Star formation in galaxy mergers: ISM turbulence, dense gas excess, and scaling relations for disks and starbusts

Abstract

Abstract. Galaxy interactions and mergers play a significant, but still debated and poorly understood role in the star formation history of galaxies. Numerical and theoretical models cannot yet explain the main properties of merger-induced starbursts, including their intensity and their spatial extent. Usually, the mechanism invoked in merger-induced starbursts is a global inflow of gas towards the central kpc, resulting in a nuclear starburst. We show here, using high-resolution AMR simulations and comparing to observations of the gas component in mergers, that the triggering of starbursts also results from increased ISM turbulence and velocity dispersions in interacting systems. This forms cold gas that are denser and more massive than in quiescent disk galaxies. The fraction of dense cold gas largely increases, modifying the global density distribution of these systems, and efficient star formation results. Because the starbursting activity is not just from a global compacting of the gas to higher average surface densities, but also from higher turbulence and fragmentation into massive and dense clouds, merging systems can enter a different regime of star formation compared to quiescent disk galaxies. This is in quantitative agreement with recent observations suggesting that disk galaxies and starbursting systems are not the low-activity end and high-activity end of a single regime, but actually follow different scaling relations for their star formation.

Star Formation Laws and Thresholds From Interstellar Medium Structure and Turbulence

“…Hydrodynamic simulations of mergers found such excess of high-density components compared to a log-normal PDF, even outside nuclear disks Bournaud et al 2011b), and PDFs from these works are well fitted by our arbitrary model using δ = 3 and m = 0.2. With feedback, we obtain…”

Star clusters in evolving galaxies

“…This situation is found in all interacting systems, with an intensity and duration that depend on the galaxies and their orbits (Renaud et al, 2009). These fully compressive tides then induce an enhancement of the total turbulence of the ISM (as observed, Irwin 1994;Elmegreen and Elmegreen 1995;Bournaud et al 2011;Ueda et al 2012), but also a change of its nature by making it compression-dominated (Renaud et al, 2014a).…”

CO (7−6), [C i] 370 μm, and [N ii] 205 μm Line Emission of the QSO BRI 1335-0417 at Redshift 4.407

“…A hallmark of a galaxy major merger is the molecular gas funneled into the inner region of the merging galaxies (Solomon & Sage 1988;Sanders et al 1988;Scoville et al 1989;Sanders & Mirabel 1996;Solomon et al 1997;Downes & Solomon 1998;Gao & Solomon 1999;Evans et al 2002) as a result of gravitational torque during the merger (Barnes & Hernquist 1996;Hopkins et al 2009). Furthermore, the non-axisymmetric tidal force also leads to gas turbulences and shocks that compress the gas into higher densities (e.g., Bournaud et al 2011). Recent cosmological simulations of galaxy mergers at high z (e.g., Sparre & Springel 2016) suggest that different gas phases have different spatial distributions, with (a) star-forming dense gas in the inner region of the merging galaxies, and (b) diffuse hot ionized gas extending to large radii.…”