# Quasars

When we view the Earth or the night sky, we

can see plants, people, cars and cities, your coffee mug, the Moon, the stars. This is all we have ever seen with our two eyes sine we've lived our whole lives seeing things with visible light. But a bee can see something that we cannot see. They see light—invisible to us—bouncing off of flowers. We have built abiotic, mechanical "eyes" which can see a very bright glow covering the entire night sky. But when we look at the night sky using just our two eyes, all we see is darkness. Just like how the bee's eyes were seeing light invisible to us, the same is true about these mechanical eyes. We can see some things, but not everything else. What's astonishing is that we've spent our whole lives seeing only the tiniest fraction of what these superior mechanical eyes can see.

The advent of infrared and radio astronomy has given us a better pair of eyes so that now, like the bee, we can see more. We no longer see a smattering of a few thousands stars set against a black canvas when we view the night sky; instead, the smattering is exploding stars and quasars emanating immense jets of light from the centers of galaxies and the canvas is a bright, colorful glow.

Around the 1950s, astronomers saw the "radio light" emanating from a far off object (which they called 3C 273) which, to them, looked like a star. At the time, we didn't have techniques like radio interferometry which would have allowed us to see every little detail of a baseball (where the baseball is the source emitting a little bit of radio waves) on the Moon. The best that we could do is to narrow down the location of this "star" (radio source) to some small region of space. Although we knew that the radio waves we were detecting must have had been coming from this tiny patch of the sky, the problem was that there were many stars located in this patch of sky—any one of them could've been responsible for the radio waves we were getting.

Astronomers got around this problem by using a clever trick. As the Moon was in transit across this tiny patch of sky, we could pinpoint very precisely the spot in that tiny patch where the Moon was blocking the radio waves. This gave us an even smaller patch of sky to look for the radio source. After doing this, astronomers pointed the most powerful visible-light telescope at the time (the 200-inch Hale telescope at the Palomar Observatory) towards this smaller patch. They spotted a single object within that patch which looked like a star—this must have been the radio source they were detecting the radio waves from. (When the Moon passed by all the other stars, the radio waves didn't get blocked out.)

Figure 1: "The 200-inch (5.1 m) Hale Telescope (f/3.3) was the world's largest effective telescope for 45 years (1948-1993). It is still a workhorse of modern astronomy. It is used nightly for a wide range of astronomical studies. On average the weather allows for at least some data collection about 290 nights a year."$$^{[1]}$$ Image credit: Caltech/Palomar Observatory

Spectra of 3C 273

Figure 2: "The spectrum of the quasar 3C 273. The strongest emission lines present are Balmer lines of hydrogen, as marked. In each case, the arrow is drawn from the rest wavelength to the observed wavelength of the line— shifted redward in each case by 15.8%. The other emission lines apparent in the spectrum are due to oxygen, helium, iron, and other elements. Credit: Michael A. Strauss, from data taken by the New Technology Telescope at La Silla, Chile; M. Türler et al. 2006, Astronomy and Astrophysics 451: L1– L4, http:// isdc.unige.ch/ 3c273/# emmi, http:// casswww.ucsd.edu/ archive/ public/ tutorial/ images/ 3C273z.gif

Tyson, Neil deGrasse; Strauss, Michael A.; Gott, J. Richard. Welcome to the Universe: An Astrophysical Tour (p. 242). Princeton University Press. Kindle Edition."$$^{[2]}$$

The 200-inch telescope had a spectroscope incorporated into it which allowed a Caltech professor named Maarten Schmidt to use spectroscopy to analyze this "star's" (at least, what we initially thought was a star) spectrum. The spectrum that he recorded came as a surprise. The peak emission lines had the same signature as hydrogen, except there was one big difference: all of the lines (each one of them) were redshifted by 15.8%. This means that 3C 273 must have been moving away from us at an extraordinary speed due to Hubble expansion (that is, the expansion of space in our universe). Given the redshift (which we know is $$Δλ=0.158$$), we can use Doppler's equation to determine the recessional velocity $$V$$. If we then plug that value of $$V$$ into Hubble's law (which we discussed in this article in more detail), we find that the distance $$D$$ that 3C 273 must be away from us is roughly 2 billion lightyears.

What was so mysterious about 3C 273 was not its enormous redshift or how far away it was. When looked at through the telescope, 3C 273 looked point-like similar to what a star looked like; it could not, for example, have had been a galaxy since galaxies looked like extended objects with some shape (as opposed to a point). What was so mysterious about this "star," 3C 273, was that it was hundreds of times brighter than entire galaxies which were roughly just as far away. How could a single lonely star outshine an entire galaxy composed of billions of stars? This was the big mystery. Clearly, 3C 273 was no star. And so, astronomers gave this peculiar object a new name. Thy called it a quasar.

