Simulations Of Major Mergers Research Articles

The growth of massive black holes in galaxy merger simulations with feedback by radiation pressure

We study the growth of massive black holes (BH) in galaxies using smoothed particle hydrodynamic simulations of major galaxy mergers with new implementations of BH accretion and feedback. The effect of BH accretion on gas in its host galaxy is modeled by depositing momentum at a rate ~ tau L/c into the ambient gas, where L is the luminosity produced by accretion onto the BH and tau is the wavelength-averaged optical depth of the galactic nucleus to the AGN's radiation (a free parameter of our model). The accretion rate onto the BH is relatively independent of our subgrid accretion model and is instead determined by the BH's dynamical impact on its host galaxy: BH accretion is thus self-regulated rather than `supply limited.' We show that the final BH mass and total stellar mass formed during a merger are more robust predictions of the simulations than the time dependence of the star formation rate or BH accretion rate. In particular, the latter depend on the assumed interstellar medium physics, which determines when and where the gas fragments to form star clusters; this in turn affects the fuel available for further star formation and BH growth. Simulations over a factor of ~ 30 in galaxy mass are consistent with the observed M_BH-sigma relation for a mean optical depth of tau ~ 25. This requires that most BH growth occur when the galactic nucleus is optically thick to far-infrared radiation, consistent with the hypothesized connection between ultra-luminous infrared galaxies and quasars. We find tentative evidence for a shallower M_BH-sigma relation in the lowest mass galaxies, sigma < 100 km/s. Our results demonstrate that feedback-regulated BH growth and consistency with the observed M_BH-sigma relation do not require that BH feedback terminate star formation in massive galaxies or unbind large quantities of cold gas.

Formation of slowly rotating early-type galaxies via major mergers: a resolution study

We study resolution effects in numerical simulations of gas-rich and gas-poor major mergers, and show that the formation of slowly rotating elliptical galaxies often requires a resolution that is beyond the present-day standards to be properly modelled. Our sample of equal-mass merger models encompasses various masses and spatial resolutions, ranging from about 200 pc and 105 particles per component (stars, gas and dark matter), i.e. a gas mass resolution of ∼105 M⊙, typical of some recently published major merger simulations, to up to 32 pc and ∼103 M⊙ in simulations using 2.4 × 107 collisionless particles and 1.2 × 107 gas particles, among the highest resolutions reached so far for gas-rich major merger of massive disc galaxies. We find that the formation of fast-rotating early-type galaxies, that are flattened by a significant residual rotation, is overall correctly reproduced at all such resolutions. However, the formation of slow-rotating early-type galaxies, which have a low-residual angular momentum and are supported mostly by anisotropic velocity dispersions, is strongly resolution-dependent. The evacuation of angular momentum from the main stellar body is largely missed at standard resolution, and systems that should be slow rotators are then found to be fast rotators. The effect is most important for gas-rich mergers, but is also witnessed in mergers with an absent or modest gas component (0–10 per cent in mass). The effect is robust with respect to our initial conditions and interaction orbits, and originates in the physical treatment of the relaxation process during the coalescence of the galaxies. Our findings show that a high-enough resolution is required to accurately model the global properties of merger remnants and the evolution of their angular momentum. The role of gas-rich mergers of spiral galaxies in the formation of slow-rotating ellipticals may therefore have been underestimated. Moreover, the effect of gas in a galaxy merger is not limited to helping the survival/rebuilding of rotating disc components: at high resolution, gas actively participates in the relaxation process and the formation of slowly rotating stellar systems.

