Explanation: About 70,000 light-years across, NGC 247 is a spiral galaxy smaller than our Milky Way. Measured to be only 11 million light-years distant it is nearby though. Tilted nearly edge-on as seen from our perspective, it dominates this telescopic field of view toward the southern constellation Cetus. The pronounced void on one side of the galaxy's disk recalls for some its popular name, the Needle's Eye galaxy. Many background galaxies are visible in this sharp galaxy portrait, including the remarkable string of four galaxies just below and left of NGC 247 known as Burbidge's Chain. Burbidge's Chain galaxies are about 300 million light-years distant. NGC 247 itself is part of the Sculptor Group of galaxies along with shiny spiral NGC 253.
Very nice APOD of the Needle Eye Galaxy and friends! So why is it called the Needle Eye Galaxy? It is because one side of its disk contains a large void like an extremely big eye of a needle!
R. Wagner-Kaiser et. al suggest that a dark matter sub-halo containing a small amount of baryonic ("normal") gas could have collided with (and plunged through?) the disk of NGC 247, pushing away the gas where the sub-halo hit and leaving a "void" at the impact site. Actually, there are a lot of small old red stars in the void, but there is no star formation there whatsoever, and no young blue stars. Read about it here!
NGC 247 is sometimes described as a dwarf galaxy, but in reality it is not so small. Its diameter is 70,000 light-years, compared with about 100,000 light-years for the Milky Way. But what makes NGC 247 a small galaxy is the fact that it is low in mass. How can we know? It's because it doesn't have a bright yellow center.
The unimpressively non-bright center of NGC 247. Credit: ESA/Hubble & NASA
Every burst of star formation will create a considerable number of small red and orange stars, which are very long-lived and very faint for their mass. Or, conversely, these small stars are impressively massive for the light they produce. Even though most of the light from bright yellow centers of galaxies will come from red giant stars (like Arcturus), bright yellow centers of galaxies always contain an enormous number of small red stars, and therefore, a lot of mass. Conversely, galaxies that lack a bright yellow center are not massive - or they don't contain a lot of baryonic mass, at least!
But while NGC 247 doesn't have an impressively large population of old red and yellow stars, it does have many young blue stars and quite a lot of star formation. Is it true that small galaxies become starburst galaxies more easily than large galaxies? Yes and no!
No, because there are very many small galaxies with no star formation and no young stars:
But small galaxies that bring their own reservoir of gas (or, alternatively, blunder into a gas cloud) can be made to convert much of this gas into stars, if the galaxy and the gas receives the right "nudging". This is something that has happened to our brilliantly starforming neighboring galaxy, the Large Magellanic Cloud.
Star formation history of the LMC, according to Sidney van den Bergh.
I copied the picture of the star formation history of the Large Magellanic Cloud from a book, The Galaxies of the Local Group by Sidney van den Bergh. The book is from the year 2000, and the figure showing the star formation history of the Large Magellanic Cloud seems to suggest that the age of the Universe is well over 15 billion years, whereas the current consensus is that it is less than 14 billion years.
Nevertheless, take a look at the bars showing the amount of star formation in the LMC at various periods in time. The most recent star formation is shown at left and the oldest star formation is at right. As you can see, the LMC formed very few stars for billions of years, until it suddenly "sprang into action" some 2 billion years ago and started forming stars at a breakneck pace.
What happened? Almost certainly, the LMC met and started interacting with the Small Magellanic Cloud, stirring up its own reservoir of gas and then beginning to steal stars and gas from the Small Magellanic Cloud via the Magellanic Bridge. Watch the video below and scroll to 2.53 to see stars flowing from the SMC to the LMC:
Click to play embedded YouTube video.
As for NGC 247, it may have interacted with its larger neighbor, NGC 253, at least in the past. That may have triggered star formation in NGC 247.
These galaxies appear to be close together and to be interacting. Probably their interaction has spurred star formation, because they are all quite blue! Note that at least three of them are not so massive, because three of them lack bright yellow centers.
Last edited by Ann on Thu Sep 05, 2024 1:49 pm, edited 2 times in total.
