Redshift 7.5 galaxy – perhaps

Observing the most distant galaxies provides vital information about the first epoch of galaxy formation. These observations are difficult, and the interpretation of results can be, well, debatable. Some studies (e.g. Ellis et al. 2013 using the Hubble Ultra Deep Field) use the flux (or lack of) of the most distant galaxies in several different filter bandpasses to detect candidate high redshift galaxies. Secure detections and redshifts of such galaxies require spectroscopy.

Shibbuya et al. (2012) used the Subaru Suprime-Cam to image four candidates, one of which has a secure emission line, and if it is Lyman-α (which is a hydrogen emission with a rest wavelength at 1216Å ) suggests a galaxy at z=7.215 with a star formation rate (SFR) of 11 solar masses per year.

Such objects not only tell us about when galaxies first formed, but also probe the surrounding intergalactic medium (IGM). In the very early universe the (hydrogen) IGM undergoes a phase transition – from neutral to ionised form – due to the first appearance of ionising stars and thus galaxies. Current studies suggest this transition occurs from z~11 to z~6, when it is complete. Lyman-α photons are sensitive to neutral hydrogen IGM abundances and hence detecting this emission line in 6 < z < 11 galaxies (called Lyman Alpha Emitters or LAEs) can tell us when this phase transition or cosmic reionisation epoch ended (and perhaps began). The exact details of how the IGM becomes ionised is not presently known.

In a recently published paper in Nature, Finkelstein et al. (2013) have proposed a galaxy, z8_GND_5296 to have a redshift of z=7.5078±0.0004.

New Hubble CANDELS image of most distant known galaxy z8_GND_5296. (Credit: NASA, ESA, V. Tilvi (Texas A&M University), S. Finkelstein (University of Texas, Austin), and C. Papovich (Texas A&M University))

The redshift is based on the identification of a high S/N emission line at 1.03 μm, which if due to hydrogen Lyman-α emission would make this the furthest spectroscopically confirmed galaxy (not including GRBs), seen some 700 million years after the Big Bang.

There is some debate over the reality of this redshift detection and some properties of the galaxy and the selection method are of interest. Forty-three photometrically-selected galaxies were surveyed, with only one, z8_GND_5296, displaying an emission line. The authors suggest that six of the survey galaxies should have had emission lines. To explain this (in line with previous lower redshift galaxies) the neutral IGM would need to rapidly increase from z=6 to z~7, yet this is not consistent with other observations that have low IGM fractions at z=7. As well the inferred SFR of z8_GND_5296 of 300 solar masses per year is a factor 30 higher than expected at z~7.

Such properties may be suggesting that high SFR is a necessary condition for Lyman-α emission in very distant galaxies, which may inhabit intergalactic space with higher than expected IGM fractions. The study of cosmic reionisation needs such information to determine how quickly UV photons from ionisation sources promote this phase transition.

Volume rendering of the neutral hydrogen density (opaque = neutral) plus galaxies (shown as yellow dots) at z=7.3. (Credit: Nick Gredin, U. Chicago)

What we need is … more data!! It is difficult to draw strong conclusions from very small sample sizes. However, just detecting a galaxy like z8_GND_5296 suggests important information on galaxy formation in general, as Riechers (2013) suggests in the accompanying News & Views article in Nature.

“The study further shows that even galaxies observed at a time when the Universe had reached only 5% of its current age may already be chemically enriched with dust and heavy elements (those heavier than hydrogen and helium), which must have been produced by an earlier generation of stars. Heavy elements such as carbon, nitrogen and oxygen produce strong emission lines, which the James Webb Space Telescope (JWST) will be able to detect with relative ease in galaxies such as z8_GND_5296, after its launch towards the end of the decade”, says Dominik A. Riechers.

For more information, see

[Glen Mackie]

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