My supervisor told me that the UV photons emitted from AGN are heavily affected by ISM (dust extinction), but not X-rays, as described in Extinction - a powerful discriminator of dust size. Since X-ray and UV are all photons with different levels of energy, why does dust (interstellar medium) affects UV more than X-rays?
Ordinary solid materials, even very thin, are almost perfectly opaque to UV. On the other hand, we use hard x-rays to image the interiors of macroscopic solid objects. Hard x-rays pass right through the dust in question. For soft x-rays it's more complicated, but they still penetrate dust much more effectively than UV.
The "NH" parameter in the quote is the nominal column density of hydrogen atoms, but it's estimated from the absorption of soft x-rays, which is mostly due to heavier elements. A "cosmic" abundance of heavier elements is assumed.
tl;dr, it’s the sizes of the grains of dust and how many there are that create wavelength dependent extinction.
Based off of the additional context you provided and reading the article you gave, it sounds like your question arises from a paragraph in the article which says
Reduced Reddening- and Extinction-to-Gas Ratios - Flat Extinction? Assuming a Galactic standard extinction curve (RV = 3.1) and a foreground screen, Maiolino et al. (2001a) determined for 19 AGNs the amount of reddening E(B - V) affecting the broad line region by comparing the observed optical/IR H broad line ratios with the intrinsic values. For these AGNs, they also determined the X-ray absorbing column densities NH from the photoelectric cutoff in their X-ray spectra. They found that for most (16 of 19) objects E(B - V) / NH is significantly lower than the Galactic standard value (approx 1.7 × 10-22 mag cm-2) by a factor ranging from a few to ~ 100 (except for 3 Low Luminosity AGNs whose physics may be intrinsically different [see Ho 1999]).
tl;dr version being that X-ray extinction seems to be significantly weaker coming from the AGN region for X-ray light than for the galactic average.
They then go on to provide two explanations:
- “Grain growth through coagulation in the dense circumnuclear region… results in a dust size distribution biased in favour of large grains and therefore a flat extinction curve”, or in other words, the dust particles themselves are different around the AGN
- “X-ray absorption and optical extinction may occur in distinct media (e.g. the X-ray absorption occurs in material located off the torus and/or accretion disk, while the optical extinction occurs in material located beyond the torus)”, or in other words, it is a matter of orientation of the AGN; there may be other, larger dust away from the AGN that don’t block optical light efficiently but that do offer some X-ray extinction, and that the x-rays in question never pass through the torus (see also Weingartner et al.)
In either case, it seems that regardless of whether or not the AGNs have “systematically larger grains” of dust, that it is the size of the dust that ultimately determines this effect. If there is some overall effect that your question implies (the which I think we would all appreciate some more sources on) then it would ultimately be the result of the population of dust sizes.