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Brandeis professor assists in Quasar discovery

By Daniel Koenigsberg

Section: News, Top Stories

January 24, 2013

The recent discovery of a new x-ray emitting quasar by a group of scientists that includes Brandeis’ own Professor John Wardle (PHYS) has challenged our understanding of the universe. Wardle and his colleagues observed the quasi-stellar radio source with the Chandra X-Ray observatory, the Hubble Space Telescope and the Very Large Array, an array of 27 radio telescopes located in New Mexico.

A quasar is a highly relativistic jet, meaning a jet that moves close to the speed of light, of electrons or positrons (the anti particle equivalent of electrons) produced at the center of a galaxy that fires perpendicular to the plane of the galaxy. They are called quasi-stellar because they appear star-like in optical light. Current models propose that they are caused by supermassive black holes at the center of the galaxy. The supermassive black holes heat the matter closest to them, called the accretion disc because the matter is falling into the black hole. Some of the energy that pulls the matter in causes matter to shoot off away from the galaxy. It is now supposed that there is a supermassive black hole at the center of every galaxy. The supermassive black hole at the center of the Milky Way Galaxy, our galaxy, is estimated to have a mass of approximately four million suns.

Most quasars emit heavily in the radio frequency because their charged particles are accelerated in circles by a magnetic field within the jet. However, those particles can collide with photons already readily available in the universe. These photons exist at the microwave frequency and are everywhere in the universe, called the Cosmic Microwave Background (CMB). The CMB is the leftover radiation from the Big Bang. In a phenomenon called Inverse Compton Scattering the charged particles in the jet can collide with the already available photons to boost their energy in such a way that they become x-rays.

The x-ray emitting quasar, named 1428+4217 for its location in the sky, is the most distant x-ray emitting quasar discovered thus far at an astonishing 12.4 billion light years away. This means that the universe was a mere 1.3 billion years old, 10 percent of it’s current age, when the light we see now was emitted from the quasar. The age of an astronomical object can be determined because objects that are farther away accelerate away from us at a faster rate.

Earlier in the universe the density and energy of photons was greater. But the universe was smaller when it was younger, because it hadn’t expanded as much yet. This relationship of volume goes by the same factor for each dimension of space. So the energy density of the photon background at the time that these x-rays were emitted is increased by a factor about 1,000. That’s why we can see this, Wardle said. Otherwise, it would be far too faint to see at all, although it is still very faint.

The argument that inverse scattering is what is causing x-ray emitting photons is not yet proven, although not entirely contested, so our ability to even see this quasar makes that argument far more powerful. It also brings to the table a plethora of questions about the current model of the universe (1.3 billion years is not a lot of time for a supermassive black hole to form). There is no reason to believe they existed at the time, Wardle said It is possible that a primordial black hole might have come across a fledgling galaxy and the two of them might have grown symbiotically, or it is possible that a young galaxy formed a black hole on its own. Not enough is known about early galaxy formation and this discovery seems to demonstrate that. How the first galaxies formed is currently a hot area of debate.

In order for x-ray quasars to be visible to us they must have a very small angle to the line of site, meaning they must be more or less pointing at us. This means that the images we produce of quasars must be de-projected on the assumption of that angle in order to find their approximate length. The de-projected length of the quasar in addition to the speed at which it is moving has much to tell us about the environment of the early universe.

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