TWENTY YEARS OF FORS TWENTY YEARS OF FORS Front cover: Messier 66 Of‘‘ all instruments at Paranal, FORS is the Swiss Army knife. ’’ Editor, Design,Typesetting: Henri M.J. Boffin © ESO 2019 TWENTY YEARS OF FORS Twenty years ago, in 1999, the first of the two FORS instrument started regular operations. If FORS1 was removed after 10 years of operations, its twin, FORS2, is still very much in use, being one of the most demanded instruments at the Very Large Telescope, despite its age. This booklet aims at celebrating these two formidable workhorses by putting together, after a description of the instruments and their productivity, a subjective collection of ESO press releases dealing with them. The first set of press releases present the history of the instruments, while the remaining ones showcase some of the astounding achievements that they allowed. In between, a few among the most iconic images ever produced by FORS1 and FORS2 are shown. The list of scientific papers on which the science press releases were based is given at the end, together with a few of the Messenger articles that described the instruments. Table of Contents The FORS Instruments 5 FORS: the most powerful eye 8 First Major Astronomical Instrument Mounted on VLT 10 A Forceful Demonstration by FORS 11 Next VLT Instrument Ready for the Astronomers 14 The Comet with a Broken Heart 17 New Image of Comet Halley in the Cold 18 ESO Observations Show First Interstellar Asteroid is Like Nothing Seen Before 21 VLT Rediscovers Life on Earth 22 Inferno World with Titanium Skies 25 The VLT Unravels the Nature of the Fastest Binary Star 26 Cosmic Sprinklers Explained 28 First Signs of Weird Quantum Property of Empty Space? 30 VLT Spectra ‘Resolve’ a Stellar Disk at 25,000 Light-Years Distance 33 How to Steal a Million Stars? 35 A Galaxy for Science and Research 36 Black Hole Hunters Set New Distance Record 38 Dark Galaxies of the Early Universe Spotted for the First Time 39 Discovering Teenage Galaxies 40 Most Distant Quasar Found 41 Cosmological Gamma-Ray Bursts and Hypernovae Conclusively Linked 43 The Most Remote Gamma-Ray Burst 44 Star Death Beacon at the Edge of the Universe 45 Bibliography 46 Unless indicated, all images are credit ESO. !3 TWENTY YEARS OF FORS The Horsehead Nebula TWENTY YEARS OF FORS The FORS Instruments FORS — the FOcal Reducer and low passive flexure compensation system) and dispersion Spectrograph — came in two is furthermore equipped with a Longitudinal copies, as some of the earliest instruments atmospheric dispersion corrector (LADC) for the Very Large Telescope at Paranal. which acts up to about 60 degrees away FORS1 had First Light in September 1998, from zenith (i.e. airmass 2). The astrometric starting regular operations in April 1999, precision reached is about 0.1 milli- with FORS2 following in April 2000. FORS1 arsecond using the high-resolution was retired in 2009, and FORS2 refurbished collimator. The instrument can also be used to combine elements from both FORSes. in spectroscopic mode with a magnitude Both instruments have been in high limit between 23 and 24, as well as in demand, and this hasn’t changed much: polarimetric mode (imaging or FORS2 is still among the most demanded spectroscopy). Built by a consortium of instruments at the VLT with 102 proposals German institutes, its name is thus a clear submitted in P102 (or 12% of all VLT ones). understatement, as these instruments should have been called FORFISMOSPOL, FORS2 is currently the Swiss army knife of for FOcal Reducer Fast Imager and low the Paranal Observatory as an instrument dispersion Single and Multi-Object covering the wide wavelength range from SpectroPOLarimeter! 330 nm to 1100 nm, with a very great sensitivity, over a field of view of 6.8 x 6.8 FORS2 spectroscopy consists of three arcmin2. It is also characterized by a modes: classical long-slit spectroscopy with superior image quality (in part due to a slits of 6.8’ length and predefined widths The two FORSes at Paranal. !5 TWENTY YEARS OF FORS Refereed publications using data from VLT instruments. FORS2 is still one of the most productive instrument. between 0.3’’ and 2.5’’ (long-slit publications, putting it in second place of all spectroscopy mode, [LSS]); multi-object La Silla Paranal instruments. Among these, spectroscopy with 19 slitlets of 20–22’’ there were 3 papers published in Nature, length each and arbitrary width created by highlighting innovative science done with it: movable slit blades (MOS mode); and multi- the first interstellar comet detected in the object spectroscopy using masks with solar system (Meech et al. 2017, Nature slitlets of almost arbitrary length, width, 552, 378; see also Micheli et al. 2018, shape and angle (MXU mode). There are Nature 559, 223), the spectroscopic 15 grisms with resolutions (for a 