The Galactic Model

By way of an introduction, galaxies are often grouped into 3 basic types, i.e. elliptical, spiral and irregular. However, to-date, most galactic models have struggled to explain all the apparent anomalies that appear to be connected with the observed rotation of the various galactic types. The following diagram simply outlines the scope of a far more complex picture within active galaxies.

1

While the following discussions will be little more than an overview of the underlying details, the main purpose is primarily to highlight the scope for potential debate within this field of astrophysics. Let us start by expanding the description of the previous types of galaxies as follows:

Galaxy
 Type
Active
Nuclei
Emission Lines X-rays Excess of Strong Radio Jets
Narrow Broad UV Far-IR
Normal no weak none weak none none none none
Starburst no yes no some no yes some no
Seyfert yes yes some some some yes few some
Quasar yes yes yes some yes yes some some
Radio yes some some some some yes yes yes

While the table above does not represent the full complexity of all the different types of galaxies observed, it is probably enough to get some idea of the scale of the problem. Although models of such galaxies have undoubtedly increased in sophistication over the last few decades, due to incredible improvements in astrophysical data, it might be argued that many of the accepted models still rest on unverified assumptions. This said, the following summary outlines the general description of the different types of galaxies outlined:

  • Normal Galaxies:
    A normal galaxy might be described as one where the total energy emitted is assumed to approximate the sum of the energy emitted from all its stars. They are generally described as being large gravitationally bound systems consisting of hundreds of billions of stars with enough gas and dust to make billions of stars, surrounded by dark matter. Our own local galaxy, the Milky Way, fits this description.

  • Starburst Galaxy:
    This is a galaxy that appears to have an exceptionally high rate of star formation compared to a normal galaxy. Observations suggest that a burst of star formation might be triggered after a collision between two galaxies. Given the rate of star formation, the gas reservoirs from which stars are formed would be used up on timescales much shorter than the age of the galaxy, such that it is usually assumed that the starburst phase has to be a temporary phase within the total lifecycle of the galaxy. However, the age of galaxies in general is still a subject of much debate.

  • Seyfert Galaxies:
    Are a class of galaxies with nuclei that produce spectral line emission from highly ionized gas and are named after the astronomer Carl Seyfert, who first identified the class in 1943. These galaxies are one of a number described having ‘Active Galactic Nuclei (AGN)’ or simply ‘active galaxies’  that are generally assumed to have a super-massive black hole, with a mass between 106 and 108 solar masses, at the centre of the galaxy.

  • Quasars:
    A ‘QUAsi-StellAr Radio’ source, i.e. quasar for short, is normally described as a very energetic and distant active galactic nucleus. As such, quasars are assumed to be extremely luminous and high redshift sources of electromagnetic energy, which on a cosmic scale are seen as points of light, similar to bright stars, rather than extended sources more typical of galaxies. The nature of these objects is still controversial, although the general consensus is that a quasar is a compact region surrounding a central super-massive black hole. In this context, the quasar is said to be powered by an accretion disc around the black hole, although alternative models, e.g. see Plasma model, may question aspects of this description.

  • Radio Galaxies:
    Are also a type of active galaxy, which are more luminous in the radio range of the EM spectrum. The radio emissions are believed to be due to a synchrotron process in which EM radiation is emitted when charged particles are subject to a radial acceleration. Again, plasma physics might provide an alternative explanation of the processes at work within these galaxies. These galaxies are generally large and elliptical in shape, which can be detected at large distances, such that they have become a valuable reference point in observational cosmology.

Based on the summary data above, it would seem that much of our understanding of galactic mechanics must depend on calculations rooted in the interpretation of the observed EM spectrum redshift. In this context, when a source of light is moving away from an observer, a redshift (z>0) occurs, while if the source moves towards the observer, a blueshift (z<0) occurs.

Note: Redshift, or blueshift, is assumed to be proportional to the relative velocity [v] between the Earth and source, which would alter as a function of time within an expanding universe. However, it is assumed that the causal mechanism of this shift, linked to velocity [v], is explained in terms of relativistic time-dilation effect, which causes a photon to be emitted with a lower frequency, i.e. longer wavelength, which results in the observed shift, i.e. red or blue. Of course, we might also assume that in an expanding universe, redshift observations would dominate. However, if we follow the logic of this argument, we appear to be led to the suggestion that different points in the universe would also be operating on different rates of time due to the perceived relativistic velocity?

The amount of ‘shift [z]’ may be defined in terms of a change in wavelength [λ] or frequency [f] between the source [S] and the observer [O]:

[1]      1

However, this shift [z] can also be interpreted as  a function of velocity [v] of the source with respect to the observer, where the propagation of the EM waves/photons in vacuum is assumed to have always been [c]. The equation below shows both the relativistic and non-relativistic form:

[2]      2

As a general description, astronomers attempt to measure the redshift of the spectral lines emitted by known elements, e.g. hydrogen, associated with bright stars within galaxies in order to determine the recessional velocity of the galaxy as a whole. However, if a star is also moving around the galactic core, the speed of rotation can be estimated by measuring the relative change in redshift as stars rotates towards and away from the observer. Finally, by taking measurements of the spectral lines from objects at different radii from the galactic centre, a plot of the rotational speed as a function of its radius can be determined. Based on Newtonian gravitational physics, we might have some initial expectation that the rotational velocity, at a given radius, might be linked to the following formulation:

2[3]      3

Based on [3], it would seem that the rotational velocity would be a function of the inverse square root of the radius. However, observations based on [1] and [2] suggest that this is not the case and  that the velocity of the stars within a galaxy tend to be generally constant across a large range of radii, as suggested by the diagram to the right. However, at this point, we first need to introduce a number of potential complexities:

  • The effective gravitational mass [M] of a galaxy changes as a function of the radius.

  • The distribution of star mass within the galaxy, as a function of radius, may not be linear.

  • We might realise that [3] only applies to stable orbits, while in practice, stars might also have a radial velocity within the spiral arms.

  • There may be other processes, besides gravity, at work within the galaxy. However, as stated, this aspect of the discussion will be deferred to the Plasma Model section.

  • There is also the consideration of dark matter thought to surround the galaxy, although some question how this speculative configuration of dark matter can provide the rotational stability of all the galactic types mentioned.

So, at this stage, it may not be unfair to say that there is some doubt as to whether the apparent confidence in the accepted model can really be justified based on the current level of verified data.