Galaxies are, on average, so widely separated in space that it is natural to think of them as isolated entities. Yet astronomers have learned over the past two decades that galaxy morphology and evolution are strongly influenced by the galaxy's environment. Galaxies can interact and merge, they can accrete and eject gas, and they can collide to trigger bouts of star formation. It is quite a challenge to understand the details of galaxy interactions when each galaxy contains billions of stars, vast quantities of dark matter, and various amounts of dust and gas. Astronomers use the power of super computers to calculate how all these materials behave during galactic interactions.
Interacting Galaxy Pair Arp 87. Click here for original source URL
A simple way to think about galaxy evolution is in terms of stellar populations. In spiral galaxies like our own, disk stars are young, often blue and luminous, and rich in heavy elements. Halo stars are consistently old, red, typically less luminous, and poor in heavy elements. Disks rotate rapidly and halos rotate very slowly. Irregular galaxies contain almost a pure disk-like population. Elliptical galaxies contain almost purely a halo-like population.
Galaxies in the local universe reflect the consequences of stellar evolution. Imagine a population of stars that formed billions of years ago. At the beginning, the overall light would be dominated by young, hot stars. As the galaxy aged, the most massive stars would evolve off the main sequence, becoming dimmer and redder. At the same time, if dust and gas is present (as is seen in spiral and irregular galaxies) new stars will continue to form, causing a mixing of stellar colors. Astronomers construct theoretical models of stellar populations to match their observations. This approach can be used to place limits on different galaxy's star formation histories.
Observations of nearby galaxies have some interesting implications for their evolution. Elliptical galaxies have little gas or dust and they have a low (or zero) rate of current star-forming activity. However, the old stars out of which ellipticals are made will lose mass by stellar winds, replenishing the interstellar medium. Thus, gas and dust are somehow being removed from elliptical galaxies. The puzzle for spirals like the Milky Way is the converse. They are actively forming stars from gas and dust in the disk. However, the star-formation rate is so rapid that it could not have been going on for the entire lifetime of the galaxy (as measured by the oldest stars in the halo). One possibility is that star formation in the disk started relatively recently and so has not had time to exhaust the gas supply; however, there is no good evidence for this. The other possibility is that gas has been added to spirals over their lifetimes. A spiral could be rejuvenated either by gas gradually falling in from intergalactic space or by swallowing small gas-rich companions. The later scenario is the most often modeled, and it matches with our observation of dwarf galaxies regularly falling into large systems. Either way, environment plays an important role in the evolution of galaxies.
Elliptical Galaxy NGC 1132 among a field of galaxies. Click here for original source URL.