Dispersed neutral hydrogen atoms (HI) in the Milky Way are the wombs of star-forming regions and the medium for most of the energy that massive stars inject during their lifetimes. Therefore, the study of the distribution and dynamics of neutral hydrogen atoms is crucial for understanding the energy and matter circulation in the interstellar medium.
Recently, a team of astronomers from Europe (JD Soler et al.) used the Effelsberg 100-meter radio telescope in Germany and the Parkes 64-meter radio telescope in Australia to observe the 21 cm emission line of neutral hydrogen atoms near the galactic disk of the Milky Way. From the obtained radio data, the researchers used the Hessian matrix analysis technique to select a series of fibrous structures, analyzed their spatial orientation with the Rayleigh statistical test method, and gave a three-dimensional imaging analysis of these "galactic blood vessels".
The related paper, titled "The Galactic dynamics revealed by the filamentary structure in atomic hydrogen emission", was published in the latest issue of the prestigious astronomical journal "Astronomy and Astrophysics" (Astronomy & Astrophysics).
In the study, astronomers also found that the orientation of the fibrous structure of the Milky Way's neutral hydrogen atoms relative to the Milky Way's galactic disk gradually changes with its distance from the center of the Milky Way. In the center of the galactic disk, most of the fibrous structures of neutral hydrogen atoms are perpendicular to the galactic disk, or there is no obvious pointing preference; while the outer side of the galactic disk is 10-18 kpc (kpc, kiloparsec, a unit for measuring distance in astronomy , equal to one thousand parsecs or 3261.56377716743353658491 light-years), the fibrous structure of neutral hydrogen atoms is parallel to the silver disk; on the outside of the silver disk, the distribution of the fibrous structure also follows the warping of the silver disk and other structures changes and may reflect the effects of interactions between the Milky Way and its nearby satellite galaxies.
The researchers speculate that in the center of the Milky Way, the vertical lift of neutral hydrogen atoms may be affected by the energy generated by the repeated explosions of supernovae in history; while in the outer reaches of the Milky Way, the horizontal distribution of the fibrous structure of neutral hydrogen atoms is likely to be the result of galactic rotation and galactic rotation. cut result.
These observations also suggest that the distribution of atomic gas in the Milky Way is similar to that of nearby spiral galaxies, where scientists have previously discovered holes that may be created by supernova winds inside the Milky Way, and mostly spiral arms and turbulence outside.
Neutral hydrogen sweeps the sky to outline the Milky Way
Hydrogen is the most abundant element in the universe and is found almost everywhere. In a single galaxy, hydrogen can exist in the form of ionized hydrogen (HII), neutral hydrogen atoms (HI), and hydrogen molecules. Generally speaking, neutral hydrogen atoms are mainly concentrated in the disk of late-type galaxies. These regions themselves are not dense enough to collapse to form stars, but they serve as the raw material reservoir for the formation of new generations of stars, in the stellar life cycle. play an important role.
Since the 1950s, scientists have used 21-centimeter radio waves to observe neutral hydrogen in the universe. This is because neutral hydrogen atoms emit radiation themselves without absorbing photons. When the electron spin and nuclear spin of a neutral hydrogen atom in the ground state interact, the electron spin will transition from a high-energy state parallel to the nuclear spin to a low-energy state antiparallel to the nuclear spin, emitting a wavelength of 21 cm, The radio line at the frequency 1420.4057517667 MHz.
Using this spectral line, astronomers can infer the distribution of neutral hydrogen atoms in the universe. In the observation, this radio wave is also not absorbed by the interstellar dust, and it is precisely because the absorption mechanism of this dust is hindered that the optical observation method is almost ineffective at distances of more than a few thousand parsecs.
From 1954 to 1958, astronomers used the Doppler frequency shift of neutral hydrogen radio to combine with the rotation curve of the Milky Way, and constructed the Milky Way Telescope to conduct the first all-sky neutral hydrogen observation, and several frontal images of the periphery. In 2005, astronomers observed through the LAB (Leiden-Argentina-Bonn) radio telescope with a resolution of 36 arc minutes and found that the Milky Way is a non-axial spiral system composed of four spiral arms, whose arm lengths are measured from the center of the Milky Way. At least 25kpc.
