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Is this (Nature 469, 504-407 (2011)) graph wrong?



 
 
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  #1  
Old November 27th 16, 02:58 AM posted to sci.astro.research
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Default Is this (Nature 469, 504-407 (2011)) graph wrong?

Hello, Can anyone help me understand this graph? The graph appears
to be incorrect to me, what am I missing?

http://www.nature.com/nature/journal...e09717_F4.html

The Y axis on left and right have two different sets of units. On
the left, there is log_10 (M_sun / (yr * Mpc^3))

On the right axis, there is: log_10 (erg / (s * Mpc^3))

Seconds and years are both just units of time, so those plots are
different by a constant value of seconds per year. Since everything
is all the same, I should be able to cancel out all of the units...
Log_10 and time and volume, so that the plot is asserting that star
formation *rate* within a region and at some time is the same as
the value of ergs within the region of the universe at a specific
time considered.

I understand that as the number of stars goes up, the value for
ergs ought to go up. But if the star formation rate went to zero,
there would still be all of the previously formed stars, so, the
erg value would not go to zero just because the universe stopped
forming stars.

So, it doesn't seem right to me that these two Y axis' can be placed
on this graph????? Isn't the left Y axis a completely different
thing from the right Y axis?

What am I missing.

rt
  #2  
Old November 28th 16, 09:27 PM posted to sci.astro.research
Craig Markwardt[_2_]
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Posts: 137
Default Is this (Nature 469, 504-407 (2011)) graph wrong?

On Saturday, November 26, 2016 at 8:58:26 PM UTC-5, wro=
te:=20
The Y axis on left and right have two different sets of units. On
the left, there is log_10 (M_sun / (yr * Mpc^3))
=20
On the right axis, there is: log_10 (erg / (s * Mpc^3))

....
I understand that as the number of stars goes up, the value for
ergs ought to go up. But if the star formation rate went to zero,
there would still be all of the previously formed stars, so, the
erg value would not go to zero just because the universe stopped
forming stars.


Stellar UV emission is dominated by O an B type stars, whose lifetime
is short (10s to few 100s million years) compared to galactic
evolutionary timescales. So in effect, no, in UV one do not see
"all" of the previously formed stars, just the very recently formed,
short-lived, massive stars.

Check out this review for more info
https://ned.ipac.caltech.edu/level5/..._contents.html

CM
 




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