Metallicity gradients at large galactocentric radii using the near-infrared Calcium triplet

We describe a new spectroscopic technique for measuring radial metallicity gradients out to large galactocentric radii. We use the DEIMOS multi-object spectrograph on the Keck telescope and the galaxy spectrum extraction technique of Proctor et al. (2009). We also make use of the metallicity sensitive near-infrared (NIR) Calcium triplet (CaT) features together with single stellar population models to obtain metallicities. Our technique is applied as a pilot study to a sample of three relatively nearby (<30 Mpc) intermediate-mass to massive early-type galaxies. Results are compared with previous literature inner region values and generally show good agreement. We also include a comparison with profiles from dissipational disk-disk major merger simulations. Based on our new extended metallicity gradients combined with other observational evidence and theoretical predictions, we discuss possible formation scenarios for the galaxies in our sample. The limitations of our new technique are also discussed.

Generation of rotationally dominated galaxies by mergers of pressure-supported progenitors

Through the analysis of a set of numerical simulations of major mergers between initially non-rotating, pressure-supported progenitor galaxies with a range of central mass concentrations, we have shown that: (1) it is possible to generate elliptical-like galaxies, with outside one effective radius, as a result of the conversion of orbital- into internal-angular momentum; (2) the outer regions acquire part of the angular momentum first; (3) both the baryonic and the dark matter components of the remnant galaxy acquire part of the angular momentum, the relative fractions depending on the initial concentration of the merging galaxies. For this conversion to occur the initial baryonic component must be sufficiently dense and/or the encounter should take place on an orbit with high angular momentum. Systems with these hybrid properties have recently been observed through a combination of stellar absorption lines and planetary nebulae for kinematic studies of early-type galaxies. Our results are in qualitative agreement with these observations and demonstrate that even mergers composed of non rotating, pressure-supported progenitor galaxies can produce early-type galaxies with significant rotation at large radii.

The merger-driven evolution of warm infrared luminous galaxies

AbstracT We present a merger-driven evolutionary model for the production of luminous (LIRGs) and ultraluminous infrared galaxies (ULIRGs) with warm infrared (IR) colours. Our results show that simulations of gas-rich major mergers including star formation, black hole growth and feedback can produce warm (U)LIRGs. We also find that while the warm evolutionary phase is associated with increased active galactic nucleus (AGN) activity, star formation alone may be sufficient to produce warm IR colours. However, the transition can be suppressed entirely – even when there is a significant AGN contribution – when we assume a single-phase interstellar medium, which maximizes the attenuation. Finally, our evolutionary models are consistent with the 25-to-60 µm flux density ratio versus LHX/LIR relation for local LIRGs and ULIRGs, and predict the observed scatter in IR colour at fixed LHX/LIR. Therefore, our models suggest a cautionary note in the interpretation of warm IR colours: while associated with periods of active black hole growth, they are probably produced by a complex mix of star formation and AGN activity intermediate between the cold star formation dominated phase and the birth of a bright, unobscured quasar.

The Self‐Regulated Growth of Supermassive Black Holes

We present a series of simulations of the self-regulated growth of supermassive black holes (SMBHs) in galaxies via three different fueling mechanisms: major mergers, minor mergers, and disk instabilities. The SMBHs in all three scenarios follow the same black hole plane (BHFP) and correlation with bulge binding energy seen in simulations of major mergers, and observed locally. Furthermore, provided that the total gas supply is significantly larger than the mass of the SMBH, its limiting mass is not influenced by the amount of gas available or the efficiency of black hole growth. This supports the assertion that SMBHs accrete until they reach a critical mass at which feedback is sufficient to unbind the gas locally, terminating the inflow and stalling further growth. At the same time, while minor and major mergers follow the same projected correlations (e.g., the MBH-σ and Magorrian relations), SMBHs grown via disk instabilities do not, owing to structural differences between the host bulges. This finding is supported by recent observations of SMBHs in pseudobulges and bulges in barred systems, as compared to those hosted by classical bulges. Taken together, this provides support for the BHFP and binding energy correlations as being more fundamental than other proposed correlations in that they reflect the physical mechanism driving the coevolution of SMBHs and spheroids.

