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

Abstract

We characterize the influence that inclination has on the shape and normalization in average dust attenuation curves derived from a sample of ∼10,000 local star-forming galaxies. To do this, we utilize aperture-matched multiwavelength data from the Galaxy Evolution Explorer, the Sloan Digital Sky Survey, the United Kingdom Infrared Telescope, and the Two Micron All-sky Survey. We separate our sample into groups according to axial ratio (b/a) and obtain an estimate of their average total-to-selective attenuation k l ( ). The attenuation curves are found to be shallower at UV wavelengths with increasing inclination, whereas the shape at longer wavelengths remains unchanged. The highest inclination subpopulation (b a 0.42 < ) exhibits an NUV excess in its average selective attenuation, which, if interpreted as a 2175Å feature, is best fit with a bump strength of 17%-26% of the MW value. No excess is apparent in the average attenuation curve of lower inclination galaxies. The differential reddening between the stellar continuum and ionized gas is found to decrease with increasing inclination. We find that higher inclination galaxies have slightly higher values of R V , although this is poorly constrained given the uncertainties. We outline possible explanations for these trends within a two component dust model (dense cloud +diffuse dust) and find that they can be naturally explained if carriers of the 2175Å feature are preferentially destroyed in star-forming regions (UV-bright regions).

Galaxy Inclination and the IRX–β Relation: Effects on UV Star Formation Rate Measurements at Intermediate to High Redshifts

“…These studies find that dust attenuation curves become flatter (grayer) as galaxy inclination increases for local galaxies (e.g., Wild et al 2011;Chevallard et al 2013;Battisti et al 2017;Salim et al 2018), which are consistent with our conclusion.…”

“…This indicates that the inclination-dependent IRX-β relation is caused by that edge-on galaxies have a higher fraction of dust optical depth from the mixture component than face-on galaxies. This is physically plausible, given that the path-length through the diffuse ISM increases with galaxy inclination, whereas the dust optical depth from around H II regions does not change due to their near-spherical shell shape (Tuffs et al 2004;Yip et al 2010;Popescu et al 2010;Wild et al 2011;Chevallard et al 2013;Battisti et al 2017).…”

MAGPHYS+photo-z: Constraining the Physical Properties of Galaxies with Unknown Redshifts

“…For the diffuse ISM attenuation curve, we utilize an additional component to account for attenuation from a 2175Å feature. The 2175Å absorption bump is a prominent feature in the Milky Way (MW) extinction curve (e.g., Draine 2003), but this feature is typically much weaker in strength or completely absent in dust attenuation curves 4 (e.g., Calzetti et al 4 The additional scattering and geometric effects at play in dust attenuation act to reduce the overall strength of the 2175Å fea-1994; Noll et al 2009;Wild et al 2011;Buat et al 2012;Kriek & Conroy 2013;Reddy et al 2015;Scoville et al 2015;Salmon et al 2016;Battisti et al 2017;Salim et al 2018) and for this reason it is often excluded in SED modeling (Charlot & Fall 2000;da Cunha et al 2008). We find that this feature is necessary to reduce systematic residuals between the observations and models in standard MAGPHYS, with an average bump strength of ∼30% the MW value being necessary for IR-detected galaxies from 0.1 z 3 (Battisti et al in prep.).…”

The Strength of the 2175 Å Feature in the Attenuation Curves of Galaxies at 0.1 < z ≲ 3

“…In local galaxies, it is seen that the attenuation curve slope becomes shallower with increasing galaxy inclination (decreasing axial ratio) (e.g., Wild et al 2011;Battisti et al 2017;Salim et al 2018). This effect can be attributed to the increase in dust column density (A V ) as a galaxy becomes more edge-on (e.g., Chevallard et al 2013).…”