Snapshots from the simulation where Tsatsi made her discovery of the "galactic rocket engine". Left: the two galaxies before the merger; right: the resulting elliptical galaxy after the merger.
Image: B. Moster / MPIA
Different
orientations for galaxy mergers: retrograde motion (top left) means
that stars in one of the progenitor galaxies (shown in dark purple)
rotate in one direction, while before the merger, the two progenitor
galaxies orbit each other in the opposite direction. The existing model
posited that elliptical galaxies with counter-rotating cores (right) can
form only in situations like this, but not in the case of prograde
motion (botto left), where both galaxies rotate, and orbit each other,
in the same direction. Image: MPIA
In this simulated integral field spectroscopic image, colors represent
motion parallel to the line of sight: from blue (fastest motions toward
us) to red (fastest motion away from us). The different types of motion
in the inner and outer regions are clearly discernible. This is how
Tsatsi first realized the simulation had produced a counter-rotating
core. Image: A. Tsatsi / MPIA
Schematic
diagram of the Meshchersky mechanism: As the core regions lose mass
during the merger, the reaction force ("rocket drive") changes their
orbits; that way, the material that ends up in the center of the
resulting elliptical galaxy rotates in the opposite way to the matter in
the outer regions. Image: MPIA
A discovery by MPIA graduate student Athanasia Tsatsi has changed
astronomers' understanding of how mergers of two galaxies can produce
unusual stellar motion in the resulting elliptical galaxies, with the
central region rotating in the direction opposite to that of the
galaxy's other stars. Previously, such differences had been thought to
be the result of an opposite ("retrograde") orientation of the galaxies
prior to their merger. Looking at a simulation of a galaxy merger,
Tsatsi discovered a different way of bringing about such
counter-rotating cores, which involve mass loss from the bodies of these
galaxies acting as a primitive galactic "rocket engine".
In so-called elliptical galaxies (which are shaped like somewhat
flattened spheres), the movement of stars can show an intriguing
pattern, with stars in the outer regions all rotating around the center
in one way, while stars in the core region jointly rotate in a
completely different direction.
Elliptical galaxies are the result of the collsion and merger of two
or more disk galaxies (such as our home galaxy, the Milky Way). Previous
explanations had assumed that counter-rotating (or, more generally,
“kinematically decoupled”) cores can form if one of the merging galaxies
has a tightly bound core region, with just the right orientation
relative to the galaxies' orbit during the merger. However, this
explanation predicts fewer counter-rotating cores than are actually
observed.
That was the situation when Athanasia Tsatsi at the Max Planck
Institute for Astronomy began to look at computer simulations of galaxy
mergers. Tsatsi's aim was to find out how the resulting galaxies would
look through astronomical instruments – but when looking through one
such “virtual instrument” at one of the simulations, she made an
unexpected discovery: The elliptical galaxy that resulted from the
simulated merger contained a counter-rotating core. But the merger
certainly did not have the specific (“retrograde”) orientation required
by the usual explanation of how such cores form!
A more detailed examination showed that the counter-rotating motion
is directly linked to a change of direction of the galactic central
regions during the merger due to the so-called Meschchersky force, or
more prosaically: due to gigantic galactic rocket engines. As the
galaxies merge, the central regions lose mass which, just like the gas
expelled by a rocket engine, can cause their motion to change.
The result of the simulated merger was consistent with the observed
examples for such counter-rotating cores: With 130 billion times the
mass of the Sun, this was one of the more massive elliptical galaxies,
where such cores were known to be more common. In the simulation, the
counter-rotating core remains distinct from its surroundings for 2
billion years after the coalescence of the two galaxies, making for a
phenomenon sufficiently persistent as to be observable in real galaxies.
Finally, the counter-rotating stars consisted mostly of older stars
that had been present before the collision, not the new generation of
stars produced during the merger; this, too, was what observations of
such systems had shown.
