Quasars are very bright objects that are generally billions of light years away. We now know that they are a kind of active galactic nuclei (AGN) and are powered by supermassive black holes in the centers of galaxies. We know this in part because they emanate from within galaxies, and their energy comes from a region only a few light years across. But how do we know the size of something from billions of light years away?

At that distance, a quasar is too small for us to measure its size directly, but we can do it indirectly by measuring the variation in a quasar’s brightness. Take, for example, the brightest quasar in our sky (in the visible spectrum) known as 3C 273. It’s redshift puts it at about 2.5 billion light years away. Because of its brightness and relative closeness, it is one of the most studied quasars. So we actually have quite a bit of data on it. One of the things we’ve observed is that its brightness oscillates over time, getting slightly brighter and dimmer about 15 times a year. We see this variability in other quasars, but 3C 273 is the most accurately measured.

Quasars plotted as a function of their size.

A varying brightness is interesting because it tells us something about the size of the source. Basically if an object is changing brightness, it can’t do that at a rate faster than the time it takes one side of the object to know what the other side is doing. That time is limited by the speed of light, so if you know the rate at which brightness varies, you know the maximum possible size of the object. Since these objects are also greatly redshifted we have to account for that as well (high redshift means time dilation), but that’s pretty easy to do. So we can take our quasar and AGN observations and calculate a maximum size for each one. Taking data for various quasars, I’ve plotted how many are at each particular size. While they vary in size, the most common size is most commonly around 5 on this graph, which equates to 100,000 AU, or about 1.5 light years across.

That might seem pretty big, but remember that these objects are brighter than our galaxy (consisting of 200 billion stars). On a galactic scale these objects are tiny. Given their large brightness and tiny size, we can say pretty confidently that they are powered by black holes. There’s other evidence to support this conclusion, but that’s a story for another time.

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Quasars were once known as quasi-stellar radio sources due to their intense but nearly point-like source of radio energy. When first discovered in the 1930s, little was known about their underlying cause. By the 1950s we knew by their redshifts that they were billions of light years away, which meant they must be unbelievably luminous. We now know that quasars are active galactic nuclei, powered by supermassive black holes in the centers of galaxies. Quasars occur when the black hole of a young galaxy consumes large quantities of gas and dust. During this active stage of the black hole the material is superheated and large jets can be emitted by the black hole. It’s thought that most, if not all, galaxies enter such a stage in their youth, but this intense active period is short-lived on a cosmic scale. So it’s unusual to see several quasars clustered together. But now we’ve found a rare quartet of quasars, and it’s raising a few eyebrows.

The discovery was presented in the latest issue of Science, which finds four quasars in a single large nebula. The nebula is about a million light years across and has a mass of about 100 billion Suns, which is surprisingly large. But the presence of four active galactic nuclei is highly unusual. Quasars make up a relatively small fraction of galaxies, so the odds of four of them being so closely packed purely by chance is about 1 in 10 million. This would seem to imply that they have some kind of common origin, but it’s unclear what that might be. The usual computer simulations don’t predict this kind of close quasar alignment.

It’s possible that this is just an odd random clustering, or it’s possible that our models of galaxy formation might have to be tweaked, but it’s too early to tell at this point.

Paper: Hennawi et al. Quasar quartet embedded in giant nebula reveals rare massive structure in distant universe. 348 (6236): 779-783 (2015)

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The wispy green clouds seen in these galaxies are unusual because they are so bright. They are emission nebula stimulated into glowing due to a burst of ultraviolet light. That by itself isn’t unusual, but the central quasars aren’t particularly bright. The brightness of these surrounding clouds indicates that the quasars were quite bright in the past. 

Given the scale of these clouds and their current brightness, the light stimulating the emission originated from the quasar tens of thousands of years ago. That might seem like a long time for a quasar to go quiet, but on a cosmic scale that’s pretty fast, and quasars don’t typically vary much in brightness. So what’s going on?

One idea is that the quasars are powered not by one supermassive black hole, but two. Two closely orbiting black holes could disrupt the flow of matter into each other, and this would cause the brightness of the quasar to vary. This would seem to be supported by the shape of the clouds themselves, which tend to have a twisted, spiral shape indicative of merged galaxies.

