arXiv subjects

A. C. Gonzalez-Garcia

Publications and source records attributed to A. C. Gonzalez-Garcia.

11 recordsLinked to original sources

arXiv2012

Light and Shadows over Petra: astronomy and landscape in Nabataean lands

The Nabateans built several monuments in Petra and elsewhere displaying a decoration with a preference for astronomical motifs, possibly as a reflection of their religion. However, due to the lack of direct written accounts and the scarcity of inscriptions we do not have a clear knowledge on the precise nature of such believes and how these reflected on the calendar or the religious time-keeping system of this ancient society. A statistical analysis of the orientation of their sacred monuments demonstrates that astronomical orientations were often part of an elaborated plan and possibly a trace of the astral nature of Nabataean religion. Petra and other monuments in the ancient Nabataean kingdom have proven to be marvellous laboratories of the interaction between landscape features and astronomical events showing impressive hierophanies on particular monuments related to cultic times and worships. Among other findings, the famous Ad Deir has shown a fascinating ensemble of light and shadow effects, perhaps connected with the bulk of Nabataean mythology, while from the impressive Urn tomb, a series of suggestive solstitial and equinoctial alignments emanate which might have lately helped its selection as the cathedral of the city. This paper demonstrates that the sky was a substantial element on Nabataean religion and reveals new evidence for cultic worship centred on the celestial sphere.

physics.hist-ph↗

arXiv2009

A minor-merger origin for inner disks and rings in early-type galaxies

Nuclear disks and rings are frequent galaxy substructures, for a wide range of morphological types (from S0 to Sc). We have investigated the possible minor-merger origin of inner disks and rings in spiral galaxies through collisionless N-body simulations. The models confirm that minor mergers can drive the formation of thin, kinematically-cold structures in the center of galaxies out of satellite material, without requiring the previous formation of a bar. Satellite core particles tend to be deposited in circular orbits in the central potential, due to the strong circularization experienced by the satellite orbit through dynamical friction. The material of the satellite core reaches the remnant center if satellites are dense or massive, building up a thin inner disk; whereas it is fully disrupted before reaching the center in the case of low-mass satellites, creating an inner ring instead.

astro-ph.CO↗

arXiv2009

The Intergalactic Stellar Population from Mergers of Elliptical Galaxies with Dark Matter halos

We present simulations of dry-merger encounters between pairs of elliptical galaxies with dark matter halos. The aim of these simulations is to study the intergalactic stellar populations produced in both parabolic and hyperbolic encounters. We model progenitor galaxies with total-to-luminous mass ratios M_T/M_L 3 and 11. The initial mass of the colliding galaxies are chosen so that M_1/M_2 and 10. The model galaxies are populated by particles representing stars, as in Stanghellini et al. (2006), and dark matter. Merger remnants resulting from these encounters display a population of unbounded particles, both dark and luminous. The number of particles becoming unbounded depends on orbital configuration, with hyperbolic encounters producing a larger luminous intracluster population than parabolic encounters. Furthermore, in simulations with identical orbital parameters, a lower M_T/M_L of the colliding galaxies produces a larger fraction of unbounded luminous particles. For each modeled collision, the fraction of unbounded to initial stellar mass is the same in all mass-bins considered, similarly to what we found previously by modeling encounters of galaxies without dark halos. The fraction of intergalactic to total luminosity resulting from our simulations is ~ 4% and ~ 6% for dark-to-bright mass rations of 10 and 2 respectively. These unbounded-to-total luminous fractions are down from 17 % that we had previously found in the case of no dark halos. Our results are in broad agreement with intergalactic light observed in groups of galaxies, while the results of our previous models without dark halos better encompass observed intracluster populations. We suggest a possible formation scenario of intergalactic stars.

astro-ph.CO↗

arXiv2006

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/sigma), 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/sigma ~ 1-2 and corresponding high h3 values, which suggested that gas dynamics and ensuing star formation might be needed in order to explain the skewness properties of ellipticals through mergers. Here we show that high V/sigma 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 disky 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.

