I've seen several popular reports of a new count of low-mass stars in elliptical galaxies (here's one).

Edit: Pursuant to several correct comments I've changed the title to agree with the actual report which is that the recount concerns elliptical galaxies---and I don't know where I got the notion that it concerned dwarf galaxies---but I am leaving my comments below intact as they represent the way I was thinking before I was corrected. Note that we are in fact talking about relatively few very massive galaxies instead of many very light ones, but the questions are largely unchanged.

My first instinct was to dismiss it as mostly interesting to those who specialize in galactic dynamics, but then it occurred to me that there must be a lot of those galaxies, and I began to wonder about the baryonic-matter/dark-matter/dark-energy balance.

My guess is that this makes little difference to the matter/dark energy part of the equation because the total matter fraction is derived from large scale measurements of cluster dynamics. But even if I am right about the matter/dark-energy thing, that leaves the question of baryonic vs. dark matter fraction.

Can anyone shed some light on this?

Also, links to pre-prints or journal articles related to this measurement would be welcome.

  • $\begingroup$ I thought they said ellipical galacies. These are usually the really big ones. They did say that they haven't observed the same thing in the Milky Way. And the survey can only go out to maybe 10 million lightyears, not a very large sampling of galaxies. $\endgroup$ Dec 4, 2010 at 2:33
  • $\begingroup$ In any case star density versus mass generally follows a power law, with the lower masses accounting for the bulk of stars, and of mass. For a number of reasons, including theoretical issues that say $\endgroup$ Dec 4, 2010 at 2:35

2 Answers 2


The fraction of baryonic matter to dark matter is not deduced only from galactic dynamics. It is also derived from big bang nucleosynthesis and from the higher multipole acoustic peaks in the CMB spectrum. I would say that the element abundance is a far more important indicator of the fraction between baryonic and dark matter.

Big-Bang nucleosynthesis

Theoretical overview of Cosmic Microwave Background anisotropy: 1.2. Results

  • $\begingroup$ Then this would be a "no" on significantly tweaking our understanding of the mass balance: the situation is constrained several ways. Aside: its been a long time since I looked at the cosmology chapters of the PDB, evidently that was a mistake. $\endgroup$ Dec 4, 2010 at 17:47
  • $\begingroup$ Yes, I don't think that it could change significantly the mass balance. $\endgroup$
    – Vagelford
    Dec 4, 2010 at 18:47

I guess I shouldn't try more than a one liner in a "comment", it looks like (enter) causes a new comment....

In any case, it is an interesting question. My reading was that the unexpectedly high abundance was for eliptical galaxies, which are usually the largest ones. So the significance may be higher than you assume. Supposedly the Milky Way (which is a spiral galaxy) doesn't show the same superabundance.

In any case stellar populations generally follow a powerlaw mass distribution function, with the lowest mass objects dominating both the numbers and the mass. It has generally been assumed that there is a low mass cutoff. I think this follows from star formation theory, which says low mass stars take too long to form (their birth clouds would be disrupted). Obviously our understanding of star birth is inadaquate. I wonder about the abundance of brown dwarfs. Perhaps the total amount of mass in dark clumps of normal matter is greater than currently assumed.

I don't think that the case for exotic dark matter to not exist has any support however. Theories of cosmology including the clumping of matter and formation of galaxy clusters do seem to require dark matter. But an increase in the amount of normal matter would be interesting.

  • $\begingroup$ I was not thinking about doing away with dark matter as it is currently thought to dominate baryonic matter by a factor of a few. $\endgroup$ Dec 4, 2010 at 2:59

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