共查询到20条相似文献，搜索用时 451 毫秒
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Duncan A. Forbes Carl J. Grillmair Williger Elson & Jean P. Brodie 《Monthly notices of the Royal Astronomical Society》1998,293(3):325-336
The Fornax cluster galaxies NGC 1399 and NGC 1404 are ideal for studying the effects of a cluster environment on globular cluster systems. Here we present new optical imaging of these two galaxies from both the Hubble Space Telescope 's Wide Field and Planetary Camera 2 and the Cerro Tololo Inter-American Observatory 1.5-m telescope. The combination of both data sets provides a unique insight on the spatial and colour distribution of globular clusters. From B − I colours, we find that both galaxies have a broad globular cluster metallicity distribution that is inconsistent with a single population. Two Gaussians provide a reasonable representation of the metallicity distribution in each galaxy. The metal-rich subpopulation is more centrally concentrated than the metal-poor one. We show that the radial metallicity gradient can be explained by the changing relative mix of the two globular cluster subpopulations. We derive globular cluster surface density profiles, and find that they are flatter (i.e., more extended) than the underlying starlight. The total number of globular clusters and specific frequency are calculated to be N =5700±500, SN =11.5±1.0 for NGC 1399, and N =725±145, SN =2.0±0.5 for NGC 1404. Our results are compared with the expectations of globular cluster formation scenarios. 相似文献
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O. González-Martín A. C. Fabian J. S. Sanders 《Monthly notices of the Royal Astronomical Society》2006,367(3):1132-1138
We report the results of a study of X-ray point sources coincident with the high-velocity system (HVS) projected in front of NGC 1275. A very deep X-ray image of the core of the Perseus cluster, made with the Chandra X-ray Observatory , has been used. We find a population of ultraluminous X-ray sources [ULXs; seven sources with L X (0.5 − 7.0 keV) > 7 × 1039 erg s-1 ]. As with the ULX populations in the Antennae and Cartwheel galaxies, those in the HVS are associated with a region of very active star formation. Several sources have possible optical counterparts found on the Hubble Space Telescope ( HST ) images, although the X-ray brightest one does not. Absorbed power-law models fit the X-ray spectra, with most having a photon index between 2 and 3. 相似文献
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Richard I. Davies Hajime Sugai & Martin J. Ward 《Monthly notices of the Royal Astronomical Society》1998,300(2):388-396
We present near-infrared emission-line images of the circumnuclear ring in NGC 1068. We have measured the Brγ fluxes in a number of star-forming complexes and derived the extinction for each of these by comparison with Hα. We investigate the star-forming histories of these regions and find that a short burst of star formation occurred coevally throughout the ring within the last 30–40 Myr, and perhaps as recently as 4–7 Myr ago. The 1–0 S(1) flux and S(1)/Brγ ratios indicate that as well as fluorescence, shock-excited H2 emission contributes to the total flux. There is excess H2 flux to the north-west where the ionization cone crosses the ring, and we show that it is possible that the non-stellar continuum from the Seyfert nucleus which produces the high-excitation lines could also be causing fluorescence at the edges of molecular clouds in the ring. The nuclear 1–0 S(1) is more extended than previously realized but only along the bar's major axis, and we consider mechanisms for its excitation. 相似文献
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P. Firth M. J. Drinkwater E. A. Evstigneeva M. D. Gregg A. M. Karick J. B. Jones S. Phillipps 《Monthly notices of the Royal Astronomical Society》2007,382(3):1342-1352
We have obtained spectroscopic redshifts of colour-selected point sources in four wide area VLT-FLAMES (Very Large Telescope-Fibre Large Array Multi Element Spectrograph) fields around the Fornax cluster giant elliptical galaxy NGC 1399, identifying as cluster members 27 previously unknown faint compact stellar systems (CSS), and improving redshift accuracy for 23 previously catalogued CSS.
