Masahiro Nagashima

Reionization and Galaxy Evolution Probed by z = 7 Lyα Emitters*

We made a narrowband NB973 (bandwidth of 200A at 9755A) imaging of the Subaru Deep Field (SDF) and found two z=7 Lyman alpha emitter (LAE) candidates down to NB973=24.9. Carrying out deep follow-up spectroscopy, we identified one of them as a real z=6.96 LAE. This has shown that galaxy formation was in progress just 750 Myr after the Big Bang. Meanwhile, the Lyman alpha line luminosity function of LAE is known to decline from z=5.7 to 6.6 in the SDF. L* at z=6.6 is 40-60% of that at z=5.7. We also confirm that the number density of z=7 LAE is only 17% of the density at z=6.6 comparing the latest SDF LAE samples. This series of significant decreases in LAE density with increasing redshift can be the result of galaxy evolution during these epochs. However, using the UV continuum luminosity functions of LAEs, those of Lyman break galaxies, and a LAE evolution model based on the hierarchical clustering, we find that galaxy evolution alone cannot explain all the decrease in density. This extra density deficit can be interpreted as the attenuation of the Lyman alpha photons from LAEs due to a rapid evolution of neutral hydrogen fraction during the ongoing cosmic reionization at z~6.6-7.

Effects of Ram Pressure from the 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 a deep potential well, or with a central pressure of ~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 (1 Mpc from the center), the SFR of the disk component drops rapidly owing 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 a shallow potential well, or with the central pressure of ~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.

The metal enrichment of elliptical galaxies in hierarchical galaxy formation models

We investigate the metal enrichment of elliptical galaxies in the framework of hierarchical models of galaxy formation. The semi-analytical model we use, which has been used to study the metal enrichment of the intracluster medium (ICM) by Nagashima et al., includes the effects of flows of gas and metals both into and out of galaxies and the processes of metal enrichment due to both type Ia and type II supernovae. We adopt a solar neighbourhood initial mass function (IMF) for star formation in discs, but consider models in which starbursts have either a solar neighbourhood IMF or a top-heavy IMF. We find that the α-element abundance in ellipticals is consistent with observed values only if the top-heavy IMF is used. This result is consistent with our previous study on the metal enrichment of the ICM. We also discuss the abundance ratio of α elements to iron as a function of velocity dispersion and metallicity. We find that models with a top-heavy IMF match the α/Fe ratios observed in typical L* ellipticals, but none of the models reproduces the observed increase of α/Fe with velocity dispersion.

The metal enrichment of the intracluster medium in hierarchical galaxy formation models

We investigate the metal enrichment of the intracluster medium (ICM) within the framework of hierarchical models of galaxy formation. We calculate the formation and evolution of galaxies and clusters using a semi-analytical model which includes the effects of flows of gas and metals both into and out of galaxies. For the first time in a semi-analytical model, we calculate the production of both α and iron-peak elements based on theoretical models for the lifetimes and ejecta of type Ia and II supernovae (SNe Ia and II). It is essential to include the long lifetimes of the SNIa progenitors in order to correctly model the evolution of the iron-peak elements. We find that if all stars form with an initial mass function (IMF) similar to that found in the solar neighbourhood, then the metallicities of O, Mg, Si and Fe in the ICM are predicted to be two to three times lower than the observed values. In contrast, a model (also favoured on other grounds) in which stars formed in bursts triggered by galaxy mergers have a top-heavy IMF reproduces the observed ICM abundances of O, Mg, Si and Fe. The same model predicts ratios of ICM mass to total stellar luminosity in clusters which agree well with observations. According to our model, the bulk of the metals in clusters are produced by L* and brighter galaxies. We predict only mild evolution of [Fe/H] in the ICM with redshift out to z∼ 1, consistent with the sparse data available on high-z clusters. In contrast, the [O/Fe] ratio is predicted to gradually decrease with time because of the delayed production of iron compared with oxygen. We find that, at a given redshift, the scatter in global metallicity for clusters of a given mass is quite small, even though the formation histories of individual clusters show wide variations. The observed diversity in ICM metallicities may thus result from the range in metallicity gradients induced by the scatter in the assembly histories of clusters of galaxies.

Clustering of Lyman Break Galaxies at z = 4 and 5 in the Subaru Deep Field: Luminosity Dependence of the Correlation Function Slope

We explored the clustering properties of Lyman break galaxies at z = 4 and 5 with an angular two-point correlation function on the basis of the very deep and wide Subaru Deep Field data. We confirmed the previous result that the clustering strength of LBGs depends on the UV luminosity in the sense that brighter LBGs are more strongly clustered. In addition, we found an apparent dependence of the correlation function slope on UV luminosity for LBGs at both z = 4 and 5. More luminous LBGs have a steeper correlation function. The bias parameter was found to be a scale-dependent function for bright LBGs, whereas it appears to be almost scale-independent for faint LBGs. Luminous LBGs have a higher bias at smaller angular scales, which decreases as the scale increases. To compare these observational results, we constructed numerical mock LBG catalogs based on a semianalytic model of hierarchical clustering combined with high-resolution N-body simulation, carefully mimicking the observational selection effects. The luminosity functions and the overall correlation functions for LBGs at z = 4 and 5 predicted by this mock catalog were found to be almost consistent with the observation. The observed dependence of the clustering on UV luminosity was not reproduced by the model, unless subsamples of distinct halo mass were considered. That is, LBGs belonging to more massive dark halos had steeper and larger amplitude correlation functions. With this model, we found that LBG multiplicity in massive dark halos amplifies the clustering strength at small scales, which steepens the correlation function. The hierarchical clustering model could therefore be reconciled with the observed luminosity dependence of the correlation function if there is a tight correlation between UV luminosity and halo mass. Our finding that the slope of the correlation function depends on luminosity could be an indication that massive dark halos hosted multiple bright LBGs.

