arXiv subjects

Takako T. Ishii

Publications and source records attributed to Takako T. Ishii.

At least 37 records · Page 2Linked to original sources

arXiv2012

Can Superflares Occur on Our Sun?

Recent observations of solar type stars with the Kepler satellite by Maehara et al. have revealed the existence of superflares (with energy of 10^33 - 10^35 erg) on Sun-like stars, which are similar to our Sun in their surface temperature (5600 K - 6000 K) and slow rotation (rotational period > 10 days). From the statistical analysis of these superflares, it was found that superflares with energy 10^34 erg occur once in 800 years and superflares with 10^35 erg occur once in 5000 years on Sun-like stars. In this paper, we examine whether superflares with energy of 10^33 - 10^35 erg could occur on the present Sun through the use of simple order-of-magnitude estimates based on current ideas relating to the mechanisms of the solar dynamo.

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arXiv2011

First Simultaneous Observation of H-alpha Moreton Wave, EUV Wave, and Filament/Prominence Oscillations

We report on the first simultaneous observation of an H-alpha Moreton wave, the corresponding EUV fast coronal waves, and a slow and bright EUV wave (typical EIT wave). Associated with an X6.9 flare that occurred on 2011 August 9 at the active region NOAA 11263, we observed a Moreton wave in the H-alpha images taken by the Solar Magnetic Activity Research Telescope (SMART) at Hida Observatory of Kyoto University. In the EUV images obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) we found not only the corresponding EUV fast "bright" coronal wave, but also the EUV fast "faint" wave that is not associated with the H-alpha Moreton wave. We also found a slow EUV wave, which corresponds to a typical EIT wave. Furthermore, we observed, for the first time, the oscillations of a prominence and a filament, simultaneously, both in the H-alpha and EUV images. To trigger the oscillations by the flare-associated coronal disturbance, we expect a coronal wave as fast as the fast-mode MHD wave with the velocity of about 570 - 800 km/s. These velocities are consistent with those of the observed Moreton wave and the EUV fast coronal wave.

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arXiv2010

Spicule Dynamics over Plage Region

We studied spicular jets over a plage area and derived their dynamic characteristics using Hinode Solar Optical Telescope (SOT) high-resolution images. The target plage region was near the west limb of the solar disk. This location permitted us to study the dynamics of spicular jets without the overlapping effect of spicular structures along the line of sight. In this work, to increase the ease with which we can identify spicules on the disk, we applied the image processing method `MadMax' developed by Koutchmy et al. (1989). It enhances fine, slender structures (like jets), over a diffuse background. We identified 169 spicules over the target plage. This sample permits us to derive statistically reliable results regarding spicular dynamics. The properties of plage spicules can be summarized as follows: (1) In a plage area, we clearly identified spicular jet features. (2) They were shorter in length than the quiet region limb spicules, and followed ballistic motion under constant deceleration. (3) The majority (80%) of the plage spicules showed the cycle of rise and retreat, while 10% of them faded out without a complete retreat phase. (4) The deceleration of the spicule was proportional to the velocity of ejection (i.e. the initial velocity).

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arXiv2008

Evolution of Anemone AR NOAA 10798 and the Related Geo-Effective Flares and CMEs

We present a detailed examination of the features of the Active Region (AR) NOAA 10798. This AR generated coronal mass ejections (CMEs) that caused a large geomagnetic storm on 24 August 2005 with the minimum Dst index of -216 nT. We examined the evolution of the AR and the features on/near the solar surface and in the interplanetary space. The AR emerged in the middle of a small coronal hole, and formed a {\it sea anemone} like configuration. H$α$ filaments were formed in the AR, which have southward axial field. Three M-class flares were generated, and the first two that occurred on 22 August 2005 were followed by Halo-type CMEs. The speeds of the CMEs were fast, and recorded about 1200 and 2400 km s$^{-1}$, respectively. The second CME was especially fast, and caught up and interacted with the first (slower) CME during their travelings toward Earth. These acted synergically to generate an interplanetary disturbance with strong southward magnetic field of about -50 nT, which was followed by the large geomagnetic storm.

