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1.
We use large volume, high resolution, N -body simulations of three different ΛCDM models, with different clustering strengths, to generate dark-matter halo merging histories. Over the reliable range of halo masses, roughly galaxy groups to rich clusters of galaxies, we quantify the number density of major mergers for two different time intervals and compare them with analytic predictions based on the extended Press–Schechter theory.  相似文献   

2.
Using high-resolution SPH simulations in a fully cosmological Λ cold dark matter context, we study the formation of a bright disc-dominated galaxy that originates from a 'wet' major merger at   z = 0.8  . The progenitors of the disc galaxy are themselves disc galaxies that formed from early major mergers between galaxies with blue colours. A substantial thin stellar disc grows rapidly following the last major merger and the present-day properties of the final remnant are typical of early-type spiral galaxies, with an i -band bulge-to-disc ratio ∼0.65, a disc scalelength of 7.2 kpc,   g − r = 0.5 mag  , an H  i linewidth ( W 20/2) of 238 km s−1 and total magnitude   i =−22.4  . The key ingredients for the formation of a dominant stellar disc component after a major merger are (i) substantial and rapid accretion of gas through cold flows followed at late times by cooling of gas from the hot phase, (ii) supernova feedback that is able to partially suppress star formation during mergers and (iii) relative fading of the spheroidal component. The gas fraction of the progenitors' discs does not exceed 25 per cent at   z < 3  , emphasizing that the continuous supply of gas from the local environment plays a major role in the regrowth of discs and in keeping the galaxies blue. The results of this simulation alleviate the problem posed for the existence of disc galaxies by the high likelihood of interactions and mergers for galaxy-sized haloes at relatively low z .  相似文献   

3.
Galaxy clusters grow by mergers with other clusters and galaxy groups. These mergers create shocks within the intracluster medium (ICM). It is proposed that particles can be accelerated to extreme energies within the shocks. In the presence of a magnetic field these particles should then form large regions emitting synchrotron radiation, creating the so-called radio relics. An example of a cluster with relics is CIZA J2242.8+5301. Here we present hydrodynamical simulations of idealized binary cluster collisions with the aim of constraining the merger scenario for this cluster. We conclude that by using the location, size and width of double radio relics we can set constraints on the mass ratios, impact parameters, time scales, and viewing geometries of binary cluster merger events.  相似文献   

4.
The physical processes that define the spine of the galaxy cluster X-ray luminosity–temperature ( L – T ) relation are investigated using a large hydrodynamical simulation of the universe. This simulation models the same volume and phases as the millennium simulation and has a linear extent of  500  h −1 Mpc  . We demonstrate that mergers typically boost a cluster along but also slightly below the L – T relation. Due to this boost, we expect that all of the very brightest clusters will be near the peak of a merger. Objects from near the top of the L – T relation tend to have assembled much of their mass earlier than an average halo of similar final mass. Conversely, objects from the bottom of the relation are often experiencing an ongoing or recent merger.  相似文献   

5.
We study the formation of galaxies in a Λ cold dark matter (ΛCDM) universe using high-resolution hydrodynamical simulations with a multiphase treatment of gas, cooling and feedback, focusing on the formation of discs. Our simulations follow eight isolated haloes similar in mass to the Milky Way and extracted from a large cosmological simulation without restriction on spin parameter or merger history. This allows us to investigate how the final properties of the simulated galaxies correlate with the formation histories of their haloes. We find that, at   z = 0  , none of our galaxies contains a disc with more than 20 per cent of its total stellar mass. Four of the eight galaxies nevertheless have well-formed disc components, three have dominant spheroids and very small discs, and one is a spheroidal galaxy with no disc at all. The   z = 0  spheroids are made of old stars, while discs are younger and formed from the inside-out. Neither the existence of a disc at   z = 0  nor the final disc-to-total mass ratio seems to depend on the spin parameter of the halo. Discs are formed in haloes with spin parameters as low as 0.01 and as high as 0.05; galaxies with little or no disc component span the same range in spin parameter. Except for one of the simulated galaxies, all have significant discs at   z ≳ 2  , regardless of their   z = 0  morphologies. Major mergers and instabilities which arise when accreting cold gas is misaligned with the stellar disc trigger a transfer of mass from the discs to the spheroids. In some cases, discs are destroyed, while in others, they survive or reform. This suggests that the survival probability of discs depends on the particular formation history of each galaxy. A realistic ΛCDM model will clearly require weaker star formation at high redshift and later disc assembly than occurs in our models.  相似文献   

