arXiv subjects

Hiroyuki Hirashita

Publications and source records attributed to Hiroyuki Hirashita.

At least 55 records · Page 3Linked to original sources

arXiv2019

Dust scaling relations in a cosmological simulation

To study the dust evolution in the cosmological structure formation history, we perform a smoothed particle hydrodynamic simulation with a dust enrichment model in a cosmological volume. We adopt the dust evolution model that represents the grain size distribution by two sizes and takes into account stellar dust production and interstellar dust processing. We examine the dust mass function and the scaling properties of dust in terms of the characteristics of galaxies. The simulation broadly reproduces the observed dust mass functions at redshift $z = 0$, except that it overproduces the massive end at dust mass $M_\mathrm{d} \gtrsim 10^{8}$ ${\rm M}_\odot$. This overabundance is due to overproducing massive gas/metal-rich systems, but we also note that the relation between stellar mass and gas-phase metallicity is reproduced fairly well by our recipe. The relation between dust-to-gas ratio and metallicity shows a good agreement with the observed one at $z=0$, which indicates successful implementation of dust evolution in our cosmological simulation. Star formation consumes not only gas but also dust, causing a decreasing trend of the dust-to-stellar mass ratio at the high-mass end of galaxies. We also examine the redshift evolution up to $z \sim~ 5$, and find that the galaxies have on average the highest dust mass at $z = 1-2$. For the grain size distribution, we find that galaxies with metallicity $\sim 0.3~ Z_\odot$ tend to have the highest small-to-large grain abundance ratio; consequently, the extinction curves in those galaxies have the steepest ultraviolet slopes.

astro-ph.GA↗

arXiv2018

Remodeling the evolution of grain size distribution in galaxies

We revisit the evolution model of grain size distribution in a galaxy for the ultimate purpose of implementing it in hydrodynamical simulations. We simplify the previous model in such a way that some model-dependent assumptions are replaced with simpler functional forms. For the first test of the developed framework, we apply it to a one-zone chemical evolution model of a galaxy, confirming that our new model satisfactorily reproduces the previous results and that efficient coagulation of small grains produced by shattering and accretion is essential in reproducing the so-called MRN grain size distribution. For the next step, in order to test if our model can be treated together with the hydrodynamical evolution of the interstellar medium (ISM), we post-process a hydrodynamical simulation of an isolated disc galaxy using the new grain evolution model. We sample hydrodynamical particles representing each of the dense and diffuse ISM phases. By this post-processing, we find that the processes occurring in the dense gas (grain growth by accretion and coagulation) are important in reproducing the grain size distribution consistent with the Milky Way extinction curve. In our model, the grain size distributions are similar between the dense and diffuse ISM, although we observe a larger dispersion in the dense ISM. Moreover, we also show that even if we degrade the grain radius resolution (with 16 grid points), the overall shape of grain size distribution (and of resulting extinction curve) can be captured.

astro-ph.GA↗

arXiv2018

Cosmological simulation with dust formation and destruction

To investigate the evolution of dust in a cosmological volume, we perform hydrodynamic simulations, in which the enrichment of metals and dust is treated self-consistently with star formation and stellar feedback. We consider dust evolution driven by dust production in stellar ejecta, dust destruction by sputtering, grain growth by accretion and coagulation, and grain disruption by shattering, and treat small and large grains separately to trace the grain size distribution. After confirming that our model nicely reproduces the observed relation between dust-to-gas ratio and metallicity for nearby galaxies, we concentrate on the dust abundance over the cosmological volume in this paper. The comoving dust mass density has a peak at redshift $z\sim 1$--2, coincident with the observationally suggested dustiest epoch in the Universe. {In the local Universe}, roughly 10 per cent of the dust is contained in the intergalactic medium (IGM), where only 1/3--1/4 of the dust survives against dust destruction by sputtering. We also show that the dust mass function is roughly reproduced at $\lesssim 10^8$ M$_\odot$, while the massive end still has a discrepancy, which indicates {the necessity of stronger feedback in massive galaxies}. %%The relation showed that accretion is essential for dusty galaxies. In addition, our model broadly reproduces the observed radial profile of dust surface density in the circum-galactic medium (CGM). While our model satisfies the observational constraints for the dust extinction {on cosmological scales}, it predicts that the dust in the CGM and IGM is dominated by large ($> 0.03~μ$m) grains, which is in tension with the steep reddening curves {observed} in the CGM.

