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1.
We investigate the conditions of star formation in the Large Magellanic Cloud (LMC). We have conducted a survey for water maser emission arising from massive young stellar objects in the 30 Doradus region (N 157) and several other H  ii regions in the LMC (N 105A, N 113 and N 160A). We have identified a new maser source in 30 Dor at the systemic velocity of the LMC. We have obtained 3–4 μm spectra, with the European Southern Observatory (ESO)-Very Large Telescope (VLT), of two candidate young stellar objects. N 105A IRS1 shows H recombination line emission, and its Spectral Energy Distribution (SED) and mid-infrared colours are consistent with a massive young star ionizing the molecular cloud. N 157B IRS1 is identified as an embedded young object, based on its SED and a tentative detection of water ice. The data on these four H  ii regions are combined with mid-infrared archival images from the Spitzer Space Telescope to study the location and nature of the embedded massive young stellar objects and signatures of stellar feedback. Our analysis of 30 Dor, N 113 and N 160A confirms the picture that the feedback from the massive O- and B-type stars, which creates the H  ii regions, also triggers further star formation on the interfaces of the ionized gas and the surrounding molecular cloud. Although in the dense cloud N 105A star formation seems to occur without evidence of massive star feedback, the general conditions in the LMC seem favourable for sequential star formation as a result of feedback. In an Appendix , we present water maser observations of the galactic red giants R Doradus and W Hydrae.  相似文献   

2.
为改善文献上惯用的表现银河系分子谱线巡视结果的完全平滑了方位信息的径向分布方法,我们发展了原子气体或分子云参量的分环银经分布图,(X—l)_i图,它在某种程度上给出了方位信息。用现存旋臂模型结合这种图我们得到的银道面旋臂区和臂间区的E(HI),E(CO),E(~(13)CO)和N/S(~(13)CO)的两维对比度约为1—2。  相似文献   

3.
A new theory for galactic arm formation shows the arms to be continually eminating from the galactic nucleus due to a continual influx of cosmic dust. In the neighborhood of the nucleus the problem is treated as a fluid flow and a simple solution is given using conservation of momentum. When rotational dynamics are included the spinning arm system is the result. This solution resolves the problem of the missing mass, accounts for warped disk galaxies and gives a probable source for the gravity waves measured by Weber which eminate from our galactic center. Reversal of arm direction is demonstrated and examples of such reversals are cited. An approximate theoretical estimate of the age of our Sun is found to be in good agreement with radio isotope dating. A general result shows why twin star systems are in such great abundance in a galaxy. It gives a model of galactic evolution which begins with only a single massive nucleus with the collapsing gas clouds forming the arms.  相似文献   

4.
恒星形成于分子云环境中。近30多年的观测研究使得天文学家对小质量恒星的形成有了相对明确的认识：小质量恒星通过坍缩、吸积和外向流的路标而形成。至于大质量恒星，其形成过程还存在着许多不确定因素，现有的观测证据表明：大质量恒星也可能通过坍缩、吸积和外向流的路标来形成，但也不排除在星团中通过中小质量恒星聚合而成的因素。大质量恒星形成与致密电离氢区(UCHII)成协较好，而与大质量恒星形成区成协的分子云环境中，既有大质量恒星也有小质量恒星形成。综述了恒星形成各个阶段的观测结果和研究现状以及成协的天体物理环境情况。未来的观测和研究重点在于，大质量恒星形成以及星团环境中的恒星形成。  相似文献   

5.
We explore the role of star clusters in the nuclear regions of galaxies through their connection with active galactic nuclei (AGN). Nuclear star clusters (NCs) are conspicuous in the centers of most nearby galaxies, all along the Hubble sequence. These clusters are probably the faint-end distribution of the central supermassive black holes (SMBHs) in massive bulges. On the other hand, star formation is known to be ongoing in the majority of Seyfert nuclei and in many low-luminosity active galactic nuclei (LLAGN). We study two samples of AGN galaxies (75 Seyferts and 26 LLAGN) in the near-ultraviolet with the Hubble Space Telescope’s Advanced Camera for Surveys. We aim to better understand the connection between the growing of the SMBH and the build-up of the bulge, and we also intend to make statistical progress and determine the properties of the population of NCs coexisting with growing SMBHs.  相似文献   

