quasars Archives

November 3, 2010December 24, 2015 by Jon Voisey

What Hanny’s Voorwerp Reveals About Quasar Deaths

The green "blob" is Hanny's Voorwerp. Credit: Dan Herbert, Peter Smith, Matt Jarvis, Galaxy Zoo Team, Isaac Newton Telescope

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Hanny’s Voorwerp is a popular topic of conversation due to its novel discovery by Hanny Van Arkel perusing images from the Galaxy Zoo project. The tale has become so well known, it was made into a comic book (view here as .pdf, 35MB). But another aspect of the story is how enigmatic the object is. Objects that are so green are rare and it lacked a direct power source to energize it. It was eventually realized a quasar in the neighboring galaxy, IC 2497 could supply the necessary energy. Yet images of the galaxy couldn’t confirm a sufficiently energetic quasar. A new paper discusses what may have happened to the source.

The evidence that a quasar must be involved comes from the green color of the voorwerp itself. Spectra of the object has shown that this coloration is due to a strong level of ionized oxygen, specifically the λ5007 line of O III. While other scenarios could account for this feature alone, the spectra also contained He II emission as well as Ne V and the lines were especially narrow. Should star formation or shockwaves energize the gas, the motions would cause Doppler broadening. An quasar powered Active Galactic Nucleus (AGN) was the best fit.

But when telescopes searched for this quasar in the galaxy, it proved elusive. Optical images from WIYN Observatory were unable to resolve the expected point source. Radio observations discovered an object emitting in this range, but far below the amount of energy necessary to power the luminous Voorwerp. Two solutions have been proposed:

“1) the quasar in IC 2497 features a novel geometry of obscuring material and is obscured at an unprecedented level only along our line of sight, while being virtually unobscured towards the Voorwerp; or 2) the quasar in IC 2497 has shut down within the last 70,000 years, while the Voorwerp remains lit up due to the light travel time from the nucleus.”

Recent observations from Suzaku have ruled out the first of these possibilities due to the lack of potassium absorption that would be expected if light from the galaxy were being absorbed in a significant amount. Thus, the conclusion is that the AGN has dropped in total output by at least two orders of magnitude, but more likely by four. In many ways, this is not entirely unexpected since quasars are plentiful in the distant universe where raw material on which to feed was more plentiful. In the present universe, quasars rarely have such material available and can’t maintain it indefinitely.

Analogs exist within our own galaxy. X-Ray Binaries (XRBs) are stellar mass black holes which form similar accretion disks and can shut down and excite on short timescales (~1 year). The authors of the new paper attempted to scale up a model XRB system to determine if the timescales would fit with the ~70,000 year upper limit imposed by the travel time. While they found a good agreement with the output from direct accretion itself (10,000–100,000 years) the team found a discrepancy in the disk. In XRBs, the material around the black hole is heated as well, and takes some time to cool down. In this case, the core of the galaxy should still retain a hot disc of material which isn’t present.

This oddity demonstrates that there is still a large amount of knowledge to be gained on the physics surrounding these objects. Fortunately, the relatively close proximity of IC 2497 allows for the potential for detailed followup studies.

July 21, 2010December 24, 2015 by Nicholos Wethington

First Quasar Gravitational Lens Discovered (w/video)

The quasi-stellar object SDSS J0013+1523 has been shown to warp the light of a background galaxy around it, producing a magnified double-image from our perspective on Earth. Image Credit: Courbin, Meylan, Djorgovski, et al., EPFL/Caltech/WMKO

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Gravitation lensing – a phenomenon that falls out of Einstein’s theory of general relativity – has been observed numerous times, making for some fantastic images of rings, arcs and crosses composed of massive galaxies light years away. As the light from a background object is bent by gravity around a foreground object, multiple, magnified images of the background object are produced from our vantage point.

For the first time, a quasar (quasi-stellar object) has been shown to gravitationally lens a galaxy behind it. About a hundred instances of gravitational lenses that consist of a foreground galaxy and a background quasar have been found, but this is the very first time where the opposite is the case; that is, a quasar bending the light from a background galaxy around it to create a multiple image of that galaxy.