By amalgamating our results with CSS from previous 2dF observations and excluding CSS dynamically associated with prominent (non-dwarf) galaxies surrounding NGC 1399, we have isolated 80 'unbound' systems that are either part of NGC 1399's globular cluster (GC) system or intracluster GCs. For these unbound systems, we find (i) they are mostly located off the main stellar locus in colour–colour space; (ii) their projected distribution about NGC 1399 is anisotropic, following the Fornax cluster galaxy distribution, and there is weak evidence for group rotation about NGC 1399; (iii) their completeness-adjusted radial surface density profile has a slope similar to that of NGC 1399's inner GC system; (iv) their mean heliocentric recessional velocity is between that of NGC 1399's inner GCs and that of the surrounding dwarf galaxies, but their velocity dispersion is significantly lower; (v) bright CSS ( M V < −11) are slightly redder than the fainter systems, suggesting they have higher metallicity; (vi) CSS show no significant trend in g '− i ' colour index with radial distance from NGC 1399. 相似文献
By amalgamating our results with CSS from previous 2dF observations and excluding CSS dynamically associated with prominent (non-dwarf) galaxies surrounding NGC 1399, we have isolated 80 'unbound' systems that are either part of NGC 1399's globular cluster (GC) system or intracluster GCs. For these unbound systems, we find (i) they are mostly located off the main stellar locus in colour–colour space; (ii) their projected distribution about NGC 1399 is anisotropic, following the Fornax cluster galaxy distribution, and there is weak evidence for group rotation about NGC 1399; (iii) their completeness-adjusted radial surface density profile has a slope similar to that of NGC 1399's inner GC system; (iv) their mean heliocentric recessional velocity is between that of NGC 1399's inner GCs and that of the surrounding dwarf galaxies, but their velocity dispersion is significantly lower; (v) bright CSS ( M
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E. Middelberg I. Agudo A. L. Roy T. P. Krichbaum 《Monthly notices of the Royal Astronomical Society》2007,377(2):731-740
We report the results from a 6-yr, multi-epoch very long baseline interferometry monitoring of the Seyfert galaxy NGC 3079. We have observed NGC 3079 during eight epochs between 1999 and 2005 predominantly at 5 GHz, but covering the frequency range of 1.7 to 22 GHz. Using our data and observations going back to 1985, we find that the separation of two of the three visible nuclear radio components underwent two decelerations. At the time of these decelerations, the flux density of one of the components increased by factors of 5 and 2, respectively. We interpret these events as a radio jet component undergoing compression, possibly as a result of a collision with interstellar medium material. This interpretation strongly supports the existence of jets surrounded by a clumpy medium of dense clouds within the first few parsec from the central engine in NGC 3079. Moreover, based on recently published simulations of jet interactions with clumpy media, this scenario is able to explain the nature of two additional regions of ageing synchrotron material detected at the lower frequencies as by-products of such interactions, and also the origin of the kpc-scale super-bubble observed in NGC 3079 as the result of the spread of the momentum of the jets impeded from propagating freely. The generalization of this scenario provides an explanation why jets in Seyfert galaxies are not able to propagate to scales of kpc as do jets in radio-loud AGN. 相似文献
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We present Very Large Array H i observations of the gas-rich, interacting spiral galaxies, NGC 1253/1253A (Arp 279). The larger of the two galaxies, NGC 1253, has a very pronounced H i ring and well-defined spiral structure. The velocity structure of the H i data shows a sudden change at the position of the spiral arms; we identify this change as evidence of a strong spiral shock and hence proceed to estimate the pattern speed, Ωp , of the spiral arms in NGC 1253. Assuming that the pattern speed is constant across the disc our derived value places the outer Lindblad resonance (OLR) at the position of the observed H i ring. As an accumulation of gas is expected at the OLR when this falls within the disc of a galaxy this agreement provides independent support for the derived value of Ωp . 相似文献
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We present kinematic observations of NGC 3923, one of the best-known examples of an elliptical galaxy with shells. Although NGC 3923 has no rotation on its major axis, it has minor axis rotation with an amplitude of some 20 km s−1 out to 25 arcsec radius. This is possibily the result of a kinematically decoupled core, but we present arguments against this hypothesis, and if the core were formed by a merger, this must have been a different merger from that which gave rise to the shells. The minor axis rotation suggests that NGC 3923 is prolate or triaxial, at least in the inner region. A prolate geometry for the whole galaxy would favour the merger model over the interaction model for the shell formation mechanism. There is other evidence, however, that the shape of the underlying galaxy changes with radius, in which case it is possible that the minor axis rotation could be confined to the galaxy core. The kinematics could reflect a triaxial mass distribution, in which the long and short axes are aligned with the photometric major and minor axes, and the axial ratios change such that the galaxy is near-prolate in the inner regions and near-oblate at larger radii. Measuring the rotation curve to about double this radius along the minor axis, as well as offset from the nucleus along the major axis at the radius of the innermost shells, would enable the true extent of the minor axis rotation to be determined. 相似文献
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R. A. Laing J. R. Canvin W. D. Cotton A. H. Bridle 《Monthly notices of the Royal Astronomical Society》2006,368(1):48-64
We present images of the jets in the nearby radio galaxy NGC 315 made with the Very Large Array at five frequencies between 1.365 and 5 GHz with resolutions between 1.5 and 45 arcsec. Within 15 arcsec of the nucleus, the spectral index of the jets is α= 0.61 . Further from the nucleus, the spectrum is flatter, with significant transverse structure. Between 15 and 70 arcsec from the nucleus, the spectral index varies from ≈0.55 on-axis to ≈0.44 at the edge. This spectral structure suggests a change of dominant particle acceleration mechanism with distance from the nucleus and the transverse gradient may be associated with shear in the jet velocity field. Further from the nucleus, the spectral index has a constant value of 0.47. We derive the distribution of Faraday rotation over the inner ±400 arcsec of the radio source and show that it has three components: a constant term, a linear gradient (both probably due to our Galaxy) and residual fluctuations at the level of 1–2 rad m−2 . These residual fluctuations are smaller in the brighter (approaching) jet, consistent with the idea that they are produced by magnetic fields in a halo of hot plasma that surrounds the radio source. We model this halo, deriving a core radius of ≈225 arcsec and constraining its central density and magnetic field strength. We also image the apparent magnetic field structure over the first ±200 arcsec from the nucleus. 相似文献
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