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A GALAXY IS COLLIDING INTO OUR GALAXY AND ITS NOT ANDROMEDA



The Milky Way galaxy with a bright band of stars stretching horizontally across the center. Dark dust lanes are visible within the band. The Sagittarius Dwarf Galaxy is highlighted with a red circle and labeled in white text, positioned slightly below the center band towards the right. Two additional galaxies are visible in the lower right corner.

You may be familiar with the satellite galaxies of the Milky Way, such as the Large and Small Magellanic Clouds, visible in the bottom right of the image above. These are dwarf galaxies, but did you know there are other satellite dwarf galaxies like the Sagittarius Dwarf Spheroidal Galaxy?

Located approx 70,000 light-years from Earth, the Sagittarius Dwarf Spheroidal Galaxy is gradually being pulled apart by the Milky Way's gravitational forces over billions of years.



In the all-sky map of the star density observed by ESA's Gaia mission above, the Sagittarius dwarf galaxy can be seen as an elongated feature below the Galactic centre and pointed downwards as an effect of our Galaxy's gravitational tug.


Astronomers have known that Sagittarius repeatedly smashes through the Milky Way’s disc, as its orbit around the galaxy’s core tightens as a result of gravitational forces.


A sequence of six images depicting the interaction between the Milky Way galaxy and the Sagittarius Dwarf Galaxy over time. The images illustrate different stages of the Sagittarius Dwarf Galaxy's passages near the Milky Way:  8 billion years ago: The Sagittarius Dwarf Galaxy is positioned above the Milky Way. 5.7 billion years ago: First passage of the Sagittarius Dwarf Galaxy near the Milky Way. 3 billion years ago: The Sagittarius Dwarf Galaxy is moving away from the Milky Way. 1.9 billion years ago: Second passage of the Sagittarius Dwarf Galaxy. 1 billion years ago: Third passage of the Sagittarius Dwarf Galaxy. Current situation: The Sagittarius Dwarf Galaxy has left visible ripples in the Milky Way. These images highlight how the gravitational interactions have shaped the structure of the Milky Way over billions of years.
Image Credit: ESA
“It is known from existing models that Sagittarius fell into the Milky Way three times – first about five or six billion years ago, then about two billion years ago, and finally one billion years ago,” says Tomás Ruiz-Lara, a researcher in Astrophysics at the Instituto de Astrofísica de Canarias (IAC) in Tenerife, Spain, and lead author of the relativity new study published in Nature Astronomy.

The Sagittarius dwarf galaxy has been orbiting the Milky Way for billions of years. As its orbit around the 10,000 times more massive Milky Way gradually tightened, it started colliding with our galaxy's disc.

An animated GIF showing the Milky Way galaxy's interaction with the Sagittarius Dwarf Galaxy over time. The animation highlights significant passages of the Sagittarius Dwarf Galaxy as it orbits and collides with the Milky Way. Key frames include:  8 billion years ago: The Sagittarius Dwarf Galaxy is positioned above the Milky Way. 1.9 billion years ago: The second passage of the Sagittarius Dwarf Galaxy near the Milky Way, showing a close interaction. The animation illustrates how these interactions have influenced the structure and evolution of the Milky Way.

These collisions are part of a series of interactions the Milky Way has experienced with various galaxies throughout its history.


The sun's orgin?

According to a prior study, the three known collisions between Sagittarius and the Milky Way were like throwing a pebble into a pond, with waves radiating outward and creating new stars. One of these star formation episodes may have given rise to the Solar System.


In fact, it seems possible that even the Sun and its planets would not have existed if the Sagittarius dwarf had not gotten trapped by the gravitational pull of the Milky Way and eventually smashed through its disc.



An artist's illustration of the Milky Way galaxy, depicting ripples on its edge. The image shows the spiral structure of the Milky Way, with bright yellow light at the center and swirling blue and white arms extending outward. Red and pink areas indicate regions of star formation. The illustration highlights the detailed and dynamic features of the galaxy's structure. The caption credits NASA JPL-Caltech and R. Hurt (SSC Caltech) for the artwork.
Artist's view of ripples on the edge of the Milky Way. Credit: NASA JPL-Caltech R. Hurt (SSC Caltech)

Researchers explain “The Sun formed at the time when stars were forming in the Milky Way because of the first passage of Sagittarius” “We don’t know if the particular cloud of gas and dust that turned into the Sun collapsed because of the effects of Sagittarius or not. But it is a possible scenario because the age of the Sun is consistent with a star formed as a result of the Sagittarius effect.”

