Nobuo Arimoto

The rapid formation of a large rotating disk galaxy three billion years after the Big Bang.

Observations and theoretical simulations have established a framework for galaxy formation and evolution in the young Universe. Galaxies formed as baryonic gas cooled at the centres of collapsing dark-matter haloes; mergers of haloes and galaxies then led to the hierarchical build-up of galaxy mass. It remains unclear, however, over what timescales galaxies were assembled and when and how bulges and disks—the primary components of present-day galaxies—were formed. It is also puzzling that the most massive galaxies were more abundant and were forming stars more rapidly at early epochs than expected from models. Here we report high-angular-resolution observations of a representative luminous star-forming galaxy when the Universe was only 20% of its current age. A large and massive rotating protodisk is channelling gas towards a growing central stellar bulge hosting an accreting massive black hole. The high surface densities of gas, the high rate of star formation and the moderately young stellar ages suggest rapid assembly, fragmentation and conversion to stars of an initially very gas-rich protodisk, with no obvious evidence for a major merger.

Kinemetry of SINS high-redshift star-forming galaxies: distinguishing rotating disks from major mergers

We present a simple set of kinematic criteria that can distinguish between galaxies dominated by ordered rotational motion and those involved in major merger events. Our criteria are based on the dynamics of the warm ionized gas (as traced by Hα) within galaxies, making this analysis accessible to high-redshift systems, whose kinematics are primarily traceable through emission features. Using the method of kinemetry (developed by Krajnovic and coworkers), we quantify asymmetries in both the velocity and velocity dispersion maps of the warm gas, and the resulting criteria enable us to empirically differentiate between nonmerging and merging systems at high redshift. We apply these criteria to 11 of our best-studied rest-frame UV/optical-selected z ~ 2 galaxies for which we have near-infrared integral-field spectroscopic data from SINFONI on the VLT. Of these 11 systems, we find that >50% have kinematics consistent with a single rotating disk interpretation, while the remaining systems are more likely undergoing major mergers. This result, combined with the short formation timescales of these systems, provides evidence that rapid, smooth accretion of gas plays a significant role in galaxy formation at high redshift.

A New View of the Dwarf Spheroidal Satellites of the Milky Way From VLT/FLAMES: Where are the Very Metal Poor Stars?

As part of the Dwarf galaxies Abundances and Radial-velocities Team (DART) program, we have measured the metallicities of a large sample of stars in four nearby dwarf spheroidal galaxies (dSphs): Sculptor, Sextans, Fornax, and Carina. The low mean metal abundances and the presence of very old stellar populations in these galaxies have supported the view that they are fossils from the early universe. However, contrary to naive expectations, we find a significant lack of stars with metallicities below [Fe/H] ~ -3 dex in all four systems. This suggests that the gas that made up the stars in these systems had been uniformly enriched prior to their formation. Furthermore, the metal-poor tail of the dSph metallicity distribution is significantly different from that of the Galactic halo. These findings show that the progenitors of nearby dSphs appear to have been fundamentally different from the building blocks of the Milky Way, even at the earliest epochs.

