Guide:
We check the weather every day when we go out. The term "weather" generally refers to various physical states in the Earth's atmosphere and related phenomena, including temperature, humidity, wind speed, wind direction, and air pressure. Just like the wind, frost, rain and snow in the Earth's atmosphere, galaxies large and small in the universe (including our Milky Way) are playing out all kinds of wonderful "weather" phenomena at every moment.
I wonder if you have ever wondered what the "weather" in the galaxy looks like? Does wind and rain in the galaxy affect our daily lives? How do astronomers study galactic "weather"?
Written by | Yong Zheng (University of California, Berkeley)
Editor-in-charge | Han Yueyang, Lü Haoran
Galactic "weather" at a glance
Just as the Earth is wrapped in a thick layer of atmosphere, the galaxies we usually see in scientific pictures (often in the optical band) have a huge but very dark gas structure on the periphery, called circumgalactic Medium. Let's call it the "atmosphere" of galaxies here. Unlike Earth's atmosphere, galaxies have very thick atmospheres and are comparable in volume to the halo of dark matter in galaxies. Take our neighbor and the Andromeda Galaxy (M31) as an example (Picture 1, right): If we think of M31's disk as a coin, then its galactic media (galactic atmosphere) would be as large as a basketball.
Figure 1: The galactic peri-galactic medium halo (galactic atmosphere) of a galaxy is much larger than the galactic disk itself. Image source: Zheng Yong
In this huge galactic atmosphere, various wonderful "weather" phenomena take turns to play out (Figure 2). From time to time, we see high-velocity outflows of gas gushing out from the center of the disk like a volcanic eruption. These gases flow outward at speeds of several hundred to several kilometers per second, and temperatures of around one million degrees Celsius, enough to melt any material on Earth. These outgoing streams of gas are either propelled by stellar winds generated by star-forming activity or by accretion feedback from the black hole at the center of the disk.
At the same time, we will also see the inward flow of gas or the return of gas precipitated from the galactic atmosphere, either rapidly or slowly, and flowed to the galactic disk at a speed of tens to hundreds of meters per second under the action of gravity. The temperature of these gases is generally between a few thousand degrees and 100,000 degrees Celsius. Just as rain moisturizes everything, these gases inward flow into the disk of galaxies a large amount of material, providing sufficient nourishment for the next generation of star-forming activity.
Figure 2: Schematic diagram of the circumstellar medium. Various dynamic activities in the peri-galactic medium can be compared to a variety of phenomena on Earth. Image source: References[1]
In fact, the phenomenon of "weather" in galaxies is quite common in various galaxies of all sizes. In addition to this, our Galaxy is in a "cloudy" state all year round, which we call high-velocity gas clouds (Figure 3). They float at an altitude of about 30,000 light-years from the disk, are made up of neutral hydrogen atoms at lower temperatures (about a few thousand to ten thousand degrees Celsius), and have a total mass of about seventy million solar masses (excluding the Magellan cloud system), accounting for about two-thousandths of the mass of the Milky Way's silver disk.
Just as we see clouds floating in the sky every day, high-speed gas clouds will take on a variety of forms. Some high-velocity gas clouds appear in clusters and slowly decompose as they move at high speeds in the galactic atmosphere; others grow strips or filamentous, or are associated with local magnetic fields or condensation processes. The physical form of these high-velocity gas clouds directly reflects the physical processes of the "weather" environment in which they reside (e.g., thermal instability, Kelvin-Helmholtz instability, and Rayleigh-Taylor instability).
Figure 3: Distribution of high-speed gas clouds in the Milky Way under the Galactic Coordinate System. The colors represent the relative velocities of high-velocity gas clouds to our solar system (the local stationary coordinate system). Positive values (yellow and orange) represent moving away from us, and negative values (green and blue) represent movement towards us. Just like the clouds we see every day, the high-speed gas clouds in the "atmosphere" of the Milky Way also take on a variety of forms. Image source: References[2]
What's in the galactic atmosphere?
