High Energy Emission from the Galactic Center

Abstract

The center of the Galaxy is a prominent source in X-rays and gamma-rays. The study of its high-energy (HE) emission is crucial in understanding the physical phenomena taking place in this dense and extreme environment, where the closest supermassive black hole (SMBH) to us, Sgr A*, is lurking nearly invisible, today, in most of the energy spectrum. These phenomena are probably common to other galactic nuclei and may explain the feedback processes between nuclear regions and galaxies, so important for the overall evolution of the Universe. The Galactic center HE emission is very complex and consists of both thermal and non thermal radiation produced by compact and extended sources, surrounded by more diffuse components. All these objects and media are interacting with each other in the narrow and dense Central Molecular Zone (CMZ). Some of them also show relevant extensions towards the Galactic poles, indicating energetic outflows that seem to link the center to the recently observed large Galactic polar structures. In spite of the fundamental advances obtained in the last twenty five years with the most sensitive X-ray and gamma-ray observatories, several questions remain open to investigations. We review here the main observational results and the open issues on the high-energy diagnostics of the Galactic nuclear activity, focusing on processes that take place in the CMZ, and in particular discussing the role of the present and past SMBH activities in powering this region and possibly the whole Galaxy.

Figures (26)

• Figure 1: The molecular and thermal content of the Central Molecular Zone of the Galaxy shown by the superposition of sub-mm to mid-IR (MIR) images: red and green show 350 $\mu$m and 70 $\mu$m emission (from Herschel) and blue shows 8 $\mu$m emission (from Spitzer). The main molecular structures are indicated. Note the longitude asymmetry between sub-mm-emitting cool diffuse dust associated with the dense gas (red), mainly distributed at positive longitudes, and the MIR warm dust heated by recently formed high-mass stars (blue), at negative ones. Figure adapted from batter25.
• Figure 2: Top view sketches of the matter inflow along the x1 and x2 orbits of the stellar bar potential (left) and of the different models of molecular matter distribution in the CMZ (right), with controversial locations of individual MCs hensha23.
• Figure 3: MeerKAT radio composite image of the GC in Galactic coordinates, showing both the thermal and non-thermal radio sources of the CMZ, with the color code giving the power-law spectral index. Figure adapted from heywoo22, original image credit I. Heywood, SARAO; color processing J.C. Munoz-Mateos, ESO.
• Figure 4: The molecular CND and the Sgr A West mini-spiral of ionized gas shown by a composite image of the 3.4 mm HCN line emission (red, BIMA) and the 3.6 cm radio continuum (green, VLA) from warm ionized gas, along with the star's IR emission (blue, Spitzer). The image is in equatorial projection centered on Sgr A$^\star$ (indicated by a white arrow) with North to the top, East to the left and scale given by the double-sided arrow. From NRAO archives (https://www.nrao.edu/archives/items/show/33428).
• Figure 5: Sketch of the SED of the Sgr A$^\star$ quiescent emission, from genzel10. Compiled data in different bands are fitted with a RIAF model (broken and dotted lines), plus a non-thermal electron component (broken-dotted line) yuan03. For the most recent and complete Sgr A$^\star$ SED data compilation see eventh22b.