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Hoag’s Object Is a Galaxy Within a Galaxy Within a Galaxy (and Nobody Knows Why)


With a flawlessly symmetrical ring encircling a red sphere of stars, Hoag's object is one of the most stunning mysteries in the universe. 


If you look closely at the serpent constellation in the northern sky, you might notice a galaxy within a galaxy within a galaxy.


This cosmic turducken is known as Hoag's object, and it has baffled astronomers since astronomer Arthur Hoag found it in 1950.


Left: Hubble Heritage WFPC2 colour image of Hoag’s Object (Image credit: NASA, ESA, and the Hubble Heritage Team). Right: The FPI Hα data drawn as contours over the greyscale HST F606W image. 


The object under consideration is a one-of-a-kind, ring-shaped galaxy 100,000 light-years across (slightly larger than the Milky Way) and 600 million light-years away from Earth. In a recent snapshot of the strange object taken by the Hubble Space Telescope and analysed by geophysicist Benoit Blanco, a dazzling ring of billions of blue stars forms a perfect circle around a much smaller and denser sphere of reddish stars. Another ring galaxy, much, much farther away, appears to say hello from the black space between the two stellar circles.


What's happening on, and what ripped Hoag's item in half? Ring galaxies account for less than 0.1% of all known galaxies, making them challenging to analyse. Hoag himself claimed that the unique ring development in the galaxy was an optical illusion caused by gravitational lensing (an effect that occurs when extremely high-mass objects bend and magnify light). This notion was later debunked by studies using larger telescopes.



Kinematic information for Hoag’s Object. Upper panels: WFPC2 F606W image (upper left) and Hα intensity map (upper right). Slit orientations and actual slit widths for the three position angles used for spectroscopy are also indicated, as well as the MPFS field of view. Lower panels: The line-of-sight velocity field (lower left) and velocity dispersion field (lower right) of Hoag’s Object from FPI and MPFS observations.


R-band negative image of Hoag’s Object obtained with the BTA 6-m telescope. The limiting surface brightness reaches µR ≈ 27.3 mag arcsec 2. 


Another widely held belief is that Hoag's object was once a more common disk-shaped galaxy, but an ancient collision with another galaxy ripped a hole through the disk's belly, permanently warping its gravitational pull. If such a collision occurred within the last 3 billion years, astronomers using radio telescopes should have been able to glimpse some of the aftermath. No such evidence has been discovered.


If a cosmic catastrophe occurred in Hoag's object's core, it must have happened so long ago that all traces of it have been washed away. With only a few other known ring galaxies to analyse (none of which exhibit the exactly symmetrical features discovered in this one), Hoag's object remains a mystery wrapped in a riddle inside an enigma — you know, like a turducken.


(Image courtesy of NASA/ESA/Hubble)

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