astro-ph↗

arXiv2006

Growth of galactic bulges by mergers. II. Low-density satellites

Satellite accretion events have been invoked for mimicking the internal secular evolutionary processes of bulge growth. However, N-body simulations of satellite accretions have paid little attention to the evolution of bulge photometric parameters, to the processes driving this evolution, and to the consistency of this evolution with observations. We want to investigate whether satellite accretions indeed drive the growth of bulges, and whether they are consistent with global scaling relations of bulges and discs. We perform N-body models of the accretion of satellites onto disc galaxies. A Tully-Fisher (M \propto V_{rot}^ {alpha_TF}) scaling between primary and satellite ensures that density ratios, critical to the outcome of the accretion, are realistic. We carry out a full structural, kinematic and dynamical analysis of the evolution of the bulge mass, bulge central concentration, and bulge-to-disc scaling relations. The remnants of the accretion have bulge-disc structure. Both the bulge-to-disc ratio (B/D) and the Sersic index (n) of the remnant bulge increase as a result of the accretion, with moderate final bulge Sersic indices: n = 1.0 to 1.9. Bulge growth occurs no matter the fate of the secondary, which fully disrupts for alpha_TF=3 and partially survives to the remnant center for alpha_TF = 3.5 or 4. Global structural parameters evolve following trends similar to observations. We show that the dominant mechanism for bulge growth is the inward flow of material from the disc to the bulge region during the satellite decay. The models confirm that the growth of the bulge out of disc material, a central ingredient of secular evolution models, may be triggered externally through satellite accretion.

astro-ph↗

arXiv2005

Harassment Origin for Kinematic Substructures in Dwarf Elliptical Galaxies?

We have run high resolution N-body models simulating the encounter of a dwarf galaxy with a bright elliptical galaxy. The dwarf absorbs orbital angular momentum and shows counter-rotating features in the external regions of the galaxy. To explain the core-envelope kinematic decoupling observed in some dwarf galaxies in high-density environments requires nearly head-on collisions and very little dark matter bound to the dwarf. These kinematic structures appear under rather restrictive conditions. As a consequence, in a cluster like Virgo ~1% of dwarf galaxies may present counter-rotation formed by harassment.

astro-ph↗

arXiv2005

Encounters between spherical galaxies I: systems without 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 non-merging 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 angular momentum different from zero are triaxial 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.

astro-ph↗

arXiv2005

Encounters between spherical galaxies II: systems with dark halo

We performe 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.

astro-ph↗

arXiv2005

Satellite Infall and the Growth of Bulges of Spiral Galaxies

For bulges of spiral galaxies, the concentration, or Sersic index, increases with bulge luminosity and bulge-to-disk ratio B/D (Andredakis, Peletier, & Balcells 1995, MNRAS, 275, 874). Does this trend trace the growth of bulges via satellite accretion? And, is satellite infall consistent with this trend? Aguerri, Balcells, & Peletier (2001, A&A, 367, 428) investigated this question with N-body simulations of the accretion of dense, spheroidal satellites. Here, we expand on that work by running N-body simulations of the accretion of satellites that have realistic densities. Satellites are modelled as disk-bulge structures with their own dark-matter halo. A realistic density scaling with the primary galaxy is ensured by using the Tully-Fisher relation. Our merger models show that most satellites disrupt before reaching the center. However, a bulge-disk decomposition of the surface density profile after the accretion shows an increase of both the B/D and the Sersic index n of the bulge. The increase in the mass and concentration of the inner Sersic component is due to inward piling up of disk material due to transient bars during the satellite orbital decay. This research is described in Eliche-Moral, Balcells, Aguerri, & Gonzalez-Garcia, 2005 (in preparation).

astro-ph↗

arXiv2004

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 bulge-less, disc-halo models to quantify the effects of the central bulge on the merger dynamics and on the structure of the remnant. Our models explore galaxy mass ratios 1:1 through 3:1, and use higher bulge mass fraction than previous studies. A full comparison of structural and dynamical properties to observations is carried out. The presence of central bulges results in longer tidal tails, oblate final intrinsic shapes, surface brightness profiles with higher Sersic index, steeper rotation curves, and oblate-rotator internal dynamics. Mergers of bulge-less 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 modeled here; they could be the result of mergers between lower luminosity ellipticals, themselves plausibly formed in disc-disc mergers.

astro-ph↗

arXiv2003

Mergers between elliptical galaxies and the thickening of the fundamental plane

We have carried out computer simulations to study the effect of merging on the fundamental plane (FP) relation. Initially, systems are spherical Jaffe models following a simple scaling relation (M/R_e^2 = constant). They have been put on the FP by imposing different M/L values. Various orbital characteristics have been considered. Our results show that the merger remnants lie very close to the FP of the progenitors. Although non-homology is introduced by the merging process, mergers among homologous galaxies leave a pre-existing FP-relation intact. As a side result we find that variations in the point of view lead to non-negligible scatter about the FP.

astro-ph↗