New study may redefine how the mass of the halo around galaxies is measured

A new study by scientists at the Raman Research Institute (RRI) and the Hebrew University of Jerusalem, Israel, has revealed that contributions from matter in the intergalactic medium may be affecting measurements of the diffuse envelope surrounding a galaxy. The study has far-reaching implications, given that the envelope holds power to make or break galaxies.

A galaxy evokes images of dust and stars sparkling within beautiful spirals. But beyond the galaxy’s outskirts lurks a diffuse, ghostly halo spreading out as far as 10-20 times the size of the galaxy. Most of the galaxy’s mass lies beyond the stars in this halo, composed of the mysterious dark matter — the invisible glue keeping the universe together — and gas. The gaseous component of the halo is called the circumgalactic medium or CGM.

The magic show of galaxies

“Imagine a street magician showing their tricks. People would slowly start gathering around … and the crowd grows,” says Dr. Kartick Sarkar, astrophysicist in the Astronomy and Astrophysics division at RRI and an author on the new paper published in The Astrophysical Journal. “Initially, people would rush in to see the show. However, as soon as they reach the boundary of the crowd, they stop.” The magician in Dr. Sarkar’s analogy is the galaxy, and the crowd is the CGM. The bigger the magician, the bigger the crowd. The region outside the CGM, not gravitationally bound by the galaxy, constitutes the intergalactic medium or IGM.

Mapping the distribution of gas in CGM is essential because CGM connects the galaxy to the cosmic web — the filamentary scaffolding pervading the universe. In doing so, CGM plays a critical role in galaxy evolution by controlling the inflow of gas into the galaxy and outflow from it. By measuring the amount of highly ionized oxygen — oxygen with five of its electrons stripped off — contained in CGM, astronomers estimate the mass of CGM. “The mass … is extremely important to learn how galaxies form,” notes Dr. Sarkar. Oxygen is one of the most abundant elements in the universe, and the particular ionized oxygen astronomers study interacts more with light, making it easy to follow its trail.

Creating shadows

Observational astronomers use light from incredibly bright cores of distant galaxies to map the CGM. When light from such a background object passes through the gas in the CGM of a foreground galaxy, certain elements absorb particular wavelengths.

Imagine driving a car at night. If a cow were in the middle of the road, one wouldn’t notice it until it’s near and the car’s headlight shines on it. But now imagine a car coming from the opposite side and the cow being between the two vehicles. No matter how far the cow is, one can distinguish it because of the silhouette the cow creates in front of the other car’s light. This is the exact mechanism by which astronomers detect ionized oxygen. By absorbing specific wavelengths, oxygen creates shadows or gaps in the light coming from a distant bright object.

But there lies an inherent problem in the observational technique. “When astronomers perform an observation, the measured ionized oxygen is the total integrated value along the line of sight,” says Dr. Sarkar. And as CGM and IGM both lie along the line of sight, there’s no way to tell apart the contribution from CGM and IGM in the observed values. Current models take all of the ionized oxygen observed to be from the CGM.

Fig 2. Artistic representation of the presence of ionised oxygen surrounding galaxies, and the principle of their observation

Contamination alert!

Now, new research from RRI uses models to suggest that a lot of the gas attributed to CGM could be coming from IGM. “We’re challenging the notion [that the entire ionized oxygen belongs to CGM],” says Dr. Sarkar. The team used a mathematical description of CGM and the gas falling into it from the IGM. They then calculated the amount of ionized oxygen each of them contains and compared it with the observations. “We’re suggesting that a  relatively small fraction of the ionized oxygen is coming from CGM and that there’s a blanket of IGM around CGM which is contributing to the observed oxygen,” says Dr. Sarkar. This contamination of CGM by IGM could lead to overestimating CGM’s mass.

Fig 3. Artistic representation of the presence of ionised oxygen in the intergalactic medium in the case of low-mass galaxies

Dr. Sarkar and colleagues got their first hint of something being amiss when they noticed that the models for CGM mass disagree with the observations for low-mass galaxies. Their current theory of IGM confounding CGM measurements applies to galaxies across masses and can help explain the discrepancy observed in lower-mass galaxies. "For high mass galaxies like our Milky Way, the CGM may contribute just 50% of the ionised oxygen, with the rest coming from the IGM. For lower mass galaxies, it can go down to 30%," says Dr. Sarkar. The study highlights the need to consider contributions from IGM when interpreting CGM observations.

The researchers are working towards upgrading their basic model into a more realistic and comprehensive one encompassing more of the parameters involved. “We’re sure there’s a discrepancy. Now we’re trying to quantify this discrepancy exactly,” says Dr. Sarkar.

Reference:

Bromberg, I., Sarkar, K.C., Ashkenazy, H., Gnat, O. and Birnboim, Y. (2025)
‘Intergalactic absorption confounding circumgalactic observations’

The Astrophysical Journal, 987(2), article 131

Available at: https://doi.org/10.3847/1538-4357/add3f8
 

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Article by Unnati Ashar

Illustrations by Arunima V

Video by Rahul Iyer