6.
Hubble Space Telescope images of a sample of 285 galaxies with measured redshifts from the Canada–France Redshift Survey (CFRS) and Autofib–Low Dispersion Spectrograph Survey (LDSS) redshift surveys are analysed to derive the evolution of the merger fraction out to redshifts z ∼1. We have performed visual and machine-based merger identifications, as well as counts of bright pairs of galaxies with magnitude differences δm ≤1.5 mag. We find that the pair fraction increases with redshift, with up to ∼20 per cent of the galaxies being in physical pairs at z ∼0.75–1. We derive a merger fraction varying with redshift as ∝(1+ z )3.2±0.6, after correction for line-of-sight contamination, in excellent agreement with the merger fraction derived from the visual classification of mergers for which m =3.4±0.6. After correcting for seeing effects on the ground-based selection of survey galaxies, we conclude that the pair fraction evolves as ∝(1+ z )2.7±0.6. This implies that an average L * galaxy will have undergone 0.8–1.8 merger events from z =1 to z =0, with 0.5 to 1.2 merger events occuring in a 2-Gyr time-span at around z ∼0.9. This result is consistent with predictions from semi-analytical models of galaxy formation. From the simple coaddition of the observed luminosities of the galaxies in pairs, physical mergers are computed to lead to a brightening of 0.5 mag for each pair on average, and a boost in star formation rate of a factor of 2, as derived from the average [O  ii ] equivalent widths. Mergers of galaxies are therefore contributing significantly to the evolution of both the luminosity function and luminosity density of the Universe out to z ∼1.  相似文献   

7.
We study close encounters involving massive main-sequence stars and the evolution of the exotic products of these encounters as common-envelope systems or possible hypernova progenitors. We show that parabolic encounters between low- and high-mass stars and between two high-mass stars with small periastrons result in mergers on time-scales of a few tens of stellar free-fall times (a few tens of hours). We show that such mergers of unevolved low-mass stars with evolved high-mass stars result in little mass-loss  (∼0.01 M⊙)  and can deliver sufficient fresh hydrogen to the core of the collision product to allow the collision product to burn for several million years. We find that grazing encounters enter a common-envelope phase which may expel the envelope of the merger product. The deposition of energy in the envelopes of our merger products causes them to swell by factors of ∼100. If these remnants exist in very densely populated environments  ( n ≳ 107 pc−3)  , they will suffer further collisions which may drive off their envelopes, leaving behind hard binaries. We show that the products of collisions have cores rotating sufficiently rapidly to make them candidate hypernova/gamma-ray burst progenitors and that ∼0.1 per cent of massive stars may suffer collisions, sufficient for such events to contribute significantly to the observed rates of hypernovae and gamma-ray bursts.  相似文献   

8.
Galaxies are believed to be in one-to-one correspondence with simulated dark matter subhaloes. We use high-resolution N -body simulations of cosmological volumes to calculate the statistical properties of subhalo (galaxy) major mergers at high redshift ( z = 0.6–5). We measure the evolution of the galaxy merger rate, finding that it is much shallower than the merger rate of dark matter host haloes at   z > 2.5  , but roughly parallels that of haloes at   z < 1.6  . We also track the detailed merger histories of individual galaxies and measure the likelihood of multiple mergers per halo or subhalo. We examine satellite merger statistics in detail: 15–35 per cent of all recently merged galaxies are satellites, and satellites are twice as likely as centrals to have had a recent major merger. Finally, we show how the differing evolution of the merger rates of haloes and galaxies leads to the evolution of the average satellite occupation per halo, noting that for a fixed halo mass, the satellite halo occupation peaks at   z ∼ 2.5  .  相似文献   

