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We recently looked at the new discovery of filament structures that were located in our neighborhood.
We also saw that the nearest open star cluster to us revealed an important part
of the filament structure of the galaxy spiral arms. Mainstream science agrees that much of
the interstellar medium is not simply filled with empty space. Instead, they see that it is a
hierarchical structure and facilitates the formation of filaments that are
governed by the galactic potential on a large scale. We already know that there is a network of
hydrogen filaments that is structured mostly parallel to the galactic plane.
In 2010, astronomers discovered a highly filamented dark cloud which they called Nessie.
They believe that it is one of the first filaments whose morphology is likely
governed by the structural dynamics of the galaxy. Because of this, they dubbed this type of
filament the bones of the Milky Way as they think that its formation, evolution,
and shape could be closely linked to the global spiral arm structure. Within molecular clouds they discovered
giant molecular filaments. These seem to have the highest masses, while exhibiting the lowest column
densities and star forming activity, making them good candidates to be intermediate descendants of the atomic
interstellar medium structures. These structures are not only associated
with spiral arms, but are also located in inter-arm regions. The recently
discovered Radcliffe wave is yet another example of filaments in our local neighborhood.
Even in their own simulations they found that giant molecular clouds often form as large filaments.
Now obviously, they see the formation process a little differently and quote
things like “In our simulations the galactic shear between the spiral arms
plays a critical role in the formation of highly filamented structures.” Their
assumption is also that these giant molecular clouds tend to sustain their
large filamentary shape, before entering the spiral arms, where they are prone to
being disrupted by local events of star formation and stellar feedback.
And yet, we have examples of large filaments that are located in our neighborhood.
Peering through our galaxy is not easy, and now astronomers think that they have
spotted a massive filament of atomic hydrogen. This is one of the largest structures ever
observed in our galaxy. It is located about 55,000 lightyears
away on the other side of the Milky Way. The research is based on data obtained
by THOR, an observational program that relies on the Very Large Array in New Mexico.
The project studies molecular cloud formation, the conversion of atomic
molecular hydrogen, the galaxy's magnetic field, and other questions related to
interstellar medium and star formation. The ultimate purpose is to determine how
the two most common types of hydrogen gas converge to create dense clouds that give rise to new stars.
These two types are atomic hydrogen and molecular hydrogen. The process of how
atomic hydrogen transitions to molecular hydrogen, is still largely unknown, which
is why finding this huge filament is so important. To put the size into context, the largest
known clouds of molecular hydrogen are around 800 lightyears in length. This
new cloud which they call Maggie, measures just shy of 4,000 lightyears long and 130 lightyears wide.
More interesting is that parts of this filament extend about 1,600 lightyears
below the plane of the Milky Way. When they analyze the speed of the
material moving in the filament, they determined that it had a mean velocity
of 54 kilometers per second. They were also able to show that the
velocities along the filament barely differ. They determined that this filament might
be remotely linked to the global spiral arm structure of the Milky Way.
They were not able to detect any carbon monoxide emissions or dust in the
filament, and there were no signs of new stars forming along the filament.
On closer inspection, the team noticed that the gas converges at various points
along the filament which led them to conclude that the hydrogen gas
accumulates into large clouds at these locations. The Maggie filament discloses a hub-like
feature in the East, on which small-scale filaments appear to converge, and a tail
that thins out towards the West. The North-Western part shows a connection to the midplane, potentially
feeding off hydrogen material located at higher latitudes. So, many of you will not be surprised by
these findings, but is there more that we can learn from these observations?
An important point to note is that, although these filaments are composed of
atomic hydrogen, that doesn't mean that they are not a plasma. Even if only one atom in 10,000 is
ionized, it can be considered a plasma. Therefore the motion will be dominated electromagnetically.
If we examine the overall structure of the filament and look at the centroid
velocity, we can see that this speeds up and slows down.
What might cause the velocity to speed up? Around these areas we also see a much
higher density of the cloud. This is likely an indi- cation that electromagnetic pinching is occurring.
This would tend to compress the material, increasing the density, and also increasing the velocity.
In larger, more dense filaments, these are probably the areas where most star formation is taking place.
At these points we would also expect Marklund convection to be strongest,
creating a greater spread of material around these points, which is what we see to some extent in
the images of the filament so far obtained. In Marklund convection, a hydrogen is the
outermost ion from the center axis which becomes deionized or recombines with an electron.
The diffuse nature of Maggie and the lack of a CO signal, except on the
smaller scale along the axis, suggests that this is a juvenile filament.
One important aspect is also that they see the movement of the material in the
filament. This once more confirms the idea, that it is the movement of this
material in the filaments, which powers the stars that reside in them.
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