arXiv subjects

Masahiro Nagashima

Publications and source records attributed to Masahiro Nagashima.

65 records · Page 4Linked to original sources

arXiv2002

Analytical Approach to the Mass Distribution Function of Subhalos and Cold Fronts in Galaxy Clusters

We construct an analytical model of the mass distribution function of subhalos in a galaxy cluster in the context of the cold dark matter (CDM) theory. Our model takes account of two important effects; the high density of the precluster region and the spatial correlation of initial density fluctuations. For subhalos with small masses, the mass distribution function that our model predicts is intermediate between the Press-Schechter (PS) mass distribution function and a conditional mass distribution function as an extension of the PS formalism. We compare the results of our model with those of numerical simulations. We find that our model predictions are consistent with the mass and velocity distribution functions of subhalos in a cluster obtained by numerical simulations. We estimate the probability of finding large X-ray subhalos that often have ``cold fronts''. The observed large X-ray subhalos and cold fronts may not always be the result of cluster mergers, but instead may be internal structures in clusters.

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arXiv2002

Analytic Approach to the Cloud-in-cloud Problem for Non-Gaussian Density Fluctuations

We revisit the cloud-in-cloud problem for non-Gaussian density fluctuations. We show that the extended Press-Schechter (EPS) formalism for non-Gaussian fluctuations has a flaw in describing mass functions regardless of type of filtering. As an example, we consider non-Gaussian models in which density fluctuations at a point obeys a χ^2 distribution with νdegrees of freedom. We find that mass functions predicted by using an integral formula proposed by Jedamzik, and Yano, Nagashima and Gouda, properly taking into account correlation between objects at different scales, deviate from those predicted by using the EPS formalism, especially for strongly non-Gaussian fluctuations. Our results for the mass function at large mass scales are consistent with those by Avelino and Viana obtained from numerical simulations.

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arXiv2001

A Solution to the Missing Link in the Press-Schechter Formalism

The Press-Schechter (PS) formalism for mass functions of the gravitationally collapsed objects is reanalyzed. It has been suggested by many authors that the PS mass function agrees well with the mass function given by $N$-body simulations, while many too simple assumptions are contained in the formalism. In order to understand why the PS formalism works well we consider the following three effects on the mass function: the filtering effect of the density fluctuation field, the peak ansatz in which objects collapse around density maxima, and the spatial correlation of density contrasts within a region of a collapsing halo because objects have non-zero finite volume. According to the method given by Yano, Nagashima & Gouda (YNG) who used an integral formula proposed by Jedamzik, we resolve the missing link in the PS formalism, taking into account the three effects. While YNG showed that the effect of the spatial correlation alters the original PS mass function, in this paper, we show that the filtering effect almost cancels out the effect of the spatial correlation and that the original PS is almost recovered by combining these two effects, particularly in the case of the top-hat filter. On the other hand, as for the peak ansatz, the resultant mass functions are changed dramatically and not in agreement especially at low-mass tails in the cases of the Gaussian and the sharp $k$-space filters. We show that these properties of the mass function can be interpreted in terms of the kernel probability $P(M_{1}| M_{2})$ in the integral formula qualitatively.

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arXiv2000

Morphology-Density Relation for Simulated Clusters of Galaxies in Cold Dark Matter Dominated Universe

We present a model to investigate the formation and evolution of cluster galaxies using cosmological N-body simulations. The N-body simulations are used to make merging history trees of dark halos. Gas cooling, star formation, supernova feedback and mergers of galaxies within dark halos are included by using simple prescriptions taken from semi-analytic models of galaxy formation. In this {\it Letter} we exam the merger-driven bulge formation model and represent the morphology-density relation of cluster galaxies at z = 0. We find that this morphological evolution model can explain the distribution of elliptical galaxies in the clusters well and cannot reproduce the distribution of S0 galaxies. This result suggests that the elliptical galaxies are mainly formed by the major mergers, while, in the S0 formation, the processes other than major mergers play important role.

