Astronomy & Space

A study looks into diffuse emission from the Cigar Galaxy.

An international group of astronomers has made X-ray observations of the Cigar Galaxy with the help of the Chandra spacecraft from NASA. The observational campaign’s findings, which were made public on the arXiv pre-print repository on March 16, provide important information about this galaxy’s diffuse emission.

The Cigar Galaxy, also known as Messier 82 or M82, is a starburst galaxy that was discovered in 1774. It is 11.73 million light years away in the constellation Ursa Major. It is one of the starburst galaxies closest to Earth and has a size of about 40,800 light years.

The Cigar Galaxy is observed to have a large-scale galactic wind at a variety of wavelengths, such as hard X-rays above a few keV. This superwind appears to be concentrated in the galaxy’s two high surface brightness regions, or clumps, and is fueled by the energy released by supernovae within the clumps, which happen about once every ten years. In previous Chandra studies of this galaxy, diffuse residual emission around the starburst disk was found, as were bright X-ray binaries that dominate the hard X-ray band.

“On a few arcsecond scales, we offer the first spatially resolved X-ray spectroscopic analysis of the 4-8 keV diffuse emission discovered in the center region of the neighboring starburst galaxy M82. The new information allows us to draw a variety of key inferences about the nature of the hot gas and its origin, as well as feedback on the interstellar medium.”

The researchers

A group of astronomers recently made the decision to investigate this diffuse emission from the Cigar Galaxy in greater depth under the direction of Kazushi Iwasawa of the University of Barcelona in Spain. They used Chandra to conduct a spectral analysis of the diffuse emission in the 4–8 keV band with spatial resolution.

“On a few arcsecond scales, we present the first spatially resolved X-ray spectroscopic study of the diffuse emission of 4–8 keV found in the central part of the nearby starburst galaxy M82.” The researchers elaborated, “The new details that we see enable a number of important conclusions to be drawn on the nature of the hot gas, its origin, and feedback on the interstellar medium.”

According to the observations, there are three distinct spectral components in the 4–8 keV diffuse emission from the Cigar Galaxy. The first, inverse Compton emission, accounts for approximately 70% of the band’s continuum luminosity. The remaining component is a metal-rich, hot gas emission, and the second one is the hard tail of the soft X-ray wind emission.

The far-infrared and radio emission morphologies of the Cigar Galaxy’s diffuse X-ray emission are comparable. This supports the hypothesis that cosmic ray electrons caused the non-thermal emission by inverse Compton scattering of far-infrared photons.

In addition, the study found hot gas in a small area close to the galactic disk. The so-called chimneys, which are collimated structures of a super bubble breaking out of the galactic disk and are marked by mid-infrared and radio voids, are filled with the gas, which appears to flow out from the eastern part of the starburst ring. It was discovered that these chimneys carried the majority of the supernova energy from the disk to the halo.

In addition, the study discovered that the Cigar Galaxy’s youngest and brightest radio and X-ray supernova remnants are located in enormous molecular clouds that have presumably just formed and are free of strong supernova feedback.

More information: K. Iwasawa et al, Origin of the diffuse 4-8 keV emission in M82, arXiv (2023). DOI: 10.48550/arxiv.2303.09637

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