共查询到20条相似文献，搜索用时 976 毫秒
1.
In this paper we discuss observational and theoretical arguments in favour of hypothesis on nomad life of active nuclei inside and outside galaxies as well as its consequences. It may be the anisotropic collapse of a supermassive star, or the disruption of a supermassive binary system after the collapse of one companion that would give birth to such nuclei. We predict the existence of veritable quasi-stellar active objects without any ghost gagalies. 相似文献
2.
3.
The conformity effect,indicating the evolution of galaxies is related to its surrounding neighbor galaxies as far as a few Mpc,is an interesting phenomenon in the modeling of galaxy and evolution.Here we study the conformity effect of HI galaxies in a matched galaxy sample between SDSS DR7 and ALFALFA surveys.By checking the probability difference for the detected H i galaxies as a function of distance around a normal or an H i galaxy,we find that this effect is significant out to 5 Mpc.It also shows a dependence on the stellar mass of galaxies,with the strength the strongest in the stellar mass range of 1010*/M⊙<1010.5.However,when the sample is confined to central galaxies in groups with virial radii smaller than 1 Mpc,the 1-halo conformity signal is still evident,while the 2-halo conformity signal is reduced to a very weak amplitude.Our results confirm the previous study in the optical band that the 2-halo term is possibly caused by the bias effect in the sample selection.Furthermore,we confirm the existence of the 1-halo conformity discovered in the optical and radio band in previous investigations.Our results provide another viewpoint on the conformity effect and hope to shed light on the co-evolution of the galaxies and their neighbors in the current galaxy formation models. 相似文献
4.
《New Astronomy Reviews》2002,46(2-7):263-271
We review the question of the age of Compact Symmetric Objects (CSOs); defined as lobe-dominated sources smaller than 1 kpc in overall size. We show that the evidence increasingly points to these objects being very young (<104 yr old). Evidence from spectral aging, energy supply arguments and, most convincingly, from long term VLBI kinematic studies, is all consistent with the ‘youth’ scenario for CSOs. From VLBI kinematic studies hotspot advance speeds in CSOs are found to be 0.1 to 0.3c and external densities estimated from ram pressure balance are ≈1 cm−3. The separate question of the subsequent evolution of CSOs and whether they are the progenitors of classical double sources is, in contrast, not yet definitively answered. However it is found that the numbers of CSOs in flux limited samples is to first order what would be expected under such a scenario. The detailed differences in CSO population density between the data and model predictions might be resolved in various ways. Possibly not all CSOs evolve into large sources, or some sources show recurrent activity or, most likely, the simplest source evolution models need modification. 相似文献
5.
The generation of magnetic fields of galaxies is usually described by the dynamo mechanism.This process is characterized by the Steenbeck-Krause-Radler equation,which is the result of averaging the magnetohydrodynamics equations by distances which are associated with the size of turbulent cells in the interstellar medium.This equation is quite difficult to solve both from an analytical and numerical point of view.For galaxies,the no-z approximation is widely used.It describes the magnetic fields in thin discs.For such objects,where it is important to study the vertical structure of the field,it is not very applicable,so it is quite useful to adopt the RZ-model,which takes into account the dependence of the distance from the equatorial plane.During our research we have obtained the critical values of the dynamo number for galaxies with large half-thickness.We have also described typical z-structure for the magnetic field.Moreover,we have demonstrated that it is possible to generate dipolar magnetic fields. 相似文献
6.
Zhong-Lue Wen Jin-Lin Han Yun-Ying Jiang National Astronomical Observatories Chinese Academy of Sciences Beijing China 《中国天文和天体物理学报》2011,(10)
We identify new strong lensing clusters of galaxies from the Sloan Digital Sky Survey III (SDSS DR8) by visually inspecting color images of a large sample of clusters of galaxies. We find 68 new clusters showing giant arcs in addition to 30 known lensing systems. Among 68 cases, 13 clusters are almost certain lensing systems with tangential giant arcs, 22 clusters are probable and 31 clusters are possible lensing systems. We also find two exotic systems with blue rings. The giant arcs have angular separatio... 相似文献
7.
