Remarks on the reduction of radial velocities within spiral galaxies

Rings and spirals in barred galaxies – II. Ring and spiral morphology

“…The first difficulty lies in the deprojection of the galaxies, since the value of the position and inclination angles can be heavily influenced by the outer parts of the arms. This is the well‐known Stock effect (Stock 1955) and may influence tightly wound spirals differently from open ones. This could introduce a bias in the analysis and thus has to be treated with particular care.…”

Uncertainties in Spiral Galaxy Projection Parameters

“…Nonaxisymmetric structures in disks, such as bars, spirals, and warps, are a major source of systematic error. Stock (1955) investigated the impact of spiral structure on the photometric estimation of position angles, particularly the case of highly inclined galaxies, finding that spiral struc-ture can introduce uncertainties in φ values of ≈ 5 • . We refer to the impact of all such nonaxisymmetric structure on the photometric projection angles as the spiral effect.…”

Using an assumption about the monotony of spiral arms to determine the orientation angles of galaxies

“…The only widely used kinematic method for determining the orientation of a galaxy consists in fitting the two‐dimensional observed velocity field with the velocity field of a model with purely circular motions. However, Stock (1955), Fridman et al (1997), Lyakhovich et al (1997) and Fridman et al (2005) showed that the presence of a bar or spirals introduces systematic deviations into the derived PA and i values owing to the presence of non‐circular motions that are reverse side of the spiral/bar structure manifested in kinematics. This method also cannot be applied to large samples of galaxies, as it requires sophisticated and time‐consuming measurements with a Fabry–Perot interferometer or slit spectrometers.…”

“…This method assumes that the galactic disc is axially symmetric, and therefore works well for early-type galaxies or galaxies with low-contrast spirals. Because this method is sensitive to the presence of a bar or spiral structure (see, for example, Stock 1955), it is usually applied to the outer parts of galaxies, where the spiral structure becomes faint. However, many galaxies have warp at the periphery and hence the result may be biased and inaccurate…”

“…This method estimates the inclination angle i from the degree of elongation of the observed elliptical galaxy isophotes, with or without allowance for the finite thickness of the disc. However, as shown by Stock (1955), the results obtained with this method may be subject to large uncertainties if the brightness distribution of the galaxy is not axially symmetric. In fact, this is the case in many observed galaxies, owing to the presence of bright spirals, a bar, or dust lanes.…”