Tag Archives: globular cluster

Some ancient Globular Clusters may not be ancient at all

Young globular clusters (ages up to 1 billion years) are indicated by the blue dots. These globular clusters are closely associated with a filamentary network of cool gas, coloured orange to white, which extends to the outer reaches of the giant galaxy at the centre of the Perseus galaxy cluster. Round or oval objects, also coloured orange to white, are smaller galaxies that are part of the same galaxy cluster. (@The University of Hong Kong)

Astronomers believe that globular clusters — found around giant galaxies in the centre of galactic clusters — are ancient relics remaining from the earliest formative stages of galaxies. But, despite this well-founded belief, the physical origins of these clusters — most common around elliptical galaxies — remains something of a mystery. 

New research conducted by Dr Jeremy Lim and his Research Assistant, Miss Emily Wong, at the Department of Physics of The University of Hong Kong (HKU) have used data collected by the Hubble Space Telescope in order to find a surprising answer to this cosmic conundrum. 

Dr Lim’s team discovered that globular clusters around the giant galaxy at the centre of the Perseus galaxy cluster are not all ancient objects. Whilst most globular clusters are believed by scientists to have formed shortly after the Universe began 13.8 billion years ago, a few thousand of the clusters studied by the team seem to have formed over at least the past 1 billion years. Even more, could have possibly formed later in cosmic history the research suggests.

These younger globular clusters seem to be associated with a complex filamentary network of cool gas which extends to the outer reaches of this giant galaxy. This seems to suggest that these clusters were born in this same network. This is significant as this cool gas is thought to have been deposited by the hot gas that infuses the entire Perseus galaxy cluster. The density of this hot gas and thus the rate at which it cools rises in the direction of the galactic cluster’s centre. 

After formation, the newly born galactic clusters are no longer bound to the network of cool gas and begin to fall inwards onto the giant galaxies. This can be considered almost analogous to raindrops condensing in clouds and falling to the ground. 

Old globular clusters (ages up to 10 billion years or more) are indicated by the red dots. These globular clusters are randomly distributed around the giant galaxy at the centre of the Perseus galaxy cluster; this galaxy is the large grey to white oval at the centre of the picture. Round or oval objects, also coloured grey to white, are smaller galaxies that are part of the same galaxy cluster. ( @The University of Hong Kong)

This inward gathering of younger globular clusters after formation in a network of cool gas is in stark contrast to the formation and dispersion of more ancient globular clusters. 

These older clusters form from gas compressed in the spiral arms of galaxies or from dense gas at the centre of galaxy clusters. After their formation, the random dispersion of older clusters across the giant galaxy is a result of them scattering off each other during the course of their orbit around this galaxy. 

Solving lingering puzzles regarding globular clusters

Globular clusters can contain anywhere from hundreds of thousands to several million stars — all of which are born at the same time. These stars are packed incredibly densely, with the clusters having spherical volumes thousands of times smaller than the diameter of our galaxy — the milky way. 

One puzzling aspect of these clusters has been the sheer numbers at which they exist — and how they could have formed at the same point in cosmic history. By showing that some of these clusters form later than others and fall into place — this new research may have solved that puzzle.

Another puzzling aspect of these global clusters is the broad range of colours they display around giant galaxies. Again, this could be a result of the clusters have different respective ages. Globular clusters likely change from blue to red as they age. This is a result of more massive stars burning through their fuel more quickly as nucleosynthesis progresses more quickly in larger stars. As these stars are bluer than smaller stars, as they die it leaves the cluster to take a redder hue. Thus, a broad range of ages would result in a broad range of colours — which is indeed what astronomers observe. 

The team’s research does indicate that despite forming at different times, both older and younger globular clusters in the Perseus galaxy share a common formation mechanism. Irrespective of age, the globular clusters span a broad range of masses — with fewer at the larger mass end of the spectrum. This similar mass trend suggests a common formation mechanism for star clusters across the mass scale regardless of the environment in which they formed. 

This sustained formation of globular clusters over a long range of time could also explain the enormous size of giant galaxies — which can be in excess of ten times that of the Milky Way. As more massive globular clusters within these galaxies endure, their more diminutive counterparts could be ripped apart during their orbits. This leaves the stars which form these smaller globular clusters to be spread through the giant galaxies contributing to their growth in size. 


Original research: ‘Sustained Formation of Progenitor Globular Clusters in a Giant Elliptical Galaxy’ by Jeremy Lim, Emily Wong, Youichi Ohyama, Tom Broadhurst & Elinor Medezinski in Nature Astronomy. 

Scientists just found the first relic galaxy — it’s remained unchanged since the early universe

According to previous calculations by astronomers, one in every thousand massive galaxies is a relic of the early universe. It’s an almost esoteric term that describes a galaxy whose properties are still largely the same as when it formed billions of years ago. Now, scientists say they’ve finally identified one for the first time.

Researchers at the Instituto de Astrofísica de Canarias (IAC) and the University of La Laguna (ULL) used ground-based telescopes to study NGC 1277, a galaxy located 225 million light-years away, in the heart of the Perseus Cluster — the largest concentration of galaxies close to the Milky Way. 

Their initial observations suggested something odd about the globular clusters surrounding NGC 1277. Globular clusters are spherical collections of ancient stars, formed at the same time as the galaxy, that orbit the galactic core as a satellite. Scientists class globular clusters in two distinct groups: red globular clusters, which are formed in massive galaxies nearer to their centers, and blue ones, which are found more toward the outskirts of the galaxy. Red globular clusters have a higher concentration of heavier elements (hence the color), while blue ones have a lower fraction of metals.

When the researchers pointed the Hubble Space Telescope towards the globular clusters surrounding NGC 1277, they struck gold. The observation revealed that the galaxy only has only red globular clusters, which have remained unchanged since they were formed many millions of years ago along with the galaxy.

