Galaxies and Super clusters

← Astro210

stellar streams

• Galaxies (r $\sim 10$ kpc)
• Groups (r $\sim$ 1Mpc)
• Supercluster (r $\sim$ 100Mpc)

\large Coma Cluster

• rich cluster dominated by elliptical Galaxies

\large Cluster ISM

• Bright x-ray emitting gas
• low density, but very large quality
• Total mass of ISM is around double that of the galaxies
• Motion through the ISM causes ram pressure stripping

Dark matter in galaxy clusters

• Use viral theorem…
• $M\approx 7.5\frac{{\sigma }^2{r}_h}{G}$
• Where $\sigma$ is a velocity distribution, and $r_h$ is radius of the cluster
• If we look at the mass to light radio of the Coma Cluster, which is 250 (in solar units), we find a very different distribution as compared to our sun ($=1$), and the Milky Way ($\sim 10$)

Large Scale Structure

• Cosmic web structure
• Sloan great wall
• Few galaxies measured far away, simply because they’re hard to measure

Galaxy collisions

• Can estimate velocity is cluster from velocity dispersion ($\sigma$) of galaxy motions
• $v\sim \sqrt{3}\sigma$
• Can then compute collision rates
• In rich clusters about a $50\%$ chance

Rates of collisions

Consider a galaxy of radii $R$ moving at velocity in, in an elapsed time $t$ it will collide with any other galaxy within a cylindrical volume $V(t)$ of radius $R^2$ and length $vt$ $V(t)(4{\pi }^2)(vt)$ If galaxy density is n then the total collisions is $N_{\star }$ and the average time to collision is $t_{\star }$ $N_{\star }=nV(t)=v(4\pi R^2)(vt)$ $N_{\star }=1\to t_{\star }=\frac{1}{nv(4\pi R)}$

Galaxies collide, but stars to not

If you repeat the previous equation but for stars, you find a $1$ in $10^8$ chance of stellar collision, in the age of the entire universe

Physics of collisions

• Raise tidal bulges and resulting tidal tails
• They are inelastic, the kinetic energy is converted into funny motions of the stars themselves