Galaxy H-Alpha Fabry-Perot System
An optical Fabry-Perot interferometer mapping ionized hydrogen velocity fields.
The Galaxy Hα Fabry-Perot System (GHaFaS) is an astronomical instrument installed on the 4.2 metre William Herschel Telescope at Roque de los Muchachos Observatory on La Palma. It produces maps, in intensity and velocity, of extended objects radiating in the H-alpha line, such as external galaxies, star-forming regions, planetary nebulae, and supernova remnants, and can also be used for a variety of other lines. Its basic aim is to make up for lost time in studying ionized hydrogen at optical wavelengths, which has received less attention than atomic hydrogen at 21 cm or molecular hydrogen at millimetre wavelengths.
- Telescope
- 4.2 metre William Herschel Telescope (WHT)
- Observatory
- Roque de los Muchachos Observatory, La Palma
- First light
- 6 July 2007
- Field of view
- 3.4 arcminutes in diameter
- Velocity resolution
- 5 km/s (11,000 mph)
- Angular resolution
- less than 1 arcsecond (seeing-limited)
Lore & Background
GHaFaS's name is a play on the acronym Galaxy Hα Fabry-Perot System and the Spanish word 'gafas' meaning spectacles. The instrument was motivated by the fact that while atomic hydrogen at 21 cm and molecular hydrogen via CO lines revolutionized astronomy, the optical emission from ionized hydrogen had received less attention. GHaFaS aims to remedy this by taking H-alpha velocity fields of galaxies at high spatial and spectral resolution.
Its performance is comparable to that of the largest radio telescope producing 21 cm maps in atomic hydrogen, the VLA, over a field of 3.4 arcminutes in diameter. It achieves a nominal velocity resolution of 5 km/s and an angular resolution limited by atmospheric seeing to less than 1 arcsecond on the WHT. A good quality map can be obtained in half a night's observing, which is considerably more time efficient than the VLA.
Comparison with ALMA, the best system for observing molecular hydrogen, is not straightforward: ALMA produces maps at considerably higher angular resolution but over a much smaller field. Thus GHaFaS is best suited to observing 'local' galaxies out to 100 Mpc (330,000,000 ly), while ALMA is superior at intermediate and high redshift in terms of angular and velocity resolutions.
Reader's Guide
GHaFaS is significant for its ability to produce high-resolution velocity fields of ionized hydrogen in extended objects, filling a gap left by radio and millimetre studies of atomic and molecular hydrogen. Its performance is comparable to the VLA's 21 cm mapping, but with greater time efficiency—a good quality map can be obtained in half a night. This makes it a powerful tool for exploring the internal kinematics of galaxies, including the corotation radii of density wave systems associated with galaxy bars, and the initial phases of superbubbles caused by stellar winds and supernovae from OB associations. While ALMA offers higher angular resolution, its smaller field limits it to intermediate and high-redshift targets, whereas GHaFaS excels at local galaxies within 100 Mpc. The instrument thus provides a unique optical complement to radio and millimetre observations, enabling detailed studies of star formation and galactic dynamics in the ionized phase of interstellar hydrogen.
Did You Know?
- GHaFaS is a play on the Spanish word 'gafas' meaning spectacles.
- First light was on 6 July 2007.
- It can produce a good quality map in half a night's observing, more time efficient than the VLA.
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