Evidence in favour of density wave theory through age gradients observed in star formation history maps and spatially resolved stellar clusters

Abstract

Quasi-stationary density wave theory predicts the existence of an age gradient across the spiral arms with a phase crossing at the co-rotation radius. We have examined evidence for such age gradients using star formation history (SFH) maps derived from lightning, a spectral energy distribution (SED) fitting procedure, and by using spatially-resolved stellar clusters. Three galaxies from the LEGUS survey were used to analyse the azimuthal offsets of spatially-resolved stellar clusters. Kernel density estimation plots of azimuthal cluster distance offsets reveal prominent central peaks and secondary peaks on the positive side, relative to the density wave for NGC 5194 and NGC 5236. These secondary downstream peaks in the cluster distributions show overall evidence for an age gradient. NGC 628 shows secondary peaks on both sides of the density wave. The cluster distributions also show an increasing spatial spread with age, consistent with the expectation that they were born in the density wave. SFH maps of 12 nearby galaxies were analysed using spirality, a matlab-based code, which plots synthetic spiral arms over FITS images. The SFH maps reveal a gradual decrement (tightening) in pitch angles with increasing age. By analysing the pitch angle differences between adjacent age bins using the error function, the average of the probabilities shows a $69{{\ \rm per\ cent}}\pm 25{{\ \rm per\ cent}}$ chance that the pitch angle values decrease (tighten) with increasing age. Thus, we see a tightening of the spiral pattern in galaxies, both when segregating stellar populations specifically by age or more generally by colour, as was shown in our previous studies.

Colour jumps across the spiral arms of Hubble Ultra Deep Field galaxies

“…The 'wavy' pattern was also detected in the pitch angle measurements of spiral arms in galaxies at z = 0 by Yu & Ho (2018); 40% of their objects (see Figure 16) with optical, N U V and F U V data present this phenomenon. This con- clusion may also explain the findings of other authors for nearby galaxies (Pour-Imani et al 2016;Miller et al 2019;Abdeen et al 2022), whose P measurements of the colour jump show smaller angles in the red than in the blue, as expected for the classic gradients. Unfortunately, the aforementioned works do not provide values for ∆R, and hence it is difficult to reproduce their results, and to compare them with those of other authors.…”

Probing the Low-Mass End of the Black Hole Mass Function via a Study of Faint Local Spiral Galaxies

“…20 Miller et al [141] go further and show that indeed, the |φ| spectral differences observed by Pour-Imani et al [139] are caused by enhanced regions of stellar light downstream from star-forming regions, as predicted by density wave theory. In turn, these results are further tested and affirmed by subsequent studies of co-rotation radii [142] and age gradients [143]. Furthermore, Smith et al [144] present relations between pitch angle, concentration, and specific star formation rate, which they say supports the fundamental plane suggested by Davis et al [52] by assuming central concentration increases with bulge mass and since star formation rate increases with increasing gas surface density [145][146][147].…”

Probing the Low-mass End of the Black Hole Mass Function via a Study of Faint Local Spiral Galaxies

“…20 Miller et al [116] go further and show that indeed, the |φ| spectral differences observed by Pour-Imani et al [114] are caused by enhanced regions of stellar light downstream from star-forming regions, as predicted by density wave theory. In turn, these results are further tested and affirmed by subsequent studies of co-rotation radii [117] and age gradients [118]. Furthermore, Smith et al [119] present relations between pitch angle, concentration, and specific star formation rate, which they say supports the fundamental plane suggested by Davis et al [43] by assuming central concentration increases with bulge mass and since star formation rate increases with increasing gas surface density [120][121][122].…”