London, Feb 7 : A new study has revealed that a spiral galaxy, which lies 15 million light years away from Earth, does not have any traces of dark matter, thus puzzling astrophysicists.
According to a report in New Scientist, the galaxy, known as NGC 4736, seems to be empty of any dark matter, which is supposed to be spread throughout the universe.
In the outer regions of most galaxies, stars orbit around the centre so fast that they should fly away. The combined mass of all the observable inner stars and gas does not exert strong enough gravity to hold onto these speeding outliers, suggesting some mass is missing.
Most astronomers believe that the missing mass is made up of some unusual invisible substance, labelled dark matter, which forms vast spherical halos around each galaxy.
But, in the spiral galaxy NGC 4736, the rotation slows down as you move farther out from the crowded inner reaches of the galaxy.
At first glance, that declining rotation curve is just what one would expect if there is no extended halo of dark matter, and no modification to gravity. As you move far away from the swarming stars of the inner galaxy, gravity becomes weaker, and so motions become more sedate.
The rotation measurements only stretch 35,000 light years out from the galactic centre, which is not far enough to confirm that first impression. So a team of astronomers in Poland developed a more sophisticated analysis.
Joanna Jalocha, Lukasz Bratek and Marek Kutschera of the Polish Academy of Science in Krakow found a way to splice the rotation curve together with another measurement: the density of hydrogen gas far from the galactic centre.
According to their combined mathematical model, ordinary luminous stars and gas can indeed account for all the mass in NGC 4736.
"If this paper is correct, then this galaxy contains very little or no dark matter," said astrophysicist Jrg Diemand of the University of California, Santa Cruz, US. "That is surprising," he added.
"It is unclear how one would form a galaxy without a dark halo, or how one could remove the halo without destroying the galaxy," said Diemand. "A galaxy without dark matter really does not fit into our current understanding of cosmology and galaxy formation," he added.