Colour image of merging galaxy

A team of scientists from different research labs calculated the age of stars. The merging galaxy has stars as old as 400 million years. The team scientists was led by Koshy George from the Indian Institute of Astrophysics

CHENNAI: Astrosat, India’s first dedicated multi-wavelength space observatory, has captured an image of a merging galaxy with stars as old as 400 million years for the second time - this time in colour! The galaxy lies at 220 million light years away.

The ultra-violet imaging telescope (UVIT), one of the five payloads of the observatory, has zoomed in to the central part of the merging galaxy named NGC 7252.

A team of scientists from different research labs across the country, who observed the galaxy, have calculated age of the the stars -- they are anywhere between 150 million years and 400 million years old. The team was led by Koshy George, a post-doctoral fellow, from the Indian Institute of Astrophysics, Bangalore.

Indian Space Research Organisation (Isro) has posted a Near UV (NUV) and Far UV (FUV) colour map as the Astrosat Picture of the Month (July) on its website. Isro launched Astrosat in September 2015.

Earlier, the telescope had captured an ultraviolet image of the loops and tails of gas and dust ripped apart from two galaxies as they merged to form NGC 7252.

The UVIT captured the galaxy in the Near UV (around 242 nanometres) and the Far UV (around 148 nanometres). When the two images are divided pixel by pixel, it gives an ‘ultraviolet colour’ image.

“Imagine a light bulb emitting all visible colour. We will perceive it as a particular colour depending on the fraction of red versus green versus blue light it emits. Similarly, dividing the FUV and NUV images will tell us what the ‘ultraviolet colour’ of NGC 7252 is and how this colour varies across the galaxy. These ‘ultraviolet colours’ are represented as red to blue in the image,” the scientists explained.

The image has a central ‘red’ region surrounded by a ‘blue’ ring, with an outer ‘red’ region. The scientists modelled this colour to calculate the corresponding ages of the stars that emit in these regions.

The bunch of stars right in the middle are around 320 million years old, while the surrounding blue ring has stars that are around 250 million years old. The even bluer clumps within the ring are only around 150 million years old. The surrounding larger region of red has stars that are more than 400 million years old.

Scientists point out that the merger of two galaxies, forming NGC 7252, is a violent event and the effects on the gas in these galaxies is complex.

The team has published a paper explaining how this merger would naturally lead to this scenario where different parts of the galaxy host stars of different ages.