Quasars are thought to be the result of a supermassive black hole at the center of a galaxy attempting to swallow up all of the matter that surrounds it. As the matter bunches up when it gets closer to the black hole, it heats up due to friction and begins to emit light across the electromagnetic spectrum. The light from a quasar can outshine an entire galaxy of stars, making it difficult to separate the light from a background galaxy from the overwhelming glare of the quasar itself.

To make this initial detection (there are surely many to follow), astronomers from the EPFL’s Laboratory of Astrophysics in cooperation with Caltech used data from the Sloan Digital Sky Survey (SDSS). They analyzed 22,298 quasars from the SDSS Data Release 7 catalog, and looked for images that had a strongly redshifted emission spectra. According to the paper announcing the results, “In these spectra, we look for emission lines redshifted beyond the redshift of the [quasar].”

In other words, a quasar that is lensing a galaxy in the background will exhibit a higher redshift than one that is not lensing a background galaxy, since the light from the galaxy and the quasar are combined in the SDSS data. So, quasars that had an expected redshift were thrown out, and a statistical analysis of quasars with emission lines that might mimic a gravitational lens eliminated many more of the objects. This left about 14 objects of the 22,298 analyzed as potential candidates. Of these 14, the team selected one to perform follow-up observations on, named SDSS J0013+1523.

SDSS J0013+1523 lies about 1.6 billion light years away, and is lensing a galaxy that is about 7.5 billion light years away from Earth. Using the Keck II telescope, they were able to confirm that SDSS J0013+1523 was indeed lensing the light from a galaxy located behind it. Hubble images of the discovery are in the works.

Here’s a video produced by the EPFL describing the results.

What is significant about this discovery – besides the novel aspect of a quasar acting as a lens – is that it will allow researchers to better refine their understanding of quasars. When light is bent around an object, it bends because of gravity, and gravity is a result of mass. So, something that is very massive will act as a stronger lens than something that is tiny, and the mass of the object doing all of the lensing work – in this case, the foreground quasar – can be determined.

Their results were published in a letter to Astronomy & Astrophysics on July 16th. The original paper is available for your perusal here.

Source: Eurekalert here and here, Arxiv paper here

February 8, 2010December 24, 2015 by Jean Tate

Caught in the Act! Merging Galaxies Create a Binary Quasar

SDSS J1254+0846 x-ray (blue), optical (yellow)(Credits: X-ray: NASA/CXC/SAO/Green et al Optical: Carnegie Obs/Magellan/Baade Telescope/Mulchaey et al)

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Excellent teamwork by astronomers working in two different wavebands – x-ray and optical – has led to the discovery of a binary quasar being created by a pair of merging galaxies.

“This is really the first case in which you see two separate galaxies, both with quasars, that are clearly interacting,” says Carnegie astronomer John Mulchaey who made observations crucial to understanding the galaxy merger.

“The model verifies the merger origin for this binary quasar system,” Thomas Cox, now a fellow at the Carnegie Observatories, says, referring to computer simulations of the merging galaxies he produced. When Cox’s model galaxies merged, they showed features remarkably similar to what Mulchaey observed in the Magellan images. “It also hints that this kind of galaxy interaction is a key component of the growth of black holes and production of quasars throughout our universe,” Cox added.

“Just because you see two galaxies that are close to each other in the sky doesn’t mean they are merging,” says Mulchaey. “But from the Magellan images we can actually see tidal tails, one from each galaxy, which suggests that the galaxies are in fact interacting and are in the process of merging.”

As Universe Today readers know, quasars are the extremely bright centers of galaxies surrounding supermassive black holes, and binary quasars are pairs of quasars bound together by the mutual gravitation of the two host galaxies’ nuclei. Binary quasars, like other quasars, are thought to be the product of galaxy mergers. Until now, however, binary quasars have not been seen in galaxies that are unambiguously in the act of merging. But images of a new binary quasar from the Carnegie Institution’s Magellan telescope in Chile show two distinct galaxies with tails produced by tidal forces from their mutual gravitational attraction.