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arXiv2007

Triggering Mechanism for the Filament Eruption on 2005 September 13 in Active Region NOAA 10808

On 2005 September 13 a filament eruption accompanied by a halo CME occurred in the most flare-productive active region NOAA 10808 in Solar Cycle 23. Using multi-wavelength observations before the filament eruption on Sep. 13th, we investigate the processes leading to the catastrophic eruption. We find that the filament slowly ascended at a speed of 0.1km/s over two days before the eruption. During slow ascending, many small flares were observed close to the footpoints of the filament, where new magnetic elements were emerging. On the basis of the observational facts we discuss the triggering mechanism leading to the filament eruption. We suggest the process toward the eruption as follows: First, a series of small flares played a role in changing the topology of the loops overlying the filament. Second, the small flares gradually changed the equilibrium state of the filament and caused the filament to ascend slowly over two days. Finally, a C2.9 flare that occurred when the filament was close to the critical point for loss of equilibrium directly led to the catastrophic filament eruption right after itself.

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arXiv2005

The ISO 170um Luminosity Function of Galaxies

We constructed a local luminosity function (LF) of galaxies using a flux-limited sample (S_170 > 0.195Jy) of 55 galaxies at z < 0.3 taken from the ISO FIRBACK survey at 170um. The overall shape of the 170-um LF is found to be different from that of the total 60-um LF (Takeuchi et al. 2003): the bright end of the LF declines more steeply than that of the 60-um LF. This behavior is quantitatively similar to the LF of the cool subsample of the IRAS PSCz galaxies. We also estimated the strength of the evolution of the LF by assuming the pure luminosity evolution (PLE): L(z) \propto (1+z)^Q. We obtained Q=5.0^{+2.5}_{-0.5} which is similar to the value obtained by recent Spitzer observations, in spite of the limited sample size. Then, integrating over the 170-um LF, we obtained the local luminosity density at 170um, ρ_L(170um). A direct integration of the LF gives ρ_L(170um) = 1.1 \times 10^8 h Lsun Mpc^{-3}, whilst if we assume a strong PLE with Q=5, the value is 5.2 \times 10^7 h Lsun Mpc^{-3}. This is a considerable contribution to the local FIR luminosity density. By summing up with other available infrared data, we obtained the total dust luminosity density in the Local Universe, ρ_L(dust)=1.1 \times 10^8 h Lsun Mpc^{-3}. Using this value, we estimated the cosmic star formation rate (SFR) density hidden by dust in the Local Universe. We obtained ρ_SFR(dust) \simeq 1.1-1.2 h \times 10^{-2} Msun yr^{-1} Mpc^{-3}$, which means that 58.5% of the star formation is obscured by dust in the Local Universe.

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arXiv2005

A model for the infrared dust emission from forming galaxies

In the early epoch of galaxy evolution, dust is only supplied by supernovae (SNe). With the aid of a new physical model of dust production by SNe developed by Nozawa et al. (2003) (N03), we constructed a model of dust emission from forming galaxies on the basis of the theoretical framework of Takeuchi et al. (2003) (T03). N03 showed that the produced dust species depends strongly on the mixing within SNe. We treated both unmixed and mixed cases and calculated the infrared (IR) spectral energy distribution (SED) of forming galaxies for both cases. Our model SED is less luminous than the SED of T03 model by a factor of 2-3. The difference is due to our improved treatment of UV photon absorption cross section, as well as different grain size and species newly adopted in this work. The SED for the unmixed case is found to have an enhanced near to mid-IR (N-MIR) continuum radiation in its early phase of the evolution (age < 10^{7.25} yr) compared with that for the mixed case. The strong N--MIR continuum is due to the emission from Si grains, which only exist in the species of the unmixed dust production. We also calculated the IR extinction curves for forming galaxies. Then we calculated the SED of a local starbursting dwarf galaxy SBS 0335-052. Our present model SED naturally reproduced the strong N--MIR continuum and the lack of cold FIR emission of SBS 0335-052. We found that only the SED of unmixed case can reproduce the NIR continuum of this galaxy. We then made a prediction for the SED of another typical star-forming dwarf, I Zw 18. We also presented the evolution of the SED of LBGs. Finally, we discussed the possibility of observing forming galaxies at z > 5.

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arXiv2005

Extinction curves expected in young galaxies

We investigate the extinction curves of young galaxies in which dust is supplied from Type II supernovae (SNe II) and/or pair instability supernovae (PISNe). We adopt Nozawa et al. (2003) for compositions and size distribution of grains formed in SNe II and PISNe. We find that the extinction curve is quite sensitive to internal metal mixing in supernovae (SNe). The extinction curves predicted from the mixed SNe are dominated by SiO2 and is characterised by steep rise from infrared to ultraviolet (UV). The dust from unmixed SNe shows shallower extinction curve, because of the contribution from large-sized (~ 0.1 um) Si grains. However, the progenitor mass is important in unmixed SNe II: If the progenitor mass is smaller than ~ 20 Msun, the extinction curve is flat in UV; otherwise, the extinction curve rises toward the short wavelength. The extinction curve observed in a high-redshift quasar (z=6.2) favours the dust production by unmixed SNe II. We also provide some useful observational quantities, so that our model might be compared with future high-z extinction curves.

