Galaxy clashes

Galaxy interactions are one of the main drivers of galaxy evolution. They may be just fliesby (i.e. the two involved galaxies pass very close to each other, but eventually end up at large distances, after this approach) or mergers (i.e. the two galaxies coalesce and form a single object). Galaxy interactions may involve (i) two galaxies that have approximately the same mass (from 1:1 up to 10:1 mass ratio), (ii) two galaxies with very different mass ratio (10:1 or more).

(i) Nearly equal-mass interactions:
Nearly equal-mass collisions between galaxies produce strongly perturbed galaxy morphologies, such as ring galaxies (e.g. the Cartwheel). At OAPd, we simulate the effects of nearly head-on and nearly equal-mass galaxy collisions on the formation and evolution of gaseous and stellar rings. These simulations are performed with both N-body/smoothed particle hydrodynamics codes (SPH, e.g. GADGET, gasoline) and N-body/adaptive mesh-refinement codes (AMR, e.g. RAMSES). In Figure 1, we show the projected gas density evolution in a set of AMR simulations of galaxy interactions. These simulations can be compared with the kinematics, the star formation history, the morphology (e.g., optical colours) and other peculiar properties of ring galaxies.
Figure 1: Mass-weighted gas density maps of the target (ring) galaxy for various runs at time t = 50 Myr after the interaction. The target galaxy is seen face-on. From the leftmost to the rightmost column: impact parameter (b) = 0, 5, 10 kpc. The top two rows show interactions with inclination angle theta = 0 degrees, while the bottom two rows with theta = 30 degrees. For each pair of rows, the first one represents encounters with initial relative velocity (between the centres of mass of the two galaxies) = 250   km s^-1, and the second one with initial relative velocity = 650   km s^-1. AMR simulations published in Fiacconi et al. (2012).

(ii) Unequal-mass collisions:
Unequal-mass collisions (such as minor mergers) induce small perturbations on the target galaxy. Such perturbations, although small, have crucial impact on the evolution of the target galaxy (e.g. they induce the formation of a bar or of an inner ring). At OAPd, we simulate minor mergers with N-body/SPH and N-body/AMR codes, to shed light on the physics of gas during and after the collision. We are interested in the formation of long-lived gaseous inner rings, and in the rejuvenation of early type galaxies (e.g. S0 galaxies) by minor mergers with gas-rich intruders. We study the importance of bar-like structures for the accretion of fresh gas after minor mergers (Figure 2).


Figure 2: Mass-weighted gas density maps of the target galaxy (a S0 galaxy) during the minor merger with a small gas-rich satellite. The superimposed green contours show the stellar density (a bar is clearly visible). From left to right: 3, 5 and 7 Gyr after the beginning of the simulation. Each panel measures 70x60 kpc. The formation of an inner gaseous ring (connected with the radial inner ultra-harmonic 4:1 resonance) is apparent from this figure.

People: Michela Mapelli, Roberto Rampazzo

Collaborations: Antonietta Marino (UniPd), Davide Fiacconi (University of Zurich), Emanuele Ripamonti (University of Milano), Lucio Mayer (University of Zurich).

Recent pubblications: Mapelli & Mayer (2012, MNRAS, 420, 1158);  Fiacconi et al. (2012, MNRAS, 425, 2255)

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