arXiv subjects

N. Arimoto

Publications and source records attributed to N. Arimoto.

At least 55 records · Page 3Linked to original sources

arXiv2006

A Potential Galaxy Threshing System in the Cosmos Field

We report on the discovery of a new potential galaxy threshing system in the COSMOS 2 square degree field using the prime-focus camera, Suprime-Cam, on the 8.2 m Subaru Telescope. This system consists of a giant elliptical galaxy with $M_V \approx -21.6$ and a tidally disrupted satellite galaxy with $M_V \approx -17.7$ at a photometric redshift of $z \approx 0.08$. This redshift is consistent with the spectroscopic redshift of 0.079 for the giant elliptical galaxy obtained from the Sloan Digital Sky Survey (SDSS) archive. The luminosity masses of the two galaxies are $3.7 \times 10^{12} \cal{M}_{\odot}$ and $3.1 \times 10^{9} \cal{M}_{\odot}$, respectively. The distance between the two galaxies is greater than 100 kpc. The two tidal tails emanating from the satellite galaxy extend over 150 kpc. This system would be the second well-defined galaxy threshing system found so far.

astro-ph↗

arXiv2006

The Cosmic Evolution Survey (COSMOS): Subaru Observations of the HST COSMOS Field

We present deep optical imaging observations of 2 square degree area, covered by the Cosmic Evolution Survey (COSMOS), made by the prime-focus Camera (Supreme-Cam) on the 8.2m Subaru Telescope. Observations were done in six broad-band [B (4459.7 AA), g' (4723.1 AA), V (5483.8 AA), r' (6213.0 AA), i' (7640.8 AA), z' (8855.0 AA)], and one narrow-band (NB816) filters. A total of 10^6 galaxies were detected to i'~26.5 mag. These data, combined with observations at u* and K-band are used to construct the photometric catalogs for the COSMOS and to measure their photometric redshifts, multi-band spectral energy distributions, stellar masses and identification of high redshift candidates. This catalog provides multi-waveband data for scientific analysis of the COSMOS survey.

astro-ph↗

arXiv2006

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 (dSph's): 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 dSph's appear to have been fundamentally different from the building blocks of the Milky Way, even at the earliest epochs.

astro-ph↗

arXiv2006

The DART imaging and CaT survey of the Fornax Dwarf Spheroidal Galaxy

As part of the DART project we have used the ESO/2.2m Wide Field Imager in conjunction with the VLT/FLAMES GIRAFFE spectrograph to study the detailed properties of the resolved stellar population of the Fornax dwarf spheroidal galaxy out to and beyond its tidal radius. We re-derived the structural parameters of the Fornax dwarf spheroidal using our wide field imaging covering the galaxy out to its tidal radius, and analysed the spatial distribution of the Fornax stars of different ages as selected from Colour-Magnitude Diagram analysis. We have obtained accurate velocities and metallicities from spectra in the CaII triplet wavelength region for 562 Red Giant Branch stars which have velocities consistent with membership in Fornax dwarf spheroidal. We have found evidence for the presence of at least three distinct stellar components: a young population (few 100 Myr old) concentrated in the centre of the galaxy, visible as a Main Sequence in the Colour-Magnitude Diagram; an intermediate age population (2-8 Gyr old); and an ancient population (> 10Gyr),which are distinguishable from each other kinematically, from the metallicity distribution and in the spatial distribution of stars found in the Colour-Magnitude Diagram. From our spectroscopic analysis we find that the ``metal rich'' stars ([Fe/H]> -1.3) show a less extended and more concentrated spatial distribution, and display a colder kinematics than the ``metal poor'' stars ([Fe/H<-1.3). There is tentative evidence that the ancient stellar population in the centre of Fornax does not exhibit equilibrium kinematics. This could be a sign of a relatively recent accretion of external material.

