Steve Willner <willner@xxxxxxxxxxxxxxx> wrote:
> spope33@xxxxxxxxxxxxxx (Steve Pope) writes:
>> Are these objects -- "giant molecular clouds" -- verified to exist?
>> Is "giant molecular cloud" the accepted term for them?
>Yes, and yes. A search in ADS finds 39 refereed papers since 1999
>with the exact phrase "giant molecular cloud" in their titles. There
>are 191 with the phrase in the abstract.
>In round numbers, giant molecular clouds have masses larger than 10^5
>solar masses and peak densities of 10^5 molecules per cubic
>centimeter. The famous Orion Nebula is on the front edge of the
>OMC-1 giant molecular cloud. Another familiar example is M 17, which
>is an H II region at the eastern edge of a gmc. I had a quick look
>for images but found only:
>You can see from this page the H II region in visible light and the
>molecular cloud to the west (right) in the far infrared IRAS image.
>I just checked the Spitzer archive, and there are lots of data on
>M 17, most publically available, so I'm surprised there hasn't been a
>press release on it.
I found the statement quoted in my first posting that a GMC
passes by every 500 million years or so interesting. This
raises some immediate questions, such as do we know when in
geologic history these things passed by, and did they leave
a record -- was there altered climate, orbital changes,
more comets? It seems they are quite massive so likely
there would be an effect.