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Here in our Universe, galaxies come in various sizes.
This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Over time, overdense clumps of matter grow richer and more massive, growing into galaxies, groups, and clusters of galaxies, while the less dense regions than average preferentially give up their matter to the denser surrounding areas. The “void” regions between the bound structures continue to expand, but the structures themselves do not.
On cosmological scales, all structures exhibit the same (5-to-1) dark matter/normal matter ratio.
This composite JWST (above left) and Hubble (lower right) image of nearby spiral galaxy NGC 628, located 32 million light-years away, showcases the gas and dust network that traces out the spiral arms and the current star-forming regions within them. For large, high-mass spiral galaxies, it’s very common to find a dark matter to normal matter ratio consistent with the cosmic average: 5-to-1. However, in extreme environments and for low mass galaxies, the ratios can be extreme, both in terms of producing galaxies overabundant and deficient in dark matter.
But on smaller scales, tiny galaxies are more dark matter dominated.
Galaxies, when we examine their stars inside, range from ultra-diffuse to ultra-compact, depending on where their stars are located. While some ultra-diffuse galaxies are rich in dark matter and others appear to be dark matter-poor, the general trend is that, with lower mass, galaxies become more and more severely dominated by dark matter, with a lower stellar mass to total mass ratio.
Credit: Sloan Digital Sky Survey, Canada-France-Hawaii Telescope and the NGVS team
After stars form, winds and radiation expel gas, leaving only dark matter, stars, and stellar corpses behind.
Galaxies undergoing massive bursts of star formation expel large quantities of matter at great speeds. They also glow red, covering the whole galaxy, thanks to hydrogen emissions. This particular galaxy, M82, the Cigar Galaxy, is gravitationally interacting with its neighbor, M81, causing this burst of activity. Although the winds and ejecta are copious, this galaxy’s high overall mass will allow it to retain most of its normal matter. Smaller mass galaxies aren’t so lucky.
Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA); Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (National Science Foundation)
With less mass overall, smaller galaxies are more dark matter dominated.
This “almost dark” galaxy, nicknamed Nube, is an incredibly diffuse galaxy found within a grouping of many other galaxies. It is thought that this ultra-diffuse galaxy, which has only a small smattering of stars inside a large mass of neutral hydrogen, owes its properties due to environmental factors. With so much hydrogen and so few stars, it represents a fascinating outlier among conventionally known galaxies. Its past star-formation history has been largely erased over the billions of years that have passed since its most recent major star-formation episode.
The most extreme low-mass galaxy is Segue 1.
Many nearby galaxies, including all the galaxies of the local group (mostly clustered at the extreme left), display a relationship between their mass and velocity dispersion that indicates the presence of dark matter. NGC 1052-DF2 is the first known galaxy that appears to be made of normal matter alone, and was later joined by DF4 in 2019. Galaxies like Segue 1, however, are particularly dark matter-rich; there are a wide diversity of properties, and dark matter-free galaxies are only poorly understood, and many question their nature.
With just 175 stellar masses worth of stars, it requires ~600,000 suns worth of additional mass.
The cosmic web that we see, the largest-scale structure in the entire Universe, is dominated by dark matter. Simulations of the large-scale structure of the Universe must include both dark matter and normal matter, including the effects of star-formation, feedback, and gas infall, as all of them are needed in order to predict the emergence of visible structures. The lowest stellar mass galaxies, owing to these effects, have the greatest amounts of dark matter relative to their surviving normal matter.
Credit: Ralf Kaehler/SLAC National Accelerator Laboratory
Its 3400-to-1 dark matter ratio, inferred previously, is the most severe ever.
Only approximately 1000 stars, totaling ~175 solar masses, are present in the entirety of dwarf galaxies Segue 1 and Segue 3, the latter of which has a gravitational mass of an impressive 600,000 Suns. The stars making up the dwarf satellite Segue 1 are circled here. As we discover smaller, fainter galaxies with fewer numbers of stars, we begin to recognize just how common these small galaxies are as well as how elevated their dark matter-to-normal matter ratios can be; there may be as many as 100 for every galaxy similar to the Milky Way, with dark matter outmassing normal matter by factors of many hundreds or even more.
But: does dark matter actually compose all of this “missing mass?”
This graph shows the number density of stars per square parsec in galaxy Segue 1, with the open circles encoding the raw data and the red circles obtained by subtracting the tidal effects of the Milky Way. The blue curve represents a smoothing spline used for the deprojection.
Only Segue 1’s stars are observable, with faster motions found near its center.
A plot of the average rotational velocity amplitude (y-axis) versus the radial distance in arc-minutes (x-axis) from the center of Segue 1. The steep rise towards the center suggests a large density of mass in the center, but few-to-no stars are located there. Either there’s a supermassive black hole, or a tremendously steep density profiled dark matter halo, with a turnover radius of no more than ~100 parsecs.
Two stellar populations, of different metallicities, suggest multiple past star-forming episodes.
By identifying which stars in a field are all at the same distance from one another, scientists can find “hidden” low mass galaxies even against the star-rich backdrop of the Milky Way. Within those galaxies, scientists can then identify whether there’s a single population of stars that formed all at once, with the same ages and metallicities, or whether multiple stellar populations exist inside.
Credit: CFHT / S. Gwyn / S. Smith
Different dark matter and supermassive black hole (SMBH) masses predict different observable signatures.
This figure shows the results of many different simulations, along with how well or how poorly they reproduce and match the observable signature of Segue 1. The dense “reverse J” shaped curve in the upper-right figure represents the best-fit set of simulations, all possessing supermassive black holes of a few hundred thousand solar masses.
Although a dark matter halo is required, simulations also favor a SMBH.
According to models and simulations, all galaxies should be embedded in dark matter haloes, whose densities peak at the galactic centers. In order to best explain the motions of the stars found at those galactic centers, however, a supermassive black hole is often required. Although intermediate mass black holes, of ~1000 to ~100,000 solar masses, are rare thus far, these objects do exist, and may be found in low-mass galaxies and/or globular clusters.
Credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI)
The best fit scenario admits a ~400,000 solar mass SMBH.
The simulated distribution of dark matter particles obeying a universal profile (orange) versus the distribution of dark matter (or overall mass) as inferred from the observations of six ultra-faint dwarf galaxies (blue). Note that what is observed is inconsistent with the most naive theoretical prediction. For galaxy Segue 1, a turnover radius of an unprecedentedly low value of only 100 parsecs (~330 light-years) is required.
Credit: Gabriel Pérez (IAC)
Without a black hole, a dense dark matter core, disfavored by simulations, is required.
If the dark mass of Segue 1 is entirely encoded by dark matter, then we require high ratios of radial velocities compared to tangential velocities (red curve) for the stars inside: an unphysical solution. If instead we include a substantial supermassive black hole (black and blue curves), the ratio remains at a more physical value of ~1.
Perhaps Segue 1 was once a “little red dot” galaxy, with outer stars stripped away by our galaxy’s gravity.
This image shows 15 of the 341 hitherto identified “little red dot” galaxies discovered in the distant Universe by JWST. Most extended little red dots are dominated by active supermassive black holes, with low stellar masses for the masses of their central black holes. It’s possible that low-mass galaxies within the Milky Way have been tidally stripped remnants of ancient little red dot galaxies.
Credit: D. Kocevski et al., Astrophysical Journal Letters accepted/arXiv:2404.03576, 2025
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.
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