Rotation curve bifurcations as indicators of close recent galaxy encounters

Rotation curves of interacting galaxies often show that velocities are either rising or falling in the direction of the companion galaxy. We seek to reproduce and analyse these features in the rotation curves of simulated equal-mass galaxies suffering a one-to-one encounter, as possible indicators of close encounters. Using simulations of major mergers in 3D, we study the time evolution of these asymmetries in a pair of galaxies, during the first passage. Our main results are: (a) the rotation curve asymmetries appear right at pericentre of the first passage, (b) the significant disturbed rotation velocities occur within a small time interval, of ~ 0.5 Gyr h^-1, and therefore the presence of bifurcation in the velocity curve could be used as an indicator of the pericentre occurrence. These results are in qualitative agreement with previous findings for minor mergers and fly-byes.

A Theoretical Interpretation of the Black Hole Fundamental Plane

We examine the origin and evolution of correlations between properties of supermassive BHs and their host galaxies using simulations of major galaxy mergers, including the effects of gas dissipation, cooling, star formation, and BH accretion and feedback. We demonstrate that the simulations predict the existence of a BH "fundamental plane" (BHFP), of the form MBH ∝ σ3.0±0.3R or MBH ∝ Mσ2.2±0.5, similar to relations found observationally. The simulations indicate that the BHFP can be understood roughly as a tilted intrinsic correlation between BH mass and spheroid binding energy, or the condition for feedback coupling to power a pressure-driven outflow. While changes in halo circular velocity, merger orbital parameters, progenitor disk redshifts and gas fractions, ISM gas pressurization, and other parameters can drive changes in, e.g., σ at fixed M*, and therefore changes in the MBH-σ or MBH-M* relations, the BHFP is robust. Given the empirical trend of decreasing Re for a given M* at high redshift (i.e., increasingly deep potential wells), the BHFP predicts that BHs will be more massive at fixed M*, in good agreement with recent observations. This evolution in the structural properties of merger remnants, to smaller Re and larger σ (and therefore larger MBH, conserving the BHFP) at a given M*, is driven by the fact that disks (merger progenitors) have characteristically larger gas fractions at high redshifts. Adopting the observed evolution of disk gas fractions with redshift, our simulations predict the observed trends in both Re(M*) and MBH(M*). The existence of this BHFP also has important implications for the masses of the very largest black holes and immediately resolves several apparent conflicts between the BH masses expected and measured for outliers in both the MBH-σ and MBH-M* relations.

Feedback‐driven Evolution of the Far‐Infrared Spectral Energy Distributions of Luminous and Ultraluminous Infrared Galaxies

We calculate infrared spectral energy distributions (SEDs) from simulations of major galaxy mergers and study the effect of AGN and starburst driven feedback on the evolution of the SED as a function of time. We use a self-consistent three-dimensional radiative equilibrium code to calculate the emergent SEDs and to make images. To facilitate a simple description of our findings, we describe our results in reference to an approximate analytic solution for the far-IR SED. We focus mainly on the luminous infrared galaxy (LIRG) and ultraluminous infrared galaxy (ULIRG) phases of evolution. We contrast the SEDs of simulations performed with AGN feedback to simulations performed with starburst driven wind feedback. We find that the feedback processes critically determine the evolution of the SED. Changing the source of illumination (whether stellar or AGN) has virtually no impact on the reprocessed far-infrared SED. We find that AGN feedback is particularly effective at dispersing gas and rapidly injecting energy into the ISM. The observational signature of such powerful feedback is a warm SED. In general, simulations performed with starburst driven winds have colder spectra and reprocess more of their emission into the infrared, resulting in higher infrared to bolometric luminosities compared to (otherwise equivalent) simulations performed with AGN feedback. We depict our results in IRAS bands, as well as in Spitzer's MIPS bands, and in Herschel's PACS bands.

