Are High-redshift Galaxies Hot? Temperature of z > 5 Galaxies and Implications for Their Dust Properties

September 2017 • 2017ApJ...847...21F

Authors • Faisst, Andreas L. • Capak, Peter L. • Yan, Lin • Pavesi, Riccardo • Riechers, Dominik A. • Barišić, Ivana • Cooke, Kevin C. • Kartaltepe, Jeyhan S. • Masters, Daniel C.

Abstract • Recent studies have found a significant evolution and scatter in the relationship between the UV spectral slope (β _UV) and the infrared excess (IRX; L _IR/L _UV) at z > 4, suggesting different dust properties of these galaxies. The total far-infrared (FIR) luminosity is key for this analysis, but it is poorly constrained in normal (main-sequence) star-forming z > 5 galaxies, where often only one single FIR point is available. To better inform estimates of the FIR luminosity, we construct a sample of local galaxies and three low-redshift analogues of z > 5 systems. The trends in this sample suggest that normal high-redshift galaxies have a warmer infrared (IR) spectral energy distribution (SED) compared to average z < 4 galaxies that are used as priors in these studies. The blueshifted peak and mid-IR excess emission could be explained by a combination of a larger fraction of metal-poor interstellar medium being optically thin to ultraviolet (UV) light and a stronger UV radiation field due to high star formation densities. Assuming a maximally warm IR SED suggests a 0.6 dex increase in total FIR luminosities, which removes some tension between the dust attenuation models and observations of the IRX-β relation at z > 5. Despite this, some galaxies still fall below the minimum IRX-β relation derived with standard dust cloud models. We propose that radiation pressure in these highly star-forming galaxies causes a spatial offset between dust clouds and young star-forming regions within the lifetime of O/B stars. These offsets change the radiation balance and create viewing-angle effects that can change UV colors at fixed IRX. We provide a modified model that can explain the location of these galaxies on the IRX-β diagram.

Links

• PREPRINT http://arxiv.org/abs/1708.07842
• NED https://ned.ipac.caltech.edu/uri/NED::InRefcode/2017ApJ...847...21F
• ELECTR https://doi.org/10.3847%2F1538-4357%2Faa886c
• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2017ApJ...847...21F
• PDF https://iopscience.iop.org/article/10.3847/1538-4357/aa886c/pdf
• PDF https://stacks.iop.org/0004-637X/847/21/pdf
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