arXiv subjects

Kentaro Nagamine

Publications and source records attributed to Kentaro Nagamine.

At least 127 records · Page 7Linked to original sources

arXiv2012

Duty Cycle and the Increasing Star Formation History of z>=6 Galaxies

We examine the duty cycle and the history of star formation (SFH) for high-redshift galaxies at z>=6 using cosmological hydrodynamic simulations. We find that, even though individual galaxies have bursty SFH, the averaged SFH between z~15 to z=6 can be characterized well by either an exponentially increasing functional form with characteristic time-scales of 70 Myr to 200 Myr for galaxies with stellar masses Ms~10^6 Msun to >10^10 Msun respectively, or by a simple power-law form which exhibits a similar mass dependent time-scales. Using the SFH of individual galaxies, we measure the duty cycle of star formation (DC_SFH); i.e., the fraction of time a galaxy of a particular mass spends above a star formation rate (SFR) threshold which would make it observable to the Hubble Space Telescope (HST) during a given epoch. We also examine the fraction of galaxies at a given redshift that are brighter than a rest-frame UV magnitude (Muv ~ -18), which is sufficient enough to make them observable (DC_Muv). We find that both DC_SFH and DC_Muv make a sharp transition from zero (for galaxies with Ms <= 10^7 Msun) to unity (for Ms > 10^9 Msun). The measured duty cycle is also manifested in the intrinsic scatter in the Ms-SFR relationship (~ 1 dex) and Ms-Muv relationship (ΔMuv ~ +-1 mag). We provide analytic fits to the DC as a function of Ms using a sigmoid function, which can be used to correct for catalogue incompleteness. We consider the effects of duty cycle to the observational estimate of galaxy stellar mass functions (GSMF) and the star formation rate density (SFRD), and find that it results in a much shallower low-mass end slopes of the GSMF and a reduction of >~ 70% of our intrinsic SFRD, making our simulation results more compatible with observational estimates.

astro-ph.CO↗

arXiv2012

Multiphase, non-spherical gas accretion onto a black hole

(Abridged) We investigate non-spherical behavior of gas accreting onto a central supermassive black hole performing simulations using the SPH code GADGET-3 including radiative cooling and heating by the central X-ray source. As found in earlier 1D studies, our 3D simulations show that the accretion mode depends on the X-ray luminosity (L_X) for a fixed density at infinity and accretion efficiency. In the low L_X limit, gas accretes in a stable, spherically symmetric fashion. In the high L_X limit, the inner gas is significantly heated up and expands, reducing the central mass inflow rate. The expanding gas can turn into a strong enough outflow capable of expelling most of the gas at larger radii. For some intermediate L_X, the accretion flow becomes unstable developing prominent non-spherical features, the key reason for which is thermal instability (TI) as shown by our analyses. Small perturbations of the initially spherically symmetric accretion flow that is heated by the intermediate L_X quickly grow to form cold and dense clumps surrounded by overheated low density regions. The cold clumps continue their inward motion forming filamentary structures; while the hot infalling gas slows down because of buoyancy and can even start outflowing through the channels in between the filaments. We found that the ratio between the mass inflow rates of the cold and hot gas is a dynamical quantity depending on several factors: time, spatial location, and L_X; and ranges between 0 and 4. We briefly discuss astrophysical implications of such TI-driven fragmentation of accreting gas on the formation of clouds in narrow and broad line regions of AGN, the formation of stars, and the observed variability of the AGN luminiosity.

astro-ph.CO↗

arXiv2012

Gas Accretion onto a Supermassive Black Hole: a step to model AGN feedback

We study the gas accretion onto a supermassive black hole (SMBH) using the 3D SPH code GADGET-3 on scales of 0.1-200 pc. First we test our code with spherically symmetric, adiabatic Bondi accretion problem. We find that our simulation can reproduce the expected Bondi accretion flow very well for a limited amount of time until the effect of outer boundary starts to be visible. We also find artificial heating of gas near the inner accretion boundary due to the artificial viscosity of SPH. Second, we implement radiative cooling and heating due to X-rays, and examine the impact of thermal feedback by the central X-ray source. The accretion flow roughly follows the Bondi solution for low central X-ray luminosities, however, the flow starts to exhibit non-spherical fragmentation due to thermal instability for a certain range of central L_X, and a strong overall outflow develops for greater L_X. The cold gas develops filamentary structures that fall into the central SMBH, whereas the hot gas tries to escape through the channels in-between the cold filaments. Such fragmentation of accreting gas can assist in the formation of clouds around AGN, induce star-formation, and contribute to the observed variability of narrow-line regions.

