Quote:
Originally Posted by anonymouse11
1. Yes that happens when people misquote each other, but the source, Cardiff Uni press release on their website is saying the same thing.
2. Where in the article does it say "The peak bombardment lags the crossing by some 2 million years. " ? If thats the case that might clear everything.
|
I don't think the Cardiff paper says that the peak bombardment lags the crossing. That is written in the other paper, here:
Quote:
|
Originally Posted by ucs.Louisiana.edu
... The peak flux times lag the Galactic plane crossing times by ~ 2 Myr and are not precisely periodic because of decreasing Galactic density as the Sun recedes from the Galactic core. The phase of the oscillations is restricted by observations which place the last previous plane crossing at ~ 1.5 Myr in the past (and the next flux peak ~ 1 Myr in the future).
|
“Variable Oort Cloud Flux Due to the Galactic Tide”
The Cardiff paper is a computer model. It is not a survey. So it produced no data which would tell us where the solar system is in relation to the galaxy. It didn't look at stars and whatnot to figure that out. It, on the other hand, modeled what should happen as we pass through the plane of the galaxy. The only two lines in the Cardiff paper I found relating to the current location of the solar system relative to the galactic plane are these two:
Quote:
|
A weak periodicity of ∼36 Myr in the cratering record is consistent with the Sun’s recent passage through the Galactic plane
|
Quote:
|
Both our position relative to the Galactic plane (Joshi 2007 and references therein) and the impact cratering record indicate that we are presently in, or very close to, the peak of an impact episode.
|
Notice the Cardiff paper gets its information on the present position of the solar system from
Joshi 2007. Understand, it is necessary for them to use another source for this data because they did not, themselves, perform a study that would give them this data. Joshi 2007 found the following:
Quote:
|
We have carried out a comparative statistical study for the displacement of the Sun from the Galactic plane (Z⊙) following three different methods... We found Z⊙ varies in a range of ~ 13 - 20 pc from the analysis is of YOCs...
|
This agrees with the many, many sources in this thread which place the solar system at about 20 pc above the galactic plane.
The reason I give all these quotes is to impress upon you that the news story which made this statement:
is most definitely in error. If the paper says "Sun’s recent passage through the Galactic plane" and the article says "Solar System will be passing through the galactic plane in the near future" then one must be wrong. The statements are mutually exclusive. They can't both be right, and since the article is reporting what the paper found it is clearly the article which is mistaken.
That said, however—the tidal forces of the galactic plane are not an instantaneously significant thing. Throughout the spiral arms of the Milky Way there are molecular clouds of high density. The solar system may approach one before or after crossing the galactic plane. While the plane itself can be said to be an exact 2D plane bisecting the galaxy, the tidal effects are not so well defined.
We are very near the plane at the moment. We are close enough that the
galactic coordinate system uses the sun as the z axes zero. Essentially: the galactic coordinate system places the sun at the vertical center of the galaxy. It is well-known that this is not exactly correct, but we are close enough for that to be a good approximation.
So, for the purposes of galactic tidal forces it is good-enough to consider us "near" the plane as the Cardiff paper does. But, at the same time, the paper recognizes explicitly that we have already crossed that line "recently".
Quote:
Originally Posted by anonymouse11
Why would it lag by 2 million years ? when the whole thing is based on the effects of intense gravity of the very thin galactic plane ?
|
I'm not sure why they claim the peak flux lags the crossing. You could read the paper and see if they say why. I do know that two-thirds of the flux is reported to happen over 15 million years (you'll see at the bottom of page 8). Saying that we are in a period of bombardment would mean only that we are within a few million years of crossing the plane which of course we are.
It should be said that these papers reporting a link between impacts and plane crossings are somewhat hypothetical while our position in the galaxy is much more rigorously known. There are many, many surveys (linked throughout this thread) which show that we are several parsecs above the galaxy's plane—that we crossed it more than 1 million years ago. These two papers which deal with impacts cannot challenge our position in the galaxy because they make no observations of stars. They did not do an astronomical survey which would show our position in the galaxy.
In short, we can have already crossed the plane yet be in a period of bombardment because the plane crossing is a kind of virtual instantaneous thing while the period of bombardment persists for millions of years.
~modest