Supermassive black holes are to be found in the nuclei of most, if not all, large galaxies, such as our galaxy the Milky Way. Because galaxies regularly interact and merge, astronomers have concluded that binary supermassive black holes have been common in the Universe, especially during its early history (when galaxy mergers were far more common). Supermassive black holes can only be detected as quasars – which are one kind of highly luminous active galactic nucleus (AGN) – when they are actively accreting matter, a process that releases vast amounts of energy across the entire electromagnetic spectrum. A leading theory of ordinary AGNs is that galaxy mergers trigger accretion, creating quasars in both galaxies (AGNs in the hearts of the giant elliptical galaxies in rich clusters are thought to be fueled by a different mechanism, cooling flow). Because most such mergers would have happened in the distant past, binary quasars and their associated galaxies are very far away and therefore difficult for most telescopes to resolve.

The binary quasar, named SDSS J1254+0846, was initially detected by the Sloan Digital Sky Survey, a multi-year, large scale astronomical survey of galaxies and quasars. Further observations by Paul Green of the Harvard-Smithsonian Center for Astrophysics and colleagues using NASA’s Chandra’s X-ray Observatory and telescopes at Kitt Peak National Observatory in Arizona and Palomar Observatory in California strongly suggest that the object was likely a binary quasar in the midst of a galaxy merger. Carnegie’s Mulchaey then used the 6.5 meter Baade-Magellan telescope at the Las Campanas observatory in Chile to obtain deeper images and more detailed spectroscopy of the merging galaxies.

The Astrophysical Journal paper on this object is: “SDSS J1254+0846: A Binary Quasar Caught in the Act of Merging” (Paul J. Green et al 2010 ApJ 710 1578-1588; arXiv:1001.1738 is the preprint).

Source: Carnegie Institution for Science

December 11, 2009December 24, 2015 by Nicholos Wethington

Fermi Spies Energetic Blazar Flare

A comparison of the Fermi images from November 2nd and December 3rd of this year, showing the brightening of 3C 454.3. Image Credit: NASA

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The blazar 3C 454.3, a bright source of gamma rays from a galaxy 7 billion light-years away just got a whole lot brighter. Observations from the Fermi gamma-ray telescope confirm that since September 15th the blazar has flared up considerably, increasing in gamma-ray brightness by about ten times in the from earlier this past summer, making it currently the brightest gamma-ray source in the sky.

3C 454.3 is a blazar, a jet of energetic particles that is caused by the supermassive black hole at the center of a galaxy. Most galaxies are thought to house a supermassive black hole at their center, and as it chomps down matter from the accretion disk that surrounds it, the supermassive black hole can form large jets that stream out light and energy in fantastic proportions. In the case of 3C 454.3, one of these jets is aimed at the Earth, which allows for us to see and study it.

This blazar has started to outshine the Vela pulsar, which because it is only 1,000 light-years away from the Earth is generally the brightest gamma-ray source in the sky. 3C 454.3 is almost twice as bright as Vela in the gamma-ray part of the spectrum, even though it lies 7 million times further away from the Earth. 3c 454.3 has also brightened significantly in the infrared, X-ray, radio and visible light.

This is not the first time the blazar has shown an increase in brightness. Over the course of observations of the blazar, it flared-up in brightness in May 2005, and again in July and August of 2007.

Dr. Erin Wells Bonning, Postdoctoral Associate at the Yale Center for Astronomy and Astrophysics, said of the recent flare in comparison with previous brightening events:

“In 2005, it reached a R-band magnitude of 12. Our peak observed R-band magnitude was 13.83, so we’re still not at the brightness of the 2005 outburst (about a factor of 5 below). On July 19, 2007, it reached a R-band magnitude of 13, not as bright as the 2005 event, but still brighter than we see it now. In 2005, there were no gamma-ray instruments to observe 3C 454.3, but the 2007 flare was observed by AGILE with a flux above 100 MeV of 3 +- 1 * 10^-6 cts/s/cm^s. The Fermi and AGILE count rates for Dec 2-3, 2009 are 6-9 times as high. So, interestingly, although it is not currently as bright optically as it was in 2007, it is a good deal brighter in gamma-rays.”

The Fermi gamma-ray space telescope (formerly GLAST) keeps tabs on the gamma-ray emissions from many sources in the sky. 3C 454.3 is just one of the top ten brightest sources of gamma-rays visible to the satellite, a list of which can be found in an article Nancy wrote in March, The Top Ten Gamma-Ray Sources from the Fermi Telescope.

