August 18, 2006 | David F. Coppedge

Early Large Spiral Galaxy Resembles Milky Way

Astronomers using adaptive optics at the Very Large Telescope (VLT) in Paranal, Chile took spectra of a galaxy at red-shift 2.38 described as an “early young galaxy” that must have, according to current theory, formed very rapidly, because it looks like the Milky Way.  The observations by Genzel et al., published in Nature,1 were described by Robert C. Kennicutt (editor of Astrophysical Journal) in the same issue of Nature2 this way:

On page 786 of this issue1, Genzel et al. present remarkable observations of what appears to be a newly formed spiral galaxy, observed when the Universe was just a fifth of its current age.  The result is doubly significant: first, it provides the most detailed glimpse so far of the formation of a galaxy similar to our own Milky Way; second, it demonstrates the power of a new generation of high-resolution instruments that use adaptive optics to study the information and evolution of far-off galaxies.

Though Kennicutt claims that our growing catalog of deep-space observations have given rise to “a self-consistent picture of the evolution of galaxies,” he did find it remarkable that such a distant galaxy would look so familiar:

The authors’ observations of BzK-15504 reveal it to be a giant spiral galaxy, with a size and mass similar to that of the Milky Way, but observed just 3 billion years after the Big Bang.  It shows many similarities to present-day spiral galaxies, with rotational properties that, again, are nearly identical to those of the Milky Way.  These similarities are notable because they imply that at least some large disk galaxies were broadly in place even at these early cosmic epochs.

He says that the spectra imply a rapid burst of star formation in this galaxy 50 times greater than that assumed in our own.  The authors of the paper, after stating the “framework” of galaxy evolution, admitted to some anomalies in the picture:

It remains unclear, however, over what timescales galaxies were assembled and when and how bulges and disks—the primary components of present-day galaxies—were formed.  It is also puzzling that the most massive galaxies were more abundant and were forming stars more rapidly at early epochs than expected from models.

Everyone thought large spiral galaxies formed late in the evolution of the cosmos.  Kennicut said, “large spiral galaxies with well-developed disks similar to the Milky Way are conspicuously absent in both observations and models of the early Universe.  These large spirals are expected to form rather late, so one would not expect to find many of them at early times,” he added.  But why there are any galaxies this large and mature at such an early age?  “Both these and other results from the same programme are challenging theorists to account for the existence of such massive and well-formed galaxies at such early cosmic epochs, he added, changing the subject to the promise of adaptive optics to answer that question.


1Genzel et al., “The rapid formation of a large rotating disk galaxy three billion years after the Big Bang,” Nature 442, 786-789(17 August 2006) | doi:10.1038/nature05052; Received 25 April 2006; Accepted 6 July 2006.
2Robert C. Kennicutt, Jr., “Astronomy: Young spirals get older,” Nature 442, 753-754(17 August 2006) | doi:10.1038/442753a; Published online 16 August 2006.

The juxtaposition of cockiness about their models and head-scratching about the particulars is what is puzzling.  To keep the model together, they have to have this galaxy, which is surely representative of billions more, forming stars and evolving so rapidly that it looks mature at one-fifth the assumed age of the universe.  This pattern of early maturity is the Cambrian Explosion of cosmology, also known as the Lumpiness Problem.  The early universe shows much more structure (lumpiness) than expected from a nearly homogeneous expansion of an initially uniform particle soup (uniform, that is, to within one part in a hundred thousandth of a degree temperature of the cosmic background radiation).  Astronomers seem to take their lumps in stride.  Sometimes, however, discretion is the better part of valor.

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Categories: Astronomy, Cosmology

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