Stars Scattered in Colossal Elliptical Galaxy Halo –“Evidence of an Ancient Collision”

 

 

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Astronomers expect that galaxies grow by swallowing smaller galaxies. But the evidence is usually not easy to see — just as the remains of the water thrown from a glass into a pond will quickly merge with the pond water, the stars in the infalling galaxy merge in with the very similar stars of the bigger galaxy leaving no trace. Such is the case with Messier 87 lies at the center of the Virgo Cluster of galaxies. It is a vast ball of stars with a total mass more than a million million times that of the Sun, lying about 50 million light-years away.


"It is very exciting to be able to identify stars that have been scattered around hundreds of thousands of light-years in the halo of this galaxy — but still to be able to see from their velocities that they belong to a common structure. The green planetary nebulae are the needles in a haystack of golden stars. But these rare needles hold the clues to what happened to the stars," concludes co-author Magda Arnaboldi (ESO, Garching, Germany).

A team of astronomers led by PhD student Alessia Longobardi at the Max-Planck-Institut für extraterrestrische Physik , Garching, Germany has applied a clever observational trick to clearly show that the nearby giant elliptical galaxy Messier 87 merged with a smaller spiral galaxy in the last billion years.

"This result shows directly that large, luminous structures in the Universe are still growing in a substantial way — galaxies are not finished yet!" says Alessia Longobardi. "A large sector of Messier 87's outer halo now appears twice as bright as it would if the collision had not taken place."

Rather than try to look at all the stars in Messier 87 — there are literally billions and they are too faint and numerous be studied individually — the team looked at planetary nebulae, the glowing shells around ageing stars. lanetary nebulae form as Sun-like stars reach the ends of their lives, and they emit a large fraction of their energy in just a few spectral lines, the brightest of which is in the green part of the spectrum. Because of this, they are the only single stars whose motions can be measured at Messier 87's distance of 50 million light-years from Earth. They behave like beacons of green light and as such they tell us where they are and at what velocity they are travelling.

Because these objects shine very brightly in a specific hue of aquamarine green, they can be distinguished from the surrounding stars. Careful observation of the light from the nebulae using a powerful spectrograph can also reveal their motions. These planetary nebulae are still very faint and need the full power of the Very Large Telescope to study them: the light emitted by a typical planetary nebula in the halo of the Messier 87 galaxy is equivalent to two 60-watt light bulbs on Venus as seen from Earth.

Just as the water from a glass is not visible once thrown into the pond — but may have caused ripples and other disturbances that can be seen if there are particles of mud in the water — the motions of the planetary nebulae, measured using the FLAMES spectrograph on the Very Large Telescope , provide clues to the past merger.

"We are witnessing a single recent accretion event where a medium-sized galaxy fell through the centre of Messier 87, and as a consequence of the enormous gravitational tidal forces, its stars are now scattered over a region that is 100 times larger than the original galaxy!" adds Ortwin Gerhard, head of the dynamics group at the Max-Planck-Institut für extraterrestrische Physik, Garching, Germany, and a co-author of the new study.

The team also looked very carefully at the light distribution in the outer parts of Messier 87 and found evidence of extra light coming from the stars in the galaxy that had been pulled in and disrupted. These observations have also shown that the disrupted galaxy has added younger, bluer stars to Messier 87, and so it was probably a star-forming spiral galaxy before its merger.

A striking example of the power and effervescence of supermassive black holes is shown in this composite Chandra X Ray Observatory image of M87 at the top of the page. The features in this image imply that outbursts and deep sounds have been generated by the black hole for eons.

The black hole located in the center of M87 is one of the most massive in the universe. The huge reservoir of hot gas in this cluster is shown in this low energy X-ray image from the Chandra X-ray Observatory (red). An optical image from the Digitized Sky Survey shows stars in M87 in blue.

A series of unevenly spaced loops and bubbles are visible in the hot gas below and to the left of the center of M87. These features are produced by small outbursts from close to the black hole about once every 6 million years. The sound waves generated by these outbursts, not visible in this image, will be incredibly deep, about 56 octaves below middle C. Because the outbursts are unevenly spaced the sound will be more like noise from the black hole rather than a harmonious musical performance.

A shock wave — similar to a sonic boom — is detected in a separate Chandra image of M87 that shows high energy X-rays. This shock was produced by a powerful outburst from the black hole about 20 million years ago. The properties of the shock, including the change in temperature and density in the gas, are consistent with classical physics. A large bubble in the X-ray gas shows another powerful outburst occurred about 50 million years earlier. The long interval between these two outbursts provides evidence for even deeper sounds, 58 or 59 octaves below middle C.

Other remarkable features are seen in M87 for the first time including narrow filaments of X-ray emission, which may be due to hot gas trapped to magnetic fields. One of these filaments is over 100,000 light years long, and extends below and to the right of the center of M87 in almost a straight line.

The Daily Galaxy via ESO

Image credit: NASA/CXC/CfA/W.Forman et al.

The Daily Galaxy via ESO

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