Further studies of neutral hydrogen radio have also determined the structure of other spiral arms of the Milky Way. In 2011, also using the LAB radio telescope, astronomers discovered a hypothetical spiral arm structure in the first quadrant of the Milky Way, which is about 15 kpc from the center of the Milky Way, which is located in the outer arm of one of the four major spiral arms of the Milky Way ( Outer Arm). In 2005, astronomers made observations of neutral hydrogen radio using the SGPS interferometric telescope with a resolution of 2 arcminutes to identify possible distant spiral arms in the fourth quadrant of the Milky Way.
In addition to the spiral arm structure, astronomers have used the radio of neutral hydrogen atoms to explain warping, flares in the galactic disk, and trace the energy input signature of main sequence stars, supernovae and other high-energy processes in the Milky Way.
Neutral hydrogen fibrils map Milky Way 'vessels'
Since the 1970s, astronomers have successively discovered various structures of the distribution of neutral hydrogen atoms in the Milky Way. In 1973, astronomers first imaged a neutral hydrogen radio image of the Milky Way's galactic disk, revealing a series of shell-like, arc-like and fibrous structures. Among them, the shell-like structure of neutral hydrogen is thought to be formed by the combined action of stellar wind and dust inside the galaxy.
The astronomers also paid particular attention to the fibrous structures that were perpendicular to the disk, saying they were "like bugs crawling out of the disk." Based on single-dish telescope observations with a resolution of 4 arcmin and 30 arcmin, astronomers found that these fibrous structures have preferential pointing along the distribution of the local interstellar medium magnetic field. This alignment may be the product of velocity anisotropy induced by the magnetic field, material collapse along the magnetic field lines, shock waves, or uneven distribution of gas density.
In 2020, in the Hi/OH/Recombination Line (THOR) survey, the astronomer Soler et al. revealed most of the fibrous in the neutral hydrogen radio through the analysis of neutral hydrogen interferometric observations based on 40′′ resolution. The structure is parallel to the galactic galactic disk, but a part of the first quadrant of the Milky Way is dominated by the vertical neutral hydrogen fiber structure. The occurrence of this situation is most likely caused by the feedback of repeated supernova explosions in history and the combined effect of the interstellar magnetic field.
In this latest neutral hydrogen fiber structure study, the researchers extend the analysis of Soler et al. (2020) to the entire galactic disk, extending across the longitude and latitude of the Milky Way. The authors also explained that the purpose of their study was not to define these fibrous structures of neutral hydrogen as objects in three-dimensional space, such as spiral arms or thin gas chains, but to study the general isotropic distribution of atomic gas in the Galactic disk. opposite sex.
The article points out that fibrous structure orientation is a feature of neutral hydrogen radio, and its dependence on angular resolution is less pronounced than other properties such as the width and length of the fibrous structure. If the fibrous structure is understood as a feature of the galaxy's intensity field, rather than a physical object, this analysis can reveal the anisotropy of the radio distribution. This feature is especially relevant given the natural axis of symmetry of the galactic disk. The gravitational potential of the galaxy, as well as the stretching caused by the circular motion and spiral arms, naturally produces a horizontally distributed fibrous structure of neutral hydrogen. Anisotropy that deviates from this natural situation may indicate energetic processes in which the gas rises or falls in the galactic disk.
Like the Soler team's research in 2020, the researchers also used the Hessian matrix technique to selectively encode the fibrous structure and used the Rayleigh technique for statistical testing.
Specifically, the researchers analyzed the distribution of neutral hydrogen atoms in the Milky Way's galactic disk by assuming a circular motion around the galactic center and a standard rotation curve. First, they focused on the orientation of the fibrous structures at the heliocentric radius to study the combined effect of angular resolution and distance, second, they compared the orientation of the fibrous structures at different heliocentric motion distances, and finally the researchers analyzed the According to the change of the radius of the galactic center, and the frontal view of the neutral hydrogen atoms pointing in the plane space of the galactic galactic disk is obtained.
In addition, the researchers speculate in the paper that the content of neutral hydrogen atoms in the fibrous structure is roughly equal in the range of the galactic center radius Rgal ≈ 15 kpc. This observation may be consistent with the neutral The mass ratio of the hot and cold phases of the hydrogen gas is related. But ultimately proving this hypothesis requires further characterization of the phases of neutral hydrogen atoms.
"Neutral hydrogen atoms still carry key information on the dynamics of the gas atoms that drive diffusion to condense into new stars," the researchers said.
Recently, a team of astronomers from Europe (JD Soler et al.) used the Effelsberg 100-meter radio telescope in Germany and the Parkes 64-meter radio telescope in Australia to observe the 21 cm emission line of neutral hydrogen atoms near the galactic disk of the Milky Way. From the obtained radio data, the researchers used the Hessian matrix analysis technique to select a series of fibrous structures, analyzed their spatial orientation with the Rayleigh statistical test method, and gave a three-dimensional imaging analysis of these "galactic blood vessels".
