Evolution along the sequence of S0 Hubble types induced by dry minor mergers (original) (raw)
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Evolution along the sequence of SO Hubble types induced by minor mergers II
Astronomy and Astrophysics, 2013
Context. Galaxy mergers are widely discussed as one possible evolution mechanism for lenticular galaxies (S0s), because even minor mergers induce structural changes that are difficult to reconcile with the strong bulge-disk coupling observed in the photometric scaling relations of these galaxies. Aims. We check if the evolution induced onto S0s by dry intermediate and minor mergers can reproduce the S0 photometric scaling relations. Methods. We analyse the bulge-disk decompositions of the collisionless N-body simulations of intermediate and minor mergers onto S0s presented previously to determine the evolution induced by the mergers in several relevant photometric planes. Results. The mergers induce an evolution in the photometric planes that is compatible with the data of S0s, even in the relations that indicate a strong bulge-disk coupling. Mergers drive the formation of the observed photometric relation in some cases and induce a slight dispersion compatible with data in others. Therefore, this evolutionary mechanism tends to preserve the observational photometric relations. In the photometric planes where the morphological types segregate, the mergers always induce evolution towards the region populated by S0s. No clear trends with the mass ratio of the encounter, the central satellite density, or the spinorbit coupling are found for the range of values studied. Long-pericentre orbits generate more concentrated disks and expanded bulges than initially, while short-pericentre orbits do the opposite. The structural coupling of the bulge and the disk is preserved or reinforced in the models because mergers trigger internal secular processes in the primary disk that induce significant bulge growth. This happens even though the encounters do not induce bars in the disks. Conclusions. Intermediate and minor mergers can be considered to be plausible mechanisms for the evolution of S0s if one includes their photometric scaling relations, because they can preserve and even strengthen any pre-existing structural bulge-disk coupling by triggering significant internal secular evolution, even without bars or dissipational effects. Satellite accretions thus seem to unavoidably entail internal secular evolution, meaning that it may be quite complex to isolate the effects of the internal secular evolution driven by mergers from the one due to purely intrinsic disk instabilities in individual early-type disks at the present.
Growth of galactic bulges by mergers. II. Low-density satellites
Astronomy & Astrophysics, 2006
Context: .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. Aims: .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. Methods: .We perform N-body models of the accretion of satellites onto disc galaxies. A Tully-Fisher (M∝ V_rotα_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. Results: . The remnants of the accretion have bulge-disc structure. Both the bulge-to-disc ratio (B/D) and the Sérsic index (n) of the remnant bulge increase as a result of the accretion, with moderate final bulge Sérsic indices: n = 1.0 to 1.9. Bulge growth occurs no matter the fate of the secondary, which fully disrupts for α_TF=3 and partially survives to the remnant center for α_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. Conclusions: .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.
Evolution induced by dry minor mergers onto fast-rotator S0 galaxies
Astronomy & Astrophysics, 2014
Numerical studies have shown that the properties of the S0 galaxies with intermediate kinematics between Fast and Slow Rotators are difficult to explain through major mergers. Aims. We investigate whether the smoother perturbation induced by minor mergers can reproduce these systems. Methods. We have analysed collisionless N-body simulations of intermediate and minor dry mergers on to S0s to determine the structural and kinematic evolution induced by the encounters. The original primary galaxies represent gas-poor Fast Rotators S0b and S0c galaxies with high intrinsic ellipticities. The original bulges are intrinsically spherical and have low rotation. Different mass ratios, parent bulges, density ratios, and orbits have been studied. Results. Minor mergers induce a lower decrease of the global rotational support (as provided by λ e ) than encounters of lower mass ratios, giving rise to S0s with intermediate properties between Fast and Slow Rotators. The resulting remnants are intrinsically more triaxial, less flattened, and span the whole range of apparent ellipticities up to ǫ e ∼ 0.8. They do not show lower apparent ellipticities in random projections than initially; on the contrary, the formation of oval distortions and the disc thickening raise the percentage of projections at 0.4 < ǫ e < 0.7. In the experiments with S0b progenitor galaxies, minor mergers tend to spin up the bulge and to decrease slightly its intrinsic ellipticity, whereas in the cases of primary S0c galaxies they keep the rotational support of the bulge nearly constant and decrease significantly its intrinsic ellipticity. The remnant bulges remain nearly spherical (B/A ∼ C/A > 0.9), but exhibit a wide range of triaxialities (0.20 < T < 1.00). In the plane of global anisotropy of velocities (δ) vs. intrinsic ellipticity (ǫ e,intr ), some of our models extend the linear trend found in previous major merger simulations towards higher ǫ e,intr values, while others clearly depart from it (depending on the progenitor S0). This is consistent with the wide dispersion exhibited by real S0s in this diagram as compared to ellipticals, which follow the linear trend drawn by major merger simulations. Conclusions. The smoother changes induced by minor mergers can explain the existence of S0s with intermediate kinematic properties between Fast and Slow Rotators that major mergers find difficult to explain. The different trends exhibited by ellipticals and S0 galaxies in the δǫ e diagram may be pointing to the different role played by major mergers in the buildup of each morphological type.