9.
The discovery of protoglobular cluster candidates in many present-day mergers allows us to understand better the possible effects of a merger event on the globular cluster system of a galaxy, and to foresee the properties of the end-product. By comparing these expectations with the properties of globular cluster systems of elliptical galaxies at the present time we can constrain merger models. The observational data indicate that (i) every gaseous merger induces the formation of new star clusters, and (ii) the number of new clusters formed in such a merger increases with the gas content of the progenitor galaxies. Low-luminosity (about M V  > −21), discy ellipticals are generally thought to be the result of a gaseous merger. As such, new globular clusters are expected to form but have not been detected to date. We investigate various reasons for the non-detection of subpopulations in low-luminosity ellipticals, i.e. absence of an old population, absence of a new population, destruction of one of the populations and, finally, an age–metallicity conspiracy that allows old and new globular clusters to appear indistinguishable at the present epoch. All of these possibilities lead us to a similar conclusion, namely that low-luminosity ellipticals did not form recently ( z  < 1) in a gas-rich merger, and might not have formed in a major merger of stellar systems at all. High-luminosity ellipticals do reveal globular cluster subpopulations. However, it is difficult to account for the two populations in terms of mergers alone and, in particular, we can rule out scenarios in which the second subpopulation is the product of a recent, gas-poor merger.  相似文献   

10.
Giant radio halos in galaxy clusters probe mechanisms of particle acceleration connected with cluster merger events. Shocks and turbulence are driven in the inter-galactic medium (IGM) during clusters mergers and may have a deep impact on the non-thermal properties of galaxy clusters. Models of turbulent (re)acceleration of relativistic particles allow good correspondence with present observations, from radio halos to γ-ray upper limits, although several aspects of this complex scenario still remain poorly understood.  相似文献   

11.
Alignments of galaxy clusters (the Binggeli effect), as well as of galaxies themselves have long been studied both observationally and theoretically. Here, we test the influence of large-scale structures and tidal fields on the shapes and alignments of cluster-size and galaxy-size dark matter haloes. We use a high-resolution N -body simulation of a Λ cold dark matter (ΛCDM) universe, together with the results of Colberg, Krughoff & Connolly, who identified filaments connecting pairs of clusters. We find that cluster pairs connected by a filament are strongly aligned with the cluster–cluster axis, whereas unconnected ones are not. For smaller, galaxy-size haloes, there also is an alignment signal, but its strength is independent of whether the halo is part of an obvious large-scale structure. Additionally, we find no measurable dependence of galaxy halo shape on membership of a filament. We also quantify the influence of tidal fields and find that these do correlate strongly with alignments of haloes. The alignments of most haloes are thus caused by tidal fields, with cluster-size haloes being strongly aligned through the added mechanism of infall of matter from filaments.  相似文献   

12.
We use the Millennium Simulation, a large, high-resolution N -body simulation of the evolution of structure in a Λ cold dark matter cosmology, to study the properties and fate of substructures within a large sample of dark matter haloes. We find that the subhalo mass function departs significantly from a power law at the high-mass end. We also find that the radial and angular distributions of substructures depend on subhalo mass. In particular, high-mass subhaloes tend to be less radially concentrated and to have angular distributions closer to the direction perpendicular to the spin of the host halo than their less massive counterparts. We find that mergers between subhaloes occur. These tend to be between substructures that were already dynamically associated before accretion into the main halo. For subhaloes larger than 0.001 times the mass of the host halo, it is more likely that the subhalo will merge with the central or main subhalo than with another subhalo larger than itself. For lower masses, subhalo–subhalo mergers become equally likely to mergers with the main subhalo. Our results have implications for the variation of galaxy properties with environment and for the treatment of mergers in galaxy formation models.  相似文献   