astro-ph↗

arXiv2006

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

[Abridged] Over the past two decades observations and theoretical simulations have established a global frame-work of galaxy formation and evolution in the young Universe. Galaxies formed as baryonic gas cooled at the centres of collapsing dark matter halos. Mergers of halos led to the build up of galaxy mass. A major step forward in understanding these issues requires well resolved physical information on individual galaxies at high redshift. Here we report adaptive optics, spectroscopic observations of a representative luminous star forming galaxy when the Universe was only twenty percent of its age. The superior angular resolution of these data reveals the physical and dynamical properties of a high redshift galaxy in unprecedented detail. A large and massive rotating proto-disk is channelling gas towards a growing central stellar bulge hosting an accreting massive black hole.

astro-ph↗

arXiv2006

A wide area survey for high-redshift massive galaxies. I. Number counts and clustering of BzKs and EROs

We have combined deep BRIz' imaging over 2x940 arcmin^2 fields obtained with the Suprime-Cam on the Subaru telescope with JKs imaging with the SOFI camera at the New Technology Telescope to search for high-redshift massive galaxies. K-band selected galaxies have been identified over an area of ~920 arcmin^2 to K_Vega=19.2, of which 320 arcmin^2 are complete to K_Vega=20. The BzK selection technique was used to obtain complete samples of ~500 candidate massive star-forming galaxies (sBzKs) and ~160 candidate massive, passively-evolving galaxies (pBzKs), both at 1.4 5 criterion we also identified ~850 extremely red objects (EROs). The surface density of sBzKs and pBzKs is found to 1.20+/-0.05 arcmin^{-2} and 0.38+/-0.03 arcmin^{-2}, respectively. Both sBzKs and pBzKs are strongly clustered, at a level at least comparable to that of EROs, with pBzKs appearing more clustered than sBzKs. We estimate the reddening, star formation rates (SFRs) and stellar masses (M_*) of the sBzKs, confirming that to K_Vega~20 median values are M_*~10^{11}M_sun, SFR 190M_sun yr^{-1}, and E(B-V)~0.44. The most massive sBzKs are also the most actively star-forming, an effect which can be seen as a manifestation of downsizing at early epochs. The space density of massive pBzKs at z~1.4-2 is 20%+/-7% that of similarly massive early-type galaxies at z~0, and similar to that of sBzKs of the same mass. We argue that star formation quenching in these sBzKs will result in nearly doubling the space density of massive early-type galaxies, thus matching their local density.

astro-ph↗

arXiv2005

MAMBO 1.2 Millimeter observations of BzK-selected star-forming Galaxies at z~2

We present MAMBO 1.2 mm observations of five BzK-pre-selected vigorous starburst galaxies at z~2. Two of these were detected at more than 99.5% confidence levels, with 1.2 mm fluxes around 1.5 mJy. These millimeter fluxes imply vigorous activity with star-formation rates (SFRs) approx. 500-1500 Msun/yr, confirmed also by detections at 24 microns with the MIPS camera on board of the Spitzer satellite. The two detected galaxies are the ones in the sample with the highest SFRs estimated from the rest-frame UV, and their far-IR- and UV-derived SFRs agree reasonably well. This is different from local ULIRGs and high-z submm/mm selected galaxies for which the UV is reported to underestimate SFRs by factors of 10-100, but similar to the average BzK-ULIRG galaxy at z~2. The two galaxies detected at 1.2 mm are brighter in K than the typical NIR-counterparts of MAMBO and SCUBA sources, implying also a significantly different K-band to submm/mm flux ratio. This suggests a scenario in which z~2 galaxies, after their rapid (sub)mm brightest phase opaque to optical/UV light, evolve into a longer lasting phase of K-band bright and massive objects. Targeting the most UV active BzKs could yield substantial detection rates at submm/mm wavelengths.

astro-ph↗

arXiv2005

The Tully-Fisher relation of intermediate redshift field and cluster galaxies from Subaru spectroscopy