L. Bassani R. C. Butler A. J. Dean G. Di Cocco F. Perotti G. Villa 《Astrophysics and Space Science》1982,82(1):199-207
Three active galaxies, generally classified as Seyferts, have been discovered recently to be powerful, low energy -ray sources. The similarity between their spectral characteristics and those of the cosmic background at -ray energies suggests that these objects could make a significant contribution to this diffuse flux. This contribution has been assessed using two different number densities of -ray-emitting Seyfert galaxies based on optical and X-ray data. The comparison of the estimated and measured diffuse -ray background intensities is used to gain a deeper understanding of the metabolism of Seyfert galaxies. 相似文献
8.
We present the main results of the Ph.D. thesis carried out by López-Sánchez (Ph.D. thesis, 2006), in which a detailed morphological, photometric and spectroscopic analysis of a sample of 20 Wolf–Rayet (WR) galaxies was performed. The main aims are the study of the star formation and O and WR stellar populations in these galaxies, and the role that interactions between low surface brightness companion objects have in the triggering of the bursts. We analyze the morphology, stellar populations, physical conditions, chemical abundances and kinematics of the ionized gas, as well as the star-formation activity of each system. 相似文献
9.
Measurements of the Hα flux from 30 neighboring dwarf galaxies are presented. After correction for absorption, these fluxes
are used to estimate the star formation rate (SFR). The SFR for 18 of the galaxies according to the Hα emission are compared
with estimates of the SFR from FUV magnitudes obtained with the GALEX telescope. These are in good agreement over the range
log[SFR] = [ −3, 0] M⨀
/year. 相似文献
10.
We have performed photometric B, V, and R observations of nine disk galaxies that presumably have abnormally low total mass-to-light (M/L) ratios for given color indices. Our data on surface photometry are used to analyze the possible causes of anomalous M/L estimates. In many cases, these can be the result of errors in photometry or rotational velocity determination but can also
reflect the real peculiarities of the stellar composition of galaxies. Comparison of the photometric and dynamical disk mass
estimates obtained by analyzing the rotational velocities shows that low M/L values for a given color index are probably real for some of the galaxies. This is primarily true of NGC 4826 (Sab), NGC
5347 (Sab), and NGC 6814 (Sb). The small number of such galaxies suggests that the stellar initial mass function is universal.
However, a small fraction of galaxies probably may have a non-typical mass function “depleted” in low-mass stars. Such galaxies
require a more careful study. 相似文献
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12.
We discuss the distribution of radial velocities of galaxies belonging to the Local Group. Two independent samples of galaxies as well as several methods of reduction from the heliocentric to the galactocentric radial velocities are explored. We applied the power spectrum analysis using the Hann function as a weighting method, together with the jackknife error estimation. We performed a detailed analysis of this approach. The distribution of galaxy redshifts seems to be non‐random. An excess of galaxies with radial velocities of ∼24 km s–1 and ∼36 km s–1 is detected, but the effect is statistically weak. Only one peak for radial velocities of ∼24 km s–1 seems to be confirmed at the confidence level of 95%. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
13.
The extremely red galaxies (ERGs) are defined in terms of their very red optical-to-near IR colours (as R − K >5 or I − K >4). Originally this selection was aimed at selecting old (>1 Gyr) passively evolving elliptical galaxies at intermediate redshift (1< z <2), but it was soon discovered that young star-forming dusty galaxies can show similar colours and therefore be selected in the same surveys. It is crucial to distinguish between these two populations because they have very different consequences on the models of galaxy formation. Here we show that old ellipticals and dusty starbursts are expected to show different colours in the ( I − K ) versus ( J − K ) diagram for redshift range 1< z <2, thus providing a useful tool to classify ERGs in large samples up to K <20. This is mainly owing to the fact that old galaxies at these redshifts have a strong 4000-Å break at λ <1.2 μm ( J band), while dusty galaxies show smoother spectral energy distributions and therefore redder J − K colours. We discuss this difference in detail both in the framework of the stellar population synthesis models and by using observed spectra. The selection criterion is also compared with the properties of ERGs of known nature. We also show that this colour selection criterion is also useful to separate the ERGs from brown dwarf stars showing similar optical-to-IR colours. 相似文献
14.
Darren J. Croton Glennys R. Farrar 《Monthly notices of the Royal Astronomical Society》2008,386(4):2285-2289
Void regions of the Universe offer a special environment for studying cosmology and galaxy formation, which may expose weaknesses in our understanding of these phenomena. Although galaxies in voids are observed to be predominately gas rich, star forming and blue, a subpopulation of bright-red void galaxies can also be found, whose star formation was shutdown long ago. Are the same processes that quench star formation in denser regions of the Universe also at work in voids?
