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Search Results: 1 - 10 of 357 matches for " Tomonori Totani "
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Size Evolution of Early-Type Galaxies and Massive Compact Objects as the Dark Matter
Tomonori Totani
Physics , 2009, DOI: 10.1093/pasj/62.1.L1
Abstract: The dramatic size evolution of early-type galaxies from z ~ 2 to 0 poses a new challenge in the theory of galaxy formation, which may not be explained by the standard picture. It is shown here that the size evolution can be explained if the non-baryonic cold dark matter is composed of compact objects having a mass scale of ~10^5 M_sun. This form of dark matter is consistent with or only weakly constrained by the currently available observations. The kinetic energy of the dark compact objects is transferred to stars by dynamical friction, and stars around the effective radius are pushed out to larger radii, resulting in a pure size evolution. This scenario has several good properties to explain the observations, including the ubiquitous nature of size evolution and faster disappearance of higher density galaxies.
Galaxy Formation by Galactic Magnetic Fields
Tomonori Totani
Physics , 1999, DOI: 10.1086/312048
Abstract: Galaxies exhibit a sequence of various morphological types, i.e., the Hubble sequence, and they are basically composed of spheroidal components (elliptical galaxies and bulges in spiral galaxies) and disks. It is known that spheroidal components are found only in relatively massive galaxies with M=10^{10-12} M_sun, and all stellar populations in them are very old, but there is no clear explanation for these facts. Here we present a speculative scenario for the origin of the Hubble sequence, in which magnetic fields ubiquitously seen in galaxies have played a crucial role. We first start from a strange observational fact that magnetic field strengths observed in spiral galaxies sharply concentrate at a few microgauss, for a wide range of galaxy luminosity and types. We then argue that this fact and the observed correlation between star formation activity and magnetic field strength in spiral galaxies suggest that spheroidal galaxies have formed by starbursts induced by strong magnetic fields. Then we show that this idea naturally leads to the formation of spheroidal systems only in massive and high-redshift objects in hierarchically clustering universe, giving a simple explanation for various observations.
TeV Burst of Gamma-Ray Bursts and Ultra High Energy Cosmic Rays
Tomonori Totani
Physics , 1998, DOI: 10.1086/311772
Abstract: Some recent experiments detecting very high energy (VHE) gamma-rays above 10-20 TeV independently reported VHE bursts for some of bright gamma-ray bursts (GRBs). If these signals are truly from GRBs, these GRBs must emit a much larger amount of energy as VHE gamma-rays than in the ordinary photon energy range of GRBs (keV-MeV). We show that such extreme phenomena can be reasonably explained by synchrotron radiation of protons accelerated to \sim 10^{20-21} eV, which has been predicted by Totani (1998a). Protons seem to carry about (m_p/m_e) times larger energy than electrons, and hence the total energy liberated by one GRB becomes as large as \sim 10^{56} (\Delta \Omega / 4 \pi) ergs. Therefore a strong beaming of GRB emission is highly likely. Extension of the VHE spectrum beyond 20 TeV gives a nearly model-independent lower limit of the Lorentz factor of GRBs, as $\gamma \gtilde 500$. Furthermore, our model gives the correct energy range and time variability of ordinary keV-MeV gamma-rays of GRBs by synchrotron radiation of electrons. Therefore the VHE bursts of GRBs strongly support the hypothesis that ultra high energy cosmic rays observed on the Earth are produced by GRBs.
Gamma-Ray Bursts, Ultra High Energy Cosmic Rays, and Cosmic Gamma-Ray Background
Tomonori Totani
Physics , 1998, DOI: 10.1016/S0927-6505(99)00069-9
Abstract: We argue that gamma-ray bursts (GRBs) may be the origin of the cosmic gamma-ray background radiation observed in GeV range. It has theoretically been discussed that protons may carry a much larger amount of energy than electrons in GRBs, and this large energy can be radiated in TeV range by synchrotron radiation of ultra-high-energy protons (\sim 10^{20} eV). The possible detection of GRBs above 10 TeV suggested by the Tibet and HEGRA groups also supports this idea. If this is the case, most of TeV gamma-rays from GRBs are absorbed in intergalactic fields and eventually form GeV gamma-ray background, whose flux is in good agreement with the recent observation.
