Astronomy | Test 4 - Part II: Galactic masses and dark matter

Dark matter

Visible portion of a galaxy lies deep in the heart of a large halo of dark matter, with there being ten times more dark matter than visible matter

Spiral galaxies and dark matter

Spiral galaxies all tend to have flat rotation curves, indicating large amounts of dark matter

Groups of galaxies

Calculating gravitational mass and measuring mass indicates huge disrepancies, indicating the presence of large amounts of non-luminous/dark matter

Local Group

Contains the Milky Way, which is an example of a poor cluster

* Few dozen members, with brighter galaxies being
* 14 elliptical
* Three spiral
* Four irregular

Mass of a cluster

* In a group of galaxies like a cluster, only important force acting between the galaxies is gravitation, which gives rise to their velocities
* Velocities can indicate the total mass inside the cluster
* More mass in the cluster, greater the forces acting on each galaxy

Missing mass problem

Out of the calculated mass of a cluster, only a fraction can be identified with matter that is emitting visible light

Dark matter in clusters

The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars

Particle motions and cluster mass

Clusters contain large amounts of X-ray emitting hot gas, with the temperature of these hot gases (*particle motions*) telling us that the cluster mass is:

* 85% dark matter
* 13% hot gas
* 2% stars

Estimating gravitational mass

* Observation of galaxies orbiting massive central elliptical galaxies: Galaxies move like bees around a hive, and from their orbits and velocities we can calculate the mass of a cluster
* Gravitational lensing: Effect a cluster has on light from objects in the background

Solutions for difference in observed/gravitational mass

Either:

* Dark matter really exists, and we are observing the effects of its gravitational attraction
* As gravity is so well tested, this is the assumed solution
* Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter

What is dark matter made of?

* *Ordinary matter (baryonic)*: It could be made up of our matter (protons, neutrons, and electrons) with its only unusual property being how dim it is
* *Nonbaryonic matter (extraordinary)*: It could be made of particles we have yet to discover

MACHOs and WIMPs

MACHOS (*massive compact halo objects*):

* Ordinary dark matter
* Dead or failed stars in halos of galaxies

WIMPS (*weakly interacting massive particles*):

* Extraordinary dark matter
* Mysterious neutrino-like particles
* Best set

Collision of celestial bodies

* Galaxies collide with each other as the average separation between galaxies is only about 20 times their diameter
* Stars almost never collide because the typical separation between them is about 10,000,000 times their diameter

Andromeda

Through the modeling of galaxy formations, models indicate that Andromeda and the Milky Way will collide and merge to make an elliptical galaxy

Supermassive black holes

Orbits of stars at the centre of the Milky Way indicate a black hole with the mass of 4 billion solar masses

Black holes in galaxies

* Many nearby galaxies (perhaps all) have supermassive black holes at their centres, which seem to be dormant active galactic nuclei
* These galaxies may have passed through a quasar-like stage earlier in time

Mass of the local supercluster

* Estimated mass of Local Supercluster is 10¹⁵ solar masses
* Corresponding ratio of visible and invisible mass for the Local Supercluster is 570
* Indicator of the presence of large amounts of dark matter in the Local Supercluster