The cosmic commute towards star and planet formation

Date:
July 7, 2020
Source:
Max-Planck-Gesellschaft
Summary:
Interconnected gas flows reveal how star-forming gas is assembled
in galaxies.



FULL STORY ==========================================================================
The molecular gas in galaxies is organised into a hierarchy of
structures. The molecular material in giant molecular gas clouds travels
along intricate networks of filamentary gas lanes towards the congested
centres of gas and dust where it is compressed into stars and planets,
much like the millions of people commuting to cities for work around
the world.


==========================================================================
To better understand this process, a team of astronomers led by Jonathan Henshaw at Max Planck Institute for Astronomy (MPIA) have measured the
motion of gas flowing from galaxy scales down to the scales of the gas
clumps within which individual stars form. Their results show that the gas flowing through each scale is dynamically interconnected: while star and
planet formation occurs on the smallest scales, this process is controlled
by a cascade of matter flows that begin on galactic scales. These results
are published today in the scientific journal Nature Astronomy.

The molecular gas in galaxies is set into motion by physical mechanisms
such as galactic rotation, supernova explosions, magnetic fields,
turbulence, and gravity, shaping the structure of the gas. Understanding
how these motions directly impact star and planet formation is difficult, because it requires quantifying gas motion over a huge range in spatial
scale, and then linking this motion to the physical structures we
observe. Modern astrophysical facilities now routinely map huge areas of
the sky, with some maps containing millions of pixels, each with hundreds
to thousands of independent velocity measurements. As a result, measuring
these motions is both scientifically and technologically challenging.

In order to address these challenges, an international team of researchers
led by Jonathan Henshaw at the MPIA in Heidelberg set out to measure gas motions throughout a variety of different environments using observations
of the gas in the Milky Way and a nearby galaxy. They detect these motions
by measuring the apparent change in the frequency of light emitted by
molecules caused by the relative motion between the source of the light
and the observer; a phenomenon known as the Doppler effect. By applying
novel software designed by Henshaw and Ph.D. student Manuel Riener (a
co-author on the paper; also at MPIA), the team were able to analyse
millions of measurements. "This method allowed us to visualise the
interstellar medium in a new way," says Henshaw.

The researchers found that cold molecular gas motions appear to fluctuate
in velocity, reminiscent in appearance of waves on the surface of
the ocean. These fluctuations represent gas motion. "The fluctuations themselves weren't particularly surprising, we know that the gas is
moving," says Henshaw. Steve Longmore, co-author of the paper, based at Liverpool John Moores University, adds, "What surprised us was how similar
the velocity structure of these different regions appeared. It didn't
matter if we were looking at an entire galaxy or an individual cloud
within our own galaxy, the structure is more or less the same." To better understand the nature of the gas flows, the team selected several regions
for close examination, using advanced statistical techniques to look
for differences between the fluctuations. By combining a variety of
different measurements, the researchers were able to determine how the
velocity fluctuations depend on the spatial scale.

"A neat feature of our analysis techniques is that they are sensitive to periodicity," explains Henshaw. "If there are repeating patterns in your
data, such as equally spaced giant molecular clouds along a spiral arm,
we can directly identify the scale on which the pattern repeats." The
team identified three filamentary gas lanes, which, despite tracing
vastly different scales, all seemed to show structure that was roughly equidistantly spaced along their crests, like beads on a string, whether
it was giant molecular clouds along a spiral arm or tiny "cores" forming
stars along a filament.

The team discovered that the velocity fluctuations associated with equidistantly spaced structure all showed a distinctive pattern. "The fluctuations look like waves oscillating along the crests of the
filaments, they have a well-defined amplitude and wavelength," says
Henshaw adding, "The periodic spacing of the giant molecular clouds
on large-scales or individual star-forming cores on small-scales is
probably the result of their parent filaments becoming gravitationally unstable. We believe that these oscillatory flows are the signature
of gas streaming along spiral arms or converging towards the density
peaks, supplying new fuel for star formation." In contrast, the team
found that the velocity fluctuations measured throughout giant molecular clouds, on scales intermediate between entire clouds and the tiny cores
within them, show no obvious characteristic scale. Diederik Kruijssen, co-author of the paper based at Heidelberg University explains: "The
density and velocity structures that we see in giant molecular clouds are 'scale-free', because the turbulent gas flows generating these structures
form a chaotic cascade, revealing ever smaller fluctuations as you zoom
in -- much like a Romanesco broccoli, or a snowflake. This scale-free
behaviour takes place between two well-defined extremes: the large scale
of the entire cloud, and the small scale of the cores forming individual
stars. We now find that these extremes have well-defined characteristic
sizes, but in between them chaos rules." "Picture the giant molecular
clouds as equally-spaced mega-cities connected by highways," says
Henshaw. "From a birds eye view, the structure of these cities, and the
cars and people moving through them, appears chaotic and disordered.

However, when we zoom in on individual roads, we see people who have
travelled from far and wide entering their individual office buildings
in an orderly fashion. The office buildings represent the dense and cold
gas cores from which stars and planets are born."

========================================================================== Story Source: Materials provided by Max-Planck-Gesellschaft. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Jonathan D. Henshaw, J. M. Diederik Kruijssen, Steven N. Longmore,
Manuel
Riener, Adam K. Leroy, Erik Rosolowsky, Adam Ginsburg, Cara
Battersby, Me'lanie Chevance, Sharon E. Meidt, Simon C. O. Glover,
Annie Hughes, Jouni Kainulainen, Ralf S. Klessen, Eva Schinnerer,
Andreas Schruba, Henrik Beuther, Frank Bigiel, Guillermo A. Blanc,
Eric Emsellem, Thomas Henning, Cynthia N. Herrera, Eric W. Koch,
Je'ro^me Pety, Sarah E. Ragan, Jiayi Sun. Ubiquitous velocity
fluctuations throughout the molecular interstellar medium. Nature
Astronomy, 2020; DOI: 10.1038/s41550-020- 1126-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200707113250.htm

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