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arXiv2004

Dust emission model of Lyman-break galaxies

Lyman-break galaxies (LBGs) contain a non-negligible amount of dust. Takeuchi et al. (2003)(T03) constructed a model of the infrared spectral energy distribution (SED) for very young galaxies by taking into account the dust size distribution in the early stage of galaxy evolution, which can be different from that of present-day evolved galaxies. We applied T03 model to LBGs and constructed their expected SED. In order to examine the grain size distribution of dust, we calculated the SEDs based on two distinct type of the distribution models: a single-sized distribution and a power-law distribution with a slope of $dN/da \propto a^{-3.5}$. We found that the single-sized and power-law dust size distributions yield a very similar detectability of LBGs at the submillimetre (submm). We also found that galaxies with a power-law dust distribution have much less flux at mid-infrared (MIR) than the other. Then, we applied the model to a gravitationally lensed LBG \cb (cB58), a unique probe of the dust emission from LBGs. Our model framework well reproduced the hot dust temperature under a natural assumption for this galaxy. We also examined the detectability of LBGs at submm wavelengths in an eight-hour deep survey by ALMA. By integrating over their redshifted SEDs with the observed luminosity functions, we obtained the contribution of LBGs to the cosmic infrared background radiation (CIRB). Although they have non-negligible amount of dust, their contribution was found to be small, especially in the FIR $\sim 200 μ$m. Thus, we need a strongly obscured population of galaxies which contains a large amount of star formation, at some epoch in the history of the universe.

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arXiv2003

A General Formulation of the Source Confusion Statistics and Application to Infrared Galaxy Surveys

Source confusion has been a long-standing problem in the astronomical history. In the previous formulation, sources are assumed to be distributed homogeneously on the sky. This fundamental assumption is not realistic in many applications. In this work, by making use of the point field theory, we derive general analytic formulae for the confusion problems with arbitrary distribution and correlation functions. As a typical example, we apply these new formulae to the source confusion of infrared galaxies. We first calculate the confusion statistics for power-law galaxy number counts as a test case. When the slope of differential number counts, γ, is steep, the confusion limits becomes much brighter and the probability distribution function (PDF) of the fluctuation field is strongly distorted. Then we estimate the PDF and confusion limits based on the realistic number count model for infrared galaxies. The gradual flattening of the slope of the source counts makes the clustering effect rather mild. Clustering effects result in an increase of the limiting flux density with \sim 10%. In this case, the peak probability of the PDF decreases up to \sim 15% and its tail becomes heavier.

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arXiv2003

Infrared Spectral Energy Distribution Model for Extremely Young Galaxies

The small grain sizes produced by Type II supernova (SN II) models in young, metal-poor galaxies make the appearance of their infrared (IR) spectral energy distribution (SED) quite different from that of nearby, older galaxies. To study this effect, we have developed a model for the evolution of dust content and the IR SED of low-metallicity, extremely young galaxies based on Hirashita et al. (2002). We find that, even in the intense ultraviolet (UV) radiation field of very young galaxies, small silicate grains are subject to stochastic heating resulting in a broad temperature distribution and substantial MIR continuum emission. Larger carbonaceous grains are in thermal equilibrium at T \simeq 50 - 100K, and they also contribute to the MIR. We present the evolution of SEDs and IR extinction of very young, low-metallicity galaxies. The IR extinction curve is also shown. In the first few Myrs, the emission peaks at λ\sim 30-50um at later times dust self-absorption decreases the apparent grain temperatures, shifting the bulk of the emission into the submillimetre band. We successfully apply the model to the IR SED of a low metallicity (1/41 Z_\odot) dwarf galaxy SBS0335-052. We find the SED, optical properties and extinction of the star forming region to be consistent with a very young and compact starburst. We also predict the SED of another extremely low-metallicity galaxy, I Zw 18, for future observational tests. Some prospects for future observations are discussed.

