Empirical determination of the shape of dust attenuation curves in star-forming galaxies

Abstract

We present a systematic study of the shape of the dust attenuation curve in star‐forming galaxies from the far‐ultraviolet (far‐UV) to the near‐infrared (NIR; ∼0.15–2 μ m), as a function of specific star formation rate (ψS) and axial ratio (b/a), for galaxies with and without a significant bulge. Our sample comprises 23 000 (15 000) galaxies with a median redshift of 0.07, with photometric entries in the Sloan Digital Sky Survey (SDSS), UKIRT Infrared Deep Sky Survey‐Large Area Survey and Galaxy Evolution Explorer‐All‐Sky Imaging Survey catalogues and emission‐line measurements from the SDSS spectroscopic survey. We develop a new pair‐matching technique to isolate the dust attenuation curves from the stellar continuum emission. The main results are: (i) the slope of the attenuation curve in the optical varies weakly with ψS, strongly with b/a, and is significantly steeper than the Milky Way extinction law in bulge‐dominated galaxies; (ii) the NIR slope is constant and matches the slope of the Milky Way extinction law; (iii) the UV has a slope change consistent with a dust bump at 2175 Å which is evident in all samples and varies strongly in strength with b/a in the bulge‐dominated sample; (iv) there is a strong increase in emission‐line‐to‐continuum dust attenuation (τV, line/τV, cont) with both decreasing ψS and increasing b/a; and (v) radial gradients in dust attenuation increase strongly with increasing ψS, and the presence of a bulge does not alter the strength of the gradients. These results are consistent with the picture in which young stars are surrounded by dense ‘birth clouds’ with low covering factor which disperse on time‐scales of ∼107 yr and the diffuse interstellar dust is distributed in a centrally concentrated disc with a smaller scaleheight than the older stars that contribute the majority of the red and NIR light. Within this model, the path‐length of diffuse dust, but not of birth‐cloud dust, increases with increasing inclination and the apparent optical attenuation curve is steepened by the differential effect of larger dust opacity towards younger stars than towards older stars. Additionally, our findings suggest that: (i) galaxies with higher star formation rates per unit stellar mass have a higher fraction of diffuse dust, which is more centrally concentrated; (ii) the observed strength of the 2175‐Å dust feature is affected predominantly by global geometry; and (iii) only highly inclined discs are optically thick. We provide new empirically derived attenuation curves for correcting the light from star‐forming galaxies for dust attenuation.

The occulting galaxy pair UGC 3995

“…7 can be explained by the mixing of transparent and opaque regions. In their comparison between galaxy-wide SEDs in Wild et al (2011) find extinction laws very similar to the Milky Way one. The measured dust reddening in this case is dominated by the optically thin parts of the galaxy discs; the reddening of the observed light is accurately retrieved but the total light absorption is not (as opaque regions are effectively not included).…”

“…The distribution of interstellar dust in spiral galaxies is still a dominant unknown in many areas of extra-galactic astrophysics, e.g., models of the spectral energy distribution (SED) of spiral galaxies (e.g., Popescu et al 2000;Popescu & Tuffs 2005;Pierini et al 2004;Baes et al 2003Baes et al , 2010Bianchi et al 2000;Bianchi 2007Bianchi , 2008Driver et al 2008;Jonsson et al 2010;Wild et al 2011;Holwerda et al 2012a;Verstappen et al, in prep. ;Holwerda et al, in prep.…”

Predicting Hα emission-line galaxy counts for future galaxy redshift surveys

“…For completeness we note that several recent studies of larger galaxy samples at both low and high redshift have found the law to be a poor description for dust attenuation at UV wavelengths (e.g. Noll et al 2009;Conroy et al 2010;Wild et al 2011;Buat et al 2011aBuat et al ,b, 2012Lee et al 2012;Reddy et al 2012). However, despite this and despite its non-physical set-up, the ease of use of the Calzetti et al (2000) law has made it a popular choice in observational analyses.…”

High-resolution, 3D radiative transfer modeling

“…With the metallicity of M 51 being solar to even super-solar locally, we would expect a prominent 2175 Å absorption feature if the metallicity plays a role in shaping the dust composition, and, thus, the shape of the attenuation curve. Other than metallicity, the absorption bump at 2175 Å appears to be sensitive to the UV radiation field (e.g., Wild et al 2011), which can alter the dust grain size distribution through destruction/coagulation processes, and/or change the ionization state of dust grains (Boulanger et al 1988;Cesarsky et al 1996). Also radiative transfer effects coupled with age-selective attenuation have been shown to affect the bump strength (Silva et al 1998;Granato et al 2000).…”