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Near IR Astronomy with the ESO VLT

Alan Moorwood European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching

 

 

Abstract:

The ESO VLT situated on Cerro Paranal in Chile comprises four 8.2m plus three 1.8m movable auxiliary telescopes providing 8 Nasmyth, 4 Cassegrain, and 1 incoherent combined focus plus a variety of interferometric options. First light was achieved with the first unit telescope (UT1) on 25th May 1998 with results which fully demonstrate the quality of both the telescope and the site. Six infrared instruments for the unit telescope foci plus three for interferometry are at various phases of design and development. Together, they provide for direct imaging in wide fields; diffraction limited imaging with adaptive optics and interferometry; polarimetry; long slit, multi-object and integral field spectroscopy at resolving powers ranging from a few hundred to 10⁵. The status and expected capabilities of these instruments is summarized together with a brief overview of the highest priority science goals which can be anticipated now.

astronomy, infrared, instrumentation

Introduction and Overview

ESO's VLT situated on Cerro Paranal in Chile comprises four 8.2m plus three 1.8m moveable auxiliary telescopes. In addition, the site will host a 2.5m wide field survey telescope to be built by the Capodimonte Observatory, Naples and operated by ESO. The site is dry; photometric more than 80$\%$ of the time and delivers a median seeing of 0.65 arcsec. The small telescopes can be combined with each other optically to form the VISA interferometer or with the large telescopes to form VIMA. The large telescopes can also be incoherently combined. Each of the large telescopes provides two Nasmyth and a Cassegrain focus. Of these twelve foci, eleven are planned to be equipped with 'common user' instruments and one will be reserved for innovative visitor instruments. UT1 will be equipped with a laser guide star system plus the NAOS Shack-Hartman adaptive optics system used with CONICA and the MACAO curvature sensing AO system to be used with SINFONI (and later interferometry and possibly CRIRES). Of the main instruments, three (ISAAC,UVES and CRIRES) plus MACAO are being developed by ESO and the rest by consortia of mostly, but not exclusively (e.g Australia is producing a fibre positioning unit) European Institutes. In the case of externally contracted instruments, ESO generally covers the capital cost while the Institutes provide manpower and expertise in exchange for guaranteed observing time. Even in the case of the subcontracted instruments, however, ESO is usually involved in the design and provides most of the detectors and part of the instrument control systems.

Current status of the project is that first light of the first 8.2m Unit Telescope was achieved on 25th May 1998. The FWHM $\simeq$ 0.32'' visible images obtained and shown at this Workshop clearly demonstrate the excellent quality of both the telescope and the site. Science Verification with a visible test camera is planned for the second half of August after which the first major instrument, FORS ( visible imaging, polarimetry, spectroscopy and MOS spectroscopy) will be installed. The first IR instrument, ISAAC (1-5$\mu$m imaging, polarimetry and spectroscopy), which has been built at ESO, will be installed in November 1998. Both of these instruments will be finally commissioned and used for Science Verification observations in early 1999 and made available to visiting astronomers in April 1999. With regard to the regular operation of the VLT it should be noted that ESO is also developing a complete data flow system comprising instrument simulators, software for pre-preparing observation/instrument sequences in the form of Observation Blocks, flexible scheduling software, data and calibration pipelines and an archive. At the beginning it is expected that about 50$\%$ of the observing will be performed by ESO in service mode and the rest with the visiting astronomer present at the Observatory. For those interested, more comprehensive information on the VLT and its instruments can be found on ESO's Web Site (http://www.eso.org/instruments/).

Infrared instruments for the VLT

Instrument Summary

The already approved infrared instruments for the VLT are:
ISAAC: 1-5$\mu$m Infrared Spectrometer and Array Camera (P.I. A. Moorwood, ESO) which is currently undergoing final testing in Garching prior to shipment to Paranal in June for installation at UT1 in Nov 1998. A derivative 1-2.5$\mu$m instrument, SOFI, is already in operation at the 3.5m NTT telescope on La Silla (Moorwood, Cuby and Lidman, 1998, The Messenger, 91,9).
CONICA: Coude Near Infrared Camera (P.I. R. Lenzen, MPIA, Heidelberg) - an historical name as this instrument will be used for diffraction limited 1-5$\mu$m imaging, polarimetry and spectroscopy with the NAOS, Nasmyth Adaptive Optics System on UT1 (P.I. G. Rousset, ONERA ). The two systems will be integrated in Europe in late 1999 and installed at the VLT in mid-2000.
SINFONI: Single Far Object Near IR Investigation (P.I. N. Thatte, MPIE, Garching) which will be used with the ESO developed MACAO AO system (P.I. D. Bonaccini, ESO) for integral field spectroscopy. SINFONI will be used by the MPIE first at Calar Alto and then transferred to the VLT in 2001.
NIRMOS: Near IR Multi-Object Spectrometer (P.I. O. Le Fèvre, LAS. Marseille ) for 1-1.8$\mu$m wide field imaging and spectroscopy at R $\simeq$ 2500 of up to 170 objects simultaneously. This instrument is in the detailed design phase together with its twin visible instrument VIMOS. It will be installed in 2001.
CRIRES: Cryogenic IR Echelle Spectrometer (P.I. G. Wiedemann, ESO) for very high (10⁵) resolution IR spectroscopy, probably in combination with a MACAO type curvature sensing AO system. Is in detailed design phase and with aim of exploiting many of the ISAAC technical developments. To be installed in 2002.
VISIR: VLT Mid Infrared Imager Spectrometer (P.I. P.-O. Lagage, SAp, Saclay) for 8-27$\mu$m imaging and spectroscopy.

