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The study of galaxies

A galaxy is a massive, gravitationally bound system consisting of stars, gas and dust (that make up the interstellar medium), and a dynamically important but poorly understood component commonly called dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way galaxy. Galaxies range from dwarfs with as few as ten million stars to giants with a hundred trillion stars, each orbiting the centre of mass of their host galaxy.

Galaxies vary in their relative amounts of stars and interstellar medium but the mass of most galaxies is dominated by dark matter, 90% being typical. It is now thought that most, if not all, galaxies contain a super-massive black hole at their centre.

As far as their optical appearance goes, galaxies have historically been classified into one of three broad classes. Elliptical galaxies have a smooth, elliptical light distribution that shows little or no detail and are characterised by having relatively little interstellar medium. Spiral galaxies are flattened disk-like systems showing beautiful, curving spiral arms, and have much more interstellar medium than the ellipticals. The third class is the irregular galaxies, generally less massive than either the ellipticals or spirals, and having an amorphous appearance. These galaxies are normally very rich in interstellar medium.

Galaxies sometimes collide and interact with each other and this can both induce very rapid bursts of star formation and transform galaxies from one type into another. Mergers of galaxies in the distant past are thought to have been fundamental in shaping their present-day appearance.

Most galaxies are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations known as groups and clusters, which, in turn usually form larger superclusters. At the largest scale, these associations are generally arranged into sheets and filaments, which are surrounded by immense voids.

THESEUS: a new window on the early Universe and the multi-messenger transient sky

May 10, 2018

THESEUS: a new window on the early Universe and the multi-messenger transient sky The European Space Agency announced the three space missions selected for ESA’s fifth medium class mission in its Cosmic Vision science programme. One of them is THESEUS, in which the Italian National Institute for Astrophysics plays a major role

The remote galaxy that is “dying out”

May 10, 2018

The remote galaxy that is “dying out” An impetuous outflow of ionized gas and carbon monoxide leaks from galaxy Xid2028, located at 9.6 billion light years from us, having a supermassive black hole in the centre. The discovery by INAF researchers.

K2-3: A system of three likely sub-Neptunes

Mar 22, 2018

K2-3: A system of three likely sub-Neptunes A team led by astronomers from INAF presented a new study aimed at determining accurate masses for the three planets in orbit aroud the M-dwarf star K2-3