UDFy-38135539

Coordinates: 3^h 32^m 38.13^s, −27° 45′ 53.9″

UDFy-38135539
Hubble Space Telescope image of UDFy-38135539.
Observation data
Constellation	Fornax
Right ascension	03h 32m 38.13s[1]
Declination	-27° 45′ 53.9″[1]
Redshift	8.55[2]
Distance	13.1 billion light-years (light travel distance)[2] ~30 billion light-years (present comoving distance)[3]
Apparent magnitude (V)	V fainter than 30.2[4] H160 = 28.1[1] (detected by HST in J and H bands)[1]
Other designations
HUDF.YD3
See also: Galaxy, List of galaxies

UDFy-38135539 (also known as "HUDF.YD3") is the Hubble Ultra Deep Field (UDF) identifier for a galaxy which has been calculated (as of October 2010^[update]) to have a light travel time of 13.1 billion years^[2] with a present comoving distance of around 30 billion light-years. The galaxy is suspected to be the second most distant object yet identified after UDFj-39546284, though UDFy-38135539 is still the most distant object spectroscopically confirmed.^[5]

It was discovered by three teams in September 2009 in sensitive infrared Hubble Space Telescope images (source UDF-38135539 in Rychard Bouwens et al.,^[1] source HUDF.YD3 in Andrew Bunker et al.^[6] and source 1721 in Ross McLure et al.^[7]), and reported in the Astrophysical Journal, and the Monthly Notices of the Royal Astronomical Society. These teams independently identified this source as a likely extremely distant galaxy due to it having no measurable light at visible wavelengths, by reason of absorption by hydrogen gas along the line of sight. Following the discovery of this candidate distant galaxy, another team targeted this object with ground-based spectroscopy to confirm its distant nature, and the measurement of a redshift is formally detailed in the 21 October 2010 article "Spectroscopic Confirmation of a Galaxy at Redshift z=8.6"^[4] in the journal Nature, authored by an international team of colleagues comprising Matthew Lehnert, Nicole Nesvadba, Mark Swinbank, Jean-Gabriel Cuby, Simon Morris, Benjamin Clement, C. J. Evans, M.N. Bremer, and Stephane Basa.

Contents 1 Detection 2 Characteristics 3 Significance 4 Subsequent discoveries 5 See also 6 References

Detection

The galaxy's image was first captured in the Hubble Ultra Deep Field, the most detailed deep space picture ever taken by the Hubble telescope, in August and September 2009.^[2][8] The image data was released to the scientific community, which led to the galaxy's detection by the teams of Bouwens,^[1] Bunker^[6] and McLure,^[7] and subsequent spectroscopic confirmation by the team of Lehnert and colleagues.^[9]

The faster that a galaxy is moving away from an observer, the more the light is skewed towards longer, redder wavelengths by the intervening expansion of the universe. This phenomenon is known as redshift, and the greater the redshift observed on Earth, the more distant the source of light is. As seen from the Hubble photo, the galaxy could have possibly been an object intrinsically red and relatively close to Earth,^[2] and therefore, confirmation using suitably sensitive spectroscopic equipment was needed.^[9] This was possible using the European Southern Observatory's SINFONI-equipped Very Large Telescope unit Yepun, located atop Cerro Paranal in Chile's Atacama Desert.^{[2][8][10][11]} Lehnert's team observed the galaxy for 16 hours, and then analysed their results over 2 months,^[9] and published their findings in Nature, in October 2010.^[4]

Characteristics

The galaxy is located in the constellation Fornax, and is estimated to have contained roughly a billion stars,^[12] although it was only at most one tenth of the diameter of our own galaxy, the Milky Way, and had less than 1% of the mass of the Milky Way's stars. According to Lehnert (of the Observatoire de Paris), it was forming the same number of stars per year as our galaxy, but they were much smaller and less massive, making it "intensely star forming".^[13] The team's analysis determined that light from the galaxy has a redshift of 8.55. For comparison, light from the previous record-holder for most distant object, the gamma-ray burst GRB 090423, has a redshift of 8.2. Light from the galaxy that we now observe on Earth was emitted 13.1 billion years ago, only 600 million years after the estimated age of the Big Bang,^[14] when the universe was only 4% of its current age.^[9]

