Deep Fabry-Perot Observations of High Velocity Clouds
Session 12 -- Interstellar medium
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

## [12.09] Deep Fabry-Perot Observations of High Velocity Clouds

J. Bland-Hawthorn (Anglo-Australian Observatory)

In recent months, we have demonstrated that it is possible to reach H$\alpha$ emission measures as low as 0.02 cm$^{-6}$ pc (4$\times$10$^{-20}$ erg cm$^{-2}$ s$^{-1}$ arcsec$^{-2}$) in six hours of dark time observation (3$\sigma$ limit) with the {\tt TAURUS-2} imaging Fabry-Perot interferometer at the f/8 Cassegrain focus of the AAT 3.9m. This deep limit assumes that the emission-line source is spectrally unresolved and completely fills the field of view (9$^{\prime}$). The etalon is kept at a fixed spacing in which case the spectrum is dispersed as a series of annular rings over the field. For our preferred choice of etalon and filters, {\tt TAURUS-2} provides $\sim$50\AA\ of spectral coverage in a single exposure at a spectral resolution close to 1\AA\ FWHM. Below emission measures (T$_e = 10^4$ K) of roughly 0.1 cm$^{-6}$ pc, there are systematic effects which can only be removed with extensive calibrations. Our new deep limit brings a number of important experiments into the realm of possibility for the first time. It has long been realized that an emission-line detection (at optical wavelengths) of a high velocity cloud would provide an important constraint on the galactic and/or metagalactic radiation fields. To date, there exists only one high velocity cloud with a possible H$\alpha$ detection (Kutryev \& Reynolds 1989). We shall present early results from a new survey which aims to detect a statistically useful number of clouds in more than one emission line.