6.
Stars form through the gravitational collapse of molecular cloud cores.Before collapsing,the cores are supported by thermal pressure and turbulent motions.A question of critical importance for the understanding of star formation is how to observationally discern whether a core has already initiated gravitational collapse or is still in hydrostatic balance.The canonical method to identify gravitational collapse is based on the observed radial density profile,which would change from Bonnor-Ebert type toward power laws as the core collapses.In practice,due to the projection effect,the resolution limit and other caveats,it has been difficult to directly reveal the dynamical status of cores,particularly in massive star forming regions.We here propose a novel,straightforward diagnostic,namely,the collapsing index(CI),which can be modeled and calculated based on the radial profile of the line width of dense gas.A meaningful measurement of CI requires spatially and spectrally resolved images of optically thin and chemically stable dense gas tracers.ALMA observations are making such data sets increasingly available for massive star forming regions.Applying our method to one of the deepest dense-gas spectral images ever taken toward such a region,namely,the Orion molecular cloud,we detect the dynamical status of selected cores.We observationally distinguished a collapsing core in a massive star forming region from a hydrostatical one.Our approach would help significantly improve our understanding of the interaction between gravity and turbulence within molecular cloud cores in the process of star formation.  相似文献   

7.
Halons are the hypothetical massive elementary components of the galactic halo. They accrete steadily on to stars; a significant internal stellar energy sink results from their presence in a low-mass star. It is shown that halons could solve the solar neutrino problem and the problem of the galactic age.  相似文献   

8.
We present an outline of our study of the effects of star formation on the different components of the interstellar medium in the discs of spiral galaxies, both globally and as a function of arm and interarm environment. We are in the process of obtaining images of 57 spiral galaxies at low inclinations, and analysing them to study the distribution of recent massive star formation, old stars, young stars, gas and dust. We will dissect the images into arm and interarm regions and compare and contrast the morphology and scale lengths within these regions inHα, HI, the near infrared, optical and (where available) CO. Modelling will show how the scale lengths are affected by star formation, how this differs between arms and interarms, and whether the Schmidt Law varies from the global values in the arm and interarm regions. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

9.
On the basis of the PLC relation (1) or the PL relation by Van den Bergh (2) and the PC relation by Deanet al. (1978), the distances of 284 galactic cepheids with photoelectric observations have been derived. The space distribution of these cepheids with 111 additional ones without photoelectric observations, is studied. In spite of the strong influence of the absorption matter, which makes a great number of distant cepheids unknown (Figure 4), a conclusion is drawn that the cepheids do not trace spiral arms with only one possible exception: the Carina arm. The cepheidz-coordinate distribution confirms the finding of Fernie (1968) that the cepheid layer is inclined towards the formal galactic plane. On the basis of cepheid space density, a number of vast star complexes (Table I) are identified in which other young objects, together with cepheids fall. The existence of these complexes is explained by star formation in giant molecular clouds. The cepheid mean period increase towards the galactic centre is most probably connected with the existence of a ring between the Sun and the centre of the Galaxy, with the highest density of hydrogen and the highest rate of star formation.  相似文献   

10.
We discuss the possible observational manifestation of the formation of massive black holes in galactic nuclei in the form of an intense high-energy neutrino flux. A short-lived (≤10 yr) hidden neutrino source results from the natural dynamicalal evolution of a central star cluster in the galactic nucleus before its gravitational collapse. The central star cluster at the final evolutionary stage consists of degenerate compact stars (neutron stars and stellar-mass black holes) and is embedded in a massive gaseous envelope produced by destructive collisions of normal stars. Multiple fireballs from frequent collisions of neutron stars give rise to a tenuous quasi-stationary cavity in the central part of the massive envelope. The cavity is filled with shock waves on which an effective cosmic-ray acceleration takes place. Allthe accelerated particles, except the secondary high-energy neutrinos, are absorbed in the dense envelope. The neutrino signal that carries information on the dynamicals of the collapsing galactic nucleus can be recorded by a neutrino detector with an effective area S∼1 km2.  相似文献   

