Antonio César González-García

Elliptical galaxies from mergers of discs

We analyse N-body galaxy merger experiments involving disc galaxies. Mergers of disc-bulge-halo models are compared to those of bulgeless, disc-halo models to quantify the effects of the central bulge on merger dynamics and the structure of the remnant. Our models explore galaxy mass ratios 1:1 through 3:1, and use higher bulge mass fractions than previous studies. A full comparison of the structural and dynamical properties with our observations is carried out. The presence of central bulges results in longer tidal tails, oblate final intrinsic shapes, surface brightness profiles with a higher Sersic index, steeper rotation curves and oblate-rotator internal dynamics. Mergers of bulgeless galaxies do not generate long-lasting tidal tails, and their strong triaxiality seems inconsistent with observations; these remnants show shells, which we do not find in models including central bulges. Giant ellipticals with boxy isophotes and anisotropic dynamics cannot be produced by the mergers modelled here; they could be the result of mergers between lower luminosity ellipticals, themselves plausibly formed in disc-disc mergers.

Encounters between spherical galaxies—II. Systems with a dark halo

We perform N-body simulations of encounters between spherical systems surrounded by a spherical halo. Following a preceding paper with a similar aim, the initial systems include a spherical Jaffe model for the luminous matter and a Hernquist model for the halo. The merger remnants from this sample are mainly slowly rotating, prolate spheroids with a radially anisotropic velocity distribution. The results are compared with real-life ellipticals and with the models without halo in Paper I. We argue that elliptical galaxies with evidence of dark matter could be formed in the field via a merger of spheroids surrounded by a dark matter halo, while ellipticals with no evidence of dark matter might be formed via a merger of two spheroids in a cluster.

Line-of-sight velocity distributions of elliptical galaxies from collisionless mergers

We analyse the skewness of the line-of-sight velocity distributions in model elliptical galaxies built through collisionless galaxy mergers. We build the models using large N-body simulations of mergers between either two spiral or two elliptical galaxies. Our aim is to investigate whether the observed ranges of skewness coefficient (h3) and the rotational support (V/σ ), as well as the anticorrelation between h3 and V, may be reproduced through collisionless mergers. Previous attempts using N-body simulations failed to reach V/σ ≈ 1‐2 and corresponding high h3 values, which suggested that gas dynamics and ensuing star formation might be needed in order explain the skewness properties of ellipticals through mergers. Here we show that high V/σ and high h3 are reproduced in collisionless spiral‐spiral mergers whenever a central bulge allows the discs to retain some of their original angular momentum during the merger. We also show that elliptical‐elliptical mergers, unless merging from a high-angular momentum orbit, reproduce the strong skewness observed in non-rotating, giant, boxy ellipticals. The behaviour of the h3 coefficient therefore associates rapidly-rotating discy ellipticals to disc-disc mergers, and associates boxy, slowly rotating giant ellipticals to elliptical‐elliptical mergers, a framework generally consistent with the expectations of hierarchical galaxy formation.

Encounters between spherical galaxies - I. Systems without a dark halo

We report here on a survey of N-body simulations of encounters between spherical galaxies. Initial systems are isotropic Jaffe models. Different sets of mass ratios, impact parameters and orbital energies are studied. Both merger remnants and systems perturbed after a nonmerging encounter are analysed and compared to real-life elliptical galaxies. The properties of merger remnants show a large variety. Merger remnants resulting from head-on encounters are mainly non-rotating prolate spheroids. Merger remnants from models with Jorb � 0 are tri-axial or mildly oblate spheroids, supported in part by rotation. The velocity distributions are biased towards the radial direction in the prolate case and the tangential direction in the oblate case. Non-mergers are affected in various ways, depending on the orbital characteristics. We conclude that many of the global properties of real-life ellipticals can, in principle, be attributed to a merger of spherical progenitors. Ke yw ords: methods: N-body simulations ‐ galaxies: elliptical and lenticular, cD ‐ galaxies: interactions ‐ galaxies: kinematics and dynamics ‐ galaxies: structure.

Astronomy in Roman Urbanism: A Statistical Analysis of the Orientation of Roman Towns in the Iberian Peninsula

The work presented in this article is part of a wide-ranging and ambitious project, started few years ago, to study the role of astronomy in Roman urban layout. In particular, the main aim is to check whether Roman cities present astronomical patterns in their orientations. The project emerged from ideas on how to properly orientate the main streets of a Roman town, as attested in a number of ancient texts and later discussions led by contemporary scholars. We present here the final conclusions of a particular study developed in the Iberian Peninsula (Roman Hispania), where the urbanism that we tend to characterize as properly Roman flourished during both the Republic and the Empire. The sample analysed includes 81 measurements of Roman urban entities spread throughout the Iberian Peninsula and is the largest dataset obtained in a specific region so far. Our results present suggestive orientation patterns that seem to point towards an astronomical intentionality.