arXiv subjects

Takashi Okamoto

Publications and source records attributed to Takashi Okamoto.

76 records · Page 5Linked to original sources

arXiv2003

Cluster Morphologies as a Test of Different Cosmological Models

We investigate how cluster morphology is affected by the cosmological constant in low-density universes. Using high-resolution cosmological N-body/SPH simulations of flat (Ω_0 = 0.3, λ_0 = 0.7, ΛCDM) and open (Ω_0 = 0.3, λ_0 = 0, OCDM) cold dark matter universes, we calculate statistical indicators to quantify the irregularity of the cluster morphologies. We study axial ratios, center shifts, cluster clumpiness, and multipole moment power ratios as indicators for the simulated clusters at z=0 and 0.5. Some of these indicators are calculated for both the X-ray surface brightness and projected mass distributions. In ΛCDM all these indicators tend to be larger than those in OCDM at z=0. This result is consistent with the analytical prediction of Richstone, Loeb, & Turner, that is, clusters in ΛCDM are formed later than in OCDM, and have more substructure at z=0. We make a Kolmogorov-Smirnov test on each indicator for these two models. We then find that the results for the multipole moment power ratios and the center shifts for the X-ray surface brightness are under the significance level (5%). We results also show that these two cosmological models can be distinguished more clearly at z=0 than z = 0.5 by these indicators.

astro-ph↗

arXiv2003

Environmental Effects on Evolution of Cluster Galaxies in a LCDM Universe

We investigate environmental effects on evolution of bright cluster galaxies in a $Λ$ dominated cold dark matter universe using a combination of dissipationless N-body simulations and a semi-analytic galaxy formation model. The N-body simulations enable us to calculate orbits of galaxies in simulated clusters. Therefore we can incorporate stripping of cold gas from galactic disks by ram pressure from intracluster medium into our model. In this paper we study how ram pressure stripping (RPS) and small starburst induced by a minor merger affect colors, star formation rates, and morphologies of cluster galaxies. We find that the RPS is not important for colors and SFRs of galaxies in the cluster core if star formation time-scale is properly chosen, because the star formation is sufficiently suppressed by consumption of the cold gas in the disks. Then observed color and SFR gradients can be reproduced without the RPS. The small starburst triggered by a minor merger hardly affects the SFRs and colors of the galaxies as well. We also examine whether these two processes can resolve the known problem that the major merger-driven bulge formation scenario predict too few galaxies of intermediate bulge-to-total luminosity ratio (B/T) in clusters. When the minor burst is taken into account, the intermediate B/T population is increased and the observed morphology gradients in clusters are successfully reproduced. When the minor burst is considered, the RPS also plays an important role in formation of the intermediate B/T galaxies. We present redshift evolution of morphological fractions predicted by our models. The predicted number ratios of the intermediate B/T galaxies to the bulge-dominated galaxies show nearly flat or slightly increasing trends with increasing redshift.

astro-ph↗

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.

astro-ph↗

arXiv2000

Evolution of Cluster Galaxies in Hierarchical Clustering Universes

Using cosmological N-body simulations of critical (SCDM) and open ($Ω= 0.3$, OCDM) cold dark matter models, we investigate evolution of cluster galaxies. From our numerical simulation, we construct merging history trees of the galaxies. By following their merging history, we show major merger fractions of the galaxies in cluster forming regions is roughly proportional to (1+z)^{4.5} at low redshifts (z < 2) and has a steep peak at $z \simeq 2.5$ and $z \simeq 3$ in SCDM and OCDM, respectively. We also show that the cluster galaxies are affected by tidal interaction after the clusters formed. Because the formation redshift of the cluster in SCDM, z_{form} = 0.15, is much more recent than that of the cluster in OCDM, z_{form} = 1.6, the cluster galaxies in SCDM show more rapid evolution by tidal interactions from z = 0.5 than those in OCDM.

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