From ESOblog: “Disentangling starlight”

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From ESOblog


Although they look like fuzzy patches of light, distant galaxies are actually made up of billions of stars and other astronomical intricacies. Telescopes are rarely powerful enough to study the individual stars in galaxies except for those closest to the Milky Way, but a team of scientists has now used the MUSE instrument on ESO’s Very Large Telescope to resolve the stars in the spiral galaxy NGC 300.

ESO MUSE on the VLT on Yepun (UT4)

By telling the story of how astronomy has reached this point, team member Martin M. Roth from the Leibniz Institute for Astrophysics Potsdam helps us understand why this result is so exciting.

Four hundred years ago, Galileo Galilei became the first person to point a telescope at the sky and prove that the hazy band of the Milky Way is actually composed of billions of individual stars. Astronomy has come a long way since then, and nowadays astronomers do not merely look at the stars, but also analyse their chemical composition, measure their rotation and velocity in space, and determine many other physical parameters to find out more about the Universe — all using a technique called spectroscopy, which is the study of the interaction of matter and light.

Stellar spectroscopy really started taking speed with the emergence of a technique called integral field spectroscopy, around the same time that I joined Leibniz Institute for Astrophysics Potsdam (AIP) as a young astronomer in the early 1990s. This technique allows astronomers to obtain a 3D view of a galaxy in just one shot. It uses an Integral Field Unit (IFU) to divide the field of view into many segments — or pixels — to obtain a more comprehensive overview of the whole. The signal from each pixel is fed into a spectrograph which generates a light spectrum for each one. The pixels in this case are rather lovingly named “spaxels”.

Even all those years ago it occurred to me that such a device could be used to disentangle the stars in crowded fields, such as in star clusters and distant galaxies, where the light from stars blends together to become a blurry blob. So by 1996, our team at Potsdam had begun to develop our own integral field spectrograph. We called it PMAS — the Potsdam Multi-Aperture Spectrophotometer.

How integral field spectroscopy works. Credit: ESO

Several research groups were developing integral field spectrographs at the same time, but the main drawback to all of them was the number of spaxels. PMAS, for example, hosted a mere 256 of them — compare this to your phone camera, which probably has something like 10–15 million pixels. This all changed dramatically with the arrival of MUSE, the Multi Unit Spectroscopic Explorer, on ESO’s Very Large Telescope (VLT). MUSE hosts an incredible 90 000 spaxels and boasts superb sensitivity.

The primary raison d’etre of MUSE is to study the origin and development of the Universe as a whole, but when ESO invited proposals for MUSE pilot studies almost five years ago, I applied to use the new instrument to try to resolve stars in the nearby spiral galaxy NGC 300. This had already been done for very nearby galaxies in what is called the Local Group but never for galaxies further afield.

Thankfully, my proposal to observe NGC 300 was chosen as one of the MUSE pilot studies, and we were given observing time! At a distance of six million light-years from the Milky Way, NGC 300 is just outside the Local Group and is what I would describe as a very “typical” spiral galaxy; finding out more about it should help us find out more about how spiral galaxies work in general.

The intricate network of pipes surrounding the 24 spectrographs of the MUSE instrument on the VLT. The instruments complexity is equaled by its power and productivity. Credit: A. Tudorica/ESO

But it wasn’t enough just to observe the galaxy using MUSE, it was also necessary to develop some software that could help us visually separate the stars in the MUSE data. A talented doctoral student within our research group created a novel tool to do this. To test the tool, we used our old PMAS spectrograph on a telescope at the Calar Alto Observatory in Spain to measure the speed of some stars in Milky Way star clusters. The tool worked perfectly!

Calar Alto Observatory located in Almería province in Spain on Calar Alto, a 2,168-meter-high (7,113 ft) mountain in Sierra de Los Filabres

We then tried out the tool with MUSE images of a cluster of Milky Way stars before the real test — would it work on NGC 300, a galaxy 800 times further away than this star cluster?

