Yet another interesting report from the European Southern Observatory (ESO):

Starving Black Hole Returns Brilliant Galaxy to the Shadows

The mystery of a rare change in the behaviour of a supermassive black hole at the centre of a distant galaxy has been solved by an international team of astronomers using ESO’s Very Large Telescope along with the NASA/ESA Hubble Space Telescope and NASA’s Chandra X-ray Observatory. It seems that the black hole has fallen on hard times and is no longer being fed enough fuel to make its surroundings shine.

Many galaxies are found to have an extremely bright core powered by a supermassive black hole. These cores make “active galaxies” some of the brightest objects in the Universe. They are thought to shine so brightly because hot material is glowing fiercely as it falls into the black hole, a process known as accretion. This brilliant light can vary hugely between different active galaxies, so astronomers classify them into several types based on the properties of the light they emit [1].

This sequence takes the viewer deep into the rather faint constellation of Cetus (The Sea Monster). In the final stages the faint active galaxy Markarian 1018 is seen, in a recent image from the MUSE instrument on ESO’s Very Large Telescope. Credit: ESO/A. Fujii/Digitized Sky Survey 2/CARS survey

Some of these galaxies have been observed to change dramatically over the course of only 10 years; a blink of an eye in astronomical terms. However, the active galaxy in this new study, Markarian 1018 stands out by having changed type a second time, reverting back to its initial classification within the last five years. A handful of galaxies have been observed to make this full-cycle change, but never before has one been studied in such detail.

The discovery of Markarian 1018’s fickle nature was a chance by-product of the Close AGN Reference Survey (CARS), a collaborative project between ESO and other organisations to gather information on 40 nearby galaxies with active cores. Routine observations of Markarian 1018 with the Multi-Unit Spectroscopic Explorer (MUSE) installed on ESO’s Very Large Telescope revealed the surprising change in the light output of the galaxy.

“We were stunned to see such a rare and dramatic change in Markarian 1018”,

said Rebecca McElroy, lead author of the discovery paper and a PhD student at the University of Sydney and the ARC Centre of Excellence for All Sky Astrophysics (CAASTRO).

The chance observation of the galaxy so soon after it began to fade was an unexpected opportunity to learn what makes these galaxies tick, as Bernd Husemann, CARS project leader and lead author of one of two papers associated with the discovery, explained:

“We were lucky that we detected the event just 3-4 years after the decline started so we could begin monitoring campaigns to study details of the accretion physics of active galaxies that cannot be studied otherwise.”

The research team made the most of this opportunity, making it their first priority to pinpoint the process causing Markarian 1018’s brightness to change so wildly. This could have been caused by any one of a number of astrophysical events, but they could rule out the black hole pulling in and consuming a single star [2] and cast doubt on the possibility of obscuration by intervening gas [3]. But the true mechanism responsible for Markarian 1018’s surprising variation remained a mystery after the first round of observations.

However, the team were able to gather extra data after they were awarded observing time to use the NASA/ESA Hubble Space Telescope, and NASA’s Chandra X-ray Observatory. With the new data from this suite of instruments they were able to solve the mystery — the black hole was slowly fading because it was being starved of accretion material.

“It’s possible that this starvation is because the inflow of fuel is being disrupted”, said Rebecca McElroy. “An intriguing possibility is that this could be due to interactions with a second supermassive black hole”.

Such a black hole binary system is a distinct possibility in Markarian 1018, as the galaxy is the product of a major merger of two galaxies — each of which likely contained a supermassive black hole in its centre.

Research continues into the mechanisms at work in active galaxies such as Markarian 1018 that change their appearance.

“The team had to work fast to determine what was causing Markarian 1018’s return to the shadows,” comments Bernd Husemann. “Ongoing monitoring campaigns with ESO telescopes and other facilities will allow us to explore the exciting world of starving black holes and changing active galaxies in more detail.”

Notes
[1] The brightest of the active galaxies are quasars, where the brilliant nucleus outshines the rest of the galaxy. Another, less extreme, class are known as Seyfert galaxies. Originally a method was developed that used brightness and the emission spectrum — the plot of the strength of radiation emitted at different wavelength — to distinguish between just two types of Seyfert galaxies, Type 1 and Type 2, but extra classifications such as Type 1.9 Seyferts have since been introduced.

[2] Such a tidal disruption event occurs when a star strays too close to a supermassive black hole and is torn apart by the extreme gravitational tidal force. This results in a sharp rise in the brightness of the central region that slowly declines over a period of years. The observed brightness variations of Markarian 1018 were found not to match the profile of such an event.