Although the galactic atmosphere also has a "weather" phenomenon like wind, frost, rain and snow on Earth, its chemical composition is completely different from the Earth's atmosphere (Figure 4). In the Earth's atmosphere, we can measure on average about 21 percent of oxygen and about 78 percent of nitrogen. The remaining one percent or so is made up of water vapor, carbon dioxide, etc. These gases are in a molecular state.
In contrast , gas in galaxies ' atmospheres is in the state of atoms or ions. As mentioned above, the temperature of gas (inward or extroverted) in the galactic atmosphere ranges from a few thousand to one million degrees. At such high temperatures, the molecular gas structure cannot exist. Even the hardest metals on Earth, placed in the high temperatures of galaxies' atmospheres, will immediately sublimate into metallic gases, existing in atomic or plasma form.
Figure 4: Comparison of the chemical composition of the Earth's atmosphere with that of galaxies. Most of the gas in the Earth's atmosphere exists in molecular form, while the material in the galactic atmosphere mostly exists in the state of atoms or plasma, picture source: Zheng Yong
As shown on the right of Figure 4, hydrogen is the most common element in galactic atmospheres, accounting for about seventy-three percent of the total baryonic material composition. It was followed by helium, which accounted for a quarter. The remaining elements, such as carbon, nitrogen, oxygen, magnesium, etc., are extremely small amounts. Astronomers refer to all of these elements as "baryons." The outgoing flow of gas we mentioned above (Figure 2) transfers this baryonic material from the disk to the galactic atmosphere, while the inward flow of gas recovers these elements from the galactic atmosphere and retransmits them to the disk to feed the next generation of star-forming activities.
We call the cycle of baryonic matter between galaxies and galactic atmospheres "cosmic baryon material cycles." This process is extremely important for the formation and evolution of galaxies. The National Research Council of the National Academy of Sciences' most recent Astro2020 Decadal Survey lists galactic atmospheres and cosmic baryon material cycles as one of the priorities of the next decade.
How do astronomers study galactic "weather"?
I don't know if you have noticed that if you are in the suburbs at night without moonlight or city lights, it is actually difficult for us to see the clouds in the sky. This is because the clouds in the sky themselves do not emit light themselves, and need to reflect, refract or obscure existing light sources to become visible to the naked eye. The situation is similar for galactic atmospheres. Because galaxies' atmospheres are extremely thin, and their density is much lower than that of the best vacuum devices on Earth (0.00001 to 0.1 ionized particles per square centimeter), it is difficult to observe galactic atmospheres with common optical telescopes. This is also why the pictures of galaxies we usually see are only the galactic disk itself.
Therefore, we need to use special means to observe galactic atmospheres, such as X-ray telescopes that can collect high-energy photons emitted by galactic atmospheres (such as the cosmic thermograni exploration program HUBS, which is under construction), and install integrated field-of-view spectrographs (such as VLT/MUSE) on large optical telescopes for long-term integration. Here I briefly introduce two commonly used methods for observing the galactic media (galactic atmosphere) of the Milky Way and neighboring galaxies:
1
radio telescope
As mentioned above, our Milky Way is in a "cloudy" state all year round, and these high-speed gas clouds can float in the Milky Way's "atmosphere" for tens of millions of years without dissipating. Because the main component of the galactic atmosphere is hydrogen (Figure 4 on the right), and the hydrogen in the high-velocity gas cloud is mainly in a neutral state, we can use the 21-centimeter spectral line generated by the ultrafine structure transition of the neutral hydrogen atom to observe the high-velocity gas cloud.
Since this spectral line falls on the radio band of the electromagnetic spectrum, we can use various radio telescopes (such as China's "Sky Eye" FAST) to observe high-speed gas clouds (Figure 5). One of my research efforts is to use radio telescopes to observe the formation and evolution of these high-speed gas clouds.
