Object 123: QSO 1331+170

Podcast release date: 29 April 2024

Right ascension: 13:33:35.8


Epoch: ICRS

Constellation: Coma Berenices

Corresponding Earth location: About 250 km south-southwest of the southernmost point of the Big Island of Hawaii

This episode's coordinates point to the object QSO 1331+170 in the constellation Coma Berenices. The letters "QSO" stand for quasistellar object, which is another name for a quasar. The digits give the location of the object in an older coordinate system.

So, a quasar is one of many different types of galaxies with an active galactic nucleus (AGN). I've talked about AGN a lot. An AGN contains a supermassive black hole, a disk of gas and dust flowing into that black hole, and jets of gas that emerge from the poles of the AGN. I'm just going to keep this description short for this episode, and I really hope that my descriptions of AGN don't get too repeptitive in this podcast series. Anyway, quasars are a subset of galaxies with AGN that are very far away and where the jets are kind-of pointed towards the Earth so that they aren't really noticeable. When quasars were first discovered in photographic plates in the mid-twentieth century, they looked like hazy star-like objects, which is why they are called quasistellar objects. These days, it doesn't make much sense to thing of these galaxies as "quasistellar", so I recommend not thinking about it too hard, or else you will hurt your brain.

QSO 1331+170 was discovered in a survey at radio wavelengths with the Molonglo Observatory in the early 1970s [1]. Initially, it did not necessarily seem that different from any other quasar. Then, shortly after its discovery, people performed spectroscopic observations of it in the visible part of the electromagnetic spectrum.

Because the expansion of the universe makes galaxies that are further from our own appear to be moving away faster than galaxies nearby, quasars seem to be moving really fast because they are really far away. Spectroscopic observations usually look for the Doppler shifting of the light emitted from or absorbed by specific atoms and molecules at specific wavelengths to determine how fast those objects are moving from the Earth, and because those wavelengths of light look like bright or dark lines in spectra, they are usually called spectral lines.

QSO 1331+170 was found to be moving so fast from the Earth that the wavelengths of the emitted spectral lines from hot interstellar gas in the galaxy were stretched to 3.09 times their original size. Astronomers like to express this wavelength stretching using a term called z where the ratio of the observed wavelength size to original wavelength size is set equal to 1+z. In other words, z equals 2.09 for QSO 1331+170 [2]. This z corresponds to a distance where the light from the quasar would take 10.5 billion years to reach the Earth. All of this, actually, was not unusual for quasars discovered in the 1970s.

What was unusual about the spectrum of QSO 1331+170 was that a few different spectral lines associated with atomic hydrogen were detected in absorption in the spectrum of the quasar, and those spectral lines were less redshifted than the quasar itself [3]. In other words, the wavelength at which light was being absorbed was shorter than the wavelength at which light was being emitted by the quasar, which meant that whatever was doing the absorbing was moving more slowly than the quasar. Moreover, the absorption was really strong, which meant that whatever was doing the absorbing contained a lot of hydrogen gas.

The people who discovered this first thought that this cold atomic hydrogen gas was somehow part of the quasar. For example, it could have been in some sort of gas clouds ejected from the quasar towards the Earth at really high velocities, thus making it look like the gas clouds were moving away less slowly that the rest of the quasar itself. However, the real explanation turned out to be that the absorbed light was created by objects that were completely unrelated to the quasar but that happened to be located between us and the quasar, and, just like more redshifted objects (where the wavelengths of light are more stretched out) are also further away, these objects absorbing light at less stretched out wavelengths were much closer to the Earth.

One of the light-absorbing objects has attracted a lot of interest because of how much light it was absorbing. This object was at a redshfit where z equals 1.78 (or where the light was stretched out to only 2.78 times its original size, or where the light from the gas took 10.0 billion years to reach the Earth) [3]. Because this object contains a lot of atomic hydrogen gas that absorbs light at the wavelengths of light characteristic of hydrogen, including a spectral line called Lyman alpha, and because the absorption lines look so dark in spectra of this absorbing object, it has been called a damped Lyman alpha system. Given how much light this object absorbs, it's probably an entire galaxy sitting between us and the quasar QSO 1331+170. I've actually discussed objects like this before in this podcast series, but I think what makes this specific damped Lyman alpha system so special is that it was discovered a really really long time ago. It wasn't the first such damped Lyman alpha system discovered, but it was close to being one of the first.

This damped Lyman alpha system became a really popular object for astronomers to look at. They basically wanted to see what other weird stuff they could find in the spectrum. One study managed to rediscover hydrogen gas absorbing light at radio wavelengths [4]. Another found carbon, oxygen, magnesium, aluminum, silicon, and iron in the damped Lyman alpha system [5]. Another found molecular hydrogen [6], which is a bit different from atomic hydrogen in that it is colder and is also much more likely to form new stars within other galaxies.

These days, astronomers have detected various intergalactic gas clouds and various galaxies absorbing light in front of various other quasars, so the damped Lyman alpha system in front of QSO 1331+170 isn't considered to be that special anymore. However, it was still one of the first damped Lyman alpha systems ever discovered, and I think it deserves recognition for its special role in the history of quasar spectroscopic studies.


[1] Hoskins, D. G. et al., Positions and some identifications for III sources of about I flux unit at 408 MHz, 1972, Australian Journal of Physics, 25, 559

[2] Ahn, Christopher P. et al., The Ninth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Baryon Oscillation Spectroscopic Survey, 2012, Astrophysical Journal Supplement Series, 203, 21

[3] Strittmatter, P. A. et al., The Absorption Line Spectrum of 1331+170, 1973, Astrophysical Journal, 183, 767

[4] Wolfe, A. M. and Davis, M. M., Detection of 21-cm absorption at z ~ 1.8 in the quasi-stellar object 1331+170., 1979, Astronomical Journal, 84, 699

[5] Chaffee, Frederic H., Jr. et al., Molecules at Early Epochs. III. The Lyman-Alpha Disk System toward 1331+170, 1988, Astrophysical Journal, 335, 584

[6] Cui, Jun et al., Molecular Hydrogen in the Damped Lyα Absorber of Q1331+170, 2005, Astrophysical Journal, 633, 649


Podcast and Website: George J. Bendo

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