As recently as the start of the 20th century, the idea that the Milky Way contained everything that existed in the Universe was predominant and astronomers were unaware of the existence of other galaxies or any kind of star systems outside our galaxy. A few observed nebulae that had been identified as clusters of stars or clouds of radiating gas were eventually too faint to be resolved, even with the best telescopes, thus their distances remained unknown. For these observed nebulae earlier on, the philosopher Immanuel Kant (1724-1804) had suggested that some might be distant systems of stars (other Milky Ways), but the evidence to support this suggestion was beyond the capabilities of the telescopes of that time.
It was only in 1924 when Edwin Hubble used a new 2.5-meter reflector telescope on Mount Wilson in California that was able to measure the distance to the Andromeda galaxy using cepheid variables—a technique pioneered by Henrietta Leavitt. Although his estimate (900,000 light-years) was about half the actual distance that we know today for the Andromeda galaxy, it was the first time that anyone had measured a distance so great in the Universe that the fundamental nature of the discovery remained intact. This measurement established the very existence of separate galaxies of stars well outside the limits of the Milky Way, marking the beginning of the study of a new scientific field: extragalactic astronomy.
We are now aware that galaxies are systems bound by gravity that were formed at the infant stages of the Universe, about 400 million years after the Big Bang (when the universe was about 3% of its current age) and contain dust, gas, and somewhat between a few million to trillion stars. Astronomers believe that nearly all—if not all—galaxies are embedded in a dark matter halo also hosting a supermassive black hole in their cores. However, not all galaxies exhibit the same morphology and most certainly they do not inhabit at the same environment.
A recent rough estimate revealed that there are somewhere between a trillion and a million, million galaxies in the observable universe. Morphologically, the vast majority of the observed bright galaxies in the universe are either spirals (galaxies with a flat, spinning disk that present a central bulge surrounded by spiral arms; a cosmic spinwheel of both old and young stars, as well as interstellar matter) or ellipticals (early-type galaxies that are fairly round or slightly elongated to ellipsoid-
However, most galaxies and stars in the Universe reside
s in galaxy groups hence they are the most important laboratories to study galaxy formation and evolution. They contain more than half of all galaxies, compared with clusters at only 2%. While people might think of them as mini-clusters, they are not simply scaled-down versions of galaxy clusters, as they possess shallow gravitational potentials and low relative velocities between their member galaxies that are conducive to the galaxy mergers and tidal interactions that drive rapid galaxy evolution. Almost all galaxies are thought to be part of a galaxy group at some stage of their evolution (so called pre-processing) before they are eventually assimilated into clusters.
In the early stage of the development of a group, galaxy interactions and mergers lead to enhanced star formation, black hole growth, and galaxy transformation. But this process is also a path to what has been thought of as “galaxy death”—large spiral galaxies are transformed into early-types with little or no star formation as an inflow of gas towards the galactic core triggers phenomena of fast star formation while a high temperature (10^6–10^8 K) halo of gas that eventually surrounds the galaxies (visible only in the X-rays) builds up. Any new galaxy that might fall in the group at this stage, will have its cool gas stripped off by the built-up hot gas halo, choking off their star formation as well, and we eventually end up with an apparently quiescent system in which the stars slowly age and most of the galaxies are “red and dead.” For galaxy groups where the dominant galaxies are mainly ellipticals, these evolutionary processes occurred earlier in the past therefore, especially in the local Universe, galaxy groups are usually thought to have settled down to a quiet retirement after the frenzied activity of collisions and explosions associated with their formation.
However, with multi-wavelength observations, a different picture emerged for astronomers: that of a violent activity. These systems are not dead, but they have instead become active in a different way: they are dominated by the activity from gas feeding into the central supermassive core igniting an active galactic nuclei. This process can produce radio emission that can launch powerful jets or winds of relativistic particles, which may travel thousands or millions of light years out into the surrounding environment driving gas flows, shocks, etc. These fast outflows from the centre of the galaxy are pushing on and heating gas near the galaxy and blowing bubbles in the gas “recycling” and transforming the galaxy and the surrounding environment. Eventually, these environmental interactions leave imprints both on the developed surrounding halo of hot gas which is seen in the X-rays and in plasmas produced in the core from the interaction with strong magnetic fields that are mainly visible in the radio wavelengths.
This is the reason why astronomers use a unified approach, combining observations in multiple wavebands in order to comprehend the process of galaxy formation. This provides a unique opportunity to learn about events that took place in a largely unobservable past combining a mosaic of information. Such vigorous behaviour in an old group of galaxies in the local universe (NGC 1550) was unexpected as an infalling galaxy seems to have set the whole group core in motion restarting galaxy evolution processes and suggests that we must re-evaluate the extent that galaxy groups can change, even today, and think again about what they might look like in the future.
The NGC 1550 galaxy group, viewed using a combination of optical, radio, and X-ray observatories. The galaxies and foreground stars are shown using multi-colour optical imaging from the PanSTARRs survey. Blue colours show X-ray emission from the halo of 10 million degree hot gas which surrounds the galaxies, imaged by NASA’s Chandra and ESA’s XMM-Newton observatories. Radio emission, detected using the Giant Metrewave Radio Telescope at 610 MHz and shown in green, traces the jets of high-energy particles thrown out by the supermassive black hole in the heart of NGC 1550 around 33 million years ago. The lopsidedness of these jets was the first clue that the group had been disturbed by the infall of a new galaxy.
Konstantinos Kolokythas & Ewan O’Sullivan image via NASA’s Chandra observatory, ESA’s XMM-Newton observatory, and the Giant Metrewave Radio Telescope (GMRT) from Pune, India.