Leo A

{{short description|Irregular galaxy in the Local Group}}

{{Sky|09|59|26.4|+|30|44|47|2250000}}

{{Infobox Galaxy

| name=Leo A

| image=Leo A Hubble WikiSky.jpg

| image_size=250px

| caption=Leo A by Hubble Space Telescope

| epoch=J2000

| type=IBm{{cite web

| title=NASA/IPAC Extragalactic Database

| work=Results for Leo A

| url=http://nedwww.ipac.caltech.edu/

| accessdate=2006-11-19 }}

| ra={{RA|09|59|26.4}}

| dec={{DEC|+30|44|47}}

| dist_ly=2.6 ± 0.1 Mly (790 ± 40 kpc){{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| doi-access=free

}}{{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 |s2cid=120973010 }}

| z=0.000067

| appmag_v=12.9

| size_v=5.1{{prime}} × 3.1{{prime}}

| constellation name=Leo

| names=Leo III, UGC 5364, DDO 69, PGC 28868

}}

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.{{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 | url=https://authors.library.caltech.edu/4492/1/ZWIpr42.pdf }} The estimated mass of this galaxy is {{nowrap|(8.0 ± 2.7) × 10⁷}} solar masses, with at least 80% consisting of dark matter.{{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 |s2cid=16639879

}} 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.{{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 |s2cid=14268757 }} The presence of RR Lyrae variables shows that the galaxy has an old stellar population that is up to 10 billion years in age.{{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 |s2cid=14883376

}}

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.{{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}}