Results 1 - 10 of 21
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[en] Recent evolutionary computations of stars in the globular clusters show that the star ascending up the red giant branch decreases its luminosity temporarily when the outward advancing hydrogen-burning shell passes over the discontinuity in the chemical compositions, which is the trace of maximum inward penetration of the convective envelope in the previous subgiant stage. To understand the physical circumstance of this luminosity drop, the semi-analytical approach using the classical red giant model was applied. The difference of the present analysis from those with usual models of the red giant is that the gradient and the discontinuity in the hydrogen profile, which were usually neglected in the classical red giant model, in the region into which the hydrogen-burning shell advances were taken into account explicitly. It was found that the luminosity would always drop at the critical stage, the stage when the hydrogen-burning shell just passes over the discontinuity. It was also pointed out that the luminosity drop depends on the time step chosen for constructing the stellar evolutionary models and computations with crude time step would fail to catch up this phenomenon. (orig.)
[en] A new galactic wind model for elliptical galaxies is presented. A periodic cycle of star formation and wind occurs in giant elliptical galaxies after an initial wind. The model explains the paucity of gas in most ellipticals and predicts a significant contribution of intermediate-age stars into the present day V-flux luminosity. During each recurrent star formation phase ellipticals flare up in blue colours, which could be observed as unusual blue objects in distant clusters of galaxies. Evolutionary models are computed for irregular and blue compact dwarf galaxies. Neither continuous star formation model nor composite star-burst one can explain consistently the observed properties such as metallicity, gas mass-to-luminosity ratio, colours, and a lack of red irregulars. A time flattening initial mass function model with a few star-bursts is suggested as the most plausible star formation history. (author)
[en] We present a new approach for population synthesis in galaxy nuclei, which makes use, exclusively, of a library of integrated spectra of star clusters. This method has given interesting results for nearby galaxy nuclei in terms of dating successive stellar generations and detecting bursts of star formation, as well as of determining the chemical enrichment. These population syntheses results for galaxy nuclei are then compared to those derived from an evolutionary method. The two different approaches are interfaced by means of theoretical M/LV ratios for single generation stellar systems computed for differing ages. We conclude that results from the population synthesis are compatible with chemical evolution models. This confrontation allows the star formation history in galaxy nuclei to be derived. (author)
[en] Stothers and Chin(1972) examined numerically thermal instability of H burning shells in very massive stars. They reported the discovery of the thermal pulses in the shell on a stage of the contracting core just after the core exhaustion. Their results, however, are inconsequential because the pulses are very feeble and the mechanism of such feeble pulses is not well known at the shell burning phases. In this note, the authors summarize the analytical method and the results on the possibility of such feeble pulses in H burning shells. (Auth.)
[en] The number ratios of horizontal branch stars to red giant stars were obtained for globular clusters and Draco dwarf galaxy and the helium abundance was estimated using model results without semiconvection zone (SCZ) and with fully developed one. The analysis was confined to the four clusters (M4, M5, M13, and 47 Tuc) and the Draco galaxy, for which fairly precise star counts had been carried out. The effect of the difference in radial distribution between horizontal and red giant branch stars were taken into account, if necessary. The statistically significant difference in R exists among these objects. The cause may be the helium abundance and/or in the development of the SCZ. In the case of the fully developed SCZ, the helium abundance for M5 and Draco is appreciably smaller than the value given by the big-bang cosmology. It may be taken as an evidence against the full development of the SCZ for the horizontal branch stars in these objects. (orig.)
[en] We present a wide field census of resolved stellar populations in the northern half of M81, conducted with Suprime-Cam on the 8 m Subaru telescope and covering an area ∼0.3 deg2. The resulting color-magnitude diagram reaches over one magnitude below the red giant branch (RGB) tip, allowing a detailed comparison between the young and old stellar spatial distributions. The surface density of stars with ages ∼<100 Myr is correlated with that of neutral hydrogen in a manner similar to the disk-averaged Kennicutt-Schmidt relation. We trace this correlation down to gas densities of ∼2 x 1020 cm-2, lower than typically probed with Hα flux. Both diffuse light and resolved RGB star counts show compelling evidence for a faint, extended structural component beyond the bright optical disk, with a much flatter surface brightness profile. The star counts allow us to probe this component to significantly fainter levels than is possible with the diffuse light alone. From the colors of its RGB stars, we estimate that this component has a peak global metallicity [M/H] ∼ -1.1 ± 0.3 at deprojected radii 32-44 kpc assuming an age of 10 Gyr and distance of 3.6 Mpc. The spatial distribution of its RGB stars follows a power-law surface density profile, I(r) ∝ r -γ, with γ ∼ 2. If this component were separate from the bulge and from the bright optical disk, then it would contain ∼10%-15% of M81's total V-band luminosity. We discuss the possibility that this is M81's halo or thick disk, and in particular highlight its similarities and differences with these components in the Milky Way. Other possibilities for its nature, such as a perturbed disk or the faint extension of the bulge, cannot be completely ruled out, though our data disfavor the latter. These observations add to the growing body of evidence for faint, complex extended structures beyond the bright disks of spiral galaxies.
