Search

Context. Determining when the first galaxies formed remains an outstanding goal of modern observational astronomy. Theory and current stellar population models imply that the first galaxies formed at least at z = 14 − 15. But to date, only one galaxy at z > 13 (GS-z13-0) has been spectroscopically confirmed. Aims. The galaxy 'HD1' was recently proposed to be a z ∼ 13.27 galaxy based on its potential Lyman break and tentative [O III] 88 μm detection with ALMA. We hereby aim to test this scenario with new ALMA Band 4 observations of what would be the [C II] 158 μm emission if HD1 is at z ∼ 13.27. Methods. We carefully analyse the new ALMA Band 4 observations and re-analyse the existing ALMA Band 6 data on the source to determine the proposed redshift. Results. We find a tentative 4σ feature in the Band 4 data that is spatially offset by 1.​​″7 and spectrally offset by 190 km s−1 from the previously reported 3.8σ '[O III] 88 μm' feature. Through various statistical tests, we demonstrate that these tentative features are fully consistent with both being random noise features. Conclusions. We conclude that we are more likely to be recovering noise features than both [O III] 88 μm and [C II] 158 μm emission from a source at z ∼ 13.27. Although we find no credible evidence of a z ∼ 13.27 galaxy, we cannot entirely rule out this scenario. Non-detections are also possible for a z ∼ 13 source with a low interstellar gas-phase metallicity or ionisation parameter and/or high gas density. Moreover, the new continuum and line upper limits provide no strong evidence for or against a lower-redshift scenario. Determining where and exactly what type of galaxy HD1 is, will now likely require JWST/NIRSpec spectroscopy. [ABSTRACT FROM AUTHOR]

We present Atacama Large Millimeter/submillimeter Array (ALMA) 2 mm continuum observations of a complete and unbiased sample of 99 870 μm selected submillimeter galaxies (SMGs) in the Extended Chandra Deep Field South (ALESS). Our observations of each SMG reach average sensitivities of 53 μJy beam−1. We measure the flux densities for 70 sources, for which we obtain a typical 870 μm-to-2 mm flux ratio of 14 ± 5. We do not find a redshift dependence of this flux ratio, which would be expected if the dust emission properties of our SMGs were the same at all redshifts. By combining our ALMA measurements with existing Herschel/SPIRE observations, we construct a (biased) subset of 27 galaxies for which the cool dust emission is sufficiently well sampled to obtain precise constraints on their dust properties using simple isothermal models. Thanks to our new 2 mm observations, the dust emissivity index is well constrained and robust against different dust opacity assumptions. The median dust emissivity index of our SMGs is β ≃ 1.9 ± 0.4, consistent with the emissivity index of dust in the Milky Way and other local and high-redshift galaxies, as well as classical dust-grain model predictions. We also find a negative correlation between the dust temperature and β, similar to low-redshift observational and theoretical studies. Our results indicate that β ≃ 2 in high-redshift dusty star-forming galaxies, implying little evolution in dust-grain properties between our SMGs and local dusty galaxy samples, and suggesting that these high-mass and high-metallicity galaxies have dust reservoirs driven by grain growth in their interstellar medium. [ABSTRACT FROM AUTHOR]

Integral field spectroscopy (IFS) surveys are changing how we study galaxies and are creating vastly more spectroscopic data available than before. The large number of resulting spectra makes visual inspection of emission line fits an infeasible option. Here, we present a demonstration of an artificial neural network (ANN) that determines the number of Gaussian components needed to describe the complex emission line velocity structures observed in galaxies after being fit with LZIFU.We apply our ANN to IFS data for the S7 survey, conducted using theWide Field Spectrograph on theANU2.3mTelescope, and the SAMI Galaxy Survey, conducted using the SAMI instrument on the 4 m Anglo-Australian Telescope. We use the spectral fitting code LZIFU (Ho et al. 2016a) to fit the emission line spectra of individual spaxels from S7 and SAMI data cubes with 1-, 2 and 3-Gaussian components. We demonstrate that using an ANN is comparable to astronomers performing the same visual inspection task of determining the best number of Gaussian components to describe the physical processes in galaxies. The advantage of our ANN is that it is capable of processing the spectra for thousands of galaxies in minutes, as compared to the years this task would take individual astronomers to complete by visual inspection. [ABSTRACT FROM AUTHOR]