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

Abstract

The density-wave theory of spiral structure proposes that star formation occurs in or near a spiral-shaped region of higher density that rotates rigidly within the galactic disk at a fixed pattern speed. In most interpretations of this theory, newborn stars move downstream of this position as they come into view, forming a downstream spiral which is tighter, with a smaller pitch angle than that of the density wave itself. Rival theories, including theories which see spiral arms as essentially transient structures, may demand that pitch angle should not depend on wavelength. We measure the pitch angle of a large sample of galaxies at several wavelengths associated with star formation or very young stars (8.0 μm, H-α line and 151 nm in the far-UV) and show that they all have the same pitch angle, which is larger than the pitch angle measured for the same galaxies at optical and near-infrared wavelengths. Our measurements in the B band and at 3.6 μm have unambiguously tighter spirals than the starforming wavelengths. In addition we have measured in the u band, which seems to fall midway between these two extremes. Thus, our results are consistent with a region of enhanced stellar light situated downstream of a starforming region.

Determining the Co-Rotation Radii of Spiral Galaxies Using Spiral Arm Pitch Angle Measurements at Multiple Wavelengths

“…In one respect, Yu & Ho (2018) disagree with the earlier results reported in Pour-Imani et al (2016)/Miller et al (2019, find that the infrared pitch angle is generally looser (not tighter, as we find in this paper, in agreement with Miller et al (2019)) than the pitch angle as measured in other wavelengths, such as B-band. This is consistent with the red spiral arm being generated by old disk stars crowded closer together by the peak of the density wave's spiral potential.…”

“…But the work of Martínez-García & González-Lópezlira (2015) does find evidence for such color gradients and they ar-gue that the spiral arms must be long-lived. Evidence for these color gradients is presented in Pour-Imani et al (2016); Miller et al (2019) using a different approach, which demonstrates that the spiral arm pitch angle in wavebands associated with star-formation is consistently larger than those associated with the stellar population. This suggests that new-born stars are found downstream of the star-formation region in a manner consistent with the density-wave theory.…”

Spirals in galaxies

“…The contrasting prediction of quasi-steady spiral structure is that the pattern speed is constant over the entire radial range of the spiral. Foyle et al (2011) rule out age gradients downstream from the spiral in their sample, but others (Chandar et al 2017, Yu & Ho 2018, Miller et al 2019, Peterken et al 2019) claim to have detected them. However, none of these careful studies was able to establish a fixed pattern speed over the entire radial extent of the spiral.…”

Spin: Ubiquitous, Fundamental, Purposeful: Its Complementary Interactions with Gravity

“…Another peculiar feature is the finding that the stars in a galaxy orbit its center at approximately the same speed regardless of where in the galaxy they are situated, unlike the predictably slower pace of orbits of the outer planets in a solar system. This feature has baffled scientists for a long time (Miller et al, 2019;Benjamin et al, 2005;Lin & Shu, 1964;Francis & Canderson, 2009). Yet another unique feature is the flaring appearance of the spiral arms (Miller et al, 2019;Benjamin et al, 2005;Lin & Shu, 1964;Francis & Canderson, 2009;Jog, 2002).…”

“…This feature has baffled scientists for a long time (Miller et al, 2019;Benjamin et al, 2005;Lin & Shu, 1964;Francis & Canderson, 2009). Yet another unique feature is the flaring appearance of the spiral arms (Miller et al, 2019;Benjamin et al, 2005;Lin & Shu, 1964;Francis & Canderson, 2009;Jog, 2002). We suggest that both of these features are the result of the unique make-up of the spiral galaxies.…”