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Japanese astronomers have presented new results of observations that suggest an unidentified X-ray source designated AX J145732−5901 is a cluster of galaxies behind the galactic plane. The findings are detailed in a research paper April 30 on the arXiv preprint server.

Launched in 1993, The Advanced Satellite for Cosmology and Astrophysics (ASCA) was an X-ray astronomy spacecraft devoted to studying distant active galaxies, galaxy clusters, sources of the cosmic X-ray background, and other phenomena. It allowed astronomers to detect faint X-ray sources even through dense matter on the galactic plane.

AX J145732−5901 is an unidentified X-ray source detected in 2001 in the ASCA Galactic plane survey. Previous observations have found that it is a heavily absorbed extended source and suggested that it may be a cluster of galaxies hiding behind the plane of our galaxy. However, no detailed spectral analysis of AX J145732−5901 has been conducted yet to confirm this assumption.

Now, a team of astronomers, led by Shigeo Yamauchi of Nara Women's University in Japan, has analyzed the data from X-ray observations of AX J145732−5901 with the Suzaku satellite. The results confirm the previous suggestions regarding the nature of this source.

"Using the results of the Galactic ridge X-ray emission and Cosmic X-ray background studies based on the Suzaku observations, we reanalyzed the ASCA data of AX J145732−5901," the paper reads.

In particular, the study found that AX J145732−5901 showcases an extended X-ray emission with a size of 14 by 10 arcminutes, which corresponds to 5.87 by 4.24 million light years. The emission is elongated in the east-west direction and seems to have local structures.

Furthermore, the X-ray spectrum of AX J145732−5901 exhibits an emission line at 5.94 keV and is heavily absorbed by interstellar matter equivalent to a hydrogen column density of around 100 sextillion atoms per square centimeter. This is several times larger than the galactic hydrogen column density. In general, the heavily absorbed X-ray spectrum of AX J145732−5901 suggests that it is an extragalactic source.

According to the paper, the X-ray luminosity of AX J145732−5901 was calculated to be about 260 tredecillion erg/s in the 1–10 keV energy band. The luminosity and angular distances of the source were estimated to be 1.8 and 1.43 billion light years.

Based on the obtained results, the authors of the paper concluded that AX J145732−5901 is a cluster of galaxies behind the galactic plane. They noted that the X-ray morphology of AX J145732−5901 indicates that it is an unrelaxed (merging) cluster.

The researchers also estimated that AX J145732−5901 has a total gas mass of about 30 trillion solar masses. They added that if the gas fraction is at a level of 15% of the total mass of clusters of galaxies, then its total mass is estimated to be around 200 trillion solar masses.

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