Willman 1

{{Use dmy dates|date=July 2021}}

{{Short description|Ultra-low luminosity dwarf galaxy}}

{{Infobox Galaxy

| image = Willman 1 area.jpg

| name = Willman 1

| epoch = J2000

| type = extreme dSph or

unusual star cluster{{Harv|Willman|Blanton|West|Dalcanton|2005}}

| ra = {{RA|10|49|22.3}}

| dec = {{DEC|+51|03|03.6}}

| dist_ly = 124 ± 23 kly (38 ± 7 kpc)

| z =

| appmag_v = 15.2 ± 0.4{{Ref_label|A|a|none}}

| size_v = 4.6{{±|0.4|0.8}}′

| constellation name = Ursa Major

| notes =

| names = SDSS J1049+5103

}}

Willman 1 is an ultra low-luminosity dwarf galaxy or a star cluster.{{Cite journal |last1=Willman |first1=Beth |last2=Geha |first2=Marla |last3=Strader |first3=Jay |last4=Strigari |first4=Louis E. |last5=Simon |first5=Joshua D. |last6=Kirby |first6=Evan |last7=Ho |first7=Nhung |last8=Warres |first8=Alex |date=2011-10-01 |title=WILLMAN 1—A PROBABLE DWARF GALAXY WITH AN IRREGULAR KINEMATIC DISTRIBUTION |url=https://iopscience.iop.org/article/10.1088/0004-6256/142/4/128 |journal=The Astronomical Journal |volume=142 |issue=4 |pages=128 |doi=10.1088/0004-6256/142/4/128 |arxiv=1007.3499 |bibcode=2011AJ....142..128W |issn=0004-6256}} Willman 1 was discovered in 2004 and is located near Ursa Major in the night sky.{{Cite web |last=Britt |first=Robert Roy |date=2004-10-25 |title=Puzzling Milky Way Companion Found |url=https://www.space.com/470-puzzling-milky-companion.html |access-date=2024-09-18 |website=Space.com |language=en}}{{Cite journal |last1=Zucker |first1=D. B. |last2=Belokurov |first2=V. |last3=Evans |first3=N. W. |last4=Kleyna |first4=J. T. |last5=Irwin |first5=M. J. |last6=Wilkinson |first6=M. I. |last7=Fellhauer |first7=M. |last8=Bramich |first8=D. M. |last9=Gilmore |first9=G. |last10=Newberg |first10=H. J. |last11=Yanny |first11=B. |last12=Smith |first12=J. A. |last13=Hewett |first13=P. C. |last14=Bell |first14=E. F. |last15=Rix |first15=H.-W. |date=2006-10-10 |title=A Curious Milky Way Satellite in Ursa Major |url=https://iopscience.iop.org/article/10.1086/508628 |journal=The Astrophysical Journal |language=en |volume=650 |issue=1 |pages=L41–L44 |doi=10.1086/508628 |arxiv=astro-ph/0606633 |bibcode=2006ApJ...650L..41Z |issn=0004-637X}} It is named after Beth Willman of Haverford College, the lead author of a study based on the Sloan Digital Sky Survey data.{{Cite book |last=Kanipe |first=Jeff |url=https://archive.org/details/chasinghubblessh00kani/page/152/mode |title=Chasing Hubble's shadows: the search for galaxies at the edge of time |date=2006 |publisher=Hill and Wang |isbn=978-0-8090-3406-2 |location=New York |pages=153 |access-date=2024-09-18}} The object is a satellite of the Milky Way, at ~120,000 light-years away.{{Cite journal |last1=Willman |first1=Beth |last2=Blanton |first2=Michael R. |last3=West |first3=Andrew A. |last4=Dalcanton |first4=Julianne J. |last5=Hogg |first5=David W. |last6=Schneider |first6=Donald P. |last7=Wherry |first7=Nicholas |last8=Yanny |first8=Brian |last9=Brinkmann |first9=Jon |date=2005 |title=A New Milky Way Companion: Unusual Globular Cluster or Extreme Dwarf Satellite? |url=https://iopscience.iop.org/article/10.1086/430214 |journal=The Astronomical Journal |language=en |volume=129 |issue=6 |pages=2692–2700 |doi=10.1086/430214 |arxiv=astro-ph/0410416 |bibcode=2005AJ....129.2692W |issn=0004-6256}} Willman 1 has an elliptical shape with the half-light radius of about 25 pc.{{Cite journal |last1=Martin |first1=Nicolas F. |last2=de Jong |first2=Jelte T. A. |last3=Rix |first3=Hans-Walter |date=2008-09-10 |title=A Comprehensive Maximum Likelihood Analysis of the Structural Properties of Faint Milky Way Satellites |url=https://iopscience.iop.org/article/10.1086/590336 |journal=The Astrophysical Journal |language=en |volume=684 |issue=2 |pages=1075–1092 |doi=10.1086/590336 |arxiv=0805.2945 |bibcode=2008ApJ...684.1075M |issn=0004-637X}} Its heliocentric velocity is approximately −13 km/s.

