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There are two major theoretical issues for the star formation law (the relation between the surface densities of molecular gas and star formation rate on a galaxy scale): (i) At low metallicity, it is not obvious that star-forming regions are rich in H2 because the H2 formation rate depends on the dust abundance; and (ii) whether or not CO really traces H2 is uncertain, especially at low metallicity. To clarify these issues, we use a hydrodynamic simulation of an isolated disc galaxy with a spatial resolution of a few tens parsecs. The evolution of dust abundance and grain size distribution is treated consistently with the metal enrichment and the physical state of the interstellar medium. We compute the H2 and CO abundances using a subgrid post-processing model based on the dust abundance and the dissociating radiation field calculated in the simulation.We find that when themetallicity is ≲ 0.4 Z⨀ (t<1 Gyr), H2 is not a good tracer of star formation rate because H2-rich regions are limited to dense compact regions. At Z ≳ 0.8 Z⨀, a tight star formation law is established for both H2 and CO. At old (t ~ 10 Gyr) ages, we also find that adopting the so-called MRN grain size distribution with an appropriate dust-to-metal ratio over the entire disc gives reasonable estimates for the H2 and CO abundances. For CO, improving the spatial resolution of the simulation is important, while the H2 abundance is not sensitive to subresolution structures at Z ≲ 0.4 Z⨀. [ABSTRACT FROM AUTHOR]

We aim at constraining the dust mass in high-z (z ≳ 5) galaxies using the upper limits obtained by Atacama Large Millimetre/submillimetre Array (ALMA) in combination with the rest-frame UV-optical spectral energy distributions (SEDs). For SED fitting, because of degeneracy between dust extinction and stellar age, we focus on two extremes: continuous star formation (Model A) and instantaneous star formation (Model B).We apply these models to Himiko (as a representative UV-bright object) and a composite SED of z > 5 Lyman break galaxies (LBGs). For Himiko, Model A requires a significant dust extinction, which leads to a high dust temperature >70K for consistency with the ALMA upper limit. This high dust temperature puts a strong upper limit on the total dust mass Md ≲ 2 × 106 M☉, and the dust mass produced per supernova (SN) md,SN ≲ 0.1 M☉. Such a low md, SN suggests significant loss of dust by reverse shock destruction or outflow, and implies that SNe are not the dominant source of dust at high z. Model B allows Md ~ 2 × 107 M☉ and md,SN ~ 0.3 M☉. We could distinguish between Models A and B if we observe Himiko at wavelength < 1.2 mm by ALMA. For the LBG sample, we obtain Md ≲ 2 × 106 M☉ for a typical LBG at z > 5, but this only puts an upper limit for md,SN as ~2 M☉. This clarifies the importance of observing UV-bright objects (like Himiko) to constrain the dust production by SNe. [ABSTRACT FROM AUTHOR]

To understand the evolution of extinction curve, we calculate the dust evolution in a galaxy using smoothed particle hydrodynamic simulations incorporating stellar dust production, dust destruction in supernova shocks, grain growth by accretion and coagulation, and grain disruption by shattering. The dust species are separated into carbonaceous dust and silicate. The evolution of grain size distribution is considered by dividing grain population into large and small grains, which allows us to estimate extinction curves. We examine the dependence of extinction curves on the position, gas density and metallicity in the galaxy, and find that extinction curves are flat at t ≳ 0.3 Gyr because stellar dust production dominates the total dust abundance. The 2175 Å bump and far-ultraviolet (FUV) rise become prominent after dust growth by accretion. At t ≳ 3 Gyr, shattering works efficiently in the outer disc and lowdensity regions, so extinction curves show a very strong 2175 Å bump and steep FUV rise. The extinction curves at t ≳ 3 Gyr are consistent with the Milky Way extinction curve, which implies that we successfully included the necessary dust processes in the model. The outer disc component caused by stellar feedback has an extinction curve with a weaker 2175 Å bump and flatter FUV slope. The strong contribution of carbonaceous dust tends to underproduce the FUV rise in the Small Magellanic Cloud extinction curve, which supports selective loss of small carbonaceous dust in the galaxy. The snapshot at young ages also explains the extinction curves in high-redshift quasars. [ABSTRACT FROM AUTHOR]

The CO-to-H2 conversion factor (XCO) 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 H2 formation on dust surfaces. In this paper, we investigate how the relation between dust-to-gas ratio and metallicity (D-Zrelation) affects the H2 and CO abundances (and XCO) of a 'molecular' cloud. For the D-Zrelation, 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 H2 formation on dust are solved consistently with the dust abundance and grain sizes. As a consequence, our models predict consistent metallicity dependence of XCO with observational data. Among various processes driving dust evolution, grain growth by accretion has the largest impact on the XCO-Z relation. The other processes also have some impacts on the XCO-Z relation, but their effects are minor compared with the scatter of the observational data at the metallicity range (Z ≳ 0.1Zʘ) where CO could be detected. We also find that dust condensation in stellar ejecta has a dramatic impact on the H2 abundance at low metallicities (≳ 0.1 Zʘ), relevant for damped Lyman α systems and nearby dwarf galaxies, and that the grain size dependence of H2 formation rate is also important. [ABSTRACT FROM AUTHOR]

