Typical spiral galaxies have their arms shaped in a form of a ‘whirlpool’, which, according to many mathematicians, is called logarithmic spiral shape. Not all galaxies are spiral, but those with the ‘arms’ also have some distinct structural properties, which are common to most of these cosmic objects, and which vary depending on some properties of a galaxy itself.

Russian scientists S.S. Savchenko and V.P. Reshetnikov from St. Petersburg State University and St. Petersburg Branch of Isaac Newton Institute of Chile, accomplished a study in which they surveyed a small sample of 50 non-barred or weakly barred grand design spiral galaxies. The data for this study was taken from Sloan Digital Sky Survey. The scientists investigated one of the main shape-related properties: the spiral arm pitch angle and how it varies according to distinct properties of those galaxies.

A paper has been presented at arXiv.org. The authors discuss the limitations of typical mathematical models which are currently used to describe the shapes of spiral galaxies: logarithmic, archimedean or hyperbolic. These models have one particular disadvantage: since the value of the real galaxy arm pitch angle varies with the distance from the galaxy center and such variation is not constant, the accuracy of its mathematical representation significantly decreases. In case of the logarithmic spiral (the most commonly used to describe shapes of spiral galaxies), its pitch angle does not change with the distance, while this angle decreases in archimedean spiral and increases in hyperbolic spiral.

Previous studies have shown that the pitch angle value is a function of a distance from the galaxy center. However, different parts of the spiral structure have to be analysed separately, or approximated by a two-dimensional Fourier analysis of the spiral pattern to make this principle work. Also, recent observations indicate that there is a potential association between the tightness of spiral structure and the shear tare of differentially rotating discs of spiral galaxies, which, in turn, is determined by their mass distribution.

With this in mind, the authors decided to verify a hypothesis, which states that, due to the influence of mass distribution, there should be a correlation between the mass of central galactic region (or so-called galaxy bulge) and its spiral arm pitch angle also.

Spiral pattern characteristics were analysed using data from Sloan Digital Sky Survey (SDSS). The sample galaxies were selected based on EFIGI morphological classification system, which permits to constrain shape-related parameters of various galaxies. At first an automatic object classification was performed, and later the results were refined manually. Fourier series-based analysis of galaxy pitch angle was performed together with photometric study of the central regions of same galaxies.

The results indicate that the pitch angle value is not constant for most of the analysed galaxies: approximately 2/3 of the sample galaxies showed pitch angle variations exceeding 20%. It is interesting that there is no shape relations between neighbor galaxies, i.e. the galaxies from approximately the same space segment may exhibit entirely different pitch angle variations. However, a correlation between the pitch angle and the galaxy surface brightness distribution was determined. The galaxies with faint central regions tend to have opened spiral arms, while bright and luminous bulges typically have tighter spiral arms. Also, tighter spiral arms were observed more often in the early-type, red and massive galaxies.

The scientists conclude in their paper, that since the mass of the central region of the galaxy is related to the mass of supermassive black hole contained there, there should be a relationship between the mass of that black hole and the pitch angle of the galaxy arms.

By Alius Noreika, Source: Technology.org