View source for Galaxy H-Alpha Fabry-Perot System

{{press release|date=January 2017}}
{{Infobox Telescope|locmapin=Canary Islands}}
The '''Galaxy Hα Fabry-Perot System''' for WHT '''(GHaFaS)''' is an astronomical instrument installed on the 4.2 metre [[William Herschel Telescope|William Herschel Telescope (WHT)]] at [[Roque de los Muchachos Observatory]] on the Canary island of [[La Palma]].<ref name=wht/> [[First light (astronomy)|First light]] was on 6 July 2007.<ref name=firstlight /> Its name is a play on the acronym: '''G'''alaxy '''Hα Fa'''bry-Perot '''S'''ystem 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 region|star forming regions]] in the Galaxy, [[Planetary nebula|planetary nebulae]], or [[Supernova remnant|supernova remnants]], as examples), which radiate in the [[H-alpha line]], emitted by [[ionized hydrogen]] 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 [[Hydrogen line|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 [[Carbon monoxide|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 velocity fields of galaxies at high spatial and spectral resolution.

The performance of '''GHaFaS''' is comparable to that of the largest [[radio telescope]] producing 21&nbsp;cm maps in atomic hydrogen: the [[Very Large Array|VLA]]. Over a field of 3.4 [[arcminutes]] in diameter, it produces a map of ionized hydrogen emission with a nominal velocity resolution of {{Convert|5|km/s|mph|abbr=unit}} and an angular resolution limited by the "seeing" due to atmospheric turbulence of less than 1 arcsecond when used on the [[William Herschel Telescope|WHT]]. These values are similar to the best figures obtainable with the [[Very Large Array|VLA]]. 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 [[Very Large Array|VLA]]. Comparison with [[Atacama Large Millimeter Array|ALMA]], the best system for observing molecular hydrogen, is not so easy. [[Atacama Large Millimeter Array|ALMA]] 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 {{Convert|100|Mpc|ly|abbr=unit|lk=on}} distance, while [[Atacama Large Millimeter Array|ALMA]] is superior at intermediate and high [[redshift]], 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 circle|corotation radii]] of the density wave systems related to galaxy bars, and to explore the initial phases of the huge [[Superbubble|superbubbles]] caused by the combined stellar winds and supernovae of the [[OB association|OB associations]] of massive young stars, among a variety of its observing achievements.

==References==
{{Reflist|refs=

<!--unused<ref name=weather>
{{cite web
 |url=http://www.ing.iac.es/Astronomy/observing/conditions/
 |title=Site Quality
 |author=Chris Benn
 |work=ING website
 |publisher=[[Isaac Newton Group]]
 |accessdate=12 Dec 2016}}
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<ref name=wht>
{{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}}
</ref>

<ref name=firstlight>
{{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}}
</ref>

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[[Category:Astrophysics]]
[[Category:Interferometers]]
[[Category:Telescopes]]
[[Category:Research]]