Cluster of Galaxies

Cluster of galaxies are the largest and massive structure in universe which are gravitationally bound together.Each cluster of galaxy contain 10 to 1000 galaxies.Galaxies in cluster have a very large peculiar velocity of 800-1000 km/s to be virialized in cluster.

Beside the galaxies it is found that there is a hot plasma gas in the clusters with the name of ICM "Inter cluster mass" which is approximately 2 times massive that the baryonic matter in galaxies.

The mysterious aspect of clusters is that if this structures are in hydrostatic equilibrium they need Dark matter with mass of 6 times more than baryonic matter in cluster.

So in this case there are three main constituents in each cluster:

1)5% galaxies 2)10% X-ray emitting hot gas 3)Dark matter.

It seems that the dark matter ,galaxies and ICM has approximately the same distribution in clusters.

Cluster of galaxies are the most massive and lately formed structure in the hierarchical structure formation model.They masses vary from 10^14 to 10^ 15 solar masses and the diameter of this clusters are from 2 to 10 Mpc.

The nearby important clusters are Virgo cluster, Hercules cluster and Coma cluster.

The galaxy clusters are a rich regions for observational cosmology and consequently theoretical cosmology.

Accreation disks

Accretion disks arises when materials in universe usually being transformed from one celestial to the other object.Accretion means the collecting of additional matter.

The place that accretion disk take place is Binary stars and active galaxy nuclei.

For example consider a white dwarf that a simple star like ours rotate around it.

because of the conservation of angular momentum and gravitational pull from White dwarf to interstellar medium and sun ,spirals appear in disk.

gas become very hot in disk due to friction and being tugged on by white dwarf and eventually looses angular momentum and falls into WD.

The important of accretion disk is that the first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc.

Neutron Star

Collapse of stars,type II supernovae and type Ib and type Ic SN caused to have neutron stars.This stars whic are completely/dominantly/ composed of neutrons have masses between 1.35~2.1 solar mass and radius of 12 km which means that neutron stars have high densities of 3.7~5.9 * 10^17 kg/m^3 .At the very high pressures involved in this collapse, it is energetically favorable to combine protons and electrons to form neutrons plus neutrinos. The neutrinos escape after scattering a bit and helping the supernova happen, and the neutrons settle down to become a neutron star, with neutron degeneracy/Pauli exclusion principle/ managing to oppose gravity.

The neutron stars are formed by high rotation speed because of conservation of angular momentum.Also because of the high speed of stars they can escape from disk of galaxies and can be found in perpendicular direction of disk galaxies.

The high density of neutron star cause to have a escape velocity of about 33% of light speed as a comparison a teaspoon of neutron star has a mass of 5*10^12 kg.

Neutron stars are believed to form in supernovae such as the one that formed the Crab Nebula (or check out this cool X-ray image of the nebula, from the Chandra X-ray Observatory).

up to today there are 2000 known neutron stars are found in milky way and Magellanic clouds.

it is also worth that they appear in binary type with 5%.

Some good links for neutron stars.

http://www.astro.umd.edu/~miller/nstar.html#formation

http://imagine.gsfc.nasa.gov/docs/science/know_l1/pulsars.html

6 puzzles for LCDM

Recently a nice review article appeared in arxivarXiv:0811.4684 by Perivolaropoulos from Greece.

The title is 6 puzzles for LCDM .In this article author pay attention to 6 problems /observational ones/that the LCDM model have problem with them.

In literature always we confront with the two problems of fine tuning and coincidence problem for LCDM ,but there are more to confront.

1)Large scale velocity flow: LCDM predicts smaller amplitude and scale for the flow.

2)Brightness of type I SuperNova at high redshifts :LCDM predicts fainter SuperNova in high redshifts.

3)Emptiness of voids:LCDM predicts more dwarf or irregular galaxies in voids.

4)Profile of Halo clusters:LCDM predicts low and shallower concentration and density.

5)Profile of galaxy halos:LCDM predicts cuspy core and low outer density while lensing and other observations predicts central core of constant density.

6)Size able population of disk galaxies:LCDM predicts less disk galaxies.

This are really interesting probes for dark energy problem

Sweet molecule in our galaxy

An interesting article with id number of 0811.3821 in arXiv is uploaded.

A group of astronomers in University Collage of London found the simplest monosacharite ,

"glycolaldehde" in a distant of 9 kpc in a hot molecular cloud in our galaxy.

The discovery is so interesting because this monosacharite is the block for generating Ribose which is the main block of RNA.

lately there are evidence for glycolaldeyde in the center of galaxy ,but because of high interaction rate and density of this part of galaxy it seemed natural to found it there.

But the detection of spectral line of glycolaldehde by IRAM radio telescope in France in region which star forming is probable is an interesting news for exoearth life science.

molecule

News in science daily:

http://www.sciencedaily.com/releases/2008/11/081125090344.htm

arXiv:0811.3821v1

Reionization

Today in IPM/Tehran/There is a lecture talk about the law frequency radio astronomy.

In the conclusion part, the lecturer state that one of the applications of law frequency radio astronomy is detecting objects in reionization era.

Now what is reionization era.

Let me do a little job to describe it.

In the history of universe two major phase transition occurred for hydrogen.

First of it was in the time of recombination/redshift z~1100 / 400,000years after big bang when the rate of combination of electron to protons became higher than the ionization of hydrogen atom so in consequence of this the hydrogen atoms formed and the universe become transparent for photons.This is the last scattering surface and the photons coming from this redshift are the cosmic microwave background radiation photons.

The second phase transition occurred in reionization era, 150 million to 1 billion year after big band/redshift of 20 to 6/ because of formation of structures high energy became free and the universe /the intergalactic medium /changes it phase and from neutral become ionized and transformed to plasma.In this case the protons and electrons which are not bound can not capture the photons so they become transparent to it.

There are different techniques to found out about this era,which in future posts I will introduce them first of all for my self like.

Techniques are:

Quasars and the Gunn-Peterson trough

CMB anisotropy and polarization

21-cm line