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

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 farinfrared (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.

Starburst to Quiescent from HST/ALMA: Stars and Dust Unveil Minor Mergers in Submillimeter Galaxies at z ∼ 4.5

“…Again, this can be understood as a result of removing the contribution from the extended irregular UV features and companion satellite galaxies that appear bluer than the dust-emitting region detected in ALMA. These results agree with the model proposed in Faisst et al (2017a) to explain the blue colors of DSFGs with high IRX.…”

“…Following this argument, the authors postulated that a concentration parameter might correlate with ΔIRX as an indicator of the decoupled UV/ FIR. Casey et al (2014b) reinforced these results, showing that the deviation from the M99 relation increases with Faisst et al (2017a) proposed that the blue colors of sources with high IRX values could be due to holes in the dust cover, tidally stripped young stars, or faint blue satellite galaxies. In addition, simulations propose recent star formation in the outskirts and low optical depths in UV-bright regions as plausible explanations of the offset (Safarzadeh et al 2017;Narayanan et al 2018).…”

The HDUV Survey: A Revised Assessment of the Relationship between UV Slope and Dust Attenuation for High-redshift Galaxies

“…This result may suggest an enhancement of silicate over carbonaceous grains that results in a steeper attenuation curve. Finally, the positions of very high redshift galaxies in the IRX−β plane, particularly those with with lower IRX than what would be predicted with an SMC-like curve, may be dictated more by geometrical considerations (e.g., Reddy et al 2006b;Faisst et al 2017) than by a physical change in the dust grain population. .…”

The CO Luminosity Density at High-z (COLDz) Survey: A Sensitive, Large-area Blind Search for Low-J CO Emission from Cold Gas in the Early Universe with the Karl G. Jansky Very Large Array

“…These studies have yielded crucial constraints on the molecular gas mass function at z∼1-3, subject to assumptions on the excitation of the CO line ladder to infer the corresponding molecular gas content. 25 They have found broad agreement with models of the CO luminosity evolution with 22 In this work, CO always refers to the most abundant isotopologue, 12 CO. 23 The dust continuum method to determine gas masses may be affected by the metallicity dependence of the dust-to-gas ratio (Sandstrom et al 2013;Berta et al 2016), trends in dust temperature with redshift (e.g., Magdis et al 2012), or galaxy population (e.g., Faisst et al 2017). 24 The ASPECS-Pilot survey simultaneously covered the CO(2-1) line in the redshift range z∼1.0-1.7, the CO(3-2) line at z∼2.0-3.1, and higher-J CO transitions at higher redshift.…”