Powerful radio jets from the black hole – which normally suppress star formation – are stimulating the production of cold gas in the galaxy's extended halo of hot gas. This newly identified supply of cold, dense gas could eventually fuel future star birth as well as feed the black hole itself. The researchers used ALMA to study a galaxy at the heart of the Phoenix Cluster, an uncommonly crowded collection of galaxies about 5.7 billion light-years from Earth.
The central galaxy in this cluster harbours a supermassive black hole that is in the process of devouring star-forming gas, which fuels a pair of powerful jets that erupt from the black hole in opposite directions into intergalactic space. Astronomers refer to this type of black-hole powered system as an active galactic nucleus (AGN).
Earlier research with NASA’s Chandra X-ray observatory revealed that the jets from this AGN are carving out a pair of giant 'radio bubbles', huge cavities in the hot, diffuse plasma that surrounds the galaxy. Previously, astronomers believed that this region would be too hot for the gas to cool and condense, preventing it from fuelling future star birth or feeding the super-massive black hole.
The latest ALMA observations, however, reveal long filaments of cold molecular gas condensing around the outer edges of the radio bubbles. These filaments extend up to eighty-two thousand light-years from either side of the AGN. They collectively contain enough material to make about 10 billion suns.
"With ALMA we can see that there's a direct link between these radio bubbles inflated by the supermassive black hole and the future fuel for galaxy growth," says Dr Helen Russell, an astronomer with the University of Cambridge's Institute of Astronomy (UK), and lead author on a paper appearing in the Astrophysical Journal. "This gives us new insights into how a black hole can regulate future star birth and how a galaxy can acquire additional material to fuel an active black hole."
Artist's impression of the galaxy at the centre of the Phoenix Cluster. Powerful radio jets from the super-massive black hole are creating giant radio bubbles (blue) in the ionized gas surrounding the galaxy. Credit: B Saxton
The new ALMA observations reveal previously unknown connections between an AGN and the abundance of cold molecular gas that fuels star birth.
"To produce powerful jets, black holes must feed on the same material that the galaxy uses to make new stars," says Michael McDonald, an astrophysicist at the Massachusetts Institute of Technology in Cambridge (USA) and co-author on the paper. "This material powers the jets that disrupt the region and quenches star formation. This illustrates how black holes can slow the growth of their host galaxies."
Without a significant source of heat, the most massive galaxies in the universe would be forming stars at extreme rates that far exceed observations. Astronomers believe that the heat, in the form of radiation and jets, from an actively feeding supermassive black hole prevents overcooling of the cluster's hot gas atmosphere, suppressing star formation. This story, however, now appears more complex. In the Phoenix Cluster, Russell and her team found an additional process that ties the galaxy and its black hole together. The radio jets that heat the core of the cluster's hot atmosphere also appear to stimulate the production of the cold gas required to sustain the AGN.
"That's what makes this result so surprising," says Brian McNamara, an astronomer at the University of Waterloo, Ontario, and co-author on the paper. "This supermassive black hole is regulating the growth of the galaxy by blowing bubbles and heating the gases around it. Remarkably, it’s also cooling enough gas to feed itself.”
This result helps astronomers understand the workings of the cosmic 'thermostat' that controls the launching of radio jets from the supermassive black hole.
"This could also explain how the most massive black holes were able to both suppress run-away starbursts and regulate the growth of their host galaxies over the past six billion years or so of cosmic history," notes Russell.
