By Mayank Chhaya

Chicago, Feb 14 (IANS) US-based Indian origin astrophysicist Sukanya Charkrabarti has found her calling 260,000 light years away in a galaxy that can barely, if at all, be seen. Galaxy X, at a distance which is modest on the cosmic scale, is where she is perfecting her theoretical skills.

Dealing with structures of the Milky Way that seem to exist more by their gravitational signature than by actual observational fact, Chakrabarti, of the University of California, Berkeley, has perfected a technique that could give scientists some key answers to deeply troubling questions. The most baffling of them relates to dark matter, which forms over 80 percent of the universe.

Dark matter is a mysterious matter that cannot be seen because it does not interact with light. Its presence is inferred by the gravitational forces it exerts on matter that can be seen.

When an object that is hundreds of thousands of light years away from the earth and happens to be dimly lit the way Galaxy X is, scientists do not have much choice except to look for what they call gravitational perturbations. Chakrabarti deployed a method to predict Galaxy X that is similar to the way Neptune was discovered 160 years ago. Then Neptune was called Planet X, and hence the name Galaxy X for now.

At the heart of Chakrabarti's work as a theoretical physicist is the conundrum arising from a major gap between what theory predicts should be the number of galaxies, known as satellite galaxies around the Milky Way, and what is actually observed. What can be seen is way less than what theory predicts.

"One of the current outstanding problems in cosmology is there's this missing satellites problem," Chakrabarti, who came to the US as a 10-year-old, told IANS in an interview here.

According to her, the current dark matter theory "is very successful at recovering the large-scale distribution of galaxies, but when you look on sub-galactic scales, it far overpredicts the number of dwarf galaxies relative to what we actually observe".

To get around the problem, Chakrabarti and colleagues developed a way to look for these dim dwarf satellite galaxies, which consist largely of dark matter. Such dark galaxies, although not visible, do cause gravitational perturbations in the huge clouds of hydrogen that are found at the edge of our Milky Way.

Asked if her work would help advance the understanding of dark matter, she said, "Yes, this is a new method that allows one to quantitatively characterise dark matter-dominated dwarf galaxies from an analysis of their tidal gravitational effects on the gas disk of the primary galaxy. Since dark matter is not directly visible, and many dwarf galaxies are very dim, it's useful to have a method that does not rely on the optical light from these systems."

"The prediction is made by analysing the gravitational perturbations. Since we expect that Galaxy X is close to the plane of the Milky Way, where there is a lot of obscuring dust and gas, we are planning to look for it in the infrared -- where it is easier to see through the dust than in the optical," Chakrabarti.

Growing up in a family of academics from Kolkata, Chakrabarti said she was always interested in studying "complex structures" and so she did consider neuroscience as a possible career option. But somewhere along she realised that she was more drawn to something way outside.

It was a tough career choice because in astrophysics women are an exception rather than a rule. "Even now, after things have improved over the past three decades, there are less than 10 percent women in astrophysics. It is not a subject that women take to for many reasons."

Chakrabarti says there has been gender-based discrimination in astrophysics which often demands an aggressive approach by those who practise it.

The growing presence of women, she said, has begun to make many in the discipline recognize that there is another way of doing science. "Something that is not aggressive but is collaborative," she said.

A polyglot, Chakrabarti speaks French, Bengali, Hindi and Sanskrit apart from English. Making light of her claim about Sanskrit, she said, "That is not entirely accurate. I was introduced to the language as a child by my parents and I can read it but that's about all."

Her findings about Galaxy X were presented recently at a meeting of the American Astronomical Society. Of course, the finding still needs to be supported by actual observational evidence.

"I am a theorist myself. I try to make very specific predictions so that observers can go out and test those predictions. If we're right, then it's fantastic. Even if we're wrong, we stand to learn something new. For instance, if we don't find Galaxy X, it may be that the model I've assumed of the Milky Way is too simple. Either way, we stand to learn something very important. If we make predictions that are not very specific, you can't really learn something new. You have to take a chance and go out on a limb," she said.