What is the oldest known object in the Universe?
This question seems to be very simple, but in fact is as complex and dangerous as only a cosmological problem can be! First of all it requires a very careful defining of almost every word included: “is”, “the oldest”, “known”, “object” and finally “in the Universe”. Let’s start.
Abell 2744 – “Pandora’s Cluster” – Composite image (Source: NASA) Credit: X-ray: NASA/CXC/ITA/INAF/J. Merten et al. Lensing: NASA/STScI; NAOJ/Subaru; ESO/VLT Optical: NASA/STScI/R. Dupke
For “is” we can assume “existing now or existing in any particular time in the history of the Universe”. This is very important. As far as we know the most distant and, so, very old objects we can observe were born a long, long time ago, shortly after the Big Bang. The problem is that we can see their light, or we can detect another interaction with visible matter, but for most cases we are not sure these bodies are still somewhere there, in the moment of time when we consider their existence. So it is convenient to adopt that “existence” means here only “occurring at some space-time coordinates”. Or that any object we can observe is real. For the same reason I assume that “the oldest” means “the one that occurred the earliest in the cosmic history”, without further wondering if the body we can observe NOW as it WAS billions years ago was really the oldest at the time when it started to radiate.
By “known” I consider as much as “observed”. I am not taking into account any hypothetical objects, even if they are strongly supported in any reliable theories. So, assume that to know is to see.
The term “object” may be the hardest of all. According to common sense “the object” can be everything that is observed in isolation and has some particular features like mass, spin, or dimensions. On the other hand this review should not be ended with the startling conclusion that the most aged things in the Universe are the original hydrogen atom or first electrons, or the cosmic microwave background radiation (CMBR) that occurred shortly after the recombination epoch when photons started to travel freely through space. One can prove that the current proper distance to particles from the CMBR, which represent the radius of the visible universe, is about 14.0 billion parsecs, making them very aged phenomena. But it will be more interesting to accept that the “object” rather has to be any complex celestial body, like star, or galaxy, or a group of galaxies.
The last problem is defining “the Universe”. The definition is now almost intuitive: let’s adopt entire observable “world” composed of space and objects like galaxies and bigger structures. They can be observed now from Earth only because light and other signals from those objects has had time to reach us since the beginning of the cosmological expansion. So we can consider a cosmological horizon – measure of the distance from which we can retrieve information. This idea sets up the size of the observable Universe. It is just the representation of the largest current distance from which light could have reached the actual observer by a specific time. The main problem is that the Universe is expanding all the time from the Big Bang, and the rate of this expansion is probably higher than the speed of light. This implies that there could be very old celestial bodies which had emitted their light very long time ago, but it was too long ago for this radiation to reach us due to the faster space expansion. So once again – the oldest existing cosmological object has to be something very tangible, something we are able to observe now.
There were a lot of good candidates for the oldest cosmic object. Few years ago astronomers announced the discovery of extremely distant example of high-energy phenomenon known as Gamma Ray Burst. But later, in 2010, another interesting discovery took place: it was so-called Hubble Ultra Deep Field galaxy named UDFy-38135539. Its light travel time was estimated to 13.7 billion years, and its actual distance to us is around 30 billion light-years. For a while it was also the most distant spectroscopically confirmed object. But by the end of 2012 scientists found new candidates for even more distant celestial bodies.
Todays’ list of the potential candidates is long but full of uncertainties. It does not satisfy particular standards for completeness. It consists of Gamma-ray burst/progenitor, galaxies, protogalaxies, and a quasar. However, its members with the highest redshifts (and, so, distances and ages) are often unconfirmed objects. The oldest confirmed galaxy, z8_GND_5296, has redshift z≅7.51 and corresponding light-travel distance about 13 Gly (billion light-years). It was discovered by the team led by Steven Finkelstein from Texas University and published in the Oct. 24 (2013) issue of the famous journal Nature. z8_GND_5296 is now seen as it was at a time of 700 million years after the Big Bang. Such discovery is not only giving the clue about the evolution of galaxies themselves, but also lots of information about conditions governing the Universe when it was only at about 5 percent of its current age.
At first scientists looked for extremely distant galaxies based on their colors extracted from the Hubble Space Telescope images. However, to measure the distance more precisely, it is better to use spectroscopy and determine redshift – the shift of a galaxy’s light at a specific wavelengths toward the red end of the spectrum, resulting from the extending path of light traveling from object to Earth due to the expansion of the Universe. Typical galaxies have redshift below 1, and even more distant so-called quasars have typical redshifts no higher than 2 – 3. To measure the redshift of z8_GND_5296 team used the high-resolution spectra from one or the largest modern optical telescopes, Keck I (Hawaii). Its extreme value had been confirmed by measuring the shift of Lyman-alpha emission line of hydrogen, which is relatively easy to detect in very distant galaxies.
What the oldest galaxy looks like now? We cannot be sure. But in its observable timeframe it was producing stars at a very high rate of about 300 sun masses per year. It is located in the constellation of Ursa Major, but can’t be observed with a naked eye. Its present distance from Earth is equal to ~30 billion light-years. The galaxy has its close neighbor galaxy that is also classified as very distant galaxy with a redshift ~ 7.2.
It is worth noting that scientist have identified many other candidates for extremely old galaxies, including some that might be even more distant, and z8_GND_5296 is only special because its distance was definitively confirmed with follow-up observations. In 2012, the best but-not-yet confirmed candidate for the oldest object in the Universe was UDFj-39546284. It is a galaxy or protogalaxy with z of 11.9 (estimated based on Hubble and Spitzer telescope data) and is now observed as it was 13.42 billion years ago, around 380 million years after the Big Bang (which is believed to date back to 13.8 billion years ago).
Author: Elzbieta Kuligowska
Reference & further reading:
Hubble Finds Granddaddy of Ancient Galaxies (Time)
Texas Astronomer Discovers Most Distant Known Galaxy (UT)
NASA Telescopes Spy Ultra-Distant Galaxy (NASA)
Oldest Object in Universe Found (Discovery)
List of the most distant astronomical objects (Wikipedia)