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1.
M. Grenon 《Astrophysics and Space Science》1989,156(1-2):29-37
Members of the Galaxy components are identified according to stellar ages, metallicities and galactic orbits. The local thin disk is found to have a maximum age of 11 billion years and a small abundance scatter partially controlled by the radial gradient of abundances. Metal-rich and old metal-poor stars belong to inner galactic populations and SMRs represent the ultimate star generation in the bulge. The thick disk forms a smooth transition between the halo and thin disk. 相似文献
2.
Markus Samland 《Astrophysics and Space Science》2002,281(1-2):305-308
The formation of a disk galaxy within a slowly growing dark halo is simulated with a new chemo-dynamical model. The model
describes the evolution of the stellar populations, the multi-phase ISM and all important interaction. I find, that the galaxy
forms radially from inside-out and vertically from top-to-bottom. The derived stellar age distributions show that the inner
halo is the oldest component, followed by the outer halo, the triaxial bulge, the halo-disk transition region and the disk.
Despite the still idealized model, the final galaxy resembles present-day disk galaxies in many aspects. In particular, the
stellar metallicity distribution in the halo of the model resembles the one of M31. The bulge in the model shows, at least
two stellar subpopulations, an early collapse population and a population that formed later out of accreted disk mass. In
the stellar metallicity distribution of the disk, I find a pronounced ‘G-dwarf problem’ which is the result of a pre-enrichment
of the disk ISM with metal-rich gas from the bulge.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
3.
K.C. Freeman 《Astrophysics and Space Science》1999,267(1-4):129-138
Away from the young disk, several classes of early type stars are found. They include (i) the old, metal-poor blue horizontal branch stars of the halo and the metal-poor tail of the thick disk; (ii) metal-rich young A stars in a rapidly rotating subsystem but with a much higher velocity dispersion than the A stars of the
young disk, and (iii) a newly discovered class of metal-poor young main sequence A stars in a subsystem of intermediate galactic rotation (Vrot ≈ 120 km s−1). The existence and kinematics of these various classes of early type stars provide insight into the formation of the metal-poor
stellar halo of the Galaxy and into the continuing accretion events suffered by our Galaxy.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
Rosemary F. G. Wyse 《Monthly notices of the Royal Astronomical Society》1998,293(4):429-433
The colours of stellar bulges and of inner stellar discs are comparable, and consistent with rather similar mean metallicities and ages. Indeed, the mean chemical abundances of the Milky Way bulge and old disc are approximately equal. Further, the scalelengths of discs and bulges are correlated. These observations imply a close relationship between discs and bulges, and may support models in which stellar bulges form from stellar discs. The present paper discusses constraints on this scenario from the stellar phase-space density of bulges and of discs. Phase-space density cannot increase in the absence of collisional processes. We show here that the maximum phase-space density of galactic bulges is higher than that of inner discs, arguing that instabilities of purely stellar discs cannot form bulges. Rather, the high densities of bulges probably reflect gaseous dissipation. Gas inflow from the disc would complicate the interpretation of the similarities in stellar colours between discs and bulges. Gas inflow from the stellar halo, if one exists, may be favoured on angular momentum grounds, but this means of formation of the bulge would provide no explanation for the relationships between disc and bulge in any one galaxy. At least in the Milky Way, the metallicity distribution of the bulge is not consistent with the bulge being built up from the dense regions of accreted satellite galaxies and/or globular clusters. 相似文献
5.
This paper has two parts: one about observational constraints related to the empirical differential oxygen abundance distribution
(EDOD), and the other about inhomogeneous models of chemical evolution, in particular the theoretical differential oxygen
abundance distribution (TDOD). In the first part, the EDOD is deduced from subsamples related to two different samples involving
(i) N=532 solar neighbourhood (SN) stars within the range, −1.5<[Fe/H]<0.5, for which the oxygen abundance has been determined
both in presence and in absence of the local thermodynamical equilibrium (LTE) approximation (Ramirez et al. in Astron. Astrophys.
465:271, 2007); and (ii) N=64 SN thick disk, SN thin disk, and bulge K-giant stars within the range, −1.7<[Fe/H]<0.5, for which the oxygen abundance
has been determined (Melendez et al. in Astron. Astrophys. 484:L21, 2008). A comparison is made with previous results implying use of [O/H]–[Fe/H] empirical relations (Caimmi in Astron. Nachr. 322:241,
2001b; New Astron. 12:289, 2007) related to (iii) 372 SN halo subdwarfs (Ryan and Norris in Astron. J. 101:1865, 1991); and (iv) 268 K-giant bulge stars (Sadler et al. in Astron. J. 112:171, 1996). The EDOD of the SN thick + thin disk is determined by weighting the mass, for assumed SN thick to thin disk mass ratio
within the range, 0.1–0.9.