Luminosity Density Evolution in the Universe and Cosmological Parameters
Tomonori Totani
Physics , 1997,
Abstract: Star formation history in galaxies is strongly correlated to their present-day colors and the Hubble sequence can be considered as a sequence of different star formation history. Therefore we can model the cosmic star formation history based on the colors of local galaxies, and comparison to direct observations of luminosity density evolution at high redshift gives a new test for the cosmological parameters which is insensitive to merger history of galaxies. The luminosity density evolution in $0$ 0.53 at 95%CL) is strongly favored. The cosmic star formation rate (SFR) at $z>2$ is also compared to the latest data of the Hubble Deep Field including new data which were not incorporated in the previous work of Totani, Yoshii, & Sato (1997), and our model of the luminosity density of spiral galaxies taking account of gas infall is consistent with the observations. Starbursts in elliptical galaxies, which are expected from the galactic wind model, however overproduce SFRs and hence they should be formed at $z \gtilde 5$ or their UV emission has to be hidden by dust extinction. The amound of metals in galactic winds and escaping ionizing photons are enough to contaminate the Ly$\alpha$ forests or to reionize the universe.
Cosmological Gamma-Ray Bursts and Evolution of Galaxies
Tomonori Totani
Physics , 1997, DOI: 10.1086/310853
Abstract: Evolution of the rate density of cosmological gamma-ray bursts (GRBs) is calculated and compared to the BATSE brightness distribution in the context of binary neutron-star mergers as the source of GRBs, taking account of the realistic star formation history in the universe and evolution of compact binary systems. We tried two models of the evolution of cosmic star formation rate (SFR): one is based on recent observations of SFRs at high redshifts, while the other is based on a galaxy evolution model of stellar population synthesis that reproduces the present-day colors of galaxies. It is shown that the binary merger scenario of GRBs naturally results in the comoving rate-density evolution of \propto (1+z)^{2-2.5} up to z ~ 1, that has been suggested independently from the compatibility between the number-brightness distribution and duration-brightness correlation. If the cosmic SFR has its peak at z ~ 1--2 as suggested by recent observations, the effective power-index of GRB photon spectrum, \alpha >~ 1.5$ is favored, that is softer than the recent observational determination of \alpha = 1.1 \pm 0.3. However, high redshift starbursts (z >~ 5) in elliptical galaxies, that have not yet been detected, can alleviate this discrepancy. The redshift of GRB970508 is likely about 2, just below the upper limit that is recently determined, and the absorption system at z = 0.835 seems not to be the site of the GRB.
Deep Near-Infrared Universe Seen in the Subaru Deep Field
Tomonori Totani
Physics , 2002,
Abstract: The Subaru Deep Field provides the currently deepest K-selected sample of high-z galaxies (K' = 23.5 at 5 sigma). The SDF counts, colors, and size distributions in the near-infrared bands are carefully compared with pure-luminosity-evolution (PLE) as well as CDM-based hierarchical merging (HM) models. The very flat faint-end slope of the SDF K count indicates that the bulk (more than 90%) of cosmic background radiation (CBR) in this band is resolved, even if we take into account every known source of incompleteness. The integrated flux from the counts is only about a third of reported flux of the diffuse CBR in the same band, suggesting that a new distinct source of this missing light may be required. We discovered unusually red objects with colors of (J-K) >~ 3-4, which are even redder than the known population of EROs, and difficult to explain by passively evolving elliptical galaxies. A plausible interpretation, which is the only viable one among those we examined, is that these are dusty starbursts at high-z (z ~ 3), whose number density is comparable with that of present-day ellipticals or spheroidal galaxies, as well as with that of faint submillimeter sources. The photometric redshift distribution obtained by BVRIz'JK' photometries is also compared with the data, and the HM model is found to predict too few high-z objects at K' >~ 22 and z >~ 2; the PLE model with reasonable amount of absorption by dust looks more consistent with the data. This result is apparently in contradiciton with some previous ones for shallower observations, and we discuss the origin of this. These results raise a question for the HM models: how to form massive objects with starbursts at such high redshifts, which presumably evolve into present-day elliptical galaxies or bulges?
