Death by black hole
Jan Scholtz, Francesco D’Eugenio, Roberto Maiolino
Cavendish astronomers have spotted one of the oldest ‘dead’ galaxies and found that a growing supermassive black hole can slowly starve a galaxy of the raw material needed for star formation. Their discovery challenges our understanding of galaxy evolution.
Black holes capture everyone’s attention, often being portrayed as cosmic villains that swallow everything in their path. However, most of the time, black holes are inactive and isolated, rarely interacting with their surroundings. One special class are so-called supermassive black holes, which are found at the centres of most galaxies in the local universe.
These giants have masses ranging from millions to billions of times that of the Sun, accounting for between 0.1% and 0.5% of the total mass of their host galaxy. At the centre of our own Milky Way sits a black hole with a mass of four million suns, a discovery that won Andrea Ghez and Reinhard Genzel the Nobel Prize in Physics in 2020.
Evidence that growing supermassive black holes influence their galaxies is a tantalising puzzle
As these black holes grow, they release enormous amounts of energy across the electromagnetic spectrum – from radio waves to gamma rays – equivalent to ten times the gravitational potential energy of the galaxy. This energy either heats up the gas within the galaxy or, through radiation pressure, drives powerful winds blasting at speeds of up to a few thousand kilometres per second. This process is predicted to remove gas from the galaxy, which is the fuel for forming new stars. Yet each growth episode only lasts up to 10 million years at a time.
Growing supermassive black holes – also known as Active Galactic Nuclei, or AGN – are key ingredients in theoretical simulations of galaxy formation, ensuring that galaxies do not grow too massive through injection of energy into a galaxy’s gas supply or its removal through outflows. The entire process of stopping formation of new stars in galaxy is often referred to as galaxy quenching. However, despite years of research, astronomers have yet to find direct evidence of this effect.
Probing ‘dead’ Pablo’s Galaxy with cutting-edge instruments
The launch of the James Webb Space Telescope (JWST) has made this problem of quenching galaxies even more challenging. We are now discovering even more ‘dead’ galaxies – those that are no longer forming new stars – that existed only a few billion years after the Big Bang. This suggests galaxies need far less time to grow to their final size and die than previously thought.
One such galaxy, called GS-10578 but nicknamed ‘Pablo’s Galaxy’ after the astronomer who first observed it in detail, is unusually massive for such an early period in the universe. It has a mass of about 200 billion solar masses, and most of its stars formed between 12.5 and 11.5 billion years ago. Pablo’s Galaxy appears to have lived fast and died young; it stopped forming new stars despite its relatively young age due to an almost total absence of the cold gas that stars need to form.
We are now discovering even more ‘dead’ galaxies – those that are no longer forming new stars – that existed only a few billion years after the Big Bang. This suggests galaxies need far less time to grow to their final size and die than previously thought.
Based on previous work also published in Nature Astronomy, the galaxy appears to be a calm, rotating disk. This indicates that it did not experience any major disruptive mergers with other galaxies. Since we can rule out a traumatic past for Pablo’s Galaxy, its lack of fresh gas must be due to the galaxy’s own evolution.
We used data from the James Webb Space Telescope and the Atacama Large Millimeter Array (ALMA) to study this galaxy in the early universe, roughly three billion years after the Big Bang, or ten billion years ago.
The imagery from the Near Infrared Camera (NIRCam) and the spectrum from the Near Infrared Spectrograph (NIRSpec) provide excellent insight into the galaxy. An image of Pablo’s galaxy and its spectrum can be seen below.
JWST/NIRCam image and NIRSpec spectrum of Pablo’s galaxy. The NIRSpec data provide us with excellent insight into the nature of the galaxy.
JWST/NIRCam image and NIRSpec spectrum of Pablo’s galaxy. The NIRSpec data provide us with excellent insight into the nature of the galaxy.
From this data, we can learn that the galaxy hosts an actively growing black hole, old stars and fast winds. Furthermore, the data provided precise measurements of its star formation history – effectively mapping out exactly when the galaxy formed its stars – while the ALMA observations measured the galaxy’s gas supply.
Starvation effect observed from supermassive black hole activity
A careful combination of the ALMA and JWST data revealed that the galaxy’s black hole is the main culprit behind this missing gas. Despite undertaking the longest ALMA observations ever conducted on this kind of distant, dead galaxy – spanning over 7 hours using over 50 antennae – we found no evidence for carbon monoxide, a key indicator for the cold molecular hydrogen required for star formation. (It is extremely challenging to detect cold molecular hydrogen at temperatures below -240°C, so we often target other molecules, such as carbon monoxide which commonly occurs in molecular hydrogen clouds but is much easier to detect.)
Rather than blowing away all its gas in a single cataclysmic event, the galaxy suffered death by a thousand cuts.
By reconstructing the galaxy’s star-formation history and gas content, our team concluded that the galaxy evolved with a ‘net-zero inflow’, meaning fresh gas never refilled its reservoir from the intergalactic medium. Rather than blowing away all its gas in a single cataclysmic event, the galaxy suffered death by a thousand cuts. The black hole repeatedly heated or expelled incoming material over multiple cycles, preventing the galaxy from replenishing itself with fresh gas and slowly strangling star formation. This breakthrough result, reported in the journal Nature Astronomy (the second Nature Astronomy article on this galaxy by the Cavendish team), explains why observing the direct impact of black hole growth on galaxy evolution has traditionally been so challenging. Each black hole growth episode does very little damage, but cumulatively, they are catastrophic to the galaxy.
Additionally, JWST spectroscopy revealed powerful winds of neutral gas streaming out from the galaxy’s supermassive black hole at 400 kilometres per second, stripping away 60 solar masses of gas every year. These numbers suggest the galaxy’s remaining fuel will be depleted in as little as 16 to 220 million years – far faster than the billion-year timescale typical for similar galaxies.
Comparison of Gas fraction (gas mass/stellar mass) against redshift.
Comparison of Gas fraction (gas mass/stellar mass) against redshift.
The phenomena may be more common than previously thought
Although Pablo’s Galaxy stopped forming stars 400 million years ago, its black hole can currently be observed in a new active growth episode. This means the current black hole activity and the observed gas outbursts did not cause the initial shutdown; instead, repeated past episodes likely kept the fuel from returning – a process known as the ’maintenance feedback mode’.
The findings help explain a growing population of massive, surprisingly old-looking galaxies observed by Webb in the early Universe. Before Webb, these were unheard of. Now we know they’re more common than we thought – and this starvation effect may be why they live fast and die young.
Looking ahead, the Cambridge team has been awarded an additional 6.5 hours of JWST observation time using the Mid-Infrared Instrument (MIRI) to observe Pablo’s galaxy. These new observations will target warmer hydrogen gas, revealing more about the exact mechanisms through which its supermassive black hole halts star formation. Concurrently, the team is expanding a number of target sources with similar high-quality ALMA and JWST observations, which will soon reveal just how common this starvation phenomenon is across the universe.
Jan Scholtz is a postdoctoral researcher at the Kavli Institute for Cosmology.
Francesco D’Eugenio is a postdoctoral researcher in the Extragalactic Research Group at the Kavli Institute for Cosmology.
Roberto Maiolino is Professor of Experimental Astrophysics at the Cavendish Laboratory and the Kavli Institute for Cosmology.