Simulations Of Galaxy Mergers Research Articles

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.

Truncated Star Formation in Compact Groups of Galaxies: A Stellar Population Study

We present the results of a study comparing the stellar populations in the elliptical galaxies of Hickson compact groups (HCGs) with those in low-density environments. We analyze a sample of intermediate-resolution (4.25 Å FWHM) spectra of 22 galaxies in HCGs and 12 in the field or in loose groups. Three different population-synthesis models and stellar population analyses are used to make the results more robust. Great care has been taken to correct for the emission contamination of the Balmer lines, used to determine the mean luminosity-weighted ages of the galaxies. We find that, on average, galaxies in HCGs are systematically older and more metal-poor than in the field, in agreement with previous works. The most interesting finding is that the low-σ galaxies in HCGs show an enhanced [Mg/Fe] ratio and depleted metallicity [Z/H] with respect to their counterparts in the field. This anomalous behavior is interpreted as evidence for the action of a mechanism that truncates the star formation (SF). Hydrodynamic simulations of galaxy mergers (Di Matteo et al. 2005) support this interpretation by predicting the quenching of SF soon after the merger event. Combining this scenario and the evidence presented here, the HCGs, generally considered to be ideal environments for galaxy-galaxy interactions, become ideal places for SF truncation.

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 co-evolution of the obscured quasar PKS 1549−79 and its host galaxy: evidence for a high accretion rate and warm outflow

We use deep optical, infrared and radio observations to explore the symbiosis between nuclear activity and galaxy evolution in the southern compact radio source PKS 1549−79 (z= 0.1523). The optical imaging observations reveal the presence of tidal tail features which provide strong evidence that the host galaxy has undergone a major merger in the recent past. The merger hypothesis is further supported by the detection of a young stellar population (YSP), which, on the basis of spectral synthesis modelling of our deep Very Large Telescope (VLT) optical spectra, was formed 50–250 Myr ago and makes up a significant fraction of the total stellar mass (1–30 per cent). Despite the core-jet structure of the radio source, which is consistent with the idea that the jet is pointing close to our line of sight, our H I 21-cm observations reveal significant H I absorption associated with both the core and the jet. Moreover, the luminous, quasar-like active galactic nucleus (AGN) (MV 6.4) at optical wavelengths and show many properties in common with narrow-line Seyfert 1 galaxies (NLS1), including relatively narrow permitted lines [full width at half-maximum (FWHM) ∼ 1940 km s−1], highly blueshifted [O III]λλ5007,4959 lines (ΔV∼ 680 km s−1) and evidence that the putative supermassive black hole is accreting at a high Eddington ratio (0.3 < Lbol/Ledd < 11). The results suggest that accretion at high Eddington ratio does not prevent the formation of powerful relativistic jets. Together, the observations lend strong support to the predictions of some recent numerical simulations of galaxy mergers in which the black hole grows rapidly through merger-induced accretion following the coalescence of the nuclei of two merging galaxies, and the major growth phase is largely hidden at optical wavelengths by the natal gas and dust. Although the models also predict that AGN-driven outflows will eventually remove the gas from the bulge of the host galaxy, the visible warm outflow in PKS 1549−79 is not currently capable of doing so. However, much of the outflow may be hidden by the material obscuring the quasar and/or tied up in hotter or cooler phases of the interstellar medium. By combining our estimates of the reddening of the quasar with the H I column derived from the 21-cm radio observations, we have also made the first direct estimate of the H I spin temperature in the vicinity of a luminous AGN: Tspin > 3000 K.

