共查询到20条相似文献，搜索用时 31 毫秒
1.
Joseph Silk 《Monthly notices of the Royal Astronomical Society》2001,324(2):313-318
Galaxy disc formation must incorporate the multiphase nature of the interstellar medium. The resulting two-phase structure is generated and maintained by gravitational instability and supernova energy input, which yield a source of turbulent viscosity that is able to compete effectively in the protodisc phase with early angular momentum loss of the baryonic component via dynamical friction in the dark halo. Provided that star formation occurs on the viscous drag time-scale, this mechanism provides a means of accounting for disc sizes and radial profiles. The star formation feedback is self-regulated by turbulent gas pressure limited percolation of the supernova remnant heated hot phase, but can run away in gas-rich protodiscs to generate compact starbursts. A simple analytic model is derived for a Schmidt-like global star formation law in terms of the cold gas volume density. 相似文献
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Paul A. Jones Ben D. Lloyd W. Bruce McAdam 《Monthly notices of the Royal Astronomical Society》2001,325(2):817-825
Centaurus B (PKS B1343−601) is one of the brightest and closest radio galaxies, with flux density ∼250 Jy at 408 MHz and redshift 0.01215, but it has not been studied much because of its position (i) close to the Galactic plane (it is also known as G309.6+1.7 and Kes 19) and (ii) in the southern sky. It has recently been suggested as the centre of a highly obscured cluster behind the Galactic plane. We present radio observations made with the Australia Telescope Compact Array and Molonglo Observatory Synthesis Telescope to study the jets and lobes. The total intensity and polarization radio images of the FR I jets are used to determine the jet brightness and width variations, magnetic field structure and fractional polarization. The equipartition pressure calculated along the jets declines rapidly over the first 1 arcmin from the galaxy reaching a constant pressure of 10−13 h −4/7 Pa in the lobes blown in the intracluster medium. 相似文献
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J. Perez P. Tissera N. Padilla M. S. Alonso D. G. Lambas 《Monthly notices of the Royal Astronomical Society》2009,399(3):1157-1166
We explore properties of close galaxy pairs and merging systems selected from the Sloan Digital Sky Survey Data Release 4 in different environments with the aim to assess the relative importance of the role of interactions over global environmental processes. For this purpose, we perform a comparative study of galaxies with and without close companions as a function of local density and host halo mass, carefully removing sources of possible biases. We find that at low- and high-local-density environments, colours and concentration indices of close galaxy pairs are very similar to those of isolated galaxies. At intermediate densities, we detect significant differences, indicating that close pairs could have experienced a more rapid transition on to the red sequence than isolated galaxies. The presence of a correlation between concentration index and colours indicates that the physical mechanism responsible for the colour transformation also operates in the transformation of the luminous matter distribution. At fixed local densities, we find a dependence of the red galaxy fraction on dark matter halo mass for galaxies with or without a close companion. This suggests the action of host halo mass related effects. Regardless of dark matter halo mass, we show that the percentage of red galaxies in close pairs and in the control sample are comparable at low- and high-local-density environments. However, at intermediate local densities, the gap in the red fraction between close pairs and the control galaxies increases from ∼10 per cent in low-mass haloes up to ∼50 per cent in the most massive ones. Interestingly, we also detect that 50 per cent of merging systems populate the intermediate local environments, with a large fraction of them being extremely red and bulge dominated. Our findings suggest that in intermediate-density environments galaxies are efficiently pre-processed by close encounters and mergers before entering higher local density regions. 相似文献
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Eric F. Bell † W. J. G. de Blok ‡ 《Monthly notices of the Royal Astronomical Society》2000,311(4):668-672
We have assessed the significance of Tully and Verheijen's bimodal Ursa Major Cluster spiral galaxy near-infrared surface brightness distribution, focusing on whether this bimodality is simply an artefact of small number statistics. A Kolmogorov–Smirnov style of significance test shows that the total distribution is fairly represented by a single-peaked distribution, but that their isolated galaxy subsample (with no significant neighbours within a projected distance of ∼80 kpc) is bimodal at the 96 per cent level. We have also investigated the assumptions underlying the isolated galaxy surface brightness distribution, finding that the (often large) inclination corrections used in the construction of this distribution reduce the significance of the bimodality. We conclude that the Ursa Major Cluster data set is insufficient to establish the presence of a bimodal near-infrared surface brightness distribution: an independent sample of ∼100 isolated, low-inclination galaxies is required to establish bimodality at the 99 per cent level. 相似文献
