On-Going Instrumentation: IRMOS

The continuing development of large format near-IR arrays, 2kx2k HgCdTe arrays for the 1-2.5µm and possibly buttable 1kx1k InSb arrays, provides an opportunity for three powerful new capabilities in the area of 1-2.5µm Multi-Object Spectroscopy (MOS).

One key near-IR MOS mode would be a high spatial resolution Integral Field Unit (IFU) that adequately samples AO corrected images, possibly equipped with coronagraphic capability. An IFU with a FOV of 5-10" and 0.05-0.1" sampling is required for studies in crowded regions and of galactic nuclei.

A second mode would be the capability to select numerous objects for spectroscopic study within a FOV consistent with AO isoplanatic patch size, about 20-30" diameter. Many applications for this type of MOS will be in crowded complex fields, so multi-slits or even multi-2D capability would be required.

The third key capability is simultaneous spectroscopy of objects over a FOV comparable to that of GMOS, particularly for studies of the distant universe. For many science applications the spectral resolution needs to be optimized to work between the OH airglow lines, between 5000 and 8000, while abundance measurements will require spectral resolutions around 30,000. This capability supports a range of scientific applications, many of which follow the themes of GMOS science to higher redshift or probe dusty environments.

The GNIRS is designed to have significant space in front of the cold slit for an IFU. Because of the "small" 1kx1k format of the GNIRS detector, an image slicer concept for the IFU, which maps slices of the image plane into a single long slit, would provide efficient use of the array and require no additional blocking filters. Two plate scales are of interest; the first of 0.12-0.15 arcsec/spatial element for non-AO use and the second of 0.03-0.04 arcsec/spatial element for use with AO.

Performance Guidelines for the IR MOS Capability:

• Wavelength range: 1-2.5µm
• Array Format: minimum of 2kx2k, goal of 4k in at least one dimension
• Resolution: R~6000 requirement, and ~30K goal
• Pixel scale/FOV:
  • IFU with 0".05 pixels, FOV 5" (or as large as practical)
    • Minimum spectral coverage of ~1000 pixels
  • Multi-slit with 0".05 pixels over minimum 30" FOV - flexibility in number, location, size, length of slitlets is highly desirable
  • Multi Object with 0".15 pixels over minimum 5'.5 FOV
• Goal to combine two or more of the above modes in a single instrument
• Slit masks or other object selection mechanism should be changeable during an observing session without warming the spectrometer or significantly interrupting operations
• Mode changes should not require warming instrument
• NIR OIWFS for tip/tilt and fast focus correction
• Study should determine optimum choice of spectral resolution for working between OH airglow lines.

Science Illustrations:

Near-infrared (JHK) photometry and moderate resolution near-infrared spectroscopy (R~6000) enables determination of stellar effective temperature, interstellar extinction and stellar luminosity of forming stars in a wide variety of environments. In practice, however, these quantities are difficult to determine, particularly in the rich, dense stellar clusters which are presently thought to dominate star formation in the Milky Way, because of source confusion and the irregular background owing to the combined effect of complex reflection and emission nebulosity. To probe the initial mass function over the full range of masses - from the hydrogen burning limit to the Eddington limit requires high spatial resolution in order to obtain accurate photometry and spectra in confused regions.

Gemini will enable detailed study of the structure of YSO envelopes and the processes of infall and outflow in the immediate vicinity (distances ranging from 10 to 1000 AU) of the star/disk system from analysis of adaptively-corrected infrared images and high resolution infrared spectroscopy. Studies of embedded YSOs in crowded regions with complex, irregular background emission requires multi-object spectroscopy over the isoplanatic patch at 2 microns using slits or optimally-designed integral field unit(s). Similarly, an equally important capability for the detailed study of nearby galaxies is a near IR IFU/multi-slit spectroscopic capability for small fields at high angular resolutions.

Moderate resolution near-infrared spectroscopy of very faint galaxies very close to bright QSOs at 1 < z < 4 will allow measurement of Balmer lines H(beta) or H(beta) and/or [OII] to determine redshifts and star formation rates. Integral field spectroscopy in the near infrared (JHK bands) with the high spatial resolution of AO-corrected images is needed to explore lines of sight as close as possible to the QSOs (the impact parameter of damped Lyman-alpha absorbers is expected to be very small).

Clusters of galaxies are the largest gravitationally-bound structures in the Universe, and their abundance and properties can provide strong constraints on cosmological parameters and the power spectrum of the mass fluctuations that give rise to large scale structure. Moreover, the study of cluster galaxies over a very large look-back-time baseline will substantially contribute to our understanding of the formation and evolution of galaxies. Near-infrared spectroscopy of the cluster galaxy candidates will determine galaxy redshifts and cluster velocity dispersions. Multi-object near-infrared spectroscopy over a field of view of several square arc-minutes is needed. Resolutions of 6000 or so are required to effectively subtract the OH sky lines, for redshift and velocity dispersion determination and to obtain information on star formation rates and galaxy ages. Galaxy surveys now show that the 1 < z < 3 region is critical to our understanding of galaxy evolution (as it appears to be for QSOs), hence multi-object 1-2.5µm band spectroscopy over as large a field as possible (>5') is vital to identify and study the characteristics of the galaxy population at these redshifts.

Having access to H(alpha) at z's as high as possible is of great importance for studies of SFR history in the universe. Having access to the K band MOS would cover the all important z~2 era. For example: studies of lenses and lensed objects, AGN and normal galaxy nuclei, the properties of groups and clusters of galaxies as f(z) and the study of galaxies close to QSO lines of sight.

Last update June 5, 1998; Ruth A. Kneale