astro-ph.GA↗

arXiv2018

Dust abundance and grain size in galaxy halos

We investigate the abundance and properties (especially, grain size) of dust in galaxy halos using available observational data in the literature. There are two major sets of data. One is (i) the reddening curves at redshifts $z\sim 1$ and 2 derived for Mg II absorbers, which are assumed to trace the medium in galaxy halos. The other is (ii) the cosmic extinction up to $z\sim 2$ mainly traced by distant background quasars. For (i), the observed reddening curves favor a grain radius of $a\sim 0.03~μ$m for silicate, while graphite is not supported because of its strong 2175 Å bump. Using amorphous carbon improves the fit to the reddening curves compared with graphite if the grain radius is $a\lesssim 0.03~μ$m. For (ii), the cosmic extinction requires $η\gtrsim 10^{-2}$ ($η$ is the ratio of the halo dust mass to the stellar mass; the observationally suggested value is $η\sim 10^{-3}$) for silicate if $a\sim 0.03~μ$m as suggested by the reddening curve constraint. Thus, for silicate, we do not find any grain radius that satisfies both (i) and (ii) unless the halo dust abundance is much larger than suggested by the observations. For amorphous carbon, in contrast, a wide range of grain radius ($a\sim 0.01$--0.3~$μ$m) is accepted by the cosmic extinction; thus, we find that a grain radius range of $a\sim 0.01$--0.03 $μ$m is supported by combining (i) and (ii). We also discuss the origin of dust in galaxy halos, focusing on the importance of grain size in the physical mechanism of dust supply to galaxy halos.

astro-ph.GA↗

arXiv2017

Exploring the Dust Population in Cold Diffuse Clouds

The formation and evolution of cold diffuse clouds (CDCs), the parent objects of dense molecular clouds, affects both the star formation process and that of larger-scale galactic evolution. We have begun a pilot study of one CDC's dust content, with the goal of quantifying the abundances of different types of dust and relating these to the relative abundance of molecular gas, the cloud's physical properties, and its general stage of development. Using photometry from AKARI and other surveys, we have extracted a sample spectral energy distribution (SED) of the CDC dust thermal emission over the near-infrared to submillimeter range. The extracted SED closely resembles others in the literature, confirming our isolation of the cloud emission from other sources along the sight line. We plan to fit this SED with dust models at each position in the cloud, automating our procedure to map out the structure of this CDC and others.

astro-ph.GA↗

arXiv2017

Populating H$_2$ and CO in galaxy simulation with dust evolution

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 H$_2$ because the H$_2$ formation rate depends on the dust abundance; and (ii) whether or not CO really traces H$_2$ 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 H$_2$ 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 the metallicity is $\lesssim 0.4$ Z$_\odot$ ($t<1$ Gyr), H$_2$ is not a good tracer of star formation rate because H$_2$-rich regions are limited to dense compact regions. At $Z\gtrsim 0.8$ Z$_\odot$, a tight star formation law is established for both H$_2$ and CO. At old ($t \sim 10$ 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 H$_2$ and CO abundances. For CO, improving the spatial resolution of the simulation is important while the H$_2$ abundance is not sensitive to sub-resolution structures at $Z\gtrsim 0.4$ Z$_\odot$.

