A **galaxy cluster** is a gravitationally bound collection of hundreds to thousands of galaxies, larger than a [[Galaxy Group]]. **Composition** (Percentages of Mass) - Galaxies: $1 \%$ - [[Intracuster Medium|ICM]]: $9 \%$ - [[Dark Matter]]: $90 \%$ > [!measure] Typical Parameters > - Member Galaxies: $N \sim 10^{2} - 10^{3}$ > - Mass: $M \sim 10^{14}-10^{15} \; {\rm M_{\odot}}$ > - Diameter: $D \sim 1-5 \; {\rm Mpc}$ ## Examples ### Bullet Cluster The [[Galaxy Cluster#Bullet Cluster]] is a smoking gun for an alternative massive and abundant species not coupled (in a detectable way currently) to electromagnetism. It is a system of galaxies which have passed through each other - In observations of the hot X-ray gas, we find that the gas and stars (which interact with each other via some complicated effective cross section) have experienced a large degree of friction during the collision, and have slowed down and become largely disrupted. - We can also map the mass distribution of two galaxies which just passed through each other using [[Gravitational Lensing#Weak Lensing|weak lensing]], and what we find is that most of the mass is invisible, and has travelled further and become less disrupted than the gas. This suggests a species which interacts with baryons and itself weakly (so less friction, can constrain self-interaction cross section of dark matter via this measurement) and not with light. Below are two views of the bullet cluster. - The first view (top-left) is a composite image of mass map (x-ray emission) and background galaxies used for the [[Gravitational Lensing#Weak Lensing|weak lensing]] analysis. - The second view (top-right & bottom) shows explicitly the mass contours obtained via [[Gravitational Lensing#Weak Lensing|weak lensing]]. ![[lensing_weak_bulletCluster.png]] ### Coma Cluster One of the first places where observed gravitational anomalies were considered to be indicative of unobserved mass. In 1933 Fritz Zwicky showed that the galaxies of the Coma Cluster were moving too fast for the cluster to be bound together by the visible matter of its galaxies $\implies$ Dark matter. > [!measure] Parameters > - $100 \; {\rm Mpc}$ away ($z \simeq 0.02$) > - $7 \times 10^{14} \; {\rm M_\odot}$ ### Norma Cluster ### Perseus cluster A galaxy cluster in the Perseus constellation. Also known as [[Catalogs#Abell|Abell]] 246. Populated by ~thousands of galaxies. Brightest X-ray cluster. Exhibits a "flattened" bubble resulting from a jet, flattening is evidence of efficient energy transfer via gravity waves (from a MAT talk, cant remember name). > [!measure] Parameters > - Recession speed: ~5000 $\mathrm{km}\,\mathrm{s}^{-1}$ > - Size: $15'$ $\simeq$ $300$ kpc (relatively small? Mike: probably just the size of the core) > - Distance: $74 \,\pu{Mpc}$ > - Redshift: $z = 0.018$ ### Pheonix Cluster ### Virgo Cluster ![[virgo.jpg|align:center|450]] A large galaxy cluster in the Virgo constellation containing [[Catalogs#M]]87 (lower left below, whose [[Black Hole#Supermassive Black Hole|SMBH]] we observed with [[Instruments#EHT]]). Its at the center of the "Virgo supercluster" which contains the [[Galaxy Group#Local Group]]. > [!measure] Parameters > - Mass: $\sim 10^{15} \; {\rm M_{\odot}}$ > - Distance: $16.5 \; {\rm Mpc}$