When we see images of galaxies outside the Milky Way, what we’re usually looking at is mostly the light from their stars. But stars are far from the only ingredient that makes up a galaxy. Think of the stars as chunks of vegetables in galactic soup.
The broth in which they float is therefore the intergalactic medium – not empty space but filled with the often tenuous, sometimes dense clouds of dust and gas that drift between the stars. Because stars are so much brighter, dust is usually second fiddle; but this dust, from which stars are born, to which stars return, can tell us a lot about the structure and activity within a galaxy.
Now four new images have been released, showing the distribution of dust in four of the galaxies closest to the Milky Way: the 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 Triangle Galaxy, a spiral galaxy 2.73 million light-years away.
Without dust and gas, galaxies as we know them would not exist. Stars form when a dense knot of matter in a cold cloud of molecular gas collapses under the effect of gravity, incorporating matter from the cloud around it. When this star dies, it ejects its outer material into the space around it, along with the new, heavier elements that it has fused together during its lifetime.
New stars that are born incorporate dust from dead stars, making each generation of stars slightly different. We are, indeed, all made of star stuff – even stars.
But the dust is not evenly distributed. Stellar winds, galactic winds, and the effects of gravity can all push and sculpt interstellar dust into complex, cavity-filled shapes. Mapping structures and the composition of the elements that make them up 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 operated by the European Space Agency between 2009 and 2013. Until Webb’s launch – which did not yet delivered its first scientific images – Herschel was the largest infrared telescope ever launched.
Like Webb, its ultra-cool operating temperature meant Herschel could peer into the far infrared, imagining some of the coldest and dustiest objects in space, down to temperatures of around -270 degrees Celsius (- 454 degrees Fahrenheit). This includes the cold clouds in which stars are born and dust in interstellar space.
However, it was less able to detect dust and more diffuse gases. To fill in 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 Infrared Astronomical Satellite (IRAS) and Explorer Cosmic Background (COBE) from NASA.
The results reveal complex interactions within the dust. Hydrogen gas appears in red; it is the most abundant element in the Universe, so there are quite a lot of them. The cavities in the dust where the fledgling stars carried it away with their intense winds appear as empty regions, surrounded by a green glow that indicates cold dust. Blue regions represent hotter dust, heated by stars or other processes.
The images also reveal new information about the complex interactions taking place in interstellar dust, the researchers said. Heavy elements such as oxygen, carbon and iron can often stick to dust grains; in the densest clouds, most of the elements are dust-bound, increasing the dust-to-gas ratio. This can affect how light is absorbed and re-emitted by dust.
However, violent processes, such as star birth or supernovae, can release radiation that breaks down dust, releasing the heavy elements into gaseous clouds. This shifts the dust/gas ratio towards gas.
Herschel’s images reveal that ratios can vary by up to a factor of 20 within a galaxy. That’s much higher than astronomers thought, important information that could help scientists better understand this cycle.
And, they’re just spectacularly pretty. Who knew Andromeda soup could be such a dazzling rainbow of colors.