Dec 01, 2011

A nearby spiral galaxy in the southern sky looks like the Milky Way—except it is twice the size.

The European Southern Observatory (ESO) has put together an image of NGC 6744, a spiral galaxy that astronomers think looks a lot like our own Milky Way: “…striking spiral arms wrapping around a dense, elongated nucleus and a dusty disc. There is even a distorted companion galaxy — NGC 6744A, seen here as a smudge to the lower right of NGC 6744, which is reminiscent of one of the Milky Way’s neighbouring Magellanic Clouds.”

The only big discrepancy is that NGC 6744 is nearly twice the size of the Milky Way—200,000 light-years across instead of 100,000 for our galaxy. This size is easily calculated from the galaxy’s measured angular diameter and its distance. The distance, too, is easily calculated from the galaxy’s measured redshift. The result is as certain as mathematics can be.

There is, however, one loose thread dangling from this tightly knit fabric of mathematical certainty: the assumption that redshift is a measure of distance. A tug of skepticism on that thread unravels the entire fabric. And there is much evidence to add weight to the tug. Many previous Pictures of the Day have featured the discordant evidence that undermines this assumption: the statistical and physical associations of high- and low-redshift objects, periodicity of redshifts, and ultra-luminosity, as well as this supersizing.

NGC 6744 is at the low end of supersizing, calculating out to be twice the size of the Milky Way despite appearing to be the same in all other respects. NGC 309, on the other hand, is a spiral with a structure similar to that of NGC 6744 but with a much larger redshift. Hence, its calculated distance is much greater, requiring much greater size for its angular diameter. If placed at the same distance as M81, one of the largest of the nearby spiral galaxies, it would be four or five times the size of M81. Because its HII (star-forming) regions are comparable in relative size (to itself) as those in NGC 6744, not just the size but the scale of the galaxy must be four or five times larger.

Astronomers place much confidence in the precision of their mathematics. But precise calculations from uncertain assumptions reminds one of the rule of thumb: garbage in, garbage out.

Mel Acheson