Figure 5: The image on the left shows neutral hydrogen in the Milky Way observed using the Arecibo Observatory, and the colors in the figure show velocity information, not the true color of the gas. We can see a large number of filamentous, strip-like gas forms (Image: Josh Peek). The picture on the right is China's "Sky Eye" FAST (Image source: Wikipedia). Thanks to fast's extremely high sensitivity and better spatial resolution than the Arecibo Observatory, as well as all-day coverage, our study of the Milky Way's high-speed gas clouds will also take it to a higher level.
2
Hubble Space Telescope
I wonder if you have ever had the experience of watching a movie in a movie theater where your eyes are often obscured by the tall man in the front row (bottom left in Figure 6)? Similar "occlusion" effects often occur in astronomical observations and are cleverly exploited. Since it is extremely difficult to directly observe the photons emitted by the galactic atmosphere, we can first observe a background point source that is bright in the ultraviolet band using the Cosmic Origin Spectrumr on the Hubble Space Telescope. When the light from the background point source passes around the galaxy in front of it, we see a partial loss of traffic. This is because the "clouds" in the ionized state of the galactic atmosphere absorb this flow, forming a "occlusion" effect similar to that in a movie theater.
By studying the lost flow rate, we can make quantitative estimates of the mass, velocity, and chemical composition of the "clouds" in the galactic atmosphere (e.g., the larger the screen area obscured by the taller the person). Many of my studies have used the Hubble Space Telescope to observe the galactic media of the Milky Way and neighboring galaxies in combination with the "occlusion" effect. Following Hubble, the next generation of infrared-optical-ultraviolet telescopes recommended by the National Research Council of the National Academy of Sciences in the "Ten-Year Survey of Astronomy and Astrophysics" will also bring a new perspective to the study of galactic atmospheres.
The "Hubble successor", the Weber Space Telescope, which has just been launched, although larger than the Hubble aperture, has made limited contributions to the study of the galactic atmosphere because it is focused on the infrared band.
Figure 6: Schematic diagram of atmospheric absorption lines of galaxies observed using the Hubble Space Telescope. Just as our vision is often obscured by tall people in the front row when watching a movie in a movie theater, we can use a similar "occlusion" effect to look for ionized "clouds" in the galactic atmosphere, image source: Zheng Yong
epilogue
"Weather" in galactic atmospheres is as common as wind, frost, rain, and snow on Earth. The difference is that wind, frost, rain and snow will dissipate in the blink of an eye, while the "weather" in the galactic atmosphere will last tens of millions of years or more. These phenomena both reflect the galaxy's past and foreshadow the future of the galaxy.
Due to the harsh observation conditions, the study of galactic atmospheres continues to promote the development of science and technology and the renewal of telescope instruments and equipment. With the preparation and construction of a new generation of radio, optical, ultraviolet and high-energy telescope equipment, the author looks forward to the next decade, twenty years... Our study of galactic atmospheres will usher in a qualitative leap forward.
bibliography:
[1] Tumlinson, Peeples & Work, , 2017,55:389-432, https://ui.adsabs.harvard.edu/abs/2017ARA%26A..55..389T/abstract
[2] Putman, Peek & Joung, , 2012, 50:491-529, https://ui.adsabs.harvard.edu/abs/2012ARA%26A..50..491P/abstract
[3]https://en.wikipedia.org/wiki/Metallicity
[4]https://en.wikipedia.org/wiki/Baryon
[5]https://en.wikipedia.org/wiki/Atmosphere_of_Earth
[6]https://en.wikipedia.org/wiki/Five-hundred-meter_Aperture_Spherical_Telescope
Edition Editor|-Koguizuki-
The reproduced content represents the views of the author only
Does not represent the position of the Institute of Physics, Chinese Academy of Sciences
The wonderful "weather" in the universe galaxy: "wind and rain" lasted for tens of millions of years | Mr. Sai Astronomy
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