Black hole velocity in galaxies
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Black Hole Velocity Dispersion in Galaxies
The velocity of stars and gas near the centers of galaxies provides crucial evidence for the presence and properties of supermassive black holes. Observations of galaxies such as M87, M31, M32, NGC 3115, and NGC 4594 show that the velocity dispersion—the spread in velocities of stars—rises sharply toward the galactic center, consistent with the gravitational influence of a massive black hole. For example, in M87, the velocity dispersion increases from about 270 km/s at 15 arcseconds from the center to around 400 km/s at just 0.5 arcseconds, indicating the presence of a central black hole with a mass of several billion solar masses Marel1994Marel1994.
The M–σ Relation: Black Hole Mass and Galaxy Velocity Dispersion
A key discovery in galaxy studies is the tight correlation between the mass of a galaxy's central black hole (Mbh) and the velocity dispersion (σ) of stars in the galaxy's bulge. This relationship, known as the M–σ relation, shows that black hole mass increases steeply with the velocity dispersion of the host galaxy. The best-fit relation is Mbh ∝ σ^3.75, and this correlation holds over a wide range of galaxy types and black hole masses, with very little scatter Gebhardt2000Graham2008Haehnelt2000+1 MORE. This suggests that the growth of black holes and the evolution of their host galaxies are closely linked.
Black Hole Kicks and Natal Velocities
Black holes can also receive "kick velocities" during events such as mergers. These kicks can reach up to several thousand kilometers per second, as observed in gravitational wave events like GW200129, where the remnant black hole was estimated to have a recoil velocity of about 1,500 km/s . However, for stellar-mass black holes in the Milky Way, typical natal kick velocities are likely below 100 km/s, as higher velocities would make such black holes harder to detect through microlensing events .
Gas and Stellar Kinematics Near Black Holes
The velocity profiles of both stars and ionized gas near galactic centers often show non-Gaussian features, with extended wings in the velocity distribution due to stars moving at high speeds close to the black hole. In M87, the central velocity dispersion of ionized gas can reach over 500 km/s, dropping rapidly with distance from the center . These high velocities are a direct result of the strong gravitational pull of the black hole.
Black Hole Feedback and Galaxy Evolution
Simulations and observations indicate that energy released by accreting black holes (quasars) can drive powerful outflows, expelling gas from the galaxy and regulating both star formation and further black hole growth. This feedback process helps explain the observed relationship between black hole mass and velocity dispersion, as it links the growth of the black hole to the properties of the host galaxy's bulge Ranjbar2023Matteo2005.
Conclusion
The velocity of stars and gas in galaxies, especially near their centers, provides strong evidence for the presence and mass of supermassive black holes. The tight correlation between black hole mass and galaxy velocity dispersion (the M–σ relation) highlights the deep connection between black holes and their host galaxies. Black hole kicks, feedback processes, and the detailed shape of velocity profiles all play important roles in shaping galaxies and their central black holes Marel1994Marel1994Gebhardt2000+6 MORE.
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