z-GAL: A NOEMA spectroscopic redshift survey of bright Herschel galaxies: II. Dust properties

We present the dust properties of 125 bright Herschel galaxies selected from the z-GAL NOEMA spectroscopic redshift survey. All the galaxies have precise spectroscopic redshifts in the range 1.3 < z < 5.4. The large instantaneous bandwidth of NOEMA provides an exquisite sampling of the underlying dust continuum emission at 2 and 3 mm in the observed frame, with flux densities in at least four sidebands for each source. Together with the available Herschel 250, 350, and 500 mu m and SCUBA-2 850 mu m flux densities, the spectral energy distribution (SED) of each source can be analyzed from the far-infrared to the millimeter, with a fine sampling of the Rayleigh-Jeans tail. This wealth of data provides a solid basis to derive robust dust properties, in particular the dust emissivity index (beta) and the dust temperature (T-dust). In order to demonstrate our ability to constrain the dust properties, we used a flux-generated mock catalog and analyzed the results under the assumption of an optically thin and optically thick modified black body emission. The robustness of the SED sampling for the z-GAL sources is highlighted by the mock analysis that showed high accuracy in estimating the continuum dust properties. These findings provided the basis for our detailed analysis of the z-GAL continuum data. We report a range of dust emissivities with beta similar to 1.5 - 3 estimated up to high precision with relative uncertainties that vary in the range 7%-15%, and an average of 2.2 +/- 0.3. We find dust temperatures varying from 20 to 50 K with an average of T-dust similar to 30 K for the optically thin case and T-dust similar to 38 K in the optically thick case. For all the sources, we estimate the dust masses and apparent infrared luminosities (based on the optically thin approach). An inverse correlation is found between T-dust and beta with beta proportional to T-dust(-0.69), which is similar to what is seen in the local Universe. Finally, we report an increasing trend in the dust temperature as a function of redshift at a rate of 6.5 +/- 0.5 K/z for this 500 mu m-selected sample. Based on this study, future prospects are outlined to further explore the evolution of dust temperature across cosmic time.