STIS-Repair > Science > Scientific Role of STIS after SM4

The Hubble Space Telescope's Space Telescope Imaging Spectrograph (STIS) was one of the most heavily utilized of HST's instruments from the time of its installation in 1997 until it suspended operations in August 2004. Plans for the next Hubble Space Telescope (HST) Servicing Mission (SM4) include a repair of STIS, and should return it to operations with capabilities similar to those it had before the 2004 failure. Even with the availability of Wide Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS), which are also slated to be installed during SM4, STIS will retain a number of unique capabilities that will make it the instrument of choice for many science programs.

Extra-solar Planets	Detection and analysis of the faint atmospheric absorption of a transiting extra-solar planet against the stellar light requires both extremely high signal-to-noise (S/N) and high confidence in the sky subtraction. The very high differential S/N ratios that can be achieved with the STIS Charge-Coupled Detector (CCD) when combined with the STIS long-slit capabilities make STIS an ideal instrument for studying these objects.
Stellar Jets and Ejecta	Stellar ejecta around individual stars are often highly structured, and may be chemically differentiated. Planetary Nebulae (PN) in particular preserve a record of the late stages of a star's mass loss. Ultra-Violet (UV) lines are essential for calculating the abundances of the element related to stellar evolution (Carbon-C, Nitrogen-N, Oxygen-O) and to progenitor populations (e.g., Neon-Ne). Spatially resolved STIS long-slit UV spectroscopy of PNs with known morphology and central star properties were planned for Cycle 13, but had not yet executed prior to the failure of STIS.
InterStellar Medium (ISM) / InterGalactic Medium (IGM)	Accurate determinations of the abundances and physical conditions of the interstellar and intergalactic gas rely upon high resolution absorption-line observations. The ability to determine accurate abundances and physical conditions improves greatly when the lines are resolved, or close to being resolved, by the instrument. For most interstellar environments, the lines have intrinsic widths of a few km/s and turbulent broadening widths of several km/s. This means that resolutions comparable to those afforded by the STIS echelles (R ~ 45,000 - 100,000) are necessary to perform the best possible abundance analyses and physical condition determinations.
Galaxies	The ability of the STIS long slits to produce spatially resolved spectra could play a pivotal role in improving our understanding of many aspects of both normal and active galaxies, allowing improved measurements of stellar populations and kinematics in these objects. STIS can be a useful complement to even the largest ground based telescopes when observing stellar clusters near the centers of complex objects such as merger remnants or starburst galaxies, as STIS enables much better background subtraction: a narrow slit reduces background contamination by a factor of ~ 10 relative to ground based spectra; the ground-based spatial resolution is not adequate to obtain good background subtraction; STIS has high sensitivity below 4200 Å where the crucial high-order Balmer lines are located. In the starburst galaxy NGC 4449, a STIS long slit was used with the G140L and G230L gratings to obtain spectra of both stellar clusters and field stars which clearly showed distinct differences between the cluster and field populations.
Black Hole Physics	The high spatial resolution and spectroscopic capability of STIS has enabled the bulk of the work done on supermassive black holes in the nuclei of nearby galaxies since 1997, when STIS was installed in HST. Large ground-based telescopes with adaptive optics and the use of laser guide stars hold promise for the future, but at present the critical resolution of a few parsecs with a stable Point Spread Function (PSF) at the center of nearby galaxies is unobtainable with any spectrograph but STIS. While many such targets have been studied using STIS, the application of improved observational techniques with STIS would especially benefit spatially resolved spectroscopy of nuclear emission-line disks in Keplerian motion. More measurements at the extremes of the mass distribution of galaxies are also needed; indeed, several such programs were approved but not done in Cycle 13.