Astronomers discover unusual spindle-like galaxies

An elliptical galaxy in prolate rotation. The galaxy
resembles the shape of a cigar, with its stars rotating around
the galaxy’s long axis, similar to a spindle. the background
image is a snapshot of a simulation by A. Tsatsi and
colleagues. Credit: J. Chang, PMO / T. Müller, HdA

Galaxies are majestic, rotating wheels of stars? Not in the
case of the spindle-like galaxies studied by Athanasia Tsatsi
(Max Planck Institute for Astronomy) and her colleagues.
Using the CALIFA survey, the astronomers found that these
slender galaxies, which rotate along their longest axis, are
much more common than previously thought. The new data
allowed the astronomers to create a model for how these
unusual galaxies probably formed, namely out of a special
kind of merger of two spiral galaxies. The results have been
published in the journal Astronomy & Astrophysics.

When most people think of , they think of majestic spiral galaxies
like that of our home galaxy, the Milky Way: billions of stars,
rotating in a flat disk similar to the way that a wheel rotates
around its central axis. But there is another kind of galaxy,
which used to be thought very rare: so-called prolate rotators,
each shaped like a cigar, which rotates along its long axis,
like a spindle.

Now, a group of astronomers led by Athanasia Tsatsi of the Max
Planck Institute for Astronomy has completed a thorough study
of these cosmic spindles. Using data from the CALIFA survey, a
systematic study that examined the velocity structure of more
than 600 galaxies, the astronomers discovered eight new prolate
rotating galaxies, almost doubling the total known number of
such galaxies (from 12 to 20). Cosmic spindles are considerably
less rare than astronomers had thought!

Given the high quality of their data, the astronomers were able
to propose a plausible explanation for how these cosmic
spindles come into existence. In general, galaxies grow when
they merge with other galaxies. Several mergers with smaller
galaxies have made our own Milky Way the stately disk it is
today. To make a cosmic , two large disk galaxies need to collide
at right angles, as shown in this animation:

The formation of an elliptical galaxy in prolate rotation.
The mechanism shown here was proposed by Athanasia Tsatsi and her
colleagues in order to explain the recent discoveries of galaxies
of this kind with the CALIFA survey. The formation involves a
polar merger of two spiral galaxies. One of the spiral galaxies
develops a marked elongated structure (a “bar,” to use the
technical term) before the merger, which gives the resulting
elliptical galaxy its cigar-like (prolate) shape. The stars of
the second spiral galaxy end up orbiting around the bar of the
first companion. Together they form a cigar-shaped elliptical
galaxy that rotates like a spindle around its long axis. Movie:
J. Chang, PMO / T. Müller, HdA

As the galaxies begin to interact via gravitational attraction,
one of them forms a bar: an elongated structure near the
center. That bar becomes the cigar-like shape of the merged
galaxy, while the orbiting stars of the other galaxy imbue the
merged galaxy with its overall sense of rotation.

The results are an interesting piece of the puzzle, explaining
a likely formation scenario for an unusual, but not all that
uncommon type of galaxy. Tsatsi’s team of researchers having
put to good use all the information contained in the CALIFA
data, the ball is now in the court of the observing astronomers
again: the merger simulations make some additional predictions
for the detailed properties of prolate rotators. These cannot
be distinguished with the current observations, but could be
tested with instruments like MUSE, the Multi Unit Spectral
Explorer at ESO’s Very Large Telescope, an 8-meter-telescope at
Paranal Observatory in Chile.

The results here will be published in the journal Astronomy
& Astrophysics
as Tsatsi et al., “CALIFA reveals
prolate rotation in massive early-type galaxies: A polar galaxy
merger origin?”

Explore further:

Astronomers spun up by galaxy-shape finding

More information: A. Tsatsi et al. CALIFA reveals
prolate rotation in massive early-type galaxies: A polar galaxy
merger origin?, Astronomy & Astrophysics (2017).
DOI: 10.1051/0004-6361/201630218 ,
https://arxiv.org/abs/1707.05130

Journal reference: Astronomy
& Astrophysics

Provided by: Max Planck
Society

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