<p>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.</p>
<p>The &quot;NH&quot; 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 &quot;cosmic&quot; abundance of heavier elements is assumed.</p>

CC BY-SA 4.0
2022-08-13T23:08:38.297Z
2022-08-13T23:08:38.297Z
John Doty
<p>tl;dr, it’s the sizes of the grains of dust and how many there are that create wavelength dependent extinction.</p>
<p>Based off of the additional context you provided and reading the <a href="https://ned.ipac.caltech.edu/level5/Sept07/Li2/Li2.html" rel="noreferrer">article</a> you gave, it sounds like your question arises from a paragraph in the article which says</p>
<blockquote>
<p>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]).</p>
</blockquote>
<p>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.</p>
<p>They then go on to provide two explanations:</p>
<ol>
<li>“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</li>
<li>“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 <a href="https://ui.adsabs.harvard.edu/abs/2002ApJ...580...88W/abstract" rel="noreferrer">Weingartner et al.</a>)</li>
</ol>
<p>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.</p>

CC BY-SA 4.0
2022-08-13T17:04:08.077Z
There's (at least) two different physical effects by which dust can interact with electromagnetic radiation; bulk absorption (attenuation) and dielectric scattering. The index of refraction of dust might be 1.1 or 1.5 in the UV (depending on density) but will deviate from 1.0 by only parts per million or less for X-rays, so scattering losses will probably be very low for X-rays. Depending on the wavelength and material type I assume that the bulk absorption lengths will be a lot longer (weaker) for X-rays as well. There are probably hard numbers out there...
uhoh
CC BY-SA 4.0
2022-08-13T16:41:53.617Z
2022-08-13T16:41:53.617Z
Justin T
<p>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  <a href="https://ned.ipac.caltech.edu/level5/Sept07/Li2/Li2.html" rel="noreferrer">Extinction - a powerful discriminator of dust size</a>. Since X-ray and UV are all photons with different levels of energy, why does dust (interstellar medium) affects UV more than X-rays?</p>

CC BY-SA 4.0
2022-08-13T13:59:38.953Z
I cannot parse "why does dust heavily affect UV less severe on other wavelengths?"
ProfRob
CC BY-SA 4.0
2022-08-13T14:29:28.070Z
@ProfRob Edited, now?
Jack the Ranger
CC BY-SA 4.0
2022-08-13T14:44:43.693Z
I think you mean "Why is there less scattering and absorption of UV light than X-rays".  But what you have written is ungrammatical and unclear.  Perhaps you could tell us where you learnt about the effect of dust on UV and X-rays?  That would help understand your doubt.
James K
CC BY-SA 4.0
2022-08-13T14:50:43.130Z
@JamesK From what I know, UV are heavily affected by dust, but (hard) X-rays are not (that's why UV telescope are not as common as others). So I was wondering why this is the case.
Jack the Ranger
CC BY-SA 4.0
2022-08-13T14:53:06.383Z
Your question and your comment are opposite. The question asks  "Why does does affect UV more than X-rays"... Your comment is "UV are heavily affected,  X-rays are not"  So I ask again - where did you learn about interstellar dust and UV/X-rays.
James K
CC BY-SA 4.0
2022-08-13T14:56:31.767Z
Adding a source to this claim would indeed be nice. It might even contain the answer implicitly
planetmaker
CC BY-SA 4.0
2022-08-13T15:14:15.433Z
@planetmaker https://ned.ipac.caltech.edu/level5/Sept07/Li2/Li2.html  This one? My supervisor told me about this,so...
Jack the Ranger
CC BY-SA 4.0
2022-08-13T15:42:42.790Z
The context of the difference between X-ray and other light absorption from that website is discussing AGN dust; are you more specifically interested in the context of AGNs? If so, it seems to give a reasonable answer in the website as to why the difference, do you find their answer unsatisfactory or would you like it expounded upon?
Justin T
CC BY-SA 4.0
2022-08-13T15:43:10.260Z
@JacktheRanger this makes for a good reference for why one should go at greater length than one-sentence questions, especially if you *do* have researched background on the question already. So use that in order to explain your question in detail (especially as UV and x-ray is an ill-defined distinction and I assume you only reference the local maximum at 216nm). Go and edit all those things in the question to make it one which can be answered
planetmaker
CC BY-SA 4.0
2022-08-13T15:54:15.043Z
@JustinT Yes, sorry, my project is about AGN, and I thought this applies to dust extinction in general. Now I'm kind of confused. When you say "AGN dust", are you referring to dust in the torus? Because my supervisor said the dust in the ISM affects the UV radiation from AGNs.
Jack the Ranger
CC BY-SA 4.0
2022-08-13T16:01:11.983Z
@JacktheRanger I think at this point I’m going to give a stab at answering; although I would still encourage you to add some of the context we’ve discussed into your original question
Justin T
CC BY-SA 4.0
2022-08-13T16:32:03.177Z
It probably also makes a difference whether we talk about the inter*stellar* or inter*galactic* medium... Mind also that the source you reference in that one comment... only shows extinction in part of the UV spectrum - and generally shows that interstellar extinction generally increases as wavelength decreases (thus exactly NOT what your topic claims)
planetmaker
CC BY-SA 4.0
2022-08-13T16:34:37.927Z
@planetmaker I actually asked my supervisor about this last time, and he told me intergalactic medium does not affect the UV light emitted from AGN, since they are mostly H and are very thin.
Jack the Ranger
<interstellar-medium><dust><active-galaxy>
Why does dust affect UV light more severe than it affects X-rays?
Jack the Ranger
# Why does dust affect UV light more severe than it affects X-rays?

