arXiv subjects

Nobuo Arimoto

Publications and source records attributed to Nobuo Arimoto.

At least 55 records · Page 3Linked to original sources

arXiv2011

Detailed Chemical Abundances of Four Stars in the Unusual Globular Cluster, Palomar 1

Detailed chemical abundances for twenty one elements are presented for four red giants in the anomalous outer halo globular cluster Palomar 1 ($R_{\rm{GC}} = 17.2$ kpc, $Z=3.6$ kpc) using high-resolution (R=36000) spectra from the High Dispersion Spectrograph (HDS) on the Subaru Telescope. Pal 1 has long been considered unusual because of its low surface brightness, sparse red giant branch, young age, and its possible association with two extragalactic streams of stars---this paper shows that its chemistry further confirms its unusual nature. The mean metallicity of the four stars, $[\rm{Fe/H}] = -0.60 \pm 0.01$, is high for a globular cluster so far from the Galactic center, but is low for a typical open cluster. The [$α$/Fe] ratios, though in agreement with the Galactic stars within the $1σ$ errors, agree best with the lower values in dwarf galaxies. No signs of the Na/O anticorrelation are detected in Pal 1, though Na appears to be marginally high in all four stars. Pal 1's neutron capture elements are also unusual: its high [Ba/Y] ratio agrees best with dwarf galaxies, implying an excess of second-peak over first-peak s-process elements, while its [Eu/$α$] and [Ba/Eu] ratios show that Pal 1's contributions from the r-process must have differed in some way from normal Galactic stars. Therefore, Pal 1 is chemically unusual, as well in its other properties. Pal 1 shares some of its unusual abundance characteristics with the young clusters associated with the Sagittarius dwarf galaxy remnant and the intermediate-age LMC clusters, and could be chemically associated with the Canis Majoris overdensity; however it does not seem to be similar to the Monoceros/Galactic Anticenter Stellar Stream.

astro-ph.GA↗

arXiv2011

Enrichment of Heavy Elements in the red giant S15-19 in the Sextans Dwarf Spheroidal Galaxy

We determined chemical abundances of the Extremely Metal-Poor (EMP) star S15-19 ([Fe/H]=-3.0) in the Sextans dwarf galaxy. While heavy neutron-capture elements (e.g., Ba) are generally deficient in EMP stars in dwarf galaxies, this object was shown to have an exceptional over-abundance of Ba ([Ba/Fe] +0.5) by a previous study, which is similar to those of r-process-enhanced stars found in the field halo. Our new high-resolution spectroscopy for this object for the blue region, however, reveals that no clear excess of r-process elements, like Eu, appears in this object. Moreover, a significant excess of carbon ([C/Fe]= +1.0) and a deficiency of Sr ([Sr/Fe] = -1.4) are found for this object. Taking the variation of radial velocities measured at the two different epochs into consideration, the origin of the excesses of heavy neutron-capture elements in S15-19 is not the r-process, but is the s-process in an asymptotic giant branch (AGB) star that was the binary companion (primary) of this object. Carbon- and s-process-enhanced material should have been transferred to the surface of S15-19 across the binary system. These results are compared with carbon-enhanced metal-poor stars in the field halo.

astro-ph.SR↗

arXiv2010

A Wide Area Survey for High-redshift Massive Galaxies. II. Near-infrared Spectroscopy of BzK-selected Massive Star-forming Galaxies

