Strong Evidence for the Density-Wave Theory of Spiral Structure in Disk Galaxies

Abstract

The density-wave theory of galactic spiral-arm structure makes a striking prediction that the pitch angle of spiral arms should vary with the wavelength of the galaxy's image. The reason is that stars are born in the density wave but move out of it as they age. They move ahead of the density wave inside the co-rotation radius, and fall behind outside of it, resulting in a tighter pitch angle at wavelengths that image stars (optical and near infrared) than those that are associated with star formation (far infrared and ultraviolet). In this study we combined large sample size with wide range of wavelengths, from the ultraviolet to the infrared to investigate this issue. For each galaxy we used an optical wavelength image (B-band: 445 nm) and images from the Spitzer Space Telescope at two infrared wavelengths (infrared: 3.6 and 8.0 µm) and we measured the pitch angle with the 2DFFT and Spirality codes (Davis et al. 2012;Shields et al. 2015). We find that the B-band and 3.6 µm images have smaller pitch angles than the infrared 8.0 µm image in all cases, in agreement with the prediction of density-wave theory. We also used images in the ultraviolet from Galaxy Evolution Explorer, whose pitch angles agreed with the measurements made at 8 µm. Because stars imaged at those wavelengths have not had time during their short lives to move out of the star-forming region.

Investigating the Origins of Spiral Structure in Disk Galaxies through a Multiwavelength Study

“…Thus, the star-forming arm is upstream from the blue arm, which is in turn upstream from the red arm. This conforms with the density-wave theory's prediction of variation in pitch angle with the image wavelength of light (Pour-Imani et al 2016). clouds before they actually crossed the maximum of the spiral potential. Although such a possibility cannot be ruled out, it does not seem to be reflected in most simulations of galactic density waves.…”

“…This issue is relevant to the current paper because it has been argued that failure to find consistent downstream displacements of observational tracers of star formation and stars of different ages is an argument against the reality of long-lasting spiral arms (Foyle et al 2011). By contrast, in previous work (Pour-Imani et al 2016) we confirmed a key prediction of the densitywave theory, that spiral-arm pitch angle varies with observation wavelength. This is in contrast to the predictions of rival theories, such as the Manifold theory (for a discussion of this theory test see Athanassoula et al 2010).…”

“…The same is true for the second plot, comparing B-band with u-band images. In contrast, we can see that FUV and B-band pitch angles (third plot) are different from each other, since in both cases the greatest numbers of galaxies have a pitch angle difference of more than 3°(see the relevant histogram), with very few found below 3°(this plot is reproduced from Pour-Imani et al (2016). The same is true for u-band pitch angles and 3.6 μm pitch angles (bottom plot), which are clearly different from each other.…”

“…Our sample of 29 galaxies is drawn from the Spitzer Infrared Nearby Galaxies Survey, which consists of imaging from the Infrared Array Camera (Fazio et al 2004). The sample is drawn from the one found in Pour-Imani et al (2016) by selecting those objects for which images in the u band and the H-α line are available. Thus the sample in this paper selects those galaxies with imaging at 3.6 μm that had available optical imaging in the B band (445 nm) and ultraviolet imaging in the u band (355 nm) as found in the NASA/IPAC Extragalactic Database (NED; see Table 1 for B band and u band image sources).…”

Spiral arms and disc stability in the Andromeda galaxy

“…From the measurements of such offsets as a function of galactocentric radius, the pattern speed of the density wave can be calculated (Egusa et al 2004;Louie et al 2013). This effect causes the pitch angle of spirals to vary with wavelength, which is indeed seen in several galaxies (Pour-Imani et al 2016 On the other hand, if spiral arms were formed due to density inhomogeneities, offsets would not occur, or they would be small and would not have a regular distribution (Grand et al 2012b,a;D'Onghia et al 2013;Baba et al 2015;Choi et al 2015). Thus, if no offsets are seen between the distributions of molecular gas and young stars, we may conclude that star formation in the compressed gas has occurred rather fast (see Egusa et al 2009), or that even the classical density wave theory cannot explain spiral structure formation (Foyle et al 2011).…”