December 15, 2004 | David F. Coppedge

Late Bloomer Galaxy Just Now Getting Into Star Formation?

According to a story in New Scientist, a “young” galaxy is just now starting its process of star formation. 

Most galaxies formed more than 10 billion years ago.  Those born later tend to be fashioned from recycled gas rich in metals that were forged by previous generations of stars.  But astronomers Trinh Thuan of the University of Virginia in Charlottesville and Yuri Izotov of the Main Astronomical Observatory in Kiev, Ukraine found the gas in the youngster, named I Zwicky 18, contains very little metal and is more like the pristine gas left over from the big bang.1

The astronomers are claiming this galaxy gives them “an excellent opportunity to study the early stages of the evolution of galaxies.”  This galaxy made Astronomy Picture of the Day December 3.


1Yuri Izotov and Trinh Thuan, “Deep Hubble Space Telescope ACS Observations of I Zw 18: a Young Galaxy in Formation,” The Astrophysical Journal, 616:768-782, 2004 December 1.

Does something sound really fishy about this story?  Like, maybe, trying to keep the story going in spite of the evidence, or trying to force-fit objects into preconceived categories of old and young?  All they found was a galaxy with a lot of gas and few stars, and low metallicity in the gas.  So?  Look for the evidence of a prior assumptions in the abstract:

The question of whether there are young galaxies in the local universe forming stars for the first time is of considerable interest for galaxy formation and cosmological studies.  There are several reasons for this.  First, cold dark matter models predict that low-mass dwarf galaxies could still be forming at the present epoch because they originate from density fluctuations considerably smaller than those giving rise to giant galaxies. Thus, the existence of young dwarf galaxies in the local universe would put strong constraints on the primordial density fluctuation spectrum.  Second, while much progress has been made in finding large populations of galaxies at high (z =+ 3) redshifts (e.g., Steidel et al. 1996), truly young galaxies in the process of forming remain elusive in the distant universe.  The spectra of those faraway galaxies generally indicate the presence of a substantial amount of heavy elements, implying previous star formation and metal enrichment.  Thus, it is important to have examples of bona fide young galaxies in the local universe because they can be used as laboratories to study star formation and chemical enrichment processes in environments that are sometimes much more pristine than those in known high-redshift galaxies.  Moreover, their proximity allows studies of their structure, metal content, and stellar populations with a sensitivity, precision, and spatial resolution that faint distant high-redshift galaxies do not allow.  Finally, in the hierarchical model of galaxy formation large galaxies result from the merging of smaller structures.  These building-block galaxies are too faint and small to be studied at high redshifts, while we stand a much better chance of understanding them if we can find local examples.

So they went into their observations with preconceived notions of what they were looking for and what it meant.  That’s a recipe for delusion.  Meanwhile, the “young” galaxies that should be at high redshifts, and presumably therefore more pristine, are nowhere to be found; they already have heavy elements.  Where is the evolution?

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