Scientific Method / Science & Exploration

Gravity’s lenses may skew surveys of early galaxies

We could drastically overcount the number of galaxies present in the early …

Gravitational lenses can distort and brighten distant galaxies

Astronomers have shown that, at high red shifts, gravitational lensing might have a dominant effect on the number of galaxies counted in upcoming surveys. As astronomers peer deeper into the sky, they will have to be careful what they count.

When counting galaxies, astronomers use the Schechter luminosity function, which gives a characteristic luminosity for a distribution of galaxies. Above this luminosity, there are exponentially fewer galaxies; below it, the number galaxies grows rapidly. Fitting the observed distribution of galaxies to the luminosity function gives an estimate of the number of galaxies in that distribution. But overcounting bright galaxies gives the wrong fit, and will throw that estimate off. And gravitational lensing might produce just this sort of overcount.

Since mass distorts space, and light travels through space, massive objects can bend light and act like giant lenses. Because of these lenses, distant objects can be magnified and, more importantly, can be made to appear brighter. The brightening of dim, distant galaxies due to gravitational lensing will make it appear as if there are more bright galaxies in a survey than there really are. Current surveys go out to a distance where the redshift of light has a value of z = 6-7. In these surveys, such as the Hubble Ultra Deep Field, around one percent of the galaxies are lensed. The authors of the new paper have calculated that, when surveys reach redshifts z > 14, up to 10 percent of galaxies could be lensed.

With new instruments, such as the James Webb Space Telescope (JWST), corrections due to gravitational lensing will be necessary for accurate survey counts. The JWST will be looking at redshifts z > 12, and the characteristic luminosity of these galaxies will be approaching the limit of detection of the telescope making imaging of the dimmer galaxies in the population almost impossible. Therefore, only bright and lensed galaxies will be seen in the high redshift surveys it performs.

The authors do note that dim galaxies that are normally inaccessible may be brought into the light of study through gravitational lenses, so there is a good side to this problem.

Nature, 2011. DOI: 10.1038/nature09619  (About DOIs).

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