In the second part, inhomogeneous models of chemical evolution for the SN thick disk, the SN thin disk, the SN thick + thin
disk, the SN halo, and the bulge, are computed assuming the instantaneous recycling approximation. The EDOD data are fitted,
to an acceptable extent, by their TDOD counterparts with the exception of the thin or thick + thin disk, where two additional
restrictions are needed: (i) still undetected, low-oxygen abundance thin disk stars exist, and (ii) a single oxygen overabundant
star is removed from a thin disk subsample. In any case, the (assumed power-law) stellar initial mass function (IMF) is universal
but gas can be inhibited from, or enhanced in, forming stars at different rates with respect to a selected reference case.
Models involving a strictly universal IMF (i.e. gas neither inhibited from, nor enhanced in, forming stars with respect to
a selected reference case) can also reproduce the data to an acceptable extent.
Our main conclusions are (1) different models are necessary to fit the (incomplete) halo sample, which is consistent with
the idea of two distinct halo components: an inner, flattened halo in slow prograde rotation, and an outer, spherical halo
in net retrograde rotation (Carollo et al. in Nature 450:1020, 2007); (2) the oxygen enrichment within the inner SN halo, the SN thick disk, and the bulge, was similar and coeval within the
same metallicity range, as inferred from observations (Prochaska et al. in Astron. J. 120:2513, 2000); (3) the fit to thin disk data implies an oxygen abundance range similar to its thick disk counterpart, with the extension
of conclusion (2) to the thin disk, and the evolution of the thick + thin disk as a whole (Haywood in Mon. Not. R. Astron.
Soc. 388:1175, 2008) cannot be excluded; (4) leaving outside the outer halo, a fit to the data related to different environments is provided
by models with a strictly universal IMF but different probabilities of a region being active, which implies different global
efficiencies of the star formation rate; (5) a special case of stellar migration across the disk can be described by models
with enhanced star formation, where a fraction of currently observed SN stars were born in situ and a comparable fraction is due to the net effect of stellar migration, according to recent results based on high-resolution
N-body + smooth particle hydrodynamics simulations (Roškar et al. in Astrophys. J. Lett. 684:L79, 2008). 相似文献
6.
Frank C. van den Bosch 《Monthly notices of the Royal Astronomical Society》2001,327(4):1334-1352
We present new models for the formation of disc galaxies that improve upon previous models by following the detailed accretion and cooling of the baryonic mass, and by using realistic distributions of specific angular momentum. Under the assumption of detailed angular momentum conservation, the discs that form have density distributions that are more centrally concentrated than an exponential. We examine the influence of star formation, bulge formation, and feedback on the outcome of the surface brightness distributions of the stars. Low angular momentum haloes yield disc galaxies with a significant bulge component and with a stellar disc that is close to exponential, in good agreement with observations. High angular momentum haloes, on the other hand, produce stellar discs that are much more concentrated than an exponential, in clear conflict with observations. At large radii, the models reveal distinct truncation radii in both the stars and the cold gas. The stellar truncation radii result from our implementation of star formation threshold densities, and are in excellent agreement with observations. The truncation radii in the density distribution of the cold gas reflect the maximum specific angular momentum of the gas that has cooled. We find that these truncation radii occur at H i surface densities of roughly 1 M⊙ pc−2 , in conflict with observations. We examine various modifications to our models, including feedback, viscosity, and dark matter haloes with constant-density cores, but show that the models consistently fail to produce bulge less discs with exponential surface brightness profiles. This signals a new problem for the standard model of disc formation: if the baryonic component of the protogalaxies out of which disc galaxies form has the same angular momentum distribution as the dark matter, discs are too compact. 相似文献
7.
