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Dwarf galaxy problem
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<h2 id="context">Context</h2> <p>For example, around 38 dwarf galaxies have been observed in the <a href="/facts/Local_Group/kjUefCLr">Local Group</a>, and only around 11 orbiting the <a href="/facts/Milky_Way/ulF6lccN">Milky Way</a>,<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a><a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> yet dark matter simulations predict that there should be around 500 dwarf satellites for the Milky Way alone.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a><a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p><p>The dwarf galaxy problem still persists, but now there are too many observed instead of too few. Based on current simulations there should be around 220 satellite galaxies in the <a href="/facts/Milky_Way/ulF6lccN">Milky Way</a>, however discoveries in 2024 point the number to be around at least 500 satellites. <a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a><a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> While the problem may have been resolved for the Milky Way when taking the <a href="/facts/Luminosity_function_(astronomy)/RRvNLgrH">luminosity function (astronomy)</a> into account, for some early-type galaxies fewer galaxies are being observed than the number predicted by simulations. The problem of finding too many galaxies may lie in the simulations used. <a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a><a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a>This implies that simulations still need to be improved upon in order to solve the dwarf galaxy problem, not just for the Milky Way but for other galaxies. <a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> The <a href="/facts/Vera_C._Rubin_Observatory/n7t9ANEu">Vera C. Rubin Observatory</a>, which finished construction in 2024, will be conducting wide field surveys of the night skies, leading to discoveries which may help scientists better understand this issue. <a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> </p> <h2 id="prospective-resolution">Prospective resolution</h2> <p>There are two main alternatives which may resolve the dwarf galaxy problem: The smaller-sized clumps of dark matter may be unable to obtain or retain the baryonic matter needed to form stars in the first place; or, after they form, dwarf galaxies may be quickly “eaten” by the larger galaxies that they orbit. </p> <h3>Baryonic matter too sparse</h3> <p>One proposal is that the smaller halos do exist but that only a few of them end up becoming visible, because they are unable to acquire enough <a href="/facts/Baryonic_matter/QFwAUWK2">baryonic matter</a> to form a visible dwarf galaxy. In support of this, in 2007 <a href="/facts/W._M._Keck_Observatory/4yhJmNde">the Keck telescopes</a> observed eight newly discovered ultra-faint Milky Way dwarf satellites of which six were around 99.9% dark matter (with a <a href="/facts/Mass-to-light_ratio/CE6P36ah">mass-to-light ratio</a> of about 1,000).<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> Density profiles from 2022 suggest that dwarf galaxies have a constant-density core, though <a href="/facts/Dark_matter/y5HgX55N">dark matter</a> simulations suggest that there should be less baryon density. <a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> </p> <h3>Early demise of young dwarfs</h3> <p>The other popular proposed solution is that dwarf galaxies may tend to merge into the galaxies they orbit shortly after star-formation, or to be quickly torn apart and <a href="/facts/Tidal_force/GJgxo7gp">tidally</a> stripped by larger galaxies, due to complicated orbital interactions. </p><p>Tidal stripping may also have been part of the problem of detecting dwarf galaxies in the first place: Finding dwarf galaxies is an extremely difficult task, since they tend to have low surface brightness and are highly diffuse – so much so that they are close to blending into background and foreground stars. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Dark_galaxy/Ja0gWgLT">Dark galaxy</a></li> <li><a href="/facts/Cold_dark_matter/vL4134Kd">Cold dark matter</a></li> <li><a href="/facts/Cuspy_halo_problem/AeTI9Lx3">Cuspy halo problem</a> (also known as "the core/cusp problem")</li> <li><a href="/facts/List_of_unsolved_problems_in_physics/vJ3d1A83">List of unsolved problems in physics</a></li></ul> <h2 id="footnotes">Footnotes</h2> <ul><li>Bullock (2010). "Notes on the Missing Satellites Problem". <a href="/facts/ArXiv_(identifier)/H6EtgnBe">arXiv</a>:<a href="https://arxiv.org/abs/1009.4505v1">1009.4505v1</a> [<a href="https://arxiv.org/archive/astro-ph.CO">astro-ph.CO</a>].</li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="http://www.space.com/scienceastronomy/060521_dwarf_galaxy.html">"The end of small galaxies"</a>. <i>SPACE.com</i>. 22 May 2006. Retrieved 22 May 2006.</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Mateo, M.L. (1998). "Dwarf Galaxies of the local group". Annual Review of Astronomy and Astrophysics. 36 (1): 435–506. arXiv:astro-ph/9810070. Bibcode:1998ARA&A..36..435M. doi:10.1146/annurev.astro.36.1.435. S2CID 119333888. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Moore, Ben; Ghigna, Sebastiano; Governato, Fabio; Lake, George; Quinn, Thomas; Stadel, Joachim; Tozzi, Paolo (1999). "Dark Matter Substructure within Galactic Halos". Astrophysical Journal Letters. 524 (1): L19 – L22. arXiv:astro-ph/9907411. Bibcode:1999ApJ...524L..19M. doi:10.1086/312287. S2CID 5644398. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Klypin, Anatoly; Kravtsov, Andrey; Valenzuela, Octavio; Prada, Francisco (1999). "Where are the missing galactic satellites?". Astrophysical Journal. 522 (1): 89–92. arXiv:astro-ph/9901240. Bibcode:1999ApJ...522...82K. doi:10.1086/307643. S2CID 12983798. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"Astronomers find long-missing dwarf galaxies—too many of them". www.science.org. Retrieved 2025-02-17. <a href="https://www.science.org/content/article/astronomers-find-long-missing-dwarf-galaxies-too-many-them" target="_blank">https://www.science.org/content/article/astronomers-find-long-missing-dwarf-galaxies-too-many-them</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"Too Many Missing Satellite Galaxies Found | Obsevation Results | Subaru Telescope". 2024-08-02. Archived from the original on 2 August 2024. Retrieved 2025-02-17. <a href="http://web.archive.org/web/20240802021058/https://subarutelescope.org/en/results/2024/06/27/3419.html" target="_blank">http://web.archive.org/web/20240802021058/https://subarutelescope.org/en/results/2024/06/27/3419.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Müller, Oliver; Pawlowski, Marcel S.; Revaz, Yves; Venhola, Aku; Rejkuba, Marina; Hilker, Michael; Lutz, Katharina (2024-04-01). "A too-many-dwarf-galaxy-satellites problem in the M 83 group". Astronomy & Astrophysics. 684: L6. arXiv:2403.08717. Bibcode:2024A&A...684L...6M. doi:10.1051/0004-6361/202348969. ISSN 0004-6361. <a href="https://www.aanda.org/articles/aa/full_html/2024/04/aa48969-23/aa48969-23.html" target="_blank">https://www.aanda.org/articles/aa/full_html/2024/04/aa48969-23/aa48969-23.html</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Kanehisa, Kosuke Jamie; Pawlowski, Marcel S.; Heesters, Nick; Müller, Oliver (2024-06-01). "A too-many dwarf satellite galaxies problem in the MATLAS low-to-moderate density fields". Astronomy & Astrophysics. 686: A280. arXiv:2405.05303. Bibcode:2024A&A...686A.280K. doi:10.1051/0004-6361/202348242. ISSN 0004-6361. <a href="https://www.aanda.org/articles/aa/full_html/2024/06/aa48242-23/aa48242-23.html" target="_blank">https://www.aanda.org/articles/aa/full_html/2024/06/aa48242-23/aa48242-23.html</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Mateo, M.L. (1998). "Dwarf Galaxies of the local group". Annual Review of Astronomy and Astrophysics. 36 (1): 435–506. arXiv:astro-ph/9810070. Bibcode:1998ARA&A..36..435M. doi:10.1146/annurev.astro.36.1.435. S2CID 119333888. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>For a detailed and up to date list see List of Milky Way's satellite galaxies. <a href="/wiki/List_of_Milky_Way%27s_satellite_galaxies" target="_blank">/wiki/List_of_Milky_Way%27s_satellite_galaxies</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Moore, Ben; Ghigna, Sebastiano; Governato, Fabio; Lake, George; Quinn, Thomas; Stadel, Joachim; Tozzi, Paolo (1999). "Dark Matter Substructure within Galactic Halos". Astrophysical Journal Letters. 