Spiral arms and disc stability in the Andromeda galaxy

Abstract

Aims. Density waves are often considered as the triggering mechanism of star formation in spiral galaxies. Our aim is to study relations between different star formation tracers (stellar UV and near-IR radiation and emission from H i, CO, and cold dust) in the spiral arms of M 31, to calculate stability conditions in the galaxy disc, and to draw conclusions about possible star formation triggering mechanisms. Methods. We selected fourteen spiral arm segments from the de-projected data maps and compared emission distributions along the cross sections of the segments in different datasets to each other, in order to detect spatial offsets between young stellar populations and the star-forming medium. By using the disc stability condition as a function of perturbation wavelength and distance from the galaxy centre, we calculated the effective disc stability parameters and the least stable wavelengths at different distances. For this we used a mass distribution model of M 31 with four disc components (old and young stellar discs, cold and warm gaseous discs) embedded within the external potential of the bulge, the stellar halo, and the dark matter halo. Each component is considered to have a realistic finite thickness. Results. No systematic offsets between the observed UV and CO/far-IR emission across the spiral segments are detected. The calculated effective stability parameter has a lowest value of Q eff 1.8 at galactocentric distances of 12-13 kpc. The least stable wavelengths are rather long, with the lowest values starting from 3 kpc at distances R > 11 kpc. Conclusions. The classical density wave theory is not a realistic explanation for the spiral structure of M 31. Instead, external causes should be considered, such as interactions with massive gas clouds or dwarf companions of M 31.

The role of spiral arms in Milky Way star formation

“…For example, Choi et al (2015) analysed resolved stellar populations to determine the starformation history as a function of position across the arms of the grand-design spiral M81 but found no evidence of the propagating star formation across the arm that is the prediction of the density-wave model. Tenjes et al (2017) find no systematic offsets between ultraviolet, infrared and CO in M31. Schinnerer et al (2017) examine the ISM properties across an arm in grand-design spiral, M51, and find no significant variations (cf.…”

Two-component gravitational instability in spiral galaxies

“…With the above correction α = 0.5 (Schaye 2004). Tenjes et al 2017). The exact two-component criterion (Rafikov 2001) and various approximations of the two-fluid criterion (Wang & Silk 1994;Romeo & Wiegert 2011) such as multicomponent approximation with molecular gas (Romeo & Falstad 2013;Romeo & Fathi 2016;Hallenbeck et al 2016;Romeo & Mogotsi 2017) were used.…”

The M31/M33 tidal interaction: a hydrodynamic simulation of the extended gas distribution

“…e Scaleheight set to 0.5 kpc. f Toomre's Q > 1.8 everywhere initially (Tenjes et al 2017) is to set to the (equivalent) Plummer scale length of M33's mass model (see Tab. 2). Then, we use these orbital parameters and an N-body representation of M31 and M33 to run a simulation forward in time.…”