The related paper, titled "The Galactic dynamics revealed by the filamentary structure in atomic hydrogen emission", was published in the latest issue of the prestigious astronomical journal "Astronomy and Astrophysics" (Astronomy & Astrophysics).
In the study, astronomers also found that the orientation of the fibrous structure of the Milky Way's neutral hydrogen atoms relative to the Milky Way's galactic disk gradually changes with its distance from the center of the Milky Way. In the center of the galactic disk, most of the fibrous structures of neutral hydrogen atoms are perpendicular to the galactic disk, or there is no obvious pointing preference; while the outer side of the galactic disk is 10-18 kpc (kpc, kiloparsec, a unit for measuring distance in astronomy , equal to one thousand parsecs or 3261.56377716743353658491 light-years), the fibrous structure of neutral hydrogen atoms is parallel to the silver disk; on the outside of the silver disk, the distribution of the fibrous structure also follows the warping of the silver disk and other structures changes and may reflect the effects of interactions between the Milky Way and its nearby satellite galaxies.
The researchers speculate that in the center of the Milky Way, the vertical lift of neutral hydrogen atoms may be affected by the energy generated by the repeated explosions of supernovae in history; while in the outer reaches of the Milky Way, the horizontal distribution of the fibrous structure of neutral hydrogen atoms is likely to be the result of galactic rotation and galactic rotation. cut result.
These observations also suggest that the distribution of atomic gas in the Milky Way is similar to that of nearby spiral galaxies, where scientists have previously discovered holes that may be created by supernova winds inside the Milky Way, and mostly spiral arms and turbulence outside.
Neutral hydrogen sweeps the sky to outline the Milky Way
Hydrogen is the most abundant element in the universe and is found almost everywhere. In a single galaxy, hydrogen can exist in the form of ionized hydrogen (HII), neutral hydrogen atoms (HI), and hydrogen molecules. Generally speaking, neutral hydrogen atoms are mainly concentrated in the disk of late-type galaxies. These regions themselves are not dense enough to collapse to form stars, but they serve as the raw material reservoir for the formation of new generations of stars, in the stellar life cycle. play an important role.
Since the 1950s, scientists have used 21-centimeter radio waves to observe neutral hydrogen in the universe. This is because neutral hydrogen atoms emit radiation themselves without absorbing photons. When the electron spin and nuclear spin of a neutral hydrogen atom in the ground state interact, the electron spin will transition from a high-energy state parallel to the nuclear spin to a low-energy state antiparallel to the nuclear spin, emitting a wavelength of 21 cm, The radio line at the frequency 1420.4057517667 MHz.
Using this spectral line, astronomers can infer the distribution of neutral hydrogen atoms in the universe. In the observation, this radio wave is also not absorbed by the interstellar dust, and it is precisely because the absorption mechanism of this dust is hindered that the optical observation method is almost ineffective at distances of more than a few thousand parsecs.
From 1954 to 1958, astronomers used the Doppler frequency shift of neutral hydrogen radio to combine with the rotation curve of the Milky Way, and constructed the Milky Way Telescope to conduct the first all-sky neutral hydrogen observation, and several frontal images of the periphery. In 2005, astronomers observed through the LAB (Leiden-Argentina-Bonn) radio telescope with a resolution of 36 arc minutes and found that the Milky Way is a non-axial spiral system composed of four spiral arms, whose arm lengths are measured from the center of the Milky Way. At least 25kpc.
Further studies of neutral hydrogen radio have also determined the structure of other spiral arms of the Milky Way. In 2011, also using the LAB radio telescope, astronomers discovered a hypothetical spiral arm structure in the first quadrant of the Milky Way, which is about 15 kpc from the center of the Milky Way, which is located in the outer arm of one of the four major spiral arms of the Milky Way ( Outer Arm). In 2005, astronomers made observations of neutral hydrogen radio using the SGPS interferometric telescope with a resolution of 2 arcminutes to identify possible distant spiral arms in the fourth quadrant of the Milky Way.
In addition to the spiral arm structure, astronomers have used the radio of neutral hydrogen atoms to explain warping, flares in the galactic disk, and trace the energy input signature of main sequence stars, supernovae and other high-energy processes in the Milky Way.
