A number of representative science programs that can be carried out with DEIMOS are summarized below. On account of its large FOV and long detector array, DEIMOS offers large gains even for single-object spectroscopy and for direct imaging. However, the focus here is on long-slit, multi-object spectroscopy. Report No. 90 for the Keck Telescope demonstrated that many of the key scientific programs of the telescope were in this category. These are the key programs that drove the size of the Nasmyth-Cassegrain field to 20'.

To assess these programs, recall that in one hour on a point source, the S/N per pixel is 21 at V = 22, 12 at V = 23, and 5 at V = 24 with a 600-line grating (Table 1.3). Excellent velocity dispersions, absorption-line strengths, and radial velocities can be measured at S/N = 25. With the multiplexing advantage of multi-slits, several-hour exposures will be feasible.

A.1 Field-Star Metallicities and Motions

• Spectra of Galactic field stars to V >= 23 mag.
• Goal is metallicities and space velocities.
• Proper motions with several-year baseline provide other velocity components.
• Binary star studies to V > 22 with a few km/sec accuracy.
• Ages of late dM stars via H&alpha emission.

A.2 Stars in the Galactic Center

• Metallicities of sub-populations in the Buldge to V > 23 (unevolved G dwarfs).
• Useful dynamics and kinematics of stars down to V = 24.
• Radial pulsation studies of faint variables to V ~ 23.
• Rough atmospheric parameters for main-sequence stars. Detailed atmospheric studies of sub-giants.

A.3 Metallicities and Kinematics of HII Regions in Nearby Galaxies

• Goals are radial variation in metallicity and detailed kinematics.
• Spectra difficult because typical HII regions are small (few arcsec), contaminated by emission from surroundings, and some lines are weak at high metallicity. High S/N and good sky-subtraction required.
• Field size of DEIMOS well-matched to nearby galaxies; many HII regions simultaneously.
• Slitlets enable background emission from target galaxy to be measured either side of HII region.
• High S/N brings out key diagnostic lines to determine excitation conditions.
• Present-day gas-phase abundances in HII regions can be compared to chemisrty of old and young stellar populations. Favors nearby galaxies, where stars can also be measured (with DEIMOS). Large DEIMOS field is therefore of value.

A.4 The Structure of Nearby Galaxies - Are Halos Triaxial?

• Triaxial halos manifest themselves through non-circular, bar-like distortion of disk rotation: the zero-velocity line typically lies away from the minor axis.
• Since inclination can be constrained by disk shape in inner regions, problem is less degenerate than for ellipticals.
• Galaxies can be mapped rapidly in strong emission lines at high dispersion.
• Unusually long slit length enables nearby large galaxies to be measured.

A.5 Globular Clusters in Nearby Galaxies

• Globular clusters are diagnostic of the earliest populations in galaxies.
• They may also represent major intermediate age star-forming events (as in the merger candidate NGC 1275).
• Need to determine metallicity, age, age dispersion, and kinematics of large numbers of globulars in a sample of nearby galaxies.
• Spectral resolution, sensitivity, and field size give DEIMOS a unique advantage.
• Classic nearby globular cluster populations are huge in the sky. At 10' radius (edge of DEIMOS field), there are 3 globulars per arcmin^2 in M87, or 600 per DEIMOS field.
• Problem is finding them against a background of roughly equal numbers of foreground Galactic stars, plus ten times more faint galaxies.
• Use DEIMOS direct-imaging mode to screen candidates for colors: eliminates most stars and two-thirds of galaxies.
• DEIMOS field size and and light grasp valuable out to Coma Cluster.

A.6 Mass Distribution in Early-Type Galaxies

• The halo mass distribution is not well-constrained in early-type galaxies.
• Measure kinematics (sigma, nu) at very large radaii.
• The limiting factor is primarily sky-subtraction, but low photon-flux is also a problem.
• The unique combination of long slit, high spectral resolution, and large light-gathering power are essential.
• Measurement of dwarf companion galaxies provides another probe of galaxy halos. Large fields on nearby galaxies required. Not limited to early-types.

