On Feb. 26, a team of astronomers announced that they have found a supermassive black hole that is about 12 billion times more massive than the Sun and appears to have formed when the universe was only 875 million years old, or 6 percent of its current age. This is a surprising find because black holes are thought to grow relatively slowly by pulling in anything that gets too close, and 875 million years is a short period of time for such a massive black hole to form.
“How do you build such a big black hole in such a short time?” asks Xue-Bing Wu of China’s Peking University, lead author of the study. “This is quite surprising because it presents serious challenges to theories of black hole growth in the early universe.”
As even light cannot escape a black hole, they must be measured indirectly. The team did this by observing a quasar, or quasi-stellar radio source, which is formed when a large amount of matter forms an accretion disc around a black hole and emits an enormous amount of energy as it falls into the black hole. Telescopes in China, Hawaii, Arizona, and Chile were used by Wu and his colleagues.
Most major galaxies, including the Milky Way, are thought to have supermassive black holes in their centers. But only some have the hot, luminous accretion discs known as quasars surrounding them. The black hole at the center of the Milky Way only has a mass of 4.31±0.38 million solar masses, while the largest black holes found so far exceed 10 billion solar masses. The newly discovered object, known as SDSS J010013.021280225.8 (or J0100+2802 for short), is the most massive and luminous quasar yet observed.
“This is the biggest monster we’ve ever detected in terms of luminosity,” says Avi Loeb, chair of the Harvard astronomy department, who was not involved in the research. It’s about 40,000 times as bright as the entire Milky Way, Loeb says, or about 429 trillion times brighter than the Sun. The brightness of the quasar tells astronomers how much the gas is being heated, which tells them how massive the black hole powering the quasar is. “We’ve seen other quasars from this period,” says Wu, “but none of them has a mass of more than three billion times that of the sun.” About 40 other such quasars with a redshift greater than 6 have been discovered.
The object looks like an ordinary star at first glance, but analyzing the light in detail revealed both the unusual emission spectrum of a quasar as well as a redshift of 6.30. As much of the observable universe is redshifted, it is known that the universe is expanding. The farther away an object is, the greater the degree of redshift. A redshift of 6.30 corresponds to a time of about 13 billion years ago. When light takes so long to reach Earth, astronomers are looking at objects as they were when the universe was much younger.
How such a massive black hole formed so fast is a mystery. “It requires either very special ways to quickly grow the black hole or a huge seed black hole,” Wu told Space.com. Accretion discs limit the growth rate of a black hole. As large amounts of gas and dust approach the black hole, they can create a “traffic jam” that slows down other material that is falling in. Heat and radiation that results from these traffic jams forces matter away from the black hole. A recent study suggests that the denser gas which might have been present at the time could have obscured some of the heat and radiation as well as provide more fuel, thereby helping matter to fall into black holes at a faster rate. But this black hole is still too large to be explained by such an effect.
Another possibility is that two black holes combined as two galaxies collided. But this only works if the black holes are nearly equal in mass. Otherwise, the imbalance would eject the combined black hole from the new galaxy. Loeb suggests that as there is no theoretical upper limit on stellar mass, the earliest stars could have been much larger, hotter, and shorter-lived than any stars observed locally. This could jump-start the formation of a supermassive black hole, and this combined with the effects of denser gas could explain the quick formation of J0100+2802. It is not known whether such supermassive stars ever formed, but the James Webb Space Telescope, which is scheduled to go into orbit in 2018, might be able to answer that question.
The newly discovered object presents an opportunity to astronomers. As a quasar’s light travels to Earth, it passes through interstellar gas that colors the light. By analyzing the wavelengths of light which arrive at Earth, scientists can determine which elements make up the gas. This can provide information to cosmologists about the star formation processes in the early universe which produced those elements.
“This quasar is the most luminous one in the early universe, which, like a lighthouse, will provide us chances to use it as a unique tool to study the cosmic structure of the dark, distant universe,” Wu said.