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arXiv2011

Steep Faint-end Slopes of Galaxy Mass and Luminosity Functions at z>=6 and the Implications for Reionisation

We present the results of a numerical study comparing photometric and physical properties of simulated z=6-9 galaxies to the observations taken by the WFC3 instrument aboard the Hubble Space Telescope. Using cosmological hydrodynamical simulations we find good agreement with observations in color-color space at all studied redshifts. We also find good agreement between observations and our Schechter luminosity function fit in the observable range, Muv<= -18, provided that a moderate dust extinction effect exists for massive galaxies. However beyond what currently can be observed, simulations predict a very large number of low-mass galaxies and evolving steep faint-end slopes from alpha_L = -2.15 at z=6 to alpha_L = -2.64 at z=9, with a dependence of |alpha_L| \propto (1+z)^0.59. During the same epoch, the normalization phi* increases and the characteristic magnitude Muv* becomes moderately brighter with decreasing redshift. We find similar trends for galaxy stellar mass function with evolving low-mass end slope from alpha_M = - 2.26 at z=6 to alpha_M = -2.87 at z=9, with a dependence of |alpha_M| \propto (1+z)^0.65. Together with our recent result on the high escape fraction of ionizing photons for low-mass galaxies, our results suggest that the low-mass galaxies are important contributor of ionizing photons for the reionisation of the Universe at z>=6.

astro-ph.CO↗

arXiv2011

Pairwise Velocities of Dark Matter Halos: a Test for the Lambda Cold Dark Matter Model using the Bullet Cluster

The existence of 1E0657-56 poses a challenge to the concordance Lambda cold dark matter model. Here we investigate the velocity distribution of dark matter halo pairs in large N-body simulations with differing box sizes (250Mpc/h-2 Gpc/h) and resolutions. We examine statistics such as the halo masses, pairwise halo velocities (v_{12}), and pair separation distances. We then compare our results to the initial conditions (ICs) required to reproduce the observational properties of 1E0657-56 in non-cosmological hydrodynamical simulations. We find that the high velocity tail of the v_{12} distribution extends to greater velocities as we increase the box size. We also find that the number of high-v_{12} pairs increases as we increase the particle count and resolution with a fixed box size, however, this increase is mostly due to lower mass halos which do not match the observed masses of 1E0657-56. We find that the redshift evolution is not very strong for the v_{12} distribution function between z=0.0 and z~0.5. We identify some pairs whose v_{12} resemble the required ICs, however, even the best candidates have either wrong halo mass ratios, or too large separations. Our simulations suggest that it is very difficult to produce such ICs at z=0.0, 0.296,& 0.489 in comoving volumes as large as (2Gpc/h)^3. Based on the extrapolation of our cumulative v_{12} function, we find that one needs a simulation with a comoving box size of (4.48Gpc/h)^3 and 2240^3 DM particles in order to produce at least one pair of halos that resembles the required v_{12} and observed masses of 1E0657-56. We find that the probability of finding a halo pair with v_{12}>=3000km/s and masses >=10^{14}Msun to be 2.76x10^{-8} at z=0.489. We conclude that either 1E0657-56 is incompatible with the concordance LCDM universe, or the ICs suggested by the non-cosmological simulations must be revised to give a lower value of v_{12}.

astro-ph.CO↗

arXiv2011

On the inconsistency between the estimates of cosmic star formation rate and stellar mass density of high redshift galaxies