James Binney 《New Astronomy Reviews》2013,57(3-4):29-51
Our Galaxy is a complex machine in which several processes operate simultaneously: metal-poor gas is accreted, is chemically enriched by dying stars, and then drifts inwards, surrendering its angular momentum to stars; new stars are formed on nearly circular orbits in the equatorial plane and then diffuse through orbit space to eccentric and inclined orbits; the central stellar bar surrenders angular momentum to the surrounding disc and dark halo while acquiring angular momentum from inspiralling gas; the outer parts of the disc are constantly disturbed by satellite objects, both luminous and dark, as they sweep through pericentre. We review the conceptual tools required to bring these complex happenings into focus. Our first concern must be the construction of equilibrium models of the Galaxy, for upon these hang our hopes of determining the Galaxy’s mean gravitational field, which is required for every subsequent step. Ideally our equilibrium model should be formulated so that the secular evolution of the system can be modelled with perturbation theory. Such theory can be used to understand how stars diffuse through orbit space from either the thin gas disc in which we presume disc stars formed, or the debris of an accreted object, the presumed origin of many halo stars. Coupling this understanding to the still very uncertain predictions of the theory of stellar evolution and nucleosynthesis, we can finally extract a complete model of the chemodynamic evolution of our reasonably generic Galaxy. We discuss the relation of such a model to cosmological simulations of galaxy formation, which provide general guidance but cannot be relied on for quantitative detail. 相似文献
8.
9.
O. K. Sil’chenko 《Astronomy Letters》2002,28(4):207-216
Our inzvestigation of the central region in NGC 4548, a bright Sb galaxy with a large-scale bar, using the Multipupil Field Spectrograph of the 6-m telescope revealed a chemically decoupled compact stellar nucleus with [Fe/H]=+0.6 and [Mg/Fe]=+0.1...+0.2 and with a mean stellar-population age of 5 Gyr. This nucleus, a probable circumnuclear disk coplanar with the global galactic disk, is embedded in the bulge whose stars are generally also young, T≈4 Gyr, although they are a factor of 2.5 more metal-poor. The bulge of NGC 4548 is triaxial and has a de Vaucouleurs surface-brightness profile; the unusual characteristics of its stellar population suggest the bulge formation or completion in the course of secular evolution in the triaxial potential of the global bar. The ionized gas within 3″ of the NGC 4548 nucleus rotates in a plane inclined to the principal symmetry plane of the galaxy, possibly, even in its polar plane, which may also result from the action of the large-scale bar. 相似文献
10.
The saturation conditions for bending modes in inhomogeneous thin stellar disks that follow from an analysis of the dispersion relation are compared with those derived from N-body simulations. In the central regions of inhomogeneous disks, the reserve of disk strength against the growth of bending instability is smaller than that for a homogeneous layer. The spheroidal component (a dark halo, a bulge) is shown to have a stabilizing effect. The latter turns out to depend not only on the total mass of the spherical component, but also on the degree of mass concentration toward the center. We conclude that the presence of a compact (not necessarily massive) bulge in spiral galaxies may prove to be enough to suppress the bending perturbations that increase the disk thickness. This conclusion is corroborated by our N-body simulations in which we simulated the evolution of near-equilibrium, but unstable finite-thickness disks in the presence of spheroidal components. The final disk thickness at the same total mass of the spherical component (dark halo + bulge) was found to be much smaller than that in the simulations where a concentrated bulge is present. 相似文献
11.
We analyze the relationship between the mass of a spherical component and the minimum possible thickness of stable stellar disks. This relationship for real galaxies allows the lower limit on the dark halo mass to be estimated (the thinner the stable stellar disk is, the more massive the dark halo must be). In our analysis, we use both theoretical relations and numerical N-body simulations of the dynamical evolution of thin disks in the presence of spherical components with different density profiles and different masses. We conclude that the theoretical relationship between the thickness of disk galaxies and the mass of their spherical components is a lower envelope for the model data points. We recommend using this theoretical relationship to estimate the lower limit for the dark halo mass in galaxies. The estimate obtained turns out to be weak. Even for the thinnest galaxies, the dark halo mass within four exponential disk scale lengths must be more than one stellar disk mass. 相似文献
12.
Markus Samland 《Astrophysics and Space Science》2003,284(2):841-844
I present a model for the formation and evolution of a massive disk galaxy, within a growing dark halo whose mass evolves
according to cosmological simulations of structure formation. The galactic evolution is simulated with a new three-dimensional
chemo-dynamical code, including dark matter, stars and a multi-phase ISM. We follow the evolution from redshift z= 4.85 until the present epoch. The energy release by massive stars and supernovae prevents a rapid collapse of the baryonic
matter and delays the maximum star formation until redshift z ≈ 1. The galaxy forms radially from inside-out and vertically from top-to-bottom. Correspondingly, the inner halo is the
oldest component, followed by the outer halo, the bar/bulge, the thick and the thin disk. The bulge in the model consists
of at least two stellar subpopulations, an early collapse population and a population that formed later in the bar.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
13.