Cluster-Cluster Microlensing as a Probe of Intracluster Stars, MACHOs, and Remnants of the First Generation Stars
Tomonori Totani
Physics , 2002, DOI: 10.1086/346205
Abstract: The galaxy cluster Abell 2152 is recently found to be forming a cluster-cluster system with another, more distant cluster whose core is almost perfectly aligned to that of A2152. We discuss the detectability of microlensing events where a single star in the source cluster behind A2152 is extremely magnified by an intracluster compact object in A2152. We show that a search with an 8m-class telescope with a wide field of view, such as the Subaru/Suprime-Cam, can probe intracluster compact objects with a wide mass range of m_{co} ~ 10^{-5}-10^{10} M_sun, including ranges that have not yet been constrained by any past observations. We expect that the event rate is biased for the background cluster than the foreground cluster (A2152), which would be a unique signature of microlensing, making this experiment particularly powerful. The sensitivity of this experiment for the mass fraction of compact objects would be 1-10% in the total dark matter of the cluster, which is roughly constant against m_{co}, with a reasonable telescope time for large telescopes (~10 nights). Therefore any compact objects in this mass range can be detected or rejected as the dominant component of the dark matter. About 10 events are expected if 20% of the cluster mass is in a form of compact objects with M ~ 1 M_sun, as claimed by the MACHO collaboration for the Milky Way halo. Other possibly detectable targets include intracluster stars stripped by galaxy interactions, and hypothetical very massive black holes (M >~ 100 M_sun) produced as remnants of the first generation stars, which might be responsible for the recently reported excess of the cosmic infrared background radiation that seems impossible to explain by normal galactic light.
A Failed Gamma-Ray Burst with Dirty Energetic Jets Spirited Away? New Implications for the GRB-SN Connection from Supernova 2002ap
Tomonori Totani
Physics , 2003, DOI: 10.1086/378936
Abstract: (Abridged) SN 2002ap is an interesting event with broad spectral features like the famous SN 1998bw / GRB 980425. Here we examine the recently proposed jet hypothesis from SN 2002ap by a spectropolarimetric observation. We show that jets should be moving at about 0.23c with a jet kinetic energy of ~5 x 10^{50} erg, a similar energy scale to the GRB jets. The weak radio emission from SN 2002ap has been used to argue against the jet hypothesis, but we show that this problem can be avoided. However, the jet cannot be kept ionized because of adiabatic cooling without external photoionization or heating source. We found that only the radioactivity of 56Ni is a possible source, indicating that the jet is formed and ejected from central region of the core collapse. Then we point out that the jet will eventually sweep up enough interstellar medium and generate shocks in a few to 10 years, producing strong radio emission that can be spatially resolved, giving us a clear test for the jet hypothesis. Discussions are given on possible implications for the GRB-SN connection in the case that the jet is real. We suggest existence of two distinct classes of GRBs from similar core-collapse events but by completely different mechanisms. Cosmologically distant GRBs (~10^{50} erg) are collimated jets generated by central activity of core collapses. SN 2002ap could be a failed GRB of this type with a large baryon load. On the other hand, much less energetic ones like GRB 980425 are rather isotropic, which may be produced by hydrodynamical shock acceleration at the outer envelope. We propose that the radioactive ionization for the SN 2002ap jet may give a new explanation also for the X-ray line features often observed in GRB afterglows.
An Interpretation of the Evidence for TeV Emission from Gamma-Ray Burst 970417a
Tomonori Totani
Physics , 2000, DOI: 10.1063/1.1370889
Abstract: The Milagrito collaboration recently reported evidence for emission of very high energy gamma-rays in the TeV range from one of the BATSE GRBs, GRB 970417a. Here I discuss possible interpretations of this result. Taking into account the intergalactic absorption of TeV gamma-rays by the cosmic infrared background, I found that the detection rate (one per 54 GRBs observed by the Milagrito) and energy fluence can be consistently explained with the redshift of this GRB at z \sim 0.7 and the isotropic total energy in the TeV range, E_{TeV, iso} >~ 10^{54} erg. This energy scale is not unreasonably large, but interestingly similar to the maximum total GRB energy observed to date, in the sub-MeV range for GRB 990123. On the other hand, the energy emitted in the ordinary sub-MeV range becomes E_{MeV, iso} \sim 10^{51} erg for the GRB 970417a, which is much smaller than the total energy in the TeV range by a factor of about 10^3. I show that the proton-synchrotron model of GRBs provides a possible explanation for these observational results. I also discuss some observational signatures expected in the future experiments from this model.
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