Teeny-Weenie Massive Galaxies

By Julianne Dalcanton | May 2, 2008 12:57 am

I’m usually not one to go all blog-happy about the latest press-release, but a recent one happened to be about a paper I had just read, so, I’ll dive on in.

To give a little background, one game that astronomers like to play is “Find the oldest galaxy around at some time”. There are a number of reasons this game has so many eager practitioners. First, the oldest galaxies at any time are probably the locations of the some of the very first collapsed structures in the universe, which make them cool. Second, the most massive galaxies we see today seem to be those that that host the oldest stellar populations — turning this around, if you hunt for the oldest things at a given time, you’re hopefully finding the most massive things at that time. Finally, it’s very tough to figure out how the population of galaxies you see at one time is related to the one you see at the present day (i.e., just because a galaxy is blue and distorted 10 Gigayears (Gyr) ago, doesn’t mean it’s not red and symmetric today). However, if you stick to studying massive galaxies that already looked old 10 Gyr ago, it’s probably a reasonably good guess that they’re comparable to the predecessors of the most massive old galaxies today, giving you one of the few cases where you can match up the two populations with reasonable confidence.

vd_compact_galaxy.pngWhich brings us to the latest paper by Pieter van Dokkum and his collaborators, on the spectroscopic and HST/NICMOS follow-up of 9 massive red galaxies from the MUSYC NIR imaging survey. These galaxies are at redshifts of 2.5-ish, corresponding to about 2-3 Gyr after the Big Bang for WMAP cosmology. They are very red, and have spectra consistent with a dormant stellar population (i.e. one that stopped forming stars roughly a Gyr previously). An earlier paper by Kriek et al estimated masses for these galaxies (assuming reasonable models of the kinds of stars and dust that are in the galaxies), and found that not only are the galaxies dormant, they’re also extremely massive.

So, now you’ve got yourself a bunch of massive galaxies, that in spite of being no more than 2-3 Gyr old, already finished all their star formation a gigayear previously. Pretty nifty, and what you’d hope to find as the precursors of the old massive galaxies we see today. That’s all well and good, but the problem is that the galaxies are just too dang small. They’re tiny. The most massive galaxies today are typically much larger than the Milky Way, but the galaxies van Dokkum is reporting on are much much smaller. (On the plot at right, the solid scale bar shows a size of 10 kpc. On the plot below, the big solid dots are van Dokkum’s sample, and the little dots are the galaxies seen today in the Sloan Digital Sky Survey.)


Ok fine, you might say. A lot can happen in the intervening 10 Gyr. However, not that much can happen, since these galaxies are already pushing the mass limit of the most massive galaxies we see today. You therefore can’t grow the galaxies by padding them out with new stars from accreted satellite galaxies, without taking them over the limit (i.e. looking at the plot on the upper left, accreting new galaxies would take the galaxies up, but would also move them to the right). Even if you could pull this off, the centers would still be too dense. You also can’t magically puff the galaxies up over time. You could imagine perhaps driving a bunch of mass out of the center of the galaxy, maybe through stellar winds, but it would take an absurd amount of mass loss, even if you believed you could get all that mass out of such a dense honking galaxy. You could also imagine trying to disrupt the galaxies somehow by puffing them up kinematically, maybe through violent mergers. However, the problem here is that the galaxies are waaaaaay too dense. If the galaxies really are as dense as the plot above suggests, then they’re going to be almost impossible to disrupt. It would be like trying to disrupt an iron ball by dropping it into a bowl of pudding.

My take on this is that some of the assumptions that went into making the plot above are wrong, because it’s very hard to imagine hiding the descendants of incredibly massive, incredibly dense galaxies somewhere in the local universe. The authors understand this, and argue that it might be a combination of issues related to measuring the radius (loss of diffuse light at large radii, radial gradients in the conversion of light to stellar mass), and possibly the initial mass function (the great “here be dragons” of all extragalactic astronomy).

