SINFONI

Go to Unit Telescope 4 (UT4, Yepun) of the Very Large Telescope (VLT) at the Paranal Observatory and SINFONI can be found conducting its own beautiful observational orchestra, thanks to its two component instruments working together: SPIFFI (SPectrometer for Infrared Faint Field Imaging), which is an infrared integral field spectrograph, and the SINFONI-AO (adaptive optics) module. George Hau and Bin Yang are responsible for arranging SINFONI’s harmony with the stars.

An integral field spectrograph (IFS) allows you to observe in three dimensions across the entirety of an astronomical object in one go: each pixel of the image is associated with a full spectrum, measuring the intensity of the light at each wavelength (the wavelength is a measurement of its colour — in some objects, it can be converted into a velocity towards or away from the Earth and tells us about its relative position in space as a result.

SINFONI uses an “image slicer” technique: as shown in the figure below, a two-dimensional image (a) is chopped into smaller components (via “optical slicing”) and re-positioned by special segmented mirrors so that they lie in a line end-to-end instead of on top of each other (b). This essentially forms a very long virtual slit. The light from the virtual slit is split by SPIFFI into its separate colours and therefore wavelengths (c), and the image is then reconstructed from the individual slices at each wavelength, i.e. the image is reconstructed from all the slices at the red end of the spectrum, then the green, etc. They are then combined to give us the final, 3D information we desire (d).

Traditional methods of obtaining spectra over the full extent of a large, extended object is both time consuming and inefficient — an IFS solves both of these problems. SINFONI is therefore the ideal instrument for looking at some of the most interesting galaxies and planetary nebulae in the Universe. Because SINFONI only uses mirrors, it can be cooled to extremely low temperatures (between -150 and -273 degrees C, or “cryogenic temperatures”). This, combined with the adaptive optics module SINFONI-AO, which also works better towards the red end of the spectrum, means that overall SINFONI is tailored for observing at infrared wavelengths.

Thanks to SINFONI-AO, the instrument can also correct for atmospheric turbulence. The distorted image of a guide star is used to measure the effect of the turbulence, and a small, deformable mirror adjusts itself in real time to correct for these distortions. For the cases where a reference star is not available, the Laser Guide Star (LGS) can be used to create a bright spot 80 kilometres above the telescope and this is used as an artificial reference star. As a result, the images reaching SPIFFI are almost as sharp as if there were no atmosphere above the VLT at all — music to our ears!

Science highlights with SINFONI

• Discovery that early Universe galaxies were dominated by normal, rather than dark, matter (eso1709)
• Discovery of by far the brightest galaxy yet found in the early Universe and strong evidence found that examples of the first generation of stars lurk within it (eso1524)
• SINFONI was used to help reveal that three billion years after the Big Bang, giant eliptical galaxies still make stars in their outskirts, but no longer in their interiors (eso1516)
• Best view yet of dusty cloud (G2) passing galactic centre black hole (eso1512)
• SINFONI was used to gather the best direct observational evidence so far supporting the theory that galaxies pull in and devour nearby material in order to grow and form stars (eso1330)
• A giant gas cloud is being ripped apart by the supermassive black hole at the centre of the Milky Way; SINFONI has been studying the cloud’s fate (eso1151, eso1332). Before that, SINFONI observed the motion of stars surrounding the black hole, which were used to measure how far away it is and its mass. (eso0846).
• SINFONI has made many observations to study the growth and evolution of galaxies in a distant past:  at the beginning, they tended to gently snack on nearby gas (eso1040, eso1330), but at a later stage in their growth, they would cannibalise other, smaller galaxies. (eso1212)
• The most distant galaxy ever measured to date was observed by SINFONI as it was only 600 million years after the Big Bang. (eso1041)