As of 2007, it was declared the least massive galaxy known, opening up a new category of ultra-low-mass galaxies, lower than the then-theoretical minimum of 10 million solar masses thought to be needed to form a galaxy.{{Cite web |last=Shiga |first=David |date=2007-06-04 |title=Smallest galaxy hints at hidden population |url=https://www.newscientist.com/article/dn11980-smallest-galaxy-hints-at-hidden-population/ |access-date=2024-09-18 |website=New Scientist |language=en-US}}

As of 2016, it is the third dimmest likely galaxy known, after Segue 1 and Virgo I, and is over ten million times less luminous than the Milky Way. It has an absolute magnitude of {{nowrap|−2.7 ± 0.7}}. Observations indicate its mass is about 0.4 million solar masses, which means that Willman's 1 mass to light ratio is around 800. A high mass to light ratio implies that Willman 1 is dominated by dark matter.{{Cite journal |last1=Loewenstein |first1=Michael |last2=Kusenko |first2=Alexander |date=2012-06-01 |title=DARK MATTER SEARCH USING XMM-NEWTON OBSERVATIONS OF WILLMAN 1 |url=https://iopscience.iop.org/article/10.1088/0004-637X/751/2/82 |journal=The Astrophysical Journal |volume=751 |issue=2 |pages=82 |doi=10.1088/0004-637X/751/2/82 |arxiv=1203.5229 |bibcode=2012ApJ...751...82L |issn=0004-637X}}{{Cite journal |last1=McDaniel |first1=Alex |last2=Ajello |first2=Marco |last3=Karwin |first3=Christopher M. |last4=Di Mauro |first4=Mattia |last5=Drlica-Wagner |first5=Alex |last6=Sánchez-Conde |first6=Miguel A. |date=2024-03-19 |title=Legacy analysis of dark matter annihilation from the Milky Way dwarf spheroidal galaxies with 14 years of Fermi -LAT data |url=https://link.aps.org/doi/10.1103/PhysRevD.109.063024 |journal=Physical Review D |language=en |volume=109 |issue=6 |page=063024 |doi=10.1103/PhysRevD.109.063024 |arxiv=2311.04982 |bibcode=2024PhRvD.109f3024M |issn=2470-0010}} It is difficult, however, to estimate the mass of such faint objects because any mass estimate is based on an implicit assumption that an object is gravitationally bound, which may not be true if the object is in a process of disruption.

The stellar population of Willman 1 consists mainly of old stars formed more than 10 billion years ago.{{Citation |title=Seven. The Experimental Hunt for Dark Matter Particles |date=2014-12-31 |work=The Cosmic Cocktail: Three Parts Dark Matter |pages=128–146 |url=https://www.degruyter.com/document/doi/10.1515/9781400850075-008/html |access-date=2024-09-18 |publisher=Princeton University Press |doi=10.1515/9781400850075-008 |isbn=978-1-4008-5007-5|url-access=subscription }} The metallicity of these stars is also very low at {{nowrap|[Fe/H] ≈ −2.1}}, which means that they contain 110 times less heavy elements than the Sun.