Hierarchical Merging Scenario Research Articles

Merging timescale for the supermassive black hole binary in interacting galaxy NGC 6240

Context. One of the mechanisms leading to the creation of a supermassive black hole (SMBH) is the so-called hierarchical merging scenario. Central SMBHs at the final phase of interacting and coalescing host-galaxies are observed as SMBH binary (SMBHB) candidates at different separations from hundreds of parsecs to megaparsecs. Aims. Today one of the strongest SMBHB candidates is the ultraluminous infrared galaxy NGC 6240 which was spatially and spectroscopically resolved in X-rays by Chandra. Dynamical calculation of central SMBHBs merging in a dense stellar environment allows us to retrace their evolution from kiloparsec to megaparsec scales. The main goal of our dynamical modeling was to reach the final, gravitational wave emission regime for the model BHs. Methods. We present direct N-body simulations with up to one million particles and relativistic post-Newtonian corrections for the SMBH particles up to 3.5 post-Newtonian terms. Results. Generally speaking, the set of initial physical conditions can strongly affect our merging time estimations. However, within a certain range of our parameters, we do not find any strong correlation between merging time and BH-to-BH mass or BH-to-bulge mass ratios. Varying the numerical parameters (like particle number – N) does not significantly change the merging time limits. From our 20 models, we find an upper limit on the merging time for central SMBHBs of less than ∼55 Myr. This precise number is only valid for our combination of initial mass ratios. Conclusions. Further detailed research of rare dual and multiple BHs in dense stellar environments (based on observational data) could clarify the dynamical co-evolution of central BHs and their host-galaxies.

SINGLE PARAMETER GALAXY CLASSIFICATION: THE PRINCIPAL CURVE THROUGH THE MULTI-DIMENSIONAL SPACE OF GALAXY PROPERTIES

We propose to describe the variety of galaxies from SDSS by using only one affine parameter. To this aim, we build the Principal Curve (P-curve) passing through the spine of the data point cloud, considering the eigenspace derived from Principal Component Analysis of morphological, physical and photometric galaxy properties. Thus, galaxies can be labeled, ranked and classified by a single arc length value of the curve, measured at the unique closest projection of the data points on the P-curve. We find that the P-curve has a "W" letter shape with 3 turning points, defining 4 branches that represent distinct galaxy populations. This behavior is controlled mainly by 2 properties, namely u-r and SFR. We further present the variations of several galaxy properties as a function of arc length. Luminosity functions variate from steep Schechter fits at low arc length, to double power law and ending in Log-normal fits at high arc length. Galaxy clustering shows increasing autocorrelation power at large scales as arc length increases. PCA analysis allowed to find peculiar galaxy populations located apart from the main cloud of data points, such as small red galaxies dominated by a disk, of relatively high stellar mass-to-light ratio and surface mass density. The P-curve allows not only dimensionality reduction, but also provides supporting evidence for relevant physical models and scenarios in extragalactic astronomy: 1) Evidence for the hierarchical merging scenario in the formation of a selected group of red massive galaxies. These galaxies present a log-normal r-band luminosity function, which might arise from multiplicative processes involved in this scenario. 2) Connection between the onset of AGN activity and star formation quenching, which appears in green galaxies when transitioning from blue to red populations. (Full abstract in downloadable version)

Formation rates of star clusters in the hierarchical merging scenario

Stars form with a complex and highly structured distribution. For a smooth star cluster to form from these initial conditions, the star cluster must erase this substructure. We study how substructure is removed using N-body simulations that realistically handle two-body relaxation. In contrast to previous studies, we find that hierarchical cluster formation occurs chiefly as a result of scattering of stars out of clumps, and not through clump merging. Two-body relaxation, in particular within the body of a clump, can significantly increase the rate at which substructure is erased beyond that of clump-merging alone. Hence the relaxation time of individual clumps is a key parameter controlling the rate at which smooth, spherical star clusters can form. The initial virial ratio of the clumps is an additional key parameter controlling the formation rate of a cluster. Reducing the initial virial ratio causes a star cluster to lose its substructure more rapidly.

SUPERDENSE MASSIVE GALAXIES IN THE NEARBY UNIVERSE

Superdense massive galaxies (re ∼ 1 kpc; M ∼ 1011 M☉) were common in the early universe (z ≳ 1.5). Within some hierarchical merging scenarios, a non-negligible fraction (1%–10%) of these galaxies is expected to survive since that epoch, retaining their compactness and presenting old stellar populations in the present universe. Using the NYU Value-Added Galaxy Catalog from the Sloan Digital Sky Survey Data Release 6, we find only a tiny fraction of galaxies (∼0.03%) with re ≲ 1.5 kpc and M⋆ ≳ 8 × 1010 M☉ in the local universe (z < 0.2). Surprisingly, they are relatively young (∼2 Gyr) and metal-rich ([Z/H] ∼0.2). The consequences of these findings within the current two competing size evolution scenarios for the most massive galaxies ("dry" mergers vs. "puffing up" due to quasar activity) are discussed.