A brief overview of the main instrumental modes is given in Table 1. Taken together, they cover a large fraction of the spatial and spectral resolution phase space with multi-object and integral field capabilities entering as major new modes opened up with the development of large format IR array detectors.

 

In addition, three interferometric instruments are at the conceptual design stage - AMBER (1-2.5$\mu$m imaging/spectroscopy), MIDI(8-13$\mu$m) and PRIMA (2-5$\mu$m phase referenced imaging and astrometry).

IR Detectors

The infrared detectors to be used in the different instruments are summarized below.

• Rockwell 1-2.5$\mu$m 1024x1024 Hawaii (ISAAC, SINFONI)
• Rockwell 1-2.5$\mu$m 2048x2048 funded by University of Hawaii, SUBARU and ESO (4 in NIRMOS)
• SBRC 1-5$\mu$m 256x256 (ISAAC)
• SBRC 1-5$\mu$m 1024x1024 Aladdin (CONICA, ISAAC, 2-3 in CRIRES)
• SBRC 8-27$\mu$m 240x340 (VISIR)

In all cases, the IRACE acquisition system, developed by ESO initially for ISAAC will probably be used. It is a modular (nominally 32 channel) high speed (20 Mpix/s), low noise (limited by detector) system employing a GByte/sec link and an Ultrasparc processor for image pre-processing (Meyer et al., 1997, The Messenger, 86, 14). Although most modes will be background limited, it is expected that some modes involving the highest spatial and spectral resolution in the near infrared will reach the limits set by the intrinsic detector noise and dark current which are now at the incredible level of only a few e^- and around 20e^-/hour respectively as measured on Rockwell Hawaii arrays with IRACE.

Performance

It is beyond the scope of this article to reproduce the expected performance of all the instruments in all their modes. Typically, however, background limited imaging in the near infrared is expected to reach point source 3$\sigma$ limits in 1 hr of around J = 25. H = 24 , Ks = 23 under the best seeing condition, and up to about 2 magnitudes fainter at the diffraction limit with AO. Continuum limits for medium resolution (R $\simeq$ 3000) spectroscopy are expected to be in the range 20-22 mag. but strongly wavelength dependent due to the OH sky lines. At this resolving power, however, a large fraction of the J and H bands can be observed between the strong OH lines allowing 1 hr line flux limits of a few x 10^-18 erg cm^-2 s^-1.

Science

The combined instrument complement of the VLT provides for a wide range of visible and infrared capabilities from deep imaging and spectroscopic surveys to high spatial and spectral resolution observations of individual objects using adaptive optics and interferometry. Science objectives are therefore expected to embrace much of contemporary astronomy and the following is only a brief summary of some of the currently envisaged science areas in which infrared observations are expected to be of particular importance.
Large scale structure. Surveys for high z clusters and evolution of structures at z $\leq$ 3. Measurement of dynamics to constrain the formation epoch and distortions in the expansion field to map dark matter and the total mass density of the Universe. (NIRMOS, ISAAC).
Cosmological parameters. Distances from Cepheids and relative distances from fundamental plane and Tully - Fisher scaling relations and supernovae. Time delay in gravitationally lensed quasars. Refine values for H₀, q₀ and $\Lambda$ (CONICA, SINFONI, ISAAC).
High redshift galaxies. Galaxy formation and evolution. Photometric redshift (in combination with visible) and emission line surveys (Ly$_{\alpha}$, H$_{\alpha}$, [OII]) plus follow-up spectroscopy to determine star formation rate as a function of z, size, morphology, mass (ISAAC, NIRMOS, CONICA, SINFONI).
Gamma-ray bursts. Spectroscopy of optical counterparts (ISAAC, CONICA, SINFONI).
Galaxy nuclei. Stellar populations/dynamics. Black hole masses. Test unified AGN theories. Quasar host galaxies. (CONICA, SINFONI, VISIR, VLTI).
Stars. Abundances, circumstellar shells, mass loss, magnetic fields (CRIRES, VISIR).
Star formation. Measure IMF to lowest masses in star forming clusters. Outflows and dynamics of protostellar discs (ISAAC, CONICA, SINFONI, CRIRES, VISIR).
Extrasolar planets. Direct detection by imaging and/or velocity modulation of planetary spectral features (CRIRES, CONICA, VISIR, VLTI).
Protoplanetary systems. Thermal imaging of protoplanetary discs (VISIR, CONICA, VLTI).
Solar system. Composition and dynamics of planetary atmospheres. Detection, chemistry and mineralogy of faint outer solar system bodies (ISAAC, CONICA, SINFONI, VISIR).