In July 2011, NASA’s Jet Propulsion Laboratory announced that two astronomy teams had detected a huge reservoir of water vapor around the quasar APM 08279+5255. The agency translated the modeled mass into a number that still repeats itself: about 140 trillion times all the water in Earth’s oceans. The same account said that the quasar released as much energy as a thousand trillion suns. Both comparisons are based on published observations, but neither is as direct as it seems. The “reservoir” is diffuse gas spread over hundreds of light years, not a liquid ocean, while the quasar is magnified by gravitational lensing in the foreground. This is also a result from 2011, not a new detection. His interest now lies in what the measurements really established about water in the young universe, and how a handful of spectral lines became one of the largest water comparisons in astronomy. The observations were fingerprints in radio light. APM 08279+5255 has a redshift of 3.91. Light from the system has been traveling for about 12 billion years, so astronomers see it as it was when the universe was only about 1.6 billion years old. Matt Bradford of NASA’s Jet Propulsion Laboratory led one of the teams. In a paper published in The Astrophysical Journal Letters, Bradford and colleagues reported on six rotational water transitions observed with Z-Spec at the Caltech Submillimeter Observatory in Hawaii. Its observing program spanned 13 nights between 2008 and 2009, for a total of 25.3 hours, and a transition was verified with the CARMA radio system in California. A separate group led by Dariusz Lis used the Plateau de Bure interferometer in France. Their independent detection of an excited water transition arrived at the same redshift. That paper also had a caveat relevant to the giant mass claim: a single radiatively excited line does not by itself provide a good measure of water abundance. The telescopes did not photograph a pond or weigh the water directly. They measured the radiation emitted at specific frequencies when water molecules changed rotational energy states. The 140 billion figure is a modeled inventory. Bradford’s team had several water lines plus carbon monoxide lines, allowing for a broader model of the molecular gas. The authors compared the spectrum to that of the much closer ultraluminous galaxy Mrk 231 and estimated an average water abundance of about 1.4 water molecules per 10 million hydrogen molecules. That sounds slim because it is. The total becomes enormous only because the region contains an extraordinary amount of molecular gas. Combining the abundance with that gas reservoir resulted in the widely cited equivalent of 140 trillion Earth’s oceans, or about 100,000 times the mass of the Sun in water vapor. JPL’s original announcement described the gas extending for hundreds of light years around the central black hole. It had an estimated temperature close to -53 degrees Celsius and a density well below Earth’s atmosphere, although it was warmer and denser than the typical molecular gas of the Milky Way. Therefore, ocean conversion is an order of magnitude communication device, not a tank measurement. It depends on the water abundance adopted, the total mass of the molecular gas, the excitation model and the correction by gravitational lensing. The black hole is not what shines A quasar is the luminous center of a galaxy whose supermassive black hole is actively feeding. The black hole itself does not emit light. Matter in the surrounding accretion flow becomes extremely hot as it loses energy and moves inward, producing radiation across much of the electromagnetic spectrum. The 2011 JPL description assigned APM 08279+5255 a black hole mass close to 20 billion suns and a luminosity comparable to a quadrillion suns. This is a comparison of energy output, not a claim that the object contains that many stars. The water helped show what that radiation was doing to the surrounding galaxy. Bradford’s paper modeled a region about 550 parsecs, or about 1,800 light years, in diameter. The X-rays heated the molecular gas, while the dust’s intense far-infrared glow pumped water molecules into higher energy states. Their subsequent transitions created the lines that telescopes detected. In my opinion, this is the most useful scientific result. The water was not simply present; It acted as a probe of the radiation field, density and temperature around an active black hole in the early universe. Gravitational lensing makes it difficult for each giant number APM 08279+5255 to appear as multiple images because a foreground galaxy bends and magnifies its light. This is gravitational lensing, the same broad effect that astronomers use in time-delay measurements of the cosmic expansion of quasars. For this source, the magnification factor has been debated. Early work considered an amplification of around 40 or more. Bradford’s team adopted a later model with an increase of around four. A lower magnification means that the quasar must be intrinsically brighter and more massive to produce the observed signal. The quadrillion sun estimate fits that lower magnification image. It should still be treated as model-dependent because changing the lens geometry changes the inferred intrinsic luminosity. The water mass and physical dimensions inherit related uncertainty. The lens does not create false spectral lines. Amplifies the light that has already been emitted. The safe conclusion is that there was water vapor at a redshift of 3.91; The lens model determines how the observed flux translates into intrinsic mass and luminosity. “Twelve billion light years away” is shorthand for light travel. For nearby objects, the distance in light years and the time it takes for light to arrive are almost interchangeable. With a redshift of 3.91, cosmic expansion makes language less orderly. Light from the quasar has been traveling for about 12 billion years, which is why NASA used it “more than 12 billion light years away.” The galaxy is now farther away than 12 billion light years according to current distance convention because space expanded as light traveled. Astronomers use several distance definitions for cosmological objects, each appropriate to a different calculation. The observational statement does not depend on the choice of a popular language distance. The measured redshift places the emission in a period when the universe was a small fraction of its current age. The discovery concerned conditions, not just quantity. Astronomers already expected there to be water in the distant universe. Previous generations of stars had made oxygen, and ordinary chemistry could combine it with hydrogen. What the teams had not previously measured at that distance was a spectrum of water that revealed this very warm, dense and strongly irradiated molecular gas. A related 2011 study led by Paul van der Werf detected four water lines in the same lens quasar. Their excitation model also found that intense infrared radiation dominated the higher energy transitions and pointed to a dark nuclear region of star formation. More lines and sharper lens models can refine the inventory. The firm result is more limited than the headline figures, but scientifically durable: Water vapor was already abundant enough to trace the physical environment around a feeding supermassive black hole when the universe was about 1.6 billion years old.
