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We report basic far-infrared (FIR) properties of eight blue compact dwarf galaxies (BCDs) observed by AKARI. We measure the fluxes at the four FIS bands (wavelengths of 65 um, 90 um, 140 um, and 160 um). Based on these fluxes, we estimate basic quantities about dust: dust temperature, dust mass, and total FIR luminosity. We find that the typical dust temperature of the BCD sample is systematically higher than that of normal spiral galaxies, although there is a large variety. The interstellar radiation field estimated from the dust temperature ranges up to 100 times of the Galactic value. This confirms the concentrated star-forming activity in BCDs. The star formation rate can be evaluated from the FIR luminosity as 0.01--0.5 $M_\odot$ yr$^{-1}$. Combining this quantity with gas mass taken from the literature, we estimate the gas consumption timescales (gas mass divided by the star formation rate), which prove to span a wide range from 1 Gyr to 100 Gyr. A natural interpretation of this large variety can be provided by intermittent star formation activity. We finally show the relation between dust-to-gas ratio and metallicity (we utilize our estimate of dust mass, and take other necessary quantities from the literature). There is a positive correlation between dust-to-gas ratio and metallicity as expected from chemical evolution models., 13 pages, 2 figures. Accepted for publication in PASJ (AKARI Special Issue No. 2)

We investigate the radio spectral energy distributions of young star-forming galaxies [1]. The radio energy emitted per supernova remnant (fluence) is constrained by using the observational radio data of SBS0335−052 (“active”) and I Zw 18 (“passive”), which are the two lowest-metallicity blue compact dwarf galaxies. The typical radio fluence is estimated to be higher than previous estimates by nearly an order of magnitude and may depend on gas density. We have also predicted the time dependence of the radio spectral index for both the active and passive cases [2] (Fig. 1). We also predict the evolution of the radio–far-infrared (FIR) relation by using our FIR radiation models [2] (Fig. 1).

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). Since at high redshift (z>5), low-mass stars cannot be dominant sources for dust grains, SNe II and PISNe, whose progenitors are massive stars with short lifetimes, should govern the dust production. Here, we theoretically investigate the extinction curves of dust produced by SNe II and PISNe, taking into account reverse shock destruction induced by collision with ambient interstellar medium. We find that the extinction curve is sensitive to the ambient gas density around a SN, since the efficiency of reverse shock destruction strongly depends on it. The destruction is particularly efficient for small-sized grains, leading to a flat extinction curve in the optical and ultraviolet wavelengths. Such a large ambient density as n_H > 1 cm^{-3} produces too flat an extinction curve to be consistent with the observed extinction curve for SDSS J104845.05+463718.3 at z=6.2. Although the extinction curve is highly sensitive to the ambient density, the hypothesis that the dust is predominantly formed by SNe at z~6 is still allowed by the current observational constraints. For further quantification, the ambient density should be obtained by some other methods. Finally we also discuss the importance of our results for observations of high-z galaxies, stressing a possibility of flat extinction curves., 8 pages, 5 figures, Accepted for publication in MNRAS

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., Comment: 24 pages, 13 figures, accepted to A&A

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., Comment: 12 pages, 8 figures, accepted for publication in MNRAS

The infrared-to-X-ray (IRX) flux ratio traces the relative importance of dust cooling to gas cooling in astrophysical plasma such as supernova remnants (SNRs). We derive IRX ratios of SNRs in the LMC using Spitzer and Chandra SNR survey data and compare them with those of Galactic SNRs. IRX ratios of all the SNRs in the sample are found to be moderately greater than unity, indicating that dust grains are a more efficient coolant than gas although gas cooling may not be negligible. The IRX ratios of the LMC SNRs are systematically lower than those of the Galactic SNRs. As both dust cooling and gas cooling pertain to the properties of the interstellar medium, the lower IRX ratios of the LMC SNRs may reflect the characteristics of the LMC, and the lower dust-to- gas ratio (a quarter of the Galactic value) is likely to be the most significant factor. The observed IRX ratios are compared with theoretical predictions that yield IRX ratios an order of magnitude larger. This discrepancy may originate from the dearth of dust in the remnants due to either the local variation of the dust abundance in the preshock medium with respect to the canonical abundance or the dust destruction in the postshock medium. The non-equilibrium ionization cooling of hot gas, in particular for young SNRs, may also cause the discrepancy. Finally, we discuss implications for the dominant cooling mechanism of SNRs in low-metallicity galaxies., Comment: 8 pages, 2 figures, 1 table. Accepted for publication in ApJ

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., 6 pages, 5 figures, Accepted for publication in Astronomy & Astrophysics

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, Accepted for publication in The Publications of the Astronomical Society of Japan. 11 pages, 9 figures

We investigate the radio spectral energy distributions (SEDs) of young star-forming galaxies and how they evolve with time. The duration and luminosity of the nonthermal radio emission from supernova remnants (SNRs) are constrained by using the observational radio SEDs of SBS 0335-052 and I Zw 18, which are the two lowest-metallicity blue compact dwarf galaxies in the nearby universe. The typical radio ``fluence'' for SNRs in SBS 0335-052, that is the radio energy emitted per SNR over its radiative lifetime, is estimated to be $\sim 6$--$22\times 10^{22} {\rm W Hz^{-1} yr}$ at 5 GHz. On the other hand, the radio fluence in I Zw 18 is $\sim 1$--$3\times 10^{22} {\rm W Hz^{-1} yr}$ at 5 GHz. We discuss the origin of this variation and propose scaling relations between synchrotron luminosity and gas density. We have also predicted the time dependence of the radio spectral index and of the spectrum itself, for both the ``active'' (SBS 0335-052) and ``passive'' (I Zw 18) cases. These models enable us to roughly age date and classify radio spectra of star-forming galaxies into active/passive classes. Implications for high-z galaxy evolution are also discussed., Comment: 18 pages, 8 figures, accepted by A&A

We examine the possibility that ram pressure exerted by the galactic wind from the Galaxy could have stripped gas from the Local Group dwarf galaxies, thereby affecting their star formation histories. Whether gas stripping occurs or not depends on the relative magnitudes of two counteracting forces acting on gas in a dwarf galaxy: ram pressure force by the wind and the gravitational binding force by the dwarf galaxy itself. We suggest that the galactic wind could have stripped gas in a dwarf galaxy located within the distance of $R_{c}\simeq 120(r_{s}/1 {kpc})^{3/2} ({\cal E}_{b}/10^{50} {erg})^{-1/2}$ kpc (where $r_{s}$ is the surface radius and ${\cal E}_{b}$ is the total binding energy of the dwarf galaxy, respectively) from the Galaxy within a timescale of Gyr, thereby preventing star formation there. Our result based on this Galactic wind model explains the recent observation that dwarfs located close to the Galaxy experienced star formation only in the early phase of their lifetimes, whereas distant dwarfs are still undergoing star formation. The present star formation in the Large Magellanic Cloud can also be explained through our Galactic wind model., 7 pages LaTeX, no figures, to appear in MNRAS

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., MNRAS, in press. 18 pages, 15 figures. Abstract abridged

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., 11 pages, 6 figures, Accepted for publication in MNRAS

We investigate the role of dust in star formation activity of extremely metal-poor blue compact dwarf galaxies (BCDs). Observations suggest that star formation in BCDs occurs in two different regimes: "active" and "passive". The "active" BCDs host super star clusters (SSCs), and are characterised by compact size, rich H2 content, large dust optical depth, and high dust temperature; the "passive" BCDs are more diffuse with cooler dust, and lack SSCs and large amounts of H2. By treating physical processes concerning formation of stars and dust, we are able to simultaneously reproduce all the above properties of both modes of star formation (active and passive). We find that the difference between the two regimes can be understood through the variation of the "compactness" of the star-forming region: an "active" mode emerges if the region is compact (with radius $\la 50$ pc) and dense (with gas number density $\ga 500$ cm$^{-3}$). The dust, supplied from Type II supernovae in a compact star-forming region, effectively reprocesses the heating photons into the infrared and induces a rapid H2 formation over a period of several Myr. This explains the high infrared luminosity, high dust temperature, and large H2 content of active BCDs. Moreover, the gas in "active" galaxies cools ($\la 300$ K) on a few dynamical timescales, producing a "run-away" star formation episode because of the favourable (cool) conditions. The mild extinction and relatively low molecular content of passive BCDs can also be explained by the same model if we assume a diffuse region (with radius $\ga 100$ pc and gas number density $\la 100$ cm$^{-3}$). We finally discuss primordial star formation in high-redshift galaxies in the context of the "active" and "passive" star formation scenario., Comment: Astronomy and Astrophysics, in press, 16 pages, 8 figures

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., 21 pages, 9 figures, accepted for publication in A&A

To interpret H_2 quasar absorption line observations in Damped Ly-alpha clouds (DLAs), we model the H_2 spatial distribution within a DLA. Based on numerical simulations of disk structures with parameters similar to those derived for such absorbers, we calculate the H_2 distribution as a function of ultraviolet background (UVB) intensity and dust-to-gas ratio. For typical values of these two quantities we find that the area in which the H_2 fraction exceeds 10^{-6} (typical observational detection limit) only covers $\la 10$% of the disk surface, i.e. H_2 has a very inhomogeneous, clumpy distribution even at these low abundance levels. This explains the relative paucity of H_2 detections in DLAs. We also show the dependence of the covering fraction of H_2 on dust-to-gas ratio and UVB intensity and we comment on the physics governing the H_2 chemical network at high redshift., Comment: 5 pages, 3 figures, 1 table, accepted by MNRAS Letter

This paper investigates the origin of the observed large variety in dust-to-gas ratio among blue compact dwarf galaxies (BCDs). By applying our chemical evolution model, we find that the dust destruction can largely suppress the dust-to-gas ratio when the metallicity of a BCD reaches $12+\log{\rm (O/H)}\sim 8$, i.e., a typical metallicity level of BCDs. We also show that dust-to-gas ratio is largely varied owing to the change of dust destruction efficiency that has two effects: (i) a significant contribution of Type Ia supernovae to total supernova rate; (ii) variation of gas mass contained in a star-forming region. While mass loss from BCDs was previously thought to be the major cause for the variance of dust-to-gas ratio, we suggest that the other two effects are also important. We finally discuss the intermittent star formation history, which naturally explains the large dispersion of dust-to-gas ratio among BCDs., 7 pages LaTeX, to appear in A&A

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., 5 pages, to be published in PASJ, August 25, 2001

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., 14 pages LaTeX, 1 PostScript figure, to appear in ApJ Letter

We model the star formation history (SFH) and the chemical evolution of the Galactic disk by combining an infall model and a limit-cycle model of the interstellar medium (ISM). Recent observations have shown that the SFH of the Galactic disk violently variates or oscillates. We model the oscillatory SFH based on the limit-cycle behavior of the fractional masses of three components of the ISM. The observed period of the oscillation ($\sim 1$ Gyr) is reproduced within the natural parameter range. This means that we can interpret the oscillatory SFH as the limit-cycle behavior of the ISM. We then test the chemical evolution of stars and gas in the framework of the limit-cycle model, since the oscillatory behavior of the SFH may cause an oscillatory evolution of the metallicity. We find however that the oscillatory behavior of metallicity is not prominent because the metallicity reflects the past integrated SFH. This indicates that the metallicity cannot be used to distinguish an oscillatory SFH from one without oscillations., 21 pages LaTeX, to appear in ApJ

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 5\sigma-confusion limit of the LMSA is fainter than 1 \mu Jy. 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., Comment: 17 pages LaTeX, PASP accepted. Abstract abridged

We analyze the star formation history (SFH) of the Galactic disk by using an infall model. Based on the observed SFH of the Galactic disk, we first determine the timescales of the gas infall into the Galactic disk ($t_{\rm in}$) and that of the gas consumption to form stars ($t_{\rm sf}$). Since each of the two timescales does not prove to be determined independently from the SFH, we first fix $t_{\rm sf}$. Then, $t_{\rm in}$ is determined so that we minimize $\chi^2$. Consequently, we choose three parameter sets: ($t_{\rm sf}$ [Gyr], $t_{\rm in}$ [Gyr]) = (6.0, 23), (11, 12), and (15, 9.0), where we set the Galactic age as 15 Gyr. All of the three cases predict almost identical star formation history. Next, we test the intermittency (or variability) of the star formation rate (SFR) along with the smooth SFH suggested from the infall model. The large value of the $\chi^2$ statistic supports the violent time-variation of the SFH. If we interpret the observed SFH with smooth and variable components, the amplitude of the variable component is comparable to the smooth component. Thus, intermittent SFH of the Galactic disk is strongly suggested. We also examined the metallicity distribution of G-dwarfs. We found that the true parameter set lies between ($t_{\rm sf}$ [Gyr], $t_{\rm in}$ [Gyr]) = (6, 23) and (11, 12), though we should need a more sophisticated model including the process of metal enrichment within the Galactic halo., Comment: 19 pages LaTeX, to appear in ApJ

We propose a limit-cycle scenario of star formation history for any morphological type of spiral galaxies. It is known observationally that the early-type spiral sample has a wider range of the present star formation rate (SFR) than the late-type sample. This tendency is understood in the framework of the limit-cycle model of the interstellar medium (ISM), in which the SFR cyclically changes in accordance with the temporal variation of the mass fraction of the three ISM components. When the limit-cycle model of the ISM is applied, the amplitude of variation of the SFR is expected to change with the supernova (SN) rate. Observational evidence indicates that the early-type spiral galaxies show smaller rates of present SN than late-type ones. Combining this evidence with the limit-cycle model of the ISM, we predict that the early-type spiral galaxies show larger amplitudes in their SFR variation than the late-types. Indeed, this prediction is consistent with the observed wider range of the SFR in the early-type sample than in the late-type sample. Thus, in the framework of the limit-cycle model of the ISM, we are able to interpret the difference in the amplitude of SFR variation among the morphological classes of spiral galaxies., Comment: 12 pages LaTeX, to appear in AJ

Infrared Imaging Surveyor (IRIS, officially Astro-F) is a satellite which will be launched in the winter of 2003. The main purpose of the IRIS mission is an all sky survey in the mid- and far-IR with a flux limit much deeper than that of IRAS. In order to examine the performance of the survey and to find a suitable set of bandpasses for tracing galaxy evolution and picking up protogalaxy candidates as effective as possible using IRIS, we estimated the FIR galaxy counts based on a simple model with various sets of cosmological parameters and evolution types.

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., Comment: 14 pages LaTeX, 2 PostScript figures, to appear in ApJ Letters

Infrared Imaging Surveyor (IRIS) is a satellite which will be launched at the beginning of 2003. One of the main purposes of the IRIS mission is an all-sky survey in the far-infrared region with a flux limit much deeper than that of IRAS. Detection of a large number of galaxies ($\sim several \times 10^6$ in the whole sky) is expected in this survey. We investigated the expected optical and near-infrared (NIR) number counts of galaxies detected by the far-infrared scanner (FIS) of IRIS (hereafter, IRIS galaxies) and possibility of optical and NIR follow-up of them. The spectral energy distribution and the luminosity function of the IRIS galaxies are modeled based on the properties of galaxies observed by IRAS. The IRIS galaxies are divided into two populations according to their infrared luminosities ($L_{IR}$); normal spirals ($L_{IR}10^{10}\LO$). The expected number counts of IRIS galaxies for both of the populations are calculated in $B$ and $H$ bands. We show that about 60 normal galaxies and about 80 starburst galaxies are detected per square degree in both of the two bands, when galaxy evolution is not taken into account. All of the normal population of IRIS galaxies are located at the redshift $z\ltsim 0.1$. As for the starburst population, we also calculated number of galaxies with a simple model of evolution. The total number of starburst population predicted by the evolution model is larger by 20% than that expected from the non-evolution model. In the evolution model, the numbers of low-z ($z3$) galaxies are 100, 20, and 0.2 per square degree, respectively., Comment: 18 pages LaTeX, 9 figures, to appear in PASJ

We discuss a physical interpretation of the relation between $M_{\rm vir}/L$ and $M_{\rm vir}$ of dwarf spheroidal galaxies (dSphs), where $M_{\rm vir}$ and $L$ are the virial mass and the total luminosity of a dSph, respectively. We used 11 dSphs in the Local Group as the sample. We find two distinct sequences on the $M_{\rm vir}/L$--$M_{\rm vir}$ plane: $M_{\rm vir}/L\propto M_{\rm vir}^{2.0}$ for dSphs with $M_{\rm vir}10^8M_\odot$. A ``discontinuity'' is seen at $M_{\rm vir}\simeq 10^8M_\odot$. We interpret the ``discontinuity'' as the threshold for the gas in dSphs to be blown away by successive supernovae. If a dSph has virial mass (most of which is dark mass) less than $10^8M_\odot$, the gas is blown away, while in a dSph of larger mass, the deep potential well prevents the blow-away mechanism from working effectively. Thus, large mass ratio of dark matter (DM) to baryonic matter (i.e., large $M_{\rm vir}/L$) is realized in a low-mass ($M_{\rm vir}10^8M_\odot$) dSphs. We further make an attempt to explain the above relation for the low-mass dSphs, $M_{\rm vir}/L\propto M_{\rm vir}^{2.0}$, based on estimate of cooling time, using the scaling laws of virial temperature, virial mass and radius of a dSph and assuming that the heating by OB-star radiation terminates the star formation activity. We succeed in deriving the above relation for the mass-to-light ratio and luminosity., 9 pages LaTeX, 1 figure, to appear in ApJ Letters

Using a one-zone model for the evolution of dust in spiral galaxies and applying the instantaneous recycling approximation to the model equations, we investigate dust-to-gas ratio of spiral galaxies. Four processes are considered; dust formation from heavy elements ejected by stellar mass loss, dust destruction in supernova remnants, dust destruction in star-forming regions, and accretion of heavy elements onto preexisting dust grains. The equations describe a simplified relation between dust-to-gas ratio and metallicity. The relation is independent of star formation rate. By comparing the theoretical relation with the observational data of nearby spiral galaxies, we show (i) that the accretion process onto the preexisting dust particles must be taken into account for the spiral galaxies; (ii) that the efficiency of dust production from heavy elements ejected by stars can be constrained by the spiral galaxies with low metallicity; (iii) that the Salpeter and Scalo initial mass functions are both consistent with the data of the spiral galaxies., Comment: 9 pages LaTeX, 3 figures, to appear in ApJ Letters

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 $\mu$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., Comment: 40 pages LaTeX2e, 11 embedded figures, to appear in PASP

Intra-cluster spaces are filled with intra-cluster medium (ICM), whose typical temperature and density are TICM ∼ 107.5 K and nICM ∼ 10−3 cm−3, respectively (e.g., Sarazin 1988). Recent Faraday rotation measurements have revealed the existence of magnetic fields in ICM with BICM ∼ a & few−10 µG (e.g., Ge & Owen 1993). In ICM, the plasma β (the ratio of gas pressure to magnetic pressure) is almost “equipartition” value as follows: $$ \beta \simeq 1 \times {{\left( {\frac{{{{B}_{{{\text{ICM}}}}}}}{{10\mu {\text{G}}}}} \right)}^{{ - 2}}}\left( {\frac{{{{n}_{{{\text{ICM}}}}}}}{{{{{10}}^{{ - 3}}}{\text{c}}{{{\text{m}}}^{{ - 3}}}}}} \right)\left( {\frac{{{{T}_{{{\text{ICM}}}}}}}{{{{{10}}^{{7.5}}}{\text{K}}}}} \right). $$ (1)

Recent studies have been revealing the properties of dwarf spheroidal galaxies (dSphs). Their low mass indicates that the dSphs may provide a clue to physical properties of the building blocks in the hierarchical structure formation. We select the Local Group dSphs as a sample. To obtain the information on the star formation history of dSphs, we investigate the relation between their metallicity and virial mass. According to our scenario, the star formation efficiency of the dSphs is low because of strong regulation. This is consistent with their high mass-to-light ratios. We also comment on the environmental effects on the dSphs.

To interpret H2 quasar absorption line observations in DLAs (damped Lyα clouds), we model the H2 spatial distribution within a DLA. Based on numerical simulations of disk structures with parameters similar to those derived for such absorbers, we calculate the H2 distribution as a function of ultraviolet background (UVB) intensity and dust-to-gas ratio. For typical values of these two quantities we find that the area in which the H2 fraction exceeds 10−6 (typical observational detection limit) only covers < 10% of the disk surface, i.e., H2 has a very inhomogeneous, clumpy distribution even at these low abundance levels. This explains the relative paucity of H2 detections in DLAs. We also show the dependence of the covering fraction of H2 on dust-to-gas ratio and UVB intensity and we comment on the physics governing the H2 chemical network at high redshift. We finally comment on our implication on the statistics of the H2 column density distribution.

We constructed a model of the infrared spectral energy distribution for very young galaxies by taking into account the dust size distribution in the early stage of galaxy evolution.

At high redshift, the universe was so young that core-collapse supernovae (SNe) are suspected to be the dominant source of dust production. However, some observations indicate that the dust production by SNe is an inefficient process, casting doubts on the existence of abundant SNe-dust in the early universe. Recently, Perley et al. (2010) reported that the afterglow of GRB 071025 - an unusually red GRB at z ~ 5 - shows evidence for the SNe-produced dust. Since this is perhaps the only high redshift GRB exhibiting compelling evidence for SNe-dust but the result could easily be affected by small systematics in photometry, we re-examined the extinction properties of GRB 071025 using our own optical/near-infrared data at a different epoch. In addition, we tested SNe-dust models with different progenitor masses and dust destruction efficiencies to constrain the dust formation mechanisms. By searching for the best-fit model of the afterglow spectral energy distribution, we confirm the previous claim that the dust in GRB 071025 is most likely to originate from SNe. We also find that the SNe-dust model of 13 or 25M_{\odot} without dust destruction fits the extinction property of GRB 071025 best, while pair-instability SNe (PISNe) models with a 170M_{\odot} progenitor poorly fit the data. Our results indicate that, at least in some systems at high redshift, SNe with intermediate masses within 10 - 30M_{\odot} were the main contributors for the dust enrichment, and the dust destruction effect due to reverse shock was negligible., 14 pages, 2 figures, Accepted for publication in ApJL

We develop one-zone galaxy formation models in the early Universe, taking into account dust formation and evolution by supernova (SN) explosions. Especially we focus on the time evolution of dust size distribution, because ${\rm H}_{2}$ formation on the dust surface plays a critical role in the star formation process in the early Universe. In the model we assume that star formation rate (SFR) is proportional to the total amount of ${\rm H}_{2}$. We consistently treat (i) the formation and size evolution of dust, (ii) the chemical reaction networks including ${\rm H}_{2}$ formation both on the surface of dust and in gas phase, and (iii) the SFR in the model. First, we find that, because of dust destruction due to both reverse and forward shocks driven by SNe, H$_{2}$ formation is more suppressed than that without dust destruction. At the galaxy age of $sim0.8 {\rm Gyr}$, for galaxy models with virial mass $M_{\rm vir}=10^{9} M_{\odot}$ and formation redshift $z_{\rm vir}=10$, the molecular fraction is 2.5 orders of magnitude less in the model with dust destruction by both shocks than that in the model without dust destruction. Second, we show that the H$_{2}$ formation rate strongly depends on the ISM density around SN progenitors. The SFR in higher ISM density is lower, since dust destruction by reverse shocks is more effective in higher ISM density. We conclude that not only the amount but also the size distribution of dust being related with the star formation activity strongly affects the evolution of galaxies in the early Universe., 23 pages, 12 figures, 3 tables, Accepted for publication in ApJ