A new image from the Hubble Space Telescope perfectly illustrates why astronomers have to be so careful with distance in space.
More than 1 billion light-years away, two galaxies float in the darkness, beautiful golden spiral-like spirals seemingly caught in the act of colliding. They are called SDSS J115331 and LEDA 2073461 and, despite appearances, they are not interacting at all.
Instead, they are separated by some distance. Their alignment is an absolutely beautiful line of sight match.
Galaxies collide in space quite often, gathering along dark matter highways to galaxy cluster nodes, where they flow toward a mutual galactic center.
This process is thought to be one way the supermassive black holes at their centers grow to billions of times the mass of the Sun: when galaxies merge, their central black holes do too.
But space is big and there’s a lot of stuff in it, so scientists have to be careful when interpreting two objects that appear to be in the same place. Are they interacting or overlapping with a large distance in between?
The full Hubble image of the overlapping galaxies SDSS J115331 and LEDA 2073461. (ESA/Hubble and NASA, W. Keel)
Distance is one of the most important tools we have to interpret the Universe around us. The size, mass, and brightness of many objects cannot be accurately measured without an accurate distance measurement.
But distances in space can also be difficult to measure. You can’t tell how far away something is just by looking at it unless you know how much light it emits.
This is why things like Type Ia supernovae, which have a known intrinsic brightness, are a useful tool for measuring distance in space.
For relatively close objects, we can use parallax, the way objects move across the sky relative to each other.
However, beyond a certain distance, individual objects become increasingly difficult to see. So scientists rely on other tools, such as the way the expanding Universe stretches light from distant objects.
This is how we know that SDSS J115331 and LEDA 2073461 are not in the middle of a giant collision, although there are other clues as well: the two galaxies are too clean; a collision would damage them.
Even in the vicinity of the Milky Way, distances can be difficult to measure. Recently, scientists discovered that a source of gamma radiation was more distant than they had thought. In this case, the shared shape and alignment of the radiation and its source allowed the scientists to make the connection.
While overlapping galaxies may not help scientists better understand galactic collisions and mergers, they can be used to understand spiral galaxies.
When a more distant galaxy is backlit, interstellar dust from a foreground galaxy can be easier to see. Scientists have used this peculiar alignment to map the distribution of interstellar dust in multiple galaxies.
It is unclear whether the Hubble image of SDSS J115331 and LEDA 2073461 will be used for this purpose. But it is incredibly beautiful to look at.
You can download wallpaper-sized versions of this image from the Hubble website.