Galactic rotation curve and the effect of density waves from data on young objects

Abstract

Based on currently available data on the three-dimensional field of space velocities of young (≤50 Myr) open star clusters and the radial velocities of HI clouds and star-forming (H II) regions, we have found the Galactic rotation curve in the range of Galactocentric distances 3 kpc < R < 12 kpc using the first six terms of the Taylor expansion of the angular velocity of Galactic rotation in Bottlinger’s equations. The Taylor terms found at the Galactocentric distance of the Sun R 0 = 7.5 kpc are: ω 0 = −27.7 ± 0.6 km s−1 kpc−1, ω 0 1 = 4.13 ± 0.07 km s−1 kpc−2, ω 0 2 = −0.912 ± 0.065 km s−1 kpc−3, ω 0 3 = 0.277 ± 0.036 km s−1 kpc−4, ω 0 4 = −0.265 ± 0.034 km s−1 kpc−5, ω 0 5 = 0.104 ± 0.020 km s−1 kpc−6. In this case, the Oort constants are A = 15.5 ± 0.3 km s−1 kpc−1 and B = −12.2 ± 0.7 km s−1 kpc−1. We have established that the centroid of the sample moves relative to the local standard of rest along the Galactic Y axis with a velocity of −6.2 ± 0.8 km s−1. A Fourier spectral analysis of the velocity residuals from the derived rotation curve attributable to density waves reveals three dominant peaks with wavelengths of 2.5, 1.4, and 0.9 kpc and amplitudes of 4.7, 2.6, and 3.6 km s−1, respectively. These have allowed us to estimate the distances between the density wave peaks, 1.9, 2.4, and 3.2 kpc as R increases, in agreement with the description of the density weave as a logarithmic spiral. The amplitude of the density wave perturbations is largest in the inner part of the Galaxy, ≈9 km s−1, and decreases to ≈1 km s−1 in its outer part. A spectral analysis of the radial velocities of young open star clusters has confirmed the presence of periodic perturbations with an amplitude of 5.9 ± 1.1 km s−1 and a wavelength λ = 1.7 ± 0.5 kpc. It shows that the phase of the Sun in the density wave is close to −π/2 and the Sun is located in the interarm space near the outer edge of the Carina-Sagittarius arm.