Mystery solved: why some galaxies appear dustier on one half than the other (Synopsis)

"He ate and drank the precious Words,
his Spirit grew robust;
He knew no more that he was poor,
nor that his frame was Dust." -Emily Dickinson

When we look at spiral galaxies, we think of grand arms, star-forming regions and dust lanes lining our perspective. But unlike face-on galaxies, where everything looks the same, galaxies that appear tilted at an angle often appear to have one half far greater in its dust-richness than the other.

The Sunflower Galaxy, Messier 63, tilted relative to our line-of-sight, with one half clearly appearing dustier than the other. Image credit: ESA/Hubble & NASA. The Sunflower Galaxy, Messier 63, tilted relative to our line-of-sight, with one half clearly appearing dustier than the other. Image credit: ESA/Hubble & NASA.

This was regarded as a mystery for a long time, but the recent Pan-chromatic Hubble Andromeda Treasury (PHAT) survey confirmed the leading picture: that the dust lanes are confined to a narrow, central region of the disk, and the dusty appearance is an optical illusion. It’s a simple result of perspective, that more of the brighter stars are hidden behind the dust from one side than the other.

The stars visible in the Andromeda galaxy, in a dust-rich region and a dust-poor region. Images credit: Illustration Credit: NASA, ESA, and Z. Levay (STScI/AURA); Science Credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), and the PHAT team, of a dusty region (top) and a relatively dust-free region (bottom). The stars visible in the Andromeda galaxy, in a dust-rich region and a dust-poor region. Images credit: Illustration Credit: NASA, ESA, and Z. Levay (STScI/AURA); Science Credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), and the PHAT team, of a dusty region (top) and a relatively dust-free region (bottom).

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