The Astrophysical Journal, 2008
Previous models of galactic disk heating in interactions invoke restrictive assumptions not necessarily valid in modern ΛCDM contexts: that satellites are rigid and orbits are circular, with slow decay over many orbital periods from dynamical friction. This leads to a linear scaling of disk heating with satellite mass: disk heights and velocity dispersions scale ∝ M sat /M disk . In turn, observed disk thicknesses present strong constraints on merger histories: the implication for the Milky Way is that < 5% of its mass could come from mergers since z ∼ 2, in conflict with cosmological predictions. More realistically, satellites merge on nearly radial orbits, and once near the disk, resonant interactions efficiently remove angular momentum while tidal stripping removes mass, leading to rapid merger/destruction in a couple of free-fall plunges. Under these conditions the proper heating efficiency is non-linear in mass ratio, ∝ (M sat /M disk ) 2 . We derive the scaling of disk scale heights and velocity dispersions as a function of mass ratio and disk gas content in this regime, and show that this accurately describes the results of simulations with appropriate "live" halos and disks. Under realistic circumstances, we show that disk heating in minor mergers is suppressed by an order of magnitude relative to the expectations of previous analyses. We show that the Milky Way disk could have experienced ∼ 5 − 10 independent 1:10 mass-ratio mergers since z ∼ 2, in agreement with cosmological models. Because the realistic heating rates are non-linear in mass, the predicted heating is dominated by the more stochastic, rare low mass-ratio mergers, and the existence of populations with little or no thick disk does not require fundamental modifications to the cosmology. This also leads to important differences in the predicted isophotal shapes of bulge-disk systems along the Hubble sequence.
Formation of S0 galaxies through mergers
Astronomy & Astrophysics, 2014
Context. Observations reveal a strong structural coupling between bulge and disc in S0 galaxies, which seems difficult to explain if they have formed from supposedly catastrophic events such as major mergers. Aims. We face this question by quantifying the bulge-disc coupling in dissipative simulations of major and minor mergers that result in realistic S0s. Methods. We have studied the dissipative N-body binary merger simulations from the GalMer database that give rise to realistic, relaxed E/S0 and S0 remnants (67 major and 29 minor mergers). We simulate surface brightness profiles of these S0-like remnants in the K band, mimicking typical observational conditions, to perform bulge-disc decompositions analogous to those carried out in real S0s. Additional components have been included when needed. The global bulge-disc structure of these remnants has been compared with real data. Results. The S0-like remnants distribute in the B/Tr eh d parameter space consistently with real bright S0s, where B/T is the bulgeto-total luminosity ratio, r e is the bulge effective radius, and h d is the disc scalelength. Major mergers can rebuild a bulge-disc coupling in the remnants after having destroyed the structures of the progenitors, whereas minor mergers directly preserve them. Remnants exhibit B/T and r e /h d spanning a wide range of values, and their distribution is consistent with observations. Many remnants have bulge Sérsic indices ranging 1 < n < 2, flat appearance, and contain residual star formation in embedded discs, a result which agrees with the presence of pseudobulges in real S0s. Conclusions. Contrary to the popular view, mergers (and in particular, major events) can result in S0 remnants with realistically coupled bulge-disc structures in less than ∼ 3Gyr. The bulge-disc coupling and the presence of pseudobulges in real S0s cannot be used as an argument against the possible major-merger origin of these galaxies.
Astronomy & Astrophysics, 2011
By means of N-body simulations we investigate the impact of minor mergers on the angular momentum and dynamical properties of the merger remnant. Our simulations cover a range of initial orbital characteristics and gas-to-stellar mass fractions (from 0 to 20%), and include star formation and supernova feedback. We confirm and extend previous results by showing that the specific angular momentum of the stellar component always decreases independently of the orbital parameters or morphology of the satellite, and that the decrease in the rotation velocity of the primary galaxy is accompanied by a change in the anisotropy of the orbits. However, the decrease affects only the old stellar population, and not the new population formed from gas during the merging process. This means that the merging process induces an increasing difference in the rotational support of the old and young stellar components, with the old one lagging with respect to the new. Even if our models are not intended specifically to reproduce the Milky Way and its accretion history, we find that, under certain conditions, the modeled rotational lag found is compatible with that observed in the Milky Way disk, thus indicating that minor mergers can be a viable way to produce it. The lag can increase with the vertical distance from the disk midplane, but only if the satellite is accreted along a direct orbit, and in all cases the main contribution to the lag comes from stars originally in the primary disk rather than from stars in the satellite galaxy. We also discuss the possibility of creating counter-rotating stars in the remnant disk, their fraction as a function of the vertical distance from the galaxy midplane, and the cumulative effect of multiple mergers on their creation.
