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?