[asac] forwarded message from Tetsuo Hasegawa

[Al Wootten]

Dear ASAC Members:

Tetsuo Hasegawa sends  the following memo as a topic of discussion in the ASAC.
Unfortunately, he will be unable to participate the telecon next Monday, but
Naomasa Nakai and Satoshi Yamamoto will participate.  

The similarity of the specs of Japanese and European enhanced correlators is
clearly stated in the minutes of the meeting on ALMA future correlator plans
in Europe and Japan held right after ASAC at Berkeley (minutes are at:
http://www.cv.nrao.edu/~awootten/mmaimcal/asac/corrminutessep00.html ).

I suggest that the ASAC correlator liaison people, Bachiller and Scoville,
lead the discussion on this topic.  -Al

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SCIENTIFIC MERIT OF THE ENHANCED CORRELATOR

A memo prepared by Tetsuo Hasegawa, Naomasa Nakai, and Satoshi Yamamoto

As we heard at the Berkeley meeting in September, the enhanced correlators
proposed by Japanese and European groups now have similar specs, i.e., total
bandwidth and number of spectral channels.  The difference between them is
in their approach (FX vs. hybrid XF architecture), which may be a topic for
the correlator working group.  We propose ASAC to discuss the scientific
merit of the enhanced correlator regardless its architecture.  In our
opinion, the enhanced correlator would change our way of thinking in
designing the observation as the introduction of multi-order Echelle
spectrographs revolutionized the optical and near-infrared spectroscopy in
the recent years.

Here are a few examples in which the enhanced correlator would have a
significant advantage over the baseline correlator:

1. Observing a forest of spectral lines in regions of massive star formation
The spectral scan of the Orion KL in the 350 GHz and 650 GHz bands made at
CSO shows a forest of lines.  With ALMA, we will be able to make images in
many spectral lines simultaneously, revealing the physical, chemical, and
kinematical structure of such regions.  This is apparently a new aspect of
the "line survey" study.  To fully appreciate the large bandwidth of the
ALMA receivers in such observations, we really need a sufficient number of
spectral channels over the entire frequency coverage provided by the
enhanced correlator.  With the sensitivity and spatial resolution of ALMA,
we will encounter similar situation for basically all regions similar to
Orion KL within the Galaxy.  They are not rare.  One can expect at least one
such region per one giant molecular cloud, i.e., thousands of them in the
Galaxy.

2. Observing the continuum emission free from the emission lines in bright
star-forming regions
Submillimeter continuum emission from dust is a good probe for column
density of gas and dust.  Accurate measurements of spectral energy
distribution (SED) of the dust emission are important to estimate the dust
temperature and the properties of the dust grains.  For this purpose, a
careful rejection of the contamination due to spectral lines is necessary,
in bright star-forming regions in particular.  To do the measurement right,
we need to use a relatively high spectral resolution to find the true
continuum level in the line forest.  A combination of a large bandwidth (= a
high sensitivity) and a sufficient spectral resolution is provided by the
enhanced correlator.

3. Searching for absorption features against bright galactic nuclei
One of the important scientific areas of ALMA will be absorption study
toward bright galactic nuclei or quasars.  The relatively quiescent gas
between the continuum source and us will be sought with an unprecedented
sensitivity.  The radial velocities of the absorbing gas clouds are not
always known accurately, and wide velocity coverage with high velocity
resolution provided by the enhanced correlator would increase our chance to
detect them.


Tetsuo HASEGAWA   tetsuo at ioa.s.u-tokyo.ac.jp
Associate Professor, Institute of Astronomy, University of Tokyo
2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
Phone +81-422-34-5032 / FAX +81-422-34-5041
New URL http://www.ioa.s.u-tokyo.ac.jp/~tetsuo/
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