Our galaxy is very old

Because our galaxy is so old, 13.4 billion years old, it has been involved in many other galaxy collisions throughout its long life. Our galaxy came into existence when the universe was just four hundred million years old, which is only a blink of an eye cosmologically speaking.

Illustration showing the Milky Way's S Shape formed by past collisions

It is because of this that not only are ripples in our galaxy , but it also appears to be S shaped. If there are any ETs out there, our galaxy may stand out like a sore thumb.




Hubble Provides a stunning shot of the dwarf galaxy

This image from the Hubble Space Telescope showcases the sagittarius dwarf irregular galaxy.

FYI, astronomers tend to name it "SagDIG" for short.

Located relatively close to us, Hubble's sharp vision reveals thousands of individual stars within SagDIG.

An image taken by the Hubble Space Telescope showing the Sagittarius Dwarf Irregular Galaxy (SagDIG). The image reveals thousands of individual stars within SagDIG, appearing as a dense field of stars with varying brightness and colors, ranging from blue to red. The background is dark, highlighting the multitude of stars. The text above the image notes that Hubble's sharp vision has provided a detailed view of this relatively nearby galaxy.

Collisions may be responsible for spiral structures

And previous studies suggested that Sagittarius has had a profound effect on how stars move in the Milky Way. Some even claim that the 10 000 times more massive Milky Way’s trademark spiral structure might be a result of the at least three known crashes with Sagittarius over the past six billion years.


A GIF showing a simulation of the Milky Way galaxy's spiral arms over time, created using supercomputer data. The image captures a snapshot labeled "time after collision = 834 million years," displaying a spiral pattern formed by blue and white dots representing stars. The x-axis is labeled "altitude of stars above/below the Galactic plane (kpc)" and the y-axis is labeled "vertical velocity of stars (km/s)." The accompanying text explains that scientific experimentation in cosmology often relies on supercomputer simulations to model distant, violent events from billions of years ago, allowing astronomers to predict and test observations.
Simulations with supercomputers, based on real data, suggest that the spiral arms of the Milky Way and its outer rings were formed by the epic collision with the Sagittarius dwarf galaxy.

Scientific experimentation in cosmology is limited to supercomputer simulations. Astronomers can create a small-scale simulation or model of distant, violent events that occurred billions of years ago using supercomputers, and observe that model in accelerated time in order to make predictions that can be tested on actual observations.


Sagdig's globular clusters

As fans of globular clusters, we were delighted to discover that this wee galaxy has some. SagDig contains at least nine known globular clusters.


One of these, M54, appears to reside at its core, while three others—Terzan 7, Terzan 8, and Arp 2—are within the main body of the galaxy.


As well as, Palomar 12, Whiting 1, NGC 2419, NGC 4147, and NGC 5634 are found within its extended stellar streams. This is an unusually low number of globular clusters, and an analysis of VVV and Gaia EDR3 data has found at least twenty more. The newly discovered globular clusters tend to be more metal-rich than previously known ones.



"It makes you wonder and ponder: with just these globular clusters alone, the odds of other Earth-like planets out there increase significantly. And that's without even considering the two trillion galaxies in the universe!" says our editor Siobhan

Moar satellite dwarf galaxies of the Milky Way

I guess this will be a great opportunity to ponder the other dwarfs in our galaxy. It would be a wee bit amusing if there were seven of them.

A diagram showing several dwarf galaxies orbiting the Milky Way. The Milky Way is at the center, with concentric circles indicating distance. Various dwarf galaxies, such as Ursa Major I, Ursa Major II, Ursa Minor Dwarf, Draco Dwarf, Sextans Dwarf, Bootes Dwarf, Sagittarius Dwarf, Sculptor Dwarf, Fornax Dwarf, Carina Dwarf, Large Magellanic Cloud, and Small Magellanic Cloud, are positioned around the Milky Way. Each galaxy is labeled, with lines indicating their relative positions and distances in light-years.ique traits and histories.

We have several dwarf galaxies orbiting the Milky Way, each with its own traits and history. The following is a brief overview of some of them:

Large Magellanic Cloud (LMC): The LMC is an irregular galaxy and the largest satellite galaxy of the Milky Way. Located about 163,000 light-years away, it has a bar structure and is rich in gas and star-forming regions, making it a site of intense star formation.