Lyα Emitters in Hierarchical Galaxy Formation

We present a new theoretical model for the luminosity functions (LFs) of Lyman alpha (Lya) emitting galaxies in the framework of hierarchical galaxy formation. We extend a semi-analytic model of galaxy formation that reproduces a number of observations for local and high-z galaxies, without changing the original model parameters but introducing a physically-motivated modelling to describe the escape fraction of Lya photons from host galaxies (f_esc). Though a previous study using a hierarchical clustering model simply assumed a constant and universal value of f_esc, we incorporate two new effects on f_esc: extinction by interstellar dust and galaxy-scale outflow induced as a star formation feedback. It is found that the new model nicely reproduces all the observed Lya LFs of the Lya emitters (LAEs) at different redshifts in z ~ 3-6. Especially, the rather surprisingly small evolution of the observed LAE Lya LFs compared with the dark halo mass function is naturally reproduced. Our model predicts that galaxies with strong outflows and f_esc ~ 1 are dominant in the observed LFs. This is also consistent with available observations, while the simple universal f_esc model requires f_esc 6, and absorption of Lya photons by neutral hydrogen in intergalactic medium (IGM) is a reasonable interpretation for the discrepancy. This indicates that the IGM neutral fraction x_HI rapidly evolves from x_HI << 1 at z < 6 to a value of order unity at z ~ 6-7, which is broadly consistent with other observational constraints on the reionization history.We present a new theoretical model for the luminosity functions (LFs) of Lyα-emitting galaxies in the framework of hierarchical galaxy formation. We extend a semianalytical model of galaxy formation that reproduces a number of observations for local and high-z galaxies, without changing the original model parameters but with the introduction of physically motivated modeling to describe the escape fraction of Lyα photons from host galaxies (f). Although previous studies using hierarchical clustering models simply assumed a constant and universal value of f, we incorporate two new effects on f: extinction by interstellar dust and galaxy-scale outflows induced as star formation feedback. It is found that the new model nicely reproduces all the observed Lyα LFs of Lyα emitters (LAEs) with redshifts in the range z ~ 3-6. In particular, the surprisingly small evolution of the observed LAE Lyα LFs as compared with the dark halo mass function is naturally reproduced. Our model predicts that galaxies with strong outflows and f ~ 1 should dominate the observed LFs. This is also consistent with available observations, whereas the simple universal f model requires f 1 in order not to overproduce the brightest LAEs. On the other hand, we find that our model significantly overpredicts LAEs at z 6, and that absorption of Lyα photons by neutral hydrogen in the intergalactic medium (IGM) provides a reasonable interpretation of this discrepancy. This indicates that the IGM neutral fraction xH I rapidly evolves from xH I 1 at z 6 to a value of order unity at z ~ 6-7, which is broadly consistent with other observational constraints on the history of reionization.

Gravitational Waves from Supermassive Black Hole Coalescence in a Hierarchical Galaxy Formation Model

We investigate the expected gravitational wave emission from coalescing supermassive black hole (SMBH) binaries resulting from mergers of their host galaxies. When galaxies merge, the SMBHs in the host galaxies sink to the center of the new merged galaxy and form a binary system. We employ a semianalytic model of galaxy and quasar formation based on the hierarchical clustering scenario to estimate the amplitude of the expected stochastic gravitational wave background due to inspiraling SMBH binaries and bursts due to the SMBH binary coalescence events. We find that the characteristic strain amplitude of the background radiation is hc(f) ~ 10-16(f/1 ?Hz)-2/3 for f 1 ?Hz just below the detection limit from measurements of the pulsar timing provided that SMBHs coalesce simultaneously when host galaxies merge. The main contribution to the total strain amplitude of the background radiation comes from SMBH coalescence events at 0 < z < 1. We also find that a future space-based gravitational wave interferometer such as the planned Laser Interferometer Space Antenna might detect intense gravitational wave bursts associated with coalescence of SMBH binaries with total mass Mtot < 107 M? at z 2 at a rate ~1.0 yr-1. Our model predicts that burst signals with a larger amplitude hburst ~ 10-15 correspond to coalescence events of massive SMBH binary with total mass Mtot ~ 108 M? at low redshift (z 1) at a rate ~0.1 yr-1, whereas those with a smaller amplitude (hburst ~ 10-17) correspond to coalescence events of less massive SMBH binaries with total mass Mtot ~ 106 M? at high redshift (z 3).

