NGC 6822
{{Short description|Galaxy in the constellation Sagittarius}}
{{Infobox Galaxy
| name = NGC 6822
| image = Image:Barnard's Galaxy.jpg
| caption = NGC 6822 from ESO's Wide Field Imager at La Silla Observatory
| epoch = J2000
| type = IB(s)m{{Cite web
| website=NASA/IPAC Extragalactic Database
| publisher = NASA and Caltech
| title=Results for object NGC 6822
| url=https://ned.ipac.caltech.edu/byname?objname=NGC+6822
| access-date=2007-03-15}}
| dist_ly = 1.63 ± 0.03 Mly (500 ± 10 kpc){{harvnb|Karachentsev|Karachentseva|Hutchmeier|Makarov|2004}}{{harvnb|Karachentsev|Kashibadze|2006}}{{harvnb|Cannon|Walter|Armus|Bendo|2006}}
| z = {{cvt|-57|±|2|km/s|lk=on}}
| size_v = 15.5{{prime}} × 13.5{{prime}}
| size = ~{{convert|2.82|kpc|ly|abbr=on|lk=on|order=flip}} (estimated){{r|ned}}
| constellation name = Sagittarius
| names = {{odlist | name= Barnard's Galaxy | IC= 4895 | name2= DDO 209 | name3= Caldwell 57 | PGC= 63616 | MCG= -02-50-006 | IRAS= 19420-1450}}
}}
NGC 6822 (also known as Barnard's Galaxy, IC 4895, or Caldwell 57) is a barred irregular galaxy approximately 1.6 million light-years away in the constellation Sagittarius. Part of the Local Group of galaxies, it was discovered by E. E. Barnard in 1884, with a six-inch refractor telescope. It is the closest non-satellite galaxy to the Milky Way, but lies just outside its virial radius. It is similar in structure and composition to the Small Magellanic Cloud. It is about 7,000 light-years in diameter.{{cite web|url=http://apod.nasa.gov/apod/|title=NGC 6822: Barnard's Galaxy|work=APOD|publisher=NASA|date=8 February 2013}}
Observational history
NGC 6822 was discovered by E. E. Barnard in 1884 using a six-inch refractor telescope.
Edwin Hubble, in the paper N.G.C. 6822, A Remote Stellar System,{{Harv|Hubble|1925}} identified 15 variable stars (11 of which were Cepheids) of this galaxy. He also surveyed the galaxy's stars distribution down to magnitude 19.4. He provided spectral characteristics, luminosities and dimensions for the five brightest "diffuse nebulae" (giant H II regions) that included the Bubble Nebula and the Ring Nebula. He also computed the absolute magnitude of the entire galaxy.
Hubble's detection of eleven Cepheid variable stars was a milestone in astronomy. Utilizing the Cepheid Period-Luminosity relationship, Hubble determined a distance of {{convert|698,000|ly|kpc|order=flip|disp=or|lk=in}}. This was the first system beyond the Magellanic Clouds to have its distance determined. (Hubble continued this process with the Andromeda Galaxy and the Triangulum Galaxy). This distance to the galaxy was way beyond Harlow Shapley's value of 300,000 light-years for the size of the universe. In the paper, Hubble concluded the "Great Debate" of 1920 between Heber Curtis and Shapley over the scale of the universe and the nature of the "spiral nebula". It soon became evident that all spiral nebulae were in fact spiral galaxies far outside our own Milky Way.
An analysis of Hubble's plates by Susan Kayser in 1966 remained the most complete study of this galaxy until 2002.{{Cite web | url=https://www.noao.edu/outreach/press/pr02/pr0202.html | title=New Image Shows Rich Neighborhood of Nearby Galaxy | access-date=2016-09-22 | archive-date=2019-03-25 | archive-url=https://web.archive.org/web/20190325032742/https://www.noao.edu/outreach/press/pr02/pr0202.html | url-status=dead }}
In 1977, Paul W. Hodge extended the list of known H II regions in Barnard to 16. Today, there are over 150 of these regions catalogued in Barnard's Galaxy.
Star formation
Observations of the galaxy show stars-forming in the dense cores of giant clouds of molecular hydrogen gas, cold enough to collapse under its own gravity.[https://www.eso.org/public/images/potw1711a/ ALMA peers into the hearts of stellar nurseries, eso.org] The distribution of hydrogen gas is disk-shaped, but mysteriously, it is angled at about 60° relative to the stellar distribution. Most of its stars formed within the last 3 to 5 billion years.
NGC 6822 has spent most of its life in relative isolation. However, it likely passed within the virial radius of the Milky Way some 3 to 4 billion years ago, which may be coincident with its increase in star formation.