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Radio astronomy is incredibly precise. This is particularly true when they’re used in combination through a process known as Very Long Baseline Interferometry (VLBI). It is so precise that by observing quasars we can measure not only changes in Earth’s rotation, but also tectonic drift between radio telescopes.

The way VLBI works is by measuring how long it takes for fluctuations in a quasar’s light to reach different radio telescopes. Since light travels at a constant rate, the difference in arrival time can be used to determine the difference in distance. Using an array of telescopes you can determine not only the distance between the telescopes, but the precise location of the signal. VBLI can determine the distance between antennas within millimeters, and the position of a radio source to within a fraction of a milliarcsecond.

Because quasars are so distant, they can be treated as fixed points of reference in space. This makes them useful for determining the motion of objects relative to them, such as the precise motion of Saturn, or the Magellanic clouds. But because they are fixed points in the sky, any shift in the location or timing of quasars must necessarily be due to a change in the position of the telescopes themselves, or the rotation of the Earth.

What’s amazing about this technique is that it uses objects billions of light years away to measure millimeter shifts in position on the Earth.

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There’s been press recently that astronomers have discovered a “spooky” or “mysterious” alignment of quasars across the universe. While such claims make great headlines, the new results aren’t spooky at all, nor are they that mysterious. They are somewhat interesting, so it’s worth discussing.

The headlines are based on a new paper in Astronomy and Astrophysics that looks at the polarization of quasars in large cosmic structures. A quasar is an intense source of energy powered by a supermassive black hole in the center of a galaxy. Often the light from a quasar is polarized, meaning it has a particular orientation. This new paper looked at the polarization of 93 quasars with good data.

It turns out that there are two ways that quasar light can be polarized. The first is due to the diffuse gas and dust between the quasar and us. Lots of light sources are polarized by the interstellar medium, and quasar signals are no exception. The second is that the quasars can be inherently polarized. This second case is useful because inherent polarization is typically either parallel or perpendicular to the rotational axis of the black hole and surrounding galaxy, so knowing the polarization you know something of a galaxy’s rotational alignment.

We can’t always tell the difference between the two types of polarization, but when the polarization is particularly sharp, it’s almost always due to an inherent polarization of the quasar. Of the 93 quasars in this particular study, 19 had sharp polarizations. So the authors looked at these 19 quasars, and found that their inherent polarizations tended to align with the structure of their galaxy clusters. On very large scales, the universe tends to be super large clumps of galaxies connected by diffuse filaments of stars and galaxies. So the quasar polarizations tended to align with the filaments. This would imply that their galaxies are also aligned with the filaments.

The odds of this happening purely by chance is about 1%, which makes it unusual, but not astoundingly so. What’s more interesting is that these filaments can span billions of light years, so the quasar alignments are over the same range. Does that make it spooky? Not really. In fact computer simulations of cosmic evolution actually predict some correlation between filaments and galaxy alignment. This new work is the first observational evidence of such an alignment. The level of observed is somewhat higher than predicted levels, which could suggest a deeper underlying mechanism, but the evidence isn’t strong enough to make that conclusion solid.

So what we’ve found here is not something spooky, it is just evidence of more complexity within the structure of the universe.

Paper: Damien Hutsemékers, et al. Alignment of quasar polarizations with large-scale structures. A&A 572, A18 (2014)

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Quasars are among the most energetic things in the universe, and are intense sources of radio waves and visible light. Because of their great distance, they appeared almost point-like to early observers. Thus they were given the name quasi-stellar radio sources, or quasars for short. For several decades the source of their great power was a mystery, but we know know that they are powered by active supermassive black holes in the centers of galaxies. As we’ve observed more quasars, we’ve found that they can vary significantly in brightness, redshift, and line spectra.

But despite this variety, there are some commonalities. One big commonality is known as Eigenvector 1, which traces correlations between emission lines of iron, oxygen and hydrogen, which in turn correlates with properties such as x-ray strength and the like. Eigenvector 1 was first discovered in 1992, so we’ve long known that it works, but why it works has been a bit of a mystery. Now a new paper in Nature may shed light on that mystery.

