Press Release

Research Highlights 2014

Million degree hot gas around giant galaxies tells a tale
Researchers at the School of Astronomy (IPM) have used the high resolution X-ray telescope, NASA’s Chandra space telescope, to discover million degree hot gas around giant elliptical galaxies at the core of galaxy groups. Most of the galaxies in the Universe live in groups and clusters of galaxies. Observations of galaxies show that they interact with each other, merge and transform from one type to another.

At the presence of a mysterious and invisible type of matter, known as dark matter, galaxies spiral towards the centre of the group, where they can merge to form a very massive galaxy. If this galaxy progressively swallows all its neighbour galaxies, groups known as “fossil” form. This has been previously shown in cosmological simulations. In the absence of dark matter, this process will be too slow for galaxies to go through this transformation within the life time of the universe, 14 giga year, and thus observations of such phenomena is an indirect evidence for the presence of dark matter.

In galaxy groups the presence of dark matter can be inferred from the presence of a hot gas heated to tens of million degree Kelvin thus emitting in X-ray. This ionised gas has a cosmic origin and is trapped in the deep potential well of the invisible dark matter, when the structures collapse under their gravity.

Using over 16 hours of Chandra X-ray space telescope time, Dr Habib Khosroshahi and his team searched for hot gas around a number of luminous galaxies which were suspected to have formed through galaxy-galaxy mergers. Targeted galaxies were at a distance of 900 million light-year and within low mass galaxy groups which were previously identified using the observations of ground based telescopes.

They reported the discovery of the hot gas that extends almost two times the visible size of these galaxies, confirming our suspicion that these giant galaxies have in fact formed at the core of galaxy groups. The X-ray emission of this size can not be originated from the galaxy itself and should therefore have a cosmic origin.

The study carried out as part of an international collaboration, confirms the hypothesis that most of the galaxy merging which builds up giant ellipticals actually takes place in collapsed groups. While giant elliptical galaxies have been previously found in X-ray luminous groups, this is the first systematic search for intergalactic hot gas around giant elliptical galaxies.

The paper is published in Monthly Notices of the Royal Astronomical Society in 2014, vol 443, 318 (DOI: 10.1093/mnras/stu1156).

Optical fossil
Figure shows contours of the hot X-ray emission from the giant elliptical galaxy suspected to have swallowed other bright galaxies in the neighbourhood, leaving fainter galaxies and the hot gas to tell a tale.
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