Elliptical galaxy
An elliptical galaxy is a galaxy that is different from other forms of galaxies in the Hubble sequence by its uniform light distribution and the absence of conspicuous structures as in spiral galaxies. Elliptical galaxies are among the oldest stellar populations in the universe. It is widely assumed that they have different fusion and interaction processes with other galaxies behind. They consist usually of old stars and contain little interstellar gas, since this has been used up long ago. Therefore, the star formation rate is very low. These galaxies are surrounded by a large number of globular clusters. The mass range of the elliptical galaxies begins with small dwarf galaxies with a few million stars and reaches the central cluster galaxies values of up to several trillion solar masses. Within a radius of about 30 million light-years of the Milky Way about 34 percent of the galaxies spirals, ellipses, and 13 percent 53 percent are irregular galaxies and dwarf galaxies.
- 2.1 Composition
- 2.2 The central black hole
- 2.3 rotation and orbits of the stars
- 2.4 Faber -Jackson relation
- 2.5 Boxlike and disk-like systems
- 2.6 shell structures
- 2.7 Halo
- 2.8 fundamental plane
Morphology
Classification according to Hubble diagram
Elliptical galaxies are relatively bright at its center, with an initially rapid and then slower decrease in brightness to the outside. The lines of equal brightness ( isophotes ) can be quite accurately described by concentric ellipses. In the Hubble sequence elliptical galaxies are distinguished by the shape of these ellipses in classes E0 ( circular) to E7 (strongly flattened ). E to the added number is determined here as a, where a is the size of the major and the minor axis b of the ellipse. It should be noted that the arrangement also depends strongly on the angle of view of our Galaxy.
Messier 32 Type: CE2
NGC 4125 Type: E6
Subdivision
In addition to the ellipticity of the elliptical galaxies can also be classified on the basis of other characteristics:
Brightness distribution
This course one forms a brightness profile of an elliptical galaxy by measuring the brightness with the distance to the center, it follows a de Vaucouleurs profile. This expresses a linear relationship between the logarithm of the intensity and the distance from the center.
The analysis of the brightness distribution is one of the most important tools to identify the properties and development of elliptical galaxies. This analysis technique the lines of equal brightness ( isophotes ) ellipses are assigned. The centers of the ellipses are usually so determined very accurately centered at the center of the galaxy. However, the ellipticity can vary with the radius. The derived ellipticity and the angle of the semimajor axis provides basic information to the Galaxy, such as the effective radius, triaxiality or a possible Isophotenverdrehung ( Isophotentwist ). At a Isophotenverdrehung the angle of the major semi-axis of the ellipse changes with increasing radius. The brightness distribution of a galaxy can show a deviation from the ideal shape of an ellipse. There are box-shaped ( boxiness ) and discoid (disk iness) systems. These distributions provide important information on the physical properties of an elliptical galaxy.
Physics
Composition
Elliptical galaxies consist mainly of older stars of Population II, which shows itself in that they have a reddish color. Small ellipses may also contain younger stars. Previously it was assumed that these galaxies contain almost no gas and dust. Through observations in X-rays, however, you could also discover hot gas with a mass of several million solar masses. In addition, up to 50% of galaxies contain a higher proportion of dust. This can be interpreted as an indication of former galaxy collisions.
Central Black Hole
Since the late 1990s, observations show increasingly clear that the center of each elliptical galaxy and each bulge of a spiral galaxy is to find a black hole, which has a few thousand of the mass of the elliptical galaxy or the bulge. In elliptical galaxies with a mass range of 1 ⋅ 106-1 ⋅ 1010 solar masses, these M -sigma relation was mentioned relationship found.
The processes in the formation and evolution of galaxies and black holes that lead to this relationship are still unclear. However, black holes seem to play an important role in the evolution of elliptical galaxies.