Gradients of Absorption-Line Strengths in Elliptical Galaxies

We have restudied line-strength gradients of 80 elliptical galaxies. Typical metallicity gradients of elliptical galaxies are Δ[Fe/H]/Δ log r -0.3, which is flatter than the gradients predicted by monolithic collapse simulations. The metallicity gradients do not correlate with any physical properties of galaxies, including central and mean metallicities, central velocity dispersions σ0, absolute B magnitudes MB, absolute effective radii Re, and dynamical masses of galaxies. By using the metallicity gradients, we have calculated mean stellar metallicities for individual ellipticals. Typical mean stellar metallicities are [Fe/H] -0.3 and range from [Fe/H] -0.8 to +0.3, which is contrary to what Gonzalez & Gorgas claimed; the mean metallicities of ellipticals are not universal. The mean metallicities correlate well with σ0 and dynamical masses, though relations for MB and Re include significant scatters. We find fundamental planes defined by surface brightnesses SBe, [Fe/H], and Re (or MB), the scatters of which are much smaller than those of the [Fe/H]-Re (or [Fe/H]-MB) relations. The [Fe/H]-log σ0 relation is nearly parallel to the [Fe/H]0-log σ0 relation but systematically lower by 0.3 dex; thus the mean metallicities are about one-half of the central values. The metallicity-mass relation or, equivalently, the color-magnitude relation of ellipticals holds not only for the central parts of galaxies but also for entire galaxies. Assuming that Mg2 and Fe1 give [Mg/H] and [Fe/H], respectively, we find [Mg/Fe] +0.2 in most of elliptical galaxies. [Mg/Fe] shows no correlation with galaxy mass tracers such as σ0, in contrast to what was claimed for the central [Mg/Fe]. This can be most naturally explained if the star formation had stopped in elliptical galaxies before the bulk of Type Ia supernovae began to occur. Elliptical galaxies can have significantly different metallicity gradients and [Fe/H], even if they have the same galaxy mass. This may result from galaxy mergers, but no evidence is found from presently available data to support the same origin for metallicity gradients, the scatters around the metallicity-mass relation, and dynamical disturbances. This may suggest that the scatters have their origin at the formation epoch of galaxies.

A mature cluster with X-ray emission at z =2 .07

We report evidence of a fully established galaxy cluster at z = 2.07, consisting of a ∼20σ overdensity of red, compact spheroidal galaxies spatially coinciding with extended X-ray emission detected with XMM-Newton. We use VLT VIMOS and FORS2 spectra and deep Subaru, VLT and Spitzer imaging to estimate the redshift of the structure from a prominent z = 2.07 spectroscopic redshift spike of emission-line galaxies, concordant with the accurate 12-band photometric redshifts of the red galaxies. Using NICMOS and Keck AO observations, we find that the red galaxies have elliptical morphologies and compact cores. While they do not form a tight red sequence, their colours are consistent with that of a 1.3 Gyr population observed at z ∼ 2.1. From an X-ray luminosity of 7.2 × 10 43 erg s −1 and the stellar mass content of the red galaxy population, we estimate a halo mass of 5.3–8 × 10 13 M� , comparable to the nearby Virgo cluster. These properties imply that this structure could be the most distant, mature cluster known to date and that X-ray luminous, elliptical-dominated clusters are already forming at substantially earlier epochs than previously known.

A high-resolution VLT/FLAMES study of individual stars in the centre of the Fornax dwarf spheroidal galaxy

For the first time we show the detailed, late-stage, chemical evolution history of a small nearby dwarf spheroidal galaxy in the Local Group. We present the results of a high-resolution (R ~ 20 000, λ = 5340–5620; 6120–6701) FLAMES/GIRAFFE abundance study at ESO/VLT of 81 photometrically selected, red giant branch stars in the central 25 of the Fornax dwarf spheroidal galaxy. We also carried out a detailed comparison of the effects of recent developments in abundance analysis (e.g., spherical models vs. plane-parallel) and the automation that is required to efficiently deal with such large data sets. We present abundances of α-elements (Mg, Si, Ca, and Ti), iron-peak elements (Fe, Ni, and Cr), and heavy elements (Y, Ba, La, Nd, and Eu). Our sample was randomly selected and is clearly dominated by the younger and more metal-rich component of Fornax, which represents the major fraction of stars in the central region. This means that the majority of our stars are 1−4 Gyr old, and thus represent the end phase of chemical evolution in this system. Our sample of stars has unusually low [α/Fe], [Ni/Fe], and [Na/Fe] compared to the Milky Way stellar populations at the same [Fe/H]. The particularly important role of stellar winds from low-metallicity AGB stars in the creation of s-process elements is clearly seen from the high [Ba/Y]. Furthermore, we present evidence of an s-process origin of Eu.

