Using data collected by the Very Large Telescope of the European Southern Observatory, a team led by the Instituto de Astrofísica de Canarias (IAC) in Spain has uncovered the reason for a dramatic increase in star formation activity in a dwarf galaxy called Leo P.

For much of its life, Leo P dwelled in the outskirts of the Virgo galaxy cluster, far away from massive galaxies and devoid of substantial reservoirs of cold molecular gas, the raw material out of which stars form. As a result, Leo P was considered a dwarf elliptical galaxy, known for hosting an old stellar population and producing few new stars.

This picture drastically changed when Leo P fell into the domain of the gigantic elliptical galaxy M87, the dominant galaxy in the Virgo cluster, about 100 million years ago.

"For the first time we have observed a minor merger event in which the merging dwarf galaxy shows not only a clear presence of cold gas, but also vigorous star formation activity" says Enriquillo Dante, researcher from the IAC and the Universidad de La Laguna (ULL) and the first author of the paper published in Nature Astronomy.

The observations using the ALMA telescope suggest that the encounter triggered gas flows that reached Leo P and compressed its gaseous reservoir. This sudden increase of the reservoir's density resulted in a dramatic intensification of star formation.

"The fact that such an inefficient star former was able to switch on its star formation activity so quickly suggests that the current episode is not the first of its kind. In the past Leo P must have been through similar events during which it became temporarily a star-forming dwarf" says David Aguerri, a researcher at the IAC who participated in the study.