| 词条 | Leo A |
| 释义 |
| name=Leo A | image=Leo A Hubble WikiSky.jpg | image_size=250px | caption=Leo A by Hubble Space Telescope | epoch=J2000 | type=IBm[1] | ra={{RA|09|59|26.4}}[1] | dec={{DEC|+30|44|47}}[1] | dist_ly=2.6 ± 0.1 Mly (790 ± 40 kpc)[2][3] | z=0.000067[1] | appmag_v=12.9[1] | size_v=5′.1 × 3′.1[1] | constellation name=Leo | names=Leo III, UGC 5364, DDO 69, PGC 28868[1] }}Leo A (also known as Leo III) is an irregular galaxy that is part of the Local Group. It lies 2.6 million light-years from Earth, and was discovered by Fritz Zwicky in 1942.[4] The estimated mass of this galaxy is {{nowrap|(8.0 ± 2.7) × 107}} solar masses, with at least 80% consisting of an unknown dark matter.[5] It is one of the most isolated galaxies in the Local Group and shows no indications of an interaction or merger for several billion years. However, Leo A is nearly unique among irregular galaxies in that more than 90% of its stars formed more recently than 8 billion years ago, suggesting a rather unusual evolutionary history.[6] The presence of RR Lyrae variables shows that the galaxy has an old stellar population that is up to 10 billion years in age.[7] The neutral hydrogen in this galaxy occupies in a volume similar to its optical extent, and is distributed in a squashed, uneven ring. The galaxy is not rotating and the hydrogen is moving about in random clumps. The proportion of elements with higher atomic numbers than helium is only about 1-2% of the ratio in the Sun. This indicates a much less complete conversion of gas into stars than in the Milky Way galaxy. The Leo A galaxy shows sign of increased star formation some time within the last 1-4 billion years, although the current level is low. There are four H II regions powered by short-lived, O-class stars.[9] References1. ^1 2 3 4 5 6 {{cite web | title=NASA/IPAC Extragalactic Database | work=Results for Leo A | url=http://nedwww.ipac.caltech.edu/ | accessdate=2006-11-19 }} 2. ^{{cite journal |author1=I. D. Karachentsev |author2=V. E. Karachentseva |author3=W. K. Hutchmeier |author4=D. I. Makarov | title=A Catalog of Neighboring Galaxies | journal=Astronomical Journal | date=2004 | volume=12745345 | issue=4 | pages=2031–2068 | bibcode=2004AJ....127.2031K | doi=10.1086/382905}} 3. ^{{cite journal |author1=Karachentsev, I. D. |author2=Kashibadze, O. G. | title=Masses of the local group and of the M81 group estimated from distortions in the local velocity field | journal=Astrophysics | date=2006 | volume=49 | issue=1 | pages=3–18 | bibcode=2006Ap.....49....3K | doi=10.1007/s10511-006-0002-6 }} 4. ^{{cite journal | last=Zwicky | first=F |date=April 1942 | title=On the Large Scale Distribution of Matter in the Universe | journal=Physical Review | volume=61 | issue=7–8 | pages=489–503 | doi=10.1103/PhysRev.61.489 |bibcode = 1942PhRv...61..489Z }} 5. ^{{cite journal | author=Brown, Warren R. |display-authors=etal | title=Stellar Velocity Dispersion of the Leo A Dwarf Galaxy |date=September 2007 | journal=The Astrophysical Journal | volume=666 | issue=1 | pages=231–235 | doi=10.1086/519547 | bibcode=2007ApJ...666..231B|arxiv = 0705.1093 }} 6. ^{{cite journal| author=Cole, Andrew |date=March 2007 |display-authors=etal|title=Leo A: A Late-blooming Survivor of the Epoch of Reionization in the Local Group|journal=The Astrophysical Journal Letters|volume=659|issue=1|pages=L17-20| doi=10.1086/516711|bibcode=2007ApJ...659L..17C|arxiv=astro-ph/0702646 }} 7. ^{{cite journal | author=Vansevičius, Vladas |date=August 2008 |display-authors=etal | title=The Full-fledged Dwarf Irregular Galaxy Leo A | journal=The Astrophysical Journal | volume=611 | issue=2 | pages=L93–L96 | doi=10.1086/423802 | bibcode=2004ApJ...611L..93V|arxiv = astro-ph/0405313 }} 8. ^{{cite web|title=A case of suspended animation?|url=http://www.spacetelescope.org/images/potw1615a/|accessdate=11 April 2016}} 9. ^{{cite journal |author1=Young, L. M. |author2=Lo, K. Y. | title=The Neutral Interstellar Medium in Nearby Dwarf Galaxies. I. Leo A | journal=Astrophysical Journal | volume=462 | pages=203–214 | doi=10.1086/177141 | date=1996| bibcode=1996ApJ...462..203Y}} External links
7 : Dwarf galaxies|Irregular galaxies|Leo (constellation)|Local Group|UGC objects|Principal Galaxies Catalogue objects|Astronomical objects discovered in 1942 |
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