arXiv subjects

Hiroyuki Hirashita

Publications and source records attributed to Hiroyuki Hirashita.

At least 109 records · Page 6Linked to original sources

arXiv2007

Origin of Chromatic Features in Multiple Quasars -Variability, Dust, or Microlensing -

Aims:In some of the lensed quasars, color differences between multiple images are observed at optical/near-infrared wavelengths. There are three possible origins of the color differences: intrinsic variabilities of quasars, differential dust extinction, and quasar microlensing. We examine how these three possible scenarios can reproduce the observed chromaticity. Methods:We evaluate how much color difference between multiple images can be reproduced by the above three possible scenarios with realistic models; (i) an empirical relation for intrinsic variabilities of quasars, (ii) empirical relations for dust extinction and theoretically predicted inhomogeneity in galaxies, or (iii) a theoretical model for quasar accretion disks and magnification patterns in the vicinity of caustics. Results:We find that intrinsic variabilities of quasars cannot be a dominant source responsible for observed chromatic features in multiple quasars. In contrast, either dust extinction or quasar microlensing can nicely reproduce the observed color differences between multiple images in most of the lensed quasars. Taking into account the time interval between observations at different wavebands in our estimations, quasar microlensing is a more realistic scenario to reproduce the observed color differences than dust extinction. All the observed color differences presented in this paper can be explained by a combination of these two effects, but monitoring observations at multiple wavebands are necessary to disentangle these.

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arXiv2007

Far-infrared dust properties in the Galaxy and the Magellanic Clouds

A recent data analysis of the far-infrared (FIR) map of the Galaxy and the Magellanic Clouds has shown that there is a tight correlation between two FIR colours: the 60 um-100 um and 100 um-140 um colours. This FIR colour relation called ``main correlation'' can be interpreted as indicative of a sequence of various interstellar radiation fields with a common FIR optical property of grains. In this paper, we constrain the FIR optical properties of grains by comparing the calculated FIR colours with the observational main correlation. We show that neither of the ``standard'' grain species (i.e. astronomical silicate and graphite grains) reproduces the main correlation. However, if the emissivity index at ~ 100--200 um is changed to ~ 1--1.5 (not ~ 2 as the above two species), the main correlation can be successfully explained. Thus, we propose that the FIR emissivity index is ~ 1--1.5 for the dust in the Galaxy and the Magellanic Clouds at ~ 100--200 um. We also consider the origin of the minor correlation called ``sub-correlation'', which can be used to estimate the Galactic star formation rate.

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arXiv2006

Fate of clumps in damped Ly$α$ systems

Recent observations have revealed that damped Ly$α$ clouds (DLAs) host star formation activity. In order to examine if such star formation activity can be triggered by ionization fronts, we perform high-resolution hydrodynamics and radiative transfer simulations of the effect of radiative feedback from propagating ionization fronts on high-density clumps. We examine two sources of ultraviolet (UV) radiation field to which high-redshift (z ~ 3) galaxies could be exposed: one corresponding to the UV radiation originating from stars within the DLA, itself, and the other corresponding to the UV background radiation. We find that, for larger clouds, the propagating I-fronts created by local stellar sources can trigger cooling instability and collapse of significant part, up to 85%, of the cloud, creating conditions for star formation in a timescale of a few Myr. The passage of the I-front also triggers collapse of smaller clumps (with radii below ~4 pc), but in these cases the resulting cold and dense gas does not reach conditions conducive to star formation. Assuming that 85% of the gas initially in the clump is converted into stars, we obtain a star formation rate of $\sim 0.25 M_\odot {yr}^{-1} {kpc}^{-2}$. This is somewhat higher than the value derived from recent observations. On the other hand, the background UV radiation which has harder spectrum fails to trigger cooling and collapse. Instead, the hard photons which have long mean-free-path heat the dense clumps, which as a result expand and essentially dissolve in the ambient medium. Therefore, the star formation activity in DLAs is strongly regulated by the radiative feedback, both from the external UV background and internal stellar sources and we predict quiescent evolution of DLAs (not starburst-like evolution).

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arXiv2006

Common Correlations between 60, 100 and 140 um Intensities in the Galactic Plane and Magellanic Clouds

We investigate the far-infrared SED of the Galaxy and the Magellanic Clouds by using the COBE (COsmic Background Explorer) / DIRBE (Diffuse InfraRed Background Experiment) ZSMA (Zodi - Subtracted Mission Average) maps at wavelengths of 60 um, 100 um and 140 um. We analyze three regions: the Galactic plane region with the Galactic latitude |b|<5 degree, the Large Magellanic Cloud (LMC) region, and the Small Magellanic Cloud (SMC) region. Because the dust optical depth is much smaller in the far-infrared than in the visible, we may observe cumulative far-infrared radiation from regions with various interstellar radiation field (IRSF) in a line of sight. As consequence of considering such an effect, we find a common far-infrared color correlation between the 140 - 100 um and 60 - 100 um intensity ratios in all the three galaxies. Although this color correlation cannot be explained by any existing model, it fits very well the far-infrared color of nearby star forming galaxies.

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arXiv2006

Search for high column density systems with gamma ray bursts

We investigate the possibility to search for metal-poor high column density (> 10^{23} cm^{-2}) clouds at high redshift (z) by using gamma ray burst (GRB) afterglows. Such clouds could be related to primeval galaxies which may cause a burst of star formation. We show that a large part of hydrogen is in molecular form in such a high column density environment. Therefore, hydrogen molecules (H2) rather than hydrogen atoms should be searched for. Then we show that infrared H2 lines are detectable for metal-poor (< 0.01 solar metallicity) high column density (log NH [cm^{-2}] > 23.5) systems at high-z without suffering dust extinction. Optical properties of dust in infrared could also be constrained by observations of high column density systems. Some possible scenarios of producing high column density systems are finally discussed in the context of galaxy evolution.

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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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arXiv2003

Star formation rate in galaxies from UV, IR, and H-alpha estimators

Infrared (IR) luminosity of galaxies originating from dust emission can be used as an indicator of the star formation rate (SFR). Inoue et al. (2000, IHK) have derived a formula for the conversion from IR luminosity to SFR by using the following three quantities: the fraction of Lyman continuum luminosity absorbed by gas (f), the fraction of UV luminosity absorbed by dust (epsilon), and the fraction of dust heating from old (>10^8 yr) stellar populations (eta). We develop a method to estimate those three quantities based on the idea that the various way of SFR estimates should return the same SFR. After applying our method to samples of galaxies, the following results are obtained. First, our method is applied to star-forming galaxies, finding that f~0.6, epsilon~0.5, and eta~0.4 as representative values. Next, we apply the method to a starburst sample, which shows larger extinction than the star-forming galaxy sample. With the aid of f, epsilon, and eta, we estimate reliable SFRs. Moreover, the H-alpha luminosity, if the H-alpha extinction is corrected by using the Balmer decrement, is suitable for a statistical analysis of SFR, because the same correction factor for the Lyman continuum extinction is applicable to both normal and starburst galaxies over all the range of SFR. The metallicity dependence of f and epsilon is also tested: Only the latter proves to have a correlation with metallicity. As an extension of our result, we show that all UV, H-alpha, and IR comoving luminosity densities at z=0 give a consistent SFR (~ 3x10^{-2}h M_sun/Mpc^3). Useful formulae for SFR estimate are listed.

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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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arXiv2001

Absorption Measurement of Hydrogen Molecules in the Early Universe

We investigate the observability of hydrogen molecules in absorption. The absorption efficiency of the hydrogen molecules become comparable with or larger than that of the dust grains in the metal-poor condition expected in the early Universe. If we can use bright infrared continuum sources behind the molecular gas clouds, the absorption measurement of the hydrogen molecules will be an important technique to explore the primordial gas clouds that are contracting into first-generation objects.

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arXiv2001

H2 Formation in Low Metallicity Galaxies

A possible formation mechanism of hydrogen molecules on a galactic scale is examined. We are interested especially in the role of hydrogen molecules for formation and evolution of primordial galaxies. Then, formation process of hydrogen molecules in a very low-metallicity galaxy (I Zw 18; the most typical metal-deficient galaxy) is studied. Adopting a recent observational result of absorption lines of hydrogen molecule in I Zw 18, we obtain the upper limit for the ionization degree in the case where hydrogen molecules can form via the H$^{-}$-process, although they are generally believed to form on the surface of dust grains. Furthermore, we present a critical ionization degree, above which H$^{-}$-process can be dominant over the formation process on the surface of grains. Interestingly, this critical ionization degree is comparable to the upper limit of the ionization degree for I Zw 18. For determination of the formation process of hydrogen molecules, future observational facilities can be useful. Thus, we examine the detectability in some wavelengths for the metal deficient galaxies. According to our estimate, near-infrared line emission of hydrogen molecule is observable in the level of 10 $μ$Jy, the free-free radio emission is in the level of mJy, and the far-infrared emission from the dust on which hydrogen molecules form can also be detected with 10-mJy level with its temperature of 16 K. The near-infrared line and the far-infrared continuum are feasible for ASTRO-F observations.

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arXiv2001

Mass-Metallicity Relation for the Local Group Dwarf Spheroidal Galaxies: A New Picture for the Chemical Enrichment of Galaxies in the Lowest Mass Range

The virial mass ($M_{\rm vir}$)-metallicity relation among the Local Group dwarf spheroidal galaxies (dSphs) is examined. Hirashita, Takeuchi, & Tamura showed that the dSphs can be divided into two distinct classes with respect to the relation between their virial masses and luminosities: low-mass ($M_{\rm vir} \la 10^8 M_\odot$) and high-mass ($M_{\rm vir} \ga 10^8 M_\odot$) groups. We see that both the mass-metallicity and the mass-luminosity relations of the high-mass dSphs are understood as a low-mass extension of giant ellipticals. On the contrary, we find that the classical galactic-wind model is problematic to apply to the low-mass dSphs, whose low binding energy is comparable to that released by several supernova explosions. A strongly regulated star formation in their formation phase is required to reproduce their observed metallicity. Such regulation is naturally expected in a gas cloud with the primordial elemental abundance according to Nishi & Tashiro. A significant scatter in the mass-metallicity relation for the low-mass dSphs is also successfully explained along with the scenario of Hirashita and coworkers. We not only propose a new picture for a chemical enrichment of the dSphs, but also suggest that the mass-metallicity and the mass-luminosity relations be understood in a consistent context.

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arXiv2001

Effect of Dust Extinction on Estimating Star Formation Rate of Galaxies: Lyman Continuum Extinction

We re-examine the effect of Lyman continuum ($λ\leq 912$ Å) extinction (LCE) by dust in H {\sc ii} regions in detail and discuss how it affects the estimation of the global star formation rate (SFR) of galaxies. To clarify the first issue, we establish two independent methods for estimating a parameter of LCE ($f$), which is defined as the fraction of Lyman continuum photons contributing to hydrogen ionization in an H {\sc ii} region. One of those methods determines $f$ from the set of Lyman continuum flux, electron density and metallicity. In the framework of this method, as the metallicity and/or the Lyman photon flux increase, $f$ is found to decrease. The other method determines $f$ from the ratio of infrared flux to Lyman continuum flux. Importantly, we show that $f \la 0.5$ via both methods in many H {\sc ii} regions of the Galaxy. Thus, it establishes that dust in such H {\sc ii} regions absorbs significant amount of Lyman continuum photons directly. To examine the second issue, we approximate $f$ to a function of only the dust-to-gas mass ratio (i.e., metallicity), assuming a parameter fit for the Galactic H {\sc ii} regions. We find that a characteristic $\hat{f}$, which is defined as $f$ averaged over a galaxy-wide scale, is 0.3 for the nearby spiral galaxies. This relatively small $\hat{f}$ indicates that a typical increment factor due to LCE for estimating the global SFR ($1/\hat{f}$) is large ($\sim 3$) for the nearby spiral galaxies. Therefore, we conclude that the effect of LCE is not negligible relative to other uncertainties of estimating the SFR of galaxies.

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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

Star Formation Efficiency in the Central 1 kpc Region of Early-Type Spiral Galaxies

It has been reported recently that there are some early-type spiral (Sa--Sab) galaxies having evident star-forming regions which concentrate in their own central 1-kpc. In such central region, is the mechanism of the star formation distinct from that in disks of spiral galaxies? To reveal this, we estimate the star formation efficiency (SFE) in this central 1-kpc star-forming region of some early-type spiral galaxies, taking account of the condition for this 1-kpc region to be self-gravitating. Using two indicators of present star formation rate (H$α$ and infrared luminosity), we estimate the SFE to be a few percents. This is equivalent to the observational SFE in the disks of late-type spiral (Sb--) galaxies. This coincidence may support the universality of the mean SFE of spiral galaxies reported in the recent studies. That is, we find no evidence of distinct mechanism of the star formation in the central 1-kpc region of early-type galaxies. Also, we examine the structure of the central star-forming region, and discuss a method for estimating the mass of star-forming regions.

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arXiv1999

Observational Test of Environmental Effects on The Local Group Dwarf Spheroidal Galaxies

In this paper, we examine whether tidal forces exerted by the Galaxy or M31 have an influence on the Local Group dwarf spheroidal galaxies (dSphs) which are their companions. We focus on the surface brightness profiles of the dSphs, especially their core radii because it is suggested based on the numerical simulations that tidal disturbance can make core radii extended. We examine the correlation for the dSphs between the distances from their parent galaxy (the Galaxy or M31) and the compactnesses of their surface brightness profiles by using a parameter ``C'' defined newly in this paper. Consequently, we find no significant correlation. We make some remarks on the origin of this result by considering three possible scenarios; tidal picture, dark matter picture, and heterogeneity of the group of dSphs, each of which has been often discussed to understand fundamental properties and formation processes of dSphs.

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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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