Feedback in simulations of disc-galaxy major mergers

Using hydrodynamic simulations of disc-galaxy major mergers, we investigate the star formation history and remnant properties when various parametrizations of a simple stellar feedback model are implemented. The simulations include radiative cooling, a density-dependent star formation recipe and a model for feedback from massive stars. The feedback model stores supernova feedback energy within individual gas particles and dissipates this energy on a time-scale specified by two free parameters; τfb, which sets the dissipative time-scale, and n, which sets the effective equation of state in star-forming regions. Via this model, feedback energy can provide pressure support to regions of gas that are thermally cold. Using a self-consistent disc galaxy, modelled after a local Sbc spiral, in both isolated and major-merger simulations, we investigate parametrizations of the feedback model that are selected with respect to the quiescent disc stability. These models produce a range of star formation histories for discs evolved in isolation, or during a major merger, yet all are consistent with the star formation relation found by Kennicutt. We suggest that this result is produced by the adopted recipe for star formation and is not a byproduct of the feedback model. All major mergers produce a population of new stars that is highly centrally concentrated, demonstrating a distinct break in the r1/4 surface density profile, consistent with previous findings. The half-mass radius and one-dimensional velocity dispersion are affected by the feedback model used. In tests with up to an order of magnitude higher resolution, the star formation history is nearly identical, suggesting that we have achieved a numerically converged star formation history. Finally, we compare our results to those of previous simulations of star formation in disc-galaxy major mergers, addressing the effects of star formation normalization, the version of smoothed particle hydrodynamics (sph) employed and assumptions about the interstellar medium. We conclude by suggesting several methods by which future studies may better constrain feedback models.

An Evolutionary Model for Submillimeter Galaxies

We calculate multiwavelength spectral energy distributions (SEDs) from simulations of major galaxy mergers with black hole feedback that produce submillimeter bright galaxies (SMGs), using the self-consistent three-dimensional radiative transfer code RADISHE. These calculations allow us to predict multiwavelength correlations for this important class of galaxies. We review star formation rates, the time evolution of the 850 μm fluxes, along with the time evolution of the MBH − M⋆ relation of the SMGs formed in the mergers. We reproduce correlations for local AGNs observed in Spitzer Space Telescope's IRAC bands, and make definitive predictions for infrared X-ray correlations. Our dynamical approach allows us to directly correlate observed clustering in the data as seen in IRAC color-color plots with the relative amount of time the system spends in a region of color-color space. We also find that this clustering is positively correlated with the stars dominating in their contribution to the total bolometric luminosity. We compare our calculated SEDs to observations of SMGs and find good agreement. We introduce a simple, heuristic classification scheme which we present in terms of the LIR/LX ratios of these galaxies, which may be interpreted as an evolutionary scheme, as these galaxies evolve in LIR/LX while transiting from a X-ray underluminous infrared bright phase (class I, LIR/LX≳ 100), through a quasar phase (class II, LIR/LX ∼ 25), to a merger remnant (class III, LIR/LX≲ 10). We find that SMGs are a broader class of systems than starbursts or quasars, traversing the range from class I to class II systems.

The star formation history of luminous infrared galaxies

Aims. We constrain the past star formation histories of a sample of 25 distant (z ∼ 0.7) luminous infrared galaxies (LIRGs) detected with the mid infrared cameras ISOCAM and MIPS onboard the ISO and Spitzer satellites. Methods. We used high-resolution VLT-FORS2 spectroscopy in addition to a comprehensive library of 200 000 model optical spectra to derive Bayesian likelihood estimates of the star formation histories of these galaxies, based on analysis of Balmer absorption lines and the 4000 A break. Results. The locus of distant LIRGs in the diagram defined by Hδ A and D4000 is roughly comparable to that of local LIRGs observed with IRAS, suggesting that no trend toward an evolution is detected between the local and distant LIRGs. We obtain similar results when using either the H8 or the Hδ A Balmer absorption-line indices in combination with D4000. By computing a birthrate parameter (b = SFR/(SFR)) of 4 ± 1, we confirme that the distant LIRGs are currently experiencing a major phase of star formation. The most likely duration of the bursts is 0.10 +0.16 -0.06 Gyr, during which the LIRGs produce ∼5-10% of their current stellar mass. No evidence was found for successive starbursts on the scale of a few times 10 7 yr, such as those predicted by some numerical simulations of major mergers. However, the high number density of those galaxies suggests that they could have experienced between two and four LIRG phases until the present epoch. This scenario is not consistent with the formation of the z ∼ 0.7 LIRGs through the continuous star formation characterizing isolated spiral galaxies as has been independently argued based on their morphology. Instead, minor mergers, tidal interactions, or gas accretion remain plausible triggering mechanisms for more than half of the distant LIRGs that do not harbor the morphology of major mergers.

