Sequential drop-out of forbidden nebular lines, with 'star-burst fade'?
-
by
JeanTate
In my experience, by far the most obvious forbidden nebular lines, in SDSS spectra of galaxies (not high-z AGNs!), are the [OII] doublet (at ~372.7 nm, rest frame; if z is above some threshold, so that this blend actually makes it, unambiguously, into the spectrum), the two [OIII] lines, at 500.7 and 465.6nm, and the [SII] doublet at 671.6 and 673.1nm. The [NII] doublet - at 654.8 and 658.3nm - is often also quite strong; however, as H-alpha sits in the middle, it can be hard to say how prominent it is, simply by eyeballing the PNG images in the SDSS Explore tool.
Having looked at, oh I don't know, a few thousand? SDSS galaxy spectra, I get the impression that, as a star-burst fades into the past, the various forbidden nebular lines fade too ... but in a sequence. For example, it seems that the [OIII] lines (obviously the 500.7nm one remains 'visible' for longer than its partner) disappear quite quickly, then the [SII] blend, finally the [OII] blend.
Does my impression have any foundation in detailed astronomical studies, of the fading of star-bursts in galaxies? If so, what are the physical mechanisms (and explanations) for it?
Posted