共查询到20条相似文献，搜索用时 31 毫秒
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Raul Jimenez Chris Flynn & Eira Kotoneva 《Monthly notices of the Royal Astronomical Society》1998,299(2):515-519
We use the Hipparcos colour–magnitude diagram of field stars with Tycho colours to make a new minimum age estimate for the Galactic disc. The method is based on fits to the red envelope of subgiants in the Hipparcos colour–magnitude diagram with synthetic isochrones covering the range of disc metal abundance. The colours and luminosities of the isochrones as a function of abundance are checked using new techniques involving 'red-clump' stars in the giant branch region and on the main sequence using G and K dwarfs. We derive a minimum disc age of 8 Gyr, in good agreement with other methods. 相似文献
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In recent years the evolution of dust-enshrouded galaxies has become an important issue in cosmology. We present the results
of a study linking the high- and low-redshift populations of such galaxies. Previously, a simple hierarchical clustering model
was found to be successful in describing the high-redshift populations of dusty galaxies. This study showed that strong evolution
could not be avoided in order to satisfy all the high-redshift infrared and submillimetre constraints. We now apply the same
model to the low-redshift data available, and find that the strong evolution inferred is compatible with the count and redshift
data we have from IRAS and ISO.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
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Nazirah N. Jetha Irini Sakelliou Martin J. Hardcastle Trevor J. Ponman Ian R. Stevens 《Monthly notices of the Royal Astronomical Society》2005,358(4):1394-1404
We present Chandra and Very Large Array observations of two galaxy clusters, Abell 160 and Abell 2462, whose brightest cluster galaxies (BCGs) host wide angle tailed radio galaxies (WATs). We search for evidence of interactions between the radio emission and the hot, X-ray emitting gas, and we test various jet termination models. We find that both clusters have cool BCGs at the cluster centre, and that the scale of these cores (∼30–40 kpc for both sources) is of approximately the same scale as the length of the radio jets. For both sources, the jet flaring point is coincident with a steepening in the host cluster's temperature gradient, and similar results are found for 3C 465 and Hydra A. However, none of the published models of WAT formation offers a satisfactory explanation as to why this may be the case. Therefore, it is unclear what causes the sudden transition between the jet and the plume. Without accurate modelling, we cannot ascertain whether the steepening of the temperature gradient is the main cause of the transition, or merely a tracer of an underlying process. 相似文献
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Agostino Renda Daisuke Kawata Yeshe Fenner Brad K. Gibson 《Monthly notices of the Royal Astronomical Society》2005,356(3):1071-1078
The chemical evolution history of a galaxy hides clues about how it formed and has been changing through time. We have studied the chemical evolution history of the Milky Way (MW) and Andromeda (M31) to find which are common features in the chemical evolution of disc galaxies as well as which are galaxy-dependent. We use a semi-analytic multizone chemical evolution model. Such models have succeeded in explaining the mean trends of the observed chemical properties in these two Local Group spiral galaxies with similar mass and morphology. Our results suggest that while the evolution of the MW and M31 shares general similarities, differences in the formation history are required to explain the observations in detail. In particular, we found that the observed higher metallicity in the M31 halo can be explained by either (i) a higher halo star formation efficiency (SFE), or (ii) a larger reservoir of infalling halo gas with a longer halo formation phase. These two different pictures would lead to (i) a higher [O/Fe] at low metallicities, or (ii) younger stellar populations in the M31 halo, respectively. Both pictures result in a more massive stellar halo in M31, which suggests a possible correlation between the halo metallicity and its stellar mass. 相似文献
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C. Tadhunter T. G. Robinson R. M. González Delgado K. Wills R. Morganti 《Monthly notices of the Royal Astronomical Society》2005,356(2):480-494
