A Jellyfish in the Sky

NUV image of JO201 from UVIT
Image Credit: Koshy George et.al

Jellyfish are gelatinous creatures of the sea, but did you know that there are giant jellyfish in the sky as well? Here we present an ultraviolet image of one such 'Jellyfish Galaxy', JO201. We know that galaxies usually cluster together because of their mutual gravity. These Galaxy Clusters can have 100s to 1000s of galaxies in them! In these clusters, the space between these galaxies is filled with very hot gas. When an external galaxy is attracted by the gravitational force of a cluster, it moves through this hot gas. When it does so, it feels this gas as a wind that is blowing against it, much like a bicyclist or a runner feels the air blowing back past them. If the galaxy is falling fast enough through this gas, then this wind can even dislodge the gas from this galaxy's gravitational pull. This stripped-off gas forms tails behind the infalling galaxy, and new stars can form out of this gas. Hence, these galaxies resemble a jellyfish with tentacles. We know a number of such jellyfish galaxies in the sky, and this image shows JO201, one such galaxy falling into the galaxy cluster Abell 85. The ultraviolet image taken by AstroSat shows the hot young stars formed from the gas that have been pushed out of this galaxy. The galaxy JO201 itself is moving in a direction towards us with an inclination towards the right. Koshy and his collaborators have used the superior resolution of UVIT to study how stars are being formed in individual parts of the tentacles of the jellyfish galaxy JO201. The paper describing their work is in https://arxiv.org/pdf/1803.06193.pdf