[3] Gas obscuration can affect the classification of an active galaxy by blocking the line of sight, drifting in front of the galaxy’s bright core like fog in front of a car’s headlights, and dimming the light passing through. This also affects the spectrum of galaxy, perhaps changing its classification.

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A report from a team using the Hubble telescope:

Astronomers observe star reborn in a flash 

Over a period of 30 years dramatic increases in the temperature of the star SAO 244567 have been observed. Now the star is cooling again, having been reborn into an earlier phase of stellar evolution. This makes it the first reborn star to have been observed during both the heating and cooling stages of rebirth.

This animation shows the fast evolution of SAO 244567. The animation starts 10 300 BC, with the star having a radius 152 times the size of the Sun and a surface temperature of about 3500 degree Celsius, giving it its orange colour. At this point of its life the star had already lost half of its initial mass.

After 10 000 years the star slowly shrinks to only 40 times the size of the Sun; at the same time its temperature rises to 6800 degree Celsius, causing its colour to change to white-yellow. As the star heats up to about 20 000 degree Celsius Helium fusion inside the star suddenly gets re-ignited — the late thermal pulse.

After the flash the star heats quickly and becomes a blue-white star with a temperature of 21 000 degree Celsius, only 4 times larger than the Sun. SAO 244567 shrinks further till it only a third the size of the Sun and a temperature of 60 000 degree Celsius; this status was reached in the year 2002. Now new observations show that the star is still blue and hot — with about 50 000 degree Celsius — but started to expand again: its size is about two third of our Sun.

Within the next few hundreds of years SAO 244567 will expand back to its giant dimensions and also change its colour to orange — as shown at the end of the animation. Credit: ESA/Hubble, L. Calçada

Even though the Universe is constantly changing, most processes are too slow to be observed within a human lifespan. But now an international team of astronomers have observed an exception to this rule.

“SAO 244567 is one of the rare examples of a star that allows us to witness stellar evolution in real time”, explains Nicole Reindl from the University of Leicester, UK, lead author of the study. “Over only twenty years the star has doubled its temperature and it was possible to watch the star ionising its previously ejected envelope, which is now known as the Stingray Nebula.”

SAO 244567, 2700 light-years from Earth, is the central star of the Stingray Nebula and has been visibly evolving between observations made over the last 45 years. Between 1971 and 2002 the surface temperature of the star skyrocketed by almost 40 000 degrees Celsius. Now new observations made with the Cosmic Origins Spectrograph (COS) on the NASA/ESA Hubble Space Telescope have revealed that SAO 244567 has started to cool and expand.

This is unusual, though not unheard-of [1], and the rapid heating could easily be explained if one assumed that SAO 244567 had an initial mass of 3 to 4 times the mass of the Sun. However, the data show that SAO 244567 must have had an original mass similar to that of our Sun. Such low-mass stars usually evolve on much longer timescales, so the rapid heating has been a mystery for decades.

Back in 2014 Reindl and her team proposed a theory that resolved the issue of both SAO 244567’s rapid increase in temperature as well as the low mass of the star. They suggested that the heating was due to what is known as a helium-shell flash event: a brief ignition of helium outside the stellar core [2].

This theory has very clear implications for SAO 244567’s future: if it has indeed experienced such a flash, then this would force the central star to begin to expand and cool again — it would return back to the previous phase of its evolution. This is exactly what the new observations confirmed. As Reindl explains:

“The release of nuclear energy by the flash forces the already very compact star to expand back to giant dimensions — the born-again scenario.”

It is not the only example of such a star, but it is the first time ever that a star has been observed during both the heating and cooling stages of such a transformation.

Yet no current stellar evolutionary models can fully explain SAO 244567’s behaviour. As Reindl elaborates:

“We need refined calculations to explain some still mysterious details in the behaviour of SAO 244567. These could not only help us to better understand the star itself but could also provide a deeper insight in the evolution of central stars of planetary nebulae.”

Until astronomers develop more refined models for the life cycles of stars, aspects of SAO 244567’s evolution will remain a mystery.

Notes

[1] The other star thought to have experienced the same type of helium flash event (see[2]) is FG Sagittae, located in the constellation Sagitta, making SAO 244567 the second of its kind. However, other objects undergoing similar “born-again” scenarios are known, including Sakurai’s Object, located in Sagittarius.

