Monday, February 15, 2010

Far out!

If you're interested in something out of the ordinary, astronomically speaking, the best place to look for the exotic may be as far away (in both space and time) as possible.

Perhaps that's why I like to consider really far out stuff, like the most distant gamma-ray burst seen yet. Or maybe I just like to get away from the depressing chaos and confusion of "modern" life.

In any case, there's always something new, just beyond the farthest thing we've seen yet. That far-out gamma-ray burst (GRB 090423) discussed in the post just linked – which resides at z~8.2, about 13 billion light-years away – has already been superseded in remoteness by 3 galaxies around z~10, 13.2 billion light-years away. z~10 objects are seen as they were only about 480 million years after the big bang. (See here for a refresher on how redshift works.)

How are high-z objects actually detected? It's surprisingly easy, in principle, even though astronomers are working at the outer limits of their instruments. The term of art for the technique used is "Lyman-break" detection, which we've discussed in detail here.

Here's the short summary of that technique. We assume that the objects of interest are galaxies composed of stars, instead of something really strange. (GRBs can be ruled out, since they appearance is very brief, and gamma-rays are far outside the range of emissions produced by stars, even given substantial redshifts.) The thing is that even the hottest stars produce relatively little output (such as x-rays) on the high-energy side of the part of the ultraviolet spectrum known as the "Lyman limit", which has a wavelength of 91.1 nm. And almost all higher energy photons will be absorbed by the interstellar medium anyhow.

At z=10, the shift factor (z+1) is 11, yielding a wavelength of 1002 nm – i. e. about 1 micron, which is in the infrared. So an object that's really at z~10 will not have any observable light to the blue side (shorter wavelength) of 1002 nm.

The main instrument used in the recently refurbished Hubble telescope to detect the high-redshift galaxies is known as the Wide Field Camera 3 (WFC3). It was installed in May 2009, and in August it did the infrared imaging on which the research described here is based. As noted, the research team identified 3 objects at z~10, using WFC3. Two of those objects were also captured in an image of the same area, using Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS).

WFC3 has three filters covering near-infrared wavelengths. The light from a galaxy at z~10 will be visible through the filter that passes only longer infrared photons, but not through the other two filters that pass only shorter (bluer) photons. So z~10 galaxies will stand out on account of their absence from images made using the shorter-wave filters.

The neat thing about this technique is that it does not require collecting a somewhat complete spectrum, which is a much more difficult feat. In principle, all sufficiently luminous z~10 galaxies should be caught. The problem is false positives – things that aren't really z~10 galaxies, such as dim reddish galaxies that are actually much closer (z<3), or even dim nearby stars.

The research that reports on this has used careful statistical tests to rule out false positives. The analysis in fact suggests that a slightly earlier study that claimed to find 20 z~10 galaxies could be all false positives.

This is the study we're discussing now:

Constraints on the First Galaxies: z~10 Galaxy Candidates from HST WFC3/IR
The first galaxies likely formed a few hundred million years after the Big Bang. Until recently, it has not been possible to detect galaxies earlier than ~750 million years after the Big Bang. The new HST WFC3/IR camera changed this when the deepest-ever, near-IR image of the universe was obtained with the HUDF09 program. Here we use this image to identify three redshift z~10 galaxy candidates in the heart of the reionization epoch when the universe was just 500 million years old. These would be the highest redshift galaxies yet detected, higher than the recent detection of a GRB at z~8.2. The HUDF09 data previously revealed galaxies at z~7 and z~8. Galaxy stellar population models predict substantial star formation at z>9-10. Verification by direct observation of the existence of galaxies at z~10 is the next step. ... Our z~10 sample suggests that the luminosity function and star formation rate density evolution found at lower redshifts continues to z~10, and pushes back the timescale for early galaxy buildup to z>10, increasing the likely role of galaxies in providing the UV flux needed to reionize the universe.

There was a more serious purpose behind this effort than simply bagging a few more objects at record-breaking distances. The ultimate goal is to understand the sequence of events when the earliest stars and galaxies formed in the early universe, and what characteristics those first stars and galaxies had.

