Ultra diffuse galaxy

<p>An ultra diffuse galaxy (UDG), or <a href="/facts/Dark_galaxy/Ja0gWgLT">dark galaxy</a>, is an extremely low luminosity <a href="/facts/Galaxy/zCyUvEf7">galaxy</a>, the first example of which was discovered in the nearby <a href="/facts/Virgo_Cluster/dBsDKTzT">Virgo Cluster</a> by <a href="/facts/Allan_Sandage/Qk1kRoTm">Allan Sandage</a> and <a href="/facts/Bruno_Binggeli/h0bmd1bs">Bruno Binggeli</a> in 1984. These galaxies have been studied for many years prior to their renaming in 2015.  Their lack of luminosity is due to the lack of star-forming gas, which results in these galaxies being reservoirs of very old stellar populations. These galaxies with lower luminosity are more likely to be found in clusters, such as the <a href="/facts/Coma_Cluster/k5Fw8qUH">coma cluster</a>, than in groups. Ultra diffuse galaxies have a similar size to the <a href="/facts/Milky_Way/ulF6lccN">Milky Way</a>, but they have masses more akin to <a href="/facts/Dwarf_galaxy/TKS7W9tA">dwarf galaxies</a>. The formation of ultra diffuse galaxies is still debated, however they're thought to have formed from the earliest of galaxies or <a href="/facts/Tidal_force/GJgxo7gp">tidal forces</a>.
</p><p>Based on discoveries confirmed in 2018, this class of galaxies includes both extremes of <a href="/facts/Dark_matter/y5HgX55N">dark matter</a> content: Some UDGs consist almost entirely of dark matter (such a galaxy may have the same size and mass as the <a href="/facts/Milky_Way/ulF6lccN">Milky Way</a> but a visible star count of only 1%), while other UDGs appear to be almost entirely free of dark matter. 
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Ultra diffuse galaxy open-in-new

Ultra diffuse galaxy