13.
We demonstrate that the large scatter in the ultraviolet (UV) colours of intermediate-mass early-type galaxies in the local Universe and the inferred low-level recent star formation (RSF) in these objects can be reproduced by minor mergers in the standard Λ cold dark matter (ΛCDM) cosmology. Numerical simulations of mergers with mass ratios ≤1:4, with reasonable assumptions for the ages, metallicities and dust properties of the merger progenitors, produce good agreement with the observed UV colours of the early-type population, if the infalling satellites are assumed to have (cold) gas fractions ≥20 per cent. Early-types that satisfy  ( NUV − r ) ≲ 3.8  are likely to have experienced mergers with mass ratios between 1:4 and 1:6 within the last ∼1.5 Gyr, while those that satisfy  3.8 < ( NUV − r ) < 5.5  are consistent with either recent mergers with mass ratios ≤1:6 or mergers with higher mass ratios that occurred more than ∼1.5 Gyr in the past. We demonstrate that the early-type colour–magnitude relations and colour distributions, in both the UV and optical spectral ranges, are consistent with the expected frequency of minor merging activity in the standard ΛCDM cosmology at low redshift. We present a strong plausibility argument for minor mergers to be the principal mechanism behind the large UV scatter and associated low-level RSF observed in early-type galaxies in the nearby Universe.  相似文献   

14.
Galaxy merger simulations have explored the behaviour of gas within the galactic disc, yet the dynamics of hot gas within the galaxy halo have been neglected. We report on the results of high-resolution hydrodynamic simulations of colliding galaxies with metal-free hot halo gas. To isolate the effect of the halo gas, we simulate only the dark matter halo and the hot halo gas over a range of mass ratios, gas fractions and orbital configurations to constrain the shocks and gas dynamics within the progenitor haloes. We find that (i) a strong shock is produced in the galaxy haloes before the first passage, increasing the temperature of the gas by almost an order of magnitude to   T ∼ 106.3 K  . (ii) The X-ray luminosity of the shock is strongly dependent on the gas fraction; it is  ≳1039 erg s−1  for halo gas fractions larger than 10 per cent. (iii) The hot diffuse gas in the simulation produces X-ray luminosities as large as  1042 erg s−1  . This contributes to the total X-ray background in the Universe. (iv) We find an analytic fit to the maximum X-ray luminosity of the shock as a function of merger parameters. This fit can be used in semi-analytic recipes of galaxy formation to estimate the total X-ray emission from shocks in merging galaxies. (v) ∼10–20 per cent of the initial gas mass is unbound from the galaxies for equal-mass mergers, while 3–5 per cent of the gas mass is released for the 3:1 and 10:1 mergers. This unbound gas ends up far from the galaxy and can be a feasible mechanism to enrich the intergalactic medium with metals.  相似文献   

15.
We present a new Monte Carlo algorithm to generate merger trees describing the formation history of dark matter haloes. The algorithm is a modification of the algorithm of Cole et al. used in the galform semi-analytic galaxy formation model. As such, it is based on the Extended Press–Schechter theory and so should be applicable to hierarchical models with a wide range of power spectra and cosmological models. It is tuned to be in accurate agreement with the conditional mass functions found in the analysis of merger trees extracted from the Λ cold dark matter Millennium N -body simulation. We present a comparison of its predictions not only with these conditional mass functions, but also with additional statistics of the Millennium Simulation halo merger histories. In all cases, we find it to be in good agreement with the Millennium Simulation and thus it should prove to be a very useful tool for semi-analytic models of galaxy formation and for modelling hierarchical structure formation in general. We have made our merger tree generation code and code to navigate the trees available at http://star-www.dur.ac.uk/~cole/merger_trees .  相似文献   

16.
The colour–magnitude relation (CMR) of cluster elliptical galaxies has been widely used to constrain their star formation histories (SFHs) and to discriminate between the monolithic collapse and merger paradigms of elliptical galaxy formation. We use a Λ cold dark matter hierarchical merger model of galaxy formation to investigate the existence and redshift evolution of the elliptical galaxy CMR in the merger paradigm. We show that the SFH of cluster ellipticals predicted by the model is quasi-monolithic , with only ∼10 per cent of the total stellar mass forming after   z ∼ 1  . The quasi-monolithic SFH results in a predicted CMR that agrees well with its observed counterpart in the redshift range  0 < z < 1.27  . We use our analysis to argue that the elliptical-only CMR can be used to constrain the SFHs of present-day cluster ellipticals only if we believe a priori in the monolithic collapse model. It is not a meaningful tool for constraining the SFH in the merger paradigm, since a progressively larger fraction of the progenitor set of present-day cluster ellipticals is contained in late-type star-forming systems at higher redshift, which cannot be ignored when deriving the SFHs. Hence, the elliptical-only CMR is not a useful discriminant between the two competing theories of elliptical galaxy evolution.  相似文献   

