post-starburst hypotheses

Today I worked out what hypotheses we are testing with the K+A environment research.

(1) Post-starburst galaxies lie in the same range of environments as "all" or "ordinary" galaxies.
(2) Post-starburst galaxies lie in the same range of environments as bulge-dominated or "early-type" galaxies.
These hypotheses were both ruled out by Quintero et al and by Blake et al; in fact hypothesis (1) was very unlikely from the start, since no spectrally or morphologically selected subsample has the same environment distribution as "all" the galaxies ("all" doesn't really have a meaning here).

Indeed, both Quintero et al and Blake et al concluded that the mean or typical environments of K+As is the same as those of disk-dominated or "spiral" or star-forming galaxies. We already know, from work like Blanton et al, that star formation is the galaxy property most tied to environment. This leads to two new hypotheses

(3-weak) Post-starburst galaxies lie in the same range of environments as disk-dominated or "spiral" galaxies.
(3-strong) Post-starburst galaxies, at every measure of A-star excess (A/K), lie in the same range of environments as the subsample of disk-dominated or "spiral" galaxies that has the same value of A/K.

Our results confirm hypothesis (3-strong): Of all the star-formation-rate indicators, it is A-star excess that predicts the environment distribution, not H-alpha (since K+As have none by definition). This fits in with the idea that A/K evolves on timescales that are long like dynamical times.

The only statistically significant deviations we find from (3-strong) are at very small physical scales, where the dynamical time is likely shorter than the lifetime of an A star.


  1. this is a somewhat stronger result
    than similar ones in the literature.
    e.g. Kauffmann et al. came to the
    same qualitative conclusion, but
    focused on the bulk of the population
    (the very lowest Halpha galaxies
    wouldn't have figured much in that
    work). this work shows that even
    the extreme outliers in the population are not caused by environmental effects.

  2. Which Kauffmann et al paper is that?

  3. don't look at it as testing the timescale that A/K evolves over. we know that A stars have a lifetime of about 0.5-1 Gyr, so A/K can't evolve much faster than that.

    look at it as testing the H-alpha timescale, (quenching timescale, if you like that word) since that is what can change rapidly, and H-alpha is what distinguishes "K+A" galaxies from disk galaxies.

    in retrospect, K+A is a misnomer. they're called that to distinguish them from elliptical galaxies, but your environment result shows they are related to disks. of course, disks also have spectra with K and A stars. "K+A" galaxies are more properly zero-H-alpha galaxies. at least they're not still being called "E+A". i dare you to try to rename them again.

    going out on a limb: A/K ratio tells you something about the star formation history over a long time scale, so something like mean age which could be related to mean overdensity (environment). while H-alpha probes the short timescale which could be related to short dynamical times. your last sentence about the deviations sounds like a toss-off, but it could be more.

    of course, this is a blog, so i could be wrong.

  4. bjw: No, it was not a toss-off; it will be one of the results of the paper. Unfortunately, we don't have any deviations at high S/N.