Breaking the Curve With Candels: A Bayesian Approach to Reveal the Non-Universality of the Dust-Attenuation Law at High Redshift

Abstract

Dust attenuation affects nearly all observational aspects of galaxy evolution, yet very little is known about the form of the dust-attenuation law in the distant Universe. Here, we model the spectral energy distributions (SEDs) of galaxies at z ∼ 1.5-3 from CANDELS with rest-frame UV to near-IR imaging under different assumptions about the dust law, and compare the amount of inferred attenuated light with the observed infrared (IR) luminosities. Some individual galaxies show strong Bayesian evidence in preference of one dust law over another, and this preference agrees with their observed location on the plane of infrared excess (IRX, L TIR /L UV ) and UV slope (β). We generalize the shape of the dust law with an empirical model, Calzetti et al. (2000), and show that there exists a correlation between the color excess E(B − V ) and tilt δ with δ = (0.62 ± 0.05) log(E(B − V ))+(0.26 ± 0.02). Galaxies with high color excess have a shallower, starburst-like law, and those with low color excess have a steeper, SMC-like law. Surprisingly, the galaxies in our sample show no correlation between the shape of the dust law and stellar mass, star-formation rate, or β. The change in the dust law with color excess is consistent with a model where attenuation is caused by by scattering, a mixed star-dust geometry, and/or trends with stellar population age, metallicity, and dust grain size. This rest-frame UV-to-near-IR method shows potential to constrain the dust law at even higher (z > 3) redshifts.

Galex–sdss–wise Legacy Catalog (Gswlc): Star Formation Rates, Stellar Masses, and Dust Attenuations of 700,000 Low-Redshift Galaxies

“…This average curve has » R 2.5 V , compared with R V =4.05 for the Calzetti curve, and agrees well with the curve derived empirically by Conroy et al (2010a): an MWlike curve with R V =2.0 instead of R V =3.1 (reducing R V makes the curve steeper). Evidence for a steeper-than-Calzetti curve has been found in other studies as well (Charlot & Fall 2000;Buat et al 2011;Hao et al 2011;Wild et al 2011;Salmon et al 2015), though not as steep as the one found here and in Conroy et al More detailed investigation of these results and their implications will be presented in a separate paper.…”

Dissecting the IRX–β dust attenuation relation: exploring the physical origin of observed variations in galaxies

“…Without prior information on the covering fraction of the dust the range of possible IRX values increases up to three orders of magnitude. Such variations are not covered anymore by two parameters, but require detailed spatially resolved information about the galaxy and full SED fitting to constrain the most likely dust attenuation curve (e.g., Noll et al 2009;Kriek & Conroy 2013;Salmon et al 2016). …”

“…Differences in SFH may play a role here (for instance young stars driving the bright UV emission in UV-selected galaxies), but there is evidence that differences in the attenuation curves may really be the key driver of the SMC IRX -β relation. Kriek & Conroy fit composite SEDs with flexible stellar population synthesis models to galaxies, while exploring attenuation curves with varying slopes and UV bump strengths (see also Salmon et al 2016). They find that star-forming galaxies at z ∼ 2 on average have an attenuation curve with a slope between MW and SMC and a weak presence of a 2175Å bump (25% of the MW bump).…”

UVI colour gradients of 0.4 < z < 1.4 star-forming main-sequence galaxies in CANDELS: dust extinction and star formation profiles

“…An offset of 0.15 mag has been subtracted from model V − I in order to locate the tracks consistently relative to galaxies in UVJ and UVI ; concerted efforts to identify the cause of this offset failed, and the correction has been applied empirically. Black arrows show the dust reddening vector and its variations determined by Salmon et al (2016), and points with different sizes mark different stellar ages, as indicated at bottom. The age effect has a steeper slope than the Calzetti vector on the UVI plane, except for the constant SFR case.…”