Shapes of Stellar Systems and Dark Halos from Simulations of Galaxy Major Mergers

Using a sample of 89 snapshots from 58 hydrodynamic binary galaxy major merger simulations, we find that stellar remnants are mostly oblate while dark matter halos are mostly prolate or triaxial. The stellar minor axis and the halo major axis are almost always nearly perpendicular. This can be understood by considering the influence of angular momentum and dissipation during the merger. If binary mergers of spiral galaxies are responsible for the formation of elliptical galaxies or some subpopulation thereof, these galaxies can be expected to be oblate and inhabit their halos with the predicted shapes and orientations. These predictions are relevant to observational studies of weak gravitational lensing, where one must stack many optically aligned galaxies in order to determine the shape of the resulting stacked mass distribution. The simple relationship between the dark and luminous matter presented here can be used to guide the stacking of galaxies to minimize the information lost.

Red mergers and the assembly of massive elliptical galaxies: the fundamental plane and its projections

Several recent observations suggest that gas-poor (dissipationless) mergers of elliptical galaxies contribute significantly to the build-up of the massive end of the red sequence at z≲ 1. We perform a series of major merger simulations to investigate the spatial and velocity structure of the remnants of such mergers. Regardless of orbital energy or angular momentum, we find that the stellar remnants lie on the fundamental plane defined by their progenitors, a result of virial equilibrium with a small tilt due to an increasing central dark matter fraction. However, the locations of merger remnants in the projections of the fundamental plane – the Faber–Jackson and Re–M* relations – depend strongly on the merger orbit, and the relations steepen significantly from the canonical scalings (L∝σ4e and Re∝M0.6*) for mergers on radial orbits. This steepening arises because stellar bulges on orbits with lower angular momentum lose less energy via dynamical friction on the dark matter haloes than do bulges on orbits with substantial angular momentum. This results in a less tightly bound remnant bulge with a smaller velocity dispersion and a larger effective radius. Our results imply that the projections of the fundamental plane – but not necessarily the plane itself – provide a powerful way of investigating the assembly history of massive elliptical galaxies, including the brightest cluster galaxies at or near the centres of galaxy clusters. We argue that most massive ellipticals are formed by anisotropic merging and that their fundamental plane projections should thus differ noticeably from those of lower mass ellipticals even though they should lie on the same fundamental plane. Current observations are consistent with this conclusion. The steepening in the L–σe relation for luminous ellipticals may also be reflected in a corresponding steepening in the MBH–σe relation for massive black holes.

Dissipationless mergers of elliptical galaxies and the evolution of the fundamental plane

We carry out numerical simulations of dissipationless major mergers of elliptical galaxies using initial galaxy models that consist of a dark matter halo and a stellar bulge with properties consistent with the observed fundamental plane. By varying the density profile of the dark matter halo (standard NFW versus adiabatically contracted NFW), the global stellar to dark matter mass ratio, and the orbit of the merging galaxies, we are able to assess the impact of each of these factors on the structure of the merger remnant. Our results indicate that the properties of the remnant bulge depend primarily on the angular momentum and energy of the orbit; for a cosmologically motivated orbit, the effective radius and velocity dispersion of the remnant bulge remain approximately on the fundamental plane. This indicates that the observed properties of elliptical galaxies are consistent with significant growth via late dissipationless mergers. We also find that the dark matter fraction within the effective radius of our remnants increases after the merger, consistent with the hypothesis that the tilt of the fundamental plane from the virial theorem is due to a varying dark matter fraction as a function of galaxy mass.

