Hα in spectra.
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mZakk
By looking at various spectral diagrams on SkyServer I've noticed that there's always a peak [brightest emission?] under Hydrogen-α (about 7k/8k Å) .
Maybe it's a stupid question, but why does this happen?Posted
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JeanTate
It's certainly not a stupid question!
Galaxies with current star-formation, or active nuclei ("AGNs"), will have spectra with emission lines (upward spikes); the strongest (spikiest) of these is often the H-alpha line (though the [OII] doublet, deep in the blue, is often a competitor, and the '5007' [OIII] line can be #1 too). For very blue galaxies, the continuum - the spectrum 'under' all the emission lines (and 'over' the absorption ones) - will rise well into the blue, peaking somewhere near 400nm perhaps; that is what 'textbook perfect' E+A (a.k.a. 'post-quench', a.k.a. 'post-starburst') galaxy spectra have.
Many quasars have spectra with continua which keep on rising, right to the blue end.
However, the spectra of typical elliptical galaxies - the "E" in "E+A" - do not (generally) have H-alpha emission lines (many have H-alpha absorption lines), and the continua are quite different. For example, AGS00002la:
Posted
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mZakk
Thanks a lot!😃
Posted
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astropixie
scientist
hi mZakk. most of the hydrogen alpha emission occurs as a result of hot, newborn stars shining light on nearby hydrogen gas clouds. the gas gets excited by the extra energy of the starlight, and in order to calm down again, the atoms of the gas release the extra energy in the form of a photon at exactly the energy that corresponds to a wavelength of 6563 angstroms. the photons have this wavelength because of the configuration of the hydrogen atom. H-alpha is produced by the electrons that jump from the third to the second energy level.
most starts are not hot enough to excite the hydrogen gas, only the biggest stars, which have short lifetimes. that's why we use hydrogen alpha as an indicator of new star formation in a galaxy.
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JeanTate
in response to astropixie's comment.
Very nice explanation, astropixie; thanks! 😃
Would you mind writing a few words about how the spectrum of a galaxy with no recent star formation, but with an AGN, would be different? Particularly one where only the NLR (narrow line region) is visible (a type 2 AGN/quasar?)
Also, to what extent do SNR (supernova remnants) contribute to H-alpha (and nebular line) emission from a galaxy, as when seen from afar?
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astropixie
scientist
in response to JeanTate's comment.
hi jean,
the presence of an active galactic nuclei (AGN) changes the overall shape of a galaxy spectrum and the ratios of different emission lines. for instance, star formation does not produce the [OIII] emission line, but this is produced in the narrow line region of an AGN. a demonstration of these properties can be found here: http://astrobites.org/2011/09/07/a-new-class-of-quasar/
supernova remnants do not contribute significantly to a galaxy's total H-alpha emission, since they are intrinsically fainter in H-alpha than the more numerous star-forming regions are, and they are usually enshrouded in dust which dims their H-alpha emission.
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JeanTate
in response to astropixie's comment.
Thanks astropixie.
A day or so after I wrote the post to which you responded, I started a new thread, very much on this topic: Automated creation of BPT diagrams, in GZ Tools?, so far not much in the way of responses.
Well, on topic at least on the emission lines part. Perhaps there's a way to do something similar with the continuum?
I'm very puzzled by this, in your post: "star formation does not produce the [OIII] emission line, but this is produced in the narrow line region of an AGN"
The spectra of Green Peas have exceptionally strong [OIII] emission, yet none contain a detectable AGN, right?
For example, DR7 ObjId 587731870165893326:
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