A Merger‐driven Scenario for Cosmological Disk Galaxy Formation

The hierarchical nature of the ?CDM cosmology poses serious difficulties for the formation of disk galaxies. To help resolve these issues, we describe a new, merger-driven scenario for the cosmological formation of disk galaxies at high redshifts that supplements the standard dissipational collapse model. In this picture, large gaseous disks may be produced from high angular momentum mergers of systems that are gas dominated, i.e., Mgas/(Mgas + M) 0.5 at the height of the merger. Pressurization from the multiphase ISM prevents the complete conversion of gas into stars during the merger, and if enough gas remains to form a disk, the remnant eventually resembles a disk galaxy. We perform numerical simulations of galaxy mergers to study how supernovae feedback strength, black hole feedback, progenitor gas fraction, merger mass ratio, and orbital geometry impact the formation of remnant disks. We find that disks can build angular momentum through mergers and the degree of rotational support of the baryons in the remnant is primarily related to feedback processes associated with star formation. Disk-dominated remnants are restricted to form in mergers that are gas dominated at the time of final coalescence. We also show that the formation of rotationally supported stellar systems in mergers is not restricted to idealized orbits, and both gas-rich major and minor mergers can produce disk-dominated stellar remnants. We suggest that the hierarchical nature of the ?CDM cosmology and the physics of the ISM may act together to form spiral galaxies by building the angular momentum of disks through gas-dominated mergers at high redshifts.

Molecular Outflows in Galaxy Merger Simulations with Embedded Active Galactic Nuclei

We study the effects of feedback from active galactic nuclei (AGN) on emission from molecular gas in galaxy mergers by combining hydrodynamic simulations which include black holes with a three-dimensional, non-local thermodynamic equilibrium (LTE) radiative transfer code. We find that molecular clouds entrained in AGN winds produce an extended CO morphology with significant off-nuclear emission, which may be detectable via contour mapping. Furthermore, kinematic signatures of these molecular outflows are visible in emission line profiles when the outflow has a large line of sight velocity. Our results can help interpret current and upcoming observations of luminous infrared galaxies, as well as provide a detailed test of subresolution prescriptions for supermassive black hole growth in galaxy-scale hydrodynamic simulations.

The Evolution of theMBH‐σ Relation

(Abridged) We examine the evolution of the black hole mass - stellar velocity dispersion (M-sigma) relation over cosmic time using simulations of galaxy mergers that include feedback from supermassive black hole growth. We consider mergers of galaxies varying the properties of the progenitors to match those expected at redshifts z=0-6. We find that the slope of the resulting M-sigma relation is the same at all redshifts considered. For the same feedback efficiency that reproduces the observed amplitude of the M-sigma relation at z=0, there is a weak redshift-dependence to the normalization that results from an increasing velocity dispersion for a given galactic stellar mass. We develop a formalism to connect redshift evolution in the M-sigma relation to the scatter in the local relation at z=0. We show that the scatter in the local relation places severe constraints on the redshift evolution of both the normalization and slope of the M-sigma relation. Furthermore, we demonstrate that cosmic downsizing introduces a black hole mass-dependent dispersion in the M-sigma relation and that the skewness of the distribution about the locally observed M-sigma relation is sensitive to redshift evolution in the normalization and slope. In principle, these various diagnostics provide a method for differentiating between theories for producing the M-sigma relation. In agreement with existing constraints, our simulations imply that hierarchical structure formation should produce the relation with small intrinsic scatter.

The Luminosity Dependence of Quasar Clustering

We investigate the luminosity dependence of quasar clustering, inspired by numerical simulations of galaxy mergers that incorporate black hole growth. These simulations have motivated a new interpretation of the quasar luminosity function. In this picture, the bright end of the quasar luminosity function consists of quasars radiating nearly at their peak luminosities, while the faint end consists mainly of very similar sources, but at dimmer phases in their evolution. We combine this model with the statistics of dark matter halos that host quasar activity. We find that, since bright and faint quasars are mostly similar sources seen in different evolutionary stages, a broad range in quasar luminosities corresponds to only a narrow range in the masses of quasar host halos. On average, bright and faint quasars reside in similar host halos. Consequently, we argue that quasar clustering should depend only weakly on luminosity. This prediction is in qualitative agreement with recent measurements of the luminosity dependence of the quasar correlation function (Croom et al.) and the galaxy-quasar cross-correlation function (Adelberger & Steidel). Future precision clustering measurements from the Sloan Digital Sky Survey (SDSS) and the Two Degree Field Survey (2dF), spanning a large range in luminosity, should provide a strong test of our model.