A.7 Field Galaxies and Galaxy Evolution

• Goal of DEEP program is spectroscopic/imaging study of 10,000+ field galaxies; velocity widths (optical Fisher-Tully and Faber-Jackson), metallicity, gas conditions, ages, and sizes (from HST or high-resolution ground-based imaging).
• Field-galaxy environments around QSOs (absorption line systems), radio galaxies, ultra luminous IR galaxies, etc.
• Tracking the interaction/merger rate versus time using redshifts of many pairs of distant galaxies.
• Studying the kinematics and gas conditions of unusual galaxies (interacting, radio, etc).
• Key new ingredient is rotation curves and velocity dispersions, made possible with DEIMOS's high spectral resolution.
• Large spectral coverage is required, since redshift not known a priori.
• High quality spectra for dynamics down to V ~ 23.

A.8 AGNs and QSO Absorption Lines

• Major AGN surveyss show surface density > 100 deg^-2 at V ~ 22. That is 5 per DEIMOS field.
• Emission line QSO and Seyfert candidate ID's to V > 24.
• Multiband imaging surveys to V > 24 for high redshift (z > 3) candidates.
• ID and redshifts of ultra-low flux radio sources to V > 24.
• Coordinated studies of deep surveys with ROSAT X-ray and radio maps (typically 40 arcmin in diameter with surface densities of candidate objects > 100 deg^-2).
• Damped Lyman-Alpha, Lyman-Alpha forest, C IV, Mg II, and intrinsic absorption line surveys of several QSO's simultaneouls to V > 23 - allows study of clustering properties via cross-correlation.
• Study of galaxy environment of many AGN's at once - host galaxy, neighbors and perturbers, groups, clusters, superclusters.

A.9 High-Redshift Group and Cluster Gakaxies

• Evolution of the fundamental plane of early-type galaxies in field and groups (velocity dispersions from spectra, colors and magnitudes, and perhaps size from imaging).
• Tracking the age and metallicity of distant cluster galaxies.
• Dynamical studies possible on bright galaxies to V = 23, z = 0.7. Have the masses of galaxies grown with time?
• Correlated radio and X-ray studies.
• Evolution of luminosity function of cluster galaxies.
• Nature of the Butcher-Oemler and Dressler-Gunn effects.
• Searches for gravitational lensing of background galaxies.
• Study of cooling flows.
• Study of cluster dynamics and superclustering.
• Study of dark matter evolution.

A.10 Cosmological Tests: Ho, qo, and the Fluctuation Spectrum

Many cosmological tests using field galaxies, especially when combined with high-resolution imaging data from HST: angular-size test, surface-brightness test, various Hubble diagram tests, volume-density tests, potential gravitational lensing and distant supernovae tests.
• Similar tests can be performed using many other classes of extragalactic objects, including cluster galaxies, radio galaxies, AGNs, etc.
• Searches for distant supernovae (determining type and redshift) in distant field and cluster samples.
• Sunyaev-Zeldovich test for Ho
• Turnaround dynamics of clusters provide another test.
• Evolution of two-point correlation function on all scales, evolution of large-scale structure topology, correlations of gas and various kinds of matter. Samples include field galaxies, groups, clusters, AGNs, QSOs, QSO absorption lines, galaxy halos, etc.
• Rate of evolution of structures in redshift space provides independent dynamical measure of Omega.

A.11 Gravitational Lensing

• Redshift surveys of gravitationally lensed arcs and arclets in several clusters of galaxies at once (dozens over several armins around each cluster).
• Use of Fisher-Tully and Faber-Jackson laws to search for gravitational lens candidates.
• Large-scale faint imaging surveys to map image distortions from lensing by major mass sources, luminous or not (hitchhiker imaging programs in other camera, when primary camera is used for spectroscopy).
• Gravitational lensing is a promising technique to map dark matter distributions in outer parts of clusters. DEIMOS field needed to probe outer distribution efficiently.