Tsatsi's discovery concerns a single case. But that is sufficient to
serve as a proof of concept, showing that the Meshchersky mechanism of
producing counter-rotating galactic cores is indeed feasible. Next, the
astronomers will need to show the likelihood of this kind of interaction
by varying the initial conditions of their galaxy collision
simulations. Should these systematic tests show that the Meshchersky
mechanism for producing counter-rotating cores is common, they would
resolve a long-standing discrepancy between the observed prevalence of
such counter-rotating cores and their assumed modes of production. But,
even now, Tsatsi's discovery has had an impact on the way future
astronomers will look at counter-rotating cores and galactic mergers –
knowing that it doesn't necessarily take special, retrograde
configurations of colliding galaxies, but that “galactic rocket engines”
could do the job just as well.
Background Information
The scientists involved are Athanasia Tsatsi, Andrea Macciò and Glenn
van de Venn (all Max Planck Institute for Astronomy) and Benjamin
Moster (at the time he created the simulations, a PhD student at MPIA;
now at Cambridge University).
The result is accepted for publication as Tsatsi et al., “A New Channel for the Formation of Kinematically Decoupled Cores in Early-type galaxies” in Astrophysical Journal Letters.
ADS Entry for the publication
Tsatsi is a graduate student at Heidelberg's International Max Planck Research School “Astronomy and Cosmic Physics” (in collaboration with the University of Heidelberg) and a Marie Curie Fellow within the DAGAL European Initial Training Network that studies the structure and evolution of galaxies.
The result is accepted for publication as Tsatsi et al., “A New Channel for the Formation of Kinematically Decoupled Cores in Early-type galaxies” in Astrophysical Journal Letters.
ADS Entry for the publication
Tsatsi is a graduate student at Heidelberg's International Max Planck Research School “Astronomy and Cosmic Physics” (in collaboration with the University of Heidelberg) and a Marie Curie Fellow within the DAGAL European Initial Training Network that studies the structure and evolution of galaxies.
Questions and Aswers
In what ways is the research described here new / important?
Tsatsi's discovery points to a solution of a long-standing problem:
How do counter-rotating cores in elliptical galaxies form? Previous
explanations could not account for the prevalence of such unusual
patterns of stellar motion; with the “galactic rocket engine”, the new
discovery posits an attractive new mechanism that could help explain the
discrepancy. It will, however, take more systematic studies to find out
whether or not the new mechanism can indeed account for the observed
number of counter-rotatig cores.
What is unusual about stellar motions in some elliptical galaxies – and what was unexplained?
Spiral galaxies such as our own Milky Way galaxy present a stately
stellar dance, with all stars orbiting the center of the galaxy in the
same direction, at a stately pace (it takes our Sun about 250 million
years to complete one orbit around the galactic center). But for a
different species, so-called elliptical galaxies, the situation can be
more complex. As the name indicates, these galaxies are shaped like
ellipsoids, and at least some of them exhibit a two-fold rotation
pattern: While the stars in their outer regions have a common preferred
direction of rotation, stars in the core region can jointly rotate in a
completely different direction – a „counter-rotating core“, or more
generally a “kinematically decoupled core”, which is apparently
completely independent from the motion of the majority of the galaxy's
stars.
The best available explanations link the existence of such
counter-rotating cores to a galaxy's formation history. Elliptical
galaxies are thought to be the result of the merger of two or more
sizable galaxies (“major merger”, see figure 1) . There is one
immediately plausible scenario for how counter-rotating cores could form
in such a merger. Imagine that at least one of the galaxies has a core
region that is fairly tightly bound by the galaxy's gravity.
Furthermore, imagine that the direction in which the two galaxies orbit
each other before merging is opposite to the direction of rotation of
stars in that tightly bound core (“retrograde merger”, cf. figure 2). In
that case, it is likely that, after the merger, the tightly bound core
will end up as the core of the new, larger galaxy, while retaining its
original sense of rotation. The surrounding stars, on the other hand,
will rotate in a different way dictated by the orbital motion of the
galaxies around each other, before the merger.
