What is Millimeter Astronomy?
There are regions in space in between stars that are so shrouded
in secrecy that no optical radiation ever escapes from their interiors! We are,
for a change, not talking about black-holes, but regions of space filled with
gas clouds, similar to the terrestrial ones in their ill-defined outlines, that
harbour some of the most exotic molecules - from vast amounts of highly active
radicals such as OH and SO and ions such as HCO+ and CH+ to such long and complex
molecules as HC13N and cyclic C3H2. Even formic acid and poly-aromatic hydrocarbons
are found in abundance in many regions. Naturally, one wonders how do all these
form? How do they survive in such tenuous regions where the radiation from the
energetic stars roam freely and in abundance making the environment harsh and
difficult for these molecules to survive? Moreover, could they have been in
a deeper way connected to the origin of life on planets like earth? Coming to
that all important question, are there extra-solar planets with life-forms?
What kind - are they intelligent? More than us?...
The story of molecules in space is also intimately connected
to what happened in the early Universe! Early on, according to the most popular
theory, the universe was filled with only lighter elements, predominantly
hydrogen but He and D were there as well. However, all heavier elements such
as C, N, O, S, Si, Na, K, Mg, Cl, Fe ... must have formed later: they must
have been cooked inside the furnaces at the cores of stars. Then, when did
the stars form? For without them there is no question of life (as we know
it) and, certainly, these fancy molecules! We do know that there are stars
and they keep forming but when did it all start? Tracing molecules back in
time and farther away from us will surely take us closer to the epoch of formation
of first generation stars and help us to understand its relation to the epoch
of Galaxy formation and evolution.
Spectral lines are the signatures of these molecules; they
are helpful to uniquely identify the molecules and to learn about the physical
conditions in the regions they exist. Most molecules have their rotational
transitions in the mm-wave band which get easily excited even in such cold
and tenuous regions they exist. Thus, mm-wave band is best suited for tracing
molecular material in the Universe, both nearby and far away!
Well, mm-wave band is important even if one discounts all
this line emission! The relic radiation of the Big-Bang called the Cosmic
Microwave Back-ground Radiation (CMBR) has a spectrum peaking in the mm-wave
band at the present epoch! Measuring its spectrum in the mm-wave band is in
itself an important achievement that confirmed this relic-radiation. But,
the mm-wave spectrum of this relic-radiation carries with it much more information
about the distant Universe. Galaxy clusters have 10^7 K hot intra-cluster
gas associated with them (in fact, the mass in the intra-cluster hot gas is
several times that of the total mass in the Galaxies in the cluster!); the
Cosmic microwave back-ground photons interact with them and gain energy through
inverse compton scattering. This introduces a distortion in the CMBR spectrum
which is referred to as Sunyaev- Zeldovich effect. The amount of distortion
depends on the size, temperature, and density of the hot cluster gas and is
independent of the red-shift of the cluster! Thus, detecting SZ effect in
clusters at high red-shifts is a good way to confirm the relic nature of CMBR.
The hot gas emits X-rays by thermal Bremstraalung: owing to sensitivity limits,
it is difficult to detect Xrays from clusters at high red shifts and the mm-wave
SZ seems the best way to detect them. But, one could measure the X-ray flux
from the nearby ones and measure the temperature and density. Then, measuring
the CMB distortion in the mm-wave spectrum one can estimate the size
of the gas-emitting region. This size along with the apparent angular extent
and brightness helps to estimate the distance to the cluster independently.
Then measuring the velocity to the cluster one can estimate the Hubble constant!
In such studies, measuring the SZ effect towards a large number of clusters
is important and accurte mm-wave continuum measurements is the key here.
Another aspect of measuring mm-wave spectrum of CMBR is studying
its anisotropy. The inhomogeneity in matter distribution at the last scattering
surface, which would eventually evolve into Galaxies, should have left their
imprints on the CMBR spectrum. Therefore, by measuring the anisotropy at various
scales one can constrain the epoch of Galaxy formation for various Galactic
evolutionary models. There are other objects in the Galaxy such as protostars
as well whose mm-wave spectral measurements are important.>
So, gear up to observe with Mopra and get introduced to the nitty-gritties
of observing at mm-waves. Prepare yourself to probe the Universe with the future
instruments of ever increasing power such as ATCA, SMA, ALMA ...
-- Ramesh Balasubramanyam
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