17.
We use two-dimensional kinematic maps of simulated binary disc mergers to investigate the  λR  -parameter, which is a luminosity-weighted measure of projected angular momentum per unit mass. This parameter was introduced to subdivide the SAURON sample of early-type galaxies in so-called fast  λR > 0.1  and slow rotators  λR < 0.1  . Tests on merger remnants reveal that  λR  is a robust indicator of the true angular momentum content in elliptical galaxies. We find the same range of  λR  values in our merger remnants as in the SAURON galaxies. The merger mass ratio is decisive in transforming fast rotators into slow rotators in a single binary merger, the latter being created mostly in an equal-mass merger. Slow rotators have a  λR  which does not vary with projection. The confusion rate with face-on fast rotators is very small. Mergers with a gas component form slow rotators with smaller ellipticities than collisionless merger remnants have, and are in much better agreement with the SAURON slow rotators. Remergers of merger remnants are slow rotators, but tend to have too high ellipticities. Fast rotators maintain the angular momentum content from the progenitor disc galaxy if merger mass ratio is high. Some SAURON galaxies have values of  λ R   as high as our progenitor disc galaxies.  相似文献   

18.
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19.
We derive analytic merger rates for dark matter haloes within the framework of the extended Press–Schechter (EPS) formalism. These rates become self-consistent within EPS once we realize that the typical merger in the limit of a small time-step involves more than two progenitors, contrary to the assumption of binary mergers adopted in earlier studies. We present a general method for computing merger rates that span the range of solutions permitted by the EPS conditional mass function, and focus on a specific solution that attempts to match the merger rates in N -body simulations. The corrected EPS merger rates are more accurate than the earlier estimates of Lacey & Cole by ∼20 per cent for major mergers and by up to a factor of ∼3 for minor mergers of mass ratio 1:104. Based on the revised merger rates, we provide a new algorithm for constructing Monte Carlo EPS merger trees, which could be useful in semi-analytic modelling. We provide analytic expressions and plot numerical results for several quantities that are very useful in studies of galaxy formation. This includes (i) the rate of mergers of a given mass ratio per given final halo, (ii) the fraction of mass added by mergers to a halo and (iii) the rate of mergers per given main progenitor. The creation and destruction rates of haloes serve for a self-consistency check. Our method for computing merger rates can be applied to conditional mass functions beyond EPS, such as those obtained by the ellipsoidal collapse model or extracted from N -body simulations.  相似文献   

20.
We employ numerical simulations of galaxy mergers to explore the effect of galaxy mass ratio on merger-driven starbursts. Our numerical simulations include radiative cooling of gas, star formation, and stellar feedback to follow the interaction and merger of four disc galaxies. The galaxy models span a factor of 23 in total mass and are designed to be representative of typical galaxies in the local universe. We find that the merger-driven star formation is a strong function of merger mass ratio, with very little, if any, induced star formation for large mass ratio mergers. We define a burst efficiency that is useful to characterize the merger-driven star formation and test that it is insensitive to uncertainties in the feedback parametrization. In accord with previous work we find that the burst efficiency depends on the structure of the primary galaxy. In particular, the presence of a massive stellar bulge stabilizes the disc and suppresses merger-driven star formation for large mass ratio mergers. Direct, coplanar merging orbits produce the largest tidal disturbance and yield the most intense burst of star formation. Contrary to naive expectations, a more compact distribution of gas or an increased gas fraction both decrease the burst efficiency. Owing to the efficient feedback model and the newer version of smoothed particle hydrodynamics employed here, the burst efficiencies of the mergers presented here are smaller than in previous studies.  相似文献   

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