Planetary nebulae in galaxy Messier 87
Messier 87 in the constellation of Virgo
Messier 87 has swallowed an entire galaxy in the last billion years
New observations with ESO’s Very Large
Telescope have revealed that the giant elliptical galaxy Messier 87 has
swallowed an entire medium-sized galaxy over the last billion years. For
the first time a team of astronomers has been able to track the motions
of 300 glowing planetary nebulae to find clear evidence of this event
and also found evidence of excess light coming from the remains of the
totally disrupted victim.
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.
But now 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."
Messier 87 lies at the centre 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.
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 [1].
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 [2].
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.
"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).
Notes
[1] Planetary 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.
The motions of the planetary nebulae along the line of sight towards or away from Earth lead to shifts in the spectral lines, as a result of the Doppler effect. These shifts can be measured accurately using a sensitive spectrograph and the velocity of the nebulae deduced.
More Information
This research was presented in a paper entitled “The build-up of the
cD halo of M87 — evidence for accretion in the last Gyr”, by A.
Longobardi et al., to appear in the journal Astronomy & Astrophysics Letters on 25 June 2015.
This work was also presented at the annual conference of the European Astronomical Society, EWASS 2015, which is being held in La Laguna, Tenerife, at the same time.
The team is composed of A. Longobardi (Max-Planck-Institut für
extraterrestrische Physik, Garching, Germany), M. Arnaboldi (ESO,
Garching, Germany), O. Gerhard (Max-Planck-Institut für
extraterrestrische Physik, Garching, Germany) and J.C. Mihos (Case
Western University, Cleveland, Ohio, USA).
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Links
Contacts:
Max-Planck-Institut für extraterrestrische Physik
Garching bei München, Germany
Tel: +49 89 30000 3022
Email: alongobardi@mpe.mpg.de
Magda Arnaboldi
ESO
Garching bei München, Germany
Tel: +49 89 3200 6599
Email: marnabol@eso.org
Ortwin Gerhard
Max-Planck-Institut für extraterrestrische Physik
Garching bei München, Germany
Tel: +49 89 30000 3539
Email: gerhard@mpe.mpg.de
Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Source: ESO