The Evolution in the Faint‐End Slope of the Quasar Luminosity Function

Using a new model for quasar lifetimes and light curves derived from numerical simulations of galaxy mergers that incorporate black hole growth, we study the faint-end slope of the quasar luminosity function (QLF) and its evolution with redshift. Our model motivates a new interpretation of the QLF in which the bright end consists of quasars radiating near their peak luminosities but the faint end is mostly made up of brighter peak luminosity quasars seen in less luminous phases of evolution. The faint-end slope of the QLF is then set by the behavior of the lifetime (light curve) of quasars with peak luminosities near the observed break when they are in less luminous stages of evolution. We determine the faint-end slope of the QLF from the quasar lifetime, based on a set of simulations that encompass a wide range of host galaxy, merger, black hole, and interstellar gas properties. Brighter peak luminosity (higher black hole mass) systems undergo more violent evolution, and gas is expelled and heated more rapidly in the final stages of quasar evolution, resulting in a flatter faint-end slope (as these objects fall below the observed break in the QLF more rapidly). Therefore, as the QLF break luminosity moves to higher luminosities with increasing redshift, implying a larger typical quasar peak luminosity, the faint-end QLF slope flattens. From our model, we predict the evolution of the faint-end slope of the QLF and find good agreement with observations. Although black holes grow in an antihierarchical manner (with lower mass black holes formed primarily at lower redshifts), in our picture the observed change in slope and differential or luminosity-dependent density evolution in the QLF is determined by the nontrivial, luminosity-dependent quasar lifetime and physics of quasar feedback, and not by changes in the shape of the underlying peak luminosity or active black hole mass distributions.

A Unified, Merger‐driven Model of the Origin of Starbursts, Quasars, the Cosmic X‐Ray Background, Supermassive Black Holes, and Galaxy Spheroids

We present an evolutionary model for starbursts, quasars, and spheroidal galaxies in which mergers between gas-rich galaxies drive nuclear inflows of gas, producing intense starbursts and feeding the buried growth of supermassive black holes (BHs) until feedback expels gas and renders a briefly visible optical quasar. The quasar lifetime and obscuring column density depend on both the instantaneous and peak luminosity of the quasar, and we determine this dependence using a large set of simulations of galaxy mergers varying host galaxy properties, orbital geometry, and gas physics. We use these fits to deconvolve observed quasar luminosity functions (LFs) and obtain the evolution of the formation rate of quasars with a certain peak luminosity, n(L_peak,z). Quasars spend extended periods of time at luminosities well below peak, and so n(L_peak) has a maximum corresponding to the 'break' in the observed LF, falling off at both brighter and fainter luminosities. From n(L_peak) and our simulation results, we obtain self-consistent fits to hard and soft X-ray and optical quasar LFs and predict many observables, including: column density distributions of optical and X-ray samples, the LF of broad-line quasars in X-ray samples and the broad-line fraction as a function of luminosity, active BH mass functions, the distribution of Eddington ratios at z~0-2, the z=0 mass function of relic BHs and total mass density of BHs, and the cosmic X-ray background. In every case, our predictions agree well with observed estimates, and unlike previous modeling attempts, we are able to reproduce them without invoking any ad hoc assumptions about source properties or distributions. We provide a library of Monte Carlo realizations of our models for comparison with observations. (Abridged)

Shapes of Luminous and Dark Matter in Hydrodynamic Simulations of Galaxy Mergers

Shapes of Luminous and Dark Matter in Hydrodynamic Simulations of Galaxy Mergers

Orbital parameters of merging dark matter halos

In order to specify cosmologically motivated initial conditions for major galaxy mergers (mass ratios $\leq$ 4:1) that are supposed to explain the formation of elliptical galaxies we study the orbital parameters of major mergers of cold dark matter halos using a high-resolution cosmological simulation. Almost half of all encounters are nearly parabolic with eccentricities $e \approx 1$ and no correlations between the halo spin planes or the orbital planes. The pericentric argument $\omega$ shows no correlation with the other orbital parameters and is distributed randomly. In addition we find that 50 % of typical pericenter distances are larger than half the halo's virial radii which is much larger than typically assumed in numerical simulations of galaxy mergers. In contrast to the usual assumption made in semi-analytic models of galaxy formation the circularities of major mergers are found to be not randomly distributed but to peak around a value of $\epsilon \approx 0.5$. Additionally all results are independent of the minimum progenitor mass and major merger definitions (i.e. mass ratios $\leq$ 4:1; 3:1; 2:1).

