Deep Field Image from JWST, showing both BlueDOGs and LRDs. Credit: NASA, ESA, CSA, STScI, Brant Robertson (UC Santa Cruz), Ben Johnson (CfA), Sandro Tacchella (Cambridge), Marcia Rieke (University of Arizona), Daniel Eisenstein (CfA)
One of the most difficult parts of astronomy is understanding how time affects it. The farther away you look in the universe, the farther back you look in time. One way this complicates things is how objects might change over time. For example, a supermassive black hole at the center of a galaxy in the early universe might appear one way to our modern telescopes, but the same supermassive black hole might appear completely differently a few billion years later.
Understanding the connection between the two objects would be difficult to say the least, but a new preprint paper on arXiv from researchers at the University of Science and Technology in South Korea describes one potential parallel, between the recently discovered “Little Red Dots” of the early universe and “BlueDOGs” of the slightly later universe.
To understand the connection, first it’s important to understand the context. Little Red Dots, or LRDs, are a relatively recent discovery by Webb in the early universe—about 600 million to 1.6 billion years after the Big Bang. They are very small, and appear “red” likely due to their high redshift and being heavily shrouded by gas and dust. While we don’t know their exact make-up yet, the current going theory is that these are early galaxies, with rapidly growing supermassive black holes at their center.
Blue Dust-Obscured Galaxies (BlueDOGs) are another recent discovery, typically found at “Cosmic Noon,” about 2 to 3 billion years after the Big Bang. They got their name from an unexpected spike in “blue” ultraviolet light that is indicative of either very active star formation or a very active central black hole.
Importantly, this light is noticeable despite the surrounding dust that give BlueDOGs part of their name. Typically the dust surrounding them would absorb most of the UV light they emit, so the fact that they are still getting light through to be observed by us means there’s a lot of it going around.
The most important idea from the paper is that LRDs and BlueDOGs represent the same type of astronomical phenomena, just at different phases of evolution. In particular, there is a noticeable similarity between the spectral energy distribution (SED) between the two classes of objects. And, importantly, given that they are spaced far apart in time, that could indicate that they really represent the same object at two different points in its life cycle.
This is important because, to be blunt, LRDs are hard to find and observe, due to being faint and really, really far away. The James Webb Space Telescope is the only system capable of detecting these faint objects with any accuracy, and observational time on humanity’s most powerful space telescope is highly in demand, to say the least. However, BlueDOGs are closer, and therefore more widely accessible with other observational platforms.
One particular BlueDOG that was the focus of the paper, and at least partially the impetus to the paper’s central idea, is ADFS-KMTDOG-102. Using the Gemini-South telescope, the researchers observed this extremely bright massive galaxy in its full spectrum. When they noticed the similarities between it and the spectrum of LRDs, it is when they started to think of them as two stages of the same object.
To be fair, we still don’t entirely know what LRDs actually are, and BlueDOGs themselves are only a relatively recent discovery as well. They could be two separate phases of the growth of massive galaxies, just obscured by gas and dust. Or it could be a quasar in a theorized state before it blows away the gas and dust surrounding it.
As researchers begin to draw further parallels between these two classes of objects, and open up the study of at least one of them to further scrutiny by more telescopes, they might be able to draw a causal timeline between two seemingly separate phenomena across billions of years of intergalactic history.
More information:
Seongjae Kim et al, The BlueDOG at Cosmic Noon: A Possible Analog to Little Red Dots?, arXiv (2025). DOI: 10.48550/arxiv.2508.19618
Journal information:
arXiv
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Universe Today
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BlueDOGs might evolve from Little Red Dots (2025, September 4)
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