Of course, the blazar 3C 454.3 is not as intrinsically bright as many of the Gamma-Ray Bursts observed by telescopes like Swift and Fermi, but it is the consistently brightest source of gamma-rays in the sky right now. Bonning said that, “While both GRBs and blazars are highly beamed toward us, the Lorentz factors (speed of particles in the jet) associated with GRBs are much higher than in blazars, causing them to appear brighter due to special relativistic effects.”

Observations 3C 454.3 are continuing in all wavelengths to capture the light curve of the event, and better understand these periodic flares. Bonning said, “The source has been relatively quiescent since it emerged from behind the Sun, and began to increase in brightness around the end of July. It then entered a bright period of fairly rapid variability, peaking every 20 days or so. The most recent, very intense, flare began around the end of November. Per our [Astronomer’s Telegram], since Nov 21, 3C 454 has increased about a factor of 3 in brightness in both optical and infrared. (B, V, and R filters are in optical wavelengths, and J and K are near-infrared). Similarly, the gamma-ray flux has increased also by a factor of 3 in the 0.1-300 GeV band over the same period.”

The cause of the intermittent flare-ups in 3C 454.3 and other blazars is still a mystery, but this current brightening will give astronomers better data as to what the possible cause could be. There seem to be no periodic events associated with the flares in blazars (with the exception of the possible “supermassive black hole binary” OJ 287).

Bonning said of a potential cause, “This is actually a very active field of research – there are numerous existing models, but no one hypothesis is clearly preferred. Perhaps particles have been shocked at some location in the blazar jet, or the jet may be precessing so that is closer to our line of sight, or there may be some other explanation.”

There will be numerous telescopes around the world zooming in on the current flare-up. According to Bonning:

“Blazars are multi-wavelength objects — their spectral energy distribution covers radio through gamma-rays, so a diverse collection of facilities will be observing 3C 454.3 during this outburst. Besides Fermi, the Italian AGILE satellite has been observing in gamma rays. The Swift X-ray telescope began monitoring in early December. The blazar monitoring group at Boston University headed by Alan Marscher is observing it with VLBA (radio; 13GHz). There is also a radio astronomy group at Michigan also observing with VLBA, as well one headed by Yuri Kovalev at Max Planck institute in Germany. There is an optical program with the ATOM telescope associated with the HESS TeV instrument in Namibia. (3C 454.3 is not bright at TeV energies, by the way.) This is not an exhaustive list by any means, but at any rate numerous facilities across the globe and operating at a wide range of energies will be taking a very close look at 3C 454.3 as it goes through this flare.”

Source: NASA press release, email interview with Erin Wells Bonning

December 1, 2009December 24, 2015 by Nicholos Wethington

Quasar Caught Building Future Home Galaxy

An artist's impression of how quasars may be able to construct their own galaxies. Image Credit: ESO/L. Calcada

The birth of galaxies is quite a complicated affair, and little is known about whether the supermassive black holes at the center of most galaxies formed first, or if the matter in the galaxy accreted first, and formed the black hole later. Observations of the quasar HE0450-2958, which is situated outside of a galaxy, show the quasar aiding a nearby galaxy in the formation of stars. This provides evidence for the idea that supermassive black holes can ‘build’ their own galaxies.

The quasar HE0450-2958 is an odd entity: normally, supermassive black holes – also known as quasars – form at the center of galaxies. But HE0450-2958 doesn’t appear to have any host galaxy out of which it formed. This was a novel discovery in its own right when it was made back in 2005. Here’s our original story on the quasar, Rogue Supermassive Black Hole Has No Galaxy.

The formation of the quasar still remains a mystery, but current theories suggest that it formed out of cold interstellar gas filaments that accreted over time, or was somehow ejected from its host galaxy by a strong gravitational interaction with another galaxy.

The other oddity about the object is its proximity to a companion galaxy, which it may be aiding to form stars. The companion galaxy lies directly in the sights of one of the quasar’s jets, and is forming stars at a frantic rate. A team of astronomers from France, Germany and Belgium studied the quasar and companion galaxy using the Very Large Telescope at the European Southern Observatory. The astronomers were initially looking to find an elusive host galaxy for the quasar.

