RRI Study Finds Intergalactic Gas May Distort Galaxy Halo Mass Measurements

The CGM acts as a critical interface between the galaxy and the cosmic web — the filamentary network of matter that forms the universe’s large-scale structure.

A groundbreaking study from the Raman Research Institute (RRI), an autonomous institute under the Department of Science and Technology (DST), has revealed that matter from the intergalactic medium (IGM) may be contaminating measurements of the diffuse gaseous envelope surrounding galaxies — known as the circumgalactic medium (CGM). The finding has major implications for how scientists estimate galaxy mass and understand galaxy evolution.

Why the CGM Matters in Galaxy Formation

Galaxies are typically imagined as swirling disks of stars, dust and spirals. Yet, far beyond their visible boundaries lies a vast, ghostly halo stretching up to 10–20 times the radius of the galaxy. This halo, made primarily of dark matter and gas, contains most of a galaxy’s mass.

The CGM acts as a critical interface between the galaxy and the cosmic web — the filamentary network of matter that forms the universe’s large-scale structure. By regulating the flow of gas in and out of galaxies, the CGM plays a central role in:

• Star formation

• Chemical enrichment

• The growth and evolution of galaxies

Astronomers estimate the mass of the CGM by measuring the amount of highly ionized oxygen (O VI) — oxygen stripped of five electrons — present around galaxies.

The Measurement Problem: CGM or IGM?

Observational astronomers rely on bright background galaxies and quasars as light sources. When this light passes through the halo of a foreground galaxy, elements such as oxygen absorb specific wavelengths, revealing their presence.

But this technique has a limitation:The oxygen measured along the line of sight includes contributions from both the CGM and the IGM — and current models assume it all comes from the CGM.

Dr. Kartick Sarkar, RRI astrophysicist and lead author of the study, explained the issue using an analogy:

“Imagine a street magician gathering a crowd. People rush in until they reach the boundary of the crowd and stop. Here, the magician is the galaxy, the crowd is the CGM, and the region beyond is the IGM.”

The study suggests that a significant share of the ionized oxygen previously attributed to the CGM may actually be coming from the surrounding IGM.

New Models Reveal Overestimation of CGM Mass

Using mathematical models of both the CGM and the inflowing IGM, the RRI team calculated how much O VI each region should contain. Their findings challenge long-standing assumptions:

• In Milky Way–like galaxies, only ~50% of the observed O VI may come from the CGM.

• In low-mass galaxies, the CGM contribution may fall to ~30%.

• The remaining fraction likely originates in the IGM – a “blanket” of gas surrounding the halo.

This contamination of CGM measurements by IGM matter may explain long-standing discrepancies between theoretical models and observations — especially in low-mass galaxies, where disagreements have been particularly stark.

A New Direction for Galaxy Evolution Research

The research marks a significant shift in the interpretation of CGM data. If CGM mass has been consistently overestimated, current theories about galaxy formation and evolution may need revision.

Dr. Sarkar noted that the team is now working with collaborators at the Hebrew University of Jerusalem to expand their model into a more comprehensive framework that includes additional physical parameters.

“We’re sure there’s a discrepancy,” he said. “Now we’re trying to quantify it exactly.”

Implications for Future Observations

The results call for:

• Greater caution when interpreting CGM mass based solely on O VI measurements

• New observational strategies to disentangle CGM and IGM contributions

• Updated theoretical models of how galaxies acquire and expel gas

The study, published in The Astrophysical Journal, opens the door to more accurate mapping of the gaseous halos that shape the lives of galaxies.