George's Random Astronomical Object

Object 4: LBQS 0058+0155

Podcast release date: 23 September 2019

Right ascension: 01:00:54.1
Declination: +02:11:36
Epoch: J2000
Constellation: Cetus
Corresponding Earth location: About halfway between Nki National Park and Lobeke National Park near the southern border of Cameroon

LBSQ 0058+0155 is one of many quasars identified primarily by its coordinates. In the case of this specific quasar, sometimes the numbers are written with just a Q or B in front, and sometimes the numbers are just written by themselves. However, the coordinates are the right ascension and declination as of 1950. Since then, the Earth's rotational axes have shifted slightly, so the astronomical coordinate system has also shifted. Many other objects have names that refer to their coordinates in a more up-to-date system, but for some reason, many quasars are identified by their old coordinates.

In any case, this is a quasar. It is a type of galaxy with an active galactic nucleus containing a supermassive black hole, an accretion disc containing gas and dust falling slowly into the black hole, and jets of gas that appear above the poles of the black hole caused by some sort of complex magnetohydrodynamic effect involving the black hole and the accretion disc.

Astronomers would describe the distance to LBSQ 0058+0155 as z equals 1.959 [1]. This is related to the Doppler shifting of the light from the galaxy caused by the expansion of the universe. The speed at which things appear to be moving can be directly related to their distance. Things that are close to the Milky Way appear to be moving slowly, while more distant galaxies appear to be moving faster. For very distant galaxies, the velocities get so stupidly large that the Doppler shifting or redshifting is described by how much the light waves get stretched. When astronomers say that z=1.959, this means that the light waves get stretched to 2.959 times their original size. It's confusing to describe the distance to the galaxy in terms of light years or parsecs, but given the value of z, it could be said that the time it took the light to travel to Earth is 10.3 billion years. However, this depends on the physics of how the universe is expanding and how dark matter works, which I'm not going to discuss in this episode.

Astronomers really like to look at the Lyman alpha line in the spectra of quasars, including LBSQ 0058+0155. This is an ultraviolet spectral line at 122 nm that can be absorbed and emitted by hydrogen gas. Quasars also produce light referred to as continuum emission that appears at all ultraviolet wavelengths and that is close to constant, but the emission at 122 nm from hydrogen gas in the quasar appears very bright compared to the continuum and looks like a bright line in the spectrum. Because of the expansion of the universe, all of this ultraviolet light gets Doppler shifted to longer wavelengths. For LBSQ 0058+0155, the Lyman alpha line emission appears at 360 nm, which is close to the edge between visible violet light and ultraviolet light.

The area of space between many quasars and Earth contains clouds of hydrogen gas. Because of how the expansion of the universe works, these gas clouds are not moving as fast as the quasar itself. The hydrogen gas absorbs light that looks to them like it is at a wavelength of 122 nm, even though it is Doppler-shifted light from the quasar and even though that wavelength of light also looks Dopper shifted as seen from Earth. Each intervening gas cloud therefore produces a drop in brightness in the spectrum of the quasar which can also be called a dark spectral line. If a lot of these gas clouds are present, they produce a lot of these dips. This is called the Lyman alpha forest because, when the spectrum of a quasar is plotted in terms of brightness versus wavelength, the graph contains a lot of spikes like trees in a forest. I myself tend to think of of Lyman alpha animals living their Lyman alpha lives among the Lyman alpha trees in the Lyman alpha forest, but except for a guy I know named Chad, most other astronomers don't share my sense of humour.

In any case, LBSQ 0058+0155 is one of the many quasars where astronomers have observed this Lyman alpha forest [1]. The structures in such a spectrum are very useful for studying the structure of gas in the universe, which in turn is important for modelling how galaxies form. However, the light from quasars as seen from Earth sometimes pass through entire galaxies, and this produces not just a small drop in brightness at one wavelength but a big drop in brightness over a broad range of wavelengths. The galaxies that absorb the light are called damped Lyman alpha systems. The term damped does not indicate that the galaxy contains a lot of water or that someone appeared to spill coffee on the spectrum but instead refers to the mathematical function used to describe how the light is absorbed over a broad range of wavelengths.

The spectrum of LBSQ 0058+0155 contains one of these damped Lyman alpha systems [1,2]. Normally, the presence of the galaxies absorbing the light is indicated solely from how it absorbs light from the quasar behind it. It would be like being able to identify where someone is standing somewhere by seeing their shadow instead of seeing the person himself or herself. However, in the case of LBSQ 0058+0155, the galaxy itself was imaged directly using the Hubble Space Telescope about 20 years ago [1]. Imaging these damped Lyman alpha systems is very tough, even with the Hubble Space Telescope, because the closer galaxies are usually very faint compared to the quasar. Imaging the damped Lyman alpha system in this quasar was a remarkable achievement, especially since people were still developing the techniques for making such images. The absorbing galaxy is an edge-on spiral galaxy at a redshift of z=0.613 [1], which corresponds to light travel time of 5.9 billion years. The light from the quasar as seen from Earth is passing above the disc of the galaxy through its halo.

While hydrogen is the most commonly obseverd element in the spectra of galaxies, other elements in interstellar gas will also absorb light to produce other dark spectral lines. This only happens if enough of the element is present, which is the case for the damped Lyman alpha system in the spectrum of LBSQ 0058+0155. Among the elements found in the closer galaxy are chromium, iron, magnesium, manganese, titanium, and zinc [1,3]. These types of measurements in this damped Lyman alpha system as well as others are important for understanding when and how heavy elements are formed in the universe. When matter formed after the Big Bang, the universe contained just hydrogen and helium; most other heavy elements were formed later in stars and are expected to accumulate over time. As a place where it is possible to find some of the weirder elements in the periodic table, LBSQ 0058+0155 is very important to measuring and modelling the formation of these heavy elements.

References:
[1] Pettini, Max et al., Si and Mn Abundances in Damped Ly\alpha Systems with Low Dust Content, 2000, Astrophysical Journal, 532, 65
[2] Chen, Hsiao-Wen et al., Abundance Profiles and Kinematics of Damped Ly\alpha Absorbing Galaxies at z \< 0.651,, 2005, Astrophysical Journal, 620, 703
[3] Guber, C. R. and Richter, P., Dust depletion of Ca and Ti in QSO absorption-line systems, 2016, Astronomy & Astrophysics, 591, A137
 

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende, which is distributed by filmmusic.io under a Creative Commons 4.0 Attribution License

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© George Bendo 2019. See the acknowledgments page for additional information.

Last update: 26 October 2019