In what is to me the most scientifically important paper of the year, astronomers today announced the discovery of 2 galaxies at redshifts > 10 and 4 galaxies with redshifts >7.7. The most distant of these galaxies was forming stars just 500,000,000 years after the Big Bang, and contributed to the re-ionizing the universe after the formation of neutral hydrogen. These galaxies were discovered using the Hubble Space Telescope and Keck II, and by taking advantage of gravitational lensing effects in three galaxy clusters. (Figure shows a selection of Hubble Space Telescope images of the cluster fields with the newly-located sources marked. credit: Stark et. al / STScI / ESA)
That was a lot of exciting information, and now that I’ve gotten some of the excitement out of my system, let me step back and tell you exactly what it means.
At time T = 0, the universe formed. Poof, we have the the Big Bang rumbles the universe into existence.
Over the course of almost 400 millennia, the universe evolved from pure energy, to atomic nuclei, to full atoms with electrons. At that last, wonderful, electron grabbing moment, at T + 372,000 years the Cosmic Microwave Background came into existence.
And then the universe went dark. If you have ever looked at an image of a dark molecular cloud, you have seen what the entire universe looked like after the electrons and nuclei got together to form neutral hydrogen and helium (with a bit of lithium and beryllium). There was nothing generating light, and astronomers refer to this time as the cosmic dark ages.
Then, at some time we are still determining, the first stars turned on, and as they heated the gas around them the universe began to glow. Have you seen images of star forming proplyds in the dense gas of the Orion nebula? This is what the early uiverse may have looked like, as the hot stars heated and ionized the neutral gas. We call this the epoch of re-ionization.
As of today, we know the epoch of re-ionization occurred no later than T+500,000,000 years.
Astronomers Daniel P. Stark, Richard S. Ellis, Johan Richard, Jean-Paul Kneib, Graham P. Smith, and Michael R. Santos used the Hubble Space Telescope and Keck II to discover these galaxies using gravitational lensing. (There is a neat collection of lead up paers here and this work occurs in the July 1 Astrophysical Journal.) Every distribution of mass has specific regions where objects appearing in those regions may be extraordinarily magnified by gravitational lensing. This team identified the regions of greatest potential gravitational lensing around 3 different galaxy clusters and looked specifically in those regions for extremely high redshifted objects. Specifically, they looked for the atomic line Lyman alpha that is strong in star forming galaxies. This line is created by electrons in hydrogen transitioning from the lowest energy shell (n=1) to the second lowest (n=2) energy shell. Normally, this line has an ultra-violet wavelength of 121.6 nanometers. The expansion of the universe, however, causes this line to get doppler shifted to the red. In the case of these extremely distant galaxies (z=8.5-10.4), the Lyman alpha line is shifted all the way to the infrared wavelength band, and appears between 1143 – 1375 nanometers.
To find this specific atomic transition, they performed spectroscopy with NIRSPEC on the 10m Keck II telescope with the slit of the spectroscope aligned along the line of maximum gravitational lensing in a cluster. Over 1 hour of telescope time went into every final spectra, and 2-5 different final spectra were obtained for each of 9 clusters. All told, 35 different telescope pointing were used and roughly 0.3 square arcminutes of sky — about the size of a decent but small crater on the moon — was observed. In this small area of sky, 6 low luminosity galaxies (fainter than Andromeda would be at the same distance) were discovered in 3 different clusters (Abell 68, Abell 1689, and Abell 2219).
Follow-up images and spectra were obtained with the Hubble Space Telescope and Keck II to confirm these were real results. “As with all work at the frontiers, skeptics may wish to see further proof that the objects we are detecting with Keck are really so distant,” confessed Ellis. However, in addition to numerous checks the team has made(described in their published scientific article, which gives 5-8 sigma confidence), there are also images of galaxies from when the universe was only a billion years old that show extremely old stellar populations. Those stars would have had to have formed at extremely high redshifts.
While 6 galaxies may not seem like a huge number, finding this many galaxies in so small an area indicates that if there is an even distribution of these systems, then there are enough of them to light up the universe and end the dark ages. So, for those of you afraid of the dark, avoid times T+373,000 to T+~500,000,000, and you’ll always have light from space to light your way.
In the coming years, planned telescopes such as the James Webb Space Telescope, and earth bound behemoths like the CalTech 30 meter will have the capabilities of seeing systems like these without relying on gravitational lenses to boost the light we can see. With in the next couple decades, astronomy may be able to push our observationally based understanding of the universe all the way back to the very first stars.
And thanks to creative thinking, hard work, and excellent use of resources, Stark and his collaborators are giving us a glimpse of what we might learn about early universe today (T+13.7 billion years and counting…)
To learn more about the Keck Telescopes, check out the Keck podcast.