IC 1101 – The supergiant galaxy

Lurking around at the centre of the Abell 2029 galaxy cluster, in the constellation Virgo, is IC 1101 -a monster galaxy that makes our Milky Way look like a grain of sand. The galaxy was discovered on 19 June 1790 by British astronomer William Herschel and is still one of the largest galaxies we know of.

Of course, in 1790 these objects were still believed to be nebulas within our own galaxy. The discovery of other galaxies outside the outskirts of the Milky Way only came in 1920, when Edwin Hubble demonstrated that Andromeda was a body outside the Milky Way by measuring Cepheid variable stars, changing our concept of the universe forever.

It soon became clear that there are countless other galaxies out there. And among these, IC 1101 is one of the most interesting. Classified as a supergiant elliptical galaxy, IC 1101 boasts a diameter of about 400,000 to 550,000 light years, which makes it one of the largest galaxies ever found. In fact, this has been for years the largest-known galaxy in the universe!*

The galaxy spans around 4 million light years across and has an extensive, diffuse halo that stretches up to 2 million light years from the core. The enormous size of IC 1101 leads most astronomer to believe that the galaxy formed from the merger of several smaller galaxies; a hypothesis supported by the scarcity of interstellar matter, which results in a low star-formation rate.

Read about the Andromeda Galaxy: Andromeda.

IC 1101’s distance from Earth is still unclear as different calculation methods across different wavelengths give different results. Initial estimations in 1980 assessed the distance from Earth at approximately 850 million light-years but more recent calculations suggest a distance of 1.2 billion light-years.

At such distance, the number of stars in the galaxy can’t be estimated accurately but astronomers believe that IC 1101 contains around 100 trillion stars, which could contribute to the galaxy’s astounding luminosity. However, the lack of interstellar matter indicates a low star-forming rate, leaving the galaxy with mostly old, metal-rich stars that give IC 1101 its distinctive yellow hue.

The galaxy’s core is one of the largest ever observed, if not the largest, with an apparent size of 2.77 arcseconds or 13,700 light years. Several radio burst have been observed from there, suggesting IC 1101, like most large galaxies, has a supermassive black hole at his core.

Hubble Space Telescope – Credit: Wikipedia

Read more about black holes: Black holes: the dark side of gravity.

The black hole’s mass is uncertain but ranges between 50 to 70 billion or 40 to 100 solar masses, depending on the calculation method used. In both cases, this is one of the largest and most massive black holes ever detected.

* After being the largest known galaxy for decades, IC 1101’s record was broken in February 2022, with the discovery of a gigantic radio galaxy dubbed Alcyoneus. Alcyoneus is located in the constellation of Lynx, about 3.5 Billion Light Years away, and spans approximately 16 Million Light Years across.