View source for I Zwicky 18

{{Short description|Galaxy in the constellation Ursa Major}}
{{Infobox Galaxy
| name               = I Zwicky 18
| image              = I Zwicky 18a.jpg
|caption             = [[Hubble Space Telescope]] (HST) image of I Zwicky 18<br />Credit: HST/[[NASA]]/[[ESA]]
| epoch              = [[J2000]]
| ra                 = {{RA|09|34|02.0}}<ref name="ned">{{NED link|id=MRK+0116|name=MRK 0116}}</ref>
| dec                = {{DEC|+55|14|28}}<ref name="ned" />
| constellation name = [[Ursa Major]]
| z                  = {{val|0.002585}}<ref name="samsonyan">{{cite journal|last1=Samsonyan |first1=Anahit |last2=Weedman |first2=Daniel |last3=Lebouteiller |first3=Vianney |last4=Barry |first4=Donald |last5=Sargsyan |first5=Lusine |title=Neon and &#91;C II&#93; 158 μm Emission Line Profiles in Dusty Starbursts and Active Galactic Nuclei |journal=The Astrophysical Journal Supplement Series |date=2016 |volume=226 |issue=1 |page=11 |doi=10.3847/0067-0049/226/1/11 |doi-access=free |arxiv=1608.02277 |bibcode=2016ApJS..226...11S}}</ref><ref name=SIMBAD>{{cite simbad|title=Mrk 116}}</ref>
| h_radial_v         = {{val|774|ul=km/s|fmt=commas}}<ref name="samsonyan" /><ref name="SIMBAD" />
| gal_v              = 
| dist_ly            = {{cvt|18.438|+/-|4.171|Mpc|Mly|sigfig=5|abbr=on|lk=on|order=flip}}<ref name="ned" />
| type               = cI;[[dwarf galaxy#Blue compact dwarf galaxies|Blue Compact Gal]],<ref name="ned" /> [[Wolf-Rayet galaxy|WR]],<ref name=schaerer>{{cite journal|last1=Schaerer |first1=D. |last2=Contini |first2=T. |last3=Pindao |first3=M. |title=New catalogue of Wolf-Rayet galaxies and high-excitation extra-galactic HII regions |journal=Astronomy and Astrophysics Supplement Series |date=1999 |volume=136 |page=35 |doi=10.1051/aas:1999197 |arxiv=astro-ph/9812347 |bibcode=1999A&AS..136...35S}}</ref><ref name="ned" /> and [[Active Galactic Nucleus|AGN]]<ref name="lambrides">{{cite journal|last1=Lambrides |first1=Erini L. |last2=Petric |first2=Andreea O. |last3=Tchernyshyov |first3=Kirill |last4=Zakamska |first4=Nadia L. |last5=Watts |first5=Duncan J. |title=Mid-infrared spectroscopic evidence for AGN heating warm molecular gas |journal=Monthly Notices of the Royal Astronomical Society |date=2019 |volume=487 |issue=2 |page=1823 |doi=10.1093/mnras/stz1316 |doi-access=free |arxiv=1808.02035 |bibcode=2019MNRAS.487.1823L}}</ref><ref name="SIMBAD" />
| appmag_v           = 15.98<ref name="ned" />
| size               = {{cvt|1.58|x|1.12|kpc|ly|sigfig=3|abbr=on|lk=on|disp=x|<br />(|)|order=flip}}<br />{{small|(''[[Galaxy#Isophotal diameter|diameter]]; 25.0 B-mag arcsec{{sup|−2}}'')}}<ref name="ned" />
| size_v             = {{val|2.0|×|2.0|ul=arcminute}}<ref name="gildepaz">{{cite journal|last1=Gil De Paz |first1=Armando |last2=Boissier |first2=Samuel |last3=Madore |first3=Barry F. |last4=Seibert |first4=Mark |last5=Joe |first5=Young H. |last6=Boselli |first6=Alessandro |last7=Wyder |first7=Ted K. |last8=Thilker |first8=David |last9=Bianchi |first9=Luciana |last10=Rey |first10=Soo-Chang |last11=Rich |first11=R. Michael |last12=Barlow |first12=Tom A. |last13=Conrow |first13=Tim |last14=Forster |first14=Karl |last15=Friedman |first15=Peter G. |last16=Martin |first16=D. Christopher |last17=Morrissey |first17=Patrick |last18=Neff |first18=Susan G. |last19=Schiminovich |first19=David |last20=Small |first20=Todd |last21=Donas |first21=José |last22=Heckman |first22=Timothy M. |last23=Lee |first23=Young-Wook |last24=Milliard |first24=Bruno |last25=Szalay |first25=Alex S. |last26=Yi |first26=Sukyoung |title=The GALEX Ultraviolet Atlas of Nearby Galaxies |journal=The Astrophysical Journal Supplement Series |date=2007 |volume=173 |issue=2 |page=185 |doi=10.1086/516636 |arxiv=astro-ph/0606440 |bibcode=2007ApJS..173..185G}}</ref><ref name="SIMBAD" />
| notes              =
| names              = {{odlist|name=I ZW 018|UGCA=166|PGC=27182|MRK=116|LEDA=27182|SDSS=J093401.92+551427.9|2MASS=J09340202+5514272}}<ref name="ned" />
}}

