arXiv subjects

Hiroyuki Hirashita

Publications and source records attributed to Hiroyuki Hirashita.

At least 91 records · Page 5Linked to original sources

arXiv2012

Properties of free-free, dust, and CO emissions in the starbursts of blue compact dwarf galaxies

The central star-forming regions in three blue compact dwarf galaxies (He 2-10, NGC 5253, and II Zw 40) were observed in the 340 GHz (880 micron) band at 5 arcsec resolution with the Submillimetre Array (SMA). Continuum emission associated with the central star-forming complex was detected in all these galaxies. The SMA 880 micron flux is decomposed into free-free emission and dust emission by using centimetre-wavelength data in the literature. We find that free-free emission contributes half or more of the SMA 880 micron flux in the central starbursts in those three galaxies. In spite of the dominance of free-free emission at 880 micron, the radio-to-far infrared (FIR) ratios in the central star-forming regions are not significantly higher than those of the entire systems, showing the robustness of radio-FIR relation. Based on the robustness of the radio-FIR relation, we argue that the free--free fraction in the 880 micron emission is regulated by the dust temperature. We also analyze the CO (J = 3--2) emission data. We find that CO is a good tracer of the total gas mass in solar-metallicity object He 2-10. Low-metallicity objects, NGC 5253 and II Zw 40, have apparently high star formation efficiencies; however, this may be an artifact of significant dissociation of CO in the low-metallicity environments. We also point out a potential underestimate of dust mass, since the dust traced by emission is biased to the most luminous high-temperature regions, particularly when a system hosts a compact star-forming region where the dust temperature is high.

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arXiv2012

Dust formation history of galaxies: a critical role of metallicity for the dust mass growth by accreting materials in the interstellar medium

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.

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arXiv2012

Impact of grain size distributions on the dust enrichment in high-redshift quasars

In high-redshift ($z>5$) quasars, a large amount of dust ($\textstyle\sim 10^{8} \mathrm{M}_{\sun}$) has been observed. In order to explain the large dust content, we focus on a possibility that grain growth by the accretion of heavy elements is the dominant dust source. We adopt a chemical evolution model applicable to nearby galaxies but utilize parameters adequate to high-$z$ quasars. It is assumed that metals and dust are predominantly ejected by Type II supernovae (SNe). We have found that grain growth strongly depends on the grain size distribution. If we simply use the size distribution of grains ejected from SNe, grain growth is inefficient because of the lack of small grains (i.e.\ small surface-to-volume ratio of the dust grains). However, if we take small grain production by interstellar shattering into consideration, grain growth is efficient enough to account for the rich dust abundance in high-$z$ quasars. Our results not only confirm that grain growth is necessary to explain the large amount of dust in high-$z$ quasars, but also demonstrate that grain size distributions have a critical impact on grain growth.

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arXiv2012

Synthesized grain size distribution in the interstellar medium

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.

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arXiv2012

Dust growth in the interstellar medium: How do accretion and coagulation interplay?

Dust grains grow in interstellar clouds by accretion and coagulation. In this paper, we focus on these two grain growth processes and numerically investigate how they interplay to increase the grain radii. We show that accretion efficiently depletes grains with radii $a\la 0.001 \micron$ on a time-scale of $\la 10$ Myr in solar-metallicity molecular clouds. Coagulation also occurs on a similar time-scale, but accretion is more efficient in producing a large bump in the grain size distribution. Coagulation further pushes the grains to larger sizes after a major part of the gas phase metals are used up. Similar grain sizes are achieved by coagulation regardless of whether accretion takes place or not; in this sense, accretion and coagulation modify the grain size distribution independently. The increase of the total dust mass in a cloud is also investigated. We show that coagulation slightly 'suppresses' dust mass growth by accretion but that this effect is slight enough to be neglected in considering the grain mass budget in galaxies. Finally we examine how accretion and coagulation affect the extinction curve: The ultraviolet slope and the carbon bump are \textit{enhanced} by accretion, while they are flattened by coagulation.

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arXiv2011

Dust properties in afterglow of GRB 071025 at z~5

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

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arXiv2011

Central free-free-dominated 880 $\micron$ emission in II Zw 40

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 ($ν\la$ several GHz; $λ\ga$ several cm) and spatial extent of dust affect the radio-FIR relation.