From top to bottom, spectra taken with the Lijiang 2.4-m telescope, the MMT and the LBT (in red, blue and black colours), respectively. For clarity, two spectra are offset upward by one and two vertical units. Although the spectral resolution varies from very low to medium, in all spectra the Lyα emission line, with a rest-frame wavelength of 1,216 Å, is redshifted to around 8,900 Å, giving a redshift of 6.30. J0100+2802 is a weak-line quasar with continuum luminosity about four times higher than that of SDSS J1148+5251 (in green on the same flux scale), which was previously the most luminous high-redshift quasar known at z = 6.42.
a, b, Transmission in Lyα and Lyβ absorption troughs (respectively a, red; b, blue) were calculated by dividing the spectrum by a power-law continuum, . The shaded band in both panels shows 1σ standard deviation. The Lyα and Lyβ absorption redshifts are given by λ/λLyα(Lyβ) − 1, where λLyα = 1,216 Å and λLyβ = 1,026 Å. The optical spectrum exhibits a deep Gunn–Peterson trough and a significant transmission peak at z = 5.99. c, Transmission in the proximity zone. The proper proximity zone for J0100+2802 (in black) extends to 7.9 ± 0.8 Mpc, a much larger value than those of other z > 6.1 quasars, including 4.9 ± 0.6 Mpc for J1148+5251 (in green), consistent with its higher ultraviolet luminosity. The transmission in c was calculated by dividing the measured spectrum by a power-law continuum plus two Gaussian fittings of Lyα and N v lines. The horizontal dotted line and the two dashed lines denote transmission values of 0, 0.1 and 1.0 respectively, while the vertical dashed line denotes the proper proximity zone size of 0.
Main panel, the black line shows the LBT optical spectrum and the red line shows the combined Magellan and Gemini near-infrared J,H,K-band spectra (from left to right, respectively). The gaps between J and H and between H and K bands are ignored due to the low sky transparency there. The magenta line shows the noise spectrum. The main emission lines Lyα, C iv and Mg ii are labelled. The details of the absorption lines are described in Fig. 8. Inset, fits of the Mg ii line (with FWHM of 5,130 ± 150 km s−1) and surrounding Fe ii emissions. The green, cyan and blue solid lines show the power law (PL), Fe ii and Mg ii components. The black dashed line shows the sum of these components in comparison with the observed spectrum, denoted by the red line. The black-hole mass is estimated to be (1.24 ± 0.19) × 1010 .
The red circle at top right represents J0100+2802. The small blue squares denote SDSS high-redshift quasars, and the large blue square represents J1148+5251. The green triangles denote CFHQS high-redshift quasars. The purple star denotes ULAS J1120+0641 at z = 7.085. Black contours (which indicate 1σ to 5σ significance from inner to outer) and grey dots denote SDSS low-redshift quasars (with broad absorption line quasars excluded). Error bars represent the 1σ standard deviation, and the mean error bar for low-redshift quasars is presented in the bottom-right corner. The dashed lines denote the luminosity in different fractions of the Eddington luminosity, LEdd. Note that the black-hole mass and bolometric luminosity are calculated using the same method and the same cosmology model as in the present Letter, and the systematic uncertainties (not included in the error bars) of virial black-hole masses could be up to a factor of three.
J0100+2802 is undetected in SDSS u,g,r bands (top row) but is relatively bright in other bands (lower three rows). It is consistent with a point source in the bands with high signal-to-noise detections. The size is 1′ × 1′ for all images. The green circle represents an angular size of 10″ in each image.
The size is 10″ × 10″. The horizontal and vertical axes denote the offsets in right ascension (ΔRA) and in declination (ΔDec.). The image, with seeing of 0.4″, shows a morphology fully consistent with a point source.
The redshifts of these three quasars are 6.30, 6.42 and 7.085, respectively. The luminosity of J0100+2802 in the ultraviolet/optical bands is about four times higher than that of J1148+5251, and seven times higher than that of ULAS J1120+0641. The photometric data are from literature for J1148+5251 and J1120+0641. The error bars show the 1σ standard deviation.
Most of them are from Mg ii, C iv and Fe ii. The labels from A to H correspond to the redshifts of absorption materials at 6.14, 6.11, 5.32, 5.11, 4.52, 4.22, 3.34 and 2.33, respectively. Studies of intervening and associated absorption systems will be discussed elsewhere.