The redshift distribution and dust properties of dusty star-forming galaxies at high redshift

Abstract

Millimeter (mm) and submillimeter (submm) continuum observations brought about a radical shift with the discovery that luminous, dusty galaxies were a thousand times more abundant in the early Universe than they are at present day. The first submm/mm detected high redshift dusty star-forming galaxies (DSFGs) were discovered in 1998, and surveys of these suggested that the peak of dust enshrouded star formation occurred at z ∼ 2, simultaneous with the peak of black hole accretion and cosmic star formation. While the history of star formation has now been measured out to z ∼ 8 in rest-frame UV surveys, little progress has been made until recently to quantify the amount of highly obscured star formation at z >3.5 in the mm/submm regime mainly because of the difficulties in obtaining robust redshifts for DSFGs. <br /> In this thesis I explore the redshifts of a sample of DSFGs selected from the South Pole Telescope (SPT) Survey. The survey covers an impressive 2500 deg² at the wavelengths of 1.4mm, 2.2mm and 3.0mm. The large area of the survey made it possible to assemble a large sample of rare and bright sources. The sources are mostly gravitationally lensed and at high redshifts. Here, we explore the high redshift nature of the SPT-DSFGs through ALMA redshift scans of 42 sources. We combine these observations with previously published and new mm/submm line and photometric data of the SPT-DSFGs to study their redshift distribution. This yields a sample of 62 sources with reliable redshifts based on either two spectral lines or one spectral line combined with photometric observations, and a median redshift is z =4.1±0.2. The redshift distribution of the SPT-DSFGs is affected by strong gravitational lensing and when correcting for the effect of gravitational lensing we find a median redshift of z =3.5±0.3. Comparing to redshift distributions selected at shorter wavelengths from the literature, we show that selection wavelength affects the shape of the redshift distribution. <br /> To complete the redshift distribution for the full sample of 91 SPT-DSFGs, we take advantage of the well sampled photometry for the full sample. We investigate different ways on finding photometric redshifts, settling on fitting spectral energy distributions. We use the sources with spectroscopic redshifts to construct a dust temperature distribution that is used as input for the spectral energy distributions. By combining the redshift distribution presented above with photometric redshifts of the remaining 29 sources without spectroscopic redshifts, we find a median redshift of z =3.7±0.1. A study of the dust temperature as a function of redshift (for the sources with spectroscopic redshifts) shows that the dust temperature is increasing with redshift, consistent with what is seen for lower redshift samples. <br /> The highest redshift galaxy found in the entire SPT-DSFG sample, SPT0311- 58 at z =6.900±0.002, is also the highest-redshift millimeter-selected DSFG discovered to date. We constrain the properties of the ISM in SPT0311-58 with a radiative transfer analysis of the dust continuum photometry and the CO and [C I] line emission. This allows us to determine the gas content without ad hoc assumptions about gas mass scaling factors. SPT0311-58 is extremely massive, with an intrinsic gas mass of M_gas =3.3±1.9×1011M_Θ. Its largemass and intense star formation is very rare for a source well into the Epoch of Reionization.