Characteristics of thick disks formed through minor mergers: stellar excesses and scale lengths
Astronomy & Astrophysics, 2011
By means of a series of N-body/SPH simulations we investigate the morphological properties of thick stellar disks formed through minor mergers with, e.g. a range of gas-to-stellar mass ratios. We show that the vertical surface density profile of the post-merger thick disk follows a sech function and has an excess in the regions furthest away from the disk mid-plane (z 2 kpc). This stellar excess also follows a sech function with a larger scale height than the main thick disk component (except at large radii). It is usually dominated by stars from the primary galaxy, but this depends on the orbital configuration. Stars in the excess have a rotational velocity lower than that of stars in the thick disk, and they may thus be confused with stars in the inner galactic halo, which can have a similar lag. Confirming previous results, the thick disk scale height increases with radius and the rate of its increase is smaller for more gas rich primary galaxies. On the contrary, the scale height of the stellar excess is independent of both radius and gas fraction. We also find that the post-merger thick disk has a radial scale length which is 10 − 50% larger than that of the thin disk. Two consecutive mergers have basically the same effect on heating the stellar disk as a single merger of the same total mass, i.e., the disk heating effect of a few consecutive mergers does not saturate but is cumulative. To investigate how thick disks produced through secular processes may differ from those produced by minor mergers, we also simulated gravitationally unstable gas-rich disks ("clumpy disks"). These clumpy disks do not produce either a stellar excess or a ratio of thick to thin disk scale lengths greater than one. Comparing our simulation results with observations of the Milky Way and nearby galaxies shows that our results for minor mergers are consistent with observations of the ratio of thick to thin disk scale lengths and with the "Toomre diagram" (the sum in quadrature of the vertical and radial versus the rotational kinetic energies) of the Milky Way. The simulations of clumpy disks do not show such agreement. We conclude that minor mergers are a viable mechanism for the creation of galactic thick disks and investigating stars at several kpc above the mid-plane of the Milky Way and other galaxies may provide a quantitative method for studying the (minor) merger history of galaxies.
2007
Multiple, sequential mergers are unavoidable in the hierarchical build-up picture of galaxies, in particular for the minor mergers that are frequent and highly likely to have occured several times for most present-day galaxies. However, the effect of repeated minor mergers on galactic structure and evolution has not been studied systematically so far. We present a numerical study of multiple, subsequent, minor galaxy mergers, with various mass ratios ranging from 4:1 to 50:1. The N-body simulations include gas dynamics and star formation. We study the morphological and kinematical properties of the remnants, and show that several so-called "minor" mergers can lead to the formation of elliptical-like galaxies that have global morphological and kinematical properties similar to that observed in real elliptical galaxies. The properties of these systems are compared with those of elliptical galaxies produced by the standard scenario of one single major merger. We thus show that repeated minor mergers can theoretically form elliptical galaxies without major mergers, and can be more frequent than major mergers, in particular at moderate redshift. This process must then have formed some elliptical galaxies seen today, and could in particular explain the high boxiness of massive ellipticals, and some fundamental relations observed in ellipticals. In addition, because repeated minor mergers, even at high mass ratios, destroy disks into spheroids, these results indicate that spiral galaxies cannot have grown only by a succession of minor mergers.
Simulations of minor mergersII. The phase‐space structure of thick discs
Monthly Notices of the Royal …, 2009
We analyse the phase-space structure of simulated thick discs that are the result of a significant merger between a disc galaxy and a satellite. Our main goal is to establish what would be the characteristic imprints of a merger origin for the Galactic thick disc. We find that the spatial distribution predicted for thick disc stars is asymmetric, seemingly in agreement with recent observations of the Milky Way thick disc. Near the Sun, the accreted stars are expected to rotate more slowly, to have broad velocity distributions, and to occupy preferentially the wings of the line-of-sight velocity distributions. The majority of the stars in our model thick discs have low eccentricity orbits (in clear reference to the pre-existing heated disc) which gives rise to a characteristic (sinusoidal) pattern for their line of sight velocities as function of galactic longitude. The z-component of the angular momentum of thick disc stars provides a clear discriminant between stars from the pre-existing disc and those from the satellite, particularly at large radii. These results are robust against the particular choices of initial conditions made in our simulations, and thus provide clean tests of the disc heating via a minor merger scenario for the formation of thick discs.
The role of mergers and interactions in driving the evolution of dwarf galaxies over cosmic time
Monthly Notices of the Royal Astronomical Society, 2020
Dwarf galaxies (M⋆ < 109 M⊙) are key drivers of mass assembly in high-mass galaxies, but relatively little is understood about the assembly of dwarf galaxies themselves. Using the NewHorizon cosmological simulation (∼40 pc spatial resolution), we investigate how mergers and fly-bys drive the mass assembly and structural evolution of around 1000 field and group dwarfs up to z = 0.5. We find that, while dwarf galaxies often exhibit disturbed morphologies (5 and 20 per cent are disturbed at z = 1 and z = 3 respectively), only a small proportion of the morphological disturbances seen in dwarf galaxies are driven by mergers at any redshift (for 109 M⊙, mergers drive under 20 per cent morphological disturbances). They are instead primarily the result of interactions that do not end in a merger (e.g. fly-bys). Given the large fraction of apparently morphologically disturbed dwarf galaxies which are not, in fact, merging, this finding is particularly important to future studies identifyi...