What Are Quasars, Blazars, and Active Galactic Nuclei?
To say that quasars are mysterious is a completely understatement. They have presented astrophysics with a great challenge that has been difficult at best to solve. So let’s explore what these objects seem to be, or depending on who you are what they could be.
The first quasar (aka a quasi-stellar radio object, a quasi-stellar source, or an interloper) to be identified was by Maarten Schmidt (from the California Institute of Technology) on March 16, 1963. The object he was examining, 3C 273, was known already to scientists (in fact the previous year saw Cyni Hazard use the moon to accurately position it) and though to be a star but Maarten calculated the distance to the object based on the redshift it displayed in its spectrum, particularly the hydrogen Balmer lines. A star normally had a redshift of 0.2% while 3C had one that was about 16%. What was shocking was the distance this redshift implied: almost 2.5 billion light years away, based on the six wavelengths the lines were redshifted from their normal positions. Why a surprise? 3C is a very luminous object and if we can see that luminosity from here then imagine how it would be if we were present at 3C. Plus the redshift implied it was moving away from us at 47,000 km/s (about 1/10 the speed of light). No star could be that bright at such a distance or display such a redshift, so what was it then? (Wall, Kruesi 24, Shipman 152-3, Fulvio 153-5)
Scientists found their answer: a supermassive black hole residing in a galaxy that is eating a lot of matter falling into the singularity around in the accretion disk. All of that matter would be ripped and heated to such high levels that it could not help but be luminous. So luminous in fact that it outshines everything in the host galaxy and appears as a bright source with energy outputs as high as 1047 ergs/s. As one gets closer to the inner portion of the disk, collisions ramp up and UV rays go up. But the further out you go, the energy between collisions is low enough to allow visible and IR light to be released. However, no matter where you are around a quasar, the material around it is heavily ionized as matter bumping into each other releases electrons, causing electric and magnetic fluxes to occur and therefore releasing synchotron radiation as well . Some of those UV photons do collide with those electrons, causing X-rays to be released, and the synchotron radiation can heat up material, further increasing the radiation deluge that these monsters put out (Wall; Kruesi 24,26, Shipman 179).
At the time of the discovery of the quasar, black holes were not accepted in the scientific community but as more evidence for them began to grow the more this explanation for quasars became acknowledged. More and more quasars were found, but a good majority existed in the past. Presently, few out there could still be functioning. As a whole, quasars seem to be dying out. Why? Moreover, with just a spectrum of the accretion disk of the SMBH and its orientation to us, what could we learn about the host galaxy? This is why little headway has been made in the field since their discovery (Wall, Kruesi 27).
To understand how an object operates, it often helps to know how it arises in the first place. Astrophysicts think that galaxies with obese black holes in their centers are correlated to the quasars we see. After all, it would require a massive object to pull in all that matter to make it as bright as we witness with quasars. In the past, the matter around the black hole was mostly basic gas and did not have the heavy materials that come from supernovas, or the violent death of a massive star. Spectrographic data seems to confirm these conditions for quasars, like ULAS J1120+6641, show lots of hydrogen, helium, and lithium but no heavy elements. It also implies that quasars’s have their black hole form first and then the stars during galactic mergers which may be why we see less quasars in the present than in the past. The merger occurs, the black hole has lots to feed on, then becomes silent (Howell, Scoles).
Researchers do have evidence of a quasar having had a merger in its past. Observations from both the Chandra and XMM-Newton X-ray Observatories found a galaxy gravitationally lensing quasar RX J1131-1231 from 6.1 billion years ago and with a mass 200 million times that of the Sun. Like all black holes, this quasar spins. However, because of the mass of the object, it twists space-time so much, known as frame dragging. It pulls iron atoms to near the speed of light and excites the electrons in them to emit photons in the radio range. Normally this would be at a level too small to detect but because of the luck in having the object lensed the light is focused. But by comparing the excitement level of the photons to the speed needed to achieve it you can calculate the spin of the quasar. Amazingly, the quasar was spinning between 67-87% that the maximum value attained by general relativity allows for. The only way the quasar could spin so fast was if it had a merger in the past increasing the angular momentum (Francis, Shipman 178).