I actually think the errant step might wind up being the assumed conversion of light to mass. The standard lore is that when you observe galaxies in the NIR, the light is dominated by old red giant branch stars, giving you a reasonably robust conversion from light into stellar mass. However, at a redshift of 2.5, there’s no way that any star in the galaxy is older than 3 Gyr. In this case, most of the red light from the galaxy will be coming from asymptotic giant branch stars, which are notoriously difficult to model. Thus, the calibration of light to mass can easily be off. AGB stars can also give you a nice red spectrum like those observed, and can be potentially centrally concentrated in response to a central burst of star formation. I think the authors have done the best that they could with their analysis, but I suspect we may have run into some of the real limits of our current modeling.

  • Ali

    Good to write about this wonderful paper. I was really excited after reading it. As you said it was a brilliant analysis and interesting results…

  • Heph

    Isn’t it almost certain that existing models, mostly derived from evolution of later generation stars/closer galaxies, will break down? It’s like taking a strength/chest size correlation graph based on men, and applying it to women … the results are likely to be surprising and contradictory …

  • Roelof

    Indeed an interesting paper. There is a known NICMOS non-linearity effect that would make these galaxies slightly larger, but also brighter, so that would not solve the problem. They do not mention whether they corrected for this effect as a letter is just too short. I agree the stellar population modeling is uncertain, but getting it wrong by a factor of ~5-10 seems a lot to ask for.

    Something else I wanted to discuss is you spelling of Dutch names. Pieter is the equivalent of and pronounced similarly as Peter, but Peiter would be pronounced as Pighter (so lighter with a P), which would be an uncommon name. The same holds for Kreik; Mariska her surname is Kriek. So this should make it easier for you to correct all my spelling mistakes in English the next time I ask for feedback from you for one of my papers…

  • http://blogs.discovermagazine.com/cosmicvariance/julianne Julianne

    Whoops! Thanks for the catching my name spelling issues. Fixed on edit…

  • Brad Holden

    One other check (that strangely did not make the press release) is that a subset of these galaxies were observed with Keck using adaptive optics. For those galaxies, the sizes turn out to be the same as when measured with NICMOS. This lends further weight to the idea that, if anything is wrong, is the conversion between how much light we see and how much mass in stars there must be.

    The typical ages that Mariska finds for these galaxies is 0.5 Gyr, which is pretty young, and the M/L values are pretty small as a consequence, typically 1 or smaller.

    These things are really weird. Some colleagues and I are looking for similar weird objects but with a much more robust determination of the mass. This could lead to some interesting results.

  • http://lablemminglounge.blogspot.com/ Lab Lemming

    At the risk of being an idiot, what is the difference between AGB stars and the red giant branch? Aren’t they all big, red, and about to die?

  • http://blogs.discovermagazine.com/cosmicvariance/julianne Julianne

    AGB stars and RGB stars are both big, red, and about to die, but AGB stars are bigger, and thus more luminous, and prone to screwing around with the amount of light you get. Internally, the RGB star has core of inert Helium (i.e. that’s not hot/dense enough to fuse into Carbon) surrounded by a shell of burning Hydrogen (where “burning”=”fusing into helium”). In an AGB star, the onion has an extra layer. The core is inert Carbon, surrounded by a shell of burning Helium and a shell of burning Hydrogen. So, you have two sources of luminosity, and less mass to support, so the star is extra luminous and extra biggified.

    And I don’t think that failure to know these differences is qualification for idiothood!!!! (Especially for someone who knows the proper distinction between physics and chemistry ;-)

  • http://lablemminglounge.blogspot.com/ Lab Lemming

    So, are the big, far away red giants like Betelgeuse and Antares AGB, while the closer, visually similar red stars like Gacrux and Arcturus RGB?

    Also, which star creates S-process nuclides, the AGB or the RGB (I seem to recall that there was a carbon-burning step required for n production, so I’m going to assume whatever the next step up from AGB is)?

    Do RGB’s turn into AGB’s when the core gets big enough to fuse?

    -the geochemist who occasionally wonders where all these isotopes (and pre-solar grains) come from

  • http://blogs.discovermagazine.com/cosmicvariance/julianne Julianne

    LL — There is a lot of s-process production that goes on in AGB stars. They tend to be pulsators as well, and drive stellar winds, so a fair bit of the freshly produced elements can make their way out. A link to start with might be here.

