H3 Line Survey Towards the Galactic Center

The H3 in the interstellar medium is a unique probe of physical and chemical environments in dense and diffuse clouds. As the observational study in many lines of sight is becoming matured, we have as many problems newly found. The overabundance of H3 in diffuse clouds is one of them [3, 4]. We undertook the absorption line survey of H3 toward the Galactic center sources to shed light on it [1]. The Galactic center sources are ideal line of sight to start the survey since they suffer heavy visual extinction of AV =25–40, the highest in the Galaxy among those obscured by diffuse clouds. The intervening clouds have different radial velocities, and the nature of them have been studied in previous spectroscopic observations from near-infrared to radio wavelengths. We observed two infrared sources, GCS 3-2 in the Quintuplet cluster and GC IRS 3 near Sgr A∗. Both have intrinsically featureless spectra, and sufficiently luminous to provide good continuum fluxes in the 3 μm region. The observations were made in June 2001 using the Infrared Camera and Spectrograph (IRCS; Tokunaga et al. 1998; Kobayashi et al. 2000) at the 8.2 m Subaru Telescope. Subaru IRCS is equipped with an echelle and a cross-dispersing gratings. The high resolution (R=20,000) spectroscopy with wide wavelength coverage is unique characteristic of the instrument. It allows us to record one third of 3.2–4.0 μm with a single exposure. This makes the system very suitable for any line-survey projects, in particular for H3 vibrational transition lines that scatter all over the 3 μm region.