Are high redshift Galaxies hot? - Temperature of z > 5 Galaxies and Implications on their Dust Properties

TL;DR

This study investigates the dust temperature and infrared properties of galaxies at redshift greater than 5, suggesting they are warmer than lower-redshift counterparts, which impacts dust modeling and IRX-$eta$ relation interpretations.

Contribution

It introduces a new perspective on the dust temperature and IR SED of high-redshift galaxies, proposing a modified model to explain their IRX-$eta$ relation behavior.

Findings

High-redshift galaxies have warmer IR SEDs than lower-redshift galaxies.

Assuming a maximally warm IR SED increases FIR luminosity estimates by 0.6 dex.

Radiation pressure causes spatial offsets affecting UV colors and IRX-$eta$ relation.

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.