恒星的Al元素丰度可以为探索星团和星系的化学演化提供重要线索.通过系统分析银河系薄盘、厚盘、核球、银晕以及M4、M5等球状星团中恒星的[Al/Fe]随恒星金属丰度[Fe/H]的变化趋势,得出银河系薄盘、厚盘和核球恒星的[Al/Fe]随着[Fe/H]的增加而缓慢下降,而球状星团M4和M5恒星的[Al/Fe]随[Fe/H]增加没有下降趋势,这暗示Ia超新星对M4和M5恒星元素丰度的贡献比较小.详细研究了银河系恒星[Al/Fe]与[Mg/Fe]、[Na/Fe]的相关性,结果表明银河系场星的[Al/Fe]与[Mg/Fe]正相关,但在球状星团M4和M5恒星中未见此相关性;银河系盘星及M4和M5等球状星团恒星的[Al/Fe]与[Na/Fe]都存在正相关. 相似文献
14.
O. Bienaymé 《Astrophysics and Space Science》1999,265(1-4):225-230
Thick discs are components intermediate between thin discs and stellar halos, they are also expected to gives clues concerning
intermediate stages of the galaxy formation. Numerical models from N-body accretion to hydrodynamical collapse may reproduce
part of kinematics or other observational facts. Consistent kinematic models of thick discs allow one to understand which
kinematic features are intrinsic to thick discs and consequently do not depend really on the precise conditions of their formation.
Establishing a dynamical continuity between the thick disc and the other stellar populations will provide the opportunity
to discriminate between formation scenarios: -free-fall collapse, halo formed as a merger of dwarf galaxies or thick disc
puff-up by galaxy harassment.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
Nozomu Kawakatu Masayuki Umemura 《Monthly notices of the Royal Astronomical Society》2002,329(3):572-578
We quantitatively scrutinize the effects of the radiation drag arising from the radiation fields in a galactic bulge in order to examine the possibility that the radiation drag could be an effective mechanism to extract angular momentum in a spheroidal system like a bulge and allow plenty of gas to accrete on to the galactic centre. For this purpose, we numerically solve the relativistic radiation hydrodynamical equation coupled with accurate radiative transfer, and quantitatively assess the radiation drag efficiency. As a result, we find that in an optically thick regime the radiation drag efficiency is sensitively dependent on the density distributions of the interstellar medium (ISM). The efficiency drops according to in an optically thick uniform ISM, where τ T is the total optical depth of the dusty ISM , whereas the efficiency remains almost constant at a high level if the ISM is clumpy . Hence, if bulge formation begins with a star formation event in a clumpy ISM, the radiation drag will effectively work to remove the angular momentum and the accreted gas may form a supermassive black hole. As a natural consequence, this mechanism reproduces a putative linear relation between the mass of a supermassive black hole and the mass of a galactic bulge, although further detailed modelling for stellar evolution is required for a more precise prediction. 相似文献
16.
B. Basu 《Astrophysics and Space Science》1986,119(1):201-206
The collapse of a homogeneous, initially spheroidal halo under self gravitation, has been considered. It is found that a weak magnetic field (as is plausible to belong to such a cloud) has little influence on the collapse, except probably sufficiently close to the centre where the gas density, and consequently the magnetic field, becomes rather high. The equatorial collapse is centrifugally balanced at a certain stage, while collapse in the perpendicular direction continues. A thick stellar disk is formed within a time-scale <3×109 yr. Brisk star formation takes place while the collapse of the gaseous disk is still in progress. This gives rise to the halo stars with low metal content and high Z-motion. A bulge is formed at the centre simultaneously. This is the first phase of formation of a disk galaxy. The thin disk is formed at a later stage as the remaining primordial gas and the gas released by the evolution of stars in the thick disk gradually settles on to it.The presented model is rather a crude one. Many aspects have not been considered, and many details have not been worked out. It is hoped that a more detailed and comprehensive model will be arrived at in the future.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984. 相似文献
17.