There are mainly two different approaches to measure the cosmic star formation history: direct star formation rate density (SFRD) and stellar mass density rhostar as functions of redshift. Compilations of current observations seem to show a disparity in the two quantities, in the sense that the integral of SFRD is higher than the observed rhostar (after considering gas recycling). Using cosmological smoothed particle hydrodynamics simulations based on the concordance Lambda cold dark matter model, we show that the two quantities become more consistent with each other when we consider the observed galaxy mass limit. The comparison between simulations and (dust corrected) observed cosmic SFRD shows a good agreement, while the observed rhostar is significantly lower than the simulation results. This can be reconciled if the current high-$z$ galaxy surveys are missing faint low-mass galaxies due to their flux limit. Our simulated GSMFs have steep low-mass end slopes of alpha < -2 at z>3, and when these numerous low-mass galaxies are included, the total rhostar matches with the integral of SFRD.

astro-ph.CO↗

arXiv2011

SPH Simulations of Black Hole Accretion: A Step to Model Black Hole Feedback in Galaxies

(Abridged) We test how accurately the smoothed particle hydrodynamics (SPH) numerical technique can follow spherically-symmetric Bondi accretion. Using the 3D SPH code GADGET-3, we perform simulations of gas accretion onto a central supermassive black hole (SMBH) of mass 10^8 M_sun within the radial range of 0.1 - 200 pc. We carry out simulations without and with radiative heating by a central X-ray corona and radiative cooling. For an adiabatic case, the radial profiles of hydrodynamical properties match the Bondi solution, except near the inner and outer radius of the computational domain. We find that adiabatic Bondi accretion can be reproduced for durations of a few dynamical times at the Bondi radius, and for longer times if the outer radius is increased. With radiative heating and cooling included, the spherically accreting gas takes a longer time to reach a steady-state than the adiabatic Bondi accretion runs, and in some cases does not reach a steady-state even within several hundred dynamical times. We find that artificial viscosity in the GADGET code causes excessive heating near the inner radius, making the thermal properties of the gas inconsistent with a physical solution. This overheating occurs typically only in the supersonic part of the flow, so that it does not affect the mass accretion rate. We see that increasing the X-ray luminosity produces a lower central mass inflow rate, implying that feedback due to radiative heating is operational in our simulations. With a sufficiently high X-ray luminosity, the inflowing gas is radiatively heated up, and an outflow develops. We conclude that the SPH simulations can capture the gas dynamics needed to study radiative feedback provided artificial viscosity alters only highly supersonic part of the inflow.

astro-ph.CO↗

arXiv2011

Gamma-ray burst rate: high-redshift excess and its possible origins

Prompted by various analyses of long (Type II) GRB rates and their relationship to the cosmic star-formation history, metallicity and luminosity function evolution, we systematically analyze these effects with a Monte Carlo code. We test various cosmic star-formation history models including analytical and empirical models as well as those derived from cosmological simulations. We also explore expressions for metallicity enhancement of the GRB rate with redshift, as presented in the literature, and discuss improvements to these analytic expressions from the point of view of galactic evolution. These are also compared to cosmological simulations on metal enrichment. Additionally we explore possible evolutionary effects of the GRB rate and luminosity function with redshift. The simulated results are tested with the observed Swift sample including the L, z, and peak flux (log N-log P) distributions. The observational data imply that an increase in the GRB rate is necessary to account for the observations at high redshift, although the form of this enhancement is unclear. A rate increase due to lower metallicity at higher redshift may not be the singular cause and is subject to a variety of uncertainties. Alternatively, evolution of the GRB luminosity function break with redshift shows promise as a possible alternative.

astro-ph.HE↗

arXiv2010

Effects of Ultraviolet Background and Local Stellar Radiation on the H_I Column Density Distribution