On the chemical evolution of the Milky Way

AbstractI discuss three different topics concerning the chemical evolution of the Milky Way (MW). 1) The metallicity distribution of the MW halo; it is shown that this distribution can be analytically derived in the framework of the hierarchical merging scenario for galaxy formation, assuming that the component sub-haloes had chemical properties similar to those of the progenitors of satellite galaxies of the MW. 2) The age-metallicity relationship (AMR) in the solar neighborhood; I argue for caution in deriving from data with important uncertainties (such as the age uncertainties in the Geneva-Copenhagen Survey) a relationship between average metallicity and age: derived relationships are shown to be systematically flatter than the true ones and should not be directly compared to models. 3) The radial mixing of stars in the disk, which may have important effects on various observables (scatter in AMR, extension of the tails of the metallicity distribution, flatenning of disk abundance profiles). Recent SPH + N-body simulations find considerable radial mixing, but only comparison to observations will ultimately determine the extent of that mixing.

X-ray temperature spectroscopy of simulated cooling clusters

Results from a large sample of hydrodynamical/N-body simulations of galaxy clusters in a ΛCDM cosmology are used to simulate cluster X-ray observations. The physical modeling of the gas includes radiative cooling, star formation, energy feedback, and metal enrichment that follow from supernova explosions. Mock cluster samples are constructed grouping simulation data according to a number of constraints which would be satisfied by a data set of X-ray measurements of cluster temperatures as expected from Chandra observations. The X-ray spectra from simulated clusters are fitted into different energy bands using the XSPEC mekal model. The biasing of spectral temperatures with respect to mass-weighted temperatures is found to be influenced by two independent processes. The first scale dependency is absent in adiabatic runs and is due to cooling, whose efficiency to transform cold gas into stars is higher for cool clusters and this in turn implies a strong dependency of the spectral versus mass-weighted temperature relation on the cluster mass. The second dependency is due to photon emission because of cool gas which is accreted during merging events and biases the spectral fits. These events have been quantified according to the power ratio method and a robust correlation is found to exist between the spectral bias and the amount of cluster substructure. The shape of the simulated temperature profiles is not universal and it is steeper at the cluster center for cool clusters than for the massive ones. This follows owing to the scale dependency introduced by cooling which implies for cool clusters higher central temperatures, in scaled units, than for massive clusters. The profiles are in good agreement with data in the radial range between ∼0.1 r vir and ∼0.4 r vir; at small radii ( r ≲ 0.1 r vir) the cooling runs fail to reproduce the shape of the observed profiles. The fit is improved if one considers a hierarchical merging scenario in which cluster cores can accrete cooler gas through merging with cluster subclumps, though the shape of the temperature profiles is modified in a significant way only in the regime where the mass of the substructure is a large fraction of the cluster mass.

Photometric scaling relations for bulges of galaxies

We describe the photometric parameters of the bulges of galaxies of different Hubble types including ellipticals, lenticulars, early and late type spirals and early type dwarf galaxies. Analyzing the distributions of various photometric parameters and two- and three-dimensional correlations between them, we find that there is a difference in the correlations exhibited by bright ($M_{\rm K} < -22$) and faint bulges, irrespective of their Hubble type. Importantly, the bright bulges, which include typically E/S0 galaxies and bulges of early type spirals, are tightly distributed around a common photometric plane (PP), while their fainter counterparts, mainly bulges of late type spirals and dwarf galaxies show significant deviation from the planar distribution. We show that the specific entropy, determined from the bulge structural parameters, systematically increases as we move from late to early Hubble types. We interpret this as evidence for hierarchical merging and passive evolution scenarios for bright and faint bulges respectively.

Galics- VI. Modelling hierarchical galaxy formation in clusters

High-resolution N-body re-simulations of 15 massive (10 14 10 15 M⊙) dark matter haloes have been combined with the hybrid galaxy formation model GalICS (Hatton et al. 2003), to study the formation and evolution of galaxies in clusters, within the framework of the hierarchical merging scenario. This paper describes the high-resolution resimulation technique used to build the dark matter halo sample, and discusses its reliability. New features incorporated in GalICS include a better description of galaxy positioning after dark matter halo merger events, a more reliable computation of the temperature of the inter-galactic medium as a function of redshift, that also takes into account the re ionisation history of the Universe, and a semi-analytic description of the ram pressure strippi ng of cold gas from galactic discs, suffered by galaxies during their motion through the diffuse hot intra-cluster medium. Within the multitude of available model results, we choose to focus here on the luminosity functions, morphological fractions and colour distributions of galax ies in clusters and in cluster outskirts, at z = 0. No systematic dependency on cluster richness is found either for the galaxy luminosity functions, morphological mixes, or colour distributions. Moving from higher density (cluster cores), to lower density environments (clust er outskirts), we detect a progressive flattening of the luminosity functions, an increase of the fr action of spirals and a decrease of that of ellipticals and S0s, and the progressive emergence of a bluer tail in the distributions of galaxy colours, especially for spirals. As compared to cluster spirals, early-type galaxies show a flatter luminosity function, and more homogeneous and redder colours. An overall good agreement is found between our results and the observations, particularly in terms of the cluster luminosity functions and morphological mixes. However, some discrepancies are also apparent, with too faint magnitudes of the brightest cl uster members, especially in the B band, and galaxy colours tendentially too red (or not blue enough) in the model, with respect to the observations. Finally, ram pressure stripping appea rs to affect very little our results.

