April 20, 2017 12:31:11 am
Using data from an array of powerful radio telescopes in the Chilean desert, an international team of astronomers has directly observed a pair of distant Milky Way-like galaxies, seen when the universe was very young — only about 8% of its current age. Among the astrophysicists who reported the findings in Science, the peer-reviewed academic journal of the American Association for the Advancement of Science, was Nissim Kanekar of National Centre for Radio Astrophysics (NCRA), Tata Institute of Fundamental Research, Pune University. The paper, [C ii] 158-µm emission from the host galaxies of damped Lyman-alpha systems, by Marcel Neeleman, Nissim Kanekar, J Xavier Prochaska, Marc Rafelski, Chris L Carilli and Arthur M Wolfe, was published on March 24. Dr Kanekar spoke to ANJALI MARAR about the discovery and its significance.
What are the key findings of your study?
Our ALMA (Atacama Large Millimetre/submillimetre Array, an astronomical interferometer of radio telescopes in the Atacama Desert of northern Chile) results have provided new insights on how normal spiral galaxies, like the Milky Way, looked in their very nascent stages, 12 billion years ago. In one of the systems, we found clear evidence of a rotating disk galaxy, similar to the Milky Way. The exciting thing is that the two newly discovered galaxies appear to be embedded in a large extended halo of neutral gas, which was suspected in the past.
What are these two galaxies like?
They are massive, dusty systems with high star formation rates, and are located quite far from the quasar line of sight. This is very different from the standard picture of such absorbers, where absorbing galaxies are small systems, located close to the quasar line of sight, and with low star formation rates. We now have deep optical images of these two galaxies with the Keck Telescope in Hawaii. These data find very little optical emission, confirming that the galaxies are very dusty, with the dust blocking our view of the starlight.
How potent are these two galaxies in giving birth to new stars?
We had expected low star formation rates from the absorbing galaxies, not more than a few solar masses per year. However, our detection of dust emission at exactly the same position as the ionized carbon emission shows directly that the star formation rates in these galaxies is very high. This is because dust emission arises from dust that’s heated by ultraviolet photons from young massive stars. A large amount of dust emission hence implies a large number of young stars, that is, a high star formation rate. Our ALMA data reveals star formation rates of about 120 solar masses per year in one galaxy and roughly 25 solar masses per year in the other. So these galaxies are extremely potent in forming stars, about 20 to 100 times more potent than the Milky Way.
Should we conclude that there is a Milky Way like galaxy taking birth?
One of the galaxies is certainly a Milky Way-like galaxy, seen very early in its life, 12 billion years ago. We are not sure about the second system right now, and are planning more ALMA observations in the next cycle to try to determine whether it too is a rotating disk. There is also excitement about recently obtained ALMA data on a few more of these absorbers, as we have already found ionised carbon emission in 3 more galaxies. With these and more data that’s coming from ALMA, we should learn much more about the processes that give rise to galaxies like the Milky Way over the next year or so.
How significant is the ionised carbon around the discs of the two galaxies?
The ionised carbon spectral line at 157.74 microns is the most important cooling line in galaxies like the Milky Way. Ionised carbon emission has never been detected in such absorbing galaxies until now, perhaps mostly due to the low sensitivity of earlier millimetre-wave radio telescopes. ALMA’s outstanding sensitivity completely changed the game and a search for ionised carbon was an obvious thing for us to try out. But we expected that any detected carbon emission would come from right on top of the quasar. What we found instead was that the ionised carbon line in both galaxies arises from very far from the line of sight to the background quasar. Since each galaxy also produces strong absorption towards its background quasar, both galaxies must be very large, massive systems. Additionally, the carbon line also helped us to obtain the velocity field of the galaxies, thereby confirming that one of the galaxies clearly has a rotating disk.
Can you explain how ALMA has helped gain crucial insights?
The Atacama Large Millimetre/submillimetre Array is a radio interferomer made up of multiple dishes, very similar to NCRA’s Giant Metrewave Radio Telescope (GMRT). The main difference between the two is that the GMRT operates at low frequencies, <= 1.4 GHz, while ALMA operates at very high frequencies, >= 90 GHz. The two telescopes provide complementary views of the universe. ALMA is special because there has never been a telescope at millimetre and sub-millimetre frequencies with this kind of sensitivity. ALMA consists of 50 parabolic dishes, each of 12-metre diameter (GMRT, by contrast, has 30 dishes of 45-m diameter), at a spectacular location, at an altitude of 5,100 m in the Atacama Desert. ALMA partner countries include the USA, Chile, Canada, Japan, and many countries in Europe. Unfortunately, India is not an ALMA partner country, which makes it very difficult for Indian astronomers to obtain observing time on ALMA.
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