An international team that included the Korea Astronomy and Space Science Institute (KASI) discovered a galaxy that emits a strong blue light even through dust.
Galaxies heavily obscured by dust usually appear red. That is because dust blocks and scatters short wavelengths such as ultraviolet (blue light), while allowing long wavelengths such as infrared (red light) to pass through more easily.
But the galaxy discovered this time was different. Named "BlueDOG (Blue-excess Dust-Obscured Galaxy)," this galaxy harbors at its center a supermassive black hole about 14 billion times the mass of the sun.
The international team identified the galaxy after conducting spectroscopic observations with the Gemini South telescope in Chile (GEMINI-South) of a peculiar object candidate found with the Korea Microlensing Telescope Network (KMTNet), which KASI operates. Spectroscopic observation is a method of dispersing light from an astronomical object by wavelength using a spectrograph such as a prism or a diffraction grating to obtain a spectrum, and analyzing it. The results were published on the 10th in the Astrophysical Journal.
The team offered two possibilities for why BlueDOG emits blue light. The first is that light from the central black hole scattered after colliding with gas and dust inside the galaxy. The second is that a recent burst of star formation inside the galaxy caused an excess in observed ultraviolet.
The team judged that both possibilities likely played a role, because a single explanation makes it difficult to fully understand this phenomenon.
BlueDOG is estimated to have existed since about 11 billion years ago, the period known as "Cosmic Noon" when galaxies and black holes grew most actively. Its mass is about 2 trillion times that of the sun, and its brightness is about 80 trillion times that of the sun. In particular, it falls into an ultraluminous class that is extremely rare in the universe.
The team said BlueDOG "may not simply be a dust-obscured galaxy, but a special object that shows a growth-spurt phase in a stage of galaxy evolution."
According to the team, BlueDOG also resembles the recently discovered "Little Red Dots (LRDs)." Known as "mystery galaxies," LRDs were found in the early universe 2 billion years earlier than BlueDOG. Both types share the common trait of simultaneous powerful black hole activity and explosive star formation. The team expected that these characteristics would be clues to uncovering the link that connects the growth processes of galaxies and black holes.