Astronomers discover a swarm of galaxies orbiting a hyperluminous galaxy

Using the Very Large Telescope and the ALMA radio telescope in Chile, a team of astronomers including researchers from the Niels Bohr Institute have discovered a swarm of galaxies orbiting a hyperluminous, vigorously stellar galaxy in the universe primitive. The observation provides important clues to how unusually bright galaxies grow and how they evolve into energetic quasars, emitting light across most of the observable universe.

A fundamental problem in astronomy is the question of how galaxies form, grow and evolve.

As part of their evolution, most galaxies seem to favor a supermassive black hole at their center. These gravitational monsters occasionally swallow gas and nearby stars, spewing excess energy in powerful jets, a phenomenon known as quasaring.

From galaxy to quasar

Many details about the transition from “normal” galaxies to quasars are still unknown. But in a new study published in Nature Communicationa team of astronomers led by Michele Ginolfi at ESO, Garching may have taken a step closer to understanding this evolution.

“Before evolving into a full-fledged quasar, some galaxies are thought to go through a very dusty and very ‘active’ phase in terms of star formation and accretion of gas onto their central supermassive black holes,” explains Ginolfi. “We set out to design an experiment to learn more about this transition phase.”

Ginolfi and his collaborators focused on an already known galaxy, W0410-0913, one of the brightest, most massive and gas-rich galaxies in the distant universe, seen 12 billion years back.

The dust is heated by energy from starlight and the central black hole, causing it to glow and divulge the galaxy through its infrared light. This has led to such galaxies being called dust-obscured hot galaxies (also colloquially known as “hot dogs”).

3D galaxies

Because the evolution of galaxies is intrinsically linked to their environment, Ginolfi and his team – whose nucleus was somewhat atypically made up of early-career researchers – decided to observe W0410-0913 with the “MUSE” instrument. at the Very Large Telescope (VLT) in Chile. This advanced tool allowed them to study a region 40 times larger than the galaxy itself.

Peter Laursen of the Cosmic Dawn Center in Copenhagen participated in the study. He explains: “Observations revealed that W0410-0913 is surrounded by a swarm of no less than 24 smaller galaxies. The cool thing about the MUSE instrument is that we can measure not only their position in the sky, but also their distance along our planet. line of sight. In other words, we can measure their 3D positions.”

Although this implies that W0410-0913 resides in a region at least ten times denser than the average universe, this is not entirely unexpected, as hot dogs are indeed believed to live in dense environments.

A galactic car accident

Moreover, while W0410-0913 is seen at a time when the universe was 1/8 of its current age, it is already ten times more massive than our own galaxy, the Milky Way. Growing such a large galaxy in such a short time and feeding a supermassive black hole requires a substantial supply of fresh matter. All of this fits well with the conventional picture that massive galaxies grow by accreting gaseous and satellite galaxies, pulled from intergalactic space by their immense gravity.

In fact, in such a dense environment, the rate of galaxy interactions and mergers should be very high. Exposed to such a bombardment, astronomers expected W0410-0913 to be a car wreck composed of chaotically swirling clumps of gas and stars.

However, by digging into old observations obtained by the ALMA radio antennas located just 300 km northeast of the VLT, Ginolfi and his colleagues were able to measure the internal movement of gas inside W0410-0913.

And here a completely different picture emerged.

Throw stones at a window

Surprisingly, ALMA observations revealed that W0410-0913 does not appear to have been disturbed by interactions with companion galaxies at all. According to observations, this gas rotates well and in an orderly fashion around the central black hole. Orderly, but surprisingly fast, with speeds reaching 500 km/s.

“By coupling the results from the two very different telescopes, we see a picture of how the most massive and dusty galaxies can evolve. These type of galaxies, a vital step in the transition from a dusty, star-forming galaxy to a quasar, tends to grow in very dense environments,” says Ginolfi. “Nevertheless, despite the expected frequent mergers with other galaxies, these gravitational interactions are not necessarily destructive – they feed the central galaxy and swirl the gas, but leave it virtually untouched. A bit like throwing small pebbles against a solid pane of glass. glass: you can scratch it, but you won’t break it…”

Michele Ginolfi’s observations offer the first clues to the multiscale process driving the evolution of the rare and extreme population of hot, dust-obscured galaxies. They grow in dense, special habitats, but interaction with their companions can be gentle.

Like a parable of that galactic car crash, the study nearly didn’t go to work at all, when Michele Ginolfi got stuck in a traffic jam in Rome, having to submit the proposal using his phone from his car , a few minutes before the deadline.