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

Abstract

The use of ultraviolet (UV) emission as a tracer of galaxy star-formation rate (SFR) is hampered by dust obscuration. The empirical relationship between UV slope, β, and the ratio between far-infrared and UV luminosity, IRX, is commonly employed to account for obscured UV emission. We present a simple model that explores the physical origin of variations in the IRX -β dust attenuation relation. A relative increase in FUV compared to NUV attenuation and an increasing stellar population age cause variations towards red UV slopes for a fixed IRX. Dust geometry effects (turbulence, dust screen with holes, mixing of stars within the dust screen, two-component dust model) cause variations towards blue UV slopes. Poor photometric sampling of the UV spectrum causes additional observational variations. We provide an analytic approximation for the IRX -β relation invoking a subset of the explored physical processes (dust type, stellar population age, turbulence). We discuss observed variations in the IRX -β relation for local (sub-galactic scales) and high-redshift (normal and dusty star-forming galaxies, galaxies during the epoch of reionization) galaxies in the context of the physical processes explored in our model. High spatial resolution imaging of the UV and sub-mm emission of galaxies can constrain the IRX -β dust attenuation relation for different galaxy types at different epochs, where different processes causing variations may dominate. These constraints will allow the use of the IRX -β relation to estimate intrinsic SFRs of galaxies, despite the lack of a universal relation.

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

“…Both SMGs and cSFGs could be part of the same global population but observed in a different phase or intensity of the stellar mass assembly, with the SMGs reflecting the peak of the process and the cSFGs being a later stage. The cSFGs are consistent with an intermediate population between z>3 SMGs and z∼2 cQGs, caught in a phase where the star formation is winding down and a compact remnant is emerging, transitioning from the region above the MS of star-forming galaxies (Barro et al 2017) to the MS (Popping et al 2017a) and eventually below it. In fact, Elbaz et al (2017) recently showed that starburst galaxies exist both above and within the MS.…”

“…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). Simple models placing a dust screen in front of a starburst have been studied to provide a detailed explanation of all the possible effects that might lead to a deviation in the IRX-β plane (Popping et al 2017b).…”

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

“…These properties ultimately depend on other physical properties of the galaxies, for example, their metallicity as shown in the case of SMC-like dust. We note that in the recent study by Popping et al (2017b), the low IRX values and red UV continuum slopes can also be explained by old and dust-poor stellar populations (see also Howell et al 2010). Observations are necessary in order to verify or disproof this solution for a given sample at high redshift.…”

Characterizing Dust Attenuation in Local Star-forming Galaxies: Inclination Effects and the 2175 Å Feature

“…However, the IRX-β correlation is seen to break down as one moves from starburst galaxies to more "normal" SFGs (Kong et al 2004;Buat et al 2005;Hao et al 2011). This breakdown has been attributed to effects of evolved stellar populations, different star formation histories, variations in the dust/star geometry, and effects from the 2175Å feature (e.g., Boquien et al 2009;Grasha et al 2013;Popping et al 2017), all of which impact IRX and/or β values and introduce scatter.…”