Adapted from Galaxy: Mapping the Cosmos by James Geach. Published by Reaktion Books, November 2014. Used with permission of the publisher.
Over the course of a week in late 1995, the Hubble Space Telescope stared at a single tiny and seemingly empty patch of sky, an area no bigger than the apparent size of a period on the printed page of a book held out at arm’s length. The patch of sky was located far out of the Milky Way’s plane and was devoid of stars, gas and dust that could obscure light from distant galaxies, offering Hubble a chance to make remarkably deep observations of the cosmos.
The week-long exposure, called the Hubble Deep Field, revealed thousands of galaxies jam-packed into that minuscule, “empty” area. Most of the galaxies were so far away that their light had taken nearly 10 billion years to reach us. Staring off into the far distance, Hubble had managed to peer right across the visible universe. Extrapolating the numbers of galaxies within the Hubble Deep Field across the entire sky, it became clear that our galaxy is but one of hundreds of billions.
But what is a galaxy? What gave rise to the rich variety we see around us today, and which processes shape their destinies? I’ve explored these questions in my new book, Galaxy: Mapping the Cosmos.
From low-mass “dwarf” systems to majestic, intricately structured spiral galaxies to the massive clusters of ancient elliptical galaxies, all the galaxies we see today are products of a nearly 14-billion-year process that began shortly after the big bang. Subtle ripples in the density of matter in the primordial universe attracted more gas and dark matter through the force of gravity, becoming the seeds from which all galaxies would grow. Understanding how these different galaxies came to be is the central goal of the field of galaxy evolution.
We know that the elliptical galaxies are the oldest, most massive galaxies in the universe. They formed most of their stars just a few billion years after the Big Bang, and they tend to reside in giant clusters: congregations of hundreds or thousands of individual galaxies moving in a common gravitational potential, like bees around a hive. They contain little or no gas, meaning new stars no longer form. Their old stars do not orbit the galaxy in a disc but instead are on random trajectories around a central hub, and this randomness is what gives an elliptical galaxy its characteristic, somewhat spherical shape.
Spiral galaxies—galaxies like our own—are much more common than ellipticals. They rotate like spinning plates and are disk-shaped, with spiral arms emanating from a central bulge. The bulge contains relatively old stars, but the disk is rich with hydrogen gas, which clumps into giant clouds that form new stars. The arms are flecked with bright blue star clusters and red-hued patches of ionized hydrogen associated with sites of active star formation.
Sometimes spiral galaxies will collide in spectacular mergers, drawn together by gravitational attraction. This process can sap the angular momentum of a spinning galaxy, causing gas to collapse to the galactic center and triggering intense bursts of star formation. Mergers are important in the process of galaxy evolution, and in the local universe the majority of the most intensely star-forming galaxies are merging systems. The fate of such systems is to eventually evolve into galaxies of the elliptical variety.
Around spirals and ellipticals we find swarms of smaller satellite galaxies called dwarfs. The most famous dwarf satellites are the Large Magellanic Cloud and the Small Magellanic Cloud, which orbit our Milky Way. Often such dwarfs are rich in gas and actively forming new stars. They tend to be irregular in shape, with the exception being dwarf “spheroidals,” which are like miniature elliptical galaxies, resembling fairly symmetric balls of stars.
At its heart, every massive galaxy (including our own) contains a supermassive black hole. The black holes in the centers of galaxies are called supermassive because they contain millions of times the mass of the sun. Usually these black holes lie dormant, but when they accrete new matter such as interstellar gas, they can awaken to become remarkably luminous, releasing vast quantities of energy back into the galaxy. We call these systems active galactic nuclei, and they appear to play a crucial role in regulating the birth of stars within a galaxy.
Our knowledge of galactic evolution and our place in the universe continues to grow with unprecedented speed, but there is still a great deal left to learn. Galaxy: Mapping The Cosmos is my story of this continuing journey of discovery.