Misc,
Are high redshift Galaxies hot? - Temperature of z > 5 Galaxies and Implications on their Dust Properties
(2017)cite arxiv:1708.07842Comment: 34 pages including Appendix, 18 figures (4 in Appendix), 5 tables (1 in Appendix). Accepted for publication in ApJ.
Abstract
Recent studies have found a significant evolution and scatter in the
IRX-$\beta$ relation at z > 4, suggesting different dust properties of these
galaxies. The total far-infrared (FIR) luminosity is key for this analysis but
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
analogs of z > 5 systems. The trends in this sample suggest that normal
high-redshift galaxies have a warmer infrared (IR) SED compared to average z <
4 galaxies that are used as prior in these studies. The blue-shifted peak and
mid-IR excess emission could be explained by a combination of a larger fraction
of the metal-poor inter-stellar medium (ISM) being optically thin to
ultra-violet (UV) light and a stronger UV radiation field due to high star
formation densities. Assuming a maximally warm IR SED suggests 0.6 dex
increased total FIR luminosities, which removes some tension between dust
attenuation models and observations of the IRX-$\beta$ relation at z > 5.
Despite this, some galaxies still fall below the minimum IRX-$\beta$ 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-$\beta$ diagram.