524 (1): L19 – L22. arXiv:astro-ph/9907411. Bibcode:1999ApJ...524L..19M. doi:10.1086/312287. S2CID 5644398. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Klypin, Anatoly; Kravtsov, Andrey; Valenzuela, Octavio; Prada, Francisco (1999). "Where are the missing galactic satellites?". Astrophysical Journal. 522 (1): 89–92. arXiv:astro-ph/9901240. Bibcode:1999ApJ...522...82K. doi:10.1086/307643. S2CID 12983798. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>"Astronomers find long-missing dwarf galaxies—too many of them". www.science.org. Retrieved 2025-02-17. <a href="https://www.science.org/content/article/astronomers-find-long-missing-dwarf-galaxies-too-many-them" target="_blank">https://www.science.org/content/article/astronomers-find-long-missing-dwarf-galaxies-too-many-them</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>"Too Many Missing Satellite Galaxies Found | Obsevation Results | Subaru Telescope". 2024-08-02. Archived from the original on 2 August 2024. Retrieved 2025-02-17. <a href="http://web.archive.org/web/20240802021058/https://subarutelescope.org/en/results/2024/06/27/3419.html" target="_blank">http://web.archive.org/web/20240802021058/https://subarutelescope.org/en/results/2024/06/27/3419.html</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Müller, Oliver; Pawlowski, Marcel S.; Revaz, Yves; Venhola, Aku; Rejkuba, Marina; Hilker, Michael; Lutz, Katharina (2024-04-01). "A too-many-dwarf-galaxy-satellites problem in the M 83 group". Astronomy & Astrophysics. 684: L6. arXiv:2403.08717. Bibcode:2024A&A...684L...6M. doi:10.1051/0004-6361/202348969. ISSN 0004-6361. <a href="https://www.aanda.org/articles/aa/full_html/2024/04/aa48969-23/aa48969-23.html" target="_blank">https://www.aanda.org/articles/aa/full_html/2024/04/aa48969-23/aa48969-23.html</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Kanehisa, Kosuke Jamie; Pawlowski, Marcel S.; Heesters, Nick; Müller, Oliver (2024-06-01). "A too-many dwarf satellite galaxies problem in the MATLAS low-to-moderate density fields". Astronomy & Astrophysics. 686: A280. arXiv:2405.05303. Bibcode:2024A&A...686A.280K. doi:10.1051/0004-6361/202348242. ISSN 0004-6361. <a href="https://www.aanda.org/articles/aa/full_html/2024/06/aa48242-23/aa48242-23.html" target="_blank">https://www.aanda.org/articles/aa/full_html/2024/06/aa48242-23/aa48242-23.html</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>Müller, Oliver; Pawlowski, Marcel S.; Revaz, Yves; Venhola, Aku; Rejkuba, Marina; Hilker, Michael; Lutz, Katharina (2024-04-01). "A too-many-dwarf-galaxy-satellites problem in the M 83 group". Astronomy & Astrophysics. 684: L6. arXiv:2403.08717. Bibcode:2024A&A...684L...6M. doi:10.1051/0004-6361/202348969. ISSN 0004-6361. <a href="https://www.aanda.org/articles/aa/full_html/2024/04/aa48969-23/aa48969-23.html" target="_blank">https://www.aanda.org/articles/aa/full_html/2024/04/aa48969-23/aa48969-23.html</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>"Too Many Missing Satellite Galaxies Found | Obsevation Results | Subaru Telescope". 2024-08-02. Archived from the original on 2 August 2024. Retrieved 2025-02-17. <a href="http://web.archive.org/web/20240802021058/https://subarutelescope.org/en/results/2024/06/27/3419.html" target="_blank">http://web.archive.org/web/20240802021058/https://subarutelescope.org/en/results/2024/06/27/3419.html</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>Simon, J.D.; Geha, M. (Nov 2007). "The Kinematics of the ultra-faint Milky Way satellites: Solving the missing satellite problem". The Astrophysical Journal. 670 (1): 313–331. arXiv:0706.0516. Bibcode:2007ApJ...670..313S. doi:10.1086/521816. S2CID 9715950. <a href="/wiki/Marla_Geha" target="_blank">/wiki/Marla_Geha</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> <li id="fn:19"><p>Sales, Laura V.; Wetzel, Andrew; Fattahi, Azadeh (2022-06-10), "Baryonic solutions and challenges for cosmological models of dwarf galaxies", Nature Astronomy, 6 (8): 897, arXiv:2206.05295, Bibcode:2022NatAs...6..897S, doi:10.1038/s41550-022-01689-w, retrieved 2025-02-17 <a href="https://arxiv.org/abs/2206.05295" target="_blank">https://arxiv.org/abs/2206.05295</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></p></li> </ol>