Abstract
ABSTRACT The relation between infrared excess (IRX) and UV spectral slope (βUV) is an empirical probe of dust properties of galaxies. The shape, scatter, and redshift evolution of this relation are not well understood, however, leading to uncertainties in estimating the dust content and star formation rates (SFRs) of galaxies at high redshift. In this study, we explore the nature and properties of the IRX–βUV relation with a sample of z = 2–6 galaxies ($M_*\approx 10^9\!-\!10^{12}\, \mathrm{M}_\odot$) extracted from high-resolution cosmological simulations (MassiveFIRE) of the Feedback in Realistic Environments (FIRE) project. The galaxies in our sample show an IRX–βUV relation that is in good agreement with the observed relation in nearby galaxies. IRX is tightly coupled to the UV optical depth, and is mainly determined by the dust-to-star geometry instead of total dust mass, while βUV is set both by stellar properties, UV optical depth, and the dust extinction law. Overall, much of the scatter in the IRX–βUV relation of our sample is found to be driven by variations of the intrinsic UV spectral slope. We further assess how the IRX–βUV relation depends on viewing direction, dust-to-metal ratio, birth-cloud structures, and the dust extinction law and we present a simple model that encapsulates most of the found dependencies. Consequently, we argue that the reported ‘deficit’ of the infrared/sub-millimetre bright objects at z ≳ 5 does not necessarily imply a non-standard dust extinction law at those epochs.
Highlights
Reliable estimates of the star formation rate (SFR) of galaxies at low and high-z are crucial for constraining the various physical processes associated with galaxy evolution at different epochs (e.g. Madau & Dickinson 2014)
LIR is derived from fitting the Dale & Helou (2002) spectral energy distribution (SED) templates to the stacked fluxes at 250, 350, and 500 μm extracted from Herschel Space Observatory Spectral and Photometric Imaging Receiver (SPIRE; Griffin et al 2010; Swinyard et al 2010) imaging of the COSMOS field. βUV is computed by fitting the photometry to a single power-law SED, fλ ∝ λβUV
LIR is estimated by fitting the Dale et al (2014) SED templates to the stacked fluxes at Herschel Photodetector Array Camera and Spectrometer (PACS; Poglitsch et al 2010) (100 and 160 μm) and SPIRE (250, 350, and 500 μm), and AzTEC (1.1 mm) bands. βUV is computed by fitting the power-law SED to the rest-UV photometry within the wavelength range 1250 < λ < 2000 Å from the Capak et al (2007) catalogue
Summary
Reliable estimates of the star formation rate (SFR) of galaxies at low and high-z are crucial for constraining the various physical processes associated with galaxy evolution at different epochs (e.g. Madau & Dickinson 2014). IRselected samples typically have complete UV detections, enabling a measurement of the IRX–βUV relation of individual galaxies These studies showed that DSFGs have systematically bluer βUV compared to the local M99 relation as well as the LBG samples at their given IRX (Penner et al 2012; Casey et al 2014a, b; Safarzadeh, Hayward & Ferguson 2017). Deviation of the DSFGs from the M99 relation appears to show a clear correlation with LIR (Casey et al 2014a, b; Narayanan et al 2018a) These findings suggest that the derived IRX–βUV relation of high-z samples may be susceptible to selection effects.
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