Results are presented from NIR spectroscopy of a sample of BzK-selected, massive star-forming galaxies (sBzKs) at 1.5<z<2.3 that were obtained with OHS/CISCO at Subaru and with SINFONI at VLT. Among the 28 sBzKs observed, Ha emission was detected in 14 objects, and for 11 of them the [NII]6583 was also measured. Multiwavelength photometry was also used to derive stellar masses and extinction parameters, whereas Ha and [NII] have allowed us to estimate SFR, metallicities, ionization mechanisms, and dynamical masses. In order to enforce agreement between SFRs from Ha with those derived from rest-frame UV and MIR, additional obscuration for the emission lines (that originate in HII regions) was required compared to the extinction derived from the slope of the UV continuum. We have also derived the stellar mass-metallicity relation, as well as the relation between stellar mass and specific SFR, and compared them to the results in other studies. At a given stellar mass, the sBzKs appear to have been already enriched to metallicities close to those of local star-forming galaxies of similar mass. The sBzKs presented here tend to have higher metallicities compared to those of UV-selected galaxies, indicating that NIR selected galaxies tend to be a chemically more evolved population. The sBzKs show specific SFRs that are systematically higher, by up to ~2 orders of magnitude, compared to those of local galaxies of the same mass. The empirical correlations between stellar mass and metallicity, and stellar mass and specific SFR are then compared with those of population synthesis models constructed either with the simple closed-box assumption, or within an infall scenario. Within the assumptions that are built-in such models, it appears that a short timescale for the star-formation (~100 Myr) and large initial gas mass appear to be required if one wants to reproduce both relations simultaneously.

astro-ph.CO↗

arXiv2009

The Globular Cluster System of the Virgo Giant Elliptical Galaxy NGC 4636: II. Kinematics of the Globular Cluster System

We present a kinematic analysis of the globular cluster(GC) system in the giant elliptical galaxy (gE) NGC 4636 in the Virgo cluster. Using the photometric and spectroscopic database of 238 GCs, we have investigated the kinematics of the GC system. The NGC 4636 GC system shows weak overall rotation, which is dominated by the red GCs. However, both the blue GCs and red GCs show some rotation in the inner region at R<4.3'. The velocity dispersion for all the GCs is derived to be sigma_p = 225{+12-9} km/s. The velocity dispersion for the blue GCs (sig=251 km/s) is slightly larger than that for the red GCs (sig=205 km/s). The velocity dispersions for the blue GCs about the mean velocity and about the best fit rotation curve have a significant variation depending on the galactocentric radius. Comparison of observed stellar and GC velocity dispersion profiles with the velocity dispersion profiles calculated from the stellar mass profile shows that the mass-to-light ratio should increase as the galactocentric distance increases, indicating the existence of an extended dark matter halo. From the comparison of the observed GC velocity dispersion profiles and the velocity dispersion profiles calculated for the X-ray mass profiles in the literature, we find that the orbit of the GC system is tangential, and that the orbit of the red GCs is slightly more tangential than that of the blue GCs. We compare the GC kinematics of NGC 4636 with those of other six gEs, finding that the kinematic properties of the GCs are diverse among gEs. We find several correlations between the kinematics of the GCs and the global parameters of their host galaxies. We discuss the implication of the results for the formation models of the GC system in gEs, and suggest a mixture scenario for the origin of the GCs in gEs.

astro-ph.CO↗

arXiv2009

The Globular Cluster System of the Virgo Giant Elliptical Galaxy NGC 4636: I. Subaru/FOCAS Spectroscopy and Database

We present a spectroscopic study of the globular clusters (GCs) in the giant elliptical galaxy NGC 4636 in the Virgo cluster. We selected target GC candidates using the Washington photometry derived from the deep CCD images taken at the KPNO 4m. Then we obtained the spectra of 164 target objects in the field of NGC 4636 using the Multi-Object Spectroscopy (MOS) mode of Faint Object Camera and Spectrograph (FOCAS) on the SUBARU 8.2m Telescope. We have measured the velocities for 122 objects: 105 GCs in NGC 4636, the nucleus of NGC 4636, 11 foreground stars, 2 background galaxies, and 3 probable intracluster GCs in the Virgo cluster. The GCs in NGC 4636 are located in the projected galactocentric radius within 10arcmin (corresponding to 43 kpc). The measured velocities for the GCs range from 300km/s to 1600km/s, with a mean value of 932_{-22}^{+25} km/s, which is in good agreement with the velocity for the nucleus of NGC 4636, 928\pm 45 km/s. The velocity dispersion of the GCs in NGC 4636 is derived to be 231_{-17}^{+15} km/s and the velocity dispersion of the blue GCs is slightly larger than that of the red GCs. Combining our results with data in the literature, we produce a master catalog of radial velocities for 238 GCs in NGC 4636. The velocity dispersion of the GCs in the master catalog is found to be 225_{-9}^{+12} km/s for the entire sample, 251_{-12}^{+18} km/s for 108 blue GCs, and 205_{-13}^{+11} km/s for 130 red GCs.

