arXiv subjects

Koki Otaki

Publications and source records attributed to Koki Otaki.

8 recordsLinked to original sources

arXiv2026

From stardust to interstellar grain growth in the first galaxies: a cosmological transition in dust evolution near z ~ 8.9

When and how did dust begin to shape galaxies? Motivated by the identification of an apparent redshift break in galaxy dust masses, suggesting substantially lower dust masses at $z \gtrsim 9$, we investigate dust enrichment during the first billion years of cosmic history. We aim to determine whether the observed evolution marks a transition in the dominant dust-production mechanism and to identify the physical conditions under which such a transition is expected to occur. Using JWST, ALMA, and NOEMA observations, we measure ultraviolet dust attenuation and dust masses. We apply a censored change-point analysis and compare the observations with dust evolution modelling. The analysis identifies a preferred transition near redshift z ~ 8.9, corresponding to about 570 Myr after the Big Bang. The evidence for a break is strongest in dust mass and dust-to-stellar mass ratio, mostly estimated from JWST NIRSpec spectrophotometric fitting but also partly from sub-mm data. The ultraviolet attenuation measurements are consistent with a transition at the same epoch but do not independently require one. The models are consistent with the onset of efficient interstellar grain growth above a characteristic metallicity. We interpret the transition near z ~ 8.9 as the emergence of grain-growth-dominated dust evolution from an earlier regime dominated by supernova-produced grains. Population III enrichment can modify the earliest chemical-enrichment history but leaves the timing of the dust transition nearly unchanged and is not required for its emergence.

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arXiv2026

Grain-size evolution and rapid dust growth in high-redshift galaxies

We present a galaxy evolution model that incorporates grain-size evolution in a multiphase interstellar medium (ISM) to investigate dust attenuation in galaxies at $z \geq 5$. Our fiducial setup assumes a low dust yield of $y_{\rm d} = 10^{-4}~\rm M_\odot$ and a small characteristic size of stellar dust of $a_0 = 0.01~\mu$m, motivated by efficient dust destruction by reverse shocks in dense ISM environments. Our model demonstrates that, even with such low dust yields, massive galaxies with $M_\ast > 10^9~\rm M_\odot$ reach high dust-to-stellar mass ratios of $M_{\rm d}/M_\ast \sim 10^{-2}$ by $z \sim 7$ because small grains supplied by SNe efficiently serve as seeds for metal accretion in the ISM. Because dust growth significantly lags behind star formation, the outer regions beyond the half-star-formation-rate radius remain relatively dust poor, allowing a non-negligible fraction of UV photons to escape without strong attenuation. We further find that dust growth becomes most efficient when the ISM is dominated by cold dense gas but still contains a modest warm component, as the former promotes metal accretion while the latter supplies additional small grains through shattering, thereby further enhancing subsequent grain growth. In particular, with a cold dense gas fraction of $\sim 90~\%$, our model predictions become broadly consistent with the dust-to-stellar mass ratios inferred for dust-rich galaxies at $z \sim 7$, as well as the upper limits for blue galaxies at $z \gtrsim 10$. Self-consistently, the model successfully reproduces the UV luminosity functions observed at both $z = 7$ and $z = 12$. Overall, this study demonstrates that a physically motivated treatment of grain growth in a multiphase ISM is essential for linking the dust content of high-redshift galaxies to their radiative properties during cosmic dawn.

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arXiv2026

Dust enrichment from core-collapse supernovae and extinction curves in the high-redshift universe

Recent JWST observations have revealed that some galaxies at $z \gtrsim 7$ generally exhibit relatively flat ultraviolet (UV) attenuation curves and a weak UV bump. These features suggest that the first dust grains formed rapidly, possibly originating from core-collapse supernovae (SNe). We investigate the time evolution of grain size distributions and extinction curves in the early phase of dust enrichment for different parameters of progenitor stars, rotation velocities, metallicity, and interstellar medium densities, including the effect of the reverse shock. We model a single starburst system assuming an initial mass function. Extinction curves are calculated from the grain size distribution for each dust species. The total dust-to-stellar mass ratio at $30 \,\mathrm{Myr}$ is $M_\mathrm{dust}/M_\star \sim 10^{-3}$ before the passage of the reverse shock, but we find it to be at most $M_\mathrm{dust}/M_\star \sim 10^{-5}$ due to the destruction effect of the reverse shock. This effect destroys grains smaller than $\sim 10\,\mathrm{nm}$ and makes amorphous carbon the dominant species, resulting in a flatter extinction curve with a wide bump at $2500\,\mathrm{\mathring{A}}$ compared to the no-reverse shock models. We find that our models are consistent with the observed attenuation curve and emissivity of high-redshift galaxies and show that the reverse shock processing significantly affects dust enrichment and grain properties such as extinction curves and emissivity in supernova yields for high-redshift galaxies.