From the spectral lines we talked about earlier, we saw that the spectrum of this quasar included emission lines associated with hydrogen that were redshifted by an amount $$Δλ=0.158$$. If we plug this value into Doppler's equation, we can determine that the quasar is moving away from us at about 16% the speed of light (due to the expansion of space). If all of the atoms comprising the object were moving away from us, as a whole, all at 16% the speed of light, then we'd except the quasar's spectrum to be discrete where all of the emission lines are identical to those of their corresponding element, except all redshifted by 15.8%. But if you look at the spectrum in the graph in Figure 2, you'll see that it's continuous (not discrete): that is, we're detecting all wavelengths of light. This means that the atoms comprising 3C 273 must be moving relative to one another. The question is: how fast?

We know that if all of these atoms had zero relative motion, their redshjift would just be $$Δλ=0.158$$. Skipping over the nitty gritty details, we can deduce from the graph the deviations (which we'll call $$Δλ_p$$) of these atoms redshifts from 0.158. If we put $$Δλ_p$$ into Doppler's equations, that gives us there relative velocities. What we find is that the gaseous and dust particles composing this object must by revolving around the center of 3C 273 at roughly $$6,000 km/s$$. That is an astonoshing speed!

Calculating the size and mass of 3C 273

Figure 3: "This image from Hubble’s Wide Field and Planetary Camera 2 (WFPC2) is likely the best of ancient and brilliant quasar 3C 273, which resides in a giant elliptical galaxy in the constellation of Virgo (The Virgin). Its light has taken some 2.5 billion years to reach us. Despite this great distance, it is still one of the closest quasars to our home. It was the first quasar ever to be identified, and was discovered in the early 1960s by astronomer Allan Sandage. The term quasar is an abbreviation of the phrase “quasi-stellar radio source”, as they appear to be star-like on the sky. In fact, quasars are the intensely powerful centres of distant, active galaxies, powered by a huge disc of particles surrounding a supermassive black hole. As material from this disc falls inwards, some quasars — including 3C 273 — have been observed to fire off super-fast jets into the surrounding space. In this picture, one of these jets appears as a cloudy streak, measuring some 200 000 light-years in length. Quasars are capable of emitting hundreds or even thousands of times the entire energy output of our galaxy, making them some of the most luminous and energetic objects in the entire Universe. Of these very bright objects, 3C 273 is the brightest in our skies. If it was located 30 light-years from our own planet — roughly seven times the distance between Earth and Proxima Centauri, the nearest star to us after the Sun — it would still appear as bright as the Sun in the sky. WFPC2 was installed on Hubble during shuttle mission STS-125. It is the size of a small piano and was capable of seeing images in the visible, near-ultraviolet, and near-infrared parts of the spectrum."$$^{[3]}$$ Image Credit: ESA/Hubble [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

What could be causing them to move so fast? To answer that question, we'll have to start off by determining the rough diameter of the distribution of these particles. Why you need the diameter to answer that question is something we'll get to in the next paragraph. When we view this quasar through our telescopes, we can see it flickering. Its brightness dims, shines, dims, shines, etc. Suppose that the diameter of 3C 273 was one light-year. As the object "shined and got brighter, we would expect that it would take one year to see the entire objet brighten up. The light emitted by the mass comprising the quasar which is closest to us (which I'll call $$m_1$$) will take $$~2×10^9\text{ years}$$ to get to us whereas the mass comprising the quasar which is farthest away from us will take $$~(2×10^9+1)\text{ years}$$ to get to us. But the fact that when we observe this quasar, it takes about a month to see it brighten means that (using this argument) the quasar must be roughly $$1$$ light-month in diameter.

That might sound very big, but actually it's comparatively small. The typical distances between stars in the Milky Way is several light-years—an unimaginably small fraction of the whole extent of the galaxy. 3C 273 sits in the center of a giant elliptical galaxy; it must occupy an incredibly small portion of this galaxy.

We now return to our original question: what is causing the gas and dust to move so fast? On the scale of light-years and beyond, it is just one force—the force of gravity—which is responsible for object's having the motion that they have. The effect of gravity caused by a large mass determines the motion of an object (i.e. gas particle) orbiting that mass. This problem is analogous to how we'd determine the mass of the Milky Way from the speeds of stars orbiting in it. In both cases, we use Newton's law of gravity and second law of motion.