astro-ph.GA↗

arXiv2017

Dust masses of $z>5$ galaxies from SED fitting and ALMA upper limits

We aim at constraining the dust mass in high-redshift ($z\gtrsim 5$) galaxies using the upper limits obtained by 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 $>70$ K for consistency with the ALMA upper limit. This high dust temperature puts a strong upper limit on the total dust mass $M_\mathrm{d}\lesssim 2\times 10^6$ M$_{\odot}$, and the dust mass produced per supernova (SN) $m_\mathrm{d,SN}\lesssim 0.1$ M$_{\odot}$. Such a low $m_\mathrm{d,SN}$ 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 $M_\mathrm{d}\sim 2\times 10^7$ M$_{\odot}$ and $m_\mathrm{d,SN}\sim 0.3$ M$_{\odot}$.} 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 $M_\mathrm{d}\lesssim 2\times 10^6$ M$_{\odot}$ for a typical LBG at $z>5$, but this only puts an upper limit for $m_\mathrm{d,SN}$ as $\sim 2$ M$_{\odot}$. This clarifies the importance of observing UV-bright objects (like Himiko) to constrain the dust production by SNe.

astro-ph.GA↗

arXiv2017

Evolution of dust extinction curves in galaxy simulation

To understand the evolution of extinction curve, we calculate the dust evolution in a galaxy using smoothed particle hydrodynamics 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 gains, 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 $t \lesssim 0.3$ 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 $t \gtrsim 3$ Gyr, shattering works efficiently in the outer disc and low density regions, so extinction curves show a very strong 2175 Å bump and steep FUV rise. The extinction curves at $t\gtrsim 3$ 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 curves 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 explain the extinction curves in high-redshift quasars.

astro-ph.GA↗

arXiv2017

Dust evolution with active galactic nucleus feedback in elliptical galaxies

We have recently suggested that dust growth in the cold gas phase dominates the dust abundance in elliptical galaxies while dust is efficiently destroyed in the hot X-ray emitting plasma (hot gas). In order to understand the dust evolution in elliptical galaxies, we construct a simple model that includes dust growth in the cold gas and dust destruction in the hot gas. We also take into account the effect of mass exchange between these two gas components induced by active galactic nucleus (AGN) feedback. We survey reasonable ranges of the relevant parameters in the model and find that AGN feedback cycles actually produce a variety in cold gas mass and dust-to-gas ratio. By comparing with an observational sample of nearby elliptical galaxies, we find that, although the dust-to-gas ratio varies by an order of magnitude in our model, the entire range of the observed dust-to-gas ratios is difficult to be reproduced under a single parameter set. Variation of the dust growth efficiency is the most probable solution to explain the large variety in dust-to-gas ratio of the observational sample. Therefore, dust growth can play a central role in creating the variation in dust-to-gas ratio through the AGN feedback cycle and through the variation in dust growth efficiency.

astro-ph.GA↗

arXiv2017

Galaxy Simulation with Dust Formation and Destruction

We perform smoothed particle hydrodynamics (SPH) simulations of an isolated galaxy with a new treatment for dust formation and destruction. To this aim, we treat dust and metal production self-consistently with star formation and supernova feedback. For dust, we consider a simplified model of grain size distribution by representing the entire range of grain sizes with large and small grains. We include dust production in stellar ejecta, dust destruction by supernova (SN) shocks, grain growth by accretion and coagulation, and grain disruption by shattering. We find that the assumption of fixed dust-to-metal mass ratio becomes no longer valid when the galaxy is older than 0.2 Gyr, at which point the grain growth by accretion starts to contribute to the nonlinear rise of dust-to-gas ratio. As expected in our previous one-zone model, shattering triggers grain growth by accretion since it increases the total surface area of grains. Coagulation becomes significant when the galaxy age is greater than $\sim$ 1 Gyr: at this epoch the abundance of small grains becomes high enough to raise the coagulation rate of small grains. We further compare the radial profiles of dust-to-gas ratio $(\mathcal{D})$ and dust-to-metal ratio $(\mathcal{D}/Z)$ (i.e., depletion) at various ages with observational data. We find that our simulations broadly reproduce the radial gradients of dust-to-gas ratio and depletion. In the early epoch ($\lesssim 0.3$ Gyr), the radial gradient of $\mathcal{D}$ follows the metallicity gradient with $\mathcal{D}/Z$ determined by the dust condensation efficiency in stellar ejecta, while the $\mathcal{D}$ gradient is steeper than the $Z$ gradient at the later epochs because of grain growth by accretion. The framework developed in this paper is applicable to any SPH-based galaxy evolution simulations including cosmological ones.

