1-8.<br /> The Cosmic Evolution Survey (COSMOS) is an equatorial 2 deg^2 field designed to probe the formation and evolution of galaxies as a function of cosmic time and large scale structure environment. To date the field has been observed with most major space and ground-based telescopes over a large fraction of the electromagnetic spectrum. However, at 850-870 um the largest survey covers only ~ 0.11 deg^2.<br /> In this work, we carried out the yet largest, 0.75 deg^2, 870 um survey of the COSMOS field, using the Large Apex BOlometer CAmera (LABOCA) at the APEX telescope.We provide a catalog with reliably detected sources and compare it with other (sub)millimeter studies in the same field. We derive the number counts and redshift distribution of the sources, which are useful to constrain models that try to follow the evolution of galaxies throughout cosmic history.<br /> We present high-resolution interferometric observations at 1.3 mm wavelength of a subsample of SMGs that we previously detected in our LABOCA imaging of the COSMOS field. The high resolution allows to unambiguously identify the location of the most likely counterparts at other wavelenghts. The conclusions from our study are: (i) 15% to 40% of SMGs observed with single-dish telescopes break up into multiple (sub)mm galaxies, (ii) identifications through statistical arguments, of counterparts to single-dish submillimeter sources could be wrong up to 30% , and (iii) the redshift distribution of SMGs shows a higher mean and broader width than what was found in previous studies. <br /> We study the average (sub)millimeter properties of large samples of galaxies that have more moderate SFRs than SMGs. They are not individually detected in (sub)millimeter maps. However, they can be studied through stacking. We implement a recently developed stacking algorithm that we test on simulations with a wide range of source densities and source clustering properties. The algorithm is applied in the COSMOS field, where the large area and a deep 2.2 um source catalog allow us to stack samples more than an order of magnitude larger than those of previous studies for similar types of galaxies. We detect the average submillimeter emission from high redshift star-forming galaxies, while high-redshift passive galaxies remain undetected, mainly due to their low number statistics. We find that at redshift 1.4 to 2.5, star-forming galaxies are four times brighter than those at lower redshifts. We study the redshift evolution of these populations, and combine this information with the stacking at radio wavelengths of the same populations, confirming that the well-known tight correlation between radio and far-infrared luminosities is also seen for these galaxy populations up to z ~ 2.