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Most Distant Galaxy Known: over 95% of the way back to the origin


Recently, a team of astronomers from the U.S., U.K. and The Netherlands have confirmed the most distant galaxy known. This galaxy had previously been estimated to have a redshift of z = 8.57, from photometric methods, that is, from the general shape of the spectrum.

EGSY8p7-a

Image: Hubble Space Telescope, NASA/STScI

More accurate redshifts are obtained by measuring particular emission or absorption lines, which have precisely known laboratory (z = 0) wavelengths.

The team measured Lyman alpha line emission, and have determined the redshift to be z = 8.68, in good agreement with the photometric redshift. The Lyman alpha line is a main transition line in neutral hydrogen that occurs at 1216 Angstroms (.1216 microns) in the rest frame. The authors observed the line in the infrared and centered at 11,776 Angstroms (1.1776 microns) on 2 separate observing nights, detecting the Lyman alpha line each night. The redshift is given by 1 + z = 11,776/1216 = 9.68, thus z for this galaxy is 8.68.

The galaxy image is thought to be somewhat magnified by intervening dark matter gravitational lensing, but less than a factor of 2, and perhaps only around 20%.

The significance here is in the detection of Lyman alpha at such a high redshift, corresponding to a time when the universe was only 600 million years old, less than 5% of its current age. Not only does this result determine the age of this earliest known galaxy, but it also provides insight into the nature of the intergalactic medium.

The cosmic microwave background radiation is the most distant source we can see. It comes from all directions, filling the universe and reflects a time when the universe was only 380,000 years old and transitioned from ionized plasma to neutral hydrogen and helium.

Later on in the universe’s evolution, as the first galaxies and stars form, hot blue stars produce ionizing ultraviolet radiation, and the neutral gas is reionized – electrons are stripped from their atoms. This process has generally thought to have completed by redshift ~ 6, at a time when the universe was around 1 billion years old.

Lyman alpha emission is not expected in a region which is still neutral, that has not yet undergone the reionization process. So the implication here is that the surrounding intergalactic medium in the neighborhood of EGSY8p7 has already been reionized at a significantly higher redshift.

The universe does not become reionized in a uniform way, rather the process would be expected to happen in “bubbles” or regions surrounding energetic galaxies with hot blue stellar populations. Eventually all the ionized regions overlap and the intergalactic medium becomes fully ionized.

This detection helps astronomers to better understand how reionization occurred.

The team’s paper is submitted to the Astrophysical Journal Letters and can be found here:

http://arxiv.org/pdf/1507.02679v2.pdf

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2 responses to “Most Distant Galaxy Known: over 95% of the way back to the origin

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