PolarS, I missed seeing your question earlier and just now realized it was there!Originally posted by polarstarus
Is there a reason why t-halo and t-disk seem to coincide with inflection points?
I dont see any connection but that's no proof there isnt one!
(I dont even see them occurring at inflection points! still have to look more at this figure and study the caption I guess)
have to go out now but will look later and see if there's
I'm back after a night's sleep and can edit in some comments on
the t-halo and t-disk stripes.
They are not essential to the Figure, but something Lineweaver added to show "the tightening network of constraints" as it says in the caption beneath the figure.
The point is that this curve---most likely the heavy solid curve (0.27, 0.73) is the right one----is what relates time in years to redshift.
there is a strict invariable connection between "size of universe" and redshift---they are the same scale with different labels so you see them in the figure drawn on the LHS and the RHS of the figure as alternative y-axis scales.
So the curve relates time in years to the size of universe and therefore to redshift.
But within our own galaxy we can measure ages of stars by their spectra! Stars that formed early have less metal, and less different colors. So we can tell how long ago, in years, the disk formed and the halo formed. Disk formed 9 billion and halo formed 12 billion years ago, say.
Now let us assume that all galaxies formed at about the same epoch.
So we can TEST our curve to see if it is reasonable!
The curve predicts what redshifts correspond to 9 billion and 12 billion. It says about z =1 and about z = 4, say. So to perform the test, we look back at galaxies with z = 1 and see if they are forming their disks.
And we look back at galaxies with z = 4 and see if they are forming their halos.
The picture shows a good fit for the heavy solid curve. If you put your finger at z = 1 on the redshift scale and move horizontally until you reach the curve, you will hit the curve at right about over the 9 billion years ago spot. Likewise for z = 4, you hit the curve right about the 12 billion years ago. So observing galaxies at z = 4 in haloformation stage, which I guess they do, tends to confirm that curve.
No big deal but reassuring