Star formation and ISM morphology in tidally induced spiral structures

Abstract

Tidal encounters are believed to be one of the key drivers of galactic spiral structure in the Universe. Such spirals are expected to produce different morphological and kinematic features compared to density wave and dynamic spiral arms. In this work, we present high resolution simulations of a tidal encounter of a small mass companion with a disc galaxy. Included are the effects of gas cooling and heating, star formation and stellar feedback. The structure of the perturbed disc differs greatly from the isolated galaxy, showing clear spiral features that act as sites of new star formation, and displaying interarm spurs. The two arms of the galaxy, the bridge and tail, appear to behave differently; with different star formation histories and structure. Specific attention is focused on offsets between gas and stellar spiral features which can be directly compared to observations. We find that some offsets do exist between different media, with gaseous arms appearing mostly on the convex side of the stellar arms, though the exact locations appear highly time dependent. These results further highlight the differences between tidal spirals and other theories of arm structure.

Young stars as tracers of a barred-spiral Milky Way

“…velocities are inherited from spiral shocks around the arm, and are influenced by nearby supernova events and cloud-cloud interactions. Similar maps were made from the live stellar disc simulations from Pettitt et al (2017), which showed a much lower variance in the mean velocities when including an inherent "old" stellar population, more in line with Gaia data. Similarly, the lack of an old population meant that attempts to measure velocity dispersions suffered from low counts in some outer bins, and would thus require a simulation with a resolved stellar component across the entire age spectrum to properly compare.…”

Bars and spirals in tidal interactions with an ensemble of galaxy mass models

“…Most of the simulations investigating spurs however use fixed spiral potentials to induce spur features. Only recently has evidence of such spurs existing in tidal spiral simulations been seen (Pettitt et al 2016(Pettitt et al , 2017, even though M51 (the poster-child of observed tidal spirals) has clear interarm spur features. In Figure 20 we show further evidence of spurs in tidal spiral arms.…”

“…The shape of the resulting spirals depend on the orbital paths and masses of the galactic components (Elmegreen et al 1991;Oh et al 2015;Pettitt et al 2016). Simulations of such interactions suggest they also induce strong bursts of star formation in the tidal arms (Noguchi & Ishibashi 1986;Di Matteo et al 2007;Gabor et al 2016;Pettitt et al 2017). These spirals behave somewhat differently to density wave and dynamic spiral patterns, in that they are semi-long lived, outlasting dynamic spirals but still winding up over time, with a pattern speed much slower than the material rotation speed.…”

Simulations of the star-forming molecular gas in an interacting M51-like galaxy

“…However, they used an isothermal equation of state for the gas, preventing them from studying the cold gas distribution or the properties of individual GMCs. More recently, Pettitt et al (2017) performed smoothed particle hydrodynamics (SPH) simulations of an interacting galaxy morphologically similar to M51 using a more sophisticated thermal treatment. However, the SPH particle mass in their simulations was 2000 M , rendering them unable to resolve all but the largest GMCs.…”