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arXiv2011

Effects of grain size distribution on the interstellar dust mass growth

Grain growth by the accretion of metals in interstellar clouds (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 demonstrated in this paper. First, we derive an analytical formula that gives the grain size distribution 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 grain 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 clouds governs the dust content in nearby galaxies unless the grain size is strongly biased to sizes larger than $\sim 0.1 \micron$ or the power index of the grain size distribution is shallower than $\sim -2.5$. The grain growth in clouds contributes to the rapid increase of dust-to-gas ratio at a certain metallicity level (called critical metallicity in Asano et al. 2011 and and Inoue 2011), which we find to be sensitive to grain size distribution. Thus, the grain growth efficiently 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 applicable for any grain size distribution and easily implemented into any framework of dust enrichment in galaxies.

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arXiv2011

The Role of Dust in the Early Universe I : Protogalaxy Evolution

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

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arXiv2010

Importance of far-infrared mapping in a spiral galaxy: AKARI observation of M81

The importance of the far-infrared (FIR) mapping is demonstrated for a face-on spiral galaxy, M81, by analyzing its imaging data at 65, 90, and 140 μm taken by AKARI. 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 μm 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 diagramis 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).

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arXiv2010

Shattering by turbulence as a production source of very small grains

The origin of grain size distribution in the interstellar medium is one of the most fundamental problems in the interstellar physics. In the Milky Way, smaller grains are more abundant in number, but their origins are not necessarily specified and quantified. One of the most efficient drivers of small grain production is interstellar turbulence, in which dust grains can acquire relative velocities large enough to be shattered. Applying the framework of shattering developed in previous papers, we show that small ($a\la 0.01~\micron$) grains reach the abundance level observed in the Milky Way in $\sim 10^8$ yr (i.e. within the grain lifetime) by shattering in warm neutral medium. We also show that if part of grains experience additional shattering in warm ionized medium, carbonaceous grains with $a\sim 0.01~\micron$ are redistributed into smaller sizes. This could explain the relative enhancement of very small carbonaceous grains with $a\sim 3$--100 Å. Our theory also explains the ubiquitous association between large grains and very small grains naturally. Some tests for our theory are proposed in terms of the metallicity dependence.

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arXiv2010

Effects of grain shattering by turbulence on extinction curves in starburst galaxies

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.

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arXiv2009

Detecting Lyman Alpha Emitters in the Sub-millimeter

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_α> 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.

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arXiv2009

Dust coagulation in star formation with different metallicities

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.

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arXiv2009

Effects of dust abundance on the far-infrared colours of blue compact dwarf galaxies

We investigate the FIR properties of a sample of BCDs observed by AKARI. By utilizing the data at wavelengths of $λ=65 μ$m, 90 $μ$m, and 140 $μ$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.

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arXiv2008

Shattering and coagulation of dust grains in interstellar turbulence

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 $a<10^{-6}$ cm coagulate in 10 Myr. In fact, the correlation among $R_V$, the carbon bump strength, and the ultraviolet slope in the observed Milky Way extinction curves can be explained by the coagulation in DC. It is possible that the grain size distribution in the Milky Way is determined by a combination of all the above effects of shattering and coagulation. Considering that shattering and coagulation in turbulence are effective if dust-to-gas ratio is typically more than $\sim 1/10$ of the Galactic value, the regulation mechanism of grain size distribution should be different between metal-poor and metal-rich environments.

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arXiv2008

Far-Infrared Properties of Blue Compact Dwarf Galaxies Observed with AKARI/Far-Infrared Surveyor (FIS)

We report basic far-infrared (FIR) properties of eight blue compact dwarf galaxies (BCDs) observed by AKARI. We measure the fluxes at the four FIS bands (wavelengths of 65 um, 90 um, 140 um, and 160 um). Based on these fluxes, we estimate basic quantities about dust: dust temperature, dust mass, and total FIR luminosity. We find that the typical dust temperature of the BCD sample is systematically higher than that of normal spiral galaxies, although there is a large variety. The interstellar radiation field estimated from the dust temperature ranges up to 100 times of the Galactic value. This confirms the concentrated star-forming activity in BCDs. The star formation rate can be evaluated from the FIR luminosity as 0.01--0.5 $M_\odot$ yr$^{-1}$. Combining this quantity with gas mass taken from the literature, we estimate the gas consumption timescales (gas mass divided by the star formation rate), which prove to span a wide range from 1 Gyr to 100 Gyr. A natural interpretation of this large variety can be provided by intermittent star formation activity. We finally show the relation between dust-to-gas ratio and metallicity (we utilize our estimate of dust mass, and take other necessary quantities from the literature). There is a positive correlation between dust-to-gas ratio and metallicity as expected from chemical evolution models.

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arXiv2008

Extinction curves flattened by reverse shocks in supernovae

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

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