(Asked by: username_0 on 2022-08-13)

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](https://ned.ipac.caltech.edu/level5/Sept07/Li2/Li2.html). Since X-ray and UV are all photons with different levels of energy, why does dust (interstellar medium) affects UV more than X-rays?

**Tags:** interstellar-medium, dust, active-galaxy

> **Comments:**
> **username_1**: I cannot parse "why does dust heavily affect UV less severe on other wavelengths?"  
> **username_0**: @username_1 Edited, now?  
> **username_2**: I think you mean "Why is there less scattering and absorption of UV light than X-rays".  But what you have written is ungrammatical and unclear.  Perhaps you could tell us where you learnt about the effect of dust on UV and X-rays?  That would help understand your doubt.  
> **username_0**: @username_2 From what I know, UV are heavily affected by dust, but (hard) X-rays are not (that's why UV telescope are not as common as others). So I was wondering why this is the case.  
> **username_2**: Your question and your comment are opposite. The question asks  "Why does does affect UV more than X-rays"... Your comment is "UV are heavily affected,  X-rays are not"  So I ask again - where did you learn about interstellar dust and UV/X-rays.  
> **username_3**: Adding a source to this claim would indeed be nice. It might even contain the answer implicitly  
> **username_0**: @username_3 https://ned.ipac.caltech.edu/level5/Sept07/Li2/Li2.html  This one? My supervisor told me about this,so...  
> **username_4**: The context of the difference between X-ray and other light absorption from that website is discussing AGN dust; are you more specifically interested in the context of AGNs? If so, it seems to give a reasonable answer in the website as to why the difference, do you find their answer unsatisfactory or would you like it expounded upon?  
> **username_3**: @username_0 this makes for a good reference for why one should go at greater length than one-sentence questions, especially if you *do* have researched background on the question already. So use that in order to explain your question in detail (especially as UV and x-ray is an ill-defined distinction and I assume you only reference the local maximum at 216nm). Go and edit all those things in the question to make it one which can be answered  
> **username_0**: @username_4 Yes, sorry, my project is about AGN, and I thought this applies to dust extinction in general. Now I'm kind of confused. When you say "AGN dust", are you referring to dust in the torus? Because my supervisor said the dust in the ISM affects the UV radiation from AGNs.  
> **username_4**: @username_0 I think at this point I’m going to give a stab at answering; although I would still encourage you to add some of the context we’ve discussed into your original question  
> **username_3**: It probably also makes a difference whether we talk about the inter*stellar* or inter*galactic* medium... Mind also that the source you reference in that one comment... only shows extinction in part of the UV spectrum - and generally shows that interstellar extinction generally increases as wavelength decreases (thus exactly NOT what your topic claims)  
> **username_0**: @username_3 I actually asked my supervisor about this last time, and he told me intergalactic medium does not affect the UV light emitted from AGN, since they are mostly H and are very thin.  

---

## 2 answers

### Answer by username_5 on 2022-08-13. Score: 6

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.


---

### Answer by username_4 on 2022-08-13. Score: 5

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](https://ned.ipac.caltech.edu/level5/Sept07/Li2/Li2.html) 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:

1. “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
2. “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.](https://ui.adsabs.harvard.edu/abs/2002ApJ...580...88W/abstract))

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.

> **Comments:**
> **username_6**: There's (at least) two different physical effects by which dust can interact with electromagnetic radiation; bulk absorption (attenuation) and dielectric scattering. The index of refraction of dust might be 1.1 or 1.5 in the UV (depending on density) but will deviate from 1.0 by only parts per million or less for X-rays, so scattering losses will probably be very low for X-rays. Depending on the wavelength and material type I assume that the bulk absorption lengths will be a lot longer (weaker) for X-rays as well. There are probably hard numbers out there...  

---