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arXiv2000

The Detection Rate of Molecular Gas in Elliptical Galaxies: Constraints on Galaxy Formation Theories

In order to constrain parameters in galaxy formation theories, especially those for a star formation process, we investigate cold gas in elliptical galaxies. We calculate the detection rate of cold gas in them using a semi-analytic model of galaxy formation and compare it with observations. We show that the model with a long star formation time-scale (~20 Gyr) is inconsistent with observations. Thus, some mechanisms of reducing the mass of interstellar medium, such as the consumption of molecular gas by star formation and/or reheating from supernovae, are certainly effective in galaxies. Our model predicts that star formation induced when galaxies in a halo collide each other reduces the cold gas left until the present. However, we find that the reduction through random collisions of satellite (non-central) galaxies in mean free time-scale in a halo is not required to explain the observations. This may imply that the collisions and mergers between satellite galaxies do not occur so often in clusters or that they do not stimulate the star formation activity as much as the simple collision model we adopted. For cD galaxies, the predicted detection rate of cold gas is consistent with observations as long as the transformation of hot gas into cold gas is prevented in halos whose circular velocities are larger than 500 km s^-1. Moreover, we find that the cold gas brought into cDs through captures of gas-rich galaxies is little. We also show that the fraction of galaxies with observable cold gas should be small for cluster ellipticals in comparison with that for field ellipticals.

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arXiv1999

What determines the slope of the colour-magnitude relation of elliptical galaxies in the hierarchical clustering scenario?

We examined the physical origin of the colour-magnitude relation of elliptical galaxies in clusters of galaxies in the hierarchical clustering scenario by using a semi-analytic model of galaxy formation. We particularly investigate the effects of the star formation process, the supernova feedback and the UV background on the formation and evolution of elliptical galaxies. It is suggested by observation that the colour-magnitude relation reflects the metallicity-luminosity relation of galaxies, and that for reproducing the colour-magnitude relation it is important to suppress the chemical enrichment of stars in faint galaxies. For suppressing the chemical enrichment, we propose some solutions. One is assuming a strong supernova feedback. Another is introducing the effect of the UV background radiation. These solutions degenerate, but they may be distinguished by studying other properties of galaxies, like the luminosity function, colour distribution, etc. Moreover, we find that the supernova feedback model in the starburst which make the bulge component is important when the star formation time-scale in disc is longer than the age of the Universe. On the other hand, when the star formation time-scale is short, the resulting colour-magnitude relation does not depend on such a starburst model because most of stars are formed not by the burst but by the ordinary star formation process in disc.

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arXiv1999

Effects of the UV background radiation on galaxy formation

We investigate the effects of the UV background radiation on galaxy formation by using the semi-analytic model including the photoionization process. The semi-analytic model is based on Cole et al. (1994) and we use almost the same parameters of their `fiducial' model. We find that the UV background mainly affects the formation of dwarf galaxies. Because of the suppression of star formation, the number density of small objects corresponding to dwarf galaxies decreases compared to the case of no UV radiation when the UV background exists until the present epoch. When the UV background vanishes at a low redshift, the number density of small objects is hardly changed but the colour becomes bluer, compared to the case of no UV radiation, because stars are newly formed after the UV background vanishes. On the other hand, the UV radiation hardly affects massive galaxies. This is because the massive galaxies are formed by mergers of small galaxies.

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arXiv1999

Is Galactic Star Formation Activity Increased During Cluster Mergers?

We have investigated the effect of pressure from intracluster medium (ICM) on disk galaxies in merging clusters. The ram-pressure on the galaxies rapidly increases when two clusters collide. This leads to stripping of the interstellar medium (ISM) and decrease of star formation rate (SFR) of the galaxies. On the other hand, the increase of SFR caused by compression of ISM is less significant. Thus, cluster merger does not trigger, but weakens star formation activity of the galaxies. In the central region of the colliding clusters, blue galaxies with high velocity should exist, although most of galaxies become red. Following the decrease of blue galaxy fraction in the clusters, the fraction of post-starburst galaxies increases. After merger, many galaxies in the cluster restart star formation activity and the segregation of blue and red galaxies becomes prominent.