Extragalactic science, cosmology, and Galactic archaeology with the Subaru Prime Focus Spectrograph

The Subaru Prime Focus Spectrograph (PFS) is a massively-multiplexed fiber-fed optical and near-infrared 3-arm spectrograph (N_fiber=2400, 380<lambda<1260nm, 1.3 degree diameter FoV), offering unique opportunities in survey astronomy. Here we summarize the science case feasible for a survey of Subaru 300 nights. We describe plans to constrain the nature of dark energy via a survey of emission line galaxies spanning a comoving volume of 9.3 (Gpc/h)^3 in the redshift range 0.8<z<2.4. In each of 6 redshift bins, the cosmological distances will be measured to 3% precision via BAO, and redshift-space distortions will be used to constrain structure growth to 6% precision. In the GA program, radial velocities and chemical abundances of stars in the Milky Way and M31 will be used to infer the past assembly histories of spiral galaxies and the structure of their dark matter halos. Data will be secured for 10^6 stars in the Galactic thick-disk, halo and tidal streams as faint as V~22, including stars with V < 20 to complement the goals of the Gaia mission. A medium-resolution mode with R = 5000 to be implemented in the red arm will allow the measurement of multiple alpha-element abundances and more precise velocities for Galactic stars, elucidating the detailed chemo-dynamical structure and evolution of each of the main stellar components of the Milky Way Galaxy and of its dwarf spheroidal galaxies. For the extragalactic program, our simulations suggest the wide avelength range will be powerful in probing the galaxy population and its clustering over a wide redshift range. We propose to conduct a color-selected survey of 1<z<2 galaxies and AGN over 16 deg^2 to J~23.4, yielding a fair sample of galaxies with stellar masses above ~10^{10}Ms at z~2. A two-tiered survey of higher redshift LBGs and LAEs will quantify the properties of early systems close to the reionization epoch.

The Iron Discrepancy in Elliptical Galaxies after ASCA

We present estimates for the iron content of the stellar and diffused components of elliptical galaxies, as derived respectively from integrated optical spectra and from ASCA X-ray observations. A macroscopic discrepancy emerges between the expected iron abundances in the hot interstellar medium (ISM) and what is indicated by the X-ray observations, especially when allowance is made for the current iron enrichment by Type Ia supernovae. This strong discrepancy, that in some extreme instances may be as large as a factor of ~20, calls into question our current understanding of supernova enrichment and chemical evolution of galaxies. We discuss several astrophysical implications of the inferred low iron abundances in the ISM, including the chemical evolution of galaxies and clusters of galaxies, the evolution of gas flows in elliptical galaxies, and the heating of the intracluster medium. Some of the consequences appear hard to accept, and in the attempt to avoid some of the difficulties we explore ways of hiding or diluting iron in the ISM of ellipticals. None of these possibilities appears astrophysically plausible, and we alternatively raise the question of the reliability of iron L line diagnostic tools that are currently used to infer abundances from X-ray spectra. Various thin-plasma emission models are shown to give iron abundances that may differ significantly, especially at low temperatures (kT 1 keV), when the iron L complex is dominated by iron ions with still many bound electrons. From a collection of ASCA and other X-ray observatory data, it is shown that current thin-plasma codes tend to give very low iron abundances when the temperature of the objects is below ~1 keV. Such objects include various types of binary stars, supernova remnants, starburst galaxies, and AGNs, with the case of galaxy groups being especially well documented. We conclude that, besides rethinking the chemical evolution of galaxies, one should also consider the possibility that existing thin-plasma models may incorporate inaccurate atomic physics for the ions responsible for the iron L complex.