11.
The theory of dissipative structures, applied to star formation systems, provides a conceptual framework for the study of the behaviour and evolution of these systems. As shown by an analysis of a model star formation process system, prolonged stationary star formation in localized areas and repetitive bursting star formation events can be understood as different behavioural modes of galactic dissipative structures. Young stellar associations with theirHII regions and molecular clouds are manifestations of the ordered distribution of matter participating in the star formation processes. A self-organization with the appearance of ordered structures is, in general, to be expected in nonequilibrium systems in which nonlinear processes occur. However, lacking a thermodynamic theory that can be applied to self-gravitating systems, the behaviour of star-forming regions can only be studied by model calculations simulating the process system within the region.  相似文献   

12.
The far outer regions of galactic disks allow an important probe of both star formation and galaxy formation. I discuss how observations of HII regions in these low gas density, low metallicity environments can shed light on the physical processes which drive galactic star formation. The history of past star formation at large radii, as traced by observations of old and intermediate-age stars, constrains the epoch at which the highest angular momentum regions of disks were in place; first results for the M31 disk suggest this occured a significant (≳ 8 Gyr) time ago. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

13.
The long-term evolution of stellar orbits bound to a massive centre is studied in order to understand the cores of star clusters in central regions of galaxies. Stellar trajectories undergo tiny perturbations, the origins of which are twofold: (i) the gravitational field of a thin gaseous disc surrounding the galactic centre, and (ii) cumulative drag arising from successive interactions of the stars with the material of the disc. Both effects are closely related because they depend on the total mass of the disc, assumed to be a small fraction of the central mass. It is shown that, in contrast to previous works, most of the retrograde (with respect to the disc) orbits are captured by the central object, presumably a massive black hole. Initially prograde orbits are also affected, so that statistical properties of the central star cluster in quasi-equilibrium may differ significantly from those deduced in previous analyses.  相似文献   

14.
We study the star formation history of normal spirals by using a large and homogeneous data sample of local galaxies. For our analysis we utilize detailed models of chemical and spectrophotometric galactic evolution, calibrated on the Milky Way disc. We find that star formation efficiency is independent of galactic mass, while massive discs have, on average, lower gas fractions and are redder than their low-mass counterparts; put together, these findings convincingly suggest that massive spirals are older than low-mass ones. We evaluate the effective ages of the galaxies of our sample and we find that massive spirals must be several Gyr older than low-mass ones. We also show that these galaxies (having rotational velocities in the 80–400 km s−1 range) cannot have suffered extensive mass losses, i.e. they cannot have lost during their lifetime an amount of mass much larger than their current content of gas+stars.  相似文献   

15.
We have produced radio maps, using the Australia Telescope Compact Array, of the central regions of six southern type 2 Seyfert galaxies (NGC 1365, 4945, 6221, 6810, 7582 and Circinus) with circumnuclear star formation, to estimate the relative contribution of star formation activity compared to activity from the active galactic nucleus (AGN). The radio morphologies range from extended diffuse structures to compact nuclear emission, with no evidence, even in the relatively compact sources, for synchrotron self-absorption. In each case the radio to far-infrared (FIR) ratio has a value consistent with star formation, and in all but one case the radio to [Fe  II ] ratio is also consistent with star formation. We derive supernova rates and conclude that, despite the presence of a Seyfert nucleus in these galaxies, the radio, FIR and [Fe  II ] line emissions are dominated by processes associated with the circumnuclear star formation (i.e. supernova remnants and H  II regions) rather than with the AGN.  相似文献   

