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Type II or NELG?



 
 
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  #1  
Old July 2nd 17, 05:09 AM posted to sci.astro.research
Eric Flesch
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Posts: 321
Default Type II or NELG?

I can't seem to find a definitive distinction between a galaxy with a
type-II narrow-line AGN core and a so-called NELG ("narrow
emission-line galaxy"). NELGs seem to have gone out of fashion
without anybody publishing them in recent years. Type-IIs have a
chequered history in that different researchers defined them in
different ways for a long time, although nowadays they are universally
regarded as narrow-line equivalents of AGN.

So in my quasar publications I make a distinction between broad-line
QSOSs and AGNs in that the latter are seen inside galaxies and the
former are not (never mind the transitional ones for now), but the
narrow-line type-II objects are not so divided. However, both kinds
are seen. To be consistent I should divide them as well.

It's naturally tempting to designate NELGs as type-II emitters within
galaxies -- it would make use of a well-known classification which has
sort of dropped out of use. Does anyone know a good reason why this
would be a bad idea, whether different forbidden lines or whatever?
  #2  
Old July 5th 17, 06:35 PM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default Type II or NELG?

In article ,
Eric Flesch writes:
I can't seem to find a definitive distinction between a galaxy with a
type-II narrow-line AGN core and a so-called NELG ("narrow
emission-line galaxy"). NELGs seem to have gone out of fashion
without anybody publishing them in recent years.


This represents a change in terminology over time. Most authors
nowadays distinguish between broad-line objects (which generally have
narrow lines as well) and narrow-line-only objects by using Type 1
and Type 2, respectively.

Type-IIs have a chequered history in that different researchers
defined them in different ways for a long time, although nowadays
they are universally regarded as narrow-line equivalents of AGN.


I might put it that "Type 2" is only _applied_ to AGN. Galaxies with
high star formation rates exhibit narrow lines, so I suppose
technically qualify as "Type 2" in some sense, but the line ratios
differ from AGN line ratios. In practice, I've not seen anyone apply
the term "Type 2" to anything other than an AGN, meaning any AGN that
lacks broad emission lines. However, nobody writes "Type 2 galaxy."
It's always "Type 2 AGN" or "Type 2 QSO" or "Type 2 Seyfert" or
something like that.

So in my quasar publications I make a distinction between broad-line
QSOSs and AGNs in that the latter are seen inside galaxies and the
former are not (never mind the transitional ones for now),


This is mostly a luminosity distinction. AGN is the general term,
encompassing all luminosities. QSO means no host galaxy is seen in
typical imaging observations. However, host galaxies can generally
be detected, at least for relatively nearby QSOs, with deep
observations and either careful image processing or use of a
coronagraph. When an AGN has its host galaxy easily visible, the
system is termed a "Seyfert galaxy." This is not a sharp
distinction; when it's important, authors will specify a dividing
line in terms of some appropriate luminosity (usually bolometric,
X-ray, or in the [O III] emission line).

It's naturally tempting to designate NELGs as type-II emitters within
galaxies


If someone used the term NELG to me, I'd take it as including both
star-forming galaxies and Seyfert galaxies that don't show broad
lines. I'm not sure that's a very useful category, but maybe it is.
(It could include galaxies that have emission lines but eith line
ratios not known well enough to classify the emission source.)

When writing, it's important to define any terminology that might be
ambiguous.

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Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #3  
Old July 5th 17, 10:09 PM posted to sci.astro.research
Eric Flesch
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Posts: 321
Default Type II or NELG?

On Wed, 05 Jul 2017, Steve Willner wrote:
If someone used the term NELG to me, I'd take it as including both
star-forming galaxies and Seyfert galaxies that don't show broad
lines. I'm not sure that's a very useful category, but maybe it is.


That's the take away for me, especially as the ~40K type-IIs that I've
got are already known to be contaminated with LINERs. I include the
type-IIs for completeness, in case a user is looking up a certain set
of them (from one paper, say) and some of them are type-II. Also, if
someone is vetting candidates to see which ones are already
classified, it's better to show the type-IIs than nothing. So I'll
proceed with my plan to integrate the NELGs with the same caveat that
they include starforming and LINER contamination. They and the
type-IIs aren't included in the count of QSOs & AGNs.

I know that deeper imaging turns up more galaxy disks, Steve, but the
QSO/AGN distinction primarily relates to the luminosity of the core
compared with the disk; the visibility of the disk is a practical
consequence of that.
  #4  
Old July 14th 17, 12:18 AM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default Type II or NELG?

In article ,
Eric Flesch writes:
I know that deeper imaging turns up more galaxy disks, Steve, but the
QSO/AGN distinction primarily relates to the luminosity of the core
compared with the disk; the visibility of the disk is a practical
consequence of that.


I agree with that except I'd write "host galaxy" in place of "disk."
Many host galaxies are ellipticals, not disks.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
 




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