astro-ph.CO↗

arXiv2009

Classification of Extremely Red Objects in the COSMOS Field

We present a study of the classification of z ~1 extremely red objects (EROs), using a combination of HST/ACS, Spitzer/IRAC, and ground-based images of the COSMOS field. Our sample includes about 5300 EROs with i-Ks>2.45 (AB, equivalently I-Ks=4 in Vega) and Ks<=21.1 (AB). For EROs in our sample, we compute, using the ACS F814W images, their concentration, asymmetry, as well as their Gini coefficient and the second moment of the brightest 20% of their light. Using those morphology parameters and the Spitzer/IRAC [3.6]-[8.0] color, the spectral energy distribution (SED) fitting method, we classify EROs into two classes: old galaxies (OGs) and young, dusty starburst galaxies (DGs). We found that the fraction of OGs and DGs in our sample is similar, about 48 percentages of EROs in our sample are OGs, and 52 percentages of them are DGs. To reduce the redundancy of these three different classification methods, we performed a principal component analysis on the measurements of EROs, and find that morphology parameters and SEDs are efficient in segregating OGs and DGs. The [3.6]-[8.0] color, which depends on reddening, redshift, and photometric accuracy, is difficult to separate EROs around the discriminating line between starburst and elliptical. We investigate the dependence of the fraction of EROs on their observational properties, and the results suggest that DGs become increasingly important at fainter magnitudes, redder colors, and higher redshifts.

astro-ph.CO↗

arXiv2008

A Suprime-Cam study of the stellar population of Ursa Major I dwarf spheroidal galaxy

We present deep and wide V, I CCD photometry of Ursa Major I (UMa I) dwarf spheroidal galaxy (dSph) in Local Group. The images of the galaxy were taken by Subaru/Suprime-Cam wide field camera, covering a field of 34\arcmin $\times$ 27\arcmin located at the centre of the galaxy. Colour-magnitude diagram (CMD) of the UMa I dSph shows a steep and narrow red giant branch (RGB), blue and red horizontal branch (HB), and main sequence (MS) stars. A well-defined main sequence turn-off (MSTO) is found to be located at V$_{0,MSTO}\sim$23.5 mag. The distance modulus is derived as $(m-M)_0=19.93\pm0.1$ (corresponding to a distance D$=96.8\pm4$ kpc) from the V-band magnitude of the horizontal branch (V$_{0,HB}=20.45\pm0.02$). The mean metallicity of the RGB stars is estimated by the V$-$I colour to [Fe/H]$\sim-2.0$. The turn-off age estimated by overlaying the theoretical isochrones reveals that most of stars in the UMa I dSph are formed at very early epoch ($\sim12$Gyrs ago). The isopleth map of stellar number density of the UMa I dSph, based upon the resolved star counts of MS, RGB, HB stars as well as blue stragglers (BS), shows that the morphology of the UMa I dSph is quite irregular and distorted, suggesting that the galaxy is in a process of disruption. The very old and metal-poor nature of the stellar population implies that the star formation history of this newly discoverd faint dSph may have been different from other well-known `classical' dSphs, which show significant stellar population of intermediate age. The stellar population of the UMa I dSph closely resembles that of Galactic old metal-poor globular cluster, but its size is typical of Galactic dSphs (r$_{e}$=188 [pc], r$_{1/2}$=300 [pc]), and the shape of its spatial density contours suggests that it is undergoing tidal disruption.

