Accurate estimation of the merger timescale of galaxy clusters is important to understand the cluster merger process and further the formation and evolution of the large-scale structure of the universe. In this paper, we explore a baryonic effect on the merger timescale of galaxy clusters by using hydrodynamical simulations. We find that the baryons play an important role in accelerating the merger process. The merger timescale decreases with increasing the gas fraction of galaxy clusters. For example, the merger timescale is shortened by a factor of up to 3 for merging clusters with gas fractions 0.15, compared with the timescale obtained with zero gas fractions. The baryonic effect is significant for a wide range of merger parameters and especially more significant for nearly head-on mergers and high merging velocities. The baryonic effect on the merger timescale of galaxy clusters is expected to have impacts on the structure formation in the universe, such as the cluster mass function and massive substructures in galaxy clusters, and a bias of "no-gas" may exist in the results obtained from the dark matter-only cosmological simulations.
We use N-body simulations and observationally-normalized relations between dark matter halo mass, stellar mass, and cold gas mass to derive robust, arguably inevitable expectations about the baryonic content of major mergers out to redshift z~2. First, we find that the majority of major mergers (m/M > 0.3) experienced by Milky Way size dark matter halos should have been gas-rich, and that gas-rich mergers are increasingly common at high redshift. Though the frequency of major mergers into galaxy halos in our simulations greatly exceeds the observed late-type galaxy fraction, the frequency of g…
Influence of baryonic physics on the merger time-scale of galaxies in N-body/hydrodynamical simulations
根据我们以前的工作（Jiang et al.（2008））中，我们在高分辨率的宇宙学水/N体模拟中研究了星系的合并时间尺度，我们研究了重子物理数值实现中的不确定性对合并时间尺度的潜在影响。在以前的工作中使用的模拟遭受过冷的问题，导致中央星系的大晕太大。这可能导致合并的时间尺度比真实宇宙中的时间尺度更短。我们运行类似的模拟，但恒星质量显着减少，以模拟另一种极端情况下的低恒星质量。我们的结果表明，合并的时间尺度是受恒星形成配方的影响很小，除了在近径向轨道的卫星，显示出22%的时间尺度在较低的恒星质量的情况下，高。由于径向轨道只占卫星轨道的一小部分，因此，Jiang等人的拟合公式是不适用的。（2008）仍然适用于合理的精度，几乎不受重子物理学中的不确定性的影响。
We present a model for the dynamical evolution of subhaloes based on an approach combining numerical and analytical methods. Our method is based on tracking subhaloes in an N-body simulation up to the last point that it can be resolved, and applying an analytic prescription for its merger timescale that takes dynamical friction and tidal disruption into account. When applied to cosmological N-body simulations with mass resolutions that differ by two orders of magnitude, the technique produces halo occupation distributions that agree to within 3%.
Different methodologies lead to order-of-magnitude variations in predicted galaxy merger rates. We examine and quantify the dominant uncertainties. Different halo merger rates and subhalo 'destruction' rates agree to within a factor ~2 given proper care in definitions. If however (sub)halo masses are not appropriately defined or are under-resolved, the major merger rate can be dramatically suppressed. The dominant differences in galaxy merger rates owe to baryonic physics. Hydrodynamic simulations without feedback and older models that do not agree with the observed galaxy mass function propag…
We develop a galaxy assignment scheme that populates dark matter halos with galaxies by tracing the most bound member particles (MBPs) of simulated halos. Several merger-timescale models based on analytic calculations and numerical simulations are adopted as the survival time of mock satellite galaxies. We build mock galaxy samples from halo merger data of the Horizon Run 4 $N$-body simulation from $z = 12-0$. We compare group properties and two-point correlation functions (2pCFs) of mock galaxies with those of volume-limited SDSS galaxies, with $r$-band absolute magnitudes of $\mathcal{M}_r -…
Using longslit, optical spectra of Ultraluminous Infrared Galaxies (ULIRGs), we measure the evolution in the star-formation intensity during galactic mergers. In individual galaxies, we resolve kpc scales allowing comparison of the nucleus, inner disk, and outer disk. We find that the strength of the Hbeta absorption line increases with the projected distance from the center of the merger, typically reaching about 9 Angstrom around 10 kpc. At these radii, the star formation intensity must have rapidly decreased about 300-400 Myr ago; only stellar populations deficient in stars more massive tha…
AMI observations towards CIZA J2242+5301, in comparison with observations of weak gravitational lensing and X-ray emission from the literature, are used to investigate the behaviour of non-baryonic dark matter (NBDM) and gas during the merger. Analysis of the Sunyaev-Zel'dovich (SZ) signal indicates the presence of high pressure gas elongated perpendicularly to the X-ray and weak-lensing morphologies which, given the merger-axis constraints in the literature, implies that high pressure gas is pushed out into a linear structure during core passing. Simulations in the literature closely matching…
The majority of galaxy mergers are expected to be minor mergers. The observational signatures of minor mergers are not well understood, thus there exist few constraints on the minor merger rate. This paper seeks to address this gap in our understanding by determining if and when minor mergers exhibit disturbed morphologies and how they differ from the morphology of major mergers. We simulate a series of unequal-mass moderate gas-fraction disc galaxy mergers. With the resulting g-band images, we determine how the time-scale for identifying galaxy mergers via projected separation and quantitativ…
