Thanks for all the information in your post about the activity around the issue of how red ellipticals seem to have puffed up since their early days.

I’d like to keep track of this issue, to follow up on my own post (here) on the subject – and thanks for your comment on it.

The whole area of galaxy evolution is getting pretty interesting, especially with all the new technology almost available to do much better studies.

Essentially, I would keep the following in mind:

- yes, the implied size growth between z~2 to 0 would imply an increase in velocity dispersion at high redshift, and there is evidence of this. Cenarro & Trujillo (2009) show an increase of ~1.3 in sigma between present-day spheroid galaxies and those at z~1.6, based on a stacked spectrum of 13 galaxies. This would support a (minor) merger scenario.

- it does not, however, seem to rule out the explanation that the red nuggets are simply the cores of today’s massive ellipticals. Hopkins (2009) argues that the accretion of low-density material over time can affect the effective radius and the surface brightness profile, but would not have a large effect on sigma.

- the observations presented in van Dokkum’s (2009) paper are really pushing up against the limits of what can be done with today’s instruments. The authors themselves acknowledge that the spectral features used to fit the velocity dispersion have low signal to noise. Moreover, calculating the mass of the galaxy relies on fitting the surface brightness profile. These high-redshift galaxies as imaged with NICMOS on Hubble are barely resolved (see Kriek et al (2009) for images) – so most of what you see in the image is just instrumental point spread function. So these model fits are basically not representative in the galaxies’ central regions, and the resulting mass has significant uncertainty.

- that said, I know some of these astronomers and would certainly trust them to do a thorough job with the data they have. So if we accept that this galaxy has the high sigma they present, it’s still only one galaxy, i.e. not exactly a “representative sample”. For that we’ll have to wait for a new generation of spectrographs with better sensitivities and spatial resolutions that will allow more reliable mass and sigma measurements.

Anyway this is kind of the way I see it. Do you think that makes sense? Sorry for the delay in replying – I wanted to read up a bit more on the subject before saying something incredibly stupid.

Great post! This subject really is fascinating. When I first heard about this problem via Julianne Dalcanton at Cosmic Variance (I’d give you the link to her post but I can’t find it) I thought the solution would simply be galaxy collisions/mergers that enlarge the galaxies. But, of course, the story isn’t so simple as the mass of these beasts seems to remain constant.

On the other hand, I don’t understand why unseen stars in the outer regions of these galaxies solves the problem. Hasn’t it been shown* that the stars we do see have high velocity dispersions? This means that the mass contained within these star’s orbits is very high, so we still have massive galaxies that are denser than their present-day siblings.

-Grad Student

*Actually, maybe this is the only case where it has been “shown.”

Van Dokkum et al 2009:
http://arxiv.org/abs/0906.2778