Center of the galaxy
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The Center of the Galaxy: A Comprehensive Overview
Unique Location and Intense Activity
The center of our galaxy, the Milky Way, is a region of significant interest due to its unique location and often intense activity. This area poses two major puzzles: the source of the enormous power in the form of radiation and high-velocity gas, and the role of massive objects in galaxy formation post-Big Bang1. Despite being relatively tame compared to other galaxies, the Milky Way's center exhibits extraordinary conditions, including high-velocity gas and momentum transfer to surrounding material1.
Obscured by Cosmic Clouds
Observations of the galactic center are challenging due to heavy obscuration by cosmic clouds, particularly in the constellations Scorpio, Ophiuchus, and Sagittarius. These clouds conceal the center from visible light observations, making it difficult to study2. However, the distribution of stars in transparent regions suggests a high stellar concentration behind the dark nebulosity2.
X-ray Observations and High-Energy Emissions
The center of our galaxy has been extensively observed in infrared and radio wavelengths. The compact radio source at the nucleus, Sagittarius A* (Sgr A*), is considered a weaker form of those found in active galaxies3. X-ray observations, particularly those made with a coded mask X-ray telescope, have provided images of the galactic center in high-energy X-rays up to 30 keV, revealing diffuse emissions and several new point sources3. These observations indicate that the nucleus's emission is weak at higher energies, with surrounding point sources dominating the region3.
Stellar Bar and Galactic Winds
Photometric evidence has established that the central bulge of the Milky Way is a stellar bar. This bar drives material into the galaxy's center, but much of the inflowing gas is expelled again in a high-pressure galactic wind4. A small portion of the gas falls onto a dense cluster of stars at the nucleus, which may contain a massive black hole4.
Supermassive Black Hole and Accretion Flow
At the center of our galaxy lies a supermassive black hole (SMBH) approximately 4 million times the mass of the Sun5. X-ray data on the accretion flow around this black hole reveal interactions with its surroundings, ruling out the possibility that quiescent X-rays are produced by coronal emission from stars or a pure radiatively inefficient accretion flow without outflows5. These observations provide crucial constraints for models of the prevalent radiatively inefficient accretion state5.
Dense Nuclear Star Cluster
The galactic center is an excellent laboratory for studying phenomena occurring in other galactic nuclei. Observations have confirmed the presence of a massive black hole, Sgr A*, surrounded by a dense nuclear star cluster6. This cluster includes a mix of evolved stars, young stars, molecular and ionized gas clouds, and a supernova-like remnant7. The interaction of these components is responsible for many phenomena in this region7.
Star Formation and Dynamics
Despite the hostile environment, several tens of early-type stars populate the central parsec of our galaxy. These stars have highly eccentric orbits and are part of a thin ring or the S-star cluster, which consists mainly of young early-type stars9. The formation of these stars is puzzling, as molecular clouds should be tidally disrupted by the SMBH before fragmenting into stars. Various scenarios, including accretion disc fragmentation and molecular cloud disruption, have been proposed to explain their formation9.
Conclusion
The center of our galaxy is a dynamic and complex region, offering insights into the processes occurring in galactic nuclei. From the presence of a supermassive black hole and dense star clusters to high-energy emissions and star formation, the Milky Way's center continues to be a focal point for astronomical research. Understanding this region not only sheds light on our galaxy but also provides a broader understanding of galactic formation and evolution.
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The centre of the Galaxy
The center of our galaxy poses unique challenges, including understanding the source of power and how galaxies formed after the big bang.
Hard X-ray images of the galactic centre
The galactic centre can be seen in high-energy X-rays up to 30 keV, with weak emission from the nucleus and a dominance of one of the surrounding point sources.
Highly CitedThe centre of the Milky Way
The central bulge of our Galaxy is a stellar bar, driven by the dynamics of the bar, which drives material into the Galaxy's centre but much of the inflowing gas is driven out again in a high-pressure galactic wind.
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The supermassive black hole at the center of our galaxy interacts with its surroundings, producing quiescent x-rays that rule out coronal emission from stars and suggest the presence of an outflow from the accretion flow onto the black hole.
Highly CitedThe galactic center massive black hole and nuclear star cluster
The Galactic Center contains a massive black hole and a dense nuclear star cluster, with recent observations revealing a 'paradox of youth' in the vicinity of Sgr A*.
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