astro-ph↗

arXiv2008

An extended star cluster at the outer edge of the spiral galaxy M33

We report a discovery of an extended globular-like star cluster, M33-EC1, at the outer edge of the spiral galaxy M33. The distance to the cluster is 890 kpc, and it lies at a 12.5 kpc projected distance from the center of M33. Old age (>~7 Gyr) and low metallicity ([M/H] <~ -1.4) are estimated on the basis of isochrone fits. Color-magnitude diagrams of stars, located in the cluster's area, photometric and structural parameters of the cluster are presented. Cluster's luminosity (M_V = -6.6) and half-light radius (r_h = 20.3 pc) are comparable to those of the extended globular clusters, discovered in more luminous Local Group galaxies, the Milky Way and M31. Extended globular clusters are suspected to be remnants of accreted dwarf galaxies, and the finding of such a cluster in the late-type dwarf spiral galaxy M33 would imply a complex merging history in the past.

astro-ph↗

arXiv2007

Deconstructing dwarf galaxies: a Suprime-Cam survey of Andromeda II

(Abridged) We present deep, sub-horizontal branch, multi-colour photometry of the Andromeda II dwarf spheroidal (And II dSph) taken with Subaru Suprime-Cam. We identify a red clump population in this galaxy, the first time this feature has been detected in a M31 dSph, which are normally characterized as having no significant intermediate age populations. We construct radial profiles for the various stellar populations and show that the horizontal branch has a nearly constant density spatial distribution out to large radius, whereas the reddest red giant branch stars are centrally concentrated in an exponential profile. We argue that these populations trace two distinct structural components in And II and show that this assumption provides a good match to the overall radial profile of this galaxy. The extended component dominates the stellar populations at large radius, whereas the exponential component dominates the inner few arcminutes. We show that the two components have very different stellar populations; the exponential component has an average age of $\sim 7 - 10$ Gyrs old, is relatively metal-rich ([Fe/H] $\sim -1$) but with a significant tail to low metallicities, and possesses a red clump. The extended component, on the other hand, is ancient ($\sim 13$ Gyrs), metal-poor ([Fe/H] $\sim -1.5$) with a narrower dispersion $σ_{\rm [Fe/H]} \simeq 0.28$, and has a well developed blue horizontal branch. The extended component contains approximately three-quarters of the light of And II and its unusual density profile is unique in Local Group dwarf galaxies. This suggests that its formation and/or evolution may have been quite different to other dwarf galaxies.

astro-ph↗

arXiv2007

The first determination of the actinide Th abundance for a red giant of the Ursa Minor dwarf galaxy

The Thorium abundance for the red giant COS82 in the Ursa Minor dwarf spheroidal galaxy is determined based on a high resolution spectrum. This is the first detection of actinides in an extra Galactic object. A detailed abundance pattern is determined for 12 other neutron-capture elements from the atomic number 39 to 68. These elements are significantly over-abundant with respect to other metals like Fe (> 1 dex) and their abundance pattern agrees well with those of the r-process-enhanced, very metal-poor stars known in the Galactic halo, while the metallicity of this object ([Fe/H] ~ -1.5) is much higher than these field stars ([Fe/H] ~ -3.0). The results indicate that the mechanism and the astrophysical site that are responsible for neutron-capture elements in COS82 is similar to that for field r-process-enhanced stars, while the condition of low mass star formation is quite different. An estimate of the age of this object based on the Th abundance ratio is discussed.

astro-ph↗

arXiv2007

Using Weak Lensing Dilution to Improve Measurements of the Luminous and Dark Matter in A1689