In this new work, the authors looked at two line spectra properties: the width of an emission line called hydrogen beta (Hβ) and the strength of an iron emission line known as Fe II. These are useful because they are known to correlate with specific properties of quasars.

The broadness of Hβ is a measure of the rotational motion of the accretion disc along our line of sight. As the material of the accretion disk rotates about the black hole, one edge of the disk is moving toward us slightly, while the other edge of the disk is moving away from us slightly. This means light from one edge is slightly blueshifted, while the other edge is slightly redshifted. This is an effect called line broadening. Since the speed of material in a black hole accretion disk is fairly consistent, the broadness of Hβ is a good measure for the orientation of the accretion disk relative to our line of sight. A broad line indicates an almost edge-on view, while a narrow one means the disk is almost face-on.

The Fe II line is connected to Eigenvector 1, and the strength of the Fe II line has been suspected to be a good measure of the efficiency at which the black hole accretes matter. Specifically, it seems to be a good measure for the Eddington ratio, which is the ratio of its rate of accretion to the maximum theoretical limit. So basically, these two lines are a measure of the orientation and energetic efficiency of quasars.

Credit: Shen and Ho

The authors then gathered the line spectra observations of 20,000 quasars and plotted them as a function of Hβ and Fe II. What they found was that the quasars cluster within a wedge. Given this wedge shape, it seems pretty clear that Fe II is, in fact a good measure of the Eddington ratio. The peak strength of Fe II aligns well with the orientation of the accretion disk.

What’s particularly striking about this graph is that it follows a similar method to that of the Hertzsprung-Russell (or HR) diagram. The HR diagram is a plot of stars by their brightness and color. When it was first developed around 1910, it revolutionized our understanding of stellar evolution by demonstrating that stars tend to cluster along a line known as the main sequence. We’ll have to wait and see if this new diagram for quasars leads to a revolution in our understanding of quasars and other active galactic nuclei, but it seems clear that this new work has a great deal of potential.

Paper: Yue Shen & Luis C. Ho. The diversity of quasars unified by accretion and orientation. Nature 513, 210–213. (2014)

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A couple days ago I wrote about a rather large cluster of quasars, and how it seemed to be larger than we’d expect for the universe as we know it. The post gathered the attention of an anonymous commenter, who pointed out an opposing view regarding this research. The rebuttal to the cluster research was published in MNRAS, and makes a rather simple claim: not all patterns are real. In other words, if you look deep enough for a pattern in your data, you are bound to find one, even if it is really just noise. 

The argument that “you can always find a pattern if you look for one” is a pretty standard rebuttal in data analysis. It is much like the old “correlation is not causation” argument. While both statements are basically true, simply stating them doesn’t buy you much. In data analysis, distinguishing the signal from the noise is a central part of the work, so we’re generally pretty careful to avoid false positives. So if you’re going to counter a research finding with “it’s just noise” you better be able to back up your claim with solid evidence. In this case, the new paper makes a pretty solid argument. As the author points out, what constitutes a “structure” depends a great deal how you define your connections between objects.

As an example, suppose you wanted to define the typical size of a person’s community. You could define a person’s community as only those people who are close friends. By that definition most communities would be quite small, usually less than a dozen. That’s probably not very accurate, so you could extend it to close friends and close acquaintances. Or perhaps even more broadly friends, acquaintances and their acquaintances. Define things too broadly and all of humanity is in the single community of Kevin Bacon. Define it too narrowly and we all look like social recluses. The reality is somewhere in the middle, with a mix of friends and acquaintances.

With quasars you can use a similar definition. Specifically, if two quasars are within a certain distance of each other, they can be considered part of the same cluster. If a third quasar is within that distance of either of the other two, it is also part of the cluster, and so on. Obviously, the distance you define for the will affect the scale of your cluster. Make that distance too small, and clusters are small and rare. Make it too large, and you get a cluster that spans most of the universe. In the original paper the minimum distance was set at 100 megaparsecs, or about 325 million light years.  That’s pretty big considering that our local cluster of galaxies (the local group) is only about 10 megaparsecs across. But is that distance too big to be meaningful? This new paper argues that it is.