We have carried out spectroscopic observations in 4 cluster fields using Subaru's FOCAS multi-slit spectrograph and obtained spectra for 103 bright disk field and cluster galaxies at $0.06 \le z \le 1.20$. Seventy-seven of these show emission lines, and 33 provide reasonably-secure determinations of the galaxies' rotation velocity. The rotation velocities, luminosities, colours and emission-line properties of these galaxies are used to study the possible effects of the cluster environment on the star-formation history of the galaxies. Comparing the Tully-Fisher relations of cluster and field galaxies at similar reshifts we find no measurable difference in rest-frame $B$-band luminosity at a given rotation velocity (the formal difference is $0.18\pm0.33 $mag). The colours of the cluster emission line galaxies are only marginally redder in rest-frame $B-V$ (by $0.06\pm0.04 $mag) than the field galaxies in our sample. Taken at face value, these results seem to indicate that bright star-forming cluster spirals are similar to their field counterparts in their star-formation properties. However, we find that the fraction of disk galaxies with absorption-line spectra (i.e., with no current star formation) is larger in clusters than in the field by a factor of $\sim3$--5. This suggests that the cluster environment has the overall effect of switching off star formation in (at least) some spiral galaxies. To interpret these observational results, we carry out simulations of the possible effects of the cluster environment on the star-formation history of disk galaxies and thus their photometric and spectroscopic properties. Finally, we evaluate the evolution of the rest-frame absolute $B$-band magnitude per unit redshift at fixed rotation velocity.

astro-ph↗

arXiv2005

Stellar Populations of Elliptical Galaxies in Virgo Cluster I. : The Data and Stellar Population Analysis

We have determined spectroscopic ages of elliptical galaxies in the Virgo cluster using spectra of very high signal-to-noise ratio (S/N/A > 100). We observed 8 galaxies with the Subaru Telescope and have combined this sample with 6 galaxies previously observed with the WHT. To determine their ages we have used a new method based on the Hgamma_sigma age indicator, which is virtually independent of the effects of metallicity. Apart from ages we have estimated abundances of various elements. In this paper we present the observations, the data reduction and the reliability of the Hgamma_sigma method. The results of this investigation are presented in a companion paper (Yamada et al. 2006).

astro-ph↗

arXiv2004

Subaru/HDS Abundances in Three Giant Stars in the Ursa Minor Dwarf Spheroidal Galaxy

With the HDS (High Dispersion Spectrograph) on the Subaru telescope, we obtained high resolution optical region spectra of three red giant stars (cos 4, cos 82, and cos 347) in the Ursa Minor dwarf spheriodal galaxy. Chemical abundances in these stars have been analysed for 26 elements including alpha-, iron-peak, and neutron capture elements. All three stars show low abundances of alpha-elements (Mg, Si, and Ca) and two stars (cos 82 and cos 347) show high abundance of Mn compared to Galactic halo stars of similar metallicity. One star (cos 4) has been confirmed to be very metal deficient ([Fe/H]=-2.7) and found to show anomalously low abundances of Mn, Cu, and Ba. In another star cos 82 ([Fe/H]=-1.5), we have found large excess of heavy neutron-capture elements with the general abundance pattern similar to the scaled solar system r-process abundance curve. These observational results are rather puzzling: low abundances of alpha-elements and high abundance of Mn seem to sugggest a significant contribution of SNe Ia at low metallicity, while there is no hint of s-process (i.e., AGB stars) contribution even at [Fe/H]=-1.5, suggesting a peculiar nucleosynthetic history of the UMi dSph galaxy.

astro-ph↗

arXiv2004

Two distinct ancient components in the Sculptor Dwarf Spheroidal Galaxy: First Results from DART

