Not all "Destruction Kings"
Hubble finds that some black holes can promote star formation
◎Reporter Wu Chunxin
In previous studies, the speed of jets or outflows generated by black hole systems is very close to the speed of light. The gas swept by the jets and outflows will be heated, and the gas in its path will be dissipated. These effects are obviously not conducive to star formation.
The outflow rate of Henize 2-10's central black hole is very low, and it does not significantly heat the gas when it interacts with the gas. Instead, its outflow passes through the compressed gas, directly producing the effect of triggering star formation.
For a long time, black holes have been considered to be "destruction kings", swallowing everything that comes close to them.
However, the black hole in the dwarf galaxy Henize 2-10 was recently observed by the Hubble Space Telescope, which is diametrically opposed to the "temperament" of other known black holes, which is promoting rather than inhibiting the formation of nearby stars.
This is a subversive discovery.
Imaging and spectral analysis of Henize 2-10 by the Hubble Space Telescope clearly shows that a stream of gas extends from the black hole to the bright star-birth region like an umbilical cord, allowing the dense cloud to form stars.
black holes that promote star formation
Henize 2-10 is located in the constellation Compass (one of the constellations in the southern sky), about 30 million light-years away from Earth, and contains only about one-tenth the number of stars in the Milky Way.
According to Lei Weihua, a professor at the School of Physics, Huazhong University of Science and Technology, the Milky Way is the product of the continuous growth and merging of primitive galaxies. Although Henize 2-10 has undergone a long cosmic evolution, it still retains its original shape. It is a small-scale galaxy and can be regarded as a The "living fossil" of the galaxy.
Do such dwarf galaxies also have a supermassive black hole at their center like other galaxies?
"10 years ago, this issue was still the focus of debate. It was thought that the X-ray and radio activity in the core of Henize 2-10 galaxy may be caused by the accretion of gas by the central black hole, or it may be from the remnant of a young supernova." Lei Weihua Say.
The observations made by the Hubble Space Telescope this time have drawn an end to the debate over whether there are massive black holes at the centers of dwarf galaxies.
The study revealed the peculiar structure of the bidirectional outflow extending from the galaxy's core to the star-forming region.
Using spectral line Doppler shifts, the researchers can measure the outflow gas velocity at different locations, further revealing the precession of the outflow.
Scientists have also observed precession of overjets in many active galactic nuclei before.
Theoretically, the precession of the jet, or outflow, could come from the twisted black hole's accretion disk.
Lei Weihua believes that the observation of the precession outflow proves that the center of Henize 2-10 must be a black hole, and it cannot come from a supernova remnant.
At the same time, there is no bulge in the center of Henize 2-10, indicating that the supermassive black hole with a mass of about one million times the sun at its center existed before the formation of the bulge. Growing is different.
"The supermassive black hole at the center of Henize 2-10 may still maintain its infantile appearance, which is of great research value and can be used to restore the original seed information similar to the supermassive black hole at the center of the Milky Way." Lei Weihua said.
For a nebula to collapse to form a star, it must be cold and dense enough.
In previous observations and theoretical studies, it was generally believed that jets or outflows from black holes at the center of galaxies would continuously heat and disperse gas wherever they passed, and would surely inhibit star formation.
For dwarf galaxies, although the observed sample of black hole activity is small, scientists still hold the same view.
Lei Weihua said that observations of Henize 2-10 found the exact opposite. The outflow generated by the activity of the central black hole triggered star formation, which was greatly unexpected by scientists.
Distinctive black hole outflow
With the development of multi-wavelength astronomy, scientists have made more and more in-depth research on galaxies, and gradually accumulated evidence shows that there is generally a supermassive black hole with a mass from one million times to ten billion times the mass of the sun in the center of normal galaxies and active galaxies.
According to Lei Weihua, the black hole itself does not emit light, but the strong gravitational field of the black hole will affect the surrounding stars or gas. For example, the supermassive black hole will capture and tear the stars close to it, suck up the stellar debris, and the mass exceeds 100 million times the mass of the sun. Black holes can directly swallow stars or gas to form accretion disks.
The black hole accretions surrounding gaseous matter to form an accretion disk.
On the accretion disk, the materials at different distances from the central black hole rotate at different speeds. These materials release gravitational potential energy through poor rotation (rotate around the black hole at different angular speeds at different radii) and heat the gas to form hot plasma. produce radiation.
Lei Weihua said that in addition to accreting matter, the black hole accretion disk system also ejects plasma, producing jets or outflows, which are very common in active galactic nuclei and microquasars.
The formation mechanisms of jets or outflows are mainly as follows: the low accretion rate accretion disk cannot be effectively cooled by radiating photons, and the hot accretion disk will eject plasma outward to form disk wind (outflow); if the accretion disk is There is a very strong ordered magnetic field, and the plasma on the disk is ejected outward through the magnetic field lines to form outflows or jets; the rotating black hole can extract the rotational energy of the black hole through the surrounding ordered magnetic field, so as to accelerate the surrounding plasma to form a collimated black hole along the direction of the two poles of the black hole. jet stream.
"The mechanism of the outflow generated by the black hole at the center of Henize 2-10 may be one of them. It is still unclear which of the above-mentioned mechanisms is dominant," Lei Weihua said.
In previous studies, the speed of jets or outflows generated by black hole systems is very close to the speed of light. The gas swept by the jets and outflows will be heated, and the gas in its path will be dissipated. These effects are obviously not conducive to star formation.
Therefore, scientists generally believe that black hole jets, outflows, inhibit star formation, at least for active galaxies.
As a dwarf galaxy, Henize 2-10 has a very low speed of outflow from its central black hole, about a few hundred kilometers per second, which is very slow relative to the speed of light, and does not significantly heat the gas when interacting with the gas, instead its outflow passes through the compressed gas , directly having the effect of triggering star formation.
Based on this, the Hubble Space Telescope observed the bidirectional outflow from Henize 2-10 extending directly into the star-forming region.
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Two possible sources of seed black holes
On April 10, 2019, the "Event Horizon Telescope" cooperation group released the first black hole photo of mankind. Through the black hole shadow scale, it was measured that M87, the black hole at the center of the giant elliptical galaxy 55 million light-years away from the earth, has a mass of about 6.5 billion times the sun. quality.
According to Lei Weihua, a professor at the School of Physics at Huazhong University of Science and Technology, it is generally believed that the black hole at the center of the galaxy and the galaxy should be in a relationship of common growth.
But the fundamental problem is that the supermassive black holes at the centers of galaxies seen today were grown through the continuous merger or accretion of matter by seed black holes in the long cosmic process.
So, what was the original seed black hole?
At present, the mainstream seed black hole models include "light seed" and "heavy seed".
"Light seed" black holes come from stellar evolution products, that is, the first generation of stars. Their metal abundance is extremely low, almost entirely composed of hydrogen and helium, and can reach several hundred times the mass of the sun, with a lifespan of only one million years, or collapse to form 100 times the mass of the sun. Black holes around the mass of the sun.
"Heavy seed" black holes come from the direct collapse of primordial gas or star clusters, and can reach thousands to hundreds of thousands of solar masses.
Lei Weihua believes that through Henize 2-10, we can get a glimpse of the activities of primitive galaxies and seed black holes in the early universe, helping scientists to more comprehensively understand the relationship between black hole feedback and galaxy evolution.