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arXiv2025

Collision frequency between dark matter subhaloes within Milky Way-like galaxies

In the standard cold dark matter (CDM) model, sub-galactic structures hierarchically collide and merge to build up larger structures. Mergers and collisions between dwarf galaxies and dark matter subhaloes (DMSHs) play an important role in the evolution and formation of structures within a massive galaxy. We investigate the collision frequency between DMSHs associated with a massive host galaxy such as the Milky Way. We analytically estimate the density distribution of DMSH pairs for the relative distance and relative velocity ($r_\mathrm{rel}$-$v_\mathrm{rel}$) and the distance from the centre of the host halo and relative velocity ($r$-$v_\mathrm{rel}$) planes, based on the distribution function of the host halo in the phase space. Then, we evaluate the collision frequencies of DMSHs by integrating the orbital evolution of DMSHs in Milky-Way-like host haloes selected from cosmological $N$-body simulations. The frequency of violent encounters, in which the relative distance of DMSHs is shorter than the sum of scale radii, is averaged as $2.1\times 10^2\,\mathrm{Gyr}^{-1}$. Since the time scale of violent encounters, $4.7\,\mathrm{Myr}$, is shorter than the dynamical time of the host halo, collisions between DMSHs occur frequently within the host halo. Although interactions between DMSHs produce pairs with higher relative velocities, the density distributions of all and colliding pairs between DMSHs provided by numerical results are approximately similar to those of the analytical model neglecting the interactions of DMSHs on $r_\mathrm{rel}$-$v_\mathrm{rel}$ plane for all pairs and $r$-$v_\mathrm{rel}$ plane for colliding pairs. We compare our results with observed colliding dwarf galaxies and provide insight into the abundance of DMSHs.

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arXiv2025

Effective supernova dust yields from rotating and non-rotating stellar progenitors

Supernovae (SNe) are believed to be the dominant sources of dust production at high redshift. However, the reverse shock generated by the interaction of the SN forward shock and the interstellar medium (ISM) significantly reduces the mass of newly formed dust in SN ejecta. This study quantifies the mass, composition, and grain size distribution of surviving dust after the passage of the reverse shock using the GRASHrev model. Our analysis covers a grid of SN models with progenitor masses $13\,M_\odot\leq m_\star\leq120M_\odot$, metallicity $-3\leq\text{[Fe/H]}\leq0$, and rotation velocities $v = 0$ and $300\,\mathrm{km\,s^{-1}}$. The SN explosions occur in a uniform ISM with densities $n_\text{ISM} = 0.05, 0.5$, and $5\text{ cm}^{-3}$. We find that the larger grains ($\gtrsim10\text{ nm}$) are more resistant to destruction by the reverse shock, with amorphous carbon dominating the surviving dust mass in most models. The surviving dust mass decreases with increasing ISM density. For non-rotating progenitors, the maximum mass of dust surviving the passage of the reverse shock is $\simeq 0.02\,M_\odot$ released by SN explosions of a $120\,M_\odot$ progenitor with $\text{[Fe/H]}=0$ in the ISM density $0.5\,\text{cm}^{-3}$, corresponding to $\simeq4\%$ of the initial dust mass before the passage of the reverse shock. Among rotating progenitors, a maximum surviving mass fraction is $\simeq5\%$ with a final dust mass $\simeq0.03\,M_\odot$ in $\text{[Fe/H]}=-1$ models. Although the reverse shock has a strong destructive impact, our results indicate that, on very short timescales, SNe can enrich the ISM with carbonaceous grains ranging in size from approximately $1\text{ nm}$ to $100\text{ nm}$ (up to $\simeq1\,\mathrm{μm}$ in non-rotating models). This is notable given the detection of the 2175 Å extinction bump in galaxies at $z>6$, suggesting the early presence of such dust.

