The Galaxy Hα Fabry-Perot System for WHT (GHaFaS) is an astronomical instrument installed on the 4.2 metre William Herschel Telescope (WHT) at

Roque de los Muchachos Observatory Roque de los Muchachos Observatory (, ORM) is an astronomical observatory located in the municipality of Garafía on the island of La Palma in the Canary Islands, Spain. The observatory site is operated by the Instituto de Astrofísica de Can ...

on the Canary island of

La Palma La Palma (, ), also known as ''La isla bonita'' () and historically San Miguel de La Palma, is the most northwesterly island of the Canary Islands, a Spanish autonomous community and archipelago in Macaronesia in the North Atlantic Ocean. La Pa ...

. First light was on 6 July 2007. Its name is a play on the acronym: Galaxy Hα Fabry-Perot System and the Spanish word "''gafas''" meaning spectacles. It produces maps, in intensity and velocity, of extended objects in the sky (which could be external galaxies, star forming regions in the Galaxy,

planetary nebulae A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionization, ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated to pla ...

, or

supernova remnants A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar mat ...

, as examples), which radiate in the H-alpha line, emitted by

ionized hydrogen A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particl ...

in interstellar space. It can also be used for a variety of other lines. The possibility to detect the emission line of interstellar neutral hydrogen at 21 centimeters wavelength revolutionized astronomy in the second half of the 20th century, as a powerful tool to explore the structure and evolution of galaxies and the star formation within them. A second revolution came, when, at millimetre wavelengths, molecular hydrogen could be detected and measured in directly using, above all, the emission from the lines of the CO molecule. Surprisingly, the emission from the third phase of interstellar hydrogen, the ionized phase, which is at optical wavelengths, has had less attention. The basic aim of GHaFaS is to make up for lost time by taking

H-alpha Hydrogen-alpha, typically shortened to H-alpha or Hα, is a deep-red visible spectral line of the hydrogen atom with a wavelength of 656.28 nm in air and 656.46 nm in vacuum. It is the first spectral line in the Balmer series and is em ...

velocity fields of galaxies at high spatial and spectral resolution. The performance of GHaFaS is comparable to that of the largest

radio telescope A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...

producing 21 cm maps in atomic hydrogen: the

VLA ''Vla'' () is a Dutch dairy product made from fresh milk. The word ''vla'' was first documented in the 13th century and originally referred to any custard-like substance covering cakes or other baked goods. The word '' vlaai'' is related and ...

. Over a field of 3.4

arcminutes A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

in diameter, it produces a map of ionized hydrogen emission with a nominal velocity resolution of and an angular resolution limited by the "seeing" due to atmospheric turbulence of less than 1 arcsecond when used on the WHT. These values are similar to the best figures obtainable with the

. However, a good quality map can be obtained in half a night's observing with GHaFaS, which is considerably more time efficient than that on the

. Comparison with

ALMA Alma or ALMA may refer to: Arts and entertainment * ''Alma'' (film), a 2009 Spanish short animated film * ''Alma'', an upcoming film by Sally Potter * ''Alma'' (Oswald de Andrade novel), 1922 * ''Alma'' (Le Clézio novel), 2017 * ''Alma'' ( ...

, the best system for observing molecular hydrogen, is not so easy.

produces maps at considerably higher angular resolution, but over a much smaller field. Thus, at the moment GHaFaS is best suited to observing "local" galaxies, out of distance, while

is superior at intermediate and high

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and increase in frequency and e ...

, in terms of angular and velocity resolutions. GHaFaS is very well suited to exploring the fine details of the internal kinematics of galaxies, as well as any phenomena related to the formation of high mass stars and their surroundings. It has been used to make the best measurements to date of the corotation radii of the density wave systems related to galaxy bars, and to explore the initial phases of the huge superbubbles caused by the combined stellar winds and supernovae of the OB associations of massive young stars, among a variety of its observing achievements.

References

{{Reflist, refs= {{cite web , url=http://www.ing.iac.es/Astronomy/observing/instruments.html , title=Overview of Instrumentation at ING , author=Chris Benn , work=ING website , publisher= Isaac Newton Group , accessdate=12 Dec 2016 {{cite web , url=http://www.ing.iac.es/PR/wht_info/whtghafas.html , title=Galaxy Hα Fabry-Perot System (GHaFaS) , author=Javier Méndez , work=ING website , publisher= Isaac Newton Group , accessdate=12 Dec 2016 Astrophysics Interferometers Telescopes Research