random wrote:Apologies for my lack of understanding here, but where did such a large gas and dust cloud come from in the first place?
Anthony just explained how gas and dust were created in the first place. Perhaps you were asking how such a large clump of gas and dust could come into existence in this particular galaxy.
It is important to understand that some galaxies are gas-rich and others are not. If a galaxy is gas-poor, it is a virtual certainty that there will be no big local concentrations of gas and dust in it, a there will be only small amounts of star formation.
Take a look at this ESO image
of the two galaxies that are bystander's avatar, NGC 4435 and NGC 4438. These galaxies are gas-poor for two reasons. They have probably been orbiting one another for quite a long time, and they are also members of a crowded part of the Virgo cluster of galaxies. For perhaps billions of years, all their interactions with each other and other galaxies have made them have undergo one bout of star formation after another, gradually using up most of the gas they were born with, and now they have converted most of their gas into stars. In this picture
, you can see NGC 4435 and 4438 close to the center of the picture, surrounded by other galaxies.
Galaxies also lose mass to the space around them, and they don't always get this gas back. Some of it is just lost. Take a look at this picture of M82
. There are tremendous outpourings of gas from this galaxy, likely due to a great number of supernovae near the center of it, and the galaxy isn't likely to get all of this gas back. And when the gas is lost, or mostly lost, there will be no more large clumps of gas and dust in this galaxy and no more massive star formation.
Take a look at this picture of Stephan's Quintet
, an interacting group of galaxies. The largest blue galaxy is a foreground galaxy and not part of the action. You can see a large "arc" of individual pink knots near the center of the picture. These are all concentrated clumps of gas and dust undergoing tremendous star formation due to a mighty collision between two galaxies. The interaction here is pretty "fresh", and at least one of the galaxies had a lot of gas to "set fire to".
M101 , the Pinwheel galaxy, is very gas-rich. It is interacting with a few small dwarf galaxies. The interaction has spawned large-scale star formation in M101. You can see some really huge pink emission nebulae in this picture
As for galaxy M33, host of NGC 604, it interacts with the large Andromeda galaxy. Andromeda could be a real bully and "suck M33 dry" pretty fast. It hasn't happened, though, probably because M33 is located at a safe distance from Andromeda. Compare M33 with hapless dwarf galaxies M32 and NGC 205 - Andromeda has stolen all the gas that originally belonged to M32, and there isn't much gas left in NGC 205. M32 is at upper left and NGC 205 at lower right in this picture