In general, galaxies can be thought of as sociable, hanging out in groups and frequently interacting. However, this recent NASA/ESA Hubble Space Telescope image highlights how some galaxies appear to be hungry loners.
This image from the Advanced Camera for Surveys of the NASA/ESA Hubble Space Telescope highlights the large and bright elliptical galaxy called ESO 306-17 in the southern sky. It appears that ESO 306-17 is surrounded by other galaxies but the bright galaxies at bottom left are thought to be in the foreground, not at the same distance in the sky. In reality, ESO 306-17 lies fairly abandoned in an enormous sea of dark matter and hot gas. Researchers are also using this image to search for nearby ultra-compact dwarf galaxies. These are mini versions of dwarf galaxies that have been left with only their core after interaction with larger, more powerful galaxies. Most ultra-compact dwarfs discovered to date are near giant elliptical galaxies in large clusters of galaxies, so it will be interesting to see if researchers find similar objects in fossil groups. Credits: NASA, ESA & M. West (ESO)
These cosmic oddities have set astronomers onto ‘the case of the missing neighbouring galaxies’. Located half a billion light-years from Earth, ESO 306-17 is a large, bright elliptical galaxy in the southern sky of a type known as a fossil group. Astronomers use this term to emphasise the isolated nature of these galaxies. However, are they like fossils – the last remnants of a once-active community – or is it more sinister than that? Did ESO 306-17 gobble up its next-door neighbours?
Gravity brings galaxies together and bigger ones swallow smaller ones. There is evidence that our own Milky Way galaxy has ‘snacked’ on numerous smaller galaxies that strayed too close. ESO 306-17 and other fossil groups may be the most extreme examples of galaxy cannibalism, ravenous systems that don’t stop until they’ve devoured all of their neighbours.
In this image, taken by Hubble’s Advanced Camera for Surveys, it appears that ESO 306-17 is surrounded by other galaxies, but the bright galaxies at bottom left are probably in the foreground, not at the same distance in the sky. In reality, ESO 306-17 lies fairly abandoned in an enormous sea of hot gas according to studies conducted by both ESA’s XMM-Newton mission and NASA’s Chandra X-ray Observatory. It is also thought to be surrounded by an even larger amount of mysterious dark matter, although no one has directly detected this yet.
When zooming in closely on ESO 306-17, faint clusters of stars can be seen through the bright shine of the galaxy’s large halo. These are globular clusters: tightly bound groups of stars that can often fend off cannibalism from larger, bullying galaxies. Studying these surrounding clusters will prove helpful to astronomers in their pursuit to put the pieces of ESO 306-17’s history together.
Researchers are also using this image to search for nearby ultra-compact dwarf galaxies. These are mini versions of dwarf galaxies that have been left with only their core after interaction with larger, more powerful galaxies. Most ultra-compact dwarfs discovered to date are near giant elliptical galaxies in large clusters of galaxies, so it will be interesting to see if researchers find similar objects in fossil groups.
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Filed under: Deep Space
Neutron stars may go completely strange thanks to dark matter
The cosmic background radiation was released about 400,000 years after the Big Bang when the first atoms were beginning to form. Scientists believe that the “clumpiness” of the background radiation is actually the cosmic blueprint from which today’s clusters of galaxies formed under the influence of gravity – a relic map of the Universe from the beginning of time.
I was never lucky enough to see or hear Allan Sandage in person, so here are two quotes about Sandage from Dennis Overbye's Lonely Hearts of the Cosmos.
Tammann usually stayed down in the control room while Sandage submitted himself to the spartan marathon of the prime focus cage. Sometimes, when the night had gotten long, Tammann would stand in front of the microphone and pour water back and forth between a pair of glasses loudly enough so that it could be heard up in the cage. “Hey, Allan, do you have to go the bathroom?”
(Pages 166-167, about a prank Gustav Tammann would play on Sandage.)
In 1967 Sandage took a trip to the University of Texas to give a talk about cosmology. Before he could speak, a young woman, a graduate student, stood up and told the audience that everything they were about to hear was wrong. Sandage was stunned and outraged – an outrage he was never to forget. “It was typical of him to recall with exaggeration,” said Tammann. “She was a graduate student. Allan was already Allan.”
(Page 175, on Beatrice Tinsley's public criticism of Allan Sandage's use of giant elliptical galaxies as standard candles.)
There are about 130 billion galaxies. If we use the smallest and largest statements shown below of the % that are spiral galaxies, we would get from 65 billion to 100 billion spiral galaxies. There is a short list at the end, of the ones I found in addition to the Milky Way. Others have said there are 400 billion other galaxies. I didn't find that number anywhere, but I guess it just goes to show that the various Astronomers and sites don't agree on the numbers. If we said 50 – 77% of 400 billion, we would have 200 – 308 billion spiral galaxies.
More than half of all observed galaxies are spiral galaxies.
About 77% of the observed galaxies in the universe are spiral galaxies. Our own galaxy, the Milky Way, is a typical spiral galaxy. Together with irregulars, spiral galaxies make up approximately 70% of galaxies in the local Universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters.
“Spiral nebula†is an old term for a spiral galaxy. Until the early 20th century, most astronomers believed that objects like the Whirlpool Galaxy were just one more form of nebula that were within our own Milky Way galaxy. The idea that they might instead be other galaxies, independent of the Milky Way, was the subject of The Great Debate of 1920, between Heber Curtis and Harvard-based Harlow Shapley. In 1926, Edwin Hubble observed Cepheid variables in several spiral nebulae, including the Andromeda Galaxy, proving that they are, in fact, entire galaxies outside our own. The term “spiral nebula†has since fallen into disuse.
Some irregular galaxies are small spiral galaxies that are being distorted by the gravity of a larger neighbour.
The Magellanic Cloud galaxies were once classified as irregular galaxies, but have since been found to contain barred spiral structures, and have been since re-classified as "SBm", a fourth type of barred spiral galaxy.
Triangulum
Whirlpool
Andromeda
Sunflower
M100, which is in the Virgo cluster
Pinwheel Galaxy – a class Sc spiral galaxy M101 (NGC 5457)
M83
ESO 269-57
NGC 1313: a starburst galaxy or barred spiral galaxy
NGC 3559, a barred spiral
These numbers sound like humongous amounts of mass until one remembers that a single galaxy consisting of ordinary stars, gas and dark matter like ours weighs in at between 600 billion and a trillion soilar masses, which means that the total mass of our galaxy is between 150,000 and 250,000 greater than the ‘paltry’ 4 million-solar mass SMB at the core of our galaxy. The total mass of giant elliptical galaxies like M87 (that would include the dark matter) could be up to 200 times higher than our galaxy, which would be up to 30,000 times greater than even its 6.6 billion solar mass SMB. These are of course only rough estimates, but they probably reflect a reasonable upper bound in the reality.