Calculating the galaxy merger rate requires both a census of galaxies identified as merger candidates, and a cosmologically-averaged `observability' timescale T_obs(z) for identifying galaxy mergers. While many have counted galaxy mergers using a variety of techniques, T_obs(z) for these techniques have been poorly constrained. We address this problem by calibrating three merger rate estimators with a suite of hydrodynamic merger simulations and three galaxy formation models. We estimate T_obs(z) for (1) close galaxy pairs with a range of projected separations, (2) the morphology indicator G-M…
Observations of quasars at redshifts z > 6 reveal that 10^9 Msol supermassive black holes (SMBHs) had already formed when the Universe was < 0.9 Gyr old. One hypothesis for the origins of these SMBHs is that they grew from the remnants of the first generation of massive stars, which formed in low-mass (~ 10^5 to 10^6 Msol) dark matter minihaloes at z > 20. This is the regime where baryonic streaming motions--the relative velocities of baryons with respect to dark matter in the early Universe--most strongly inhibit star formation by suppressing gas infall and cooling. We investigate the impact …
We present a new 2 Ms Chandra observation of the cluster merger Abell 2146, which hosts two huge M~2 shock fronts each ~500 kpc across. For the first time, we resolve and measure the width of cluster merger shocks. The best-fit width for the bow shock is 17+/-1 kpc and for the upstream shock is 10.7+/-0.3 kpc. A narrow collisionless shock will appear broader in projection if its smooth shape is warped by local gas motions. We show that both shock widths are consistent with collisionless shocks blurred by local gas motions of 290+/-30 km/s. The upstream shock forms later on in the merger than t…
Massive galaxy clusters are the most violent large scale structures undergoing merger events in the Universe. Based upon their morphological properties in X-rays, they are classified as un-relaxed and relaxed clusters and often host (a fraction of them) different types of non-thermal radio emitting components, viz., haloes, mini-haloes, relics and phoenix within their Intra Cluster Medium (ICM). The radio haloes show steep (alpha = -1.2) and ultra steep (alpha < -1.5) spectral properties at low radio frequencies, giving important insights on the merger (pre or post) state of the cluster. Ultra…
The rotation curves of some star forming massive galaxies at redshift two decline over the radial range of a few times the effective radius, indicating a significant deficit of dark matter (DM) mass in the galaxy centre. The DM mass deficit is interpreted as the existence of a DM density core rather than the cuspy structure predicted by the standard cosmological model. A recent study proposed that a galaxy merger, in which the smaller satellite galaxy is significantly compacted by dissipative contraction of the galactic gas, can heat the centre of the host galaxy and help make a large DM core.…
The expected event rate of lensed gravitational wave sources scales with the merger rate at redshift $z\geq 1$, where the optical depth for lensing is high. It is commonly assumed that the merger rate of the astrophysical compact objects is closely connected with the star formation rate, which peaks around redshift $z\sim 2$. However, a major source of uncertainty is the delay time between the formation and merger of compact objects. We explore the impact of delay time on the lensing event rate. We show that as the delay time increases, the peak of the merger rate of gravitational wave sources…
[abr.] Using the multi-integral-field spectrograph GIRAFFE at VLT, we previsouly derived the stellar-mass Tully-Fisher Relation (smTFR) at z~0.6, and found that the distant relation is systematically offset by roughly a factor of two toward lower masses. We extend the study of the evolution of the TFR by establishing the first distant baryonic TFR. To derive gas masses in distant galaxies, we estimate a gas radius and invert the Schmidt-Kennicutt law between star formation rate and gas surface densities. We find that gas extends farther out than the UV light from young stars, a median of ~30%.…
Andromeda II (And II) has been known for a few decades but only recently observations have unveiled new properties of this dwarf spheroidal galaxy. The presence of two stellar populations, the bimodal star formation history (SFH) and an unusual rotation velocity of And II put strong constrains on its formation and evolution. Following Lokas et al. (2014), we propose a detailed model to explain the main properties of And II involving (1) a gas-rich major merger between two dwarf galaxies at high redshift in the field and (2) a close interaction with M31 about 5 Gyr ago. The model is based on N-…
When a satellite galaxy falls into a massive dark matter halo, it suffers the dynamical friction force which drag it into the halo center and finally it merger with the central galaxy. The time interval between entry and merger is called as the dynamical friction timescale (T_df). Many studies have been dedicated to derive T_df using analytical models or N-body simulations. These studies have obtained qualitative agreements on how T_df depends on the orbit parameters, and mass ratio between satellite and host halo. However, there are still disagreements on the accurate form of T_df . In this p…