1’’-slit) from identification of the gravitational wave 260 to 2600, which may be combined with source (Pian et al. 2017, Nature 551, 67) three different order-separating filters to and the first detection of titanium oxide in avoid second-order contamination. the atmosphere of an exoplanet (Sedaghati et al. 2017, Nature 549, 238; see also The FORS instruments are among the most Nikolov et al. 2018, Nature 557, 526). Over successful instruments in Paranal. Thus the years, FORS2 led to 29 Nature papers FORS1 led to 1022 refereed publications, and 10 Science papers. Apart from the while for FORS2 this number is 1511. In above-mentioned topics, these papers dealt 2017 alone, FORS2 led to 107 refereed with a variety of topics, including asteroids, Number of refereed publications per VLT instrument published in 2017. !6 TWENTY YEARS OF FORS binary stars, neutron stars, supernovae, 2008, A&A 478, 83; Popesso et al. 2009, black holes, gamma-ray bursts, and A&A 494, 443; Balestra et al. 2010, A&A quasars. All the modes of FORS2 were used 512, A12) and of the Chandra Deep Field- for these discoveries. South (Szokoly et al. 2004, ApJS 155, 271), which are based on MXU observations. A presentation of the FORS science can be Looking at the most recent years, the most found in Rupprecht et al. (2010, Messenger cited papers are about Ly-alpha emitters in 140, 2). At that time, it was already stated the early universe (MXU), spectropolarimetry that “If we look at the number of citations of of massive stars, photometry studies of VLT papers, it is symptomatic that at least young stellar regions, astrometric studies of one of the two FORS instruments was brown dwarfs and transmission involved in eight of the ten most cited VLT spectroscopy of exoplanets. Hence, again, papers.” this shows that all modes of FORS2 are used and lead to high impact science. Among the most cited FORS2 papers, one finds obviously the spectroscopic study of The fraction of modes used by FORS2 the GOODS-South field (Vanzella et al. between 2009 and end 2016 is shown in the 2005, A&A 434, 53; 2006, A&A 454, 423; table below. Mode Number of OBs % of total Number of Runs % of total IMG 3521 27 437 31 LSS 4255 33 389 28 MOS 366 3 50 4 MXU 2395 19 222 16 IMG,PRE 634 5 117 8 PMOS 1707 13 142 10 IPOL 432 3 48 3 HIT 34 0.3 9 0.6 Total: 12944 1414 Statistics on the usage of different FORS2 modes. IMG=imaging; IMG,PRE=pre-imaging for MOS and MXU; PMOS=spectro-polarimetry; IPOL=polarimetry imaging; HIT=high time-resolution imaging (now decommissioned). It is interesting to note that much of the mes use relatively short exposure times (and current science done with FORS2 differ from some do time monitoring), where we are what was initially foreseen (e.g., FORS2 SV mostly photon-noise limited and where the in Renzini & Rosati 1999, Messenger 98, 24) read-out time of the CCD should be as small and therefore leads to different require– as possible. ments. For example, many current program– NGC 3190 TWENTY YEARS OF FORS FORS: The Most Powerful Eye In 1991, and following a one-year preparatory elements (pixels) of the very sensitive CCD phase after which a number of astronomical detectors. This is necessary because the institutes were invited to participate in a technique of adaptive optics does not (yet) tendering for two types of VLT instruments, work in visual light so the size of visual images ESO's Finance Committee agreed that will be determined by the instantaneous contracts for these should be placed with two atmospheric turbulence (which is particularly consortia of leading European astronomical small at the site chosen for the VLT observatory, 5 February 1992 institutes. the Paranal mountain in the Chilean Atacama Desert). In this way, FORS will permit a better The selection process was very competitive concentration of the light so that the faintest and several proposals of high quality emanated possible objects can be observed. A special, from the community, all of which conformed "flexure-compensation" system will ensure that with the very tough technical requirements. FORS will remain extremely stable during the The stringent financial limits also played an very long exposure times anticipated for the important role in the present decision. The detection and observation of such faint objects. chosen consortia have proven merits in the instrumental field and can look forward to great By means of various optical elements which challenges and opportunities during the can be inserted into the light beam, FORS can coming years. A consortium of the Heidelberg State Observatory and the University Observatories of Göttingen and Munich will construct two copies of a FOcal Reducer/low dispersion Spectrograph FORS, to be installed at the Cassegrain foci of VLT Unit Telescopes 1 (in 1996) and 3 (in 1998).