This image shows Neptune observed with the MUSE instrument at ESO’s Very Large Telescope (VLT). Credit: ESO/P. Irwin et al.
There’s no getting around it: our Solar System’s gas giants all have big, conspicuous spots on their faces. These include Jupiter’s Great Red Spot, Saturn’s Great White Spot, Uranus’ Great Dark Spot, and Neptune’s Great Dark Spot. Far from blemishes or features that tarnish the planets’ natural beauty, these “spots” are caused by massive storms or other processes in the planets’ atmospheres. While they are extremely large by Earth standards, they are difficult to study by anything other than robotic probes that can get close to the planet.
Neptune’s Great Dark Spot was not discovered until NASA’s Voyager 2probe flew past the planet in 1989 on its way to the edge of the Solar System. Decades later, scientists are still unsure how this storm originated or what mechanisms drive it today. Using the ESO’s Very Large Telescope (VLT), a team of astronomers was able to observe the Great Dark Spot for the first time using a ground-based telescope. Their results provided the most detailed data on the spot to date and some interesting insights into the nature and origin of this mysterious feature.
This image of the planet Neptune was obtained during the testing of the Narrow-Field adaptive optics mode of the MUSE/GALACSI instrument on ESO’s Very Large Telescope. The corrected image is sharper than a comparable image from the NASA/ESA Hubble Space Telescope. Credit: ESO/P. Weilbacher (AIP)
In 2007, the European Southern Observatory (ESO) completed work on the Very Large Telescope (VLT) at the Paranal Observatory in northern Chile. This ground-based telescope is the world’s most advanced optical instrument, consisting of four Unit Telescopes with main mirrors (measuring 8.2 meters in diameter) and four movable 1.8-meter diameter Auxiliary Telescopes.
Recently, the VLT was upgraded with a new instrument known as the Multi Unit Spectroscopic Explorer (MUSE), a panoramic integral-field spectrograph that works at visible wavelengths. Thanks to the new adaptive optics mode that this allows for (known as laser tomography) the VLT was able to recently acquire some images of Neptune, star clusters and other astronomical objects with impeccable clarity.
In astronomy, adaptive optics refers to a technique where instruments are able to compensate for the blurring effect caused by Earth’s atmosphere, which is a serious issue when it comes to ground-based telescopes. Basically, as light passes through our atmosphere, it becomes distorted and causes distant objects to become blurred (which is why stars appear to twinkle when seen with the naked eye).
Images of the planet Neptune obtained during the testing of the Narrow-Field adaptive optics mode of the MUSE/GALACSI instrument on ESO’s Very Large Telescope. Credit: ESO/P. Weilbacher (AIP
One solution to this problem is to deploy telescopes into space, where atmospheric disturbance is not an issue. Another is to rely on advanced technology that can artificially correct for the distortions, thus resulting in much clearer images. One such technology is the MUSE instrument, which works with an adaptive optics unit called a GALACSI – a subsystem of the Adaptive Optics Facility (AOF).
The instrument allows for two adaptive optics modes – the Wide Field Mode and the Narrow Field Mode. Whereas the former corrects for the effects of atmospheric turbulence up to one km above the telescope over a comparatively wide field of view, the Narrow Field mode uses laser tomography to correct for almost all of the atmospheric turbulence above the telescope to create much sharper images, but over a smaller region of the sky.
This consists of four lasers that are fixed to the fourth Unit Telescope (UT4) beaming intense orange light into the sky, simulating sodium atoms high in the atmosphere and creating artificial “Laser Guide Stars”. Light from these artificial stars is then used to determine the turbulence in the atmosphere and calculate corrections, which are then sent to the deformable secondary mirror of the UT4 to correct for the distorted light.
Using this Narrow Field Mode, the VLT was able to capture remarkably sharp test images of the planet Neptune, distant star clusters (such as the globular star cluster NGC 6388), and other objects. In so doing, the VLT demonstrated that its UT4 mirror is able to reach the theoretical limit of image sharpness and is no longer limited by the effects of atmospheric distortion.
The most powerful laser guide star system in the world sees first light at the Paranal Observatory. Credit: ESO
This essentially means that it is now possible for the VLT to capture images from the ground that are sharper than those taken by the Hubble Space Telescope. The results from UT4 will also help engineers to make similar adaptations to the ESO’s Extremely Large Telescope (ELT), which will also rely on laser tomography to conduct its surveys and accomplish its scientific goals.
