arXiv subjects

Hajime Susa

Publications and source records attributed to Hajime Susa.

48 records · Page 3Linked to original sources

arXiv2003

Quasar Mesolensing - Direct Probe to Substructures around Galaxies -

Recently, ``CDM crisis'' is under discussion. The main point of this crisis is that number of substructures presented by cosmological N-body simulations based on CDM scenario for structure formation is much larger than observed substructures. Therefore, it is crucial for this crisis to discriminate whether expected number of CDM substructures really exist but non-luminous or do not exist. In this paper, we present a new idea to detect such invisible substructures by utilizing a gravitational lensing. Here, we consider quasars that are gravitationally lensed by a foreground galaxy. A substructure around the lensing galaxy may superposed on one of the lensed images of such quasars. In this situation, additional image splitting should occur in the image behind the substructure, and further multiple images are created. This is ``quasar mesolensing''. We estimate separation and time delay between further multiple images due to quasar mesolensing. The expected value is $1 \sim 30$ milli-arcsecond for the separation and future fine resolution imaging enable us to find invisible substructures, and is $1 \sim 10^3$ second for the time delay and high-speed monitoring of such quasar will be able to find ``echo''-like variation due to quasar mesolensing in intrinsic variability of the quasar. Furthermore, we evaluate that the optical depth for the quasar mesolensing is $\sim 0.1$. Consequently, if we monitor a few multiple quasars, we can find ``echo''-like variation in one of the images after intrinsic flux variations of quasars.

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arXiv2003

Formation of Dwarf Galaxies during the Cosmic Reionization

We reanalyze the photoevaporation problem of subgalactic objects irradiated by UVB radiation in a reionized universe. For the purpose, we perform three-dimensional radiation smoothed-particle-hydrodynamics calculations, where the radiative transfer is solved by a direct method and also the non-equilibrium chemistry of primordial gas including H$_2$ molecules is incorporated. Attention is concentrated on radiative transfer effects against UVB for the formation of subgalactic objects with $T_{\rm vir}\la 10^4$K. We consider the reionization model with $z_{\rm reion} \approx 7$ and also the earlier reionization model ($z_{\rm reion} \approx 17$) inferred by the WMAP. We find that the star formation is suppressed appreciably by UVB, but baryons at high-density peaks are self-shielded even during the reionization, forming some amount of stars eventually. In that sense, the photoevaporation for subgalactic systems is not so perfect as argued by one-dimensional spherical calculations. The final stellar fraction depends on the collapse epoch and the mass of system, but almost regardless of the reionization epoch. For instance, a few tenths of formed stars are born after the cosmic reionization in $z_{\rm reion} \approx 7$ model, while more than 90% stars are born after the reionization in the WMAP reionization model. Thus, effects of UVB feedback on the substructure problem with CDM scenario should be evaluated with careful treatment of the radiative transfer. The star clusters formed at high-density peaks coalesce with each other in a dissipationless fashion in a dark matter potential, resultantly forming a spheroidal system. As a result, these low-mass galaxies have large mass-to-light ratios such as observed in dwarf spheroidals in the Local Group.

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arXiv2000

UV Background-Induced Bifurcation of the Galactic Morphology

Based upon a novel paradigm of the galaxy formation under UV background, the evolutionary bifurcation of pregalactic clouds is confronted with observations on elliptical and spiral galaxies. The theory predicts that the dichotomy between the dissipational and dissipationless galaxy formation stems from the degree of self-shielding from the UV background radiation. This is demonstrated on a bifurcation diagram of collapse epochs versus masses of pregalactic clouds. Using the observed properties, the collapse epochs are assessed for each type of galaxies with attentive mass estimation. By the direct comparison of the theory with the observations, it turns out that the theoretical bifurcation branch successfully discriminates between elliptical and spiral galaxies. This suggests that the UV background radiation could play a profound role for the differentiation of the galactic morphology into the Hubble sequence.