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arXiv2003

The Luminosity Function of IRAS PSCz Galaxies

We estimated the luminosity function (LF) of IRAS galaxies in the PSCz catalogue. The faint end of the PSCz LF is slightly steeper than that of the LF derived by Saunders et al. (1990; S90). Using an analytical form for the LF used by S90, we obtain the following parameters: α= 1.23 \pm 0.04, L_*=(8.85 \pm 1.75) \times 10^8 h^{-2} L_\odot, σ=0.724 \pm 0.010, and ϕ_* = (2.34 \pm 0.30) \times 10^{-2} h^3 Mpc^{-3}. We also examined the evolution in the sample by a simple assumption ϕ_*(z) \propto (1+z)^P, and found P=3.40 \pm 0.70. It does not affect the three parameters, α, L_*, and σ, but ϕ_*(z=0) is overestimated up to \sim 15% if we ignore evolution. We estimated the temperature dependence of the LF. The LFs of warm and cool galaxies are quite different: the LF of warm galaxies has a very steep faint end with α=1.37. We also discuss a lump found at the brightest end of the LF.

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arXiv2001

Plasma Ejections from a Light Bridge in a Sunspot Umbra

We present conspicuous activities of plasma ejections along a light bridge of a stable and mature sunspot in NOAA 8971 on 2000 May 02. We found the ejections both in the H-alpha (10^4 K) images obtained with the Domeless Solar Telescope (DST) at Hida Observatory, and in the 171 A (Fe IX/X, ~10^6 K) images obtained with the Transition Region and Coronal Explorer (TRACE). Main characteristics of the ejections are as follows: (1)Ejections occur intermittently and recurrently, (2)The velocities and the timings of the 171 A-ejections are the same as those of H-alpha ejections, (3)The appearance of ejections are different from one another, i.e. the H-alpha ejections have jet-like appearance, while that of 171 A-ejections is like a loop.

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arXiv2001

Impact of Future Submillimeter and Millimeter Large Facilities on the Studies of Galaxy Formation and Evolution

We investigate what we can learn about galaxy formation and evolution from the data which will be obtained by the forthcoming large submillimeter/millimeter facilities, mainly by the Atacama Submillimeter Telescope Experiment (ASTE) and the Atacama Large Millimeter Array/Large Millimeter and Submillimeter Array (ALMA/LMSA). We first calculate the source counts from 350 um to 3 mm using the empirical infrared galaxy number count model of Takeuchi et al. (2001). Based on the number counts, we evaluate the source confusion and determine the confusion limit at various wavebands as a function of the characteristic beam size. At submillimeter wavelengths, source confusion with the 10-15-m class facilities becomes severe at 0.1 to 1 mJy level, and astrometry and flux measurements will be difficult. However, we show that very a large-area survey of submillimeter sources brighter than 10-50 mJy can provide a unique constraint on infrared galaxy evolution at z = 1-2. We also find that the 5σ-confusion limit of the LMSA is fainter than 1 μJy. The source counts at such faint flux levels give important information of the epoch of galaxy formation. We then show that multiband photometry from the infrared (by ASTRO-F) to the millimeter can be utilized as a redshift estimator. In addition, we compare the observed 1.4, 5, and 8-GHz source counts with our model counts to examine the contribution of star forming galaxies to the faint radio galaxies.

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arXiv2000

Exploring Galaxy Evolution from Infrared Number Counts and Cosmic Infrared Background

Recently reported infrared (IR) galaxy number counts and cosmic infrared background (CIRB) all suggest that galaxies have experienced a strong evolution sometime in their lifetime. We statistically estimate the galaxy evolution history from these data. We find that an order of magnitude increase of the far-infrared (FIR) luminosity at redshift z = 0.5 - 1.0 is necessary to reproduce the very high CIRB intensity at 140 um reported by Hauser et al. (1998). z \sim 0.75 and decreases to, even at most, a factor of 10 toward z \sim 5, though many variants are allowed within these constraints. This evolution history also satisfies the constraints from the galaxy number counts obtained by IRAS, ISO and, roughly, SCUBA. The rapid evolution of the comoving IR luminosity density required from the CIRB well reproduces the very steep slope of galaxy number counts obtained by ISO. We also estimate the cosmic star formation history (SFH) from the obtained FIR luminosity density, considering the effect of the metal enrichment in galaxies. The derived SFH increases steeply with redshift in 0 < z < 0.75, and becomes flat or even declines at z > 0.75. This is consistent with the SFH estimated from the reported ultraviolet luminosity density. In addition, we present the performance of the Japanese ASTRO-F FIR galaxy survey. We show the expected number counts in the survey. We also evaluate how large a sky area is necessary to derive a secure information of galaxy evolution up to z \sim 1 from the survey, and find that at least 50 - 300 deg^2 is required.