The overwhelming majority of the mass comprising 3C 273 must be comprised at its center and we can therfore treat the mass of 3C 273 as a single point mass $$M$$. The net force $$\sum{\vec{F}}$$ acting on a gaseous particle orbiting around this mass will be just the gravitational force:

$$\sum{\vec{F}}=F_g.\tag{1}$$

Substituting for $$\sum{\vec{F}}$$ and $$F_g$$, we have

$$ma_{\text{gas particle}}=G\frac{(M_{\text{3C 273}})(m_{\text{gas particle}})}{r^2}.\tag{2}$$

This gas particle will rotate around the point mass $$M$$ in roughly a circle and, therefore, $$a_{\text{gas particle}}=\frac{v^2_{\text{gas particle}}}{r}$$. Substituting this into the equation above, we get

$$m_{\text{gas particle}}\frac{v^2}{r}=G\frac{(M_{\text{3C 273}})(m_{\text{gas particle}})}{r^2}.\tag{3}$$

Let's cancel the mass $$m$$ on both sides and rearrange the equation in terms of $$M_{\text{3C 273}}$$ to get

$$M_{\text{3C 273}}=\frac{rv_{\text{gas particle}}^2}{G}.\tag{4}$$

We know that these gas particles are traveling at speeds of about $$6,000km/s=6×10^6m/s$$ and the radius $$r$$ of their orbits is about $$1\text{ light-month}=7.88×10^{14}m$$. If we substitute the results into Equation (4), we find that the mass $$M_{\text{3C 273}}$$ of the quasar is roughly $$2×10^8$$ solar masses.

What is a quasar?

The only way that $$2×10^8$$ solar masses could be crammed into a space of only $$1$$ light-month in diameter (or less) is if this object were a blackhole. The typical blackhole in a galaxy was formed by all the mass of the inner core of a supergiant star collapsing into a single point of zero size. Such black holes—which are fairly ubiquitous in even just the Milky Way—are called stellar black holes. Their mass is identical to that of the stellar core before it collapsed. Since the largest supergiant stars have an upper size limit of roughly a few dozen solar masses, we do not expect the mass of a commonplace stellar black hole to be much more massive than a few dozen solar masses. This is expected even if we account for the fact that black holes can grow and get more massive by sucking up infalling matter since interstellar space is mostly empty. The black hole associated with 3C 273 is an entirely different kind of black hole than a stellar black hole and is called a supermassive backhole.

The quasar 3C 273 is therefore an object which consists of an immense disk of gas and dust with a supermassive black hole in the center of this disk. Earlier, we asked the question: how could it be that this object can outshine an entire galaxy? The short answer is: because the tidal forces exerted on the accretion disk by the blackhole are so extraordinary. The sections of the quasar's acretion disk which are closer to the blackhole move with greater tangential speeds than the out sections. This causes portions of the acretion disk to rub against each other and exert frictional forces on each other. The frictional forces causes the gas to heat up to hundreds of millions of degrees and to glow brighter than entire galaxies.

Figure 4: Illustration of the various different components of a quasar.$$^{[4]}$$ Image credit: Futurism.

Near the center of the accretion disk where the supermassive black hole is, gaseous particles are ejected at an angle perpendicular to the accretion disk and galactic plane from both sides (the "top" and "bottom") of the accretion disk illustrated in Figure 4. The gas is ejected so energetically that it coalesces into strands of superheated plasma which can extend up to millions of light-years away from the galaxy. That, to me, is somewhat remarkable that "thin" strands of gaseous plasma extend for millions of light-years through the mostly empty depths of intergalactic space.

Quasars and cosmic evolution

Quasars tell a story of cosmic evolution. And telescopes are time machines: for example, if we are looking at a star one billion light-years away, we are seeing it as it was a billion years ago.

Figure 5: A 3-d map obtained by the Sloan Digital Survey of billions of light years of our local universe. Each dot is an entire galaxy and this survey shows millions of them. As you can see from this image, on the scale of billions of light-years the galaxies of congregated into a "web"-like structure of filaments and strands known as the cosmic web. Image Credit: M. Blanton and SDSS

The Sloan Digital Survey mapped out the locations of about 2,000,000 galaxies and 400,000 quasars as shown in Figure 5. According to this survey and others, nearly all the quasars in the heavens are billions of light-years away. We find almost no quasars closer to us and less far away. The observations indicate that there were many hundreds of thousands of quasars in the younger universe; but in the older universe (the one we're in) there are hardly any quasars at all. This means that the cosmos must have had changed since then.

Let's try to think about how this cosmic evolution unfolded. We know from the Sloan Digital Survey that the centers of many young galaxies (400,000 of them) were active: which is just a fancy way of saying that their central supermassive black-holes were still busy sucking up matter from their surrounding accretion disk and, also, still spitting out large streams of superheated plasma into the background of intergalactic space. But the fact that, despite the ubiquity of quasars we see in the younger universe, we see hardly any quasars at all in the older universe implies the following: all of the matter comprising the accretion disks of old, primordial quasars must have had gotten swallowed by their supermassive black holes. Our Milky Way is just one example (among all the countless nearby old galaxies) of a galaxy which, despite having a central supermassive black hole, is no longer active.