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We calculate the rate at which dark matter haloes merge to form higher mass systems. Two complementary derivations using Press–Schechter theory are given, both of which result in the same equation for the formation rate. First, a derivation using the properties of the Brownian random walks within the framework of Press–Schechter theory is presented. We then use Bayes' theorem to obtain the same result from the standard Press–Schechter mass function. The rate obtained is shown to be in good agreement with results from Monte Carlo and N -body simulations. We illustrate the usefulness of this formula by calculating the expected cosmological evolution in the rate of star formation that is due to short-lived, merger-induced starbursts. The calculated evolution is well-matched to the observed evolution in ultraviolet luminosity density, in contrast to the lower rates of evolution that are derived from semi-analytic models that do not include a dominant contribution from starbursts. Hence we suggest that the bulk of the observed ultraviolet starlight at z >1 arises from merger-induced starbursts. Finally, we show that a simple merging-halo model can also account for the bulk of the observed evolution in the comoving quasar space density. 相似文献
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M. Elmouttie M. Krause R. F. Haynes & K. L. Jones 《Monthly notices of the Royal Astronomical Society》1998,300(4):1119-1126
We have used the Swedish ESO Submillimeter Telescope to observe the molecular gas in the Circinus galaxy using the CO(1 → 0) transition as a tracer. The central region and major axis have been mapped and several other points were also observed. The gas in the galaxy is concentrated towards the nucleus, the peak being coincident with the radio/optical core. The inclination of the molecular galactic disc is more comparable to that of the radio continuum than to that of the large-scale H i emission. Evidence for an anomalous spur structure pointing radially away from the galactic centre is presented, and may indicate a causal link between it and similar features seen in optical lines and radio continuum. Our data suggest the presence of a central molecular ring or disc with radius 300 ± 50 pc and a rotation velocity of about 200 km s−1 (assuming i = 73°). The dynamical mass of the nucleus is estimated to be no greater than 3.9 × 109 M⊙ . Assuming that the distribution of gas varies smoothly in the outer regions, we calculate the mass of molecular gas in the galaxy to be at least M mol = 1.1 × 109 M⊙ , and the star-forming efficiency to be 11 ± 2 L⊙ M⊙ −1 . These results imply that Circinus is undergoing a massive central starburst which may be, at least partially, responsible for its extended minor axis emission seen in several wavebands. 相似文献
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Michael Boylan-Kolchin Chung-Pei Ma Eliot Quataert 《Monthly notices of the Royal Astronomical Society》2008,383(1):93-101
The time-scale for galaxies within merging dark matter haloes to merge with each other is an important ingredient in galaxy formation models. Accurate estimates of merging time-scales are required for predictions of astrophysical quantities such as black hole binary merger rates, the build-up of stellar mass in central galaxies and the statistical properties of satellite galaxies within dark matter haloes. In this paper, we study the merging time-scales of extended dark matter haloes using N -body simulations. We compare these results to standard estimates based on the Chandrasekhar theory of dynamical friction. We find that these standard predictions for merging time-scales, which are often used in semi-analytic galaxy formation models, are systematically shorter than those found in simulations. The discrepancy is approximately a factor of 1.7 for M sat / M host ≈ 0.1 and becomes larger for more disparate satellite-to-host mass ratios, reaching a factor of ∼3.3 for M sat / M host ≈ 0.01. Based on our simulations, we propose a new, easily implementable fitting formula that accurately predicts the time-scale for an extended satellite to sink from the virial radius of a host halo down to the halo's centre for a wide range of M sat / M host and orbits. Including a central bulge in each galaxy changes the merging time-scale by ≲10 per cent. To highlight one concrete application of our results, we show that merging time-scales often used in the literature overestimate the growth of stellar mass by satellite accretion by ≈40 per cent, with the extra mass gained in low mass ratio mergers. 相似文献
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M. Ruiz D. M. Alexander S. Young J. Hough S. L. Lumsden C. A. Heisler † 《Monthly notices of the Royal Astronomical Society》2000,316(1):49-57
We present near-infrared broad-band polarization images of the nuclear regions of the Circinus galaxy in the J , H and K bands. For the first time the south-eastern reflection cone is detected in polarized light, which is obscured at optical wavelengths behind the galactic disc. This biconical structure is clearly observed in J - and H -band polarized flux, whilst in the K band a more compact structure is detected. Total flux J − K and H − K colour maps reveal a complex colour gradient toward the south-east direction (where the Circinus galactic disc is nearer to us). We find enhanced extinction in an arc-shaped structure, at about 200 pc from the nucleus, probably part of the star formation ring.