Helen Russell et al. ALMA Observations of Massive Molecular Gas Filaments Encasing Radio Bubbles in the Phoenix Cluster The Astrophysical Journal DOI: 10.3847/1538-4357/836/1/130
Press release courtesy of National Radio Astronomy Observatory (NRAO)
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered a surprising connection between a supermassive black hole and the galaxy where it resides.This gives us new insights into how a black hole can regulate future star birth Helen RussellALMA
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On Feb. 9-20, NASA’s OSIRIS-REx spacecraft will activate its onboard camera suite and commence a search for elusive “Trojan” asteroids. Trojans are asteroids that are constant companions to planets in our solar system as they orbit the sun, remaining near a stable point 60 degrees in front of or behind the planet.News Article Type: Homepage ArticlesPublished: Thursday, February 9, 2017 - 08:01
Astronomy: Intermediate-mass black hole found
Nature 542, 7640 (2017). doi:10.1038/542175a
Authors: Kayhan Gültekin
The existence of medium-sized black holes has long been debated. Such an object has now been discovered in the centre of a dense cluster of stars, potentially enhancing our understanding of all black holes. See Letter p.203
An intermediate-mass black hole in the centre of the globular cluster 47 Tucanae
Nature 542, 7640 (2017). doi:10.1038/nature21361
Authors: Bülent Kızıltan, Holger Baumgardt & Abraham Loeb
Intermediate-mass black holes should help us to understand the evolutionary connection between stellar-mass and super-massive black holes. However, the existence of intermediate-mass black holes is still uncertain, and their formation process is therefore unknown. It has long been suspected that black holes with masses 100 to 10,000 times that of the Sun should form and reside in dense stellar systems. Therefore, dedicated observational campaigns have targeted globular clusters for many decades, searching for signatures of these elusive objects. All candidate signatures appear radio-dim and do not have the X-ray to radio flux ratios required for accreting black holes. Based on the lack of an electromagnetic counterpart, upper limits of 2,060 and 470 solar masses have been placed on the mass of a putative black hole in 47 Tucanae (NGC 104) from radio and X-ray observations, respectively. Here we show there is evidence for a central black hole in 47 Tucanae with a mass of solar masses when the dynamical state of the globular cluster is probed with pulsars. The existence of an intermediate-mass black hole in the centre of one of the densest clusters with no detectable electromagnetic counterpart suggests that the black hole is not accreting at a sufficient rate to make it electromagnetically bright and therefore, contrary to expectations, is gas-starved. This intermediate-mass black hole might be a member of an electromagnetically invisible population of black holes that grow into supermassive black holes in galaxies.
Astroparticle physics: Dark matter remains elusive
Nature 542, 7640 (2017). doi:10.1038/542172a
Authors: Xiangdong Ji
WIMPs, or weakly interacting massive particles, are the leading candidates for dark matter, the 'missing' mass in the Universe. An experiment has obtained no evidence for such particles, despite an impressive increase in sensitivity.
LIGO’s underdog cousin ready to enhance gravitational-wave hunt
Nature 542, 7640 (2017). http://www.nature.com/doifinder/10.1038/542146a
Author: Davide Castelvecchi
It missed the historic discovery, but the Virgo lab in Italy is now primed to extend LIGO’s reach and precision.
Astronomy: A leisurely way to visit the stars
Nature 542, 7640 (2017). doi:10.1038/542140c
Plans to explore the nearest star system rely on light sails — reflective panels that are propelled by light. These craft travel so fast that they will have little time to explore their destination, but altering the way the sails are used could help.An
For the past 15 years, scientists have been eagerly anticipating the data from Gaia. The first portion of information from the satellite was released three months ago and is freely accessible to everyone. This dataset of unprecedented quality is a catalogue of the positions and brightness of a billion stars in our Milky Way galaxy and its environs.
What Gaia has sent to Earth is unique. The satellite’s angular resolution is similar to that of the Hubble Space Telescope, but given its greater field of view, it can cover the entire sky rather than a small portion of it. In fact, Gaia uses the largest number of pixels to take digital images of the sky for any space-borne instrument. Better still, the Observatory has not just one telescope but two, sharing the one metre wide focal plane.
Unlike typical telescopes, Gaia does not just point and stare: it constantly spins around its axis, sweeping the entire sky in less than a month. Therefore, it not only measures the instantaneous properties of the stars, but also tracks their changes over time. This provides a perfect opportunity for finding a variety of objects, for example stars that pulsate or explode - even if this is not what the satellite was primarily designed for.
The Cambridge team concentrated on the area around the Magellanic Clouds and used the Gaia data to pick out pulsating stars of a particular type: the so-called RR Lyrae, very old and chemically un-evolved. As these stars have been around since the earliest days of the Clouds’ existence, they offer an insight into the pair’s history. Studying the Large and Small Magellanic Clouds (LMC and SMC respectively) has always been difficult as they sprawl out over a large area. But with Gaia’s all-sky view, this has become a much easier task.
Around the Milky Way, the clouds are the brightest, and largest, examples of dwarf satellite galaxies. Known to humanity since the dawn of history (and to Europeans since their first voyages to the Southern hemisphere) the Magellanic Clouds have remained an enigma to date. Even though the clouds have been a constant fixture of the heavens, astronomers have only recently had the chance to study them in any detail.
The Magellanic Clouds can be seen just above the horizon and below the arc of the Milky Way - D Erkal
Whether the clouds fit the conventional theory of galaxy formation or not depends critically on their mass and the time of their first approach to the Milky Way. The researchers at Cambridge’s Institute of Astronomy found clues that could help answer both of these questions.
