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Tom Mazanec
2015-Oct-08, 02:18 PM
How are Blazars and Quasars and Seyfert Galaxies the same? How are they different? Is the difference because of position (Ex are Seyferts just closer? Are Blazars just "pointed" this way?) or are they distinct classes?

antoniseb
2015-Oct-08, 02:52 PM
The Wikipedia entries on these things are pretty good. They are all connected to activity with the central supermassive black hole, but as you noted, Blazars are pointing their jet at the observer (us), and quasars are more brightly active than Seyferts, but the dividing line between them is kind of fuzzy and arbitrary.

StupendousMan
2015-Oct-08, 04:12 PM
The reason that there are many different words which usually end up pointing to the same type of object is historical: in the old days, some astronomers who used optical telescopes noticed strange features in a small number of galaxies; some astronomers using radio telescopes noticed strange features in a small number of objects; etc. Each group created its own word to refer to the outliers. Only many years later did it become clear that all these "strange" objects were in large part the same set.

I could have stopped at "the reason is history" and answered about 90% of all questions about astronomical jargon. Sigh.

Jeff Root
2015-Oct-08, 06:23 PM
I saw a magazine article many years ago (Sci-Am?) that
tried to divide up all those different categories by angle.
It looked terribly arbitrary. I find StupendousMan's
explanation very satisfying as an antidote to that.

-- Jeff, in Minneapolis

ngc3314
2015-Oct-08, 09:17 PM
I have some material online (http://astronomy.ua.edu/keel/agn/) dealing with these issues, especially this introduction (http://astronomy.ua.edu/keel/agn/text.html) and the linked image gallery. StupendousMan's points are key - for example, here are optical spectra of many of the AGN categories, showing how things with different names can be very similar when observed in another spectral band.

http://astronomy.ua.edu/keel/agn/spectra.gif

StupendousMan
2015-Oct-09, 12:48 AM
I think it was a bit unfair to focus on a "mean quasar" when there are so many friendly ones ...

Jean Tate
2015-Oct-09, 01:27 AM
I have some material online (http://astronomy.ua.edu/keel/agn/) dealing with these issues, especially this introduction (http://astronomy.ua.edu/keel/agn/text.html) and the linked image gallery. StupendousMan's points are key - for example, here are optical spectra of many of the AGN categories, showing how things with different names can be very similar when observed in another spectral band.
Very cool, thank you ngc3314! :)

That there's nothing in the spectrum of the BL Lac (a blazar) is no surprise, nor that at least one Balmer line is obvious, in emission, in all the other spectra (although its presence in the 'normal galaxy' is a bit odd; the galaxy looks like it could be an ETG, with no ongoing star-formation). Except for that mean quasar ... where did the H-alpha emission go? Also, the LINER's H-alpha/[NII] complex looks as if the H-alpha is somewhat broadened (or perhaps that's due to instrumental resolution?).

Curious fact for regular CQuestians: without reading what's in any of ngc3314's links, I was able to quickly work out that these spectra are all in the 'rest frame' (the blazar may be an exception), and that I could identify almost all the obvious features in each spectrum (including the 400nm 'cliff'), with no reference to their wavelenghts! :eek: Clearly, I have been spending far too much of my time looking at galaxies and squinting at their (optical) spectra (mostly SDSS ones) ... :p

StupendousMan
2015-Oct-09, 11:36 AM
Except for that mean quasar ... where did the H-alpha emission go?

Look at the wavelength scale underneath the mean quasar's spectrum more carefully.

ngc3314
2015-Oct-09, 01:05 PM
Yes - the redshift range Francis et al. used for their mean spectrum was high enough that their data didn't go very far to the red in the emitted frame.

(Or "if you're going to be mean, we're going to window your energies").

Jean Tate:Sharp eyes on the LINER H-alpha profile - NGC 4579 was one of the first known LINERs with weak but clear broad H-alpha emission (sometimes called LINER 1's, but that way lies madness and it's barely walking distance). The "normal galaxy" is the nuclear region of NGC 3368, which has an old stellar population and weak LINER-like emission (at the level which is almost always present in spirals of early Hubble types, Sa-Sb).