The CO-to-H2 conversion factor (XCO) 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 H2 formation on dust surfaces. In this paper, we investigate how the relation between dust-to-gas ratio and metallicity (D-Zrelation) affects the H2 and CO abundances (and XCO) of a 'molecular' cloud. For the D-Zrelation, 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 H2 formation on dust are solved consistently with the dust abundance and grain sizes. As a consequence, our models predict consistent metallicity dependence of XCO with observational data. Among various processes driving dust evolution, grain growth by accretion has the largest impact on the XCO-Z relation. The other processes also have some impacts on the XCO-Z relation, but their effects are minor compared with the scatter of the observational data at the metallicity range (Z ≳ 0.1Zʘ) where CO could be detected. We also find that dust condensation in stellar ejecta has a dramatic impact on the H2 abundance at low metallicities (≳ 0.1 Zʘ), relevant for damped Lyman α systems and nearby dwarf galaxies, and that the grain size dependence of H2 formation rate is also important. [ABSTRACT FROM AUTHOR]

Recent Atacama Large Millimetre/submillimetre Array (ALMA) observations of high-redshift normal galaxies have been providing a great opportunity to clarify the general origin of dust in the Universe, not biased to very bright special objects even at z > 6. To clarify what constraint we can get for the dust enrichment in normal galaxies detected by ALMA, we use a theoretical model that includes major processes driving dust evolution in a galaxy; that is, dust condensation in stellar ejecta, dust growth by the accretion of gas-phase metals and supernova destruction. Using the dust emission fluxes detected in two normal galaxies at z > 6 by ALMA as a constraint, we can get the range of the time-scales (or efficiencies) of the above mentioned processes. We find that if we assume extremely high-condensation efficiency in stellar ejecta (fin ≳0.5), rapid dust enrichment by stellar sources in the early phase may be enough to explain the observed ALMA flux, unless dust destruction by supernovae in those galaxies is stronger than that in nearby galaxies. If we assume a condensation efficiency expected from theoretical calculations (fin ≲0.1), strong dust growth (even stronger than assumed for nearby galaxies if they are metal-poor galaxies) is required. These results indicate that the normal galaxies detected by ALMA at z > 6 are biased to objects (i) with high dust condensation efficiency in stellar ejecta, (ii) with strong dust growth in very dense molecular clouds or (iii) with efficient dust growth because of fast metal enrichment up to solar metallicity. A measurement of metallicity is crucial to distinguish among these possibilities. [ABSTRACT FROM AUTHOR]

Extinction curves, especially those in the Milky Way (MW), the Large Magellanic Cloud (LMC), and the Small Magellanic Cloud (SMC), have provided us with a clue to the dust properties in the nearby Universe. We examine whether or not these extinction curves can be explained by well-known dust evolution processes. We treat the dust production in stellar ejecta, destruction in supernova shocks, dust growth by accretion and coagulation, and dust disruption by shattering. To make a survey of the large parameter space possible, we simplify the treatment of the grain size distribution evolution by adopting the "two-size approximation," in which we divide the grain population into small (≲0.03 μm) and large (≳0.03 μm) grains. It is confirmed that the MW extinction curve can be reproduced in reasonable ranges for the time-scale of the above processes with a silicate-graphite mixture. This indicates that the MW extinction curve is a natural consequence of the dust evolution through the above processes. We also find that the same models fail to reproduce the SMC/LMC extinction curves. Nevertheless, this failure can be remedied by giving higher supernova destruction rates for small dust particles dust and considering amorphous carbon for carbonaceous dust; these modifications in fact fall in line with previous studies. Therefore, we conclude that the current dust evolution scenario composed of the aforementioned processes is successful in explaining the extinction curves. All the extinction curves favor efficient interstellar processing of dust, especially strong grain growth by accretion and coagulation. [ABSTRACT FROM AUTHOR]

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 1010 cm-3, instead of the typical density 105 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 ≿0.1M⊙. 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. [ABSTRACT FROM AUTHOR]

Ground-based observations at terahertz (THz) frequencies are a newly explorable area of astronomy in the coming decades. 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 for a 10-m dish), as compared to space or airborne THz telescopes. Thus, highresolution 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-µm lines, which are scientifically relevant tracers of star formation and stellar feedback. Those lines are the brightest lines in the THz windows, so they are suitable for the initiation of groundbased 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, the uniqueness of which is detecting 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. [ABSTRACT FROM AUTHOR]

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 ~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 ~10-3 if this process lasts ≥10/(nH/10³ cm-3) Myr, where nH 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. [ABSTRACT FROM AUTHOR]

The small grain sizes produced by Type II supernova 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 JR SED of low-metallicity, extremely young galaxies based on recent work by Hirashita et al. We find that, even in the intense ultraviolet radiation field of very young galaxies, small silicate grains are subject to stochastic heating resulting in a broad temperature distribution and substantial mid-infrared (MIR) continuum emission. Larger car- bonaceous grains are in thermal equilibrium at T ≃ 50-100 K, and they also contribute to the MIR. We present the evolution of SEDs and JR extinction of very young, low-metallicity galaxies. The JR extinction curve is also shown. In the first few Myr, the emission peaks at λ ∼ 30-50 μm; 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 SBS 0335-052, a low-metallicity (1/41 Z[SUB⊙] dwarf galaxy with an unusually strong MIR flux. We find the SED, optical properties and extinction of the star-forming region to be consistent with a very young (age ≃ 6.5 × 10[SUP6] yr) and compact (radius ≃ 20 pc) starburst. We also predict the SED of another extremely low-metallicity galaxy, IZw 18, for future observational tests. We estimate the HR luminosity of IZw 18 to be low as L[SUBFIR] ∼ 10[SUP7]-10[SUP7.5] L[SUB⊙] depending on the uncertainty of dust mass. Some prospects for future observations are discussed. [ABSTRACT FROM AUTHOR]