arXiv subjects

Daisuke Toyouchi

Publications and source records attributed to Daisuke Toyouchi.

21 records · Page 2Linked to original sources

arXiv2016

Bimodal Distribution of Galactic Disk Stars on the [alpha/Fe]-[Fe/H] Plane as a Possible Evidence of Discontinuous Radial Migration History

We investigate the role of radial migration history of stars in chemical evolution of a disk galaxy, in particular in understanding the origin of their bimodal distribution on the [alpha/Fe]-[Fe/H] plane. For this purpose, we examine the three different models with no, continuous, and discontinuous radial migration, respectively. We find that for the model with radial migration, the [alpha/Fe] ratios of stars in outer disk regions decrease more rapidly with time than the model without radial migration, because the associated net transfer of intermediate and old disk stars from inner to outer disk regions increases the rate of Type Ia relative to that of Type II supernovae in the latter regions. Moreover, in the model assuming rapid and discontinuous radial migration, its effect on the stellar abundances at larger radii is significant enough to provide the large difference in the evolution of stars on the [alpha/Fe]-[Fe/H] plane between inner and outer disk regions. As a result we obtain the bimodal distribution of disk stars on the [alpha/Fe]-[Fe/H] plane as observed in the Galactic stellar disk, thereby implying that the event of discontinuous radial migration may play a key role in reproducing the observed bimodality of stars on this abundance-ratio diagram. We discuss possible mechanisms causing such discontinuous radial migration in the early evolution of the Galactic disk, including the event of minor merging of a relatively massive satellite onto the stellar disk.

astro-ph.GA↗

arXiv2015

Gas Inflow and Outflow Histories in Disk Galaxies as Revealed from Observations of Distant Star-Forming Galaxies

We investigate gas inflow and outflow histories in Milky Way-like disk galaxies, to get new insights into the baryonic processes in galaxy formation and evolution. For this purpose, we solve the equations for the evolutions of the surface mass densities of gas and metals at each radius in a galactic disk, based on the observed structural properties of distant star-forming galaxies, including the redshift evolution of their stellar mass distribution, their scaling relation between the mass of baryonic components, star formation rate (SFR) and chemical abundance, as well as the supposed evolution of their radial metallicity gradients (RMGs). We find that the efficiency of gas inflow for a given SFR decreases with time and that the inflow rate is always nearly proportional to the SFR. For gas outflow, although its efficiency for a given SFR is a decreasing function of time, similarly to gas inflow, the outflow rate is not necessarily proportional to the SFR and the relation between the outflow rate and SFR strongly depends on the evolution of the adopted RMG. We also find that the results on the outflow rate can be reproduced in the framework of momentum-driven (energy-driven) wind mechanism if the RMG is steepening (flattening) with time. Therefore if the well measured RMGs and their evolution for Milky Way-like galaxies are obtained from future observations, then our results will be useful to constrain the main driving mechanism for their galactic outflows.

astro-ph.GA↗

arXiv2014

On the Chemical and Structural Evolution of the Galactic Disk

We study the detailed properties of the radial metallicity gradient in the stellar disk of our Galaxy to constrain its chemical and structural evolution. For this purpose we select and analyze $\sim$ 18,500 disk stars taken from two datasets, the Sloan Digital Sky Survey (SDSS) and the High-Accuracy Radial velocity Planetary Searcher (HARPS). On these surveys we examine the metallicity gradient, $Δ$[Fe/H]/$ΔR_{\rm g}$, along the guiding-center radii, $R_{\rm g}$, of stars and its dependence on the [$α$/Fe] ratios, to infer the original metallicity distribution of the gas disk from which those stars formed and its time evolution. In both sample sources, the thick-disk candidate stars characterized by high [$α$/Fe] ratios ([$α$/Fe] $>$ 0.3 in SDSS, [$α$/Fe] $>$ 0.2 in HARPS) are found to show a positive $Δ$[Fe/H]/$ΔR_{\rm g}$, whereas the thin-disk candidate stars characterized by lower [$α$/Fe] ratios show a negative one. Furthermore, we find that the relatively young thin-disk population characterized by much lower [$α$/Fe] ratios ([$α$/Fe] $<$ 0.2 in SDSS, [$α$/Fe] $<$ 0.1 in HARPS) shows notably a flattening $Δ$[Fe/H]/$ΔR_{\rm g}$ with decreasing [$α$/Fe], in contrast to the old one with higher [$α$/Fe] ratios ([$α$/Fe] $\sim$ 0.2 in SDSS, [$α$/Fe] $\sim$ 0.1 in HARPS). The possible implication for early disk evolution is discussed, in the context of galaxy formation accompanying the rapid infall of primordial gas on the inner disk region, which can generate a positive metallicity gradient, and the subsequent chemical evolution of the disk, which results in a flattening effect of a metallicity gradient at later epochs.

astro-ph.GA↗