We present a new approach to investigate the content and spatial distribution of dust in structurally unresolved star-forming galaxies from the observed dependence of integrated spectral properties on galaxy inclination .
Motivated by the observation that different stellar populations reside in different spatial components of nearby star-forming galaxies , we develop an innovative combination of generic models of radiative transfer in dusty media with a prescription for the spectral evolution of galaxies , via the association of different geometric components of galaxies with stars in different age ranges .
We start by showing that a wide range of radiative transfer models all predict a quasi-universal relation between slope of the attenuation curve at any wavelength , from the ultraviolet to the near-infrared , and V -band attenuation optical depth in the diffuse interstellar medium ( ISM ) , at all galaxy inclinations .
This relation predicts steeper ( shallower ) dust attenuation curves than both the Calzetti and Milky-Way curves at small ( large ) attenuation optical depths , which implies that geometry and orientation effects have a stronger influence on the shape of the attenuation curve than changes in the optical properties of dust grains .
We use our new , combined radiative transfer and spectral evolution model to interpret the observed dependence of the \textnormal { H } \alpha / \textnormal { H } \beta ratio and ugrizYJH attenuation curve on inclination in a sample of about 23 000 nearby star-forming galaxies , which we correct for systematic biases by developing a general method based on importance sampling .
From the exploration of the model parameter space by means of a Bayesian Markov Chain Monte Carlo technique , we measure the central face-on B -band optical depth of this sample to be \hbox { $ \tau _ { B \perp } $ } \approx 1.8 \pm 0.2 ( corresponding to an angle-average \hbox { $ \langle \hat { \tau } ^ { \textnormal { ISM } } _ { V } \rangle _ { \theta } $ } \approx 0.3 ) .
We also quantify the enhanced optical depth towards newly formed stars in their birth clouds , finding this to be significantly larger in galaxies with bulges than in disc-dominated galaxies , while \tau _ { B \perp } is roughly similar in both cases .
This can arise if , for example , galaxies with significant bulges have higher central star formation efficiencies than their disc-dominated counterparts at fixed specific star formation rate , and dustier stellar birth clouds because of the higher metallicity .
We find that over 80 percent of the attenuation in galaxies in our sample is characteristic of that affecting thin-disc stars in radiative transfer models .
The median unattenuated V -band luminosity ratio of thick-disc to thin-disc stars is 0.1–0.2 , in good agreement with the results from spatially resolved studies of nearby edge-on disc galaxies .
Finally , we show that neglecting the effect of geometry and orientation on attenuation can severely bias the interpretation of galaxy spectral energy distributions , as the impact on broadband colours can reach up to 0.3–0.4 mag at optical wavelengths and 0.1 mag at near-infrared ones .
This paper also contains an original application of Gaussian Random Processes to extend the wavelength range of dust attenuation curves .