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arXiv1998

Effects of Ram-Pressure from Intracluster Medium on the Star Formation Rate of Disk Galaxies in Clusters of Galaxies

Using a simple model of molecular cloud evolution, we have quantitatively estimated the change of star formation rate (SFR) of a disk galaxy falling radially into the potential well of a cluster of galaxies. The SFR is affected by the ram-pressure from the intracluster medium (ICM). As the galaxy approaches the cluster center, the SFR increases to twice the initial value, at most, in a cluster with high gas density and deep potential well, or with a central pressure of $\sim 10^{-2} cm^{-3} keV$ because the ram-pressure compresses the molecular gas of the galaxy. However, this increase does not affect the color of the galaxy significantly. Further into the central region of the cluster ($\lesssim 1$ Mpc from the center), the SFR of the disk component drops rapidly due to the effect of ram-pressure stripping. This makes the color of the galaxy redder and makes the disk dark. These effects may explain the observed color, morphology distribution and evolution of galaxies in high-redshift clusters. By contrast, in a cluster with low gas density and shallow potential well, or the central pressure of $\sim 10^{-3} cm^{-3} keV$, the SFR of a radially infalling galaxy changes less significantly, because neither ram-pressure compression nor stripping is effective. Therefore, the color of galaxies in poor clusters is as blue as that of field galaxies, if other environmental effects such as galaxy-galaxy interaction are not effective. The predictions of the model are compared with observations.

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arXiv1998

Effects of Merging Histories on Angular Momentum Distribution of Dark Matter Haloes

The effects of merging histories of proto-objects on the angular momentum distributions of the present-time dark matter haloes are analysed. An analytical approach to the analysis of the angular momentum distributions assumes that the haloes are initially homogeneous ellipsoids and that the growth of the angular momentum of the haloes halts at their maximum expansion time. However, the maximum expansion time cannot be determined uniquely, because in the hierarchical clustering scenario each progenitor, or subunit, of the halo has its own maximum expansion time. Therefore the merging history of the halo may be important in estimating its angular momentum. Using the merger tree model by Rodrigues & Thomas, which takes into account the spatial correlations of the density fluctuations, we have investigated the effects of the merging histories on the angular momentum distributions of dark matter haloes. It was found that the merger effects, that is, the effects of the inhomogeneity of the maximum expansion times of the progenitors which finally merge together into a halo, do not strongly affect the final angular momentum distributions, so that the homogeneous ellipsoid approximation happens to be good for the estimation of the angular momentum distribution of dark matter haloes. This is because the effect of the different directions of the angular momenta of the progenitors cancels out the effect of the inhomogeneity of the maximum expansion times of the progenitors.

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arXiv1996

The Effect of Spatial Correlations on Merger Trees of Dark Matter Haloes

The effects of spatial correlations of density fluctuations on merger histories of dark matter haloes (so-called `{\it merger trees}') are analysed. We compare the mass functions of dark haloes derived by a new method for calculating merger trees, that proposed by Rodrigues \& Thomas (RT), with those given by other methods such as the Block model, the Press-Schechter formula and our own formula in which the mass functions are analytically expressed in a way that takes into consideration the spatial correlations. It is found that the mass functions given by the new method are well fit by those given by our formula. We believe that new method (RT) {\it naturally} and correctly takes into account the spatial correlations of the density fluctuations due to a calculated, grid-based realisation of the density fluctuations and so is very useful for estimating the merger tree accurately in a way that takes into consideration spatial correlations. Moreover, by applying our formula, we present an analytic expression which reproduces the mass function derived by the Block model. We therefore show clearly why and how the mass functions given by the new method and the Block model are different from each other. Furthermore, we note that the construction of merger trees is sensitive to the criterion of collapse and merging of overlapped haloes in cases in which two or more haloes happen to overlap. In fact, it is shown that the mass function is very much affected when the criterion of overlapping is changed.

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