Left: The “cluster” showing only members of the cluster with large dots. Right: The same region with smaller dots including non-cluster quasars. Credit: Clowes/Nadathur

To demonstrate this, the author looked at just how likely false positives were under this definition. They ran simulations of randomly distributed quasars, so that they satisfied the condition of homogeneity, then calculated how often a super-large cluster appeared by chance alone. The result was that such super clusters were rather common. Finding a cluster 4 billion light years wide isn’t that unusual. So it’s hard to say that the cluster is a legitimate structure. It certainly shouldn’t be seen as very unusual.

On their blog, the author hammers the point home by visually comparing the cluster with other quasars in the region, as you can see in the figure. If you only include members of the cluster and make your dots big, then it seems pretty clear. If you make the dots smaller and include all quasars in the region, the cluster doesn’t look quite as convincing.

Sometimes a mystery raises deep questions about our understanding of the cosmos. Sometimes it simply fades away when you look at things a bit more carefully.

Paper: Seshadri Nadathur. Seeing patterns in noise: gigaparsec-scale ‘structures’ that do not violate homogeneity.  MNRAS 434 (1): 398-406. doi: 10.1093/mnras/stt1028 (2013)

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Quasars are intense sources of radio energy that appear as almost starlike points. For this reason, when they were first discovered in 1939 they came to be known as quasi-stellar radio sources, or quasars for short. Early on it was not entirely clear what these objects were. They were incredibly energetic, and they also tended to have very large redshifts, which implied that they were very far away. It was also noticed that quasars weren’t randomly scattered across the sky, but instead tended to clump together in groups.

We now know that quasars are one example of what we call active galactic nuclei. They are powered by supermassive black holes in the centers of galaxies. This explains why they tend to be seen in groups. Since galaxies tend to clump into clusters and superclusters, their corresponding supermassive black holes are therefore also seen in clusters. Because of this, we can get a better understanding of how galaxies cluster across the universe by observing the distribution of quasars. Direct surveys of galaxies have been made out to about 6 billion light years, but quasars are so bright we can take that a bit further.

When we do studies of quasar clustering, we find a bit of a mystery. As a paper in MNRAS demonstrated, there are clustered structures of quasars that span more than 4 billion light years. Given big bang cosmology, and the presumption that the universe is homogeneous and isotropic, we wouldn’t expect clustering structures larger than about a billion light years across. Just how such huge quasar clusters could have formed is a mystery.

We have lots of evidence to support the big bang, so this mystery isn’t enough to overturn the big bang model, but it does show that there are still things we don’t understand about the cosmos. Things that span 4 billion light years apparently.

Paper: Roger G. Clowes, et. al. A structure in the early Universe at z ∼ 1.3 that exceeds the homogeneity scale of the R-W concordance cosmology. Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/sts497 (2013)

Note: for another view of this work, check out this post.

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Quasars are among the most energetic things in the universe, and are intense sources of radio waves and visible light. They typically have large redshifts, and are therefore very far away. Although they can be thousands of times brighter than the entire Milky Way galaxy, the source of their energy is relatively small, a fraction of our galaxy’s size, so they look almost point-like in the way stars do. For this reason they were called quasi-stellar radio sources, or quasars for short. For a long time the source of a quasar’s power was a mystery, and models developed to explain them were controversial. We now know that they are driven by black holes in the center of galaxies, and are part of a larger class of objects known as active galactic nuclei, or AGNs. These AGNs are driven by supermassive black holes.

When a supermassive black hole eats nearby gas, dust and stars, it is known as an active galactic nucleus (AGN). These active black holes have superheated material swirling around them in a circular disk, known as an accretion disk. They also have jets of gas and dust shooting out from their polar regions at a large fraction of the speed of light. The jets are formed by a fairly complex process, but basically some of the material from the accretion disk gets so much energy from the heat, magnetic fields, and such that instead of falling into the black hole it shoots away and escapes.

