Galaxy Zoo Talk

AGZ0002ccv: A quenched spectrum?

  • ChrisMolloy by ChrisMolloy

    Not that I know much about spectrum, but could AGZ0002ccv be a quenched galaxy?

    enter image description here

    Here's the spectrum.

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

    Well spotted! 😃

    The Balmer series are all 'in absorption', blue-ward of H-eta, and all but H-alpha have clear, broad 'in absorption' wings, with narrow 'in emission' peaks (check out the DR12 interactive spectrum). Perhaps this indicates the presence of a (not too strong) narrow-line AGN? Or perhaps it's star-formation having turned off too recently for the prominent emission lines to have faded enough yet?

    I think mlpeck knows of some analyses which are rather straight-forward to do (400nm cliff?), analyses which could indicate more clearly what's going on.

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

    The traditional criteria for a post-starburst spectrum are a combination of strong absorption in the higher order Balmer lines combined with weak or no emission in [O II] and/or Hα. Goto and most subsequent researchers use the Hδ index as the absorption line index of choice. This one has Lick HδA=5.3±0.6Å, which is pretty strong absorption, but when there are also strong emission lines present the usual interpretation is we're looking at a dusty starburst galaxy where the youngest population is hidden. We could also be looking at an AGN and those are perhaps associated with quenching of star formation, but in this case the line ratios are consistent with hot stars as the ionization source.

    I've been considering doing a final series of posts to Quench Talk on selecting post-starburst samples from SDSS using SQL queries in CasJobs. I have some slight concern that an administrator will stumble across the site, notice the complete absence of activity, and shut it down though, so I don't plan to invest a lot of effort in it. Maybe I should just post here.

    Unfortunately also the queries rely on line index measurements from the MPA pipeline. Those were officially deprecated as of DR10 and no BOSS spectra have been processed. No other group that provided value added data to SDSS performed line index measurements.

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  • ChrisMolloy by ChrisMolloy

    Thanks Jean Tate, mlpeck, for your replies. Always very informative.

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  • mlpeck by mlpeck

    Slightly more detailed followup -- I upgraded to the latest distribution on my Linux PC this weekend and that's almost always a mistake. At least I didn't in the end have to do a fresh install.

    Anyway, Kauffmann and collaborators made extensive use of the "Hδ - D4000" plane as a diagnostic of stellar age and burst strength. Here is an example from Kauffmann et al. (2003). She didn't pioneer the use of this particular diagram by the way; that appears to have been Balogh et al. (1999) and there were earlier variations that used broad band colors instead of the 4000Å break strength.

    This diagram shows several starburst models as thin lines. A fresh starburst starts at the left endpoints, and as it ages it loops up and to the right, gradually fading into the red sequence after a few Gyr. That's if it passively evolves after the starburst ends. The points in the diagram are continuously starforming and passively evolving galaxies. The galaxy in this discussion lies near the upper left tip of the locus of starforming galaxies.

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    For anyone interested in the part of the spectrum where these indexes come from here's a zoomed in view of a high S/N spectrum in my collection of true post-starbursts. I've marked the blue and red wings of the D4000n index and the Hδ line and the sidebands that are used to mark the continuum in the Lick system. The other absorption lines are a mix of higher order Balmer lines and a Ca II (ionized calcium) doublet.

    enter image description here

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