'''I Zwicky 18''' is a [[dwarf galaxy#Blue compact dwarf galaxies|blue compact dwarf]] and [[Wolf-Rayet galaxy|Wolf–Rayet]] [[galaxy]] located about {{convert|18.44|Mpc|e6ly|sigfig=4|abbr=off|lk=on}} away in the [[constellation]] [[Ursa Major]].<ref name="ned" /><ref name=schaerer/> The galaxy was first identified by Swiss [[astronomer]] [[Fritz Zwicky]] in a 1930s photographic [[astronomical surveys|survey]] of galaxies.<ref>{{cite book |author=Ken Croswell |title=The Universe at Midnight: Observations Illuminating the Cosmos |page=[https://archive.org/details/universeatmidnig00kenc/page/111 111] |url=https://archive.org/details/universeatmidnig00kenc|url-access=registration |publisher=Free Press |year=2001 |isbn=0684859319 }}</ref> It also possess an [[active galactic nucleus]].<ref name="lambrides"/>

==Age==
Studies at the [[Palomar Observatory]] some 40 years ago led astronomers to believe that the galaxy erupted with star formation billions of years after its galactic neighbours. Galaxies resembling I Zwicky 18's youthful appearance are typically found only in the early universe. Early observations with the [[Hubble Space Telescope]] suggested an age of 500 million years old for I Zwicky 18.<ref>{{cite web |date=December 1, 2004 |title=Hubble Uncovers a Baby Galaxy in a Grown-Up Universe |url=https://hubblesite.org/contents/news-releases/2004/news-2004-35.html |publisher=NASA}}</ref>

Astronomers have hypothesised the existence of young dwarf galaxies in a [[cold dark matter]]-dominated Universe. Dwarfs as young as 500 million years would be, however, much fainter than I Zwicky 18. I Zwicky 18 is thus unlikely to have formed all its stars recently in a cold dark matter Universe.<ref>{{Cite journal|last1=Babul|first1=A.|last2=Rees|first2=M. J.|date=1992-03-15|title=On dwarf elliptical galaxies and the faint blue counts|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=255|issue=2|pages=346–350|doi=10.1093/mnras/255.2.346|issn=0035-8711|doi-access=free}}</ref><ref>{{Cite journal|last1=Benitez-Llambay|first1=Alejandro|last2=Fumagalli|first2=Michele|date=2021-11-01|title=The Tail of Late-forming Dwarf Galaxies in ΛCDM|journal=The Astrophysical Journal Letters|volume=921|issue=1|pages=L9|doi=10.3847/2041-8213/ac3006| arxiv=2110.08279|bibcode=2021ApJ...921L...9B |s2cid=239016084 |issn=2041-8205 |doi-access=free }}</ref>