An image of the Large Magellanic Cloud (LMC), showing two glowing gas clouds. On the left is NGC 2020, a blue gas cloud with a ring-like structure, and on the right is NGC 2014, a red gas cloud with a more irregular shape. The background is filled with numerous stars. The text above the image describes the LMC as an irregular galaxy and the largest satellite galaxy of the Milky Way, located about 163,000 light-years away. It has a bar structure and is rich in gas and star-forming regions, making it a site of intense star formation. The caption notes the two gas clouds within the LMC.
Two different glowing gas clouds in the Large Magellanic Cloud, NGC 2014 (red) and NGC 2020 (blue)

Small Magellanic Cloud (SMC): Or the Wee Magellanic Cloud I like to call it. The SMC is another irregular galaxy, located around 200,000 light-years away. It's smaller and less massive than the LMC, but also active in star formation. Both the LMC and SMC are connected to the Milky Way by a bridge of gas called the Magellanic Stream.

A recent image of NGC 346, a star-forming region within the Small Magellanic Cloud (SMC), taken by the James Webb Space Telescope (JWST). The image showcases a dynamic and colorful nebula with bright stars scattered throughout. The nebula features intricate structures and filaments in shades of pink, purple, and orange, set against a dark star-filled background. The text beneath the image describes NGC 346 as one of the most dynamic star-forming regions in nearby galaxies.
This is a region known as NGC 346 inside SMC , which is one of the most dynamic star-forming regions in nearby galaxies. This shot was recently taken by JWST.

SMC may actually be divided into two distinct segments, according to astrophysicists D. S. Mathewson, V. L. Ford, and N. Visvanathan. According to their theory, a smaller part of the SMC is located behind the main body as seen from Earth, separated by about 30,000 light-years. The SMC was split due to an interaction with the Large Magellanic Cloud (LMC) in the past, and the two sections continue to move apart. This smaller remnant has been dubbed the Mini Magellanic Cloud (MMC).


Further research in 2023 confirmed that the SMC consists of two separate structures separated by around 5 kiloparsecs (approximately 16,300 light-years).


Canis Major Dwarf Galaxy: This is an irregular galaxy located about 25,000 light-years away. It's the closest known satellite galaxy to the Milky Way and is the galaxy is heavily disrupted by the gravitational forces of the Milky Way, which has stripped away much of its material. Hence, the disruption has scattered its stars across a wide area, making the CMa galaxy appear as a faint and diffuse collection of stars rather than a distinct, cohesive structure. As well, it is located behind the dense star fields and dust of the Milky Way's disk, further complicating efforts to image it clearly.

Ursa Minor Dwarf Galaxy: A spheroidal galaxy about 200,000 light-years away, the Ursa Minor Dwarf is characterized by its low star formation rate and is predominantly composed of older stars.

Draco Dwarf Galaxy: This is another spheroidal galaxy located approximately 260,000 light-years away. It is one of the most dark matter-dominated galaxies known, with very little visible matter.

Sculptor Dwarf Galaxy: About 290,000 light-years away, the Sculptor Dwarf is a spheroidal galaxy that has an older stellar population with little to no recent star formation activity.


Fornax Dwarf Galaxy: Situated around 460,000 light-years from the Milky Way, the Fornax Dwarf is larger than many other spheroidal galaxies and contains several globular clusters. It has a mix of old and intermediate-age stars.


A Hubble Space Telescope image of four globular clusters in the Fornax Dwarf galaxy. Each cluster is shown against a dark background, with stars ranging in color from red and yellow to white. The images progress from left to right, showing clusters that increase in star density and brightness towards the center. The central regions of the clusters become increasingly bright and densely packed with stars.
Hubble Space Telescope image of four of the globular clusters in Fornax Dwarf - S. Larsen (Radboud University, the Netherlands)
  • Age: Fornax's clusters are ancient, with ages estimated to be around 10-12 billion years. This places them among the oldest known star clusters, forming during the early epochs of the universe.

  • Metallicity: The clusters have a range of metallicities, generally lower than those found in globular clusters within the Milky Way.

Because the universe was very young at the time of its formation, between 3.8 and 1.8 billion years ago, this lower metallicity is consistent with the idea that the Fornax Dwarf formed stars over a longer period of time and from more primordial gas.