We study the impact of ultraviolet background (UVB) radiation field and the local stellar radiation on the H_I column density distribution f(N_HI) of damped Ly-alpha systems (DLAs) and sub-DLAs at z=3 using cosmological smoothed particle hydrodynamics simulations. We find that, in the previous simulations with an optically thin approximation, the UVB was sinking into the H_I cloud too deeply, and therefore we underestimated the f(N_HI) at 19 < log(N_HI) < 21.2 compared to the observations. If the UVB is shut off in the high-density regions with n_gas > 6 x 10^{-3} cm^{-3}, then we reproduce the observed f(N_HI) at z=3 very well. We also investigate the effect of local stellar radiation by post-processing our simulation with a radiative transfer code, and find that the local stellar radiation does not change the f(N_HI) very much. Our results show that the shape of f(N_HI) is determined primarily by the UVB with a much weaker effect by the local stellar radiation and that the optically thin approximation often used in cosmological simulation is inadequate to properly treat the ionization structure of neutral gas in and out of DLAs. Our result also indicates that the DLA gas is closely related to the transition region from optically-thick neutral gas to optically-thin ionized gas within dark matter halos.

astro-ph.CO↗

arXiv2010

Luminosity Distribution of Gamma-Ray Burst Host Galaxies at redshift z=1 in Cosmological Smoothed Particle Hydrodynamic Simulations: Implications for the Metallicity Dependence of GRBs

We study the relationship between the metallicity of gamma-ray burst (GRB) progenitors and the probability distribution function (PDF) of GRB host galaxies as a function of luminosity using cosmological hydrodynamic simulations of galaxy formation. We impose a maximum limit to the gas metallicity in which GRBs can occur, and examine how the predicted luminosity PDF of GRB host galaxies changes in the simulation. We perform the Kolmogorov-Smirnov test, and show that the result from our simulation agrees with the observed luminosity PDF of core-collapse supernovae (SNe) host galaxies when we assume that the core-collapse SNe trace star formation. When we assume that GRBs occur only in a low-metallicity environment with $Z\lesssim 0.1 \Zsun$, GRBs occur in lower luminosity galaxies, and the simulated luminosity PDF becomes quantitatively consistent with the observed luminosity PDF. The observational bias against the host galaxies of optically dark GRBs owing to dust extinction may be another reason for the lower luminosities of GRB host galaxies, but the observed luminosity PDF of GRB host galaxies cannot be reproduced solely by the dust bias in our simulation.

astro-ph.CO↗

arXiv2010

Escape fraction of ionizing photons from high-redshift galaxies in cosmological SPH simulations

Combing the three-dimensional radiative transfer (RT) calculation and cosmological SPH simulations, we study the escape fraction of ionizing photons (f_esc) of high-redshift galaxies at z=3-6. Our simulations cover the halo mass range of M_h = 10^9 - 10^12 M_sun. We postprocess several hundred simulated galaxies with the Authentic Radiative Transfer (ART) code to study the halo mass dependence of f_esc. In this paper, we restrict ourselves to the transfer of stellar radiation from local stellar population in each dark matter halo. We find that the average f_esc steeply decreases as the halo mass increases, with a large scatter for the lower mass haloes. The low mass haloes with M_h ~ 10^9 M_sun have large values of f_esc (with an average of ~ 0.4), whereas the massive haloes with M_h ~ 10^11 M_sun show small values of f_esc (with an average of ~ 0.07). This is because in our simulations, the massive haloes show more clumpy structure in gas distribution, and star-forming regions are embedded inside these clumps, making it more difficult for the ionizing photons to escape. On the other hand, in low mass haloes, there are often conical regions of highly ionized gas due to the shifted location of young star clusters from the center of dark matter halo, which allows the ionizing photons to escape more easily than in the high-mass haloes. By counting the number of escaped ionizing photons, we show that the star-forming galaxies can ionize the intergalactic medium at z=3-6. The main contributor to the ionizing photons is the haloes with M_h < 10^10 M_sun owing to their high f_esc. The large dispersion in f_esc suggests that there may be various sizes of H{\sc ii} bubbles around the haloes even with the same mass in the early stages of reionization. We also examine the effect of UV background radiation field on f_esc using simple, four different treatment of UV background.

astro-ph.CO↗

arXiv2010

Lyman-alpha Emitters and Lyman-break Galaxies at z=3-6 in Cosmological SPH Simulations

