On the missing 2175 Angstroem-bump in the Calzetti extinction curve

TL;DR

This paper explains the absence of the 2175 Angstrom bump in the Calzetti extinction curve by modeling dust in an inhomogeneous turbulent medium, showing turbulence and UV destruction of carriers as key factors.

Contribution

It provides a physical explanation for the missing bump in the Calzetti curve by linking turbulence and dust destruction to observed extinction features.

Findings

Turbulence explains the flatter Calzetti curve and missing bump.

Critical column density for bump destruction is N_H >= 10^21 cm^-2.

Density contrast aligns with Galactic cold neutral medium properties.

Abstract

The aim of the paper is to give a physical explanation of the absence of the feature in the Calzetti extinction curve. We analyze the dust attenuation of a homogeneous source seen through a distant inhomogeneous distant screen. The inhomogeneities are described through an idealized isothermal turbulent medium where the probability distribution function (PDF) of the column density is log-normal. In addition it is assumed that below a certain critical column density the carriers of the extinction bump at 2175 Angstroem are being destroyed by the ambient UV radiation field. Turbulence is found to be a natural explanation not only of the flatter curvature of the Calzetti extinction curve but also of the missing bump provided the critical column density is N_H >= 10^21 cm^-2. The density contrast needed to explain both characteristics is well consistent with the Mach number of the cold neutral medium of our own Galaxy which suggests a density contrast sigma_(rho/<rho>) 6.