Reionization Zone Galaxy Reveals Highly Evolved Characteristics
Using a multi-instrument methodology, Danish astronomers have discovered a young galaxy in a surprisingly advanced evolutionary state. Situated on the edge of observable space, deep within the ‘epoch of reionization’ (EOR), A1689-zD1 has a calculated redshift of z ≈ 7.5 (VLT X-Shooter spectrograph), making it one of the furthest objects ever scrutinised. (see below)
Due to the vast cosmic distances light has to travel, coupled with their low intrinsic luminosities, high red-shift galaxies are notoriously difficult to locate and study. This physical limitation can be highly problematic for astronomers; however, A1689-zD1 lies directly behind a lensing cluster, Abell 1689, therefore providing a nine-fold magnification: ‘because of the gravitational amplification, [A1689-zD1] is one of the brightest candidate z > 7 galaxies known’, say the team.
Originally identified in 2008 (Hubble/Spitzer), researchers were able to expand their initial restframe-UV imaging analysis due to the galaxy’s rich composition, using ALMA and ESO’s Very Large Telescope.
To date, ‘characterising’ deep field objects within the UV range has proved problematic, although as researchers point out: ‘The detailed properties of these galaxies could be measured from dust and cool gas emission at far-infrared wavelengths.’ However, ‘galaxies have to become sufficiently enriched in dust and metal’ for this to be a realistic proposition.
For young EOR galaxies, such extensive enrichment—provided by supernovae activity—was thought highly unlikely, although the team report a total star formation rate ≈ 12 M⊙y-1 (solar masses per year), a dust mass of 4 x 107 M⊙ and a dust-to-gas ratio of 0.0017, making A1689-zD1 physically comparable to much older galactic structures like the Milky Way—‘Instead of a young, dust-poor galaxy, these measurements suggest an evolved system.’
Time is too early to suggest whether dust rich, high metallicity, galaxies are typical during these early comsmological periods; however, in light of this latest discovery, researchers are positive: ‘though it is magnified by a factor of 9.3, it was detected in only brief observations with ALMA. This promises a reasonable detection rate for L* galaxies in unlensed fields at these redshifts.’ Further research should help ascertain vital mechanisms required for such rapid, post Big Bang, evolution.
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