We study the properties of Lyman-alpha emitters (LAEs) and Lyman-break galaxies (LBGs) at z=3-6 using cosmological SPH simulations. We investigate two simple scenarios for explaining the observed Ly-a and rest-frame UV luminosity functions (LFs) of LAEs: (i) the "escape fraction" scenario, in which the "effective" escape fraction (including the IGM attenuation) of Ly-a photons is f_Lya ~0.1 (0.15) at z=3 (6), and (ii) the "stochastic" scenario, in which the fraction of LAEs that are turned on at z=3 (6) is \Cstoc ~0.07 (0.2) after correcting for the IGM attenuation. Our comparisons with a number of different observations suggest that the stochastic scenario is preferred over the escape fraction scenario. We find that the mean values of stellar mass, metallicity and black hole mass hosted by LAEs are all smaller in the stochastic scenario than in the escape fraction scenario. In our simulations, the galaxy stellar mass function evolves rapidly, as expected in hierarchical structure formation. However, its evolution is largely compensated by a beginning decline in the specific star formation rate, resulting in little evolution of the rest-frame UV LF from z=6 to 3. The rest-frame UV LF of both LAEs and LBGs at z=3 & 6 can be described well by the stochastic scenario provided the extinction is moderate, E(B-V) ~0.15, for both populations, although our simulation might be overpredicting the number of bright LBGs at z=6. We also discuss the correlation function and bias of LAEs. The Ly-a LFs at z=6 in a field-of-view of 0.2 deg^2 show a significantly larger scatter owing to cosmic variance relative to that in a 1 deg^2 field, and the scatter seen in the current observational estimates of the Ly-a LF can be accounted for by cosmic variance.

astro-ph↗

arXiv2010

Cross-Correlation between Damped Ly-alpha Systems and Ly-break Galaxies in Cosmological SPH Simulations

We calculate the cross-correlation function (CCF) between damped Ly-a systems (DLAs) and Lyman break galaxies (LBGs) using cosmological hydrodynamic simulations at z=3. We compute the CCF with two different methods. First, we assume that there is one DLA in each dark matter halo if its DLA cross section is non-zero. In our second approach we weight the pair-count by the DLA cross section of each halo, yielding a cross-section-weighted CCF. We also compute the angular CCF for direct comparison with observations. Finally, we calculate the auto-correlation functions of LBGs and DLAs, and their bias against the dark matter distribution. For these different approaches, we consistently find that there is good agreement between our simulations and observational measurements by Cooke et al. and Adelberger et al. Our results thus confirm that the spatial distribution of LBGs and DLAs can be well described within the framework of the concordance Lambda CDM model. We find that the correlation strengths of LBGs and DLAs are consistent with the actual observations, and in the case of LBGs it is higher than would be predicted by low-mass galaxy merger models.

astro-ph↗

arXiv2010

Multicomponent and Variable Velocity Galactic Outflow in Cosmological Hydrodynamic Simulations

We develop a new ``Multicomponent and Variable Velocity'' (MVV) galactic outflow model for cosmological smoothed particle hydrodynamic (SPH) simulations. The MVV wind model reflects the fact that the wind material can arise from different phases in the interstellar medium (ISM), and the mass-loading factor in the MVV model is a function of galaxy stellar mass. We find that the simulation with the MVV outflow has the following characteristics: (i) the intergalactic medium (IGM) is hardly heated up, and the mean IGM temperature is almost the same as in the no-wind run; (ii) it has lower cosmic star formation rates (SFRs) compared to the no-wind run, but higher SFRs than the constant velocity wind run; (iii) it roughly agrees with the observed IGM metallicity, and roughly follows the observed evolution of Omega(Civ); (iv) the lower mass galaxies have larger mass-loading factors, and the low-mass end of galaxy stellar mass function is flatter than in the previous simulations. Therefore, the MVV outflow model mildly alleviates the problem of too steep galaxy stellar mass function seen in the previous SPH simulations. In summary, the new MVV outflow model shows reasonable agreement with observations, and gives better results than the constant velocity wind model.

