Cosmic collision provides clues to mystery of passive galaxies

By ESO137001_400x600

Astrophysicists from Durham University have been observing a distant spiral galaxy as it is stripped of its interstellar gas while racing into a vast galaxy cluster, in the hope that it will provide insight into a long-standing puzzle about galactic evolution.

Lead-author Michele Fumagalli and his team, which includes Professor Richard Bower, both of the university’s Centre for Extragalactic Astronomy and the Institute for Computational Cosmology, used a new instrument on the European Southern Observatory’s Very Large Telescope (VLT) to monitor ESO 137-001 as it plummets at high speed into the Norma Cluster in Triangulum Australe or ‘the southern triangle’, a small constellation at a distance of 200 million light-years from Earth.

As ESO 137-001 speeds through the vast cloud of hot gas enclosing the cluster at several million kilometres per hour, it experiences a pressure due to bombardment by the gas particles. This in turn leads to so-called ram-pressure stripping – a loss of material under the action of a drag force.

The Multi Unit Spectroscopic Explorer (MUSE) instrument, which is located on the eight-metre Unit Telescope in the Paranal Observatory in northern Chile, was highly effective at surveying the event, producing a high resolution visible-wavelength image of ESO 137-001 after just a single hour of observation.

This is impressive considering that the cosmic smash is occurring behind a large amount of dust and gas from our own galaxy, the Milky Way, which obscures it significantly.

To get a clear signal, astronomers also have to compensate for the wavefront distortions introduced by the earth’s turbulent atmosphere, using a technique called adaptive optics. MUSE has an advanced in-built system called GALACSI that greatly improves the resolution.

Fumagalli et al were able to calculate the distribution and spatial motion of the gas in exceptional detail

The instrument was in fact designed to take a spectrum for each pixel in the image – around 90,000 spectra for every frame. From these colour-bands, Fumagalli et al were able to calculate the distribution and spatial motion of the gas in exceptional detail.

The team established that the stripping process had removed the interstellar medium from the outer disk of the galaxy in its entirety. Gravitational attraction is weaker there because the star clusters are less dense than in the inner regions.

A shortage of gas has a great impact on the evolution of a galaxy; intracluster gas is the raw material for star formation, so deficiency stints galactic growth. Young stars are blue when observed in the optical region of the electromagnetic spectrum, hence active galaxies emit light of a bluish hue. But when star-formation halts, old, low-mass stars remain, and they are redder in colour. Such galaxies are referred to by astronomers as passive, or ‘red and dead’.

Fumagalli explained the importance of the research, which  was published in Monthly Notices of the Royal Astronomical Society and funded by the Science and Technology Facilities Council: “It is one of the major tasks of modern astronomy to find out how and why galaxies in clusters evolve from blue to red over a very short period of time.

“Catching a galaxy right when it switches from one to the other allows us to investigate how this happens.”

Co-author Matteo Fossati added: “We believe the stripping of gas from the galaxy, caused by the ram-pressure, is halting the galaxy’s star formation.”

The researchers were also able to find additional evidence that it is the cluster gas, and not gravity, causing galactic stripping.

There is no doubt that MUSE is a powerful instrument that will continue to make exciting discoveries about the distant universe.

Photograph: NASA/ESA/CXC

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