Updated

Updated · spacedaily.com · Jul 3

Yale-Led Team Finds Third Dark-Matter-Free Galaxy at 6.5 km/s, Backing Collision Theory

Updated

Updated · spacedaily.com · Jul 3

Yale-Led Team Finds Third Dark-Matter-Free Galaxy at 6.5 km/s, Backing Collision Theory

Q: A line of ghost-like galaxies has been found. What cosmic event could be responsible for their creation?

The most likely cause is a long-ago, high-speed collision between two gas-rich galaxies. Astronomers say the newly identified galaxy DF9, like DF2 and DF4 before it, appears to contain little or no dark matter and sits along the same narrow, straight trail in the NGC 1052 field. That alignment is the key clue. In the leading “bullet dwarf” scenario, the collision stripped and shocked ordinary gas while the dark matter passed through with little interaction. The gas then compressed and later collapsed into a string of small, faint galaxies made mostly of normal matter. The measurements support that idea. DF9’s stars move at only about 6.5 kilometers per second, far slower than expected if a normal dark matter halo were present. That means its visible stars account for nearly all of its mass, making it another strong example of a dark-matter-poor galaxy formed in an unusual event rather than by standard galaxy growth. Researchers say this strengthens the case that dark matter is a real substance, because these galaxies look like rare exceptions created by a specific violent encounter, not evidence that gravity works differently everywhere. An alternative explanation remains under discussion: the galaxies may be tidal dwarf galaxies formed from debris in a major merger involving NGC 1052. But based on the straight-line arrangement and the prediction that more such galaxies should lie along the trail, the high-speed collision scenario is currently the favored explanation.

3 articles · Updated · spacedaily.com · Jul 3

NGC 1052-DF9’s stars show a velocity dispersion of about 6.5 kilometers per second, a Yale-led team found, making it the third known galaxy whose mass appears to be almost entirely its stars.
Keck observations over nearly 11 hours indicate DF9’s motions fit a stellar mass of roughly 100 million Suns, far below the roughly 24 kilometers per second expected if a normal dark matter halo were present.
DF9 joins DF2 and DF4 along a narrow galaxy trail in the NGC 1052 field, matching a prediction that multiple dwarfs formed together after a violent collision stripped ordinary matter from dark matter.
The result still carries sizable uncertainty: a modest amount of dark matter remains possible, and the measurement cannot yet distinguish the favored collision scenario from a tidal-dwarf origin.
Astronomers say the broader significance is that rare galaxies missing dark matter support dark matter as a separable substance; the next key test is to find leftover gas along the trail.

Sources

spacedaily.com9h ago

Yale-led Team Discovers Third Galaxy, NGC 1052-DF9, Lacking Dark Matter

Big Think9h ago

Is it finally time to take dark matter-free galaxies seriously? - Big Think

Astronomy Magazine9h ago

Do all galaxies have dark matter? | Astronomy.com

How does finding galaxies with no dark matter actually strengthen the case that this mysterious substance is real?

A line of ghost-like galaxies has been found. What cosmic event could be responsible for their creation?

NGC 1052-DF9 and the Trail of Dark Matter-Deficient Galaxies: Evidence for the "Bullet Dwarf" Collision Scenario

Overview

In June 2026, astronomers discovered NGC 1052-DF9 (DF9), a galaxy that appears to be largely devoid of dark matter. This surprising finding challenges the long-held belief that galaxies need dark matter halos to form and stay together. The confirmation that DF9, like its neighbor DF2, lacks dark matter strengthens the idea that these galaxies may have formed through an unusual cosmic event. This discovery suggests that stars and galaxies can exist outside traditional dark matter halos and supports the view that dark matter is a real, physical substance acting independently from normal matter.

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