Simulations of Dust in Interacting Galaxies. I. Dust Attenuation

A new Monte Carlo radiative transfer code, SUNRISE, is used in conjunction with hydrodynamic simulations of major galaxy mergers to calculate the effects of dust in such systems. Dust has a profound effect on the emerging radiation, consistent with observations of dust absorption in starburst galaxies. The dust attenuation increases with luminosity such that at peak luminosities ~90% of the bolometric luminosity is absorbed by dust. We find that our predictions agree with observed relationships between the UV spectral slope and the fraction of light absorbed by dust (IRX-β) and observational estimates of the optical depth as a function of intrinsic B-band or UV luminosity. In general, the detailed appearance of the merging event depends on the stage of the merger and the geometry of the encounter. The fraction of bolometric energy absorbed by the dust, however, is a robust quantity that can be predicted from the intrinsic properties bolometric luminosity, baryonic mass, star formation rate, and metallicity of the simulated system. This paper presents fitting formulae, valid over a wide range of masses and metallicities, from which the absorbed fraction of luminosity (and consequently also the infrared dust luminosity) can be predicted. The attenuation of the luminosity at specific wavelengths can also be predicted, albeit with a larger scatter due to the variation with viewing angle. These formulae for dust attenuation are consistent with earlier studies and would be suitable for inclusion in theoretical models, e.g., semianalytic models, of galaxy formation and evolution.

Generating Hot Gas in Simulations of Disk-Galaxy Major Mergers

We report on the merger-induced generation of a shock-heated gas wind and the formation of a remnant gas halo in simulations of colliding disk galaxies. The simulations use cosmologically motivated initial conditions and include the effects of radiative cooling, star formation, stellar feedback, and the nonadiabatic heating of gas. The nonadiabatic heating, i.e., shocks generated in the final merger forces gas out of the central region of the merger remnant and into the dark matter halo. We demonstrate that the amount of heating depends on the size of the progenitor disk galaxy as well as the initial orbit on which the galaxies are placed. On the basis of these dependencies, we motivate a possible recipe for including this effect in semianalytic models of galaxy formation.

Major mergers of galaxy haloes: cuspy or cored inner density profile?

We present the results from a series of collisionless N-body simulations of major mergers of galaxy dark matter haloes with density profiles having either inner cusps or cores. Our simulations range from 2 × 10 5 to 10 7 particles, allowing us to probe the phase-space distribution of dark matter particles in the innermost regions (less than 0.005 virial radii) of cold dark matter haloes, a subject of much recent debate. We find that a major merger of two cored haloes yields a cored halo and does not result in a cuspy profile seen in many cosmological simulations. This result is unchanged if we consider mergers with parent mass ratios of 3 : 1 instead of 1 : 1. Mergers of a cuspy halo with either a cored halo or a second cuspy halo of equal mass, on the other hand, produce cuspy haloes with a slightly reduced inner logarithmic slope. Cuspy haloes, once formed, therefore appear resilient to major mergers. We find the velocity structure of the remnants to be mildly anisotropic, with a Maxwellian velocity distribution near the centre but not in the outer portions of the final haloes. Violent relaxation is effective only during the early phase of mergers, with phase mixing likely to be the dominant relaxation process at late times.

Elliptical Galaxies: Darkly Cloaked or Scantily Clad?

Planetary nebulae (PNe) may be the most promising tracers in the halos of early-type galaxies. We have used multi-object spectrographs on the WHT and the VLT, and the new Planetary Nebula Spectrograph on the WHT, to obtain hundreds of PN velocities in a small sample of nearby galaxies. These ellipticals show weak halo rotation, which may be consistent with ab initio models of galaxy formation, but not with more detailed major merger simulations. the galaxies near L* show evidence of a universal declining velocity dispersion profile, and dynamical models indicate the presence of little dark matter within 5 Reff—implying halos either not as massive or not as centrally concentrated as CDM predicts.