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arXiv2000

Tests of Statistical Methods for Estimating Galaxy Luminosity Function and Applications to the Hubble Deep Field

We studied the statistical methods for the estimation of the luminosity function (LF) of galaxies. We focused on four nonparametric estimators: $1/V_{\rm max}$ estimator, maximum-likelihood estimator of Efstathiou et al. (1988), Chołoniewski's estimator, and improved Lynden-Bell's estimator. The performance of the $1/V_{\rm max}$ estimator has been recently questioned, especially for the faint-end estimation of the LF. We improved these estimators for the studies of the distant Universe, and examined their performances for various classes of functional forms by Monte Carlo simulations. We also applied these estimation methods to the mock 2dF redshift survey catalog prepared by Cole et al. (1998). We found that $1/V_{\rm max}$ estimator yields a completely unbiased result if there is no inhomogeneity, but is not robust against clusters or voids. This is consistent with the well-known results, and we did not confirm the bias trend of $1/V_{\rm max}$ estimator claimed by Willmer (1997) in the case of homogeneous sample. We also found that the other three maximum-likelihood type estimators are quite robust and give consistent results with each other. In practice we recommend Chołoniewski's estimator for two reasons: 1. it simultaneously provides the shape and normalization of the LF; 2. it is the fastest among these four estimators, because of the algorithmic simplicity. Then, we analyzed the photometric redshift data of the Hubble Deep Field prepared by Fernández-Soto et al. (1999) using the above four methods. We also derived luminosity density $ρ_{\rm L}$ at $B$- and $I$-band. Our $B$-band estimation is roughly consistent with that of Sawicki, Lin, & Yee (1997), but a few times lower at $2.0 < z < 3.0$. The evolution of $ρ_{\rm L}(I)$ is found to be less prominent.

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arXiv1998

The IRIS Far-Infrared Galaxy Survey : Expected Number Count, Redshift, and Perspective

Infrared Imaging Surveyor (IRIS) is a satellite which will be launched in the beginning of 2003. One of the main purposes of the IRIS mission is an all-sky survey at far-infrared (FIR) with a flux limit much deeper than that of IRAS. In order to examine the performance of the survey, we estimated the FIR galaxy counts in four (50, 70, 120, and 150 $μ$m) bands based on some models. We adopted a multicomponent model which consists of cirrus and starburst components for galaxy spectra, and the nearby FIR luminosity function derived from that of IRAS galaxies. We derived the number counts, redshift distributions, and infrared diffuse background radiation spectra for i) no evolution, ii) pure luminosity evolution, iii) pure density evolution with $q_0 = 0.1$ and 0.5. We found that a large number of galaxies ($\sim {\rm a few} \times 10^6$ in the whole sky) will be detected in this survey. With the aid of a vast number of detection, we will detect the effect of galaxy evolution, and evaluate the amplitude of evolution at least in the nearby universe in the IRIS survey, though it will be still difficult to constrain which type of evolution takes place from the number count alone. We also studied the estimation of redshifts of detected galaxies by their infrared colors alone. Although significant contamination takes place among nearby faint galaxies and high-$z$ ones, we found that rough estimation of galaxy redshift can be practicable by jointly using present and future optical surveys.

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arXiv1998

Photometric Properties of Kiso Ultraviolet-Excess Galaxies in the Lynx-Ursa Major Region

We have performed a systematic study of several regions in the sky where the number of galaxies exhibiting star formation (SF) activity is greater than average. We used Kiso ultraviolet-excess galaxies (KUGs) as our SF-enhanced sample. By statistically comparing the KUG and non-KUG distributions, we discovered four KUG-rich regions with a size of $\sim 10^\circ \times 10^\circ$. One of these regions corresponds spatially to a filament of length $\sim 60 h^{-1}$ Mpc in the Lynx-Ursa Major region ($α\sim 9^{\rm h} - 10^{\rm h}, δ\sim 42^\circ - 48^\circ$). We call this ``the Lynx-Ursa Major (LUM) filament''. We obtained $V(RI)_{\rm C}$ surface photometry of 11 of the KUGs in the LUM filament and used these to investigate the integrated colors, distribution of SF regions, morphologies, and local environments. We found that these KUGs consist of distorted spiral galaxies and compact galaxies with blue colors. Their star formation occurs in the entire disk, and is not confined to just the central regions. The colors of the SF regions imply that active star formation in the spiral galaxies occurred $10^{7 - 8}$ yr ago, while that of the compact objects occurred $10^{6-7}$ yr ago. Though the photometric characteristics of these KUGs are similar to those of interacting galaxies or mergers, most of these KUGs do not show direct evidence of merger processes.

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