The Cosmic Origins Spectrograph and the Future of Ultraviolet Astronomy

I describe the capabilities of the Cosmic Origins Spectrograph, scheduled for May 2009 installation on the Hubble Space Telescope. With a factor-of-ten increase in far-UV throughput for moderate resolution spectroscopy, COS will enable a range of scientific programs that study hot stars, AGN, and gas in the interstellar medium, intergalactic medium, and galactic halos. We also plan a large-scale HST Spectroscopic Legacy Project for QSO absorption lines, galactic halos, and AGN outflows. Studies of next-generation telescopes for UV/O astronomy are now underway, including small, medium, and large missions to fill the imminent ten-year gap between the end of Hubble and a plausible launch of the next large mission. Selecting a strategy for achieving these goals will involve hard choices and tradeoffs in aperture, wavelength, and capability.

💡 Research Summary

The paper presents a comprehensive overview of the Cosmic Origins Spectrograph (COS), slated for installation on the Hubble Space Telescope (HST) in May 2009, and discusses its implications for the future of ultraviolet (UV) astronomy. COS is designed to deliver a ten‑fold increase in far‑UV (FUV) throughput compared with the existing Space Telescope Imaging Spectrograph (STIS), particularly in the 1150–1775 Å band. This dramatic gain in efficiency translates into a 1–2 mag improvement in limiting magnitude for a given exposure time, enabling high‑signal‑to‑noise observations of faint background sources such as quasars (QSOs) and low‑surface‑brightness interstellar structures.

Two spectroscopic modes are offered: a medium‑resolution mode (R≈20,000) optimized for broad wavelength coverage and high photon collection efficiency, and a high‑resolution mode (R≈45,000) suited for detailed line‑profile work. Both modes employ advanced multi‑grating optics and a high‑quantum‑efficiency microchannel plate detector with low‑noise readout electronics, ensuring that the instrument’s performance is limited primarily by astrophysical backgrounds rather than detector noise.

The scientific program enabled by COS is organized around four primary themes. First, the instrument will revolutionize the study of hot massive stars and white dwarfs by providing precise measurements of key UV resonance lines (e.g., C IV, N V, Si IV), thereby constraining wind dynamics, atmospheric composition, and mass‑loss rates. Second, COS will probe active galactic nuclei (AGN) with unprecedented detail, allowing researchers to dissect broad‑ and narrow‑line regions, map outflow kinematics, and quantify feedback processes that regulate galaxy evolution. Third, the spectrograph’s sensitivity to weak absorption features makes it ideal for mapping the interstellar medium (ISM), circumgalactic medium (CGM), and intergalactic medium (IGM) through Lyman‑α, O VI, Ne VIII, and other high‑ionization lines. By assembling large samples of QSO sightlines, COS will enable statistical studies of metallicity, temperature, and velocity structure across cosmic time. Fourth, the authors outline a large‑scale “HST Spectroscopic Legacy Project” that will systematically acquire medium‑resolution spectra of hundreds of QSOs and foreground galaxies, creating a legacy database that will serve the community for decades.

Beyond the immediate capabilities of COS, the paper addresses the looming observational gap that will arise when HST ceases operations (anticipated around 2019) and before a next‑generation flagship UV/optical mission can be launched. To bridge this ten‑year hiatus, the authors propose a tiered roadmap comprising small, medium, and large missions. Small‑satellite concepts such as CUTE (Colorado Ultraviolet Transit Experiment) and SPARCS (Star‑Planet Activity Research CubeSat) focus on niche science cases—primarily exoplanet atmospheric transits and stellar activity—at modest cost. Medium‑class telescopes (1–2 m aperture) would retain COS‑like throughput while extending wavelength coverage to the near‑UV (≈1000–3000 Å) and offering modest improvements in spectral resolution. Finally, large‑aperture flagship concepts (≥8 m) aim to combine broadband UV capability with high‑resolution spectroscopy (R > 30,000), enabling transformative studies of faint galaxies, the epoch of reionization, and detailed CGM/IGM tomography. The authors emphasize that each tier involves trade‑offs among aperture size, wavelength range, instrument complexity, and budget, and that community consensus will be required to prioritize the most compelling scientific drivers.

In summary, COS represents a quantum leap in HST’s UV spectroscopic performance, opening new windows on stellar winds, AGN feedback, and the diffuse baryonic web that connects galaxies. Simultaneously, the paper lays out a forward‑looking strategy to sustain UV astronomy through a coordinated suite of missions that will preserve and extend the scientific momentum generated by COS, ensuring that the field remains vibrant during the post‑Hubble era.