    So, some, but not all, RGB stars will become AGB stars. The inert core doesn’t have sufficient pressure support to hold it up against gravity, so it collapses continuously, becoming denser and hotter as it does so. In higher mass stars, the density and temperature get hot enough that the Helium can start fusing into Carbon. In lower mass stars, the collapsing core never quite makes it, and fusion in the core is over.

  • Fermi-Walker Public Transport


    Very interesting. Regarding M/L, wouldn’t stellar metallicity be a factor ? That is
    since these stars are on the whole more likely to be metal deficient then their present day counteraprts, then I would think that this could affect the stellar luminosity and give a different M/L then if we assumed a solar-type metallicity.
    If I recall correctly, metal deficiency tend to make a star bluer and brighter.

  • http://lablemminglounge.blogspot.com/ Lab Lemming

    Wow, this is great- I could read it all afternoon.

    specifically, I had no idea that C/O ratios changed so much as starts evolved- I thought they were either C stars or O stars as a result of initial composition. Very cool.

    Just one more totally unrelated question:
    I was reading (and blogging about) a geochemistry paper that used isotopic constraints to suggest that the solar system-generating supernova was a Type II with a neutrino-driven wind.

    What is a neutrino-driven wind, and how can particles that pass through everything drive?

  • Professor R

    This was a superb post, drawing attention to a paper I was completely unaware of, and discussing it at a level some of us can really learn from…
    Why can’t there be more posts (and blogs) like this one? Cormac

  • http://kea-monad.blogspot.com Kea

    Pitkanen wrote a post on this article already. Maybe there is no mistake.

  • Ben

    Neutrinos: Just before a core-collapse supernova occurs, the precursor’s core becomes too massive for electron degeneracy to support it, and electrons and protons combine to form neutrons and neutrinos. The neutron core also liberates its thermal energy in neutrino-antineutrino radiation. Even though neutrinos have a very small cross-section, the interior of the precursor is so dense and hot that this blast of neutrinos starts to blow away the star – the neutrinos are actually the driving force of the supernova. I recall vaguely that without capturing some fraction of the neutrino luminosity, simulations of SNe just can’t get the explosion to explode; the energetics don’t work.

    Tiny massive galaxies: It seems a problem if these things are actually that massive. However, even without that high-profile result, if you crank down the mass and accept all their other characteristics, if they are truly quiescent, they are somewhat surprising as very small-radius objects in which star formation has quenched very early. I’ll claim (although I’m open to correction) that there are few or no obvious direct descendants of such objects in the local universe either (there are lots of small galaxies like dwarf ellipticals, but I don’t believe those are old enough). So you would have to merge them away or subsequently add stars around them (to make them cores of bigger old galaxies, or bulges). Or they’re not really quiescent. All of those are possible, it remains to be seen which, if any, are believable.

  • Brian

    Great post, Julianne. Thanx. A determined sleuth might look for slightly more evolved versions of such galaxies at 9, 8, and 7 Gyr.

  • Superstring

    If Halton Arp is correct, these are not at cosmic distances, They are nearby compact young galaxies.

  • http://blogs.discovermagazine.com/cosmicvariance/julianne Julianne

    The odds that Halton Arp is correct on this point are about the same as the LHC destroying the world.

  • KS

    Very nice post – I stumbled upon this blog by accident but will surely read it regularly. There is another paper on astro-ph by Buitrago, Trujillo and Conselice claiming similar things – they claim two orders of magnitude evolution in the mass surface density. Anyways – we discussed it at our astro-ph group yesterday and no one believed it. The mass is likely incorrect – they claim they use the MAraston models but theu don’t use the existing IRAC data (rest-frame near-IR) where the BC03 and Maraston models actually differ. I also think their measurements of sizes is incorrect. Mancini, working with Cimatti, in Italy is finding that the sizes are grossly underestimated in these analysis.

    Anyways – while the interpretationis interesting, I think these papers are wrong. They are not doing a careful job with the analysis which is leading the field astray – well may be not astray but it would be nice to see better analysis of these data which will put this “exciting” discovery to rest ;)


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