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arXiv2000

Collapse of Low Mass Clouds in the Presence of UV Radiation Field

The collapse of marginally Jeans unstable primordial gas clouds in the presence of UV radiation field is discussed. Assuming that the dynamical collapse proceeds approximately in an isothermal self-similar fashion, we investigate the thermal evolution of collapsing central core until H$_2$ cooling dominates photoheating and the temperature drops to below $10^4$K. Consequently, the mass of the cooled core is evaluated as $M_{cool}=3.6\times 10^6 M_\odot(I_{21}/1)^{-0.32}$. This scale depends only on the incident UV intensity, and provides a lower limit to the mass of collapsed objects in the UV radiation field.

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arXiv2000

Formation of Primordial Galaxies under UV background Radiation

The pancake collapse of pregalactic clouds under UV background radiation is explored with a one-dimensional sheet model. Here, attention is concentrated on elucidating the basic physics on the thermal evolution of pregalactic clouds exposed to diffuse UV radiation. So, we treat accurately the radiation transfer for the ionizing photons, with solving chemical reactions regarding hydrogen molecules as well as atoms. The self-shielding against UV radiation by H$_2$ Lyman-Werner bands, which regulates the photo-dissociation of hydrogen molecules, is also taken into account. As a result, it is found that when the UV background radiation is at a level of $10^{-22} (ν/ν_L)^{-1} {\rm erg~ s^{-1} cm^{-2} Hz^{-1} str^{-1}}$, the cloud evolution bifurcates with a critical mass as $M_{\rm SB} = 2.2\times 10^{11} M_\odot [(1+z_c)/5]^{-4.2}$, where $z_c$ is the final collapse epoch. A cloud more massive than $M_{\rm SB}$ cools below $5\times 10^3$K due to H$_2$ line emission at the pancake collapse and would undergo the initial starburst. The pancake possibly evolves into a virialized system in a dissipationless fashion. Consequently, this leads to the dissipationless galaxy formation at $3\lsim z_c \lsim 10$. A cloud less massive than $M_{\rm SB}$ cannot cool by H$_2$ emission shortly after the pancake collapse, but could cool in the course of shrinking to the rotation barrier. This is likely to lead to the dissipational galaxy formation at relatively low redshifts as $0\lsim z_c \lsim 4$. The present results provide a solid physical mechanism which controls the star formation efficiency in the pregalactic clouds. In the context of a standard CDM cosmology, $M_{\rm SB}$ lies between 1$σ$ and 2$σ$ density fluctuations.

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arXiv1999

Formation and Disruption of Cosmological Low Mass Objects

We investigate the evolution of cosmological low mass (low virial temperature) objects and the formation of the first luminous objects. First, the `cooling diagram' for low mass objects is shown. We assess the cooling rate taking into account the contribution of H_2, which is not in chemical equilibrium generally, with a simple argument of time scales. The reaction rates and the cooling rate of H_2 are taken from the recent results by Galli & Palla (1998). Using this cooling diagram, we also estimate the formation condition of luminous objects taking into account the supernova (SN) disruption of virialized clouds. We find that the mass of the first luminous object is several times 10^7 solar mass, because smaller objects may be disrupted by the SNe before they become luminous. Metal pollution of low mass (Ly-alpha) clouds also discussed. The resultant metallicity of the clouds is about 1/1000 of the solar metallicity.

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arXiv1998

Thermal and Dynamical Evolution of Primordial Gas Clouds - On the Formation of First Luminous Objects -

We investigate the thermal and dynamical evolution of primordial gas clouds in the universe after decoupling. Comparing the time-scale of dynamical evolution with that of fragmentation, we can estimate the typical fragmentation scale. We propose the following scenario of the formation process of first luminous objects consisting of large number stars. First, by pancake collapse of the overdensity regions in the expanding universe or collision between clouds in potential wells, quasi-plane shocks form. If the shock-heated temperature is higher than about $10^4$ K, the postshock gas cools down to several hundred K by \Hbun line cooling, and the shock-compressed layer fragments into filamentary clouds. The filamentary cloud collapses dynamically once more and fragments into cloud cores. Finally, a primordial star forms in a cloud core. We show that the minimum mass of the first star is essentially determined by the Chandrasekhar mass. Also, we investigate the dynamical collapse of cloud cores by numerical simulation and show that the evolution paths of the central regions of the cores depend only very weakly on the total core mass. After mass accretion, a massive star may be formed in a core, since the estimated mass accretion rate is very large. In such a case, it may be possible for many massive stars form almost simultaneously. Then the clouds can be luminous objects. On the other hand, if the shock-heated temperature is lower, effective star formation is delayed significantly.