We have found evidence for the presence of two distinct ancient stellar components (both geq 10 Gyr old) in the Sculptor dwarf spheroidal galaxy. We used the ESO Wide Field Imager (WFI) in conjunction with the VLT/FLAMES spectrograph to study the properties of the resolved stellar population of Sculptor out to and beyond the tidal radius. We find that two components are discernible in the spatial distribution of Horizontal Branch stars in our imaging, and in the [Fe/H] and v_hel distributions for our large sample of spectroscopic measurements. They can be generally described as a ``metal-poor'' component ([Fe/H]< -1.7) and a ``metal-rich'' component ([Fe/H]> -1.7). The metal-poor stars are more spatially extended than the metal-rich stars, and they also appear to be kinematically distinct. These results provide an important insight into the formation processes of small systems in the early universe and the conditions found there. Even this simplest of galaxies appears to have had a surprisingly complex early evolution.

astro-ph↗

arXiv2004

Present-day scaling relations of submm galaxies: origin of spheroidal sytems

We analyse the spectral energy distributions (SEDs) of 23 submm galaxies and 3 ISO-detected EROs, all of which have the spectroscopic redshifts, by using an evolutionary SED model of starbursts. This SED model allows us to investigate intrinsic properties of starbursts, such as the starburst age and the mean stellar metallicity, as it takes into account the chemical evolution. Also, the intrinsic size of the starburst region is estimated from observed SEDs. Using this SED model, we predict colour, magnitude, and size of present-day descendants of submm galaxies, and derive scaling relations, such as the present-day colour-magnitude and size-magnitude relations. We argue that submm galaxies are the progenitors of present-day elliptical galaxies, provided that the initial mass function (IMF) of submm galaxies is slightly flatter than the Salpeter IMF. In this case, we find that 1) the mean present-day magnitude of submm galaxies is similar to that of L* elliptical galaxies, 2) the present-day colour-magnitude relation is consistent with that of elliptical galaxies, 3) the present-day size-magnitude relation of elliptical galaxies can be reproduced if massive submm galaxies consist of multiple starburst regions. We estimate the effect of feedback in submm galaxies as a function of starburst age. It is found that starburst regions in submm galaxies are likely to be self-regulated; i.e. the effect of feedback is nearly balanced with the self-gravity of starburst regions.

astro-ph↗

arXiv2002

HIPPARCOS age-metallicity relation of the solar neighbourhood disc stars

We derive age-metallicity relations (AMRs) and orbital parameters for the 1658 solar neighbourhood stars to which accurate distances are measured by the HIPPARCOS satellite. The sample stars comprise 1382 thin disc stars, 229 thick disc stars, and 47 halo stars according to their orbital parameters. We find a considerable scatter for thin disc AMR along the one-zone Galactic chemical evolution (GCE) model. Orbits and metallicities of thin disc stars show now clear relation each other. The scatter along the AMR exists even if the stars with the same orbits are selected. We examine simple extension of one-zone GCE models which account for inhomogeneity in the effective yield and inhomogeneous star formation rate in the Galaxy. Both extensions of one-zone GCE model cannot account for the scatter in age - [Fe/H] - [Ca/Fe] relation simultaneously. We conclude, therefore, that the scatter along the thin disc AMR is an essential feature in the formation and evolution of the Galaxy. The AMR for thick disc stars shows that the star formation terminated 8 Gyr ago in thick disc. As already reported by Gratton et.al. (2000) and Prochaska et.al. (2000), thick disc stars are more Ca-rich than thin disc stars with the same [Fe/H]. We find that thick disc stars show a vertical abundance gradient. These three facts, the AMR, vertical gradient, and [Ca/Fe]-[Fe/H] relation, support monolithic collapse and/or accretion of satellite dwarf galaxy as thick disc formation scenario.

astro-ph↗

arXiv2002

Extended Star Formation in Dwarf Spheroidal Galaxies: the Cases of Draco, Sextans, and Ursa Minor