[2] Helium flash events, also known as late thermal pulses, occur late in the evolution of about 25% of low- to medium-mass stars. After evolving off the main sequence, these stars enter the red giant phase, where the star expands dramatically. Various changes occur in the star’s chemical and physical composition during this phase, until it has burnt most of the helium available in its core, which is by then composed of carbon and oxygen. Helium fusion continues in a thin shell around the core, but then turns off as the helium becomes depleted. This allows hydrogen fusion to start in a layer above the helium layer. After enough additional helium accumulates, helium fusion is reignited, leading to a thermal pulse which eventually causes the star to expand, cool and brighten temporarily.

A team of astronomers using the Hubble space telescope release a new finding:

Hubble discovers rare fossil relic of early Milky Way 

A fossilised remnant of the early Milky Way harbouring stars of hugely different ages has been revealed by an international team of astronomers. This stellar system resembles a globular cluster, but is like no other cluster known. It contains stars remarkably similar to the most ancient stars in the Milky Way and bridges the gap in understanding between our galaxy’s past and its present.

Terzan 5, 19 000 light-years from Earth, has been classified as a globular cluster for the forty-odd years since its detection. Now, an Italian-led team of astronomers have discovered that Terzan 5 is like no other globular cluster known.

The team scoured data from the Advanced Camera for Surveys and the Wide Field Camera 3 on board Hubble, as well as from a suite of other ground-based telescopes [1]. They found compelling evidence that there are two distinct kinds of stars in Terzan 5 which not only differ in the elements they contain, but have an age-gap of roughly 7 billion years [2].

This sequence takes the viewer from a wide view of the Milky Way to the central regions, where many bright star forming regions and star clusters can be seen. The final view is a close-up of the sky around the star cluster Terzan 5 taken with Hubble, the Very Large Telescope at ESO’s Paranal Observatory and the Keck Telescope. Credit: Nick Risinger (skysurvey.org)/DSS/Hubble. Music: Johan B. Monell

The ages of the two populations indicate that the star formation process in Terzan 5 was not continuous, but was dominated by two distinct bursts of star formation.

“This requires the Terzan 5 ancestor to have large amounts of gas for a second generation of stars and to be quite massive. At least 100 million times the mass of the Sun,” explains Davide Massari, co-author of the study, from INAF, Italy, and the University of Gröningen, Netherlands.

Its unusual properties make Terzan 5 the ideal candidate for a living fossil from the early days of the Milky Way. Current theories on galaxy formation assume that vast clumps of gas and stars interacted to form the primordial bulge of the Milky Way, merging and dissolving in the process.

“We think that some remnants of these gaseous clumps could remain relatively undisrupted and keep existing embedded within the galaxy,” explains Francesco Ferraro from the University of Bologna, Italy, and lead author of the study. “Such galactic fossils allow astronomers to reconstruct an important piece of the history of our Milky Way.”

While the properties of Terzan 5 are uncommon for a globular cluster, they are very similar to the stellar population which can be found in the galactic bulge, the tightly packed central region of the Milky Way. These similarities could make Terzan 5 a fossilised relic of galaxy formation, representing one of the earliest building blocks of the Milky Way.

This assumption is strengthened by the original mass of Terzan 5 necessary to create two stellar populations: a mass similar to the huge clumps which are assumed to have formed the bulge during galaxy assembly around 12 billion years ago. Somehow Terzan 5 has managed to survive being disrupted for billions of years, and has been preserved as a remnant of the distant past of the Milky Way.

“Some characteristics of Terzan 5 resemble those detected in the giant clumps we see in star-forming galaxies at high-redshift, suggesting that similar assembling processes occurred in the local and in the distant Universe at the epoch of galaxy formation,“ continues Ferraro.

Hence, this discovery paves the way for a better and more complete understanding of galaxy assembly.

“Terzan 5 could represent an intriguing link between the local and the distant Universe, a surviving witness of the Galactic bulge assembly process,” explains Ferraro while commenting on the importance of the discovery.

The research presents a possible route for astronomers to unravel the mysteries of galaxy formation, and offers an unrivaled view into the complicated history of the Milky Way.

Notes

[1] The researchers also used data from the Multi-conjugate Adaptive Optics Demonstrator at ESO’s Very Large Telescope and the Near Infrared Camera 2 at the W. M. Keck Observatory.

[2] The two detected stellar populations have ages of 12 billion years and 4.5 billion years respectively.

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