The material in the accretion disk is swirling around the black hole in a tight circle, which means all the charged particles in the disk are being accelerated. When you accelerate charges, they give off radio waves along their direction of motion, known as synchrotron radiation. If you look at an AGN from a bit of an angle, you would see intense visible light from the superheated accretion disk, and depending on your angle you might (or might not) also see intense radio waves, which makes it a quasar.

Because of their distance, quasars can be quite difficult to study. In particular it can be difficult to study the dynamics of these active black holes. This challenge is made worse because of their tremendous size. Since they are millions of solar masses, the features of quasars tend to change very slowly on human scales. The jets of AGNs, for example, evolve over centuries.

Fortunately, quasars have a smaller version, known as microquasars. A microquasar is a stellar-mass black hole or neutron star with a stellar companion. The companion is close enough that some of its outer material is captured by the black hole or neutron star. The captured material forms an accretion disk, just as supermassive black holes due. Because these binary systems give off intense x-rays as well, they are more commonly known as x-ray binaries. When they give off intense radio waves as well, they are known as microquasars.

Because microquasars are physically similar to regular quasars, with a compact massive core, accretion disk and jets, the dynamics of the two are likewise similar. But since the microquasar is stellar mass rather than a million solar masses, the timescale of a microquasar is much smaller. This means we can watch a system change over days rather than centuries.

You can see an example of this in the image above which shows the radio jets of the microquasar SS433 as they change over time. You can see the jets expand in a corkscrew pattern. This is because the accretion disk is precessing over time, and the jets spiral over time.

By observing the dynamics of microquasars we gain a glimpse into the complex dynamics of supermassive black holes and the mysterious quasars they drive.

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While they were once a mystery, we now know quasars are driven by black holes in the center of galaxies, and are part of a larger class of objects known as active galactic nuclei, or AGNs.  What makes quasars distinctive is that they are very bright, and their light is highly redshifted.  This latter property means that they are also very far away.  Because of their great distance, the light from quasars began their journey billions of years ago.  This means the study of quasars allow us to understand the earliest examples of active black holes.  The great distance of quasars also poses a challenge, because it is difficult to measure the properties of an object billions of light years away.  But sometimes a bit of luck will allow astronomers to make some good observations.  Such is the case of a recent paper in Nature which measures the rotation of a quasar’s black hole.

The lensed x-ray light from the quasar. Circles indicate regions used in the analysis. Credit: R. C. Reis, et al.

The particular quasar in question has a redshift of z = 0.658, which means its light left the quasar about six billion years ago.  It also happens to be behind a much closer galaxy from our viewpoint.  You might think that might make observing the quasar worse, but in away it is actually a good thing. The mass of the close galaxy acts as a gravitational lens, bending the light of the quasar a bit, and focusing it in our direction.  This means we actually see more light from the quasar than we would if the closer galaxy wasn’t there.  Of course the galaxy also distorts the light coming from the quasars, so the team had to reconstruct the image of the quasar using the gravitationally lensed light.

Doing that, they then had a strong enough x-ray source from the quasar to analyze its rotation.  This is done by looking at light reflected off its accretion disk.  The region around the supermassive black hole of the quasar generates intense x-rays, and some of it reflects off material in the accretion disk.  The motion of the accretion disk around the black hole means that some of the reflected light is redshifted more than the average for the quasar, and some less.  By measuring this difference it is possible to measure the rotation of the accretion disk, which in turn allows us to determine the rotation of the black hole.

The rotation of a black hole is often given as a parameter known as “a”.  This parameter can have a value between 0 (no rotation) and 1 (maximum possible rotation).  Sometimes in the popular press it is stated that the maximum rotation rate is the speed of light, but that isn’t quite how things work.  So we’ll stick with “a” as a measure of its rotation.  What the authors found was that this particular quasar had a rotation of at least a = 0.66, and very possibly as high as a = 0.87.  This is a very high rotation rate, and it likely means this particular black hole formed as a merger between two smaller black holes.