The results turned out to be better than we could have ever imagined! We could see individual stars with incredible clarity and gaseous regions, such as supernova remnants, planetary nebulae, and ionised hydrogen regions were revealed. Amazingly, we could even see dim background galaxies through NGC 300! MUSE is special because it can look at light with a wide range of wavelengths, making many different objects and colours visible.

This picture of the spectacular southern spiral galaxy NGC 300 was taken using the Wide Field Imager (WFI) at ESO’s La Silla Observatory in Chile.

ESO WFI LaSilla 2.2-m MPG/ESO telescope at La Silla, 600 km north of Santiago de Chile at an altitude of 2400 metres

It was assembled from many individual images through a large set of different filters over many observing nights, spanning several years. The main purpose of this extensive observational campaign was to get an unusually thorough census of the stars in the galaxy, counting both the number and varieties of stars and marking regions, or even individual stars, that warrant deeper and more focussed investigation. But such a rich data collection will also have many other uses for years to come.

The images were mostly taken through filters that transmit red, green or blue light. These were supplemented by images through special filters that allow through only the light from ionised hydrogen or oxygen gas and highlight the glowing clouds in the galaxy’s spiral arms. The total exposure time amounted to around 50 hours.

Credit: ESO

The new MUSE images of NGC 300 laid over the WFI image, with individual stars clearly visible. Credit: ESO

After so many years of preparation, involving the hard work of so many individuals, it’s fair to say that we were overwhelmed when we received the NGC 300 data. But we have merely scratched the surface of a gold mine. We have so much more data to analyse that we have gathered a team of enthusiastic astronomers to go through it, all keen to discover what lies beyond what we once thought was impossible. And through it all, I keep reminding myself that this was a pilot study.

Not only do we hope to use MUSE to look at even more galaxies, ESO is currently building an instrument called 4MOST that will be dedicated to disentangling starlight and imaging up to 2400 individual stars per single exposure in the Milky Way. The goal is to study millions of stars in the attempt to unravel our galaxy’s formation history and evolution, as part of a vibrant field of research called “galactic archaeology”.

But MUSE is already enabling “extragalactic archaeology” for the first time ever. With its ability to collect huge amounts of light and create incredibly sharp images, ESO’s Extremely Large Telescope will be able to take extragalactic archaeology even further, investigating individual stars in other galaxies to help us find out more about the Universe.

See the full article here .


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ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre EEuropean Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

ESO VLT at Cerro Paranal in the Atacama Desert, •ANTU (UT1; The Sun ),
•KUEYEN (UT2; The Moon ),
•MELIPAL (UT3; The Southern Cross ), and
•YEPUN (UT4; Venus – as evening star).
elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo,

ESO LaSilla
ESO/Cerro LaSilla 600 km north of Santiago de Chile at an altitude of 2400 metres.

ESO VLT 4 lasers on Yepun

ESO Vista Telescope
ESO/Vista Telescope at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level.

ESO/NTT at Cerro LaSilla 600 km north of Santiago de Chile at an altitude of 2400 metres.

ESO VLT Survey telescope
VLT Survey Telescope at Cerro Paranal with an elevation of 2,635 metres (8,645 ft) above sea level.

ALMA Array
ALMA on the Chajnantor plateau at 5,000 metres.

ESO/E-ELT,to be on top of Cerro Armazones in the Atacama Desert of northern Chile. located at the summit of the mountain at an altitude of 3,060 metres (10,040 ft).

APEX Atacama Pathfinder 5,100 meters above sea level, at the Llano de Chajnantor Observatory in the Atacama desert.

Leiden MASCARA instrument, La Silla, located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)

Leiden MASCARA cabinet at ESO Cerro la Silla located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)

ESO Next Generation Transit Survey at Cerro Paranel, 2,635 metres (8,645 ft) above sea level

SPECULOOS four 1m-diameter robotic telescopes 2016 in the ESO Paranal Observatory, 2,635 metres (8,645 ft) above sea level

ESO TAROT telescope at Paranal, 2,635 metres (8,645 ft) above sea level

ESO ExTrA telescopes at Cerro LaSilla at an altitude of 2400 metres