TITLE: A warp in the Milky Way linked to galactic collision!
Do you know what Pringles and the Milky Way Galaxy have in common?
The shape -in some sense. I know, dear. I know that in
high school it was taught us to think about our galaxy as a flat one, in which all the
stars orbited pretty on the same plane. I'm sorry, but that was quite incorrect.
Recently, an international team of astronomers published a very important paper,
by which they brought us new knowledge about the Milky Way and, in particular,about its 3D shape.
Ladies and gentlemen, fasten your seatbelts because we are going to break down
all of your convictions about the shape of our galaxy!
We wish to take a closer look at what they found. For the sake of the video, we will try to analyze
some pieces of the paper's abstract. What's an abstract?
An abstract is a brief summary of a research article, thesis, review, conference proceeding,
or any in-depth analysis of a particular subject and is often used to help the
reader quickly ascertain the paper's purpose. In the first lines, we can read:
“Previous analyses of large databases of Milky Way stars
have revealed the stellar disk of our Galaxy to be warped and that this
imparts a strong signature on the kinematics of stars beyond the solar neighbourhood.”
So. First of all. Which previous analyses are they talking about?
Well, we could cite some studies and researches led by astronomers like
Bosma and Binney, but I guess this would not include some interesting information.
But the fact is that, among the astronomy community, it is quite a common thing to say
that disk warps are features of spiral galaxies. Moreover, the presence of a wrap in the outer
Milky Way disk has been long-established. Also, while the origin of the Galactic warp still
invites controversy, the fact that the stellar warp follows the same topology as the gaseous one
is evidence that the warp is gravitationally induced!
This is the first thing to note: the warp is gravitationally induced.
Interactions with massive satellite galaxies can affect the outskirts of galaxy disks,
and the most likely candidates to create this warped outer disk in the Milky Way
are the Sagittarius dwarf spheroidal galaxy and the Magellanic Clouds.
Let's go ahead:
“However, due to the limitation of accurate distance estimates, many attempts to explore the
extent of these Galactic features have generally been restricted to a volume near the Sun.
By combining Gaia DR2 astrometric solution and stellar abundances from the APOGEE survey,
we present the most detailed and radially expansive study yet of the vertical
and radial motions of stars in the Galactic disk.”
Ok, let's stop here for a while and let's see if we can comment something on these lines.
What's GAIA? ... Gaia is an ambitious mission to chart
a three-dimensional map of our Galaxy, the Milky Way, in the process revealing the composition,
formation and evolution of the Galaxy. Gaia provides unprecedented positional and radial
velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census
of about one billion stars in our Galaxy! During the mapping, Gaia will detect and very
accurately measure the motion of each star in its orbit around the centre of the Galaxy. Much
of this motion was imparted upon each star during its birth; studying this motion allows astronomers
to peer back in time, to when the Galaxy was first forming. By constructing a detailed map
of the stars, Gaia will provide a crucial tool to study the formation of our Galaxy, the Milky Way.
With Gaia, for the first time, we have a large amount of data on a vast amount stars,
the motion of which is measured so precisely that we can try to understand the large
scale motions of the galaxy and model its formation history, this is something unique.
This really is the Gaia revolution.
...And what's APOGEE? APOGEE is a famous
survey among the astronomy community. The APO Galactic Evolution Experiment 1
(APOGEE-1) employed high-resolution infrared spectroscopy to penetrate the dust
that obscures significant fractions of the disk and bulge of our Galaxy. APOGEE surveyed over
100,000 red giant stars across the full range of the Galactic bulge, bar, disk, and halo. APOGEE-1
generated precise radial velocities and detailed chemical abundances, providing unprecedented
insights into the dynamical structure and chemical history of the Galaxy. In conjunction with the
planet-finding surveys, Kepler and CoRoT, APOGEE unravels problems in fundamental astrophysics.
So we learnt that, of course, in order to study the warp of the Milky Way, we have to know
something about the stars' kinematics. That's why we needed astrometric data from GAIA and APOGEE!
Most of all, GAIA is the best astrometry we have at the moment, and it reaches the best accuracy
in the study of star position and velocities. It is collecting such a huge quantity of data that,
as to speak, it will feed this and the next generation of astronomers!