Later observations with the [[Hubble Space Telescope]] found faint and old stars contained within the galaxy, suggesting its star formation started at least one billion years ago and possibly as much as ten billion years ago. The galaxy, therefore, may have formed at the same time as most other galaxies.<ref name="Hubble2007">{{cite press release  |date=October 16, 2007 |title=Hubble shows 'baby' galaxy is not so young after all |url=http://www.spacetelescope.org/news/heic0716/ |location=Hubblesite |publisher=NASA |access-date=2015-03-24 }}</ref>

==Contents==
[[Image:Hubble - infant galaxy.jpg|thumb|left|I Zwicky 18 and its component or companion '''I Zwicky 18 C''']]
[[Astronomical spectroscopy|Spectroscopic]] observations with ground-based telescopes have shown that I Zwicky 18 is almost exclusively composed of [[hydrogen]] and [[helium]], the main ingredients created in the [[Big Bang]]. The galaxy's primordial makeup suggests that its rate of star formation has been much lower than that of other galaxies of similar age. The galaxy has been studied with most of NASA's telescopes, including the [[Spitzer Space Telescope]], the [[Chandra X-ray Observatory]], the [[Far Ultraviolet Spectroscopic Explorer]] (FUSE),<ref name="hubble2007">{{cite web |title=Hubble Finds 'Dorian Gray' Galaxy |url=https://hubblesite.org/contents/news-releases/2007/news-2007-35.html |access-date=2007-10-16 |work=HubbleSite NewsCenter}}</ref> and the [[James Webb Space Telescope]].<ref>{{Cite web |date=2024-03-26 |title=A duo of starbursts in I Zwicky 18 |url=https://esawebb.org/images/potm2403a/ |access-date=2024-03-26 |website=ESA/Webb}}</ref> However, it remains a mystery why I Zwicky 18 formed so few stars in the past, and why it is forming so many new stars right now.<ref name="hubble2007" />

In 2015 a study found a very large region of ionised helium in this small galaxy, which tends to be more frequent in very distant galaxies with low presence of metals. The ionization of helium implies the presence of objects emitting a radiation intense enough to knock electrons off the helium atoms.<ref>{{cite web |title=IZw18: the galaxy that reveals the history of the universe |publisher=Instituto de Astrofísica de Andalucía |url=http://www.iaa.es/content/izw18-galaxy-reveals-history-universe |date=March 23, 2015 |access-date=March 24, 2015 |archive-date=March 27, 2015 |archive-url=https://web.archive.org/web/20150327000140/http://www.iaa.es/content/izw18-galaxy-reveals-history-universe |url-status=dead }}</ref> Scientists therefore hypothesise that I Zwicky 18 must contain [[Population III]] stars, which are almost absent in all other galaxies in the local universe.<ref>{{cite journal |author1=C. Kehrig |author2=J.M. Vilchez |author3=E. Perez-Montero |author4=J. Iglesias-Paramo |author5=J. Brinchmann |author6=D. Kunth |author7=F. Durret |author8=F.M. Bayo |title=The extended HeIIλ4686-emitting region in IZw18: clues for peculiar ionizing sources
|journal=The Astrophysical Journal Letters |volume=801 |number=2 |date=March 10, 2015 |doi=10.1088/2041-8205/801/2/L28 |arxiv=1502.00522 |bibcode = 2015ApJ...801L..28K |page=L28|s2cid=119257004 }}</ref>

==Distance==
Astronomers at the [[Space Telescope Science Institute]] in [[Baltimore, Maryland]] and the [[European Space Agency]] discerned the distance by observing [[Cepheid variable]] stars within I Zwicky 18.  These massive flashing stars pulse in a regular rhythm. The timing of their pulsations is directly related to their brightness. The team determined the [[Apparent magnitude|observed brightness]] of three Cepheids and compared it with the [[Absolute magnitude|actual brightness]] predicted by theoretical models. These models were calculated specifically for I Zwicky 18's deficiency in heavy elements, indicating the galaxy's stars formed before these elements were abundant in the universe. The Cepheid distance was also validated through the observed brightness of the brightest red stars older than one billion years.<ref name="Hubble2007" />

