Shedding light on the formation mechanism of shell galaxy NGC 474 with MUSE

Stellar shells around galaxies could provide precious insights into their assembly history. However, their formation mechanism remains poorly empirically constrained, regarding in particular the type of galaxy collisions at their origin. We present MUSE at VLT data of the most prominent outer shell of NGC 474, to constrain its formation history. The stellar shell spectrum is clearly detected, with a signal-to-noise ratio of ∼65 pix−1. We used a full spectral fitting method to determine the line-of-sight velocity and the age and metallicity of the shell and associated point-like sources within the MUSE field of view. We detect six globular cluster (GC) candidates and eight planetary nebula (PN) candidates that are all kinematically associated with the stellar shell. We show that the shell has an intermediate metallicity, [M/H] = −0.83−0.12+0.12, and a possible α-enrichment, [α/Fe] ∼ 0.3. Assuming the material of the shell comes from a lower mass companion, and that the latter had no initial metallicity gradient, such a stellar metallicity would constrain the mass of the progenitor at around 7.4 × 108 M⊙, implying a merger mass ratio of about 1:100. However, our census of PNe and earlier photometry of the shell would suggest a much higher ratio, around 1:20. Given the uncertainties, this difference is only significant at the ≃1σ level. We discuss the characteristics of the progenitor, and in particular whether the progenitor could also be composed of stars from the low-metallicity outskirts of a more massive galaxy. Ultimately, the presented data do not allow us to put a firm constraint on the progenitor mass. We show that at least two GC candidates possibly associated with the shell are quite young, with ages below 1.5 Gyr. We also note the presence of a young (∼1 Gyr) stellar population in the center of NGC 474. The two may have resulted from the same event.

Splash bridge models of inclined, gas-rich, direct galaxy collisions

ABSTRACT Splash bridges are formed from the direct inelastic collision of gas-rich galaxies. Recent multiwavelength observations of the Taffy galaxies, UGC 12914/15, have revealed complicated gas structures in the bridge. We have upgraded the sticky particle simulation code of Yeager & Struck by adding: the ability to adjust the relative inclination of the gas discs, the ability to track cloud–cloud collisions over time, and additional cooling processes. Inclination effects lead to various morphological features, including filamentary streams of gas stripped from the smaller galactic disc. The offset of disc centres at impact determines whether or not these streams flow in a single direction or multiple directions, even transverse to the motion of the two galaxies. We also find that, across many types of direct collision, independent of the inclination or offset, the distributions of weighted Mach numbers and shock velocities in colliding clouds relax to a very similar form. There is good evidence of prolonged turbulence in the gas of each splash bridge for all inclinations and offsets tested, as a result of continuing cloud collisions, which in turn are the result of shearing and differentially accelerated trajectories. The number distribution of high velocity shocks in cloud collisions, produced in our low inclination models, are in agreement with those observed by Appleton et al. in the Taffy Galaxies with ALMA.

Timescales of major mergers from simulations of isolated binary galaxy collisions

A six-dimensional parameter space based on high-resolution numerical simulations of isolated binary galaxy collisions has been constructed to investigate the dynamical friction timescales, τmer, for major mergers. Our experiments follow the gravitational encounters between ∼600 pairs of similarly massive late- and early-type galaxies with orbital parameters that meet the predictions of the Λ-cold dark matter (ΛCDM) cosmology. We analyse the performance of different schemes for tracking the secular evolution of mergers, finding that the product of the intergalactic distance and velocity is best suited to identify the time of coalescence. In contrast, a widely used merger-time estimator such as the exhaustion of the orbital spin is shown to systematically underpredict τmer, resulting in relative errors that can reach 60% for nearly radial encounters. We find that the internal spins of the progenitors can lead to total variations in the merger times larger than 30% in highly circular encounters, whereas only the spin of the principal halo is capable of modulating the strength of the interaction prevailing throughout a merger. The comparison of our simulated merger times with predictions from different variants of a well-known fitting formula has revealed an only partially satisfactory agreement, which has led us to recalculate the values of the coefficients of these expressions to obtain relations that fit major mergers perfectly. The observed biases between data and predictions, which do not only apply to the present work, are inconsistent with expectations from differences in the degree of idealisation of the collisions, their metric, spin-related biases, or the simulation set-up. This indicates a certain lack of accuracy of the dynamical friction modelling, arising perhaps from a still incomplete identification of the parameters governing orbital decay.