astro-ph.CO↗

arXiv2010

Infrared Emission from High-Redshift Galaxies in Cosmological SPH Simulations

We compute the infrared (IR) emission from high-redshift galaxies in cosmological smoothed particle hydrodynamics simulations by coupling the output of the simulation with the population synthesis code `GRASIL' by Silva et al. Based on the stellar mass, metallicity and formation time of each star particle, we estimate the full spectral energy distribution of each star particle from ultraviolet to IR, and compute the luminosity function of simulated galaxies in the Spitzer broadband filters for direct comparison with the available Spitzer observations.

astro-ph.CO↗

arXiv2010

Effects of cosmological parameters and star formation models on the cosmic star formation history in LambdaCDM cosmological simulations

We investigate the effects of the change of cosmological parameters and star formation (SF) models on the cosmic SF history using cosmological smoothed particle hydrodynamics (SPH) simulations based on the cold dark matter (CDM) model. We vary the cosmological parameters within 1-sigma error from the WMAP best-fit parameters, and find that such changes in cosmological parameters mostly affect the amplitude of the cosmic SF history. At high redshift (hereafter high-z), the star formation rate (SFR) is sensitive to the cosmological parameters that control the small-scale power of the primordial power spectrum, while the cosmic matter content becomes important at lower redshifts. We also test two new SF models: 1) the `Pressure' model based on the work by Schaye & Dalla Vecchia (2008), and 2) the `Blitz' model that takes the effect of molecular hydrogen formation into account, based on the work by Blitz & Rosolowsky (2006). Compared to the previous conventional SF model, the Pressure model reduces the SFR in low-density regions and shows better agreement with the observations of the Kennicutt-Schmidt law. This model also suppresses the early star formation and shifts the peak of the cosmic SF history toward lower redshift, more consistently with the recent observational estimates of cosmic SFR density. The simulations with the new SF model also predict lower global stellar mass densities at high-z, larger populations of low-mass galaxies and a higher gas fraction in high-z galaxies. Our results suggest that there is room left in the model uncertainties to reconcile the discrepancy that was found between the theory and observations of cosmic SF history and stellar mass density. Nevertheless, our simulations still predict higher stellar mass densities than most of the observational estimates.

astro-ph.CO↗

arXiv2010

Large-Scale Outflows from AGN: a link between central black holes and galaxies

We summarize the results from numerical simulations of mass outflows from AGN. We focus on simulations of outflows driven by radiation from large-scale inflows. We discuss the properties of these outflows in the context of the so-called AGN feedback problem. Our main conclusion is that this type of outflows are efficient in removing matter but inefficient in removing energy.

astro-ph.GA↗

arXiv2009

On the Feedback Efficiency of Active Galactic Nuclei

(Abridged) We measure and analyze the energy, momentum, and mass feedback efficiencies due to radiation from AGN in relatively large scale outflows. Our measurements are based on the axisymmetric and time-dependent radiation-hydrodynamical simulations recently presented in Kurosawa & Proga. These simulations follow dynamics of gas under the influences of the AGN radiation and the gravity of the central 10^8 Msun black hole on scales from ~0.01 to ~10pc. We compare our Mdot_a-rho_o relation with that predicted by the Bondi accretion model. For high luminosities comparable to the Eddington limit, the power-law fit (Mdot_a ~ rho_o^q) to our models yields q=~0.5 instead of q=1.0 which is predicted by the Bondi model. This difference is caused by the outflows which are important for the overall mass budget at high luminosities. The maximum momentum and mass feedback efficiencies found in our models are ~10^-2 and ~10^-1, respectively. However, the outflows are much less important energetically: their thermal and kinetic powers in units of the radiative luminosity are ~10^-5 and ~10^-4, respectively. The efficiencies do not increase monotonically with the accretion luminosity but rather peak around the Eddington limit beyond which a steady state disk-wind-like solution exists. Our energy feedback efficiencies are significantly lower than 0.05, which is required in some cosmological and galaxy merger simulations. The low feedback efficiencies found here could have significant implications on the mass growth of super massive black holes in the early universe. We stress however that we have not considered the innermost parts of the accretion and outflow where radiation and matter interact most strongly. The feedback from this region could have efficiencies significantly above the low values found here.

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