While this is a plausible scenario, it can only explain some of the
counter-rotating cores. In total, more than half of the more massive
elliptical galaxies contain kinematically decoupled central regions.
This is significantly more than the retrograde merger scenario can
explain. After all, by pure chance, one would expect a retrograde motion
for the more tightly bound of the two galaxies in only about half of
the cases – and only some of those mergers are thought to result in a
counter-rotating core.
How did Tsatsi's discovery of an alternative production mechanism for counter-rotating cores come about?
Tsatsi's aim was to analyze simulations that show the formation of an
elliptical galaxy by the merger of two spiral galaxies, and to
reconstruct how the resulting galaxy would look to astronomical
observers: What would such observers find if they analyzed their
astronomical images and spectroscopic measurements? Such a
reconstruction is a key step if one wants to compare predictions from
these simulations with observations of actual galaxies. The simulations
in question were created by Benjamin Moster, then also graduate student
at MPIA and now at Cambridge University. They are based on the
cosmological simulation code GADGET developed by Volker Springel and
colleagues, which simulates a galaxy as a collection of a great many
particles representing the galaxy's stars, gas and dark matter content.
The code is particularly suitable for running in parallel, on a great
number of processors at once, enabling detailed, yet large-scale
simulations.
The main observational technique featured in Tsatsi's program is
known as integral field spectroscopy. This type of observation allows
astronomers to take spectra of many different regions of a galaxy,
splitting light from each region into myriads of different colors. As
stars move towards or away from the observer, the starlight is shifted
towards shorter or longer wavelengths, respectively (a Doppler shift,
more concretely a blueshift or redshift). Such a wavelength shift can be
identified in a star's spectrum. In this way, integral field
spectroscopy allows astronomers to reconstruct which parts of the galaxy
are, on average, moving towards us and which parts are moving away.
Based on such observations, astronomers can reconstruct stellar motion
within a galaxy, which in turn gives them valuable information about the
distribution of the galaxy's mass.
When Tsatsi reconstructed integral field spectroscopic observations
for one particular simulation, she noticed an unusual fact. The
kinematic map showing stellar motion within the galaxy indicated that
the central region was moving in a different way from the rest of the
galaxy (cf. figure 3). In other words: the galaxy evidently contained a
counter-rotating core. But this had been a merger in which the two
colliding galaxies rotate in the same direction as that of their orbit
around each other – a prograde merger, and thus a merger of a kind
deemed incapable of producing a counter-rotating core (see figure 2).
When Tsatsi had a closer look, she could see directly what had escaped
the attention of all previous astronomers who had looked at the
simulation: As the core regions of the two galaxies orbit each other,
there is a particular time at which their orbital direction changes.
This change in direction happens just as the galaxies are shedding mass
in the form of stars while they interact via their mutual gravitational
attraction (cf. figure 4).
What is the Meshchersky mechanism?
While searching the available scientific literature, Tsatsi realized
that there was a precedent for the effect she had observed in the
simulated galaxy collision. It is closely related to a special case of a
problem studied intensively by the Russian mathematician Ivan
Vsevolodovich Meshchersky (sometimes also spelled “Mestschersky”):
point-like bodies, whose masses can change over time, moving under each
other's gravitational influence. In such a situation, the influence of
lost mass can change a body's direction of motion – resulting in the
so-called Meshchersky force. The best-known example is rocket
propulsion, where the rocket's loss of mass as it expels hot gases in
one direction is accompanied by a reactive force (and hence an
acceleration) in the opposite direction (see figure 3). That was why,
even in ordinary (prograde) collisions, counter-rotating cores could
form: the mass loss experienced by the galactic bodies acted like a
gigantic rocket engine, and could be sufficiently strong so as to change
the direction of rotation for the stars in the new galaxy's core (the
remnant of the two colliding galaxies' central region). Tsatsi dubbed
this way of producing counter-rotating cores the Meshchersky mechanism.