So how's this warp look like? Do we have any information about it?
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Let's go back to the abstract for a while: “We map stellar velocity with respect to
their Galactocentric radius, angular momentum, and azimuthal angle and assess their relation to
the warp. A decrease in vertical velocity is discovered at Galactocentric radius
R = 13 kpc. We also discovered that trends in the vertical velocity with
azimuthal angle are not symmetric about the peak, suggesting the warp to be lopsided. To explain the
global trend in vertical velocity, we built a simple analytical model of the Galactic warp.
Our best fit yields a starting radius of 8.87 kpc and precession rate of 13.57km s−1
kpc−1. These parameters remain consistent across stellar age groups, a result that
supports the notion that the warp is the result of an external, gravitationally induced phenomenon. “
Basically, the warp model that the team proposes is the one that we are showing now: (TO THE
As you can see, there is the milky way and you can notice the warp. The interesting thing here
is that scientists calculated that it moves across the galaxy every 440 million years, so every 440
million years the warp moves around milky way, (a phenomenon called precession) which is roughly
around the double amount of time it takes for the solar system to move across the galaxy.
This means that the warp moves a little bit slower than the typical motion of the stars,
which also implies that the stars go through the warp most likely going up
and down or possibly above and below the galactic plane and then return back to their normal motion,
when they leave the warp behind because these stars do seem to move faster than the warp.
According to the model, the stars of our galaxy are“ gliding ”on the warp , like
surfers ride the waves of the sea. In regards to the shape of this warp,
it seems to be lopsided as the scientists refer to it, which implies that one of the side is a
little bit more pronounced than the other side with one side experiencing something a little
bit more dramatic than the opposite side. The scientists were also able to identify
where this warp officially starts and the distance from the centre of the
galaxy that they suggest is roughly around 8.87 kiloparsec light-years away.
now the current estimates for the distance to the galactic centre from the solar system
put us at around 8 kpc which also means that we are probably not really inside the warp.
The other unusual discovery was that the younger stars seem to be more affected by
this than the older stars: stars that were 6 to 9 billion years old were not as warped
or not as affected by this warp as the stars that were less than three billion years old, and here
the scientists believe that it's something to do with the passage of the nearby dwarf galaxy
(most likely Sagittarius dwarf galaxy) that did happen three billion years ago but the exact
reasons for this and the exact explanations for these differences are currently not really clear.
However, we are dead in the water for all that concerns both the
geometry of the warp and the explanations for it. In fact, later in the paper, It is stated that:
“While the origin of the Galactic warp understandably remains a complex puzzle,
simply defining the geometry of the warp is a problem that is also far from resolved,
with a variety of potential models posited for its shape .”
So far, we said that one of the cause for the Milky Way to be warped could be the presence
of Sgr dwarf galaxy. But how is this possible?
Well, scientists believe that our Galaxy and Sgr galaxy are actually colliding.
Don't be scared: nothing to worry about..for now! In fact, despite the two are far enough from each
other, we are talking about collision in a more general sense. We could call it an interaction.
Galaxies are held together by mutual gravity and orbit around a common centre.
Interactions between galaxies are quite common, especially between giant and satellite galaxies.
This is often the result of a galaxy drifting too close to one another, to the point where
the gravity of the satellite galaxy will attract one of the giant galaxy's primary spiral arms.
In this sense, astronomers believe that Sgr Dwarf Galaxy plays an important
role in the warping of our Milky Way. Anyway, this is still a matter of debate,
and maybe in the following years, we will have new data that will help us to understand
more about the pretty warped galaxy we call home.
"We're entering an age in astronomy, especially in galactic astronomy,
in which we are measuring the movement of the stars at such a level of precision that we can map
their past orbital paths and start to understand how they may have been affected at earlier times
and how other galaxies approaching our own interacted with stars as they were being born,"
Borja Anguiano, a post-doctoral research associate at UVA and one of the authors of the article says.
"This level of precision has opened a new door to understanding our galaxy's past
and how it was assembled."
You can find the article "Exploring the Galactic Warp through Asymmetries
in the Kinematics of the Galactic Disk," by Cheng and his colleagues, on the Astrophysical Journal.
"This video ends here! Thanks for watching everyone! Did you know about the warp of the Milky
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