Tidal dwarf galaxies (TDGs) are recycled objects that form within the collisional debris of interacting/merging galaxies. They are expected to be devoid of non-baryonic dark matter, since they can form only from dissipative material ejected from the discs of the progenitor galaxies. We investigate the gas dynamics in a sample of six bona-fide TDGs around three interacting and post-interacting systems: NGC 4694, NGC 5291, and NGC 7252 ("Atoms for Peace"). For NGC 4694 and NGC 5291 we analyse existing HI data from the Very Large Array (VLA), while for NGC 7252 we present new HI observations from…
Gas-rich galaxy mergers are more easily identified by their disturbed morphologies than mergers with less gas. Because the typical gas fraction of galaxy mergers is expected to increase with redshift, the under-counting of low gas-fraction mergers may bias morphological estimates of the evolution of galaxy merger rate. To understand the magnitude of this bias, we explore the effect of gas fraction on the morphologies of a series of simulated disc galaxy mergers. With the resulting g-band images, we determine how the time-scale for identifying major and minor galaxy mergers via close projected …
Employing a simplified version of the Israel-Stewart formalism of general-relativistic shear-viscous hydrodynamics, we explore the evolution of a remnant massive neutron star of binary neutron star merger and pay special attention to the resulting gravitational waveforms. We find that for the plausible values of the so-called viscous alpha parameter of the order $10^{-2}$, the degree of the differential rotation in the remnant massive neutron star is significantly reduced in the viscous timescale, $\alt 5$\,ms. Associated with this, the degree of non-axisymmetric deformation is also reduced qu…
We compare assembly of DM halos with and without baryons, within the context of cosmological evolution in the LCDM WMAP3 Universe (baryons+DM, BDM model, and pure DM, PDM model). In representative PDM and BDM models we find that baryons contribute decisively to the evolution of the central region, leading to an isothermal DM cusp, and to a flat DM density core -- the result of heating by dynamical friction of the substructure during a quiescent evolution epoch. This process ablates the cold gas from an embedded disk, cutting the star formation rate by ~10, and heats up the spheroidal gas and s…
Galaxy clusters grow by mergers with other clusters and galaxy groups. These mergers create shocks within the intracluster medium (ICM). It is proposed that within the shocks particles can be accelerated to extreme energies. In the presence of a magnetic field these particles should then form large regions emitting synchrotron radiation, creating 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 …
Gravitational lensing observations of massive X-ray clusters imply a steep characteristic density profile marked by a central concentration of dark matter. The observed mass fraction within a projected radius of 150 kpc is twice that found in state-of-the-art dark matter simulations of the standard Lambda-CDM cosmology. A central baryon enhancement that could explain this discrepancy is not observed, leaving a major puzzle. We propose a solution based on the merger histories of clusters. A significant fraction of the final dark matter content of a cluster halo originates within galaxy-sized ha…
By means of the abundance matching technique, we infer the local stellar and baryonic mass-halo mass (Ms-Mh and Mb-Mh) relation for central blue and red galaxies separately in the mass range Ms~10^8.5-10^12.0 Msun. The observational inputs are the SDSS central blue and red Galaxy Stellar Mass Functions reported in Yang et al. 2009, and the measured local gas mass-Ms relations for blue and red galaxies. For the Halo Mass Function associated to central blue galaxies, the distinct LCDM one is used and set up to exclude: (i) the observed group/cluster mass function (blue galaxies are rare as cente…
We measure the fraction of Luminous Red Galaxies (LRGs) in dynamically close pairs (with projected separation less than 20 $h^{-1}$ kpc and velocity difference less than 500 km s$^{-1}$) to estimate the dry merger rate for galaxies with $-23 < M(r)_{k+e,z=0.2} +5 \log h < -21.5$ and $0.45 < z < 0.65$ in the 2dF-SDSS LRG and QSO (2SLAQ) redshift survey. For galaxies with a luminosity ratio of $1:4$ or greater we determine a $5σ$ upper limit to the merger fraction of 1.0% and a merger rate of $< 0.8 \times 10^{-5}$ Mpc$^{-3}$ Gyr$^{-1}$ (assuming that all pairs merge on the shortest possible tim…
[Abridged] [...] We study the mass and spin evolution of massive black holes within a semianalytical galaxy-formation model that follows the evolution of dark-matter halos along merger trees, as well as that of the baryonic components (hot gas, stellar and gaseous bulges, and stellar and gaseous galactic disks). This allows us to study the mass and spin evolution of massive black holes in a self-consistent way, by taking into account the effect of the gas present in galactic nuclei both during the accretion phases and during mergers. Also, we present predictions, as a function of redshift, for…
We compute the effect of an orbiting gas disc in promoting the coalescence of a central supermassive black hole binary. Unlike earlier studies, we consider a finite mass of gas with explicit time dependence: we do not assume that the gas necessarily adopts a steady state or a spatially constant accretion rate, i.e. that the merging black hole was somehow inserted into a pre--existing accretion disc. We consider the tidal torque of the binary on the disc, and the binary's gravitational radiation. We study the effects of star formation in the gas disc in a simple energy feedback framework. The d…