Evolution of UV–NIR structural properties of cluster galaxies

We study the structure and the internal colour gradients of cluster galaxies from UV to NIR restframe, in the redshift range $z=0.21{-}0.64$. Structural parameters (half light radius ${r_{\rm e}}$, mean surface brightness $ _{\rm e}$ and Sersic index ${n}$) are derived for galaxies in the clusters ${\rm A\,209}$ at ${z = 0.21}$ and ${\rm EIS\,0048}$ at ${z = 0.64}$. This data set, together with previous data for the cluster ${\rm AC\,118}$ at ${z=0.31}$, constitutes the first large (${N \sim 270}$) sample of cluster galaxies whose internal structure in UV, optical (OPT) and NIR ( U -, V - and H -band restframe) can be investigated up to a look-back time of ${\sim }6~\rm Gyr$ ($\Omega_m=0.3$, $\Omega_\Lambda=0.7$ and ${ H_0= 70~\rm km~s^{-1}~ Mpc^{-1} }$). Galaxies are classified as spheroids or disks according to the shape of the light profile, and the evolution of the two populations are investigated separately. On average, both spheroids and disks are more concentrated at longer wavelengths: the galaxy sizes become smaller from UV to NIR, while Sersic indices increase. This trend shows an evolution in disks, where the mean ratio of optical to NIR Sersic indices decreases from ${z=0.31}$ to ${z=0.64}$. Colour gradients are on average negative at all redshifts and are stronger in disks than in spheroids. But while for spheroids both ${\rm grad\,(UV{-}OPT)}$ and ${\rm grad\,(OPT{-}NIR)}$ are only weakly dependent on redshift, the optical-NIR gradients of disks become significantly smaller at ${z=0.64}$. Colour gradients and central colours are compared with models of metallicity, age, and dust extinction gradients. Metallicity turns out to be the primary driver of colour gradients in spheroids, the age gradient being constrained to be smaller than ${\sim} 25\%$. For disks, two kinds of models fit the present data: (i) age gradients (in the range [30, 50]%) with significant dust extinction, and (ii) “pure” dust models, in which the gradients of colour excess are a factor of two higher in ${\rm EIS\,0048}$ than in the other clusters. Since colour gradients of disks seem not to correlate significantly with inclination, we argue that age gradient models could represent a more likely explanation of the present data, in agreement with what expected on the basis of hierarchical merging scenarios.

Scaling Relations of Field Spirals at intermediate Redshift

In the last few years, galaxies at redshifts up to z~1 have become accessible for medium-resolved spectroscopy thanks to the new generation of 10m-class telescopes. With kinematic and photometric information on spiral galaxies in this regime, well-known scaling relations like the Tully-Fisher relation (TFR) can be studied over half a Hubble time. By comparison to local samples, these studies facilitate simultaneous tests of the hierarchical merging scenario and stellar population models. Using the Very Large Telescope, we obtained spatially resolved rotation curves of 78 spiral galaxies in the FORS Deep Field (FDF), covering all Hubble types from Sa to Sm/Irr at redshifts 0.1<z<1.0. We find evidence for a B-band luminosity increase of up to 2mag for low-mass spirals, whereas the most massive galaxies are of the same luminosity as their local counterparts. In effect, the TFR slope decreases significantly. This would explain the discrepant results of previous observational studies. We also present the velocity-size relation and compare it to the predictions of numerical simulations based on the hierarchical merging scenario.

Magnetization of the Intergalactic Medium by Primeval Galaxies

We review what has recently been learned from starburst-driven superwinds, galaxy-scale outflows, dwarf galaxy surveys, and evidence for early-universe merging scenarios. We apply this knowledge to simplified model populations of dwarf galaxies in the primeval galaxy era to demonstrate how a substantial fraction of the intergalactic medium can be permeated with magnetic fields by dwarf galaxy outflow winds in the volcanic early Mechanisms of both magnetic field regeneration and diffusion are discussed as they apply to galaxies at both primeval and later epochs of the universe. We propose that, in the past, magnetic field strengths in outflow superbubbles and halos were stronger than those now prevailing in the disks of current-epoch galaxies such as the Milky Way. We also show how subsequent acausal diffusion of magnetic fields (which we have not modeled) can propagate the fields over great distances in the universe within a Hubble time. We conclude that dwarf galaxies can effectively seed the intergalactic medium with magnetic fields for a wide range of outflow and galaxy density parameters. If the first galaxies form at or before z~10 in a bottom-up hierarchical merging scenario, a substantial fraction of the intergalactic medium will become permeated with a magnetic field at the present epoch. This intergalactic field seeding is largely accomplished by z~6.