The phenomenon of ‘naked quasars’ has been reported before, but each time further observations are made, a host galaxy is found for the object. Energy streaming from the quasars can obscure a faint galaxy that is hidden behind dust, so the astronomers used the VLT spectrometer and imager for the mid-infrared (VISIR). Mid-infrared observations readily detect dust clouds. They combined these observations with new images obtained from the Hubble Space Telescope in the near-infrared.

Observations of HE0450-2958, which lies 5 billion light years from Earth, confirmed that the quasar is indeed without a host galaxy, and that the energy and matter streaming out of the jets is pointed right at the companion galaxy. This scenario is ramping up star formation in that galaxy: 340 solar masses of stars a year are formed in the galaxy, one-hundred times more than for a typical galaxy in the Universe. The quasar and the galaxy are close enough that they will eventually merge, finally giving the quasar a home.

David Elbaz of the Service d’Astrophysique, who is the lead author of the paper which appeared in Astronomy & Astrophysics, said “The ‘chicken and egg’ question of whether a galaxy or its black hole comes first is one of the most debated subjects in astrophysics today. Our study suggests that supermassive black holes can trigger the formation of stars, thus ‘building’ their own host galaxies. This link could also explain why galaxies hosting larger black holes have more stars.”

‘Quasar feedback’ could be a potential explanation for how some galaxies form, and naturally the study of other systems is needed to confirm whether this scenario is unique, or a common feature in the Universe.

Source: ESO, Astronomy & Astrophysics

November 16, 2009December 24, 2015 by Nicholos Wethington

35 Radio Observatories Link to Break Record

Ever wondered what the largest telescope on the Earth is? Well, this coming Wednesday and Thursday of this week, the largest telescope ever assembled here will take observations for a whole day. How big is the telescope? About the size of the whole Earth! 35 radio telescopes on 7 continents will link together for one whole day in an effort to observe distant quasars as part of an initiative to improve the reference frame that scientists use to measure positions in the sky.

Radio telescopes in Asia, Australia, Europe, North America, South America, Antarctica, and in the Pacific will all be linked together to measure the same 243 quasars over a 24-hour period. Quasars are galaxies that have a supermassive black hole at the center, which has strong emissions in the radio spectrum. The quasars being monitored are so far away from the Earth that they appear to be motionless in the sky. This makes them a perfect candidate for setting up a grid in the sky to use as a frame of reference, against which the positions of other objects can be determined.

This monitoring session comes out of a meeting of the International Astronomical Union in August, during which it was decided to start using a set of 295 quasars as a celestial reference frame starting January 1st, 2010. This is not a new reference frame to be used by astronomers – the current one was adopted in 1998 – but an important update to the existing reference frame, the International Celestial Reference Frame.

The session, called the Very Large Astrometry Session is coordinated by the International VLBI Service for Geodesy and Astrometry. Several of the participating observatories will have live webcams running during the event (check for the observatory in your language!), and a public outreach page on the event, hosted by the Bordeaux Observatory, can be found here. The public outreach page will post images as they are taken during the session, with information about observation coordinates.

Radio telescopes like the Very Long Base Array in the United States already link together observatories that are far apart to take observations. This technique is called very long baseline radio interferometry (VLBI), and allows for the use of smaller telescopes that are distant from one another to be linked together and have the same angular resolution as if they were one larger telescope. Doing these observations all in one go will reduce some of the errors that occur when disparate observatories take images at different times.

The previous record for radio observatories linked together to create a larger telescope for one monitoring session is 23. That means that this observing session will beat that record by a whopping 12 additional observatories. Even with this unprecedented amount of observatories monitoring the quasars, there will be a few gaps in the sky, mostly in the Southern hemisphere. Only 243 of the total 295 quasars in the reference frame will be observed this week, though that will break another record for the amount of objects observed in one session using this method. The image below depicts the locations of the participating observatories.

By taking data on the 243 selected quasars, astronomers will be able to more accurately pinpoint objects in the sky in all wavelengths, and gather more precise data. For instance, many objects of scientific interest are monitored by separate telescopes operating in the visible, radio, x-ray and infrared wavelengths. Having a more accurate frame of reference to tell these different telescopes where to point in the sky will improve the ability of the different telescopes to gather information from the same place in space.

Source: NRAO, IVS