A robust age indicator for old stellar populations

We derive new spectral H_gamma index definitions which are robust age indicators for old and relatively old stellar populations and thus have great potential for solving the age-metallicity degeneracy of galaxy spectra. To study H_gamma as a function of age, metallicity and resolution, we used a new spectral synthesis model which predicts SEDs of single-age, single-metallicity stellar populations at resolution FWHM=1.8A (which can be smoothed to different resolutions), allowing direct measurements of the equivalent widths of particular absorption features. We find that the H_gamma strong age disentangling power strongly depends strongly on the adopted resolution and galaxy velocity dispersion. We propose a system of indices which are completely insensitive to metallicity and stable against resolution, allowing the study of galaxies up to ~300 km/s. Observational spectra of very high S/N and relatively high dispersion, are required to gain this unprecedented age discriminating power. Once such spectra are obtained, accurate and reliable estimates for the luminosity-weighted average stellar ages of these galaxies will become possible for the first time, without assessing their metallicities. We measured this index for two globular clusters, a number of low-luminosity elliptical galaxies and a standard S0 galaxy. We find a large spread in the average stellar ages of a sample of low-luminosity ellipticals. In particular these indices yield 4 Gyr for M32, in agreement with the age provided by an extraordinary fit to the full spectrum of this galaxy that we achieve here.We derive new spectral Hγ index definitions that are robust age indicators for old and relatively old stellar populations and therefore have great potential for solving the age-metallicity degeneracy of galaxy spectra. To study this feature as a function of age, metallicity, and resolution, we have used a new spectral synthesis model that predicts spectral energy distributions of single-age, single-metallicity stellar populations at resolution FWHM~1.8 A (which can be smoothed to different resolutions), allowing direct measurements of the equivalent widths of particular absorption features. We show that Hγs strong age-disentangling power is due to a compensating effect: at a specified age, Hγ strengthens with metallicity owing to an adjacent metallic absorption, but on the other hand, the adopted pseudocontinua are depressed by the effects of strong neighboring Fe I lines on both sides of Hγ. Despite the fact that this effect depends strongly on the adopted resolution and galaxy velocity dispersion σ, we propose a system of indicators that are completely insensitive to metallicity and stable against resolution, allowing the study of galaxies up to σ ~ 300 km s-1. An extensive analysis of the characteristics of these indices indicates that observational spectra of very high signal-to-noise ratio and relatively high dispersion are required to gain this unprecedented age-discriminating power. Once such spectra are obtained, accurate and reliable estimates for the luminosity-weighted average stellar ages of these galaxies will become possible for the first time, without assessing their metallicities. We measured this index for two globular clusters, a number of low-luminosity elliptical galaxies, and a standard S0 galaxy. We find a large spread in the average stellar ages of a sample of low-luminosity ellipticals. In particular, these indices yield 4 Gyr for M32. This value is in excellent agreement with the age provided by the extraordinary fit to the full spectrum of this galaxy that we achieve in this paper.

The FMOS-COSMOS Survey of Star-forming Galaxies at z ~ 1.6. I. Hα-based Star Formation Rates and Dust Extinction

We present the first results from a near-IR spectroscopic survey of the COSMOS field, using the Fiber Multi-Object Spectrograph on the Subaru telescope, designed to characterize the star-forming galaxy population at 1.4 < z < 1.7. The high-resolution mode is implemented to detect Hα in emission between 1.6-1.8 μm with f Hα gsim 4 × 10–17 erg cm–2 s–1. Here, we specifically focus on 271 sBzK-selected galaxies that yield a Hα detection thus providing a redshift and emission line luminosity to establish the relation between star formation rate and stellar mass. With further J-band spectroscopy for 89 of these, the level of dust extinction is assessed by measuring the Balmer decrement using co-added spectra. We find that the extinction (0.6 lsim A Hα lsim 2.5) rises with stellar mass and is elevated at high masses compared to low-redshift galaxies. Using this subset of the spectroscopic sample, we further find that the differential extinction between stellar and nebular emission E star(B – V)/E neb(B – V) is 0.7-0.8, dissimilar to that typically seen at low redshift. After correcting for extinction, we derive an Hα-based main sequence with a slope (0.81 ± 0.04) and normalization similar to previous studies at these redshifts.