[en] We investigate the environmental effect on the metal enrichment of star-forming galaxies (SFGs) in the farthest spectroscopically confirmed and X-ray-detected cluster, CL J1449+0856 at z = 1.99. We combined Hubble Space Telescope/WFC3 G141 slitless spectroscopic data, our thirteen-band photometry, and a recent Subaru/Multi-object InfraRed Camera and Spectrograph (MOIRCS) near-infrared spectroscopic follow-up to constrain the physical properties of SFGs in CL J1449+0856 and in a mass-matched field sample. After a conservative removal of active galactic nuclei, stacking individual MOIRCS spectra of 6 (31) sources in the cluster (field) in the mass range 10 ≤ log(M/M_⊙) ≤ 11, we find a ∼4σ lower [N ii]/Hα ratio in the cluster than in the field. Stacking a subsample of 16 field galaxies with Hβ and [O iii] in the observed range, we measure an [O iii]/Hβ ratio fully compatible with the cluster value. Converting these ratios into metallicities, we find that the cluster SFGs are up to 0.25 dex poorer in metals than their field counterparts, depending on the adopted calibration. The low metallicity in cluster sources is confirmed using alternative indicators. Furthermore, we observe a significantly higher Hα luminosity and equivalent width in the average cluster spectrum than in the field. This is likely due to the enhanced specific star formation rate; even if lower dust reddening and/or an uncertain environmental dependence on the continuum-to-nebular emission differential reddening may play a role. Our findings might be explained by the accretion of pristine gas around galaxies at z = 2 and from cluster-scale reservoirs, possibly connected with a phase of rapid halo mass assembly at z > 2 and of a high galaxy merging rate
[en] We investigate the stellar population properties of a sample of 24 massive quenched galaxies at identified in the COSMOS field with our Subaru/Multi-object Infrared Camera and Spectrograph near-IR spectroscopic observations. Tracing the stellar population properties as close to their major formation epoch as possible, we try to put constraints on the star formation history, post-quenching evolution, and possible progenitor star-forming populations for such massive quenched galaxies. By using a set of Lick absorption line indices on a rest-frame optical composite spectrum, the average age, metallicity [Z/H], and α-to-iron element abundance ratio [α/Fe] are derived as , , and , respectively. If our sample of quenched galaxies at is evolved passively to z = 0, their stellar population properties will align in excellent agreement with local counterparts at similar stellar velocity dispersions, which qualifies them as progenitors of local massive early-type galaxies. Redshift evolution of stellar population ages in quenched galaxies combined with low redshift measurements from the literature suggests a formation redshift of , around which the bulk of stars in these galaxies have been formed. The measured [α/Fe] value indicates a star formation timescale of Gyr, which can be translated into a specific star formation rate of prior to quenching. Based on these findings, we discuss identifying possible progenitor star-forming galaxies at . We identify normal star-forming galaxies, i.e., those on the star-forming main sequence, followed by a rapid quenching event, as likely precursors of the quenched galaxies at presented here.
[en] We present deep Hubble Space Telescope Wide Field Camera 3 (HST/WFC3) slitless spectroscopic observations of the distant cluster Cl J1449+0856. These cover a single pointing with 18 orbits of G141 spectroscopy and F140W imaging, allowing us to derive secure redshifts down to M140 ∼ 25.5 AB and 3σ line fluxes of ∼5 × 10–18 erg s–1 cm–2. In particular, we were able to spectroscopically confirm 12 early-type galaxies (ETGs) in the field up to z ∼ 3, 6 of which are in the cluster core, which represents the first direct spectroscopic confirmation of quiescent galaxies in a z = 2 cluster environment. With 140 redshifts in a ∼6 arcmin2 field, we can trace the spatial and redshift galaxy distribution in the cluster core and background field. We find two strong peaks at z = 2.00 and z = 2.07, where only one was seen in our previously published ground-based data. Due to the spectroscopic confirmation of the cluster ETGs, we can now reevaluate the redshift of Cl J1449+0856 at z = 2.00, rather than z = 2.07, with the background overdensity being revealed to be sparse and sheet-like. This presents an interesting case of chance alignment of two close yet unrelated structures, each one preferentially selected by different observing strategies. With 6 quiescent or early-type spectroscopic members and 20 star-forming ones, Cl J1449+0856 is now reliably confirmed to be at z = 2.00. The identified members can now allow for a detailed study of galaxy properties in the densest environment at z = 2