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