Ann wrote: ↑Thu Sep 05, 2024 8:04 am
Very nice APOD of the Needle Eye Galaxy and friends! So why is it called the Needle Eye Galaxy? It is because one side of its disk contains a large void like an extremely big eye of a needle!
R. Wagner-Kaiser et. al suggest that a dark matter sub-halo containing a small amount of baryonic ("normal") gas could have collided with (and plunged through?) the disk of NGC 247, pushing away the gas where the sub-halo hit and leaving a "void" at the impact site.
[...]
Ann
Great - ThanX Ann!
OK, if you're interested:
A dark matter subhalo is essentially a smaller, dense clump of dark matter embedded in a larger dark matter halo. Imagine a large ball of invisible substance surrounded by many smaller balls of the same substance. These smaller balls are the subhalos.
Subhalos usually form during the formation of the structure of the universe. When the universe was very young and hot, it consisted of a dense plasma of particles. As the universe expanded and cooled, small density fluctuations formed. These density fluctuations attracted more matter through their gravity, leading to the formation of larger and larger structures. The largest of these structures are the galaxy clusters, and inside them are galaxies, which in turn are surrounded by large halos of dark matter. The smaller density fluctuations that did not become galaxies formed the subhalos. These subhalos can play an important role in the formation and evolution of galaxies. It is believed that subhalos can serve as seed nuclei to initiate the formation of stars.
...so much for the theory...
My favorite theory is a little less in the dark but rather quite clearly visible nearby (bottom left of the picture):
"The glittering specks in this image, resembling a distant flock of flying birds, are the stars that make up the dwarf galaxy ESO 540-31. Captured in this new image from the NASA/ESA Hubble Space Telescope, the dwarf galaxy lies just over 11 million light-years from Earth, in the constellation of Cetus (The Whale)." https://esahubble.org/images/potw1336a/
Ann wrote: ↑Thu Sep 05, 2024 8:04 am
Very nice APOD of the Needle Eye Galaxy and friends! So why is it called the Needle Eye Galaxy? It is because one side of its disk contains a large void like an extremely big eye of a needle!
R. Wagner-Kaiser et. al suggest that a dark matter sub-halo containing a small amount of baryonic ("normal") gas could have collided with (and plunged through?) the disk of NGC 247, pushing away the gas where the sub-halo hit and leaving a "void" at the impact site.
[...]
Ann
Great - ThanX Ann!
OK, if you're interested:
A dark matter subhalo is essentially a smaller, dense clump of dark matter embedded in a larger dark matter halo. Imagine a large ball of invisible substance surrounded by many smaller balls of the same substance. These smaller balls are the subhalos.
Subhalos usually form during the formation of the structure of the universe. When the universe was very young and hot, it consisted of a dense plasma of particles. As the universe expanded and cooled, small density fluctuations formed. These density fluctuations attracted more matter through their gravity, leading to the formation of larger and larger structures. The largest of these structures are the galaxy clusters, and inside them are galaxies, which in turn are surrounded by large halos of dark matter. The smaller density fluctuations that did not become galaxies formed the subhalos. These subhalos can play an important role in the formation and evolution of galaxies. It is believed that subhalos can serve as seed nuclei to initiate the formation of stars.
...so much for the theory...
My favorite theory is a little less in the dark but rather quite clearly visible nearby (bottom left of the picture):
"The glittering specks in this image, resembling a distant flock of flying birds, are the stars that make up the dwarf galaxy ESO 540-31. Captured in this new image from the NASA/ESA Hubble Space Telescope, the dwarf galaxy lies just over 11 million light-years from Earth, in the constellation of Cetus (The Whale)." https://esahubble.org/images/potw1336a/
Wow, Jac! I don't know what kind of history NGC 247 and ESO 540-31 have with one another, but really - ESO 540-31 looks exactly like the sort of galaxy that could have punched a hole in the disk of NGC 247 by plunging right through it! And ESO 540-31 is also at the right distance from NGC 247 to be the real culprit (unlike NGC 253, which is really too far away and too massive to do no more damage than punch a hole in the lightweight Needle Eye galaxy while interacting with it)!
NGC 247 and Friends