Rotation and orbits of the stars
Earlier it was often assumed that this flattening is due to rotation about and elliptical galaxies are spherical or oblate spheroids. This is now recognized as not universally valid. There are, especially among the most luminous elliptical galaxies little or no rotating systems, which still appear flattened. The orbits of the stars are no ellipses or other closed forms, but the stars lead independent movement in all three main axes through. Their, triaxial ', that is, in three directions different expansion, produced by a direction depending on the variation of the speeds of its star, that is, an elliptical galaxy is not stabilized by rotation, but rather by the chaotic motion of the star gas. This star gas is in a relaxed state, which is not achieved by collisions but by Violent relaxation.
Faber -Jackson relation
With the Faber -Jackson relation is an empirical relation between luminosity L and velocity dispersion σ in elliptical galaxies observed. This relationship is particularly advantageous because the velocity dispersion of the stars can be relatively easily determined by spectroscopic analyzes. About the determined absolute magnitude can then calculate the distance to the galaxy.
Boxlike and disk-like systems
By analyzing the distribution of brightness could find that many elliptical galaxies did not show the ideal shape of an ellipse. These systems can be classified depending on the shape of the isophotes in box-like ( boxy shape) and disk-like ( disky shape). This showed that at this subdivision attach other physical properties.
Approximately 70 % to 90 % of the elliptical galaxies are disk-like, while 10 to 20 % show a box-like structure. The box-like systems have a larger mass-to- light ratio than the disk -like. In the disk-like systems, the rotation is sculpting than the box-shaped. So here outweighs a minor component of motion, which does not exist in the other type. The disk-like systems are weak radio emitters, while there is a broad distribution of radio luminosity at the box-shaped type. A similar picture can be found at the X- ray range. At the box-shaped galaxies nuclei were often discovered that move against the general direction of rotation ( counter-rotating cores ).
Shell structures
Some ellipticals shell structures were ( shells ) discovered in the form of arcs in the outer regions of the galaxy. The sheets are centered on the center of the galaxy. These structures can be detected by contrast-enhanced images. These bows are probably remnants of a galaxy has been collected.
Properties of a galaxy like Kastenförmigkeit, shell structures and opposing cores indicate that it is a result of a completed merger of two or more galaxies.
Halo
Elliptical galaxies are surrounded by diffuse stellar halos and a large number of globular clusters. While it is very easy for spiral galaxies, based on the rotation of the galaxy derive a proportion of dark matter in the halo, this is not possible with elliptical galaxies, there is no clear rotational motion exists. Through analyzes of the X-ray radiation from the hot gas, a high proportion of dark matter could be discovered in the halo for elliptical galaxies, a dark - matter content of about 90 % was determined for example for the galaxy NGC 4472.
Fundamental plane
The fundamental plane is a relationship between the effective radius, the average brightness and the overall velocity dispersion of normal elliptical galaxies. These three parameters are interdependent, a parameter can be calculated once the other two parameters are determined. In a three-dimensional coordinate system, the measured values of the parameters of different elliptical galaxies form a plane.
Occurrence
The fraction of elliptical galaxies in the total number of galaxies is strongly dependent on the environment. In rich galaxy clusters nearly half of the galaxies are elliptical, while the proportion falls in low galaxy density regions below 10%. Also found in the center of many galaxies, a particularly massive elliptical galaxy or a similar as a cD galaxy called galaxies form.
There are many similarities between some fainter elliptical galaxies and the central bulges of spiral galaxies. A distinction must be elliptical galaxies of the faint dwarf spheroidal galaxies, although they also have elliptical shape, but follow other contexts between brightness and size.
Formation
Most of the stars in elliptical galaxies are old. Elliptical galaxies usually contain very little interstellar matter, may arise from the new star. Due to her age and high central stellar density was often assumed that elliptical galaxies formed already around 10 billion years ago by the rapid collapse of a single large gas cloud. This idea can, however, difficult to explain the small rotation of many elliptical galaxies and is not readily compatible with a basic assumption of modern cosmology, namely that structures are formed as galaxies by merging of smaller units.
In addition, there are from observation and theory signs that the merger of two spiral galaxies an elliptical galaxy is formed. This would also explain the existence of counter-rotating cores, which are found in every third galaxy. But this mechanism also can not explain all the properties of all today's elliptical galaxies. Emergence period and mechanism of elliptical galaxies are still the subject of intense research and not necessarily for all such galaxies same.