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The Discovery Of UDFy-38135539: Z ~ 8 In The Hubble Ultra-Deep Field


Located in the constellation Fornax, UDFy-38135539 (a.k.a “HUDF.YD3”) is the furthest confirmed object we know of in the Universe (a title that won’t probably last long). Calculated to have a light travel time of 13.1 billion years (Bouwens et al. 2009; Lehnert et al. 2010), UDFy-38135539 lies a little over 4 billion parsecs away, corresponding to a redshift* of z ~ 8.6; UDFy-38135539 z = 8.5549±0.0002 (Lehnert et al. 2010), when the Universe was a mere 600 million years old; residing in a part of the Universe which provides scant evidence for observational astronomers of the early Universe. Previously, the highest spectroscopically confirmed redshift was measured to be z = 6.96 (Iye et al. 2006), where prior Bouwens et al.’s (2009) study only two or three convincing candidates at redshifts just greater than z = 7 existed (see Bouwens et al. 2008; Bradley et al. 2008; Richard et al. 2008; Oesch et al. 2009a).

N.B. Redshift (and blue shift) may be defined as the relative difference between the observed and emitted wavelengths (or frequency) of light emitted by an object. The emitted wavelengths are calibrated via laboratory observations. In astronomy, it is customary to refer to this change using a dimensionless quantity z. If λ represents the wavelength (where f represents the frequency: λf = c where c is the speed of light), then z is defined by: .

It could be argued that galaxies in the early Universe made a significant impact on our current astronomical vistas, when they assembled their first generations of stars (Lehnert et al. 2010). Energetic photons emitted by young, massive stars (Population III stars) in primordial galaxies ionized the intergalactic medium surrounding their host galaxies, in the process clearing sight-lines along which the light of the young galaxies could escape. This process fundamentally altered the physical state of the intergalactic gas in the Universe continuously until the present day (Mesinger & Furlanetto 2007; Choudhury et al. 2009).

The importance of UDFy-38135539 not only comes with its distance. But with what it can tell astronomers about the early epochs of the Universe. The galaxy happens to reside at a very special time in cosmic history when the properties of gas in the universe were changing rapidly – the so-called “age of recombination” – and therefore this galaxy and others like it may teach us a lot about the early history of the universe.

Observations of the cosmic microwave background (CMB; […/scott/cmb_intro.html]), and of galaxies and quasars at the highest redshifts, suggest that the Universe was re-ionised through a complex process that was completed about a billion years after the Big Bang, by redshift z = 6 (Fan et al. 2006; Komatsu et al. 2010). Detecting ionizing Lyman-alpha (Ly-α; []) photons from increasingly distant galaxies places important constraints on the timing, location and nature of the sources responsible for re-ionisation. The detection of Ly-α photons emitted from UDFy-38135539 are a milestone, the first of such example at such extreme distances in the Universe i.e. less than 600 million years after the Big Bang.

The pre-stellar period that followed recombination is referred to as the Dark Ages. Although it was optically transparent, absorption by neutral atomic hydrogen made it opaque to ultraviolet (UV) radiation. The period of star birth that followed initiated the reionization epoch: the universe’s first stars were massive, and their intense ultraviolet radiation ionized hydrogen, eventually filling space with a UV-transparent plasma.

The UV-transparent “bubble” that surrounded UDFy-38135539 shows that, 600 million years after the Big Bang, stars in galaxies had almost completed the process of hydrogen re-ionisation. Current theoretical models and computer simulations suggest the first galaxies could have formed as early as 200 million years after the Big Bang. The discovery makes UDFy-38135539 the first known galaxy observed during the re-ionization epoch, and those involved believe it will help scientists better understand the era.

Journal References:

  • Lehnert, M.D. et al. (2010) Spectroscopic Confirmation Of A Galaxy At Redshift Z = 8.6Nature467 (7318) pp. 940-942.
  • Komatsu, E. et al (2011) Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation. The Astrophysical Journal: Supplement, 192 (2) Article I.D.: 18.
  • Trenti, M et al. (2011) The Brightest of Reionizing Galaxies Survey: Design & Preliminary ResultsThe Astrophysical Journal: Letters727 (2) Article I.D.: L39.

Suggested Further Reading:

  • Brandenberger, R.H. (2010) Cosmology Of The Very Early UniverseMcGill University, HETC Lecture Series-2010/3.

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