16.
C23 UV spectroscopy of the PG1159-type star NGC7094 C26 Variations of the radio synchrotron spectral index in the interstellar medium of M33 C38 Angular Momentum Evolution of Young Brown Dwarfs and Low Mass Stars C48 The radio halo of the nearby starburst galaxy NGC 253 C95 Signatures of early metal enrichment in Damped-Lyman Alpha systems C113 CO 4 → 3 and [CI] 1 → 0 in the centers of NGC4945 and Circinus C115 Ratio of atomic and molecular gas and gravitational stabilty in the disk of M51 C130 The Interstellar Mediumat Early Cosmic Times: Molecular Gas in Distant Quasar Host Galaxies C188 Probing the interstellar medium in distant galaxies with SPICA/ESI C191 The evolution of spectral energy distributions of galaxies over cosmic times C197 Observations of 60Fe in the Galaxy with INTEGRAL/SPI C204 Evolution of Interstellar Clouds in a hot Gas Environment C205 The effect of clouds in a galactic wind on the evolution of gas-rich dwarf galaxies C206 Energy and element deposit into the interstellar medium during the lives of massive stars C209 The distribution and kinematics of massive stars in the inner Galaxy mapped with SPI/INTEGRAL 26Al 1.8 MeV line observations C213 PDR modelling of the Galactic FIR line emission C239 Towards a complete picture of the molecular ISM in local Luminous Infrared Galaxies: first results from the JCMT/IRAM line survey C242 The Search for the Very High-redshift Tail of Submillimeter Galaxies  相似文献   

17.
《New Astronomy Reviews》2002,46(8-10):535-539
The COMPTEL observations of the galactic 1.809 MeV emission attributed to the radioactive decay of 26Al have confirmed the diffuse nature of this interstellar emission line. One of the most significant features of the reconstructed intensity pattern is a flux enhancement towards Cygnus. This region is fairly young and contains a wealth of massive stars, most of them grouped in the Cygnus OB associations. Multi-frequency model fitting strongly supports the hypothesis of massive stars and their descendent supernovae being the dominant sources of interstellar 26Al as observed by COMPTEL. Massive stars and supernovae are known to impart a large amount of kinetic energy into their surroundings causing shock regions and large cavities in the ISM. In addition, a significant fraction of the electro-magnetic radiation of these stars is emitted in the EUV regime leading to photoionisation of the surrounding medium. We applied a population synthesis model in combination with an 1D model of expanding superbubbles to the Cygnus OB associations. Besides the expected 1.809 MeV flux and the γ-ray line intensity due to interstellar 60Fe we compute the sizes and expansion parameters of the expected HI-structures and the free–free emission intensities due to the photoionizing radiation from massive stars within this region of the sky. We discuss our present understanding of the Cygnus region with respect to the massive star census. Our model assigns about 70% of the 1.809 MeV intensity to six known OB associations, about 20% to known isolated sources and roughly 10% to an unknown diffuse component.  相似文献   

18.
The effects of the passage of a spiral arm through the disc of the giant Virgo Sc galaxy NGC 4321 are investigated with Hubble Space Telescope WFPC2 images in two colours. Concentrating on a portion of the southern spiral arm of NGC 4321, we have applied a new program to solve for the star formation histories in the arm and interarm regions separately. The observational uncertainties and the variable crowding across the spiral arm are taken into account using the results of artificial star tests. In the interarm regions the data are consistent with a constant star formation rate for the last 50 Myr while the stars in the arm region show a star formation rate four times larger than in the interarm regions in the last 5 Myr.  相似文献   

19.
We discuss here the spectroscopic properties of the Seyfert 2 galaxy NGC 7130 (= IC 5135). Emission line regions were isolated and line ratios were measured and fitted with photoionization models; this allowed to discriminate between thermal and non thermal ionization in the circumnuclear regions. Massive star formation is likely to occur at projected distances from the nucleus ≫ 2 kpc: line profiles in these regions suggest the presence of outflows of gas due to stellar winds from hot massive stars.  相似文献   

20.
In order to study magnetic field generation in galaxies with active processes such as intensive star formation, supernovae explosions, etc, a model is needed to differentiate between the properties of interstellar medium in different parts of the galactic disk. In this paper we consider galactic dynamo equations with stochastic coefficients where the parameters responsible for dissipation randomly depend on time and spatial coordinates and are distributed around two values corresponding to aweakly heated neutral component and a hot ionized component. Ionized gas is assumed to be concentrated in small regions evenly distributed over the galactic disk plane. The ratio of the total area of such regions to the entire disk plane corresponds to the mean surface star-formation density in the given region of the galactic disk. Unlike in our previous papers, we take into account the dissipation in the disk plane. We have obtained numerical estimates of the exponential growth rate for different numbers of areas containing ionized gas. We show that the influence of the fluctuations on the magnetic field behavior has a threshold nature; intensive star formation leads to the destruction of large scale magnetic field structures.  相似文献   

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