Luminosity‐dependent Quasar Lifetimes: Reconciling the Optical and X‐Ray Quasar Luminosity Functions

We consider implications of our new model of quasar lifetimes and light curves for the quasar luminosity function (LF) at different frequencies and redshifts. In our picture, quasars evolve rapidly and the lifetime depends on both their instantaneous and peak luminosities. The bright end of the LF traces the peak intrinsic quasar activity, but the faint end consists of quasars that are either undergoing exponential growth to much larger masses and luminosities or are in sub-Eddington quiescent states going into or coming out of a period of peak activity. The break in the observed LF corresponds directly to the maximum in the intrinsic distribution of peak luminosities, which falls off at both brighter and fainter luminosities. We study this model using simulations of galaxy mergers that successfully reproduce a wide range of observed quasar phenomena, including the observed column density distribution. By combining quasar lifetimes and the distribution of maximum quasar luminosities determined from the observed hard X-ray LF with the corresponding luminosity and host system-dependent column densities, we produce the expected soft X-ray and B-band LFs. Our predictions agree exceptionally well with the observed LFs at all observed luminosities over the redshift range considered (z ? 1) without invoking any ad hoc assumptions about an obscured population of sources. Our results also suggest that observed correlations in hard X-ray samples between the obscured fraction of quasars and luminosity can be explained in the context of our model by the expulsion of surrounding gas due to heating from accretion feedback energy as a quasar nears its peak luminosity and final black hole mass.

Luminosity‐dependent Quasar Lifetimes: A New Interpretation of the Quasar Luminosity Function

We propose a new interpretation of the quasar luminosity function (LF) derived from physically motivated models of quasar lifetimes and light curves. In our picture, quasars evolve rapidly, and their lifetime depends on both their instantaneous and peak luminosities. We study this model using simulations of galaxy mergers that successfully reproduce a wide range of observed quasar phenomena. With lifetimes inferred from the simulations, we deconvolve the observed quasar LF from the distribution of peak luminosities and show that they differ qualitatively, unlike the simple models of quasar lifetimes used previously. We find that the bright end of the LF traces the intrinsic peak quasar activity but that the faint end consists of quasars that are either undergoing exponential growth to much larger masses and higher luminosities, or are in sub-Eddington quiescent states going into or coming out of a period of peak activity. The in the LF directly corresponds to the maximum in the intrinsic distribution of peak luminosities, which falls off at both brighter and fainter luminosities. Our interpretation of the quasar LF provides a physical basis for the nature and slope of the faint-end distribution, as well as the location of the break luminosity.

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.

L.O.S.V.D.'s of elliptical galaxies from mergers of disk galaxies

Modern kinematic diagnostics from high-S/N spectroscopy provide useful tests on the collisionless merger hypothesis for the formation of elliptical galaxies. Bender, Saglia &amp; Gerhard (1994) show that the skewness of the line-of-sight velocity distributions (LOSVD) has opposite sign to the rotation velocity and follows a distinct pattern. Previous N-body experiments (Bendo &amp; Barnes 2000, Naab &amp; Burkert 2001) disagree as to whether the relation is reproduced by the models.We present first results from an ongoing effort to explain the skewness properties of the LOSVD's of ellipticals. We use large N-body simulations of disk-disk galaxy mergers (. So far, it is unclear whether the disagreement is related to the lack of rotation in the bulge components of our initial model galaxies, or to a more general shortcoming of dissipationless mergers to reproduce the properties of real ellipticals.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

A Galaxy Merging Sequence Traced by X-rays

We are studying a sample of nearly 20 nearby (cz≲13,000 km s−1) IR-luminous interacting/merging galaxies observed with the ACIS in the Chandra archive (e.g., Gao et al. 2003; Ptak et al. 2003). Most galaxies in the sample are luminous infrared galaxies (LIGs) and nearly half of them are ultraluminous infrared galaxies (ULIGs) with bolometric luminosities comparable to QSOs. Based on multiwavelength data and numerical simulations of gas-rich galaxy mergers, we have attempted to arrange them in a merger sequence perspective and compared their broadband X-ray emission here with the optical images (mostly DSS).