We present high-quality long-slit spectra for three nearby powerful radio galaxies – 3C 293, 3C 305 and PKS 1345+12. These were taken with the aim of characterizing the young stellar populations (YSP), and thereby investigating the evolution of the host galaxies, as well as the events that triggered the activity. Isochrone spectral synthesis modelling of the wide wavelength coverage spectra of nuclear and off-nuclear continuum-emitting regions have been used to estimate the ages, masses and luminosities of the YSP component, taking full account of reddening effects and potential contamination by activity-related components. We find that the YSP make a substantial contribution to the continuum flux in the off-nuclear regions on a radial scale of 1–20 kpc in all three objects. Moreover, in two objects we find evidence for reddened post-starburst stellar populations in the near-nuclear regions of the host galaxies. The YSP are relatively old (0.1–2 Gyr), massive (109 < M YSP < 2 × 1010 M⊙ ) and make up a large proportion (∼1–50 per cent) of the total stellar mass in the regions of the galaxies sampled by the observations. Overall, these results are consistent with the idea that the nuclear activity of active galactic nuclei in some radio galaxies is triggered by major gas-rich mergers. Therefore, these radio galaxies form part of the subset of early-type galaxies that is evolving most rapidly in the local Universe. Intriguingly, the results also suggest that the radio jets are triggered relatively late in the merger sequence, and that there is an evolutionary link between radio galaxies and luminous/ultraluminous infrared galaxies. 相似文献
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Slavek M. Rucinski 《Monthly notices of the Royal Astronomical Society》2009,395(4):2299-2303
The time sequence of 105 spectra covering one full orbital period of AA Dor has been analysed. Direct determination of V sin i for the sdOB component from 97 spectra outside of the eclipse for the lines Mg ii 4481 Å and Si iv 4089 Å clearly indicated a substantially smaller value than estimated before. Detailed modelling of line-profile variations for eight spectra during the eclipse for the Mg ii 4481 Å line, combined with the out-of-eclipse fits, gave V sin i = 31.8 ± 1.8 km s−1 . The previous determinations of V sin i , based on the He ii 4686 Å line, appear to be invalid because of the large natural broadening of the line. With the assumption of the solid-body, synchronous rotation of the sdOB primary, the measured values of the semi-amplitude K 1 and V sin i lead to the mass ratio q = 0.213 ± 0.013 which in turn gives K 2 and thus the masses and radii of both components. The sdOB component appears to be less massive than assumed before, M 1 = 0.25 ± 0.05 M⊙ , but the secondary has its mass–radius parameters close to theoretically predicted for a brown dwarf, M 2 = 0.054 ± 0.010 M⊙ and R 2 = 0.089 ± 0.005 R⊙ . Our results do not agree with the recent determination of Vŭcković et al. based on a K 2 estimate from line-profile asymmetries. 相似文献
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I. G. McCarthy A. Babul R. G. Bower M. L. Balogh 《Monthly notices of the Royal Astronomical Society》2008,386(3):1309-1331
X-ray clusters are conventionally divided into two classes: 'cool core' (CC) clusters and 'non-cool core' (NCC) clusters. Yet relatively little attention has been given to the origins of this apparent dichotomy and, in particular, to the energetics and thermal histories of the two classes. We develop a model for the entropy profiles of clusters starting from the configuration established by gravitational shock heating and radiative cooling. At large radii, gravitational heating accounts for the observed profiles and their scalings well. However, at small and intermediate radii, radiative cooling and gravitational heating cannot be combined to explain the observed profiles of either CC or NCC clusters. The inferred entropy profiles of NCC clusters require that material is 'pre-heated' prior to cluster collapse in order to explain the absence of low-entropy (cool) material in these systems. We show that a similar modification is also required in CC clusters in order to match their entropy profiles at intermediate radii. In CC clusters, this modification is unstable, and an additional process is required to prevent cooling below a temperature of a few keV. We show that this can be achieved by adding a self-consistent active galactic nuclei (AGN) feedback loop in which the lowest entropy, most rapidly cooling material is heated and rises buoyantly to mix with material at larger radii. The resulting model does not require fine-tuning and is in excellent agreement with a wide variety of observational data from Chandra and XMM–Newton , including entropy and gas density profiles, the luminosity–temperature relation and high-resolution spectra. The spread in cluster core morphologies is seen to arise because of the steep dependence of the central cooling time on the initial level of pre-heating. Some of the other implications of this model are briefly discussed. 相似文献