astro-ph.GA↗

arXiv2017

Effects of dust evolution on the abundances of CO and H$_2$

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 $α$ systems and nearby dwarf galaxies, and that the grain size dependence of H$_2$ formation rate is also important.

astro-ph.GA↗

arXiv2016

Dust evolution processes constrained by extinction curves in nearby galaxies

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 ($\lesssim 0.03~μ$m) and large ($\gtrsim 0.03~μ$m) 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 carbonaceous dust and considering amorphous carbon for carbonaceous dust; these modification fall in fact 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.

astro-ph.GA↗

arXiv2016

Millimeter-sized grains in the protostellar envelopes: where do they come from?

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.

astro-ph.SR↗

arXiv2016

Formation, Evolution, and Revolution of Galaxies by SKA: Activities of SKA-Japan Galaxy Evolution Sub-SWG

Formation and evolution of galaxies have been a central driving force in the studies of galaxies and cosmology. Recent studies provided a global picture of cosmic star formation history. However, what drives the evolution of star formation activities in galaxies has long been a matter of debate. The key factor of the star formation is the transition of hydrogen from atomic to molecular state, since the star formation is associated with the molecular phase. This transition is also strongly coupled with chemical evolution, because dust grains, i.e., tiny solid particles of heavy elements, play a critical role in molecular formation. Therefore, a comprehensive understanding of neutral-molecular gas transition, star formation and chemical enrichment is necessary to clarify the galaxy formation and evolution. Here we present the activity of SKA-JP galaxy evolution sub-science working group (subSWG) Our activity is focused on three epochs: z \sim 0, 1, and z > 3. At z \sim 0, we try to construct a unified picture of atomic and molecular hydrogen through nearby galaxies in terms of metallicity and other various ISM properties. Up to intermediate redshifts z \sim 1, we explore scaling relations including gas and star formation properties, like the main sequence and the Kennicutt-Schmidt law of star forming galaxies. To connect the global studies with spatially-resolved investigations, such relations will be plausibly a viable way. For high redshift objects, the absorption lines of HI 21-cm line will be a very promising observable to explore the properties of gas in galaxies. By these studies, we will surely witness a real revolution in the studies of galaxies by SKA.

astro-ph.GA↗

arXiv2016

Revisiting the lifetime estimate of large presolar grains in the interstellar medium

Some very large (>0.1 um) presolar grains are sampled in meteorites. We reconsider the lifetime of very large grains (VLGs) in the interstellar medium focusing on interstellar shattering caused by turbulence-induced large velocity dispersions. This path has never been noted as a dominant mechanism of destruction. We show that, if interstellar shattering is the main mechanism of destruction of VLGs, their lifetime is estimated to be $\gtrsim 10^8$ yr; in particular, very large SiC grains can survive cosmic-ray exposure time. However, most presolar SiC grains show residence times significantly shorter than 1 Gyr, which may indicate that there is a more efficient mechanism than shattering in destroying VLGs, or that VLGs have larger velocity dispersions than 10 km s$^{-1}$. We also argue that the enhanced lifetime of SiC relative to graphite can be the reason why we find SiC among $μ$m-sized presolar grains, while the abundance of SiC in the normal interstellar grains is much lower than graphite.

astro-ph.GA↗

arXiv2015

First-generation Science Cases for Ground-based Terahertz Telescopes

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.

astro-ph.GA↗

arXiv2015

Dust processing in elliptical galaxies

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.

astro-ph.GA↗

arXiv2015

The origin of dust extinction curves with or without the 2175 A bump in galaxies: The case of the Magellanic Clouds

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.

astro-ph.GA↗