Varun Putcha (firstname.lastname@example.org) sent the following paper
Universal Reionization: And the Case of the Missing Dwarf Galaxies
Why do so many dwarf galaxies near the, Milky Way not appear?
3 extremely tiny *ghost* galaxies have been discovered by the Hubble Space telescope (HST). These dwarf galaxies all share the same birth date, 13 billion years ago (when they first started making stars). They also all abruptly stopped making stars, all in the first 1 billion years of the universe! To understand this we must first understand what a “galaxy” is.
Galaxies are massive clusters of stars. They may have up to trillions of stars.
There are 4 main types of galaxies, elliptical galaxies, spiral galaxies, starburst galaxies, and irregular galaxies. It is believed that most galaxies have a supermassive black hole in the center of them. Most galaxies are organized into groups and clusters. These in turn form Super clusters. There are over 170,000,000,000 (170 billion) galaxies in our observable universe, and possibly more in the theoretical wider universe.
Then there is a whole other subcategory of galaxies called dwarf galaxies. It is believed that most galaxies are really dwarf galaxies. Dwarf galaxies do NOT have a black hole, and compared to larger galaxies, are very tiny. Many dwarf galaxies are absorbed and ripped apart into bigger galaxies. And almost all normal galaxies have hundreds of dwarf galaxies orbiting them. Imagine how many of them dwarf galaxies are! Now that you know what galaxies and dwarf galaxies are, let’s get back to the question, why do many extremely ancient, pristine “ghost” galaxies have so few stars?
2 main theories have emerged as the most likely explanation to this intriguing phenomenon. One suggests that it happened during the second age of reionization, while the other suggests some other causes, mainly a supernova. These ideas both intrigued me, and are very interesting.
“These galaxies are all ancient and they’re all the same age, so you know something came down like a guillotine and turned off the star formation at the same time in these galaxies,” said Tom Brown of the Space Telescope Science Institute in Baltimore, Md., the study’s leader. “The most likely explanation is reionization.”
This all happened in the second stage of reionization. Reionization began in the first 1 billion years after the big bang. During this stage of the universe, radiation from the first stars knocked electron off primeval hydrogen atoms. This caused them to become transparent to UV radiation. Many big galaxies were massive enough to shield themselves from the deadly radiation, causing star production to continue. The same radiation that sparked reionization seems to have squelched the star making capabilities in numerous dwarf galaxies, including Leo IV, one of the 3 galaxies found by the HST. (Next Page A Leo IV comparison)
(See Leo IV Image as a separate post)
As you can see in the image, f you look at the section where Leo IV is, it barely has any stars once you remove the background galaxies! (100- 1000) A typical dwarf galaxy would have300-1000x that amount! These are the effects of reionization.
Dwarf galaxies like Leo IV did not have enough mass to stop the dreaded UV radiation. Their remaining gas was stripped away, and star making stopped. In this particular situation, the dark matter outnumbers the matter by a factor of 100! In normal dwarf galaxies, this ratio is 10 to 1. This all suggests that the dark matter was a byproduct of whatever happened.
If this is all true than it must a have been a deadly time. Now onto theory 2!
Some people believe that a massive internal supernova blew out any gas these dwarf galaxies once had. This theory is obviously supported by the fact that all the stars are ancient. Although this theory does not have backing from the HST like theory 1, it has some proof. Some stellar gas remains have been fund hovering around the location of the galaxies. Although most people believe these are recent, some believe they are ancient. If so, why did they not create stars? That is a flaw in the theory. Most astronomers support Theory 1 though.
Agreements between theories
Both theories agree though, that this may explain why we only see 12-20 dwarf galaxies around the Milky Way, even though computer simulations say there should be thousands. Reionization and supernovae are/were deadly forces that ripped through the universe; they didn’t have the mass to stop it. This was a dramatic era of our universe with many things happening, this may be some of the first casualties we may discover, and is probably not the only type too.
This is a key component of understanding the early universe, and astronomers are frantically looking for a reason that they didn’t survive. Maybe it is because they didn’t form black holes like other major galaxies, or maybe they were hit with a giant explosion… We may not know, until telescope many times stronger than the HST peer back into time and reveal the secrets of our past.