The E/SO sequence of a cluster defines a boundary redward of which a reliable weak lensing signal can be obtained from background galaxies, uncontaminated by cluster members. For bluer colors, both background and cluster members are present, reducing the distortion signal by the proportion of unlensed cluster members. In deep Subaru and HST/ACS images of A1689 the tangential distortion of galaxies with bluer colors falls rapidly toward the cluster center relative to the lensing signal of the red background. We use this dilution effect to derive the cluster light profile and luminosity function to large radius, with the advantage that no subtraction of far-field background counts is required. The light profile declines smoothly to the limit of the data, r<2Mpc/h, with a constant slope, dlog(L)/dlog(r)=-1.12+-0.06, unlike the lensing mass profile which steepens continuously with radius, so that M/L peaks at an intermediate radius, ~100kpc/h. A flatter behavior is found for the more physically meaningful ratio of dark-matter to stellar-matter, when accounting for the color-mass relation of cluster members. The cluster luminosity function has a flat slope, alpha=-1.05+-0.07, independent of radius and with no faint upturn to M_i'<-12. We establish that the very bluest objects are negligibly contaminated by the cluster V-i'<0.2, because their distortion profile rises towards the center following the red background, but offset higher by ~20%. This larger amplitude is consistent with the greater estimated depth of the faint blue galaxies, z~=2.0 compared to z~=0.85 for the red background, a purely geometric effect related to cosmological parameters. Finally, we improve upon our earlier mass profile by combining both the red and blue background populations, clearly excluding low concentration CDM profiles.

astro-ph↗

arXiv2006

The stellar content of the isolated transition dwarf galaxy DDO210

We use Subaru Suprime-Cam and VLT FORS1 photometry of the dwarf galaxy DDO210 to study the global stellar content and structural properties of a transition-type galaxy (with properties intermediate between dwarf irregular and dwarf spheroidal systems). This galaxy is sufficiently isolated that tidal interactions are not likely to have affected its evolution in any way. The colour-magnitude diagrams of DDO210 show a red giant branch (RGB) population (with an RGB bump), a bright asymptotic giant branch population, a red clump, young main sequence stars and blue-loop stars. The youngest stars formed within the last 60 Myrs and have a distinct radial distribution compared to the main population. Whereas the overall stellar spatial distribution and HI spatial distribution are concentric, the young stars are offset from the center of DDO210 and are coincident with a `dent' in the HI distribution. The implied recent star formation rate required to form the young population is significantly higher than the derived current star formation rate, by a factor of > 10. Most of the stars in DDO210 are found in a red clump, and its mean I-band magnitude suggests that the majority of stars in DDO210 have an average age of 4^{+2}_{-1} Gyr. Given this age, the colour of the RGB implies a mean metallicity of [Fe/H] ~ -1.3. By comparing the shape of the red clump with models for a variety of star formation histories, we estimate that an old (> 10 Gyr) stellar population can contribute ~ 20 - 30% of the stars in DDO210 at most. The unusual star formation history of DDO210, its low mass estimate and its isolated nature, provide insight into how star formation proceeds in the lowest mass, unperturbed, dwarf galaxy haloes.

astro-ph↗

arXiv2006

A Subaru/Suprime-Cam wide-field survey of globular cluster populations around M87 - II: Colour and spatial distribution

We have performed a wide-field imaging survey of the globular cluster (GC) populations around M87 out to a radius of ~ 0.5 Mpc with Suprime-Cam on the 8.2m Subaru Telescope. By investigating the GC colour distribution as a function of distance from M87 and NGC 4552, another luminous Virgo elliptical in our survey field, it is found that clear bimodality ((V-I)_peak ~ 1.0 and 1.2) is seen only in the inner regions (< 10 kpc) of the host galaxies and that it becomes weaker with radius due to the decreasing contribution of the red GC (V-I > 1.1) subpopulation. It is also found (both around M87 and NGC 4552) that while the spatial distribution of the red GCs is as centrally concentrated as the host galaxy halo light distribution, the distribution of the blue GCs (V-I < 1.1) tends to be more extended. However, the blue GC distribution around M87 is not as extended as the Virgo cluster mass density profile. Based on these facts, we argue that the contribution of GCs associated with the Virgo cluster (e.g., intergalactic GCs) is not significant around M87 and is not the primary reason for the high S_N value of M87. Instead, most of the blue GCs around luminous ellipticals, as well as the red GCs, are presumed to be associated with the host galaxy. We model the radial profile of GC surface densities out to ~ 0.5 Mpc from M87 by a superposition of the GC populations associated with M87 and with NGC 4552, suggesting the existence of an additional intergalactic GC (i-GC) population independent of the luminous ellipticals. We obtain a crude estimate of i-GC specific frequency S_N = 2.9 (+4.2, -1.5), which may indicate the contribution of tidally captured GCs in the GC population of M87 is low to be consistent with the high S_N value of M87.