As we discussed here and here, at the time of "recombination", about 380,000 years after the big bang, most of the hydrogen and helium gas in the universe was in the form of neutral (un-ionized) atoms. Much later (relatively speaking), in what is known as the "reionization" period, this gas became ionized again by the intense light from young, very hot stars and galaxies. This probably happened over a period of several hundred million years and was essentially complete by 900 million years after the big bang.

What is not so clear is when the reionization (and hence the first stars) began, or how rapidly it proceeded. The best way to understand this process is to determine the numbers of galaxies per unit volume of space throughout this period, and the intrinsic brightness of these galaxies.

Astronomers have gradually been able to make reliable counts of the number of bright galaxies for z<7, corresponding to ~780 million years after the big bang and later. Of course, many galaxies for even much smaller z are too dim to be visible, but it is possible to count the number of galaxies, having at least a minimum intrinsic brightness, per unit volume of space. There is a relatively simple function that fits the data and describes the number of galaxies we can see in a given patch of sky at different redshift values.

We can see more galaxies at smaller values of z not only because the necessary intrinsic brightness decreases with z, but also because there are actually increasing numbers of galaxies having any given intrinsic brightness as the universe grows older – up to a point. This is simply because galaxies themselves became larger and brighter over time as new stars formed.

Another way to view the accumulated data is in terms of the rate of star formation. The more rapidly stars are forming, the more rapidly galaxies grow to reach any particular intrinsic brightness. When read in this way, the data show that the number of stars being formed per year increased monotonically from as far back as we can tell up to a peak with z between 2 and 3, and that the rate of star formation drops after z~2, 3.3 billion years after the big bang.

However, the actual data, so far, become very sparse for z>6. If stars were forming at z~10 at the same rate as they do at z~6, astronomers should find 20±5 galaxies at z~10. Or, using z~7 as a baseline, there should be 9±3 galaxies at z~10. Instead, there were only 3. (Keep in mind this is all for a patch of sky of the same size.)

What this means is that for z>7, stars were forming at even slower rates. And in fact, if one extrapolates the function for the number of observable galaxies back to z~10, 2 or 3 fits very nicely. Further, if that is the case, then the energy emitted by galaxies at z~10 is only about 13% of what would be needed to completely ionize the interstellar gas. Which means, finally, that the largest part of reionization occurred more than 500 million years after the big bang.

Astronomers would, however, like to know much more than that. Ideally one would like more precisely the rates of star formation during the whole time period beginning with the first stars, perhaps 200 or 300 million years after the big bang. As well as other things, such as the distribution of shapes and sizes of galaxies in that period.

Such information isn't important just for its own sake, either. It will also help astronomers answer other questions, such as: What were the characteristics of the very first stars and galaxies, and how did they form? How was dark matter distributed at that time, and how did that affect the formation of stars and galaxies? What role did black holes play in the formation and evolution of galaxies?

As impressive as the recent upgrades to Hubble's instruments have been, they are still at the limits of their capability. So astronomers are eagerly awaiting the James Webb Space Telescope – a much bigger piece of equipment (with a 6.5 m diameter mirror) than Hubble – now scheduled for launch in 2014.
R. J. Bouwens, G. D. Illingworth, I. Labbe, P. A. Oesch, M. Carollo, M. Trenti, P. G. van Dokkum, M. Franx, M. Stiavelli, V. Gonzalez, & D. Magee (2009). Constraints on the First Galaxies: z~10 Galaxy Candidates from HST
WFC3/IR Nature arXiv: 0912.4263v2

Update: The published paper may be found here: A candidate redshift z ≈ 10 galaxy and rapid changes in that population at an age of 500

Further reading:

New-found Galaxies May Be Farthest Back In Time And Space Yet (1/3/09)

Hubble Spots Oldest Galaxies Yet (1/5/10)

Earliest Known Galaxies Spied in Deep Hubble Picture (1/5/10)

Hubble Reaches the 'Undiscovered Country' of Primeval Galaxies (1/5/10)

Hubble Ultra Deep Field 2009 detects earliest galaxies (1/6/10)

Oldest Galaxies Show Stars Came Together in a Hurry (1/15/10)

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