# 4.1.13. Ebv: Galactic interstellar extinction model¶

This model can be used to correct fluxes of optical/UV data for interstellar reddening in our Galaxy. The reddening is caused by dust absorption and scattering (extinction) in the interstellar medium. The model uses the extinction curve of Cardelli et al. (1989). including the update for near-UV given by O’Donnell et al. (1994).

The extinction law can be expressed as:

where, and are the total extinctions (measured in magnitude) at wavelength and the -band, respectively. The wavelength- dependent parameters and are provided by the aforementioned papers. The scalar is defined as the ratio of total extinction to selective extinction , where is the total extinction at the -band. So the selective extinction represents a colour excess, which is commonly denoted as . For Milky Way, the typical value for is reported to be 3.1. Thus, by specifying and , the extinction can be derived, which in turn gives us the unreddened flux () from the observed reddened flux () using . Note that at energies above the Lyman limit, the transmission of the model is set to 1 in SPEX, thus it can be used alongside the Galactic interstellar X-ray absorption models in SPEX.

This extinction model is currently the best model for the Milky Way. However, the extinction curve of the Milky Way has a famous broad bump at 2175 . The origin of this bump is still not fully understood. Interestingly, this feature (due to dust) is seen in the Milky Way and not in other galaxies like in AGN. If one would use the ebv model to correct for extinction in the host galaxy of an AGN, it is best to remove this bump from the model as it can have an effect on the fit to the data. The bump can be removed by setting the mway parameter to 0.

The parameters of the model are:
ebv : The colour excess . The value is set by the user.
rv : The scalar . Default (recommended) value: 3.1
fcov : The covering factor of the absorber. Default value: 1 (full covering)
mway : Include the Milky Way broad bump (1 = Yes, 0 = No).

Recommended citation: Cardelli et al. (1989) and O’Donnell et al. (1994).