We compare the luminosity function of void galaxies in the 2dF Galaxy Redshift Survey, to those from a galaxy formation model built on the Millennium simulation. We show that a global star formation suppression mechanism in the form of low-luminosity 'radio-mode' active galactic nuclei (AGN) heating is sufficient to reproduce the observed population of void early types. Radio-mode heating is environment independent other than its dependence on dark matter halo mass, where, above a critical mass threshold of approximately Mvir ∼ 1012.5 M⊙ , gas cooling on to the galaxy is suppressed and star formation subsequently fades. In the Millennium simulation, the void halo mass function is shifted with respect to denser environments, but still maintains a high-mass tail above this critical threshold. In such void haloes, radio-mode heating remains efficient and red galaxies are found; collectively these galaxies match the observed space density without any modification to the model. Consequently, galaxies living in vastly different large-scale environments but hosted by haloes of similar mass are predicted to have similar properties, consistent with observations. 相似文献
We compare the luminosity function of void galaxies in the 2dF Galaxy Redshift Survey, to those from a galaxy formation model built on the Millennium simulation. We show that a global star formation suppression mechanism in the form of low-luminosity 'radio-mode' active galactic nuclei (AGN) heating is sufficient to reproduce the observed population of void early types. Radio-mode heating is environment independent other than its dependence on dark matter halo mass, where, above a critical mass threshold of approximately M
15.
V. A. Hagen-Thorn L. V. Shalyapina G. M. Karataeva V. A. Yakovleva 《Astronomy Letters》2003,29(3):133-141
We present the results of photometric and spectroscopic observations of UGC 4892—a candidate for polar-ring galaxies. We show that the feature in the brightness distribution described by Whitmore et al. (1990) as a possible polar ring is actually a companion galaxy that interacts with the giant main galaxy. The velocity fields of the gas and stellar components can be explained by assuming that the gas lost by the companion galaxy and captured by the main galaxy forms a gaseous stream rotating around the latter in a plane almost perpendicular to its principal plane. 相似文献
16.
Irini Sakelliou 《Monthly notices of the Royal Astronomical Society》2000,318(4):1164-1170
In clusters of galaxies, the reaction of the intracluster medium (ICM) to the motion of the co-existing galaxies in the cluster triggers the formation of unique features, which trace their position and motion. Galactic wakes, for example, are an apparent result of the ICM/galaxy interactions, and they constitute an important tool for deciphering the motion of the cluster galaxies.
In this paper we investigate whether Bondi–Hoyle accretion can create galactic wakes by focusing the ICM behind moving galaxies. The solution of the equations that describe this physical problem provides us with observable quantities along the wake at any time of its lifetime. We also investigate which are the best environmental conditions for the detectability of such structures in the X-ray images of clusters of galaxies.
We find that significant Bondi–Hoyle wakes can only be formed in low-temperature clusters, and that they are more pronounced behind slow-moving, relatively massive galaxies. The scalelength of these elongated structures is not very large: in the most favourable conditions a Bondi–Hoyle wake in a cluster at the redshift of z =0.05 is 12 arcsec long. However, the X-ray emission of the wake is noticeably strong: the X-ray flux can reach ∼30 times the flux of the surrounding medium. Such features will be easily detectable in the X-ray images of nearby, relatively poor clusters of galaxies by the Chandra and XMM-Newton satellites. 相似文献
In this paper we investigate whether Bondi–Hoyle accretion can create galactic wakes by focusing the ICM behind moving galaxies. The solution of the equations that describe this physical problem provides us with observable quantities along the wake at any time of its lifetime. We also investigate which are the best environmental conditions for the detectability of such structures in the X-ray images of clusters of galaxies.
We find that significant Bondi–Hoyle wakes can only be formed in low-temperature clusters, and that they are more pronounced behind slow-moving, relatively massive galaxies. The scalelength of these elongated structures is not very large: in the most favourable conditions a Bondi–Hoyle wake in a cluster at the redshift of z =0.05 is 12 arcsec long. However, the X-ray emission of the wake is noticeably strong: the X-ray flux can reach ∼30 times the flux of the surrounding medium. Such features will be easily detectable in the X-ray images of nearby, relatively poor clusters of galaxies by the Chandra and XMM-Newton satellites. 相似文献
17.