Firstly, the RR Lyrae stars detected by Gaia were used to trace the extent of the Large Magellanic Cloud. The LMC was found to possess a fuzzy low-luminosity ‘halo’ stretching as far as 20 degrees from its centre. The LMC would only be able to hold on to the stars at such large distances if it was substantially bigger than previously thought, totalling perhaps as much as a tenth of the mass of the entire Milky Way.
An accurate timing of the clouds’ arrival to the galaxy is impossible without knowledge of their orbits. Unfortunately, satellite orbits are difficult to measure: at large distances, the object’s motion in the sky is so minute that it is simply unobservable over a human lifespan. In the absence of an orbit, Dr Vasily Belokurov and colleagues found the next best thing: a stellar stream.
Streams of stars form when a satellite - a dwarf galaxy or a star cluster - starts to feel the tidal force of the body around which it orbits. The tides stretch the satellite in two directions: towards and away from the host. As a result, on the periphery of the satellite, two openings form: small regions where the gravitational pull of the satellite is balanced by the pull of the host. Satellite stars that enter these regions find it easy to leave the satellite altogether and start orbiting the host. Slowly, star after star abandons the satellite, leaving a luminous trace on the sky, and thus revealing the satellite’s orbit.
“Stellar streams around the Clouds were predicted but never observed,” explains Dr Belokurov. “Having marked the locations of the Gaia RR Lyrae on the sky, we were surprised to see a narrow bridge-like structure connecting the two clouds. We believe that at least in part this ‘bridge’ is composed of stars stripped from the Small Cloud by the Large. The rest may actually be the LMC stars pulled from it by the Milky Way.”
The researchers believe the RR Lyrae bridge will help to clarify the history of the interaction between the clouds and our galaxy.
"We have compared the shape and the exact position of the Gaia stellar bridge to the computer simulations of the Magellanic Clouds as they approach the Milky Way”, explains Dr Denis Erkal, a co-author of the study. "Many of the stars in the bridge appear to have been removed from the SMC in the most recent interaction, some 200 million years ago, when the dwarf galaxies passed relatively close by each other. “We believe that as a result of that fly-by, not only the stars but also hydrogen gas was removed from the SMC. By measuring the offset between the RR Lyrae and hydrogen bridges, we can put constraints on the density of the gaseous Galactic corona.”
Composed of ionised gas at very low density, the hot Galactic corona is notoriously difficult to study. Nevertheless, it has been the subject of intense scrutiny because scientists believe it may contain most of the missing baryonic - or ordinary - matter. Astronomers are trying to estimate where this missing matter (the atoms and ions that make up stars, planets, dust and gas) is. It’s thought that most, or even all, of these missing baryons are in the corona. By measuring the coronal density at large distances they hope to solve this conundrum.
During the previous encounter between the Small and Large Magellanic Cloud, both stars and gas were ripped out of the Small Cloud, forming a tidal stream. Initially, the gas and stars were moving at the same speed. However, as the Clouds approached our Galaxy, the Milky Way’s corona exerted a drag force on both of them. The stars, being relatively small and dense, punched through the corona with no change in their speed. However, the more tenuous neutral hydrogen gas slowed down substantially in the corona. By comparing the current location of the stars and the gas, taking into account the density of the gas and how long the Clouds have spent in the corona, the team estimated the density of the corona. Dr. Erkal concludes, “Our estimate showed that the corona could make up a significant fraction of the missing baryons, in agreement with previous independent techniques. With the missing baryon problem seemingly alleviated, the current model of galaxy formation is holding up well to the increased scrutiny possible with Gaia.”
Vasily Belokurov et al. “Clouds, Streams and Bridges. Redrawing the blueprint of the Magellanic System with Gaia DR1”. Monthly Notices of the Royal Astronomical Society; 8th Feb. 2017; DOI:10.1093/mnras/stw3357
The Magellanic Clouds, the two largest satellite galaxies of the Milky Way, appear to be connected by a bridge stretching across 43,000 light years, according to an international team of astronomers led by researchers from the University of Cambridge. The discovery is reported in the journal Monthly Notices of the Royal Astronomical Society (MNRAS) and is based on the Galactic stellar census being conducted by the European Space Observatory, Gaia.We believe that at least in part this 'bridge' is composed of stars stripped from the Small Cloud by the LargeVasily BelokurovV Belokurov, D Erkal, A MellingerPale white veils and the narrow bridge between the clouds represent the distribution of the RR Lyrae stars
The text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.