New images show the dust in nearby galaxies, and you have never seen them like this

When we see images of galaxies outside the Milky Way, what we usually see is first and foremost the light from their stars. But stars are far from the only ingredient that makes up a galaxy. Think of stars as the pieces of vegetables in the galactic soup.

The broth in which they float is the intergalactic medium – not voids, but filled with often thin, sometimes dense clouds of dust and gas drifting between the stars. Because stars are so much brighter, the dust is usually second fiddle; but that dust, from which stars are born, to which stars return, can tell us much about the structure and activity of a galaxy.

Now, four new images have been released showing the distribution of dust in four of the galaxies closest to the Milky Way: Large and small Magellanic clouds, dwarf galaxies orbiting our own; The Andromeda Galaxy, a large spiral galaxy at a distance of 2.5 million light-years; and the Triangulum Galaxy, a spiral galaxy 2.73 million light-years away.

large Magellanic sky herschelThe great Magellanic cloud. (ESA, NASA, NASA-JPL, Caltech, Christopher Clark / STScI, S. Kim / Sejong University, T. Wong / UIUC)

Without dust and gas, galaxies as we know them would not exist. Stars form when a dense knot of material in a cold cloud of molecular gas collapses under gravity, incorporating material from the cloud around it. When that star dies, it throws its outer material back into the space around it, with the new, heavier elements it fused together during its lifetime.

New stars that are born contain the dust from dead stars, which makes each subsequent generation of stars a little different. In fact, we are all made of star stuff – even the stars.

But the dust is not evenly distributed. Stellar winds, galactic winds and the effects of gravity can all press and shape interstellar dust into complex shapes filled with cavities. Mapping the structures and the composition of the elements in them is a crucial tool for understanding the formation of… well… pretty much everything.

The new images, unveiled at the 240th meeting of the American Astronomical Society, were obtained by the Herschel Space Observatory run by the European Space Agency between 2009 and 2013. Until the launch of Webb – who has not yet submitted his first scientific images – Herschel was the largest infrared telescope ever launched.

small magellanic sky herschelThe small Magellanic cloud. (ESA, NASA, NASA-JPL, Caltech, Christopher Clark / STScI, S. Stanimirovic / UW-Madison, N. Mizuno / Nagoya University)

Like Webb, its ultra-cold operating temperature meant that Herschel could look into the far infrared, and image some of the coldest and dustiest objects in space, down to temperatures around -270 degrees Celsius (-454 degrees Fahrenheit). This includes the cold clouds where stars are born and the dust in interstellar space.

However, it was less good at detecting more diffuse dust and gas. To fill the gaps, a team of astronomers led by Christopher Clark of the Space Telescope Science Institute used data from three other retired telescopes: ESA’s Planck and NASA’s Infrared Astronomical Satellite (IRAS) and Cosmic Background Explorer (COBE).

The results reveal complex interactions in the dust. Hydrogen gas appears in red; it is the most numerous element in the universe, so there is quite a lot of it. Cavities in the dust where newborn stars have blown it away with their intense winds, appear as empty areas, surrounded by a green glow indicating cold dust. Blue areas represent warmer dust, heated by stars or other processes.

triangulum galaxies herschelTriangulum galaxies. (ESA, NASA, NASA-JPL, Caltech, Christopher Clark / STScI, E. Koch / University of Alberta, C. Druard / University of Bordeaux)

The images also reveal new information about the complex interactions that take place in interstellar dust, the researchers said. Heavy elements such as oxygen, carbon and iron can often attach to dust grains; in the densest clouds, most of the elements are bound to dust, which increases the ratio of dust to gas. This can affect the way light is absorbed and re-emitted by dust.

However, violent processes, such as starbursts, or supernovae, can release radiation that breaks apart the dust, releasing the heavy elements back into gaseous clouds. This tips the ratio of dust to gas back to gas.

Herschel images reveal that ratios can vary by up to a factor of 20 in a galaxy. It is much higher than astronomers thought, important information that can help scientists better understand this cycle.

And they are just spectacularly pretty. Who knew that Andromeda soup could be such a dazzling rainbow of colors.