Potential Formation Sites of Super–Star Clusters in Ultraluminous Infrared Galaxies

Recent observational results on high spatial resolution images of ultraluminous infrared galaxies (ULIGs) have revealed very luminous, young, compact, and heavily obscured super-star clusters in their central regions, suggested to be formed by gas-rich major mergers. By using stellar and gaseous numerical simulations of galaxy mergers, we first demonstrate that the central regions of ULIGs are the most promising formation sites of super-star clusters owing to the rather high gaseous pressure of the interstellar medium. Based on simple analytical arguments, we then discuss the possibility that super-star clusters in a ULIG can be efficiently transferred into the nuclear region owing to dynamical friction and consequently merge with one another to form a single compact stellar nucleus with a seed massive black hole. We thus suggest that multiple merging between super-star clusters formed by nuclear starbursts in the central regions of ULIGs can result in the formation of massive black holes.

The Fundamental Plane and Merger Scenario. I. Star Formation History of Galaxy Mergers and Origin of the Fundamental Plane

We perform numerical simulations of galaxy mergers between star-forming and gas-rich spirals in order to explore the origin of the fundamental plane (FP) of elliptical galaxies. We consider, in particular, that the origin of the slope of the FP is essentially due to the nonhomology in the structure and kinematics of elliptical galaxies and, accordingly, investigate structural and kinematic properties of elliptical galaxies formed by dissipative galaxy merging with star formation. We found that the rapidity of star formation, which is defined as the ratio of dynamical timescale of a merger progenitor to the timescale of gas consumption by star formation, is a key determinant for nonhomology parameters, such as the density profile of the stellar component, the relative importance of global rotation in kinematics, and the ratio of total dynamical mass to luminous mass, in merger remnants. We furthermore found that this result does not depend so strongly on initial intrinsic spins of progenitor disks and orbital energy and angular momentum of mergers. These results strongly suggest that the structural and kinematic nonhomology observed in elliptical galaxies can be closely associated with the difference in star formation history between elliptical galaxies formed by dissipative galaxy merging. Based on these results, we discuss a close physical relation between the origin of the FP and the star formation history of elliptical galaxies.

Ellipticals with Kinematically Distinct Cores:V−IColor Images with WPFC2

We have analyzed Hubble Space Telescope WFPC2 F555W and F814W (i.e., V and I) images for 15 elliptical galaxies with kinematically distinct cores. For each of them? we have derived surface brightness and isophotal parameter profiles in the two bands, color maps, and radial profiles in V-I. Most galaxies show patchy dust absorption close to their nuclei, However, there are generally no indications of homogeneous, diffuse dust components close to the nuclei. The nuclear colors in the unobscured regions are most likely representative of the central stellar populations. We have detected photometric evidence for faint stellar disks, on scales of a few tens to a few arcseconds, in seven galaxies, namely NGC 1427, 1439, 1700, 4365, 4406, 4494, and 5322, In NGC 1700, the isophotes are slightly boxy at the scale of the counterrotating component and disky at larger radii. We find no difference in V-I color greater than 0.02 mag between these disks and the surrounding galactic regions. Hence, the stellar populations in the kinematically distinct cores are not strongly deviant from the population of the main body. Specifically, there is no evidence for a dominating population of blue, very metal weak stars as predicted by some of the formation scenarios. This argues against models in which small galaxies fall in and survive in the nuclei, unless supermassive black holes are present. These would likely disrupt the accreted small systems. For one galaxy, NGC 4365, the innermost region is bluer than the surrounding regions. This area extends to similar to 15 pc and contains a luminosity of similar to 2.5 x 10(6) L.. If interpreted as a stellar population effect, an age difference of similar to 3-4 Gyr, or an [Fe/H] variation of about 0.2 dex? is derived. The nuclear intensity profiles show a large variety: some galaxies have steep cusp profiles, while others have shallow cusps and a ''break radius.'' The nuclear cusps of galaxies with kinematically distinct cores follow the same trends as the nuclei of normal galaxies. We have not been able to identify a unique, qualifying feature in the WFPC2 images that distinguishes the galaxies with kinematically distinct cores from the kinematically normal cares. It is possible that statistical differences exist: possibly, the kinematically distinct cores have a higher fraction of nuclear disks. The similarity of both types of cores puts strong constraints on the formation scenarios. Simulations of galaxy mergers, with the inclusion of star formation and nuclear black holes, are needed to resolve the question of how these structures may have formed. Spectra with high spatial resolution are needed to study the nuclear structure of the distinct component in detail.