Hierarchical Formation of Galaxies with Dynamical Response to Supernova-Induced Gas Removal

We reanalyze the formation and evolution of galaxies in the hierarchical clustering scenario. Using a semianalytic model (SAM) of galaxy formation described in this paper, which we hereafter call the Mitaka model, we extensively investigate the observed scaling relations of galaxies among photometric, kinematic, structural, and chemical characteristics. In such a scenario, spheroidal galaxies are assumed to be formed by a major merger and subsequent starburst, in contrast to the traditional scenario of monolithic cloud collapse. As a new ingredient of SAMs, we introduce the effects of dynamical response to supernova-induced gas removal on size and velocity dispersion, which play an important role in dwarf galaxy formation. In previous theoretical studies of dwarf galaxies based on the monolithic cloud collapse given by Yoshii & Arimoto and Dekel & Silk, the dynamical response was treated in the extremes of a purely baryonic cloud and a baryonic cloud fully supported by surrounding dark matter. To improve this simple treatment, in our previous paper we formulated the dynamical response in more realistic, intermediate situations between the above extremes. While the effects of dynamical response depend on the mass fraction of removed gas from a galaxy, the amount of the gas that remains just after major merger depends on the star formation history. A variety of star formation histories are generated through the Monte Carlo realization of merging histories of dark halos, and it is found that our SAM naturally makes a wide variety of dwarf galaxies and their dispersed characteristics as observed. It is also found that our result strongly depends on the adopted redshift dependence of thestar formation timescale, because it determines the gas fraction in high-redshift galaxies for which major mergers frequently occur. We test four star formation models. The first model has a constant timescale of star formation independent of redshift. The last model has a timescale proportional to the dynamical timescale of the galactic disk. The other models have timescales intermediates of these two. The last model fails to reproduce observations, because it predicts only a small amount of the leftover gas at major mergers, therefore giving too weak a dynamical response on size and velocity dispersion of dwarf spheroidals. The models, having a constant timescale of star formation or a timescale very weakly dependent on redshift, associated with our SAM, succeed in reproducing most observations from giant to dwarf galaxies, except that the adopted strong supernova feedback in this paper does not fully explain the color-magnitude relation under the cluster environment and the Tully-Fisher relation. A direction for overcoming this remaining problem is also discussed.

The origin of globular cluster systems from cosmological simulations

We investigate the structural, kinematical and chemical properties of globular cluster systems (GCSs) in galaxies of different Hubble types in a self-consistent manner based on high-resolution cosmological N-body simulations combined with semi-analytic models of galaxy and globular cluster (GC) formation. We focus on correlations between the physical properties of GCSs and those of their host galaxies for ∼10 5 simulated galaxies located at the centres of dark matter haloes (i.e. we do not consider satellite galaxies in subhaloes). Our principal results, which can be tested against observations, are as follows. The majority (∼90 per cent) of GCs currently in haloes are formed in low-mass galaxies at redshifts greater than 3 with mean formation redshifts of z = 5.7 (12.7Gyr ago) and 4.3 (12.3Gyr ago) for metal-poor GCs (MPCs) and metal-rich GCs (MRCs), respectively. About 52 per cent of galaxies with GCs show clear bimodality in their metallicity distribution functions, though less luminous galaxies with M B fainter than -17 are much less likely to show bimodality owing to little or no MRCs. The number fraction of MRCs does not depend on Hubble type but is generally smaller for less luminous galaxies. The specific frequencies (S N ) of GCSs are typically higher in ellipticals (S N ∼ 4.0) than in spirals (S N ∼ 1.8), and higher again (S N ∼ 5.0) for galaxies located at the centres of clusters of galaxies. The total number of GCs per unit halo mass does not depend strongly on M B or Hubble type of the host galaxy. The mean metallicities of MPCs and MRCs depend on M B such that they are higher in more luminous galaxies, though the dependence is significantly weaker for MPCs. The spatial distributions of MRCs are more compact than those of MPCs and we find that the half-number radii of MPCs (r e,mpc ) correlate with the halo masses (M h ) such that r e,mpc oc M h 0.18 . There is no significant difference in velocity dispersions between MPCs and MRCs. We qualitatively compare our results to observational data where possible. Finally, we discuss these results in the wider context of galaxy formation and evolution.

Environmental Effects on Evolution of Cluster Galaxies in a Λ-dominated Cold Dark Matter Universe

We investigate environmental effects on evolution of bright cluster galaxies (L > L*) in a ?-dominated cold dark matter universe using a combination of dissipationless N-body simulations and a semianalytic 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 (RP) from the 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 (SFRs), and morphologies of cluster galaxies. These processes are new ingredients in our model and have not been studied sufficiently. We find that the RPS is not important for colors and SFRs of galaxies in the cluster core if the star formation timescale 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 hierarchical clustering models based on 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. Without the minor burst, the RPS cannot increase the intermediate B/T population. On the other hand, 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. We conclude that these trends are inevitable when bulges are formed through mergers. We discuss whether our results conflict with observationally suggested NS0/NE evolution in clusters, which is a decreasing function of redshift.