The Hubble Space Telescope observations seem to confirm this also. After tuning into the IR portion of the spectrum, where the extreme brightness of a quasar doesn't completely blot out its host galaxy, Hubble looked at 11 quasars that were partially obscured by dust (which further helped lower the quasar brightness) and also about 12 billion light-years away. images seem to show that all the host galaxies are in the process of merging, and at such an early stage of the Universe's life. According to Eilat Glikman (Middlebury College) and C. Megan Urry (Yale University), the authors of the research, quasars seem to peak at this time, then start to die off (Rzetelny "The," STScl "Teenage").
And then there is Markarian 231 (Mrk 231), the closest quasar to the Earth at 600 million light-years away. After examining UV readings made by Hubble, scientists found that drops occurred in the data. That would only happen if something was absorbing the UV light, which is generated by the SMBH's accretion disc. What could do that? Another black hole, acquired possible from a merger in the past. The two black holes are 150 million solar masses and 4 million solar masses and complete an orbit every 1.2 years. Further data showed a huge outflow of material caused the black hole to cut-off its food supply via the jets shooting out from it as far as 8,000 light-years away and going as fast as 620 miles per second. The amount sent off combined with the star presence of Mrk 231 indicates this active galactic nuclei is nearing the end of its active phase (STScl "Double", Gemini).
Another piece of proof for past mergers came from quasar 3C 186, located 8 billion light years away with a mass of 1 billion solar masses. Scientists spotted this quasar and noticed how it was offset from the host galaxy, then using spectroscopy concluded that it was not only a quasar but also moving at a fast pace of 4.7 million miles an hour and was 35,000 light years away. A huge amount of energy would be required to launch the quasar out, like...a merger, where one black hole was way larger than the other and so launched the companion out of the galaxy it resided (Klesman "Astronomers").
One astronomical mystery that ended up being indirect evidence for these mergers was found by Hanny van Arkel, a citizen using the Galaxy Zoo website for classifying space objects. She found a strange green filament in space and dubbed it Hanny's Voorwerp (Dutch for Hanny's object). Turns out, they seem to be around quasars that were active in the past but no longer are and are a relic from that heavy active time. UV radiation hits these remnants and that is what excites them into being green. What could have prompted such a change in a quasar? If it had merged with another galaxy and caused a huge spike in activity before settling down. The filaments seen should eventually fall into the newly merged objects and make an even bigger galaxy (STScl "Dead").
So we know that it is possible for quasars to have mergers in the past, but how can we learn more about them? What other information could we use to help us differentiate them from one another? Scientists have a main sequence of sorts with quasars to help them, much like the H-R diagram associated with stars. But why does it exist? As it turns out, it is possible to show how the viewing angle (or how it is oriented with respect to us) and the amount of material entering the black hole can be used to explain it. Work by Yue Shen of the Carnegie Institute for Science and Luis Ho of the Kavli Institute for Astronomy and Astrophysics looked at over 20,000 quasars from the Sloan Digital Sky Survey. After applying many statistics to the information they found that the Eddington ratio, or how efficient a black hole is eating at the matter surrounding it because of gravitational force fighting light pressure is one of the key components. Another is how much you are viewing it an angle for if the quasar is flat against the sky you see all its action but if it is edge on to you then you will see little activity. With both of these in hand, a better understanding of the possible growth of quasars may be achieved (Carnegie).
However, it should be mentioned that evidence for the SMBHs in their host galaxies growing with them versus merging into them exists. Most SMBHs seen in quasars are 0.1-0.2% of the host galaxy's bulge at the center, based off luminosity versus mass charts. Of course, you got oddballs for this piece of evidence also. Take for example NGC 1277, whose SMBH is 59% the mass of that galactic bulge, according to a study by Renico van den Bosch (from the Max Planck Institute for Astronomy). Totallying in at 17 billion solar masses, it is a beast. What could it mean? (Kruesi 28).