A dense star field with numerous stars against a dark background. The stars range from faint to bright, with a prominent cluster of stars forming a loose, spherical shape near the center. A bright yellow star is visible on the left side, and a bright blue star is on the right side. The central cluster is slightly more concentrated with a faint, hazy appearance, indicating a galaxy or star cluster.
Fornax by ESO/Digitized Sky Survey 2

Leo I Dwarf Galaxy: This spheroidal galaxy is approx 820,000 light-years away. Leo I has experienced some recent star formation and has a relatively young stellar population compared to other dwarf spheroidals. Interestingly, studies suggest it may have a black hole around the same mass of the Milky Way's SagA*.

"You have a very small galaxy falling into the Milky Way, yet its black hole is nearly as massive as the one in the Milky Way."
An illutration of Leo 1 Dwarf Galaxy
Leo I Dwarf Galaxy - Image credit: ESA / Gaia / DPAC / SDSS.

Leo II Dwarf Galaxy: Located about 690,000 light-years away, Leo II is a spheroidal galaxy that also shows signs of relatively recent star formation.

Sextans Dwarf Galaxy: This spheroidal galaxy is about 280,000 light-years from the Milky Way. It has a very low luminosity and low star formation rate.


Carina Dwarf Galaxy: Roughly 330,000 light-years away, the Carina Dwarf is notable for having undergone several bursts of star formation separated by long periods of inactivity.


Each of these dwarf galaxies contributes to our understanding of galaxy formation and evolution, and their interactions with the Milky Way continue to shape our galaxy's structure and star formation history.

Local Group

Many of these satellite galaxies are within the Local Group. The Local Group extends further out and includes larger galaxies such as Andromeda and the Triangulum galaxy.

A diagram of the Local Group of galaxies. It shows a cylindrical shape with many labeled galaxies and dwarf galaxies. The Andromeda Galaxy and its companions are on the left, while the Milky Way and its companions are on the right. Each galaxy is labeled with its name and distance in kiloparsecs (kpc). The text "LOCAL GROUP" runs vertically along the left side.
Credit: D’ ANDREW Z. COLVIN (CC BY-SA 3.0)

Among the 35 confirmed satellite galaxies of the Andromeda Galaxy, M32 and M110 are particularly noteworthy.


Credit: https://www.achintthomas.photography/blog/2020/10/25/astrophotography-andromeda-galaxy

And if you're wondering, there is evidence to suggest that both M32 and M110 have interacted or collided with Andromeda in the past:


  • M32: Studies indicate that M32 might have undergone interactions with Andromeda, which could explain its compact and peculiar structure. There is evidence that M32 might be the remnant of a larger galaxy that was stripped of its outer layers by Andromeda's gravitational forces.


  • M110: There is also evidence suggesting that M110 has interacted with Andromeda. Observations of tidal streams and disturbances in M110's structure indicate that it might have experienced gravitational interactions with its larger neighbour.





Infrared image of the Triangulum Galaxy taken from Spitzer Space Telescope - By NASA/JPL-Caltech

The Triangulum has fewer known satellite galaxies compared to Andromeda.


LGS 3: This is one of the notable satellite galaxies of Triangulum. Studies suggest that LGS 3 might have had interactions with Triangulum in the past. Observations of its stellar population provide insights into its history and possible interactions.


Andromeda XXI and Andromeda XXII: Though primarily considered satellites of Andromeda, these dwarf galaxies are also in proximity to Triangulum and could potentially be influenced by its gravitational field.


Just like our own, the spiral structure of Triangulum itself provides evidence of past interactions. Its well-defined spiral arms and star-forming regions suggest a history of gravitational influences, possibly as well from Andromeda or other nearby galaxies.


Similar to the tidal streams observed around Andromeda and our own galaxy, Triangulum might exhibit faint stellar streams indicating past gravitational interactions.


Closing thoughts

The ongoing smashing of these wee galaxies has led to the creation of stars and stellar remnants. Without these interactions, the Milky Way's spiral structure wouldn't have been affected, which resulted in the formation of spiral arms and star clusters. This shows that these dwarf galaxies play a crucial role in galactic formation. And it very well could be that without these wee galaxies, the sun may not have been born. If you enjoyed this article, we certainly enjoyed writing it. It took several days to research and write, so please consider sharing it—your support would be much appreciated. Regards, the Space Ponder team.
 

Curated resources for pondering.

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