Galaxy formation and evolution: low-surface-brightness galaxies

We investigate in detail the hypothesis that low-surface-brightness