Spiral arm

In contrast, the spiral structure of flocculent galaxies comprises numerous small fragments of arms that are not connected to each other.

In addition to increased brightness, spiral arms are characterised by an increased concentration of interstellar gas and dust, bright stars and star clusters, active starburst, a bluer colour, and an enhanced magnetic field strength in galaxies.

[2] While spiral arms are primarily identifiable due to their young stellar population, there also exists an increased concentration of old stars within them.

[4][7] The appearance and expression of spiral branches in a galaxy may vary depending on the part of the electromagnetic spectrum in which it is observed.

In the red and near-infrared, older stars contribute more, which makes the spiral arms appear smoother, but less contrasted.

For example, in Hubble's classification scheme, spiral galaxies are divided into types Sa, Sb, Sc.

In contrast, the spiral structure of flocculent galaxies consists of numerous small fragments of arms that are not connected to each other.

[18] The distinction between the two main types of spiral arms appears to be related to fundamental physical differences between them.

Apparently, the galaxies in these clusters are subject to ram pressure, which results in the rapid loss of gas.

Conversely, magnetic fields can influence the movement of gas within the galaxy and contribute to the formation of spiral arms.

Additionally, their bulge contributes less to the total luminosity, they have a lower velocity dispersion in the centre, and their rotation curves appear to be more increasing.

Nevertheless, spiral arms can be observed, for instance, when mapping the distribution of neutral hydrogen or molecular clouds.

However, since galaxies themselves rotate differentially rather than as solid bodies, any structure in the disc should curve significantly and disappear in approximately one to two revolutions.

The two most prevalent solutions to this issue are the stochastic self-propagating star formation model (SSPSF) and the density wave theory, which describe disparate variants of the spiral structure.

Due to the low velocity of matter at a distance from the galaxy, tidal tails appear to persist for an extended period of time.

[47] The SSPSF model posits that spiral arms emerge when starburst becomes active within a region of the galaxy.

The presence of young, bright stars in this region has the effect of influencing the surrounding interstellar medium.

For instance, a supernova explosion generates a shockwave in the gas, thereby facilitating the spread of star formation across the galactic disk.

[49][50] Given that such spiral arms are only visible due to young stars, they have a minimal impact on the mass distribution within the galaxy and are rarely observed in the infrared.

The influence of this mechanism results in the formation of a large-scale, ordered spiral structure, which is also observed in the infrared.

[51][52][53] The concentration of stars in the spiral arm increases by a mere 10–20%, yet this relatively modest change in gravitational potential has a profound impact on the gas dynamics.

The gas accelerates, and shock waves can occur in it, appearing as dark dust lanes in the arms.

However, it is possible to do so, for instance, by detecting a specific corotation radius, which is a region where the spiral arm moves at the same speed as the stars.

[54][55] It is hypothesised that density waves are created and maintained by the bars of galaxies or by tidal force of their satellites.

According to this theory, the gravitational influence of the bar causes the orbits of the stars to be arranged in a certain way, creating spiral arms and moving along them.

The name of the theory is related to the fact that in this model the stars moving in spiral arms form a manifold in phase space.

If spiral arms were material entities, due to differential rotation, they would twist very rapidly to the point where they would be impossible to observe.

Since 1927, this question has been addressed by Bertil Lindblad, who in 1961 correctly concluded that the spiral arms arise due to gravitational interaction between the stars in the disc.

Subsequently, in 1964, Chia-Chiao Lin and Frank Shu proposed the theory that spiral arms can be conceptualised as density waves.

[62][63] Despite the considerable successes of the density wave theory, the physical nature of spiral arms remains a topic of debate, with no clear consensus yet reached.