astro-ph↗

arXiv2006

A Subaru/Suprime-Cam wide-field survey of globular cluster populations around M87 - I: Observation, data analysis, and luminosity function

In this paper and a companion paper, we report on a wide-field imaging survey of the globular cluster (GC) populations around M87 carried out with Suprime-Cam on the 8.2m Subaru telescope. Here we describe the observations, data reduction, and data analysis and present luminosity functions of GC populations around M87 and NGC 4552, another luminous Virgo elliptical in our survey field. The imaging data were taken in the B, V, and I bands with a sky coverage out to 0.5 Mpc from the M87 centre. GC candidates were selected by applying a colour criterion on the B-V and V-I diagram to unresolved objects, which greatly reduces contamination. The data from control fields taken with Subaru/Suprime-Cam were also analyzed for subtraction of contamination in the GC sample. We investigate GC luminosity functions (GCLFs) within 10' (~ 45 kpc) from the host galaxy centre in detail. The V-band turnover magnitude (V_TO) is estimated to be 23.62 +-0.06 mag and 23.56 +- 0.20 mag for the GC population in M87 and NGC 4552, respectively. The GCLF is found to be a function of GC colour; V_TO of the red GC subpopulation (V-I > 1.1) is fainter than that of the blue GC subpopulation (V-I < 1.1) in both M87 and NGC 4552, as expected if the colour differences are primarily due to a metallicity effect and the mass functions of the two subpopulations are similar. The radial dependence of the GCLF is also investigated for the GC population in M87. The GCLF of each subpopulation at 1' < R < 5' is compared with that at 5' < R < 10' but no significant trend with distance is found in the shape of the GCLF. We also estimate GC specific frequencies (S_N) for M87 and NGC 4552. The S_N of the M87 GC population is estimated to be 12.5 +- 0.8 within 25'. The S_N value of the NGC 4552 GC population is estimated to be 5.0 +- 0.6 within 10'.

astro-ph↗

arXiv2006

Chemical Abundances in the Secondary Star of the Black Hole Binary V4641 Sagittarii (SAX J1819.3-2525)

We report on detailed spectroscopic studies performed for the secondary star in the black hole binary (micro-quasar) V4641 Sgr in order to examine its surface chemical composition and to see if its surface shows any signature of pollution by ejecta from a supernova explosion. High-resolution spectra of V4641 Sgr observed in the quiescent state in the blue-visual region are compared with those of the two bright well-studied B9 stars (14 Cyg and $ν$ Cap) observed with the same instrument. The effective temperature of V4641 Sgr (10500 $\pm$ 200 K) is estimated from the strengths of He~{\sc i} lines, while its rotational velocity, $\it v$ sin $\it i$ (95 $\pm$ 10 km s${}^{-1}$), is estimated from the profile of the Mg~{\sc ii} line at 4481 Å. We obtain abundances of 10 elements and find definite over-abundances of N (by 0.8 dex or more) and Na (by 0.8 dex) in V4641 Sgr. From line-by-line comparisons of eight other elements (C, O, Mg, Al, Si, Ti, Cr, and Fe) between V4641 Sgr and the two reference stars, we conclude that there is no apparent difference in the abundances of these elements between V4641 Sgr and the two normal late B-type stars, which have been reported to have solar abundances. An evolutionary model of a massive close binary system has been constructed to explain the abundances observed in V4641 Sgr. The model suggests that the progenitor of the black hole forming supernova was as massive as ~ 35 Msun on the main-sequence and, after becoming a ~ 10 Msun He star, underwent "dark" explosion which ejected only N and Na-rich outer layer of the He star without radioactive $^{56}$Ni.

