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Category Archives: 17. Quasars and active galaxies

From an ESO press release, October 26, 2016:

An international team of astronomers has discovered glowing gas clouds surrounding distant quasars, active galaxies less than two billion years after the Big Bang. This new survey by ESO’s Very Large Telescope indicates that halos around quasars are far more common than expected. The properties of the halos in this surprising find are also in striking disagreement with currently accepted theories of galaxy formation in the early Universe.

Bright halos around distant quasars

Credit: ESO/Borisova et al.

The study involved 19 quasars, selected from among the brightest that are observable with the telescope’s MUSE instrument. Previous studies have shown that around 10% of all quasars examined were surrounded by halos, made from gas known as the intergalactic medium. These halos extend up to 300,000 light-years away from the centers of the quasars. This new study, however, has thrown up a surprise, with the detection of large halos around all 19 quasars observed – far more than the two halos that were expected statistically.

The newly detected halos also revealed another surprise: they consist of relatively cold intergalactic gas – approximately 10,000 degrees Celsius. This revelation is in strong disagreement with currently accepted models of the structure and formation of galaxies, which suggest that gas in such close proximity to galaxies should have temperatures upwards of a million degrees.

Links: full ESO press release, including video animations.

From an article on

The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory, located at 4000 m above sea level on the slopes of Mexico’s Volcán Sierra Negra, is the newest tool available to visualize the most energetic phenomena in the Universe, such as supernovae, neutron star collisions and active galactic nuclei.


Credit: HAWC Collaboration

In March 2015, construction was completed on HAWC’s 300th and final detector tank (each holding 200,000 liters of water), and the observatory will soon begin collecting data at full capacity.

It is a joint project between U.S. and Mexican scientists, with some participation from Polish and Costa Rican scientists.

Links: article; HAWC Observatory home.

A recent article in Nature describes how a supermassive black hole’s spin has been measured via the gravitational lens of a foreground galaxy, that fortuitously lies along the same line-of-sight. The new measurements have enabled astronomers to find that a supermassive black hole powering a distant quasar has grown through coherent, rather than random, episodes of mass accretion (see Section 17.3, p. 458-459). The following text is a digest of Guido Risalti’s summary in Nature‘s ‘News and views’ section, March 13, 2014.

Supermassive black holes are simple systems. They are characterized by just two quantities, their mass and their spin. Whereas the total amount of accretion and any mergers that a supermassive black hole undergoes are encoded in its mass, how this mass was assembled is encoded in its spin. A few ordered accretion events or mergers of large black holes produce high spins, and short, random accretion processes produce low spins. Measuring these spins is therefore a major goal of extragalactic astronomy: the spins of supermassive black holes hold a key to understanding the evolution of their host galaxies.

But how can we measure the spins? According to Einstein’s general theory of relativity, a black hole’s gravitational field twists space-time around it. Such twisting depends on the black hole’s spin, so measuring the twisting allows the spin to be estimated. The signature of space-time distortion is imprinted on the emission of radiation from regions close to the black hole’s event horizon – the surface beyond which no radiation can escape. The best way to perform such a measurement is to observe X-rays reflected by the disk.

In their study, R. C. Reis and colleagues break new ground by obtaining a spin measurement of a quasar at a distance of more than 6 billion light years from Earth, from a time when the Universe was about half its current age. This remarkable result was possible owing to the exceptional nature of the observed source – a quadruply imaged, gravitationally lensed quasar.


The light from the distant quasar is both magnified and split into four different images by the gravitational field of a foreground elliptical galaxy (the lens) that, by chance, is on the line of sight of the quasar. For this reason, the authors could analyse four ‘copies’ of the X-ray spectrum of the quasar, each with an intensity significantly magnified by the lens. The resulting X-ray spectra have a quality that matches the best that has been obtained for nearby sources, and allowed a robust measurement of the black hole’s spin. As it turns out, the spin is large (close to the highest possible value that theory predicts), suggesting that the black hole acquired its mass through coherent phases of mass accretion.

Links: The Nature article (behind paywall); a widefield view of the lens via U. Michigan press release.

Astronomers have discovered a distant quasar illuminating a vast nebula of diffuse gas, revealing for the first time part of the network of filaments thought to connect galaxies in a cosmic ‘web’. Researchers at the University of California, Santa Cruz, led the study, published January 19 in the journal, Nature. Using the 10-meter Keck I telescope in Hawaii, the researchers detected a very large, luminous nebula of gas extending about 2 million light-years across intergalactic space.


Credit: S. Cantalupo (UCSC); Joel Primack (UCSC); Anatoly Klypin (NMSU)

The standard cosmological model of structure formation in the Universe predicts that galaxies are embedded in a cosmic web of matter, most of which (about 84 percent) is invisible dark matter. This web is seen in the results from computer simulations of the evolution of structure in the Universe, which show the distribution of dark matter on large scales, including the dark matter halos in which galaxies form and the cosmic web of filaments that connect them. Gravity causes ordinary matter to follow the distribution of dark matter, so filaments of diffuse, ionized gas are expected to trace a pattern similar to that seen in dark matter simulations.

Until now, these filaments have never been seen. Intergalactic gas has been detected by its absorption of light from bright background sources, but those results don’t reveal how the gas is distributed. In this study, the researchers detected the fluorescent glow of hydrogen gas resulting from its illumination by intense radiation from the quasar.

The hydrogen gas illuminated by the quasar emits ultraviolet light known as Lyman alpha radiation. The distance to the quasar is so great (about 10 billion light-years) that the emitted light is “stretched” by the expansion of the Universe from an invisible ultraviolet wavelength to a visible shade of violet by the time it reaches the Keck telescope and the spectrometer used for this discovery. Knowing the distance to the quasar, the researchers calculated the wavelength for Lyman alpha radiation from that distance and built a special filter to get an image at that wavelength.

Links: further images and information via the full Keck Observatory press release.