Finding the edge of a galaxy that spans 1.7 million light-years
In short: How do you measure the size of an object that blends into the very fabric of space? For the universe's largest galaxy, the boundary is so faint it was previously invisible. New ultra-deep imaging has finally allowed astronomers to map its outer limits.
The galaxy IC 1101 is classified as a brightest cluster galaxy (BCG), a type of cosmic giant formed through hierarchical galaxy formation. Over billions of years, these systems grow by absorbing smaller galaxies. The galaxy's dim stellar halo merges seamlessly with the intracluster medium, which comprises the gas and stars occupying the gaps between cluster galaxies. This seamless transition makes it incredibly difficult to determine where a galaxy ends and the rest of the cluster begins.
To address this issue, Carlos Marrero-de la Rosa headed a group that utilised extremely deep imagery from the Isaac Newton Telescope's (INT) Wide Field Camera. They targeted the galaxy cluster Abell 2029 to find the edge of one of the most extended stellar systems ever observed. Although a prior study of a comparable system indicated a stellar reach of 607 kiloparsecs, these fresh findings verify IC 1101 as the biggest known galaxy, stretching approximately 520 kiloparsecs, or nearly 1.7 million light-years.
The primary challenge in these measurements is light pollution from foreground stars. Scattered light from these stars can create a glow that washes out the faint regions astronomers need to see. The researchers constructed a precise model of the point spread function for the telescope by employing numerous calibration stars to mitigate this effect. They then systematically subtracted the scattered light from more than 250 individual stars, one by one, to reveal the galaxy's true outskirts. The results indicate that IC 1101 contains an estimated 3.4 trillion solar masses in stars.
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Sources
- Summary by Intellect · source: New Measurements of the Largest Galaxy in the Universe IC 1101
- Also: Astronomers finally measure the full size of the largest known galaxy
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