Using Spektr-RG and Chandra space observatories, Chinese astronomers have investigated a nearby spiral galaxy known as NGC 7793. Results of the new study, published March 13 on the arXiv preprint server, deliver important insights into the properties of the galaxy’s hot gaseous halo.

Located some 12.2 million light years away, NGC 7793 (also known as PGC 73049) is an unbarred spiral galaxy in the Sculptor Group of galaxies. NGC 7793 is about 30,000 light years in diameter, has a stellar mass of approximately 3.2 billion solar masses, and is one of the brightest galaxies of the Sculptor Group.

Although NGC 7793 was discovered two centuries ago and has been extensively studied since then, its galactic X-ray emission remains relatively underexplored, especially for the measurement of halo gas. A team of astronomers led by Lin He of the Nanjing University in China recently decided to change this, and explored the diffuse X-ray emission of this galaxy. For this purpose, they analyzed the data from Spektr-RG’s eROSITA telescope and Chandra’s Advanced CCD Imaging Spectrometer (ACIS).

“Based on the early released eROSITA and archival Chandra observations, we have studied the diffuse soft X-ray emission in and around a nearby low-mass, moderately inclined Sd galaxy, NGC 7793,” the researchers wrote in the paper.

He’s team detected extraplanar X-ray emission in the energy band 0.4–2.3 keV of eROSITA, and 0.5–2 keV of Chandra, visible on both sides of the galactic plane. The finding indicates that NGC 7793 harbors a hot gaseous halo, and it is assumed that stellar feedback plays a dominant role in forming and shaping this hot gas in and around such low-mass spiral galaxies as NGC 7793.

The study found that radial profiles of NGC 7793 exhibit a peculiar dip, which cannot be explained by neutral hydrogen absorption and is more prominent in higher energy bands. The astronomers noted that this dip may arise from a bubble-like structure or is a manifestation of the fluctuating intragroup medium (as the galaxy resides in the western end of the Sculptor Group).

According to the paper, the hot gas of NGC 7793 has a temperature of approximately 0.18 keV and its mass is estimated to be at a level of 10 million solar masses. It was found that the hot gas has an unabsorbed 0.5–2 keV luminosity of about 130 undecillion erg/s and extends up to a distance of around 19,500 light years from the center of the galaxy.

The researchers concluded that the estimated hot gas luminosity also underlines the role of continuous star formation as the most likely provider of hot gas replenishment.

Summing up the results, the authors of the paper noted that further investigation of NGC 7793 should be conducted in order to better understand its hot circumgalactic medium. This could be performed using the Hot Universe Baryon Surveyor (HUBS) X-ray microcalorimeter.

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