The gravitational collapse of molecular clouds or cloud cores is expected to lead to the formation of stars that begin their lives in a state of rapid rotation. It is known that, in at least some specific cases, rapidly rotating, slf-gravitating bodies are subject to instabilities that cause them to assume ellipsoidal shapes. In this paper we investigate the consequences of such instabilities on the angular momentum evolution of a star in the process of formation from a collapsing cloud, and surrounded by a protostellar disk, with a view toward applications to the formation of the Solar System. We use a specific model of star formation to demonstrate the possibility that such a star would become unstable, that the resulting distortion of the star would generate spiral density waves in the circumstellar disk, and that the torque associated with these waves would regulate the angular momentum of the star as it feeds angular momentum to the disk. We conclude that the angular momentum so transported to the disk would not spread the disk to, say, Solar System dimensions, by the action of the spiral density waves alone. However, a viscous disk could effectively extract stellar angular momentum and attain Solar System size. Our results also indicate that viscous disks could feed mass and angular momentum to a growing protostar in such a manner that distortions of the star would occur before gravitational torques could balance the influx of angular momentum. In other situations (in which the viscosity was small), a gap could be cleared between the disk and star. 相似文献
18.
Cristina Chiappini 《Astrophysics and Space Science》2002,281(1-2):253-256
According to the two-infall model for the chemical evolution of the Galaxy the halo and bulge formed on a relatively short
timescale (0.8–1.0 Gyr) out of the first infall episode, whereas the disk accumulated much more slowly and ‘inside-out’ during
a second independent infall episode. We explored the effects of a threshold in the star formation process, during both the
halo and disk phases. In the comparison between model predictions and available data, we have focused our attention on abundance
gradients as well as gas, stellar and star formation rate distributions along the disk. We suggest that the mechanism for
the formation of the halo leaves detectable imprints on the chemical properties of the outer regions of the disk, whereas
the evolution of the halo and the inner disk are almost completely disentangled. This is due to the fact that the halo and
disk densities are comparable at large Galactocentric distances and therefore the gas lost from the halo can substantially
contribute to building up the outer disk. We predict that the abundance gradients along the Galactic disk have increased in
time during the first billion years of the disk evolution and remained almost constant in the last ~5Gyrs.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
19.
《New Astronomy》2002,7(4):161-169
In the usual and most widespread textbook picture of the Milky Way Galaxy, disk stars like the Sun are referred to as Population I, the spheroidal or halo component in turn as Population II. The latter is thought of as the pressure-supported, metal-poor relic of the early Galaxy, with renewed interest in recent years in the search for dark matter via microlensing. Modelling the putative massive compact halo objects however, faces the problem that the stellar halo is generally considered to consist of only a few billion solar masses. Here we present observational evidence that even this low budget may be a factor ten too high. If so, this immediately implies that the classical population II of halo stars is fairly irrelevant, not only in the dark matter context, but, in particular, in models of the formation and evolution of the Milky Way Galaxy. 相似文献
20.
Bing Zhang Rosemary F. G. Wyse 《Monthly notices of the Royal Astronomical Society》2000,313(2):310-322
We investigate a model of disc galaxies whereby viscous evolution of the gaseous disc drives material inwards to form a protobulge. We start from the standard picture of disc formation through the settling of gas into a dark halo potential well, with the disc initially coming into centrifugal equilibrium with detailed conservation of angular momentum. We derive generic analytic solutions for the disc–halo system after adiabatic compression of the dark halo, with free choice of the input virialized dark halo density profile and of the specific angular momentum distribution. We derive limits on the final density profile of the halo in the central regions. Subsequent viscous evolution of the disc is modelled by a variation of the specific angular momentum distribution of the disc, providing analytic solutions to the final disc structure. The assumption that the viscous evolution time-scale and the star formation time-scale are similar leads to predictions of the properties of the stellar components. Focusing on small 'exponential' bulges, i.e., ones that may be formed through a disc instability, we investigate the relationship between the assumed initial conditions, such as halo 'formation', or assembly, redshift z f , spin parameter λ , baryonic fraction F , and final disc properties such as global star formation time-scale, gas fraction, and bulge-to-disc ratio. We find that the present properties of discs, such as the scalelength, are compatible with a higher initial formation redshift if the redistribution by viscous evolution is included than if it is ignored. We also quantify the dependence of final disc properties on the ratio F λ , thus including the possibility that the baryonic fraction varies from galaxy to galaxy, as perhaps may be inferred from the observations. 相似文献