Astronomer Tomotsugu Goto of the Japan Aerospace Exploration Agency (JAXA) has used the Subaru telescope to identify a distant quasar powered by a massive black hole. The quasar is almost 12.7 billion light-years away from Earth in the direction of the constellation Cancer the Crab. It is the most distant one ever found by a Japanese researcher and the eleventh most distant quasar currently known.
The discovery of the object, called SDSSJ084119.52+290504.4, demonstrates a successful strategy for finding distant black holes and puts strong constraints on the early history of the universe.
Quasars are black holes in the act of swallowing material from surrounding space and emitting huge outbursts of energy. They are rare, and astronomers must search over a wide area of sky to find them. To locate candidate quasars, Goto searched the database of the Sloan Digital Sky Survey (SDSS) to find objects that have the same color in visible light as quasars at a distance of 12.7 billion light years. He found 300 candidates among the 180 million objects scattered in the sky in the 6,670-square-degree area of the SDSS (this covers roughly one-sixth of the sky). By observing these candidates in infrared light using the Apache Point 3.5-meter telescope and the United Kingdom Infrared Telescope on Mauna Kea, Goto was able to eliminate stars in our own galaxy that have similar visible colors to quasars.
Goto then observed the remaining 26 candidates with the Faint Object Camera and Spectrograph (FOCAS) on the Subaru telescope to find what he was looking for – a quasar 12.7 billion light years away. Xiaohui Fan from the University of Arizona and his collaborators who discovered the ten most distant quasars known have been the only other researchers successful in finding such distant quasars.
Goto’s newly discovered quasar has a black hole that is probably 2 billion times more massive than the Sun. So far, no one has proposed a theory of how such a massive black hole could have formed only a billion years after the birth of the universe.
In addition, the quasar¹s spectrum shows that much of the hydrogen between the quasar and Earth is ionized. This suggests that something had converted neutral hydrogen to ionized hydrogen before the universe was even a billion years old, a mysterious event known as the reionization of the universe.
The most promising key to understanding to the riddle of reionization is ultraviolet radiation. It comes from either stars or massive black holes. However, since reionization occurred over 12 billion years ago, getting reliable observational evidence has been a challenge.
Quasars are ideal for probing the epoch of reionization because they are distant and shine brightly and stably over long periods of time. Gamma ray bursts are also extremely distant and bright, and many researchers have used them successfully to probe reionization. However, as their name implies, gamma-ray bursts only happen occasionally, and do not last a long time.
Reionization of the universe is a patchy affair, progressing faster in regions with more ionizing sources. To truly understand how this process occurs, it is important to find probes of reionization in as many directions as possible and at a range of distances. Goto hopes to repeat his success with even more distant quasars.
This research will be published in the Monthly Notices of the Royal Astronomical Society.
Object Name: SDSSJ084119.52+290504.4
Distance and Redshift: Due to the expansion of the universe, light from distant objects is stretched and shifted to longer wavelengths. The redshift of the quasar is 5.96, which corresponds to a distance of 12.7 billion light-years. The universe itself is thought to be 13.7 billion years old.
Light from Black Holes: Black holes cannot be seen directly because they are so dense that light cannot escape from their gravitational pull. However, matter falling into a black hole heats up from friction as it swirls around the event horizon of the black hole at great speeds. The hot material radiates strongly in ultraviolet and visible light.
Quasars: Quasars are light from matter falling into a very massive and distant black hole.
Calculating the Black Hole Mass: The maximum amount of light emitted by matter falling into a black hole depends on the mass of the black hole. If the matter is falling into the black hole in a spherically symmetrical shell, this maximum brightness can be calculated. This brightness is called the Eddington luminosity. By assuming that the quasar’s brightness is equivalent to the Eddington luminosity, researchers can estimate the mass of the black hole.
Measuring Hydrogen Absorption: Neutral hydrogen can be ionoized and split into separate electrons and protons by absorbing light of particular wavelengths. The spectrum of the quasar shows some traces of residual light between the wavelengths of 800 to 830 nanometers. This indicates that there was not enough neutral hydrogen to absorb this light completely, suggesting that the universe was already substantially ionized 12.7 billion years ago.
Reionization: The universe came into existence 13.7 billion years ago in a state of extreme heat. As it expanded and cooled, elementary particles began to form and combine. By the time 300,000 years had passed, the universe was cool enough for electrons and protons to combine and form electrically neutral hydrogen atoms. However, today most of the hydrogen in the universe has been ionized and split into separate electrons and protons.
Figure 1: The spectrum of the quasar from Subaru telescope¹s Faint Object Camera and Spectrograph. The observed wavelengths of hydrogen emission lines indicate a redshift of 5.96, which corresponds to a distance of 12.7 light years.
Figure 2: Images of the quasar in i-band (750 nanometers; left), z-band (890 nanometers: middle), and J-band (1300 nanometers; right). The quasar is fainter at shorter wavelengths as is typical for quasars at its distance. (i-and z-band data are from the Sloan Digital Sky Survey. J-band data are from the United Kingdom Infrared Telescope.)
Figure 3: The CCD image of the spectrum. The fact that light is detected between 800 and 830 nanometers indicates that much of the hydrogen between Earth and the quasar must be ionized.
Research References:
Gunn J. E., Peterson B. A., 1965, ApJ, 142, 1633
Adelman-McCarthy J. K., et al., 2006, ApJS, 162, 38
Fan X. et al., 2003 AJ, 125, 1649
Goto T. 2006, astro-ph/0606493
Subaru is an 8.2 meter optical-infrared telescope on Mauna Kea, Hawaii, operated by the National Astronomical Observatory of Japan, a member institute of Japan’s National Institutes of Natural Science.
