Indian Scientists Reveal How Supermassive Black Holes Shape The Fate Of Galaxies And The Birth Of Stars

In the vast stretches of the Universe, where galaxies shimmer with billions of stars, some appear mysteriously quiet. A new study led by astronomers at the Indian Institute of Astrophysics (IIA) suggests that supermassive black holes play a crucial role in shaping the fate of their host galaxies. The researchers found that powerful outflows of gas and radiation from these central black holes can suppress the formation of new stars, thereby influencing how galaxies grow and evolve.

Active Galactic Nuclei: The energetic hearts of galaxies

Active galactic nuclei (AGN), the energetic cores of galaxies powered by matter falling onto supermassive black holes, emit enormous amounts of radiation and, in some cases, high-speed jets of charged particles and plasma. At the centre of a galaxy sits a supermassive black hole, often weighing about 10¹⁰ times the mass of our Sun (one solar mass equals 2 × 10³³ grams). Being extremely gravitational, it pulls in matter from its surrounding host galaxy through a process known as accretion.

As gas flows from the outer regions toward the centre, the black hole becomes active, releasing intense radiation and, in a few cases, powerful jets. These emissions play a crucial role in regulating star formation within the galaxy, a key factor in shaping its evolution over cosmic time. Published in the Astrophysical Journal, the study reveals that both radiation and jets work as a formidable team to push gas out from galactic centres—gas that would otherwise fuel the birth of new stars.

In an exclusive interview with ETV Bharat, Payel Nandi, the lead author of the study and PhD student at IIA, along with Professor CS Stalin, co-author of the study, shared detailed insights into their research.

Speaking about her inspiration behind this study, Payel Nandi said, “My PhD aimed to understand how black holes grow along with their host galaxies. Initially, we studied individual sources showing such phenomena, but soon realised that there was no statistical study identifying what actually triggers the outflows. While some studies linked them to radiation and others to jets, there were very few comprehensive analyses. That motivated us to create a pure sample of AGN using high-resolution data.”

Star formation in galaxies

Nandi added, “We found that outflows of warm ionised gas are widespread in AGN. Radiation from the black hole is the main driver, but galaxies with radio jets exhibit faster and more energetic outflows.”

Professor Stalin said, “We believe that this process injects energy into the interstellar medium of the host galaxy, regulating star formation—either boosting or suppressing it. One of the key drivers is outflows, which are streams of gas expelled from the vicinity of supermassive black holes at the centre of galaxies. What remained unclear was whether radiation or jets play the dominant role in regulating star formation. The study reveals that radiation primarily drives these outflows, while jets, present in only about 10 per cent of AGN, create additional effects beyond those caused by radiation. This will also have a major impact on future follow-up studies.”

A schematic illustrating how AGN with both jets and radiation drive stronger outflows compared to AGN powered solely by radiation, resulting in differing effects on their host galaxies. (Credit: Payel Nandi)

The team also found that, due to outflows in these galaxies, star formation is almost completely shut down in the central regions. When asked whether the rest of the galaxy continues to form stars or if this suppression could spread outward over time, Nandi said, “We haven’t explored that much. Our work was confined to identifying the driver of the outflow and studying the central <1 kiloparsec (kpc) region. For reference, a typical galaxy spans several tens of kpc.”

By examining the stellar populations and infrared colour diagnostics of these galaxies, the team confirmed that it is black hole activity—not new star formation—that drives these winds. The result points to a process known as negative AGN feedback, where the energy released by the black hole suppresses star formation in its surroundings.

Galaxies with radio jets experience stronger black hole-driven winds

In response to how radiation and radio jets work together rather than independently, Professor Stalin explained, “AGN emits radiation across the spectrum—from low-energy radio waves to high-energy gamma rays. Only about 10 per cent show strong radio emission in the form of large-scale relativistic jets. We divided our sample into radio-quiet AGN without jets and radio-loud AGN with jets. Comparing their outflow properties, we found that sources with jets had much stronger outflow kinematics. This shows that while radiation is the main driver, jets amplify the effect. We carefully selected our targets and control sample to ensure accuracy.”

The study revealed that outflows are more than twice as common in galaxies with radio emission (56 per cent) compared to those without (25 per cent). These gas streams, moving at speeds up to 2,000 km per second, can even escape the galaxy’s gravity, stripping it of the gas needed for new star formation.

Black hole luminosity correlates with outflow energy, enhanced by jets

The researchers also found a clear link between the outflow energy and the black hole’s luminosity—a relationship that was even stronger in galaxies with radio jets. They concluded that jets don’t start the process but act as accelerators, boosting the impact of radiation and driving more gas out.

A schematic representation illustrating how AGN (shown within the blue dotted box) interact with their host galaxies through outflowing gas, influencing them via positive feedback (indicated by the yellowish cloud) and negative feedback (indicated by the bluish cloud). The blue dotted box encloses the AGN components, including radiation from the accretion disk and jets, which together drive the outflowing gas. Note: The orientation of the jets and outflowing gas is illustrative only and does not indicate a specific angular relationship. (Credit: Payel Nandi, Thesis)

The discussion focused on how this relationship could help model galaxy evolution. Professor Stalin said that higher jet power increases the likelihood of outflows, allowing astronomers to study more powerful systems. By examining galaxies across a range of jet powers, researchers can probe different mass and distance scales, effectively screening the entire galaxy population and gaining a complete picture of their evolution.

Analysing 500 galaxies across optical and radio wavelengths to study AGN outflows

The team analysed more than 500 nearby galaxies using archival data from two of the world’s premier observatories—the Sloan Digital Sky Survey (SDSS), which observes the Universe in optical light, and the Very Large Array (VLA), which studies the sky in radio wavelengths. “This study underscores how essential it is to combine data across multiple wavelengths to capture the full picture of galaxy evolution,” says C S Stalin.

Speaking about the biggest challenges of working with large multi-wavelength datasets, Nandi said, “The most difficult part was analysing and detecting the outflows, which took the longest time. Outflows are usually studied through spectra, particularly the oxygen line at 5007 angstroms. Extracting science information from these spectra is challenging as it depends on factors like signal-to-noise ratio, instrument resolution, and other effects. Automated Python codes or pipelines can sometimes alter the statistics, so we used multiple fitting methods and also verified the fits and residuals manually.”

Mapping the hidden gas around black holes: The next steps in galaxy research

Discussing the next steps, Professor Stalin explained, “We began with large-scale observations using the Ultraviolet Imaging Telescope (UVIT) built by IIA, in collaboration with other institutes aboard ISRO's AstroSat (launched by ISRO in 2015), which provided preliminary insights. Next, we aim to use high-resolution spectroscopy from international facilities to study the central gas—its ionisation state and whether it is cold or warm. This involves combining data from the James Webb Space Telescope for infrared observations, radio data to trace jets, the Atacama Large Millimetre/submillimeter Array (ALMA) for molecular gas, and optical observations for ionised and atomic gas. Integrating these multi-wavelength datasets will allow us to map all gas components comprehensively.”

Nandi added that while such multi-telescope mapping offers a detailed and complete view, it is time-consuming and costly, often limiting studies to one or two galaxies rather than large samples. Stalin concluded that with rapid advancements and increasing availability of new instruments—along with many in development—the future of astronomy is highly promising, even as competition grows more challenging.