And then a new mystery grew. Komberg, Kravtsov, and Lukash, three scientists working on a joint Astro Space Center and New Mexico University study, looked at quasars that form a Large Quasar Group (LQG). What is this exactly? For this study, they were chosen as groups of 10 or more quasars that were at least twice the density of local quasar groups and that had solid redshift values. This was all done to ensure that reliable trends could be found by removing background data. After this parsing, only 12 groups were analyzed. The scientists concluded that the quasars may have acted as matter density sites in the past much like how galaxies seem to follow a dark matter web. Why this is the case is unclear but it could have its origins in the early universe. The LQGs also seem to correspond to areas where large elliptical galaxies (which are considered very old) reside. This makes sense if quasars are from the past and potentially evolved into this. There is even possible evidence that current galaxy superclusters may have origins from LQGs (Komberg et al).
But wait, there is more! Using the Very Large Telescope in Chile, Damien Hutsemekers found that out of 93 known quasars from the early universe (when it was 1/3 its current age), 19 of them had their rotational axis lined up nearly parallel to each other. This somehow happened despite them being billions of light-years away. The axis also happen to point along the path of the cosmic web the quasar resides on. And the chances of this being a false finding are less than 1%. What does it mean? Who knows... (Ferron "Active," ESO).
Looking For Patterns
Scientists realized that they had too many questions and needed something to help lay out the information in a meaningful way. So they came up with an HR diagram equivalent for quasars, using 20,000 found by the Sloan Digital Sky Survey. Like the famous star diagram that shows interesting evolutionary characteristics for stars, this quasar diagram also found a pattern. Yes, the Eddington ratio is shown to play a role, but also the angle of the quasar with respect to us. When you plot the spectrum line width against the Eddington ratio, one realizes that there is a color relationship as well. And they make a nice wedge shape also. Hopefully, it can lead to the same type of understandings that the HR diagram did (Rzetelny "Massive").
But of course a new mystery is always waiting in the wings. Take SDSS J1011-5442, a quasar that seemingly disappeared. According to a study by Jessie Runnoe (University of Penn State) released at the January 2016 AAS Meeting, hydrogen alpha emissions were studied for a group of objects by SDSS from 2003 to 2015. In 5442's case, those emissions went down by a factor of 50 and now it looks like a normal galaxy. Why did it stop? The answer remains unknown but it is likely that all the material surrounding the immediate vicinity of the quasar has been consumed and now without food they are shutting down (Eicher, Raddick).
Another mystery lies in a study done by Hai Fu and the team at the University of Iowa. In their July 31, 2017 article in the Astrophysical Journal, 4 quasars were discovered in dust heavy star forming galaxies. They found that all of them were kicking material out at high energy so...maybe this was an early process that started star formation. But quasars are not known for being found in these conditions, so maybe these ones are low density regions which allow us a glimpse into their inner workings. This then may imply that more quasars exist than we know of...for now (Klesman "Quasars").
It is worth mentioning that an alternate method for quasar activity has been put out. Called the cold gas accretion theory, it states that quasars can be fed through cosmic filaments, which come from the structure around galaxies courtesy of dark matter. This doesn't eliminate mergers as a possible growth mechanism but it does provide a plausible alternative, according to Kelly Holley-Bockelmann (an assistant professor of physic and astronomy from Vanderbilt University) (Ferron "How").
It is also important to note a major alternate theory to all the above has been postulated by scientists who study steady-state theory, or the idea that the universe is eternal and is constantly creating new matter. Based on the work of these scientists, the redshift seen is actually a prediction of what an observer would see if new matter was being created. This implies that quasars are actually the source of new matter being created, similar to the hypothetical white hole. Not many consider this idea to be serious however. Still, it is important to consider all the possibilities especially when you deal with something as strange as a quasar.
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