First Galaxies Fast and Compact
The old picture: after the big bang, matter is diffuse. Out of the darkness, stars slowly begin to form, as the first galaxies take shape. Galaxies start out large and slowly grow more dense and structured over billions of years. The new picture: the first galaxies are very compact and dense, spinning rapidly, with stars forming at a prodigious rate. The compact galaxies spin twice as fast as “mature” galaxies closer to us.
This change in thinking was expressed by reports found on Science Daily and Space.com. A look at the original paper in Astrophysical Journal Letters puts the surprises in context.1 Pieter van Dokkum et al made observations with the Hubble Space Telescope and the Keck Observatory at redshift z=2.3 in near-infrared light. They found nine galaxies that seemed more compact than usual and did not find any of the large galaxies common at lower redshifts. They deduced that the distant galaxies are 0.9 kiloparsecs in diameter (compared to 5 kpc for nearby galaxies), and that the compact galaxies rotate twice as fast. They did, however, list 5 possible sources of error. Their field of view was limited, for instance, and “the stellar ages and masses of the galaxies have large uncertainties.” Most important, they said, studies at redshifts greater than 2 are “typically based on photometric redshifts, which are poorly calibrated for faint, red galaxies.”
Nevertheless, they felt confident that their survey rules out “monolithic” models of galaxy evolution (i.e., “in which early-type galaxies are assembled at the same time as their stars”). They viewed their results as “the most direct evidence to date for an essentially hierarchical assembly history for massive galaxies.”
One of the unsolved problems in the paper was how to get large mature galaxies out of early compact ones. How would a dense, compact object grow outward by a factor of six? Galaxy mergers seem insufficient to do the trick. Another problem is how they formed in the first place. For that, the astronomers invoked one of their favorite fudge factors – dark matter:
Van Dokkum speculated on how these small, crowded galaxies formed. He said, one way could have involved an interaction in the emerging universe between hydrogen gas and dark matter — an invisible form of matter that accounts for most of the universe’s mass. Shortly after the Big Bang, the universe contained an uneven landscape of dark matter. He said that hydrogen gas could have been trapped in puddles of the invisible material which began spinning rapidly in dark matter’s gravitational whirlpool, forming stars at a furious rate.
1. van Dokkum et al, “Confirmation of the Remarkable Compactness of Massive Quiescent Galaxies at z~2.3: Early-Type Galaxies Did not Form in a Simple Monolithic Collapse,” Astrophysical Journal Letters,677:L5�L8, 2008 April 10, DOI: 10.1086/587874.
You gotta love the imagination of some scientists. Nobody has a clue what dark matter is, or whether it even exists. This guy has the imaginary stuff forming puddles and whirlpools. Let him connect his “mysterious unknown stuff” (02/28/2008) with empirical observations before telling us it will make stars form at a furious rate.
Claims like the ones in this paper should always be taken with a grain of salt. Many laymen read such things on Science Daily or other popular news sources and have no idea what the astronomers are talking about, let alone what it means. Something to watch for is the element of surprise. Why were the astronomers surprised by what they found? What did they actually find?
Measurements such as this are extremely difficult to make. Astronomers are trying to interpret very faint objects near the limit of observability. It becomes hard to establish where the noise stops and the signal begins. The team was honest enough in the original paper to list five major sources of error that could invalidate the claims that these galaxies are unusually compact. To us, each source of error seemed significant.
Another caution is that observations at cosmological distances are very much tied into the theories employed to make the observations. What does an infrared blob with a redshift of 2.3 (inferred to represent something at a given distance and age) actually represent? Why were they focusing on these things instead of other things? In the 1920s, recall, Edwin Hubble thought that galaxies began as ellipticals and evolved into spirals. In later years some astronomers reversed the sequence. Lately astronomers have been finding more structure, more density, and more “maturity” the farther back they look.
It appears that this team was somewhat eager to substantiate the hierarchical model of galaxy evolution over the monolithic model. But how do they know there are not other possibilities? And how do they know other sources of error, unknown to us today, might confuse what they think they saw? Consider that about 20 years ago, many astronomers were caught off guard by the discovery of gravitational lensing. The bending of light by intervening galaxies, they realized, can seriously compromise the interpretation of distant objects. It was something few had ever considered. No one knows whether another phenomenon might be introducing systematic errors into the observations today.
That being said, let’s assume they are correct, and that these distant galaxies are in fact more compact than expected. It is noteworthy that the astronomers were surprised to see tight and dense structures so close to the assumed big bang. Simplistic models would have predicted otherwise. Creation astronomers might want to consider how this survey might fit a “top-down” model for galaxy formation. Humphreys’ “white hole cosmology” predicted, for example, that distant galaxies would appear from earth to be changing rapidly due to gravitational time dilation. We’ll leave such considerations to those interested. The lesson for our purposes is that different assumptions allow for different interpretations consistent with the very same empirical observations. Things are not always what they seem. Some cosmologists have their scientific method backward. They work according to the inverted principle, “No observation should be considered legitimate until confirmed by theory.”
Observation should trump theory in science. It may not be possible to observe something completely free of bias, but a good first step would be to state one’s biases up front as far as one is aware of them.