These goals include the study of supermassive black holes (SMBHs) at the centers of distant galaxies, jets from young stars, globular clusters, supernovae, the planets and moons of the Solar System, and extra-solar planets. In short, the use of adaptive optics – as tested and confirmed by the VLT’s MUSE – will allow astronomers to use ground-based telescopes to study the properties of astronomical objects in much greater detail than ever before.
In addition, other adaptive optics systems will benefit from work with the Adaptive Optics Facility (AOF) in the coming years. These include the ESO’s GRAAL, a ground layer adaptive optics module that is already being used by the Hawk-I infrared wide-field imager. In a few years, the powerful Enhanced Resolution Imager and Spectrograph (ERIS) instrument will also be added to the VLT.
Between these upgrades and the deployment of next-generation space telescopes in the coming years (like the James Webb Space Telescope, which will be deploying in 2021), astronomers expect to bringing a great deal more of the Universe “into focus”. And what they see is sure to help resolve some long-standing mysteries, and will probably create a whole lot more!
And be sure to enjoy these videos of the images obtained by the VLT of Neptune and NGC 6388, courtesy of the ESO:
Artist's impression of the star cluster NGC 3201 orbiting an black hole with about four times the mass of the Sun. Credit: ESO/L. Calçada
Astronomers have been fascinated with globular clusters ever since they were first observed in 17th century. These spherical collections of stars are among the oldest known stellar systems in the Universe, dating back to the early Universe when galaxies were just beginning to grow and evolve. Such clusters orbit the centers of most galaxies, with over 150 known to belong to the Milky Way alone.
One of these clusters is known as NGC 3201, a cluster located about 16,300 light years away in the southern constellation of Vela. Using the ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile, a team of astronomers recently studied this cluster and noticed something very interesting. According to the study they released, this cluster appears to have a black hole embedded in it.
For the sake of their study, the team relied on the Multi Unit Spectroscopic Explorer (MUSE) instrument on the VLT to observe NGC 3201. This instrument is unique because of the way it allows astronomers to measure the motions of thousands of far away stars simultaneously. In the course of their observations, the team found that one of the cluster’s stars was being flung around at speeds of several hundred kilometers an hour and with a period of 167 days.
“It was orbiting something that was completely invisible, which had a mass more than four times the Sun — this could only be a black hole! The first one found in a globular cluster by directly observing its gravitational pull.”
This finding was rather unexpected, and constitutes the first time that astronomers have been able to detect an inactive black hole at the heart of a globular cluster – meaning that it is not currently accreting matter or surrounded by a glowing disc of gas. They were also able to estimate the black hole’s mass by measuring the movements of the star around it and thus extrapolating its enormous gravitational pull.
From its observed properties, the team determined that the rapidly-moving star is about 0.8 times the mass of our Sun and the mass of its black hole counterpart to be around 4.36 times the Sun’s mass. This put’s it in the “stellar-mass black hole” category, which are stars that exceeds the maximum mass allowance of a neutron star, but are smaller than supermassive black holes (SMBHs) – which exist at the centers of most galaxies.
This finding is highly significant, and not just because it was the first time that astronomers have observed a stellar-mass black hole in a globular cluster. In addition, it confirms what scientists have been suspecting for a few years now, thanks to recent radio and x-ray studies of globular clusters and the detection of gravity wave signals. Basically, it indicates that black holes are more common in globular clusters than previously thought.
“Until recently, it was assumed that almost all black holes would disappear from globular clusters after a short time and that systems like this should not even exist!” said Giesers. “But clearly this is not the case – our discovery is the first direct detection of the gravitational effects of a stellar-mass black hole in a globular cluster. This finding helps in understanding the formation of globular clusters and the evolution of black holes and binary systems – vital in the context of understanding gravitational wave sources.”
This find was also significant given that the relationship between black holes and globular clusters remains a mysterious, but highly important one. Due to their high masses, compact volumes, and great ages, astronomers believe that clusters have produced a large number of stellar-mass black holes over the course of the Universe’s history. This discovery could therefore tell us much about the formation of globular clusters, black holes, and the origins of gravitational wave events.