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arXiv2024

Cosmological evolution of dark matter subhaloes under tidal stripping by growing Milky Way-like galaxies

We present the findings of a comprehensive and detailed analysis of merger tree data from ultra-high-resolution cosmological $N$-body simulations. The analysis, conducted with a particle mass resolution of $5 \times 10^3 h^{-1} M_{\odot}$ and a halo mass resolution of $10^7 h^{-1} M_{\odot}$, provides sufficient accuracy to suppress numerical artefacts. This study elucidates the dynamical evolution of subhaloes associated with the Milky Way-like host haloes. Unlike more massive dark matter haloes, which have been extensively studied, these subhaloes follow a distinct mass evolution pattern: an initial accretion phase, followed by a tidal stripping phase where mass is lost due to the tidal forces of the host halo. The transition from accretion to stripping, where subhaloes reach their maximum mass, occurs around a redshift of $z\simeq1$. Smaller subhaloes reach this point earlier, while larger ones do so later. Our analysis reveals that over 80 per cent of subhaloes have experienced mass loss, underscoring the universality of tidal stripping in subhalo evolution. Additionally, we derived the eccentricities and pericentre distances of subhalo orbits from the simulations and compare them with those of nearby satellite galaxies observed by the Gaia satellite. The results demonstrate a significant alignment between the orbital elements predicted by the cold dark matter model and the observed data, providing robust support for the model as a credible candidate for dark matter.

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arXiv2024

A universal scaling relation incorporating the cusp-to-core transition of dark matter haloes

The dark matter haloes associated with galaxies have hitherto established strong correlations within a range of observed parameters, known as scaling relations of dark matter haloes. The origin of these scaling relations still contains significant ambiguities and requires comprehensive exploration for complete understanding. Utilising the correlation between the concentration and mass of dark matter haloes inferred from cosmological $N$-body simulations based on the cold dark matter paradigm ($c$-$M$ relation), we derive theoretical scaling relations among other physical quantities such as the surface mass density, the maximum circular velocity, and the scale radius of the dark matter halo. By comparing theoretical and observed scaling relations at various mass scales, it is found that the scaling relations observed in dwarf galaxies and galaxies originate in the $c$-$M$ relation of the dark matter halo. We predict that this theoretical scaling relation is also established in galaxy clusters. Moreover, we propose a novel theoretical scaling relation that incorporates the effects of the cusp-to-core transition, which is supposed to occur in cold dark matter haloes. Our discussion concludes with the exploration of potential observational verification of the cusp-to-core transition process in dark matter haloes.

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arXiv2023

Frequency of the dark matter subhalo collisions and bifurcation sequence arising formation of dwarf galaxies

The cold dark matter (CDM) model predicts galaxies have 100 times more dark matter mass than stars. Nevertheless, recent observations report the existence of dark-matter-deficient galaxies with less dark matter than expected. To solve this problem, we investigate the physical processes of galaxy formation in head-on collisions between gas-containing dark matter subhaloes (DMSHs). Analytical estimation of the collision frequency between DMSHs associated with a massive host halo indicates that collisions frequently occur within 1/10th of the virial radius of the host halo, with a collision timescale of about 10 Myr, and the most frequent relative velocity increases with increasing radius. Using analytical models and numerical simulations, we show the bifurcation channel of the formation of dark-matter-dominated and dark-matter-deficient galaxies. In the case of low-velocity collisions, a dark-matter-dominated galaxy is formed by the merging of two DMSHs. In the case of moderate-velocity collisions, the two DMSHs penetrate each other. However the gas medium collides, and star formation begins as the gas density increases, forming a dwarf galaxy without dark matter at the collision surface. In the case of high-velocity collisions, shock-breakout occurs due to the shock waves generated at the collision surface reaching the gas surface, and no galaxy forms. For example, the simulation demonstrates that a pair of DMSHs with a mass of 10^9 Msun containing gas of 0.1 solar metallicity forms a dark-matter-deficient galaxy with a stellar mass of 10^7 Msun for a relative velocity of 200 km/s.

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