Most of the things that we know about the Universe today is primarily because of the radiation they emit, reflect or absorb. The human eyes are sensitive to the visual range (400 to 750nm) of electromagnetic spectrum. But that does not by any chance mean that the astronomical objects can emit radiations only at this range. In fact they emit at a broad range of wavelengths, starting from highly energetic X-rays till the cold long Radio waves. And each of these wavelengths bring to us unique informations about our Universe. X-rays and Gamma Rays reveals the high-energy phenomena of black holes, supernova remnant, hot gas etc while Ultra-Violet rays tells us about hot stars and quasars. Visible range shows us the warmer stars, planets, nebulae and galaxy. Through Millimeter range and InfraRed, we can look at the cool stars, dusty regions, core of our galaxy and regions of Starbirth and using Radio Radiation, it is possible to study the cold molecular clouds and Big Bang’s left over radiation.
When it comes to observing a far-away object with good resolution, the problem is that we need to have very very big telescope dishes. Bigger the dish, better the resolution. But to make such big steerable dishes to stand on ground maintaining perfect surface accuracy is not a joke! So, scientists have come up with something very smart and interesting and we call it ‘Interferometry’. The idea is to cover up a whole large area with small small telescope dishes (as smaller is easier to maintain and cheaper to consruct) and add up the feeds from all these dishes to get one high resolution final image. There are many such interferometers that are made up of small dishes which are scattered around all over the world, enabling it to have a resolution equivalent to having a big-dish telescope with a diameter as big as that of our earth.
ALMA or Atacama Large Millimeter Array is the next big Astronomy Project that the scientific world is looking forward for. It will be observing the universe at a wavelength 1000 times longer than the visible range. An international collaboration of Europe, North America, East Asia and the host country Chile – ALMA is a mm and sub-mm range ground-based telescope in the Atacama Desert at an altitude of 5000m. With 54 12m antennas and 12 7m antennas, it has a total collective area of 6500 sq meters and a maximum baseline range of 18km, hoping to get 10 to 100 times better sensitivity and 10 to 100 times better angular resolution compared to the existing facilities in this wavelength range. It will be the highest, largest and the most expensive ground-based observatory of mankind.
Though mm and submm ground based observation is vulnerable to the opacity due to water vapor resuling in a reduced transparency of the sky specially to the shorter wavelength range; but it has numerous scientic motivations. Most of the 150 molecules found in Inter-Stellar Medium emit in this range, CO transmission lines are very strong and can be used to probe distant galaxies, IR dust thermal bump of high-redshift star forming galaxies is redshifted to this range and moreover this particular wavelength range encompasses the high-frequency tail of free-free emission (can trace HII regions), high-frequency tail of synchrotron emission (tracing radio galaxies) and Rayleigh Jeans Region (traces star formation).
Last week in a University of Göttingen Seminar on Supermassive Blackholes, I presented a talk on ‘Prospects of AGN and SMBH-Growth Studies with ALMA’. The presentation is attached here –
It will give you a brief overview of some of the most important things that can be done in Extragalactic Astrophysics using ALMA, if you are interested. I will just talk about one fancy idea which will be ‘another step for mankind’ if and when materialized. And that is to have a picture of the Black Hole in the center of our galaxy. Well, it is not possible to see a black hole, of course. We know, we are bound to the ground only because of the Earth’s Gravitational Force. This gravitational force for a black hole is so ssso high that forget matter, even light can not escape from it. So, there is no possibility of observing a Black Hole in the traditional sense. But we can capture an image of it’s shadow, if we have a telescope with very high angular resolution.
The whole idea is that – light when passing nearby a massive body, gets deflected (Yes, Einstein proved it mathematically and Eddington showed this experimentally). So, light from the background, while passing through the vicinty of that massive black hole will be deflected too (or will fall into it, if the distance of it’s path is not very far from the black hole). And when we look from earth, there should be a tiny region behind that black hole which we can not see – it will be like a shadow of our Black Hole. Now, to detect that shadow, we need very very high resolution. As shown on a paper of 2000, the shadow that we can see will look something like the figure below, depending on the resolution of our telescope.
As the pic above shows, it will be possible for us to detect the shadow if we get a resolution of something like 16 micro-arcseconds. ALMA does not have so high resolution. None of the existing telescopes at this range does. But by collaborating with and combining all the mm and submm telescopes on the face of earth, scientists have calculated that we will be achieving a resolution as good as 10 micro-arcsecond (which basically is an ammmazing achievement and Black Hole Pic capture is possible with it). So, let us wait and see. May be one day soon, well, verry soon, we will have a picture of our Black Hole!!
Fancy instruments!! Fancy scientists!! Fancy Ideas!!
Ohh! I love it 🙂
PS: Just in case you wish to visit the ALMA site, there is this very nice video: