Search

Ground-based observations at terahertz (THz) frequencies are a newly explorable area of astronomy for the next ten years. We discuss science cases for a first-generation 10-m class THz telescope, focusing on the Greenland Telescope as an example of such a facility. We propose science cases and provide quantitative estimates for each case. The largest advantage of ground-based THz telescopes is their higher angular resolution (~ 4 arcsec for a 10-m dish), as compared to space or airborne THz telescopes. Thus, high-resolution mapping is an important scientific argument. In particular, we can isolate zones of interest for Galactic and extragalactic star-forming regions. The THz windows are suitable for observations of high-excitation CO lines and [N II] 205 um lines, which are scientifically relevant tracers of star formation and stellar feedback. Those lines are the brightest lines in the THz windows, so that they are suitable for the initiation of ground-based THz observations. THz polarization of star-forming regions can also be explored since it traces the dust population contributing to the THz spectral peak. For survey-type observations, we focus on ``sub-THz'' extragalactic surveys, whose uniqueness is to detect galaxies at redshifts z ~ 1--2, where the dust emission per comoving volume is the largest in the history of the Universe. Finally we explore possibilities of flexible time scheduling, which enables us to monitor active galactic nuclei, and to target gamma-ray burst afterglows. For these objects, THz and submillimeter wavelength ranges have not yet been explored., 39 pages, 17 figures, accepted for publication in PASJ (as a review paper)

The central star-forming region in a blue compact dwarf galaxy, II Zw 40, was observed in the 340 GHz ($880 \micron$) band at $\sim 5$ arcsec (250 pc) resolution with the Submillimetre Array (SMA). A source associated with the central star-forming complex was detected with a flux of $13.6\pm 2.0$ mJy. The structure is more extended than the beam in the east-west direction. The SMA 880 $\micron$ flux is analyzed by using theoretical models of radio spectral energy distribution along with centimetre interferometric measurements in the literature. We find (i) that the SMA 880 $\micron$ flux is dominated ($\sim 75$ per cent) by free-free emission from the central compact star-forming region, and (ii) that the contribution from dust emission to the SMA 880 $\micron$ flux is at most $4\pm 2.5$ mJy. We also utilize our models to derive the radio--FIR relation of the II Zw 40 centre, suggesting that free-free absorption at low frequencies ($\nu\la$ several GHz; $\lambda\ga$ several cm) and spatial extent of dust affect the radio-FIR relation.

Grain growth by the accretion of metals in interstellar clou ds (called ‘grain growth’) could be one of the dominant processes that determine the dust content in galaxies. The importance of grain size distribution for the grain growth is demo nstrated in this paper. First, we derive an analytical formula that gives the grain size distr ibution after the grain growth in individual clouds for any initial grain size distribution. The time-scale of the grain growth is very sensitive to grain size distribution, since the grai n growth is mainly regulated by the surface to volume ratio of grains. Next, we implement the results of grain growth into dust enrichment models of entire galactic system along with the grain formation and destruction in the interstellar medium, finding that the grain growth in c louds governs the dust content in nearby galaxies unless the grain size is strongly biased to sizes larger than � 0.1 µm or the power index of the grain size distribution is shallower t han � 2.5. The grain growth in clouds contributes to the rapid increase of dust-to-gas r atio at a certain metallicity level (called critical metallicity in Asano et al. 2011 and Inoue 2 011), which we find to be sensitive to grain size distribution. Thus, the grain growth efficient ly increase the dust mass not only in nearby galaxies but also in high-redshift quasars, whose metallicities are larger than the critical value. Our recipe for the grain growth is applicabl e for any grain size distribution and easily implemented into any framework of dust enrichment in galaxies.

The importance of the far-infrared (FIR) mapping is demonstrated for a face-on spiral galaxy, M81, by analysing its imaging data at 65, 90 and 140 μm taken by AKARI. The basic products are the dust temperature map, the dust optical depth map and the colour-colour diagram. The main features are as follows. (i) The dust temperature derived from the total fluxes at 90 and 140 μm reflects the relatively low temperatures seen in the interarm and spiral arms excluding the warm spots rather than the high temperatures in warm spots and the centre. This indicates that the total FIR luminosity is dominated by the dust heated by the general interstellar radiation field. (ii) The galaxy is more extended at 140 μm than at the other shorter wavelengths, which reflects the radial dust temperature gradient. (iii) The dust optical depth derived from the FIR mapping is broadly consistent with that estimated from the FIR-to-ultraviolet luminosity ratio. (iv) The FIR colour-colour diagram is useful to identify a 'contamination' of warm dust. The existence of small-scale warm star-forming regions is supported in the bright spots along the spiral arms. This contamination also leads to an underestimate of dust optical depth (or dust column density).

We aim at understanding the statistical properties of luminous sub-millimeter (submm) galaxies (SMGs) in the context of cosmological structure formation. By utilizing a cosmological N-body simulation to calculate the distribution of dark halos in the Universe, we consider the dust enrichment in individual halos by Type II supernovae (SNe II). The SN II rate is estimated under a star formation activity which is assumed to occur on a dynamical timescale in the dark matter potential. Our simple framework successfully explains the luminosity function, the typical star formation rate, and the typical dust mass of an observational SMG sample at z∼3. We also examine the clustering properties of SMGs, since a positive cross correlation between SMGs and Lyα emitters (LAEs) is indeed observed by a recent observation. In the simulation, we select SMGs by FIR dust luminosity >1012L⊙, while LAEs are chosen such that the age and the virial mass are consistent with the observed LAE properties. The SMGs and LAEs selected in this way show a spatial cross correlation whose strength is consistent with the observation. This confirms that the SMGs really trace the most clustered regions at z∼3 and that their luminosities can be explained by the dust accumulation as a result of their star formation activities. We extend our prediction to higher redshifts, finding that a statistical sample of submm galaxies at z≥6 can be obtained by ALMA with a 100 arcmin2 survey. With the same survey, a few submm galaxies at z∼10 may be detected.

Using the results from a previously developed Lyman Alpha/continuum production/transmission and dust enrichment model for Lyman Alpha Emitters (LAEs), based on cosmological SPH simulations, we assess the detectability of their dust-reprocessed sub-millimeter (submm) radiation. As supernovae (rather than evolved stars) control dust formation and destruction processes, LAEs are relatively dust-poor with respect to local galaxies: they have low dust-to-gas ratios (0.05 times the dust-to-gas ratio of the Milky Way) in spite of their relatively high metallicity, which is approximately 10-50% of the solar value. Using the derived escape fraction of ultraviolet (UV) continuum photons we compute the UV luminosity absorbed by dust and re-emitted in the far infrared. The LAE submm fluxes correlate with their Lyman Alpha luminosity: about (3%, 1%) at z=(5.7, 6.6) of the LAEs in our simulated sample (those with with Log L_\alpha > 43.1) would have fluxes at 850 micrometer (the optimal band for detection) in excess of 0.12 mJy and will be therefore detectable at 5 sigma with ALMA with an integration time of only 1 hour. Such detections would open a new window on the physical conditions prevailing in these most distant galaxies.

We investigate shattering and coagulation of dust grains in turbulent interstellar medium (ISM). The typical velocity of dust grain as a function of grain size has been calculated for various ISM phases based on a theory of grain dynamics in compressible magnetohydrodynamic turbulence. In this paper, we develop a scheme of grain shattering and coagulation and apply it to turbulent ISM by using the grain velocities predicted by the above turbulence theory. Since large grains tend to acquire large velocity dispersions as shown by earlier studies, large grains tend to be shattered. Large shattering effects are indeed seen in warm ionized medium (WIM) within a few Myr for grains with radius $a\ga 10^{-6}$ cm. We also show that shattering in warm neutral medium (WNM) can limit the largest grain size in ISM ($a\sim 2\times 10^{-5} \mathrm{cm}$). On the other hand, coagulation tends to modify small grains since it only occurs when the grain velocity is small enough. Coagulation significantly modifies the grain size distribution in dense clouds (DC), where a large fraction of the grains with $a, 15 pages, 10 figures, accepted for publication in MNRAS

We investigate the size-density relation of H ii regions in blue compact dwarf galaxies (BCDs) by compiling observational data of their size (Di) and electron density (ne). We find that the size-density relation follows a relation with constant column density (ne ∝ Di−1) rather than with constant luminosity (ne ∝ Di−1.5). Such behavior resembles that of Galactic H ii regions, and may imply an underlying “scale-free” connection. Because this size-density relation cannot be explained by static models, we model and examine the evolution of the size-density relation of H ii regions by considering the star formation history and pressure-driven expansion of H ii regions. We find that the size-density relation of the entire BCD sample does not result from an evolutionary sequence of H ii regions but rather reflects a sequence with different initial gas densities (or “hierarchy” of density). We also find that the dust extinction of ionizing photons is significant for the BCD sample, despite their blue optical colors. This means that as long as the emission from H ii regions is used to trace massive star formation, we would miss the star formation activity in dense environments even in low-metallicity galaxies such as BCDs.

We reconsider the origin and processing of dust in elliptical galaxies. We theoretically formulate the evolution of grain size distribution, taking into account dust supply from asymptotic giant branch (AGB) stars and dust destruction by sputtering in the hot interstellar medium (ISM), whose temperature evolution is treated by including two cooling paths: gas emission and dust emission (i.e. gas cooling and dust cooling). With our new full treatment of grain size distribution, we confirm that dust destruction by sputtering is too efficient to explain the observed dust abundance even if AGB stars continue to supply dust grains, and that, except for the case where the initial dust-to-gas ratio in the hot gas is as high as $\sim 0.01$, dust cooling is negligible compared with gas cooling. However, we show that, contrary to previous expectations, cooling does not help to protect the dust; rather, the sputtering efficiency is raised by the gas compression as a result of cooling. We additionally consider grain growth after the gas cools down. Dust growth by the accretion of gas-phase metals in the cold medium increase the dust-to-gas ratio up to $\sim 10^{-3}$ if this process lasts >10/(n_H/10^3 cm^{-3}) Myr, where $n_\mathrm{H}$ is the number density of hydrogen nuclei. We show that the accretion of gas-phase metals is a viable mechanism of increasing the dust abundance in elliptical galaxies to a level consistent with observations, and that the steepness of observed extinction curves is better explained with grain growth by accretion., 15 pages, 11 figures, accepted for publication in MNRAS

The Large and Small Magellanic Clouds (LMC and SMC, respectively) are observed to have characteristic dust extinction curves that are quite different from those of the Galaxy (e.g., strength of the 2175 A bump). Although the dust composition and size distribution of the Magellanic Clouds (MCs) that can self-consistently explain their observed extinction curves have been already proposed, it remain unclear whether and how the required dust properties can be achieved in the formation histories of the MCs. We therefore investigate the time evolution of the dust properties of the MCs and thereby derive their extinction curves using one-zone chemical evolution models with formation and evolution of small and large silicate and carbonaceous dust grains and dusty winds associated with starburst events. We find that the observed SMC extinction curve without a conspicuous 2175 A bump can be reproduced well by our SMC model, if the small carbon grains can be selectively lost through the dust wind during the latest starburst about 0.2 Gyr ago. We also find that the LMC extinction curve with a weak 2175 A bump can be reproduced by our LMC model with less efficient removal of dust through dust wind. We discuss possible physical reasons for different dust wind efficiencies between silicate and graphite and among galaxies., Comment: 31 pages, 13 figures, accepted for the publication in ApJ

Full calculations of the evolution of grain size distribution in galaxies are in general computationally heavy. In this paper, we propose a simple model of dust enrichment in a galaxy with a simplified treatment of grain size distribution by imposing a `two-size approximation'; that is, all the grain population is represented by small (grain radius a < 0.03 micron) and large (a > 0.03 micron) grains. We include in the model dust supply from stellar ejecta, destruction in supernova shocks, dust growth by accretion, grain growth by coagulation and grain disruption by shattering, considering how these processes work on the small and large grains. We show that this simple framework reproduces the main features found in full calculations of grain size distributions as follows. The dust enrichment starts with the supply of large grains from stars. At a metallicity level referred to as the critical metallicity of accretion, the abundance of the small grains formed by shattering becomes large enough to rapidly increase the grain abundance by accretion. Associated with this epoch, the mass ratio of the small grains to the large grains reaches the maximum. After that, this ratio converges to the value determined by the balance between shattering and coagulation, and the dust-to-metal ratio is determined by the balance between accretion and shock destruction. With a Monte Carlo simulation, we demonstrate that the simplicity of our model has an advantage in predicting statistical properties. We also show some applications for predicting observational dust properties such as extinction curves., 14 pages, 12 figures, accepted for publication in MNRAS

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 \lambda \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., Comment: MNRAS in press, pages, 6 figures, using mn2e.cls. Abstract abridged

Stars form out of molecular gas and supply dust grains during their last evolutionary stages; in turn hydrogen molecules (H2) are produced more efficiently on dust grains. Therefore, dust can drastically accelerate H2 formation, leading to an enhancement of star formation activity. In order to examine the first formation of stars and dust in galaxies, we model the evolution of galaxies in the redshift range of 55) galaxies in sub-millimetre and near-infrared bands. We find that: i) ALMA can detect dust emission from several thousands of galaxies per square degree, and ii) NGST can detect the stellar emission from 10^6 galaxies per square degree. Further observational checks of our predictions include the integrated flux of metal (oxygen and carbon) lines. We finally discuss possible color selection strategies for high-redshift galaxy searches., Comment: 19 pages, LaTeX, 10 figures, accepted to MNRAS

The CO-to-H$_2$ conversion factor ($X_\mathrm{CO}$) is known to correlate with the metallicity ($Z$). The dust abundance, which is related to the metallicity, is responsible for this correlation through dust shielding of dissociating photons and H$_2$ formation on dust surfaces. In this paper, we investigate how the relation between dust-to-gas ratio and metallicity ($\mathcal{D}$--$Z$ relation) affects the H$_2$ and CO abundances (and $X_\mathrm{CO}$) of a `molecular' cloud. For the $\mathcal{D}$--$Z$ relation, we adopt a dust evolution model developed in our previous work, which treats the evolution of not only dust abundance but also grain sizes in a galaxy. Shielding of dissociating photons and H$_2$ formation on dust are solved consistently with the dust abundance and grain sizes. As a consequence, our models {predict consistent metallicity dependence of $X_\mathrm{CO}$ with observational data}. Among various processes driving dust evolution, grain growth by accretion has the largest impact on the $X_\mathrm{CO}$--$Z$ relation. The other processes also have some impacts on the $X_\mathrm{CO}$--$Z$ relation, but their effects are minor compared with the scatter of the observational data at the metallicity range ($Z\gtrsim 0.1$ Z$_\odot$) where CO could be detected. We also find that dust condensation in stellar ejecta has a dramatic impact on the H$_2$ abundance at low metallicities ($\lesssim 0.1$ Z$_\odot$), relevant for damped Lyman $\alpha$ systems and nearby dwarf galaxies, and that the grain size dependence of H$_2$ formation rate is also important., Comment: 18 pages, 7 figures, accepted for publication in MNRAS

We apply the time evolution of grain size distributions by accretion and coagulation found in our previous work to the modelling of the wavelength dependence of interstellar linear polarization. We especially focus on the parameters of the Serkowski curve $K$ and $\lambda_{\max}$ characterizing the width and the maximum wavelength of this curve, respectively. We use aligned silicate and non-aligned carbonaceous spheroidal particles with different aspect ratios $a/b$. The imperfect alignment of grains with sizes larger than a cut-off size $r_{V,\rm cut}$ is considered. We find that the evolutionary effects on the polarization curve are negligible in the original model with commonly used material parameters (hydrogen number density $n_\mathrm{H}=10^3$ cm$^{-3}$, gas temperature $T_\mathrm{gas}=10$~K, and the sticking probability for accretion $S_\mathrm{acc}=0.3$). Therefore, we apply the tuned model where the coagulation threshold of silicate is removed. In this model, $\lambda_{\max}$ displaces to the longer wavelengths and the polarization curve becomes wider ($K$ reduces) on time-scales $\sim (30 - 50) (n_\mathrm{H}/10^3 \mathrm{cm}^{-3})^{-1}$ Myr. The tuned models at $T < 30 (n_\mathrm{H}/10^3 \mathrm{cm}^{-3})^{-1} $ Myr and different values of the parameters $r_{V,\rm cut}$ can also explain the observed trend between $K$ and $\lambda_{\max}$. It is significant that the evolutionary effect appears in the perpendicular direction to the effect of $r_{V,\rm cut}$ on the $K$ - $\lambda_{\max}$ diagram. Very narrow polarization curves can be reproduced if we change the type of particles (prolate/oblate) and/or vary $a/b$., Comment: 9 pages, 6 figures, accepted for publication in MNRAS

We compare theoretical dust yields for stars with mass 1 Msun < mstar < 8 Msun, and metallicities 0.001 < Z < 0.008 with observed dust production rates (DPR) by carbon- rich and oxygen-rich Asymptotic Giant Branch (C-AGB and O-AGB) stars in the Large and Small Magellanic Clouds (LMC, SMC). The measured DPR of C-AGB in the LMC are reproduced only if the mass loss from AGB stars is very efficient during the carbon-star stage. The same yields over-predict the observed DPR in the SMC, suggesting a stronger metallicity dependence of the mass-loss rates during the carbon- star stage. DPR of O-AGB stars suggest that rapid silicate dust enrichment occurs due to efficient hot-bottom-burning if mstar > 3 Msun and Z > 0.001. When compared to the most recent observations, our models support a stellar origin for the existing dust mass, if no significant destruction in the ISM occurs, with a contribution from AGB stars of 70% in the LMC and 15% in the SMC., 12 pages, 8 figures, accepted by MNRAS

The ubiquity of planets poses an interesting question: when first planets are formed in galaxies. We investigate this problem by adopting a theoretical model developed for understanding the statistical properties of exoplanets. Our model is constructed as the combination of planet traps with the standard core accretion scenario in which the efficiency of forming planetary cores directly relates to the dust density in disks or the metallicity ([Fe/H]). We statistically compute planet formation frequencies (PFFs) as well as the orbital radius ($$) within which gas accretion becomes efficient enough to form Jovian planets. The three characteristic exoplanetary populations are considered: hot Jupiters, exo-Jupiters densely populated around 1 AU, and low-mass planets such as super-Earths. We explore the behavior of the PFFs as well as $$ for the three different populations as a function of metallicity ($-2 \leq$[Fe/H]$\leq -0.6$). We show that the total PFFs increase steadily with metallicity, which is the direct outcome of the core accretion picture. For the entire range of the metallicity considered here, the population of the low-mass planets dominates over the Jovian planets. The Jovian planets contribute to the PFFs above [Fe/H]-1. We find that the hot Jupiters form at lower metallcities than the exo-Jupiters. This arises from the radially inward transport of planetary cores by their host traps, which is more effective for lower metallicity disks due to the slower growth of the cores. The PFFs for the exo-Jupiters exceed those for the hot Jupiters around [Fe/H]-0.7. Finally, we show that the critical metallicity for forming Jovian planets is [Fe/H]-1.2, which is evaluated by comparing the values of $$ between the hot Jupiters and the low-mass planets. The comparison intrinsically links to the different gas accretion efficiency between them., Comment: 10 pages, 3 figures, 5 tables, accepted for publication in ApJ

Polycyclic aromatic hydrocarbons (PAHs) are some of the major dust components in the interstellar medium (ISM). We present our evolution models for the abundance of PAHs in the ISM on a galaxy-evolution timescale. We consider shattering of carbonaceous dust grains in interstellar turbulence as the formation mechanism of PAHs while the PAH abundance can be reduced by coagulation onto dust grains, destruction by supernova shocks, and incorporation into stars. We implement these processes in a one-zone chemical evolution model to obtain the evolution of the PAH abundance in a galaxy. We find that PAH formation becomes accelerated above certain metallicity where shattering becomes efficient. For PAH destruction, while supernova shock is the primary mechanism in the metal-poor environment, coagulation is dominant in the metal-rich environment. We compare the evolution of the PAH abundances in our models with observed abundances in galaxies with a wide metallicity range. Our models reproduce both the paucity of PAH detection in low metallicity galaxies and the metallicity-dependence of the PAH abundance in high-metallicity galaxies. The strong metallicity dependence of PAH abundance appears as a result of the strong metallicity dependence of the dust mass increase by the accretion of metals onto dust grains, which are eventually shattered into PAHs. We conclude that the observational trend of the PAH abundance can be a natural consequence of shattering of carbonaceous grains being the source of PAHs. To establish our scenario of PAH formation, observational evidence of PAH formation by shattering would be crucial., 13 pages, 7 figures, 2 tables, accepted for publication in MNRAS

We compare theoretical dust yields for stars with mass 1 Msun 3 Msun and Z > 0.001. When compared to the most recent observations, our models support a stellar origin for the existing dust mass, if no significant destruction in the ISM occurs, with a contribution from AGB stars of 70% in the LMC and 15% in the SMC.

Core-collapse supernovae (SNe) are believed to be the first significant source of dust in the Universe. Such SNe are expected to be the main dust producers in young high-redshift Lyman $\alpha$ emitters (LAEs) given their young ages, providing an excellent testbed of SN dust formation models during the early stages of galaxy evolution. We focus on the dust enrichment of a specific, luminous LAE (Himiko, $z\simeq 6.6$) for which a stringent upper limit of $52.1~\mu$Jy ($3\sigma$) has recently been obtained from ALMA continuum observations at 1.2 mm. We predict its submillimetre dust emission using detailed models that follow SN dust enrichment and destruction and the equilibrium dust temperature, and obtain a plausible upper limit to the dust mass produced by a single SN: $m_\mathrm{d,SN} < 0.15$--0.45 M$_\odot$, depending on the adopted dust optical properties. These upper limits are smaller than the dust mass deduced for SN 1987A and that predicted by dust condensation theories, implying that dust produced in SNe are likely to be subject to reverse shock destruction before being injected into the interstellar medium. Finally, we provide a recipe for deriving $m_\mathrm{d,SN}$ from submillimetre observations of young, metal poor objects wherein condensation in SN ejecta is the dominant dust formation channel., Comment: 10 pages, 3 figures, accepted for publication in MNRAS

We re-examine the effect of Lyman continuum ($\lambda \leq 912$ \AA) 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., Comment: 18 papges, 11 figures, accepted by ApJ

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 $\mu$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., Comment: 12pages, no figure. PASJ (Accepted; tex-file is replaced)

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., 18 pages LaTeX, PASJ in press. Abstract abridged

Infrared (IR) dust emission from galaxies is frequently used as an indicator of star formation rate (SFR). However, the effect of the dust-to-gas ratio (i.e., amount of the dust) on the conversion law from IR luminosity to SFR has not so far been considered. Then, in this paper, we present a convenient analytical formula including this effect. In order to obtain the dependence on the dust-to-gas ratio, we extend the formula derived in our previous paper, in which a theoretical formula converting IR luminosity to SFR was derived. That formula was expressed as ${\rm SFR}/(M_\odot~{\rm yr}^{-1})=\{3.3\times 10^{-10}(1- \eta)/(0.4-0.2f+0.6\epsilon)\} (L_{\rm IR}/L_\odot)$, where f is the fraction of ionizing photons absorbed by hydrogen, $\epsilon$ is the efficiency of dust absorption for nonionizing photons, $\eta$ is the cirrus fraction of observed dust luminosity, and $L_{\rm IR}$ is the observed luminosity of dust emission in the 8-1000-$\mu$m range. Our formula explains the IR excess of the Galaxy and the Large Magellanic Cloud. In the current paper, especially, we present the metallicity dependence of our conversion law between SFR and $L_{\rm IR}$. This is possible since both f and $\epsilon$ can be estimated via the dust-to-gas ratio, which is related to metallicity. We have confirmed that the relation between the metallicity and the dust-to-gas ratio is applied to both giant and dwarf galaxies. Finally, we apply the result to the cosmic star formation history. We find that the comoving SFR at z=3 calculated from previous empirical formulae is underestimated by a factor of 4-5., Comment: 8 pages LaTeX, to appear in A&A

Because of their brightness, gamma-ray burst (GRB) afterglows are viable targets for investigating the dust content in their host galaxies. Simple intrinsic spectral shapes of GRB afterglows allow us to derive the dust extinction. Recently, the extinction data of GRB afterglows are compiled up to redshift $z=6.3$, in combination with hydrogen column densities and metallicities. This data set enables us to investigate the relation between dust-to-gas ratio and metallicity out to high redshift for a wide metallicity range. By applying our evolution models of dust content in galaxies, we find that the dust-to-gas ratio derived from GRB afterglow extinction data are excessively high such that they can be explained with a fraction of gas-phase metals condensed into dust ($f_\mathrm{in}$) $\sim 1$, while theoretical calculations on dust formation in the wind of asymptotic giant branch stars and in the ejecta of Type II supernovae suggest a much more moderate condensation efficiency ($f_\mathrm{in}\sim 0.1$). Efficient dust growth in dense clouds has difficulty in explaining the excessive dust-to-gas ratio at metallicities $Z/\mathrm{Z}_\odot, Comment: 7 pages, 3 figures, published in MNRAS

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$\alpha$ 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., Comment: accepted by AJ

We construct a new algorithm for estimating the star formation rate (SFR) of galaxies from their infrared (IR) luminosity by developing the theory of the IR emission from a dusty H II region. The derived formula is ${\rm SFR}/(\MO {\rm yr^{-1}})=\{3.3\times 10^{-10}(1-\eta)/ (0.4-0.2f+0.6 \epsilon)\}(L{\rm ^{obs}_{IR}}/\LO)$, where $f$ is the fraction of ionizing photons absorbed by hydrogen, $\epsilon$ is the efficiency of dust absorption for nonionizing photons from OB stars, and $\eta$ is the cirrus fraction of observed IR luminosity. The previous conversion formulae of SFR from the IR luminosity is applicable to only the case where the observed IR luminosity is nearly equal to the bolometric luminosity (starburst galaxies etc.), except for some empirical formulae. On the other hand, our theoretical SFR is applicable to galaxies even with a moderate star formation activity. That is, our simple and convenient formula is significantly useful for estimating the SFR of various morphologies and types of galaxies --- from early elliptical to late spiral and irregular galaxies, or from active starburst to quiescent galaxies --- as far as they have neither an abnormal dust-to-gas ratio nor an evident active galactic nucleus., Comment: 5 pages, no figure, accepted by PASJ

We discuss the time variation of the dust-to-gas mass ratio in spiral galaxies, using the multiphase model of interstellar medium. The typical timescale of the phase change of an interstellar gas is ~107-8 yr in spiral galaxies. Since the phase transition changes the filling factor of the cold gas where the dust growth occurs, the dust growth rate varies on that timescale. In order to examine the response of the dust-to-gas ratio to the phase transition, we construct a model of the time evolution of the dust-to-gas ratio. We adopt the three-phase model for the interstellar gas and the Ikeuchi-Tomita model for the mass exchange between the phases. According to the model, three types of solutions are possible: (1) all the gas is transformed to a hot gas; (2) a stable stationary state of three phases is realized; and (3) the filling factor of each phase cyclically changes. For the three types of solutions, the dust-to-gas ratio behaves as follows: for solution 1, almost all the dust is destroyed (the dust-to-gas ratio becomes ~0); for solution 2, the dust-to-gas ratio converges to a stationary state; and for solution 3, the dust-to-gas ratio varies cyclically in response to the phase transition. In the case of solution 3, the amplitude of the variation of the dust-to-gas ratio is large (nearly an order of magnitude) if the dust growth timescale is shorter than the phase transition timescale. This condition is easily satisfied in spiral galaxies. However, it is difficult for dwarf galaxies to realize the condition, because their small metallicity makes the dust growth timescale long.

We examine the dust-to-gas ratio as a function of metallicity for dwarf galaxies [dwarf irregular galaxies (dIrrs) and blue compact dwarf galaxies (BCDGs)]. Using a one-zone model and adopting the instantaneous recycling approximation, we prepare a set of basic equations which describes processes of dust formation and destruction in a galaxy. Four terms are included for the processes: 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. Solving the equations, we compare the result with observational data of nearby dIrrs and BCDGs. The solution is consistent with the data within the reasonable ranges of model parameters constrained by the previous examinations. This means that the model is successful in understanding the dust amount of nearby galaxies. We also show that the accretion rate of heavy element onto preexisting dust grains is less effective than the condensation of heavy elements in dwarf galaxies., Comment: 14 pages LaTeX, 4 figures, to appear in ApJ

We consider evolution of interstellar clouds in Local Group dwarf spheroidal galaxies (dSphs) in the context of their observed star formation histories. The Local Group dSphs generally experienced initial bursts of star formations in their formation epochs ($\sim 15$ Gyr ago), when hot gas originating from the supernovae can make the cold interstellar clouds evaporate. We find that the maximum size of evaporating cloud is 10 pc. Thus, clouds larger than 10 pc can survive during the initial star formation. These surviving clouds can contribute to the second star formation to produce ``intermediate-age ($\sim$ 3--10 Gyr ago) stellar populations.'' Assuming that collisions between clouds induce star formation and that the timescale of the second star formation is a few Gyr, we estimate the total mass of the clouds. The total mass is about $10^{4}M_\odot$, which is 1--3 orders of magnitude smaller than the typical stellar mass of a present dSph. This implies that the initial star formation is dominant over the second star formation, which is broadly consistent with the observed star formation histories. However, the variety of the dSphs in their star formation histories suggests that the effects of environments on the dSphs may be important., Comment: 14 pages LaTeX, no figures, to appear in ApJ

We analyzed 10 dwarf spheroidal galaxies (dSphs) in the Local Group, and found two distinct sequences on the Mvir/L - Mvir plane: Mvir/L ∝ Mvir1.6 for Mvir < 108M⊙ whereas Mvir/L ≃ const. for Mvir > 108M⊙ (Mvir and L are the virial mass and the total luminosity of a dSph, respectively). We interpret the discontinuity as the threshold for the gas in dSphs to be blown away by successive supernovae. We succeeded in giving a quantitative explanation of the discontinuity mass of 108M⊙ as the blow-away condition. We further derived the above relation for the low-mass dSphs, assuming that the initial star formation rate of the dSphs is proportional to the inverse of the cooling time. The relation of high-mass dSphs is also explained along with the same consideration, with the condition that the gas cannot be blown away.

We investigate the dark matter (DM) content in the Local Group dwarf spheroidal galaxies (dSphs) by examining the correlations among their physical quantities. Two origins of the large velocity dispersions of the dSphs are possible: [1] the existence of DM and [2] tidal heating by the Galaxy. The correlation tests support both [1] and [2]. We finally mention circumstantial evidence for the existence of DM in the dSphs.

We construct a dust evolution model taking into account the grain size distribution, and investigate what kind of dust processes determine the grain size distribution at each stage of galaxy evolution. In addition to the dust production by type II supernovae (SNeII) and asymptotic giant branch (AGB) stars, we consider three processes in the ISM: (i) dust destruction by SN shocks, (ii) metal accretion onto the surface of preexisting grains in the cold neutral medium (CNM) (called grain growth), and (iii) grain-grain collisions (shattering and coagulation) in the warm neutral medium (WNM) and CNM. We found that the grain size distribution in galaxies is controlled by stellar sources in the early stage of galaxy evolution, and that afterwards the main processes that govern the size distribution changes to those in the ISM. Since shattering produces a large abundance of small grains (consequently, the surface-to-volume ratio of grains increases), it enhances the efficiency of grain growth, contributing to the significant increase of the total dust mass. Grain growth creates a large bump in the grain size distribution around a ~ 0.01 \mu m. Coagulation occurs effectively after the number of small grains is enhanced by shattering, and the grain size distribution is deformed to have a bump at a ~ 0.03 - 0.05 \mu m at t ~ 10 Gyr. We conclude that the evolutions of the total dust mass and the grain size distribution in galaxies are closely related to each other, and the grain size distribution changes considerably through the galaxy evolution because the dominant dust processes which regulate the grain size distribution change., Comment: 20 pages, 11 figures, accepted for publication in MNRAS

Dust grains grow their sizes in the interstellar clouds (especially in molecular clouds) by accretion and coagulation. Here we model and test these processes by examining the consistency with the observed variation of the extinction curves in the Milky Way. We find that, if we simply use the parameters used in previous studies, the model fails to explain the flattening of far-UV extinction curve for large $R_V$ (flatness of optical extinction curve) and the existence of carbon bump even in flat extinction curves. This discrepancy is resolved by adopting a `tuned' model, in which coagulation of carbonaceous dust is less efficient (by a factor of 2) and that of silicate is more efficient with the coagulation threshold removed. The tuned model is also consistent with the relation between silicon depletion (indicator of accretion) and $R_V$ if the duration of accretion and coagulation is >100(n_H/10^3 cm^{-3})^{-1} Myr, where n_H is the number density of hydrogen nuclei in the cloud. We also examine the relations between each of the extinction curve features (UV slope, far-UV curvature, and carbon bump strength) and $R_V$. The correlation between UV slope and $R_V$, which is the strongest among the three correlations, is well reproduced by the tuned model. For far-UV curvature and carbon bump strength, the observational data are located between the tuned model and the original model without tuning, implying that the large scatters in the observational data can be explained by the sensitive response to the coagulation efficiency. The overall success of the tuned model indicates that accretion and coagulation are promising mechanisms of producing the variation of extinction curves in the Milky Way, although we do not exclude possibilities of other dust-processing mechanisms changing extinction curves., 10 pages, 7 figures, accepted for publication in MNRAS

Grain growth during star formation affects the physical and chemical processes in the evolution of star-forming clouds. We investigate the origin of the millimeter (mm)-sized grains recently observed in Class I protostellar envelopes. We use the coagulation model developed in our previous paper and find that a hydrogen number density of as high as $10^{10}~{\rm cm^{-3}}$, instead of the typical density $10^5~{\rm cm^{-3}}$, is necessary for the formation of mm-sized grains. Thus, we test a hypothesis that such large grains are transported to the envelope from the inner, denser parts, finding that gas drag by outflow efficiently "launches" the large grains as long as the central object has not grown to $\gtrsim 0.1$ M$_{\odot}$. By investigating the shattering effect on the mm-sized grains, we ensure that the large grains are not significantly fragmented after being injected in the envelope. We conclude that the mm-sized grains observed in the protostellar envelopes are not formed in the envelopes but formed in the inner parts of the star-forming regions and transported to the envelopes before a significant mass growth of the central object, and that they survive in the envelopes., Comment: 8 pages, 4 figures, 1 table, accepted for publication in PASJ

This paper investigate what is the main driver of the dust mass growth in the interstellar medium (ISM) by using a chemical evolution model of galaxy with metals (elements heavier than helium) in dust phase in addition to the total amount of metals. We consider asymptotic giant branch (AGB) stars, type II supernovae (SNe II) and the dust mass growth in the ISM as the sources of dust, and SN shocks as the destruction mechanism of dust. Further, to describe the dust evolution precisely, our model takes into account the age and metallicity (the ratio of metal mass to ISM mass) dependence of the sources of dust. We particularly focused on the dust mass growth, and found that the dust mass growth in the ISM is regulated by the metallicity. To quantify this aspect, we introduce a "critical metallicity", which is a metallicity at which the contribution of stars (AGB stars and SNe II) equals that of the dust mass growth in the ISM. If the star formation timescale is shorter, the value of the critical metallicity is higher, but the galactic age at which the metallicity reaches the critical metallicity is shorter. From observations, it was expected that the dust mass growth was the dominant source of dust in the Milky Way and dusty QSOs at high redshifts. By introducing the critical metallicity, it is clearly shown that the dust mass growth is the main source of dust in such galaxies with various star formation timescales and ages. The dust mass growth in the ISM is regulated by metallicity, and we stress that the critical metallicity works as an indicator to judge whether the grain growth in the ISM is dominant source of dust in a galaxy, especially because of the strong and nonlinear dependence on the metallicity., 10 pages, 8 figures, accepted for publication in Earth, Planets, and Space

We examine a synthetic way of constructing the grain size distribution in the interstellar medium (ISM). First we formulate a synthetic grain size distribution composed of three grain size distributions processed with the following mechanisms that govern the grain size distribution in the Milky Way: (i) grain growth by accretion and coagulation in dense clouds, (ii) supernova shock destruction by sputtering in diffuse ISM, and (iii) shattering driven by turbulence in diffuse ISM. Then, we examine if the observational grain size distribution in the Milky Way (called MRN) is successfully synthesized or not. We find that the three components actually synthesize the MRN grain size distribution in the sense that the deficiency of small grains by (i) and (ii) is compensated by the production of small grains by (iii). The fraction of each {contribution} to the total grain processing of (i), (ii), and (iii) (i.e., the relative importance of the three {contributions} to all grain processing mechanisms) is 30-50%, 20-40%, and 10-40%, respectively. We also show that the Milky Way extinction curve is reproduced with the synthetic grain size distributions., 10 pages, 6 figures, accepted for publication in Earth, Planets, and Space

We report our recent theoretical work on the evolution of grain size distribution and grain abundance, particularly focusing on high-redshift (high-z) galaxies. We consider dust production in supernovae and shattering in interstellar turbulence. We show that observed extinction curves at high z can be reproduced if the dust supplied from supernovae is processed by shattering in interstellar turbulence. We also calculate the evolution of grain abundance and compare our theoretical results with the observed dust abundances in high-redshift quasars. We find that shattering is crucial to activate grain mass increase by the accretion of gas-phase metals since small-grain production by shattering raises the grain growth rate significantly. This means that the evolution of grain size distribution plays a critical role in governing the dust abundance at high z.

We construct a semi-analytical model which includes a novel implementation of dust size evolution and the resulting H2 formation on dust grains in the early universe. We investigate the cosmic star formation rate and the reionization history, taking into acount the initial mass function transition from Pop III to Pop II stars due to dust cooling. We derive the star formation efficiencies for various host dark matter halo masses and discuss effects of the value of critical dust-to-gas mass ratio, Dcr, on the reionization history. We conclude that the reionization completes before z = 6 in the case of the fiducail value Dcr = 7.4×10−8.

We apply the supernova (SN) extinction curves to reproduce the observed properties of SSTJ1604+4304 which is a young infrared (IR) galaxy at z ~ 1. The SN extinction curves used in this work were obtained from models of unmixed ejecta of Type II supernovae for the Salpeter initial mass function with a mass range from 8 to 30 M ⊙ or 8 to 40 M ⊙ . The effect of dust distributions on the attenuation of starlight is investigated by performing the χ 2 fitting method against various dust distributions. These are the commonly used uniform dust screen, the clumpy dust screen and the internal dust geometry. We add to these geometries three scattering properties, namely, no scattering, isotropic scattering and forward-only scattering. Judging from the χ 2 values, we find that the uniform screen models with any scattering property provide good approximations to the real dust geometry. Internal dust is inefficient to attenuate starlight and thus cannot be the dominant source of the extinction. We show that the SN extinction curves reproduce the data of SST J1604+4304 comparable to or better than the Calzetti extinction curve. The Milky Way extinction curve is not in satisfactory agreement with the data unless several dusty clumps are in the line of sight. This trend may be explained by the abundance of SN-origin dust in these galaxies; SN dust is the most abundant in the young IR galaxy at z ~ 1, abundant in local starbursts and less abundant in the Galaxy. If dust in SST J1604+4304 is dominated by SN dust, the dust production rate is ~0.1M ⊙ per SN.

Dust grains can be efficiently accelerated and shattered in warm ionized medium (WIM) because of the turbulent motion. This effect is enhanced in starburst galaxies, where gas is ionized and turbulence is sustained by massive stars. Moreover, dust production by Type II supernovae (SNe II) can be efficient in starburst galaxies. In this paper, we examine the effect of shattering in WIM on the dust grains produced by SNe II. We find that although the grains ejected from SNe II are expected to be biased to large sizes ($a\ga 0.1 \micron$, where $a$ is the grain radius) because of the shock destruction in supernova remnants, the shattering in WIM is efficient enough in $\sim 5$ Myr to produce small grains if the metallicity is nearly solar or more. The production of small grains by shattering steepens the extinction curve. Thus, steepening of extinction curves by shattering should always be taken into account for the system where the metallicity is solar and the starburst age is typically larger than 5 Myr. These conditions may be satisfied not only in nearby starbursts but also in high redshift ($z>5$) quasars., 12 pages, 11 figures, accepted for publication in MNRAS

Dust grains coagulate into larger aggregates in dense gas. This changes their size distribution and possibly affects the thermal evolution of star-forming clouds. We here investigate dust coagulation in collapsing pre-stellar cores with different metallicities by considering the thermal motions of grains. We show that coagulation does occur even at low metallicity $\sim 10^{-6} \Zsun$. However, we also find (i) that the H$_2$ formation rate on dust grains is reduced only after the majority of H$_2$ is formed; and (ii) that the dust opacity is modified only after the core becomes optically thick. Therefore, we conclude that the effects of dust coagulation can safely be neglected in discussing the temperature evolution of the pre-stellar cores for any metallicity as long as the grain motions are thermal., 7 pages, 3 figures. Accepted for publication in MNRAS

We investigate the FIR properties of a sample of BCDs observed by AKARI. By utilizing the data at wavelengths of $\lambda =65 \mu$m, 90 $\mu$m, and 140 $\mu$m, we find that the FIR colours of the BCDs are located at the natural high-temperature extension of those of the Milky Way and the Magellanic Clouds. This implies that the optical properties of dust in BCDs are similar to those in the Milky Way. Indeed, we explain the FIR colours by assuming the same grain optical properties, which may be appropriate for amorphous dust grains, and the same size distribution as those adopted for the Milky Way dust. Since both interstellar radiation field and dust optical depth affect the dust temperature, it is difficult to distinguish which of these two physical properties is responsible for the change of FIR colours. Then, in order to examine if the dust optical depth plays an important role in determining the dust temperature, we investigate the correlation between FIR colour (dust temperature) and dust-to-gas ratio. We find that the dust temperature tends to be high as the dust-to-gas ratio decreases but that this trend cannot be explained by the effect of dust optical depth. Rather, it indicates a correlation between dust-to-gas ratio and interstellar radiation field. Although the metallicity may also play a role in this correlation, we suggest that the dust optical depth could regulate the star formation activities, which govern the interstellar radiation field. We also mention the importance of submillimetre data in tracing the emission from highly shielded low-temperature dust., Comment: 12 pages, 8 figures, accepted for publication in MNRAS