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arXiv1998

Evolution of Primordial Protostellar Clouds --- Quasi-Static Analysis ---

The contraction processes of metal-free molecular clouds of starlike mass (or cloud cores) are investigated. We calculate radiative transfer of the H_2 lines and examine quasi-static contraction with radiative cooling. Comparing two time-scales, the free-fall time t_ff and the time-scale of quasi-static contraction t_qsc (nearly equal to t_cool, the cooling time) of these cores, we find that the ratio of the two time-scales t_ff/t_qsc, i.e., the efficiency of cooling, becomes larger with contraction even under the existence of cold and opaque envelopes. In particular, for fragments of primordial filamentary clouds, for which t_ff is nearly equal to t_qsc at the fragmentation epoch, they collapse dynamically in the free-fall time-scale. This efficiency of cooling is unique to line cooling.

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arXiv1998

The Thermal Evolution of the Postshock Layer in Pregalactic Clouds

We re-examine the thermal evolution of the postshock layer in primordial gas clouds. Comparing the time scales, we find that the evolutionary paths of postshock regions in primordial gas clouds can be basically understood in terms of the diagram drawn in the ionization degree vs temperature plane. The results obtained from the diagram are independent of the density in the case that we do not include photodissociation and photoionization. We also argue that the diagram is not only relevant to the case of the steady postshock flow, but also to the isochorically cooling gas.

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arXiv1996

Fragmentation of the Primordial Gas Clouds and the Lower Limit on the Mass of the First Stars

We discuss the fragmentation of primordial gas clouds in the universe after decoupling. Comparing the time scale of collapse with that of fragmentation, we obtain the typical mass of a fragment both numerically and analytically. It is shown that the estimated mass gives the minimum mass of a fragment which is formed from the primordial gas cloud and is essentially determined by the Chandrasekhar mass.

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arXiv1996

The fragment mass scale of the primordial gas clouds I

We investigated the thermal evolution of non-spherical primordial gas clouds of mass $ M=10^6M_\odot $. We studied two cases: 1) prolate and oblate clouds without angular momentum, 2) initially spherical, oblate and prolate clouds with angular momentum. In the spherical case, the magnitude of the angular momentum is the key quantity which determines the fragment mass. The fragment mass is found to be $M \gsim 100 M_\odot$ for $10^{-2} \lsim λ\lsim 1$, where $λ$ is the cosmological spin parameter. For the oblate shape of initial gas clouds, the angular momentum is almost always not important, In the case of prolate clouds with some angular momentum, collapse proceeds in the same way as spherical case, and the bounce occurs to form a disk due to the angular momentum. In any case, the estimated ``fragment mass'', must be above $100 M_\odot$ for the primordial gas clouds with typical angular momentum and typical oblateness or prolateness.

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arXiv1994

Probability Distribution of Primordial Angular Momentum and Formation of Massive Black Holes

We consider the joint probability distribution function for the mass contrast and angular momentum of over-density regions on the proto- galactic scale and investigate the formation of massive black holes at redshift $z\gsim10$. We estimate the growth rate of the angular momentum by the linear perturbation theory and the decay rate by the Compton drag and apply the Press-Schechter theory to obtain the formation rate of massive black holes, assuming the full reionization of the universe at $z=z_{ion}\gg 10$. We find the correlation between the mass contrast and angular momentum vanishes in the linear theory. However, application of the Press-Schechter theory introduces a correlation between the mass contrast and angular momentum of bound objects. Using thus obtained probability distribution, we calculate the mass fraction of black holes with $M\sim10^6-10^8M_{\odot}$ in the universe. We find that it crucially depends on the reionization epoch $z_{ion}$. Specifically, for the standard CDM power spectrum with the COBE normalization, the condition $z_{ion}\gsim 500$ must be satisfied to reproduce the observed number density of QSOs.

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