Quick on the heels of NASA’s showcasing of the first images taken by a reborn Hubble Space Telescope come a pair of papers posted to astro-ph showing a glimpse of Hubble’s potential new power. These papers, by a collaboration of US, Swiss and Dutch astronomers, report the detection of galaxies using Hubble’s new optical/infrared camera WFC3 out to staggeringly high redshifts of 8-9. If confirmed, this shows that Hubble can now detect radiation from galaxies when the Universe was just a few hundred million years old. The first anything at those distances was spotted just a few months ago, when satellite SWIFT caught a gamma-ray burst that was confirmed to have erupted at redshift of 8.2.
The reason this wasn’t possible until now is related to the technique used to identify these high-redshift objects. Spectra of galaxies that contain a lot of bright young stars have a very characteristic sharp drop in intensity in the ultraviolet at a wavelength called the Lyman limit. Photons with shorter wavelengths (and resulting higher energies) are easily absorbed by the neutral hydrogen that is abundant in the interstellar and intergalactic media, causing a sudden drop in a its brightness. Galaxies displaying this effect in their spectra are called Lyman break galaxies.
As we look back further into the past of the expanding Universe, features in the spectra of galaxies, including the characteristic Lyman break, are shifted to longer wavelengths. Because the Lyman break is such a strong and essentially well-understood feature, its location on the wavelength scale is a very strong diagnostic for determining the distance to a galaxy. At high redshifts, where the galaxies are too faint to get spectra, we can use imaging to determine the approximate location of the break using a sequence of images of a galaxy in different filters and inspecting in which filter the galaxy disappears from view. This can give a reasonable estimate of distance to the faintest of galaxies.
Rychard Bouwens of Lick Observatory, the lead author on one of the papers, has previously applied this technique to identify galaxies at redshifts of 7-8 by imaging faint objects from the Hubble Ultra Deep Field in several optical and near-infrared filters using Hubble’s NICMOS camera. The newly commissioned WFC3 has a larger field of view, better resolution and is far more sensitive to faint targets than NICMOS, and this has allowed Bouwens, Swiss astronomer Pascal Oesch and their collaborators to detect much fainter galaxies at these redshifts, substantially increasing the sample size for future study.
With an improved filter set that gives better coverage in the infrared, WFC3 is also better equipped to help rule out false detections. Bouwens and colleagues were able to use WFC3’s improved coverage to spot those galaxies whose Lyman breaks are redshifted further into the infrared than ever before, resulting in the tentative detection of five galaxies at redshift 8-8.5, when the Universe was just 600 million years old.
This is a great early result for the upgraded Hubble Space Telescope showing clearly why astronomers wanted the final servicing mission to go ahead, despite the cost and risk of sending the ageing Shuttle to the observatory. Hubble’s successor, the infrared-optimised James Webb Space Telescope, will be ideally suited to studying the Universe at high redshifts, but it won’t be launched until mid-2014. This recent Hubble upgrade will make sure that our access to the optical and infrared wavelength regime from space, which provides numerous advantages over ground-based observations, is maintained up to the launch of JWST.
R. J. Bouwens, G. D. Illingworth, P. A. Oesch, M. Stiavelli, P. van Dokkum, M. Trenti, D. Magee, I. Labbe, M. Franx, & M. Carollo (2009). z~8 galaxies from ultra-deep WFC3/IR Observations over the HUDF ApJL arXiv: 0909.1803v1
P. A. Oesch, R. J. Bouwens, G. D. Illingworth, C. M. Carollo, M. Franx, I. Labbe, D. Magee, M. Stiavelli, M. Trenti, & P. G. van Dokkum (2009). z~7 Galaxies in the HUDF: First Epoch WFC3/IR Results ApJL (submitted) arXiv: 0909.1806v1
Bouwens, R., & Illingworth, G. (2006). Rapid evolution of the most luminous galaxies during the first 900 million years Nature, 443 (7108), 189-192 DOI: 10.1038/nature05156