astro-ph↗

arXiv2006

Deep Extragalactic Surveys around the Ecliptic Poles with AKARI (ASTRO-F)

AKARI (formerly ASTRO-F) is an infrared space telescope designed for an all-sky survey at 10-180 (mu)m, and deep pointed surveys of selected areas at 2-180 (mu)m. The deep pointed surveys with AKARI will significantly advance our understanding of galaxy evolution, the structure formation of the Universe, the nature of the buried AGNs, and the cosmic infrared background. Here we describe the important characteristics of the AKARI mission: the orbit, and the attitude control system, and investigate the optimum survey area based on the updated pre-flight sensitivities of AKARI, taking into account the cirrus confusion noise as well as the surface density of bright stars. The North Ecliptic Pole (NEP) is concluded to be the best area for 2-26 (mu)m deep surveys, while the low-cirrus noise regions around the South Ecliptic Pole (SEP) are worth considering for 50-180 (mu)m pointed surveys to high sensitivities limited by the galaxy confusion noise. Current observational plans of these pointed surveys are described in detail. Comparing these surveys with the deep surveys with the Spitzer Space Telescope, the AKARI deep surveys are particularly unique in respect of their continuous wavelength coverage over the 2-26 (mu)m range in broad-band deep imaging, and their slitless spectroscopy mode over the same wavelength range.

astro-ph↗

arXiv2006

Origin of Two Distinct Populations in Dwarf Spheroidal Galaxies

We study the chemical and kinematic properties of the first galaxies which formed at a high redshift, using high resolution cosmological numerical simulations, and compared them with the recent observational results for the Sculptor dwarf spheroidal galaxy by Tolstoy et al., who found two distinct stellar populations: the lower metallicity stars are more spatially extended and possess a higher velocity dispersion than the higher metallicity stars. Our calculations reproduce these observations as the result of a steep metallicity gradient, within a single populations, induced by dissipative collapse of the gas component. We also predict strong [N/O] enhancements in the lowest metallicity stars in dwarf spheroidals, due to the preferential retention of ejected gas from intermediate mass stars, compared to Type II supernovae.

astro-ph↗

arXiv2005

Spectroscopically Confirmed Large-Scale Structures Associated to a z=0.83 Cluster

(Abridged) We present a discovery of definitive large-scale structures around RXJ0152.7--1352 at z=0.83 based on spectroscopic redshifts. In our previous papers, we reported a photometric identification of the large-scale structures at z~0.8. A spectroscopic follow-up observation was carried out on 8 selected regions covering the most prominent structures to confirm their association to the main cluster. In six out of the eight fields, a well isolated peak is identified in the distribution of spectroscopic redshifts at or near the cluster redshift. This is strong evidence for the presence of large-scale structures associated to the main cluster at z=0.83. We then investigate stellar populations of galaxies in the structures. The composite spectra are constructed from a number of red member galaxies on the colour-magnitude sequence. We consider three representative environments -- cluster, group, and field. The ``cluster'' red galaxies do not show any sign of on-going or recent star formation activities and the passive evolution can naturally link them to the present-day red sequence galaxies in the Sloan Digital Sky Survey. In contrast, the red galaxies in ``groups'' and in the ``field'' tend to show signs of remaining and/or recent star formation activities characterized by weak [OII] emissions and/or strong H delta absorptions. Our current data seem to favour a scenario that star formation is truncated in a short time scale (<1Gyr). This would imply that galaxy-galaxy interactions are responsible for the truncation of star formation.

astro-ph↗