Distance to the Antennae Galaxies
The Antennae Galaxies are among the closest known merging galaxies. The merging pair of galaxies, NGC 4038 and NGC 4039, started to interact a few hundred million years ago, creating one of the most impressive sights in the night sky. They are considered by scientists as the archetypal merging galaxy system and are used as a standard with which to validate theories about galaxy evolution.
The ground-based image (left) is taken by Robert Gendler and shows the two merging galaxies and their impressive long tidal tails.
The Hubble Advanced Camera for Surveys image (right) shows a portion of the southern tidal tail. The main visible component consists of young blue stars recently formed out of the gas accumulating along the spine of the tail, and many red background galaxies can also be seen. The region is also filled by thousands of faint red stars, just visible when zooming in on the image.
By measuring the colours and brightnesses of red giant stars in the system, the scientists found that the Antennae Galaxies are much closer to us than previously thought: 45 million light-years instead of the previous best estimate of 65 million light-years. These red stars cannot be intrinsically brighter than a certain threshold, providing a measure for their absolute magnitude. Comparing the observed magnitudes with this absolute value allowed for deriving the new distance of the Antennae Galaxies.