Galaxy Zoo Talk

Curiosity

  • Zebx by Zebx

    Hi all. 😃 I've some question about galaxy spectra. In some spectra(like this) I read "Galaxy Starforming". But in other spectra very similar to the example I found "Starbust" or "AGN". My questions are two(maybe more depending on the answers):

    1. How can I establish if the galaxy is starbust or "normal" starforming? I've think "maybe starbust has a higher OII emission?"
    2. Everytime I find a high Halpha and NII emission I'm faced with an agn right?

    Thanks, and sorry for inaccuracies 😕

    Posted

  • JeanTate by JeanTate in response to Zebx's comment.

    Welcome to Galaxy Zoo, Zebx!

    I think it's really cool that you're trying to 'read' spectra; it's a very challenging thing to do.

    The first thing to note is how the automated spectroscopic SDSS pipeline decides what sort of spectrum it's got. It does this, basically, by seeing which of ~30 different 'templates' are the best fit. And 'best fit' means a quite specific type of matching. For us, in GZ, the key templates are "GALAXY", and "QSO". Once this top level assignment is made, various sub-classes can be determined: "AGN", "BROADLINE", "STARFORMING", and "STARBURST". Sometimes more than one is given, e.g. "BROADLINE AGN".

    I'm a little hazy on exactly how these finer classes are determined, but the general rules are sorta like this:

    • if the estimated line width of at least the (prominent) H Balmer lines (you know what these are, right?) is greater than some threshold, it's BROADLINE (this doesn't apply to the nebular/forbidden lines)
    • if the ratio of the [OIII]5007 and H-beta emission lines is above a certain number, it's an AGN; if not, either STARFORMING or STARBURST (an [NII]6583 to H-alpha ratio may be used too)
    • STARFORMING vs STARBURST? I don't know.

    Not part of the selection - as far as I know - but a good guide anyway: STARFORMING and STARBURST galaxies do not have 'high ionization' emission lines; AGN can (but not necessarily). [NeIII]3868 is an example, as is the HeII line (I think).

    This method is generally OK, but it has some serious shortcomings; for example:

    • the spectrum is taken of only the central 2" (BOSS) or 3" (older SDSS) part
    • while this mostly includes the nucleus, it doesn't always; if it doesn't, no chance of AGN
    • a galaxy may have a quite boring 'nuclear' (actually at least part of a bulge, if it's a spiral) spectrum, but an exciting one (STARFORMING, say) elsewhere
    • a great many spiral galaxies (and some ellipticals?) have both significant star-formation and an AGN in their central regions; the SDSS classification ignores this

    To your questions:

    How can I establish if the galaxy is starbust or "normal" starforming? I've think "maybe starbust has a higher OII emission?"

    I don't know, but would guess that a STARBURST has stronger emission lines in general

    Everytime I find a high Halpha and NII emission I'm faced with an agn right?

    No. It's the ratio of the emission lines which is the 'tie-breaker', not their strength (although if the H-alpha line is broad, it's an AGN).

    Hope this helps, and happy hunting! 😃

    Posted

  • Zebx by Zebx in response to JeanTate's comment.

    Yeah I like to understand as more as I can from spectrum, "see what can't be seen". 😃 So in conclusion what I read on the spectrum information depends on how sdss classified it. But maybe the difference between starburst or starforming could be more understandable looking at other datas, in addition to spectrum. I will collect some example of agn and a "what-it-looks-like" an agn although this will allow me to understand only indicatively, but I suppose that the ratios you mentioned depends on several factors so it would be require more in-depth knowledge of the matter 😄 I'm studying that at university so I hope for a better understanding in the future.
    Thanks for answer! 😉

    Posted

  • JeanTate by JeanTate in response to Zebx's comment.

    SDSS itself provides extensive information on the spectroscopic pipeline; here is a link to main page of DR12's (latest data release).

    So, to clarify what I wrote re AGN STARFORMING etc (from this page):

    GALAXY identified with a galaxy template; can have subclasses:

    STARFORMING set based on whether the galaxy has detectable emission lines that are consistent with star-formation according to the criteria: log10(OIII/Hα) < 0.7 – 1.2(log10(NII/Hα) + 0.4)

    STARBURST set if the galaxy is star-forming but has an equivalent width of Hα greater than 50 Å

    AGN set based on whether the galaxy has detectable emission lines that are consistent with being a Seyfert or LINER: log10(OIII/Hα) > 0.7 – 1.2(log10(NII/Hα) + 0.4)

    If any galaxies or quasars have lines detected at the 10-sigma level with sigmas > 200 km/sec at the 5-sigma level, the indication “BROADLINE” is appended to their subclass.

    The STARFORMING/AGN divide was originally proposed by Baldwin, Phillips & Terlevich (Astronomical Society of the Pacific, Publications, vol. 93, Feb.-Mar. 1981, p. 5-19); this NED page has a brief introduction (the criteria have been somewhat changed since 1981).

    "OIII" refers to the [OIII]5007 line; "NII" to the [NII]6583.

    Oh, and there's an error (two actually): "log10(OIII/Hα)" should be "log10(OIII/Hβ)". This page has it right:

    When the signal-to-noise of the line measurements permits doing so, spectra classified as galaxies and quasars are sub-classified into AGN and star-forming galaxies based upon measured [OIII]/Hβ and [NII]/Hα line ratios, and galaxies with very high equivalent width in Hα are sub-classified as starburst objects.

    Hope this helps, and happy hunting! 😃

    Posted

  • Zebx by Zebx

    Thank you very much! I really have to look to a lot of things yet, but I like all of that 😃

    Posted