The light travel distance of the light that we observe from UDFy-38135539 is more than 4 billion parsecs^[12] (13.1 billion light years), and it has a luminosity distance of 86.9 billion parsecs (about 283 billion light years).^[15] There are a number of different distance measures in cosmology, and both "light travel distance" and "luminosity distance" are different from the comoving distance or "proper distance" generally used in defining the size of the observable universe^[16][17] (comoving distance and proper distance are defined to be equal at the present cosmological time, so they can be used interchangeably when talking about the distance to an object at present, but proper distance increases with time due to the expansion of the universe, and is the distance used in Hubble's law; see Uses of the proper distance for more on the physical meaning of this notion of 'distance'). The luminosity distance is related to a factor called the "comoving transverse distance" by the equation , where z is the redshift, and the comoving transverse distance is itself equal to the radial comoving distance (i.e., comoving distance between an object and ourselves) in a spatially flat universe.^[18][19] So with = 86.9 billion parsecs and z=8.55, the comoving distance would be about 9.1 billion parsecs (about 30 billion light years).^[3]

The infrared light that we now observe from the galaxy was emitted as ultraviolet radiation toward the end of an era when the universe was filled with atomic hydrogen, which absorbed at ultraviolet wavelengths. Because the galaxy's own light alone would not have been intense enough to ionize a large region and render it transparent, scientists suspect that a population of smaller, undetected galaxies, contributed to the reionization that makes UDFy-38135539 visible.^[9]

Significance

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 reionization. Theoretical models and computer simulations suggest the first galaxies could have formed as early as 200 million years after the Big Bang.^[20]

The discovery makes UDFy-38135539 the first known galaxy observed during the reionization epoch, and those involved believe it will help scientists better understand the era.^[2] Caltech astronomer Brant Robertson, commenting on the study, stated that the "galaxy happens to reside at a very special time in cosmic history when the properties of gas in the universe were changing rapidly, and therefore this galaxy and others like it may teach us a lot about the early history of the universe".^[13] Michele Trenti, an astronomer who was not involved in the study but provided commentary published with the report, says that the discovery of the distant galaxy represents a "fundamental leap forward in observational cosmology".^[13]

Subsequent discoveries

Scientists hope to find older galaxies; however, closer to the Big Bang, fewer exist and they are dimmer on average. They will therefore be increasingly difficult to find, since they would be very faint with fewer observable stars.^[20] Trenti says that new "most distant" record holders will soon be announced, but only incremental distance gains will be realized until NASA's James Webb Space Telescope becomes operational in 2018.

The James Webb telescope should be able to detect galaxies more than 13.4 billion light years away, less than 300 million years after the Big Bang. Bremer states that it, and eventually the European Extremely Large Telescope, which will have a mirror five times the diameter of Yepun's,^[11] and is tentatively scheduled for completion in 2018, will enable more detailed study of galaxies at such great distances.^[21] Lehnert states that this discovery is not "the limit, perhaps not even that close to it".

Trenti says redshift 8.6 is likely to be as high as we can reach with the current generation of telescopes, but that with Hubble, "it might be possible to find some galaxies up to redshift 10".^[20] Candidates with higher redshifts than UDFy-38135539's have been reported, but not yet confirmed with light spectrum instruments.^[2] Astronomers believe they have other candidates of similar distance which they hope to confirm soon.^[11]

See also

Other known most distant objects

• GRB 090423 is a gamma ray burst, which previously held the record for most distant object.
• IOK-1 is the previous record holder for the commonly accepted most distant galaxy.
• The methods used to determine the distances to very distant cosmic objects are described in the "Cosmic distance ladder"

References

Preceded by GRB 090423	Most distant astronomical object 2010 — 2011	Succeeded by UDFj-39546284
Preceded by IOK-1	Most distant galaxy 2010 — 2011	Succeeded by UDFj-39546284