“Globular cluster systems are very sensitive to the history of galaxy formation” explained Michael Beasley, the first author of the article and a researcher at Instituto de Astrofísica de Canarias (IAC), who said that “this is the first time a galaxy so massive has been observed with so few blue globular clusters.”

NGC 1277 is composed of a thousand billion stars, and its relatively close proximity to Earth provided an excellent opportunity to find a rare wonder of the cosmos. “The galaxy NGC 1277 gives us a unique opportunity to study a “primitive” galaxy in the “local” universe” adds Ignacio Trujillo, another of the article’s authors.

According to the researchers, when the galaxy formed, it used to birth stars at a rate of 1,000 per year, whilst the Milky Way is currently forming only one star per year. The team believes that the reason why the galaxy has remained unchanged during all this time, retaining its original form and composition, is because it formed as a satellite to the central galaxy of the Perseus cluster. Since the cluster absorbed most of the material that could have fallen into NGC 1277, this dynamic has caused the relic galaxy to evolve differently.

“We argue that the colour distribution of the cluster system of NGC 1277 indicates that the galaxy has undergone little (if any) mass accretion after its initial collapse, and use simulations of possible merger histories to show that the stellar mass due to accretion is probably at most ten percent of the total stellar mass of the galaxy. These results confirm that NGC 1277 is a genuine relic galaxy and demonstrate that blue clusters constitute an accreted population in present-day massive galaxies,” the authors wrote in the journal Nature.

The authors hope to find more relic galaxies in the future using the Hubble Space Telescope and its successor, the James Webb Space Telescope.

Scientific reference: BEASLEY, Michael A. et al. “A single population of red globular clusters around the massive compact galaxy NGC 1277”, Nature. DOI: 10.1038/nature25756

Hubble takes brilliant picture of young star population in elderly company

The great pics from Hubble just never end! This time, the brave telescope offered an impressive view of the center of globular cluster NGC 6362. The image of this spherical collection of stars takes a deeper look at the core of the globular cluster, which contains a high concentration of stars with different colors.

Click the pic for full size.

Seeing what appears to be young stars came as quite a surprise, considering that globular clusters are composed of old stars, which, at around 10 billion years old, are way older than the Sun. These clusters are quite common both in our galaxy (over 150 found so far) and in other galaxies. Also, globular clusters are among the oldest objects directly observable in the known Universe, making them living fossils, extremely useful in understanding how galaxies work.

The accepted theory at the moment is that all stars in a globular cluster are about the same age; however, new, high precision measurements performed in numerous globular clusters, primarily with the Hubble Space Telescope have made some astrophysicists doubt this theory. In particular, there appear to be younger, bluer stars, amidst older ones. Researchers dubbed them blue stragglers and NGC 6362 has lots of them.

It’s unclear at the moment how they appear, but since they are usually found in the core regions of clusters, where the concentration of stars is large, the most plausible explanation seems to be that they form as a result of stellar collisions or transfer of material between stars in binary systems.

NGC 6362 is located about 25 000 light-years from Earth in the constellation of Ara (The Altar), and it was discovered all the way back in 1826 by British astronomer James Dunlop.

An artist's conception of a black hole in globular cluster. (c) NRAO/AUI/NSF

Black hole pair in star cluster defy scientific expectations

Black holes are simply the worst neighbors to have around, as they wreck havoc in their vicinity. Scientific belief states, however, that in a globular cluster, which is a massive spherical conglomeration of thousands of thousands of stars, you can’t have more than one black hole, if any. New findings however show that there’s no safe neighborhood, as data suggests black holes may reside in pairs in star clusters.

“Before this work, there were zero black holes known in Milky Way globular clusters, so even finding one would have been exciting,” said lead study author Jay Strader, an astronomer at Michigan State University in East Lansing.

An artist's conception of a black hole in globular cluster. (c) NRAO/AUI/NSF

An artist’s conception of a black hole in globular cluster. (c) NRAO/AUI/NSF

Initially, the researchers used the recently upgraded Very Large Array (VLA), a network of radio dishes near Socorro, N.M., to look for an intermediate-mass black hole at the center of M22, a globular cluster 10,000 light years away. Now, although they didn’t find the central black hole they were after in the first place, they stumbled across something a whole lot more valuable. Intermediate-mass black holes have a mass of around a few thousand times that of our sun. Supermassive blackholes, commonly found at the center of galaxies, weigh millions to billions times the mass of our sun. In M22 the astronomers found a pair of even lighter black holes that form from the collapse of massive stars – each are only 10 to 20 times the mass of the sun.

These black holes are both binary systems, each possessing a companion star from which they are ripping matter from like a vampire. Matter from the stars, like gas and dust, is drained and collected by the black hole, much like water swirling down a drain.

“One of the most interesting aspects of this work is that we found the black holes via radio emission,” Strader said. “All the other stellar-mass black holes in our galaxy have been discovered by X-ray emission rather than radio. We hypothesize that the reason our sources haven’t been seen in previous X-ray searches is that they aren’t accreting very much matter at all, so they don’t produce the hot accretion disks that glow in the optical and X-rays.

Theories say that that there may be few or no black holes in a globular cluster, as gravitational interactions between black holes in the cluster would eject almost all the black holes in short order. The two black holes, named M22-VLA1 and M22-VLA2, provide counter evidence to these theories. Strader and colleagues estimate that there could be as many as 100 low-mass black holes in the globular cluster.

“Future computer simulations of the evolution of globular clusters with populations of black holes should help address this issue,” Strader said.

He added, “My personal view is that it’s likely that other clusters also have black holes that we just haven’t found yet.”

The astronomers from the U.S., England and Australia announced their discovery in a study published in the October 4 issue of Nature.