Star formation and chemical enrichment histories of the dwarf spheroidal galaxies (dSphs) Draco, Sextans, and Ursa Minor are investigated by means of chemical evolution models and a simulation code for colour-magnitude diagrams (CMDs). The CMD simulation code is designed to fully consider effects of the chemical evolution on stellar evolution and photometric properties. Comparisons between the chemical evolution models and the observed abundance patterns reveal that the star formation rates were very low (1-5% of that of the solar neighbourhood disc) and that the initial star formation continued for a long duration (> 3.9-6.5 Gyr) in these dSphs. This star formation history can reproduce morphologies of the observed CMDs, such as narrow red giant branches and red horizontal branches. Hence, both of the abundance patterns and the morphologies of the CMDs can be explained by the star formation histories characterised by the low star formation rate and the long duration of the star formation period. Because of the low star formation rates, plenty of gas remains at the final epoch of star formation. We suggest that gas stripping by the Galaxy results in termination of star formation in the dSphs.

astro-ph↗

arXiv1999

Self-enrichment in Omega Centauri

The origin of abundance spreads observed in omega Centauri is studied in the context of the self-enrichment scenario. Five chemical evolution models are constructed and are compared with empirical metallicity distribution of $ω$ Cen. After a series of simulations, it is found that neither of closed-box, outflow, nor infall models can reproduce the empirical metallicity distribution of omega Cen, while a modified outflow model with a bimodal initial mass function (IMF) gives a metallicity distribution that fits closely to the empirical ones. In the modified outflow model, long-lived stars are assumed to form after the first explosion of type II supernovae (SNII) in a proto-cloud. The modified outflow model involves gas infall at the very first chemical evolution. Thus we conclude that self-enrichment causes the abundance dispersion in omega Cen. A success of the outflow model with the bimodal IMF implies that low mass stars in a globular cluster (GC) should have formed in the gas already enriched by the first generation of SNII. This scenario, originally proposed by Cayrel (1986), can explain a lack of globular clusters with [Fe/H] > -2.2 in the Milky Way Galaxy.

astro-ph↗

arXiv1999

Metal Enrichment in the Early Galactic Halo

An early history of metal enrichment in the Galactic halo is studied. We investigate chemical inhomogeneity by using a stochastic chemical evolution model. The model confronts with metallicity distribution function of long-lived halo stars which is found to be a clue to obtain the best model prescriptions. We find that the star formation in the halo virtually terminated by around 1 Gyr and that the halo has never been chemically homogeneous in its star formation history. This conclusion does not depend whether mass loss from the halo is taken into account or not. Observed ratios of alpha-elements with respect to iron do not show scatters on a [alpha/Fe]-[Fe/H] plane, but this does not imply that interstellar matter in the halo was homogeneous because a chemical evolution path on this diagram is degenerate in the star formation rate. On the other hand, apparent spread of [Sr/Fe] ratio among metal-poor halo stars does not reflect an inhomogeneous metal enrichment, instead it is due to a sharp increase in a production rate of strontium that is probably synthesised in slightly less massive stars than progenitor of iron-producing SNII.

astro-ph↗

arXiv1999

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.

astro-ph↗

arXiv1998

New Near-Infrared Spectroscopy of the High Redshift Quasar B 1422+231 at z=3.62

We present new near-infrared (rest-frame UV-to-optical) spectra of the high redshift, gravitationally lensed quasar B 1422+231 (z=3.62). Diagnostic emission lines of FeII, [OIII]5007, and Hb, commonly used to determine the excitation, ionization, and chemical abundances of radio-quiet and radio-loud quasars, were detected. Our new data show that the ratio FeII(UV)/Hb=18.1+-4.6 and FeII(optical)/Hb=2.3+-0.6 are higher than those reported by Kawara et al. (1996) by factors of 1.6 and 3.3, respectively, although the ration [OIII]5007/Hb=0.19+-0.02 is nearly the same between the two measurements. The discrepancy of the line flux ratios between the measurements is likely due to improved data and fitting procedures rather that to intrinsic variability. While approximately half of the high-z quasars observed to date have much more extreme FeII(optical)/Hb ratios, the line ratio measured for B 422+231 are consistent with the observed range of FeII(optical) ratios of low-z quasars.

astro-ph↗