What makes this interesting is that it hints at early supermassive black holes forming by mergers.  By the time the universe was 7 billion years old, two supermassive black holes had formed, then merged to produce this fast-rotating supermassive black hole.  Of course this is only one example, so we don’t know if such fast-rotating quasars are common or exceptional.

What we do know is that even early black holes can rotate very fast indeed.

Paper:  R. C. Reis, M. T. Reynolds, J. M. Miller & D. J. Walton. Reflection from the strong gravity regime in a lensed quasar at redshift z = 0.658. Nature. doi:10.1038/nature13031 (2014)

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In 1916, Eddington demonstrated that there was a limit to how bright a stable star could be.  The basic idea is that the atmosphere of a star is being gravitationally attracted by the mass of the star (giving it weight), and this weight is balanced by the pressure of the deeper layer of the star.  For a star to be stable, the weight and pressure must be equal, so the star doesn’t collapse inward or push the atmosphere outward.
We typically think of pressure as being due to gas and such, but light can also exert pressure on a material.  We don’t notice light pressure in our daily lives because it is so small.  Even in our Sun, the pressure on the atmosphere is relatively small, so the weight of our Sun’s atmosphere is mostly balanced by the pressure of the plasma in the layer underneath it.  But if the Sun were brighter, the light it emits would push harder against the particles of the atmosphere.  What Eddington showed is that there is a limit where the pressure of a star’s light on the atmosphere is large enough to balance the gravitational weight of the stellar atmosphere entirely, known as the Eddington luminosity limit.  If the star were any brighter, the light of the star would push away the outer layers of the atmosphere, thus causing the star to lose mass.

The galaxy M83. Credit: ESO/IDA/Danish 1.5 m/R. Gendler, S. Guisard and C. Thöne

This same limit is often thought to hold for other objects, such as active galactic nuclei (AGNs) powered by black holes, but it also isn’t an absolute limit.  When Eddington derived the limit he assumed a star that is spherical and non-rotating.  Black holes are known to rotate, and their accretion disks are not spherical, so there have been proposed models that allow AGNs and other black holes to emit more power than the Eddington limit.  There have been searches for such super-Eddington luminosity, but so far results have been inconclusive.

Now a new paper in Science has analyzed the energy output of a black hole in the galaxy M83, and found that it has emitted sustained levels of energy beyond the Eddington limit.  Since the black hole was in a quiet phase when it was observed, the team could make an accurate determination of its mass by analyzing its accretion disk.  They then looked at the effect of its jets, which were produced in an active period.  They found that the energy of the jets clearly exceeded the Eddington limit.

By demonstrating that energy generated near black holes can exceed the Eddington limit, the authors have demonstrated that black holes can affect their environment in a more powerful way that originally thought.

Paper:  Soria R, Long KS, Blair WP, et al. Super-Eddington Mechanical Power of an Accreting Black Hole in M83. Science. (2014)

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Quasars are among the most energetic things in the universe, and are intense sources of radio waves and visible light.  They typically have large redshifts, and are therefore very far away.  Although they can be thousands of times brighter than the entire Milky Way galaxy, the source of their energy is relatively small, a fraction of our galaxy’s size.  For a long time the source of a quasar’s power was a mystery, and models developed to explain them were controversial.  We now know that they are driven by black holes in the center of galaxies, and are part of a larger class of objects known as active galactic nuclei, or AGNs.

While quasars are interesting in their own right, they are also used as an astronomical tool.  Because they are both very bright and very distant, they can be used as reference points when measuring the positions of celestial objects.  With large radio telescope arrays we can measure the position of quasars to within a milliarcsecond.  Since quasars are also bright at visible wavelengths, astronomical images that include a quasar are then known to a similar precision.

This technique was used recently to measure the motion of stars in the Large Magellanic Cloud.  With images from the Hubble telescope, the positions of stars in the LMC were measured relative to various quasars in the field of view.  Over time this allowed for the proper motion of stars to be determined.  What was found is that the stars in the LMC rotate around the central region with a period of about 250 million years.  Interestingly, that’s also about the same period as stars in our own galaxy.

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