And be sure to enjoy this ESO podcast explaining the recent discovery:
Images from the Hubble Ultra Deep Field (HUDF). Credit: NASA/ESA/S. Beckwith (STScI)/HUDF Team
Since its deployment in 1990, the Hubble Space Telescope has given us some of the richest and most detailed images of our Universe. Many of these images were taken while observing a patch of sky located in the Fornax constellation between September 2003 and January 2004. This region, known as the Hubble Ultra Deep Field (HUDF), contains an estimated 10,000 galaxies, all of which existed roughly 13 billion years ago.
Looking to this region of space, multiple teams of astronomers used the MUSE instrument on the ESO’s Very Large Telescope (VLT) to discover 72 previously unseen galaxies. In a series of ten recently released studies, these teams indicate how they measured the distance and properties of 1600 very faint galaxies in the Ultra Deep Field, revealing new information about star formation and the motions of galaxies in the early Universe.
The original HUDF images, which were published in 2004, were a major milestone for astronomy and cosmology. The thousands of galaxies it observed were dated to less than just a billion years after the Big Bang, ranging from 400 to 800 million years of age. This area was subsequently observed many times using the Hubble and other telescopes, which has resulted in the deepest views of the Universe to date.
One such telescope is the European Southern Observatory‘s (ESO) Very Large Telescope, located in the Paranal Observatory in Chile. Intrinsic to the studies of the HUDF was the Multi Unit Spectroscopic Explorer (MUSE), a panoramic integral-field spectrograph operating in the visible wavelength range. It was the data accumulated by this instrument that allowed for 72 new galaxies to be discovered from this tiny area of sky.
The MUSE HUDF Survey team, which was led by Roland Bacon of the Centre de recherche astrophysique de Lyon (CRAL) and the National Center for Scientific Research (CNRS), included members from multiple European observatories, research institutes and universities. Together, they produced ten studies detailing the precise spectroscopic measurements they conducted of 1600 HUDF galaxies.
This was an unprecedented accomplishment, given that this is ten times as many galaxies that have had similar measurements performed on them in the last decade using ground-based telescopes. As Bacon indicated in an ESO press release:
“MUSE can do something that Hubble can’t — it splits up the light from every point in the image into its component colors to create a spectrum. This allows us to measure the distance, colors and other properties of all the galaxies we can see — including some that are invisible to Hubble itself.”
The galaxies detected in this survey were also 100 times fainter than any galaxies studied in previous surveys. Given their age and their very dim and distant nature, the study of these 1600 galaxies is sure to add to any already very richly-observed field. This,in turn, can only deepen our understanding of how galaxies formed and evolved during the past 13 billions years.
The 72 newly-discovered galaxies that the survey observed are known as Lyman-alpha emitters, a class of galaxy that is extremely distant and only detectable in Lyman-alpha light. This form of radiation is emitted by excited hydrogen atoms, and is thought to be the result of ongoing star formation. Our current understanding of star formation cannot fully explain these galaxies, and they were not visible in the original Hubble images.
Thanks to MUSE’s ability to disperse light into its component colors, these galaxies became more apparent. As Jarle Brinchmann – an astronomer at the University of Leiden and the University of Porto’s (CAUP) Institute of Astrophysics and Space Sciences, and the lead author of one of the papers – described the results of the survey:
“MUSE has the unique ability to extract information about some of the earliest galaxies in the Universe — even in a part of the sky that is already very well studied. We learn things about these galaxies that is only possible with spectroscopy, such as chemical content and internal motions — not galaxy by galaxy but all at once for all the galaxies!”
Another major finding of this survey was the systematic detection of luminous hydrogen halos around galaxies in the early Universe. This finding is expected to give astronomers a new and promising way to study how material flowed in and out of early galaxies, which was central to early star formation and galactic evolution. The series of studies produced by Bacon and his colleagues also indicate a range of other possibilities.
These include studying the role faint galaxies played during cosmic reionization, the period that took place between 150 million to billion years after the Big Bang. It was during this period, which followed the “dark ages” (380 thousand to 150 million years ago) that the first stars and quasars formed and sent ionizing radiation throughout the early Universe. And as Roland Bacon explained, the best may yet be to come:
“Remarkably, these data were all taken without the use of MUSE’s recent Adaptive Optics Facility upgrade. The activation of the AOF after a decade of intensive work by ESO’s astronomers and engineers promises yet more revolutionary data in the future.”
Even before Einstein proposed his groundbreaking Theory of General Relativity – which established that space and time are inextricably linked – scientists have understood that probing deeper into the cosmic field is to also probe farther back in time. The farther we are able to see, the more we are able to learn about how the Universe evolved over the course of billions of years.