Neutral hydrogen fibrils map Milky Way 'vessels'
Since the 1970s, astronomers have successively discovered various structures of the distribution of neutral hydrogen atoms in the Milky Way. In 1973, astronomers first imaged a neutral hydrogen radio image of the Milky Way's galactic disk, revealing a series of shell-like, arc-like and fibrous structures. Among them, the shell-like structure of neutral hydrogen is thought to be formed by the combined action of stellar wind and dust inside the galaxy.
The astronomers also paid particular attention to the fibrous structures that were perpendicular to the disk, saying they were "like bugs crawling out of the disk." Based on single-dish telescope observations with a resolution of 4 arcmin and 30 arcmin, astronomers found that these fibrous structures have preferential pointing along the distribution of the local interstellar medium magnetic field. This alignment may be the product of velocity anisotropy induced by the magnetic field, material collapse along the magnetic field lines, shock waves, or uneven distribution of gas density.
In 2020, in the Hi/OH/Recombination Line (THOR) survey, the astronomer Soler et al. revealed most of the fibrous in the neutral hydrogen radio through the analysis of neutral hydrogen interferometric observations based on 40′′ resolution. The structure is parallel to the galactic galactic disk, but a part of the first quadrant of the Milky Way is dominated by the vertical neutral hydrogen fiber structure. The occurrence of this situation is most likely caused by the feedback of repeated supernova explosions in history and the combined effect of the interstellar magnetic field.
In this latest neutral hydrogen fiber structure study, the researchers extend the analysis of Soler et al. (2020) to the entire galactic disk, extending across the longitude and latitude of the Milky Way. The authors also explained that the purpose of their study was not to define these fibrous structures of neutral hydrogen as objects in three-dimensional space, such as spiral arms or thin gas chains, but to study the general isotropic distribution of atomic gas in the Galactic disk. opposite sex.
The article points out that fibrous structure orientation is a feature of neutral hydrogen radio, and its dependence on angular resolution is less pronounced than other properties such as the width and length of the fibrous structure. If the fibrous structure is understood as a feature of the galaxy's intensity field, rather than a physical object, this analysis can reveal the anisotropy of the radio distribution. This feature is especially relevant given the natural axis of symmetry of the galactic disk. The gravitational potential of the galaxy, as well as the stretching caused by the circular motion and spiral arms, naturally produces a horizontally distributed fibrous structure of neutral hydrogen. Anisotropy that deviates from this natural situation may indicate energetic processes in which the gas rises or falls in the galactic disk.
Like the Soler team's research in 2020, the researchers also used the Hessian matrix technique to selectively encode the fibrous structure and used the Rayleigh technique for statistical testing.
Specifically, the researchers analyzed the distribution of neutral hydrogen atoms in the Milky Way's galactic disk by assuming a circular motion around the galactic center and a standard rotation curve. First, they focused on the orientation of the fibrous structures at the heliocentric radius to study the combined effect of angular resolution and distance, second, they compared the orientation of the fibrous structures at different heliocentric motion distances, and finally the researchers analyzed the According to the change of the radius of the galactic center, and the frontal view of the neutral hydrogen atoms pointing in the plane space of the galactic galactic disk is obtained.
Figure 1: Longitude-line-of-sight velocity diagram of the Milky Way galactic disk: The author has marked the main spiral arm structure of the Milky Way with different lines. The colors in the image on the left represent the radiation intensity of neutral hydrogen atoms, where numerous fibrous structures are hidden. The figure on the right shows the orientation of different fiber structures using Rayleigh statistics.
Figure 2: The researchers used visualization technology to highlight the determined fiber structure from the background radiation, and selected three velocity channels to make a color map with 3D effect. Like the network of blood vessels in a giant beast.
Some industry insiders commented that the distribution and orientation of these slender "vessels" tells the story of where and how the energy after the death of stars is returned to the interstellar medium in the long history of the Milky Way. It may also contain important information about gas accretion toward the galactic disk, cosmic ray-induced energy diffusion in the interstellar medium, and "galactic fountain" events produced by massive supernova explosions.In addition, the researchers speculate in the paper that the content of neutral hydrogen atoms in the fibrous structure is roughly equal in the range of the galactic center radius Rgal ≈ 15 kpc. This observation may be consistent with the neutral The mass ratio of the hot and cold phases of the hydrogen gas is related. But ultimately proving this hypothesis requires further characterization of the phases of neutral hydrogen atoms.
"Neutral hydrogen atoms still carry key information on the dynamics of the gas atoms that drive diffusion to condense into new stars," the researchers said.
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