Thursday, August 5, 2010

Hβ absorption

There might be a problem with the modelling of the Hβ absorption line in stellar population models. Two examples I know of are:
- Figure 2 of Walcher et al. 2009 : the largest residuals when fitting the integrated spectrum of a globular cluster with some of the most accurate stellar population modelling available occur around the Hbeta line. Actually not even in the center of the line, but more in the red wing.
- Figure 4 of Schombert&Rakos 2009 : these authors use spectroscopic measurements of ages and metallicities of real galaxies to compute their expected colours. They then compare these to the actually observed colours and find an offset in the sense that the spectroscopic ages are younger. Is this just a manifestation of the known bias of SSP-equivalent ages to underestimate the age of the observed galaxies or does it indicate a problem with the Hbeta line?

Incomplete line lists, while possible of course, cannot explain the problem by themselves. The stellar population models used above are based on observed stars. Thus the problem must (?) be in the stellar evolution.

Is there more evidence for or against a problem with the Hbeta line? If problem, where does it come from?


  1. To continue this discussion, I have found something similar in the SDSS fits... and Im working with Jarle Brinchmann to sort out where this problem lies and determine to what extent this error occurs.

  2. We found good agreement between all colors plus
    line indices and pop models, except for H-beta.
    We concluded that H-beta is the problem, either
    in the models or in the data. The data accuracy
    has reached the level that I can think of no
    systematics that would distort the H-beta values
    (for example the excellent Trager Coma spectra).

    We outlined all this in our most recent papers,
    but they have been ignored by the community.
    Prof. Rakos has retired, and I have returned
    to my work on LSB galaxies. But when this is
    figured out, I hope to have my "I told you so"

  3. Hi, all!

    Yep, we've been gnawing at H-beta for eons, in terms of its strange low EW in some MR globular clusters. The latest from over here is that we rule out (1) abundance ratio effects and (2) M dwarf active chromospheres. "Normal" stellar pops effects are already ruled out. The Schiavon & Rose hypothesis of active M giants is still viable and is our current favorite.


    About "profile fitting" I'm not up to speed on what has been done. Such a thing seems fraught with peril; most observers do not keep a very tight control on instrumental resolution (or, if they do, they don't publish that information). So separating "core" from "wings" in galaxy spectra rings all kinds of alarm bells in my head.


  4. my 2 cents, from the modeling point of view:

    - when Hbeta in models are weaker than data, i'd suspect of either the presence of blue stragglers (related to binarity?) or some extended horizontal branch. neither is well modeled, as far as i know.

    - if Hbeta is stronger in models, could it be the presence of gas (emission) in galaxies, unaccounted for in models? or the presence of stars with chromospheric emission...

  5. Yes, H beta has been a concern pretty much forever.

    If we are talking globular clusters, and just overall strength, the only mystery I'm aware of is the collection of metal-rich ones with really weak H beta. The latest I'm aware of is this paper:

    Wherein we reject abundance ratio effects and also active Mdwarf chromospheres. General nebular emission has been ruled for quite a while, maybe even since Hesser & Smith in the 1970's.

    The Schiavon & Rose hypothesis of flaring AGB stars is still valid & we favor that one, as the MP clusters don't have such stars. That could use an observational H-alpha monitoring program . . hint, hint.

    Regarding galaxies, I've never had great trouble with Hbeta. ALL age indicators are in conflict, at some level, so I'm not a bit surprised at the Schombert/Rakos result. There is no particular reason that models which produce good colors will also produce accurate index strengths.

    Blue stragglers are significant at the 10% level for ages. I'm not sure that we are down to that level of worry, yet.