We model the polarized flux images with the scattering and torus model of Young et al., with the same basic input parameters as used by Alexander et al. in the spectropolarimetry modelling of Circinus. The best fit to the polarized flux is achieved with a torus radius of ∼16 pc, and a visual extinction A V , through the torus, to the near-infrared emission regions of >66 mag. 相似文献
We model the polarized flux images with the scattering and torus model of Young et al., with the same basic input parameters as used by Alexander et al. in the spectropolarimetry modelling of Circinus. The best fit to the polarized flux is achieved with a torus radius of ∼16 pc, and a visual extinction A
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Rowena K. Malbon C. M. Baugh C. S. Frenk C. G. Lacey 《Monthly notices of the Royal Astronomical Society》2007,382(4):1394-1414
We incorporate a model for black hole growth during galaxy mergers into the semi-analytical galaxy formation model based on ΛCDM proposed by Baugh et al. Our black hole model has one free parameter, which we set by matching the observed zero-point of the local correlation between black hole mass and bulge luminosity. We present predictions for the evolution with redshift of the relationships between black hole mass and bulge properties. Our simulations reproduce the evolution of the optical luminosity function of quasars. We study the demographics of the black hole population and address the issue of how black holes acquire their mass. We find that the direct accretion of cold gas during starbursts is an important growth mechanism for lower mass black holes and at high redshift. On the other hand, the re-assembly of pre-existing black hole mass into larger units via merging dominates the growth of more massive black holes at low redshift. This prediction could be tested by future gravitational wave experiments. As redshift decreases, progressively less massive black holes have the highest fractional growth rates, in line with recent claims of 'downsizing' in quasar activity. 相似文献
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T. F. Laganá R. A. Dupke L. Sodré Jr G. B. Lima Neto F. Durret 《Monthly notices of the Royal Astronomical Society》2009,394(1):357-366
X-ray observations of galaxy clusters have shown that the intra-cluster gas has iron abundances of about one-third of the solar value. These observations also show that part (if not all) of the intra-cluster gas metals was produced within the member galaxies. We present a systematic analysis of 20 galaxy clusters to explore the connection between the iron mass and the total luminosity of early- and late-type galaxies, and of the brightest cluster galaxies (BCGs). From our results, the intra-cluster medium (ICM) iron mass seems to correlate better with the luminosity of the BCGs than with that of the red and blue galaxy populations. As the BCGs cannot produce alone the observed amount of iron, we suggest that ram-pressure plus tidal stripping acts together to enhance, at the same time, the BCG luminosities and the iron mass in the ICM. Through the analysis of the iron yield, we have also estimated that SN Ia are responsible for more than 50 per cent of the total iron in the ICM. This result corroborates the fact that ram-pressure contributes to the gas removal from galaxies to the ICM, being very efficient for clusters in the temperature range 2 < kT (keV) < 10 . 相似文献
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M. Sol Alonso Diego G. Lambas Patricia Tissera Georgina Coldwell 《Monthly notices of the Royal Astronomical Society》2006,367(3):1029-1038
We analyse star formation rates (SFRs) derived from photometric and spectroscopic data of galaxies in pairs in different environments using the 2-degree field galaxy redshift survey (2dFGRS) and the Sloan digital sky survey (SDSS). The two samples comprise several thousand pairs, suitable to explore into detail the dependence of star formation activity in pairs on orbital parameters and global environment. We use the projected galaxy density derived from the fifth brightest neighbour of each galaxy, with a convenient luminosity threshold to characterize environment in both surveys in a consistent way. Star formation activity is derived through the η parameter in 2dFGRS and through the SFR normalized to the total mass in stars, SFR/ M * , given by Brinchmann et al. in the SDSS-second data release (SDSS-DR2). For both galaxy pair catalogs, the star formation birth rate parameter is a strong function of the global environment and orbital parameters. Our analysis on SDSS pairs confirms previous results found with the 2dFGRS where suitable thresholds for the star formation activity induced by interactions are estimated at a projected distance r p = 100 h −1 kpc and a relative velocity Δ V = 350 km s−1 . We observe that galaxy interactions are more effective at triggering important star formation activity in low- and moderate-density environments with respect to the control sample of galaxies without a close companion. Although close pairs have a larger fraction of actively star-forming galaxies, they also exhibit a greater fraction of red galaxies with respect to those systems without a close companion, an effect that may indicate that dust stirred up during encounters could affect colours and, partially, obscure tidally induced star formation. 相似文献
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