Abstract
ABSTRACT We study the relation between the UV slope, β, and the ratio between the infrared- and UV luminosities (IRX) of galaxies from TNG50, the latest installment of the IllustrisTNG galaxy formation simulations. We select 7280 star-forming main-sequence (SFMS) galaxies with stellar mass ≥109 M⊙ at redshifts 0 ≤ z ≤ 4 and perform radiative transfer with skirt to model effects of interstellar medium dust on the emitted stellar light. Assuming a Milky Way dust type and a dust-to-metal ratio of 0.3, we find that TNG50 SFMS galaxies generally agree with observationally derived IRX–β relations at z ≲ 1. However, we find a redshift-dependent systematic offset with respect to empirically derived local relations, with the TNG50 IRX–β relation shifting towards lower β and steepening at higher redshifts. This is partially driven by variations in the dust-uncorrected UV slope of galaxies, due to different star formation histories of galaxies selected at different cosmic epochs; we suggest the remainder of the effect is caused by differences in the effective dust attenuation curves of galaxies as a function of redshift. We find a typical galaxy-to-galaxy variation of 0.3 dex in infrared excess (IRX) at fixed β, correlated with intrinsic galaxy properties: galaxies with higher star formation rates, star formation efficiencies, gas metallicities and stellar masses exhibit larger IRX values. We demonstrate a degeneracy between stellar age, dust geometry, and dust composition: z = 4 galaxies with a Small Magellanic Cloud dust type follow the same IRX–β relation as low-redshift galaxies with MW dust. We provide a redshift-dependent fitting function for the IRX–β relation for MW dust based on our models.
Highlights
Determining the cosmic star formation rate density (SFRD) and its evolution with redshift is of critical importance for our understanding of galaxy formation and evolution
We study the relation between the UV–slope, β, and the ratio between the infrared– and UV– luminosities (IRX) of galaxies from TNG50, the latest installment of the IllustrisTNG galaxy formation simulations
Higher mass star–forming main–sequence (SFMS) galaxies generally have higher gas mass and dust–to–gas fractions, causing more dust obscuration and higher infrared excess (IRX). Putting all of this together, we find that the location of different tracks in the IRX–β plane is best determined by the specific star formation rate (sSFR) and the star–formation efficiency of galaxies, where the sSFR is a good indicator of variations in stellar population age and the shift along β, and the star–formation efficiency might be a good indicator of variations in the stellar–to–dust geometry and the change in slope
Summary
Determining the cosmic star formation rate density (SFRD) and its evolution with redshift is of critical importance for our understanding of galaxy formation and evolution. Measurements of star formation rates (SFRs) at z > 3 are often based on the ultraviolet (UV) emission as a tracer for the abundance of young stars in a galaxy, For local UV–bright starburst galaxies, an empirically derived tight relation between the rest–frame UV continuum slope β (where we assume a power law: fλ ∝ λβ) and the ratio between IR and UV luminosity, called the infrared excess (IRX) has been found (Meurer et al 1999) This IRX-β relation provides a connection between the shape of the UV continuum (the "color" of the galaxy) and the amount of dust attenuation. If this relation proved to hold for various galaxy types at different redshifts, it could be used as a reliable tool to account for dust attenuation where only UV data is available and IR data is missing (e.g. as in Bouwens et al 2011, Oesch et al 2014, McLeod et al 2015)
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