[mmaimcal] Re: CO and the MMA

[Al Wootten]

Lorne Avery writes:
 > Hi Al;
 > 
 > I'd be interested in your reaction to a statement attribute to Leo Blitz
 > to the effect that the "SKA will be 5 times more sensitive to CO at z>3.6
 > than the MMA/LSA".  This is in connection with the SKA science case that is
 > being prepared for the upcoming US decadal review and is likely to be 
 > promoted as a strong selling point for the SKA.
Well, I don't know what is the basis for his claim.  Let us recall that the
1-0 line has not been successfully observed in any high redshift galaxy.
The science which has come out of the 1-0 line has not been cosmological
science, really.  I don't think that there is much reason to expect SKA to
contribute very much, though it may out armwrestle the MMA for 1-0
(I haven't tried to calculate this as SKA is so chimerical I have no idea what
parameters to use).
 > 
 > As you may know, there is strong interest in Canada in joining the MMA/LSA
 > and I have been involved in trying to estimate the sensitivity of
 > the MMA/LSA to high-z CO in star-bursting galaxies.  On the basis of my
 > numbers, I am surprised by Leo's assertion.  I'm pretty sure that it's not
 > generally true.  Possibly for low-density (n~ 10**2) gas (z>4) where only CO 1-0
 > is thermally excited, this is true, but in general?
We are going through just such calculations.  At the moment, they aren't
quite ready for public release, but take a look at the WWW page
http://www.cv.nrao.edu/~awootten/mmaimcal/zlines.html
to see what various folks at NRAO have been doing.  I suspect that Leo is
considering CO as seen locally in evolved galaxies.  But if we take 
evolution into account (I think only Silk and Spaans have done this, Sept
97 ApJL) I think we have to expect that the power will be in the higher
J lines.  The background radiation will be stronger by a fair amount for
one thing.  But lacking such a calculation including evolution, Min and
Radford have calculated that the MW itself--unevolved--should be observable
with the MMA/LSA out to z>3, and perhaps much greater than 3 (if we take a
favorable cosmology).  The SKA folks have a picture which shows M101 at
z of several, undetectable by the MMA while easily seen by SKA; M101 isn't
a particularly bright CO galaxy I understand, so Min is doing a similar
thing with M51 and the MW.  I am forwarding your email to our discussion
group and I'll forward responses to you.  I'd be equally interested in your
calculations.
 > 
 > Have you thought about this? Do you have any quantitative numbers that might
 > refute this claim?  Or am I mistaken in my estimates?
 > 
 > I'm not interested in getting into a slugging match over this, but I think it's
 > important that the SKA claims be correct.
Even so, they are left with a single CO line until z=8.22 when 2-1 shifts
down to 25 GHz (I thought SKA stopped at 20 GHz), so I don't think that a
SKA result on 1-0 CO will be a particularly physically interesting one.
What will be interesting will be the comparison of CO in various transitions
with those of water, [C I], [C II], [N II] and the other important cooling
lines which SKA will never approach.  SKA just isn't the instrument of choice
for the investigation of molecular gas at high redshift, even if it can measure
one puny line.

We'll continue this discussion...

Clear skies, (which they are finally becoming here on Kitt Peak)
Al
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| Alwyn  Wootten   (http://www.cv.nrao.edu/~awootten/)	 |
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