When telescopes get really, really bigPosted: October 19, 2009
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In our first post exploring galaxy evolution, we saw how observing galaxies at different distances from us is crucial for our understanding of how galaxies form and evolve. It also naturally follows that the larger the range of distances we can study, the better we can constrain our theories. So it’s only natural that astronomers have always been hunting for the most distant galaxies – it’s a sort of high-flying game in the astronomy community, and breaking the record for the most distant galaxy observed is no mean feat.
More distant objects appear on average fainter, and they are harder to detect. So traditionally one looks at technological improvements in order to make advancements in this area. For example, a larger telescope has a wider light-collecting area. Therefore it’s more sensitive, and is able to detect fainter objects in a given time. One can also observe a region of sky for a longer period of time, which again increases the number of photons that we collect. Astronomers call this deep imaging.
Recently, the public release of very deep imaging from the Hubble space telescope‘s new Wide Field Camera 3 generated a rush of papers which were precisely looking for very high-redshift galaxies. Look for example at Bunker et al., McLure et al., Oesch et al., among others, which were mostly submitted within a few hours of each other, and just a few days after the data was publicly released – astronomy doesn’t get much more immediately competitive (or stressful?) than this! The work of these particular papers requires not only deep imaging, but also a wide range in terms of electromagnetic spectrum – i.e., they need sensitive images of the same region of the sky in different colours, and the redder the better.
These papers detected galaxies at redshifts between 7 and 8.5. Or, in more common units, these galaxies are at least 12,900,000,000 light-years away. That means the light that was detected by the Hubble space telescope, and on which these papers and scientific analysis are based, left those galaxies 12,9000,000,000 years ago. The Universe was only around 778,000,000 years old by then. To go back to our previous analogy, it’s like looking at a snapshot of me when I was less than 2 years old. Admittedly I had bathed by then, but that’s still very very young!
These papers are interesting and important in their own right, but what prompted me to come and tell you all of this was actually the work of Bradac et al., which has the same goal as the above, it uses the same basic techniques as the above, but it cheats. And it’s the way it cheats that makes it really rather neat.
Bradac and co-authors use not only human-made telescopes, but also harvest the power of gravitational lensing to turn a galaxy cluster (the Bullet Cluster) into an enormous cosmic telescope. We have covered here before how mass affects space which in turn affects the way light travels. Matter can act to focus light from distant objects – and galaxy clusters have a lot of matter. This makes them rich and exciting playgrounds for astronomers who have now long used gravitational lensing to probe the distant Universe.
Bradac and friends have additionally showed just how effective it can be at measuring the density and properties of distant galaxies, and how much there is to gain from a given image (with a given sensitivity limit) when there is a strong and appropriately focused cosmic lens in the field of view. Distant galaxies are also magnified – their angular size on the sky increases, compared to an unlensed image – and that allows us to look at them in more detail, and study their properties. As a bonus, given that cosmic telescopes often produce more than one lensed image of any one given galaxy, they can use these multiple images to help with the distance measurement and avoid some contamination.
They didn’t set any distance records as the wavelength range of their imaging wasn’t quite right for that. But with the right imaging, the right clusters and the right analysis, the authors argue that this is the way forward for this sort of study. Galaxy evolution is hard and full of technical challenges, so using galaxy clusters as gigantic telescopes can certainly go a long long way.
M. Bradač, T. Treu, D. Applegate, A. H. Gonzalez, D. Clowe, W. Forman, C. Jones, P. Marshall, P. Schneider, & D. Zaritsky (2009). Focusing Cosmic Telescopes: Exploring Redshift z~5-6 Galaxies with the Bullet Cluster 1E0657-56 Accepted for publication in ApJL arXiv: 0910.2708v1
R. J. McLure, J. S. Dunlop, M. Cirasuolo, A. M. Koekemoer, E. Sabbi, D. P. Stark, T. A. Targett, & R. S. Ellis (2009). Galaxies at z = 6 – 9 from the WFC3/IR imaging of the HUDF Submitted to MNRAS arXiv: 0909.2437v1
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 submitted to ApJL arXiv: 0909.1806v1
Andrew Bunker, Stephen Wilkins, Richard Ellis, Daniel Stark, Silvio Lorenzoni, Kuenley Chiu, Mark Lacy, Matt Jarvis, & Samantha Hickey (2009). The Contribution of High Redshift Galaxies to Cosmic Reionization: New
Results from Deep WFC3 Imaging of the Hubble Ultra Deep Field Submitted to MNRAS arXiv: 0909.2255v2