The Hubble data suggest that I Zwicky 18 is 59 million [[light-year]]s from Earth. The galaxy's larger-than-expected distance may explain why astronomers have had difficulty detecting older, fainter stars within the galaxy. The faint, old stars in I Zwicky 18 are almost at the limit of Hubble's resolution and sensitivity.<ref>{{cite web |title=Baby Galaxy Not So Young After All |url=http://www.esa.int/Our_Activities/Space_Science/Hubble_shows_baby_galaxy_is_not_so_young_after_all |publisher=[[European Space Agency]] |date=16 October 2007 }}</ref>

==Classification==
I Zwicky 18 is classified as a dwarf irregular galaxy and also a [[Dwarf galaxy#Blue compact dwarf galaxies|blue compact galaxy]]<ref>{{cite journal |author1=R. Contreras Ramos |author2=F. Annibali |author3=G. Fiorentino |author4=M. Tosi |author5=A. Aloisi |author6=G. Clementini |author7=M. Marconi |author8=I. Musella |author9=A. Saha |author10=R. P. van der Marel |title=Spatial distribution and evolution of the stellar populations and candidate star clusters in the Blue Compact Dwarf I Zwicky 18 Galaxy |journal=[[The Astrophysical Journal]] |arxiv=1106.5613 |date=28 June 2011 |bibcode = 2011ApJ...739...74C |doi = 10.1088/0004-637X/739/2/74 |volume=739 |issue=2 |page=74|s2cid=118459969 }}</ref> and a [[starburst galaxy]].<ref>{{cite journal |author1=Dorottya Szecsi |author2=Norbert Langer |title= Massive Star Evolution in the Dwarf Galaxy I Zwicky 18 |journal=Massive Stars: From Alpha to Omega |date=1 June 2013 |page=103 |bibcode=2013msao.confE.103S }}</ref> I Zwicky 18 is much smaller than our [[Milky Way]] and a mere {{convert|1.58|kpc|ly|sigfig=3|abbr=off|lk=on}} across.<ref name="ned" /> I Zwicky 18 may still be creating [[Metallicity#Stars|Population III stars]]—[[spectroscopy]] shows that its stars are composed almost entirely of [[hydrogen]] and [[helium]], with heavier [[chemical element|elements]] almost completely absent.<ref>{{cite journal|author1=Lequeux, J.|author2=Viallefond, F.|title=H I observations and star formation in the blue compact galaxy IZw 18|journal=Astronomy and Astrophysics|volume=91|issue=3|pages=269–275|year=1980|bibcode=1980A&A....91..269L}}</ref> The concentrated bluish-white knots embedded in the heart of the galaxy are two major starburst regions where stars are forming at a furious rate. The wispy blue filaments surrounding the central starburst regions are bubbles of gas that have been blown away by stellar winds and supernova explosions from a previous generation of hot, young stars. This gas is now heated by intense ultraviolet radiation unleashed by a new generation of hot, young stars.<ref>{{cite web |title=Hubble Finds Mature Galaxy Masquerading as Toddler |url=http://m.hubblesite.org/gallery/pr2007035a |publisher=NASA |year=2006 |access-date=2015-03-26 |archive-date=2015-04-02 |archive-url=https://web.archive.org/web/20150402102831/http://m.hubblesite.org/gallery/pr2007035a |url-status=dead }}</ref>

==See also==
* [[Peekaboo Galaxy]]

==References==
{{reflist}}

==External links==
* {{Cite APOD |title=I Zwicky 18: Young Galaxy |date=3 December 2004 }}
* [https://www.newscientist.com/article.ns?id=dn7056 Astronomers claim first 'dark galaxy' find]
* [http://www.spacetelescope.org/videos/heic0716a/ Zoom in I Zwicky 18.] {{Webarchive|url=https://web.archive.org/web/20150924124635/http://www.spacetelescope.org/videos/heic0716a/ |date=2015-09-24 }}

{{Ursa Major}}

[[Category:Irregular galaxies]]
[[Category:Dwarf irregular galaxies]]
[[Category:Zwicky objects|1-18]]
[[Category:Principal Galaxies Catalogue objects|27182]]
[[Category:UGCA objects]]
[[Category:Ursa Major]]
[[Category:Markarian galaxies]]