Is the Milky Way a collection of merged galaxies, or has it only been one galaxy that was created from the interstellar media?
1$\begingroup$ Neither, but both! $\endgroup$– WalterSep 7, 2014 at 19:45
$\begingroup$ @Walter - I sense an answer coming. . . ? $\endgroup$– HDE 226868 ♦Sep 10, 2014 at 0:04
$\begingroup$ @HDE226868 I don't think you should hold your breath. $\endgroup$– LDC3Sep 10, 2014 at 0:33
The subject of galaxy (including the Milky Way) formation is still hotly debated by the professionals. But is is quite clear that the process is a hierchical mess. The standard picture involves cold dark matter (CDM), which first forms structures (sheets, filaments, haloes) into which the baryons fall. Since the baryons can dissipate and cool by radiation), some of them accumulate at the centre of the haloes, where they form a rotating disc (since angular momentum cannot be radiated away). Stars form (roughly) whenever the gas density is high enough.
This all is happening on various scales. First, smaller galaxies form, but mergers quickly lead to the formation of larger ones. Most stars in galactic discs actually formed in situ (rather than in a progenitor galaxies). Accreted stars (from merged progenitors) are typically not in a disc-structure, but form a spheroidal structure (stellar halo, bulge).
A major merger (mass ratios 1:3 to 1:1) destroys the stellar discs and leads to the formation of an elliptial galaxy. The Milky Way galaxy cannot have had a merger with mass ratio 1:1 to 1:4 in the last few Gyr, and even a 1:10 merger should have left some detectable traces.
Great link in your answer, @LCD3. In addition, I'm going to cover some things about the continuing evolution of the Milky Way:
The Milky Way is currently merging with one or more of its satellite galaxies (I say "one or more" because several objects in its vicinity, such as the Virgo Stellar Stream, may or may not be galaxies). The major merger is with the Sagittarius Dwarf Galaxy, which undergoing a 100 million year+ merger with the Milky Way. It sill eventually be completely torn apart, and become part of the Milky Way.
There is also evidence that this has happened in the past (see this paper) and it could happen in the future. The Milky Way has a lot of satellite galaxies, and even though they are far away, closer satellite galaxies could have been consumed in the past. The eventual merger with Andromeda will also add to the Milky Way's growth, as the two become a new galaxy.
List of Milky Way satellite galaxies
Sagittarius Dwarf Spheroidal Galaxy
Note: I am looking for more non-Wikipedia sources, and I will update this answer as soon as I can.
If we look at M31 (the Andromeda galaxy) and consider that it has most likely had a similar history to our own (similar size, age and make-up) then I would say that it is a galaxy which initially formed by itself and later collected other galaxies. M31 has a distinct distortion on its inner region, like a kink. This is quite possibly the remnant of an ealier interaction with another galaxy. Some evidence has been proposed that this interaction was with the Milky Way, although this remains unverified.
This simulation probably gives the best answer.
The simulation is a 2 1/2 min video of the creation of a large galaxy from interstellar dust by collecting smaller galaxies. The time span is probably in hundreds of millions of years.
$\begingroup$ Could you summarize it here? If the video is ever removed your answer becomes useless. $\endgroup$– stevenvhSep 17, 2014 at 15:31