Image:ALMA peers into the hearts of stellar nurseries.jpg|Composite image from data by 2.2-meter MPG/ESO and ALMA with star-forming regions identified.{{cite web|title=ALMA peers into the hearts of stellar nurseries|url=https://www.eso.org/public/images/potw1711a/|website=www.eso.org|access-date=13 March 2017}}
Image:Glowing Gas Cloud in the Star-Forming Region of Galaxy NGC 6822 (Hubble).jpg|The Hubble-X Star-forming region.
Image:Region of NGC 6822.jpg|Closeup of another star-forming region
Image:NGC 6822 (NIRCam image) (potm2307c).jpg|NASA/ESA/CSA James Webb Space Telescope image of NGC 6822
Image:NGC_6822_(potm2309a).jpg|JWST NIRCam’s view of NGC 6822 {{cite news |url=https://esawebb.org/images/potm2309a/|title=JWST NIRCam's view of NGC 6822 |date=October 13, 2023}}
Image:Euclid’s view of irregular galaxy NGC 6822 ESA25170768.jpg|NGC 6822 imaged by Euclid
See also
- Large Magellanic Cloud, another irregular galaxy within the Local Group
Notes
Sources
{{refbegin}}
- {{citation
| last1=Cannon
| first1=John M.
| last2=Walter
| first2=Fabian
| last3=Armus
| first3=Lee
| last4=Bendo
| first4=George J.
| last5=Calzetti
| first5=Daniela|author5-link= Daniela Calzetti
| last6=Draine
| first6=Bruce T.
| last7=Engelbracht
| first7=Charles W.
| last8=Helou
| first8=George
| last9=Kennicutt Jr.
| first9=Robert C.
| last10=Leitherer
| first10=Claus
| last11=Roussel
| first11=Helene
| last12=Bot
| first12=Caroline
| last13=Buckalew
| first13=Brent A.
| last14=Dale
| first14=Daniel A.
| last15=De Blok
| first15=W. J. G.
| last16=Gordon
| first16=Karl D.
| last17=Hollenbach
| first17=David J.
| last18=Jarrett
| first18=Thomas H.
| last19=Meyer
| first19=Martin J.
| last20=Murphy
| first20=Eric J.
| last21=Sheth
| first21=Kartik
| last22=Thornley
| first22=Michele D.
| title=The Nature of Infrared Emission in the Local Group Dwarf Galaxy NGC 6822 as Revealed by Spitzer
| periodical=The Astrophysical Journal
| date=December 2006
| volume=652
| issue=2
| pages=1170–1187
| bibcode=2006ApJ...652.1170C
| doi=10.1086/508341
|arxiv = astro-ph/0608249 | s2cid=10287455
| display-authors=8
}}
- {{citation
| last=Hubble
| first=Edwin P.
| author-link=Edwin Hubble
| title=NGC 6822, a remote stellar system
| periodical=Astrophysical Journal
| date=December 1925
| volume=62
| pages=409–433
| bibcode=1925ApJ....62..409H
| doi=10.1086/142943
| doi-access=free
}}
- {{citation
| last1=Karachentsev
| first1=I. D.
| last2=Karachentseva
| first2=V. E.
| last3=Hutchmeier
| first3=W. K.
| last4=Makarov
| first4=D. I.
| title=A Catalog of Neighboring Galaxies
| periodical=Astronomical Journal
| date=April 2004
| volume=127
| issue=4
| pages=2031–2068
| bibcode=2004AJ....127.2031K
| doi=10.1086/382905
| doi-access=free
}}
- {{citation
| last1=Karachentsev
| first1=I. D.
| last2=Kashibadze
| first2=O. G.
| title=Masses of the local group and of the M81 group estimated from distortions in the local velocity field
| periodical=Astrophysics
| date=January 2006
| volume=49
| issue=1
| pages=3–18
| bibcode=2006Ap.....49....3K
| doi=10.1007/s10511-006-0002-6
| s2cid=120973010
}}
- Robert Burnham Jr., Burnham's Celestial Handbook: An observer's guide to the universe beyond the solar system, vol 3, p. 1559
{{refend}}
External links
{{Commons category|NGC 6822}}
- [http://www.eso.org/public/news/eso0938/ The Milky Way's Tiny but Tough Galactic Neighbour] — ESO Press Release
- [http://www.noao.edu/outreach/aop/observers/n6822.html NGC 6822: Barnard's Galaxy] {{Webarchive|url=https://web.archive.org/web/20090603091535/http://www.noao.edu/outreach/aop/observers/n6822.html |date=2009-06-03 }}
- {{WikiSky}}
- [http://messier.seds.org/xtra/ngc/n6822.html SEDS] – NGC 6822
{{Sky|19|44|57.70|-|14|48|12.0|1630000}}
{{Catalogs|NGC=6822|C=57|IC=4895|PGC=63616}}
{{Caldwell catalogue}}
{{NGC70}}
Category:Barred irregular galaxies