T. P. Prabhu 《Journal of Astrophysics and Astronomy》1980,1(2):129-154
A classification scheme is proposed for the central regions of Sérsic-Pastoriza galaxies based on high resolution photographs of 50 objects in the integrated light (4000 Å-8700 Å). Structures of two different linear scales are recognized: (1) nucleus(?1 kpc) and (2) perinuclear formation (～l.5 kpc). The perinuclear formation is weak in classκ while the nucleus is too faint to detect in class ι. In the intermediate classesε andσ both the components are bright. Classε has an elliptical perinuclear formation with little gas while the classσ consists of bright H II complexes and dust. Observations of a few galaxies in the infrared and the blue ends of the image tube response show that the nucleus is generally redder than its surroundings. Equal intensity contours and the luminosity profiles are presented for the central regions of 27 galaxies. A comparison of their axial ratios with those of the parent galaxies indicates that the perinuclear formations are prolate or barlike. The dependence of the peak surface brightness of the central formation on the size of the bar is investigated as also the dependence of the central surface brightness of the bar on the size of the bar. The following major conclusions are drawn:
- The peak central surface brightness of the perinuclear formation varies as the square of the bar length. This relation implies that the bar induces the infall of gas from the bar-disk region.
- The formations of classσ move towards classε as star formation ceases and the massive stars die.
- The classι differs from classσ in the intensity of the burst of star formation. Low luminosity of the parent galaxies in classι implies less infall of gas and higher domination of the bar potential on the perinuclear formation. Thus the classι structures are more prolate than those of classσ.
- The central brightness of the bars varies directly as the length of the bar.
18.
J. M. Mas-Hesse D. Kunth H. Atek G. Östlin C. Leitherer A. Petrosian D. Schaerer 《Astrophysics and Space Science》2009,320(1-3):35-38
The Lyα line is an important diagnostic of star formation and activity in galaxies. The analysis of Lyα is complicated due to the resonant nature of the line and radiative transfer effects. High spectral resolution studies of local starburst galaxies with the unprecedented UV capabilities of the HST have shown that this line is either seen in absorption or in emission and in the latter case with a P?Cygni profile indicative of a large scale outflow of neutral gas. Moreover, HST imaging obtained with HST-ACS of a sample of 6 star-forming galaxies has revealed that a substantial fraction of the Lyα photons are diffused far away from the emissive knots. Since the importance of Lyα for tracing large scale structure, correlation functions, and galaxy formation is recognized, Lyα will remain a very important probe of the distant universe for the foreseeable future, and it is therefore imperative to acquire a better understanding of what mechanisms regulate our ability to detect this line. 相似文献
19.
A.F.M. Moorwood P.P. Van der Werf J. G. Cuby E. Oliva 《Astrophysics and Space Science》1999,266(1-2):255-260
The deepest survey for H emitting galaxies at z 2.2has recently been made in narrow-band ( 1%) filters around2.1m using SOFI at the ESO NTT telescope. An effective area of 100 sq. arcmin and a comoving volume of 9000 Mpc3 (forH0 = 50, q0 = 0.5) has been covered to a volume weighted 3 line flux limit of 5 × 10-17 erg s-1cm-2. Our survey covered the WFPC2 and STIS fields in the HubbleDeep Field South and an anonymous field about 30 deg. away. Thefaintest limit reached was 3 × 10-17 erg s-1 cm-2 onthe WFPC2 field. In total, 10 convincing candidates with deduced starformation rates in the range 9 - 50 M/yr and an equal number ofmarginal ones have been identified for confirmation and follow-upspectroscopy with ISAAC at the VLT. Based on a very preliminaryanalysis we compare our results with those of earlier surveys andbriefly discuss some possible implications for the form of theevolution out to z 2 and the effects of clustering. 相似文献
20.
G. Soucail 《Astrophysics and Space Science》1990,171(1-2):283-287
Photon-counting techniques have been applied to a study of the movement of the image of an unresolved star at the 50 cm telescope of the Royal Academy of Sciences, at the Roque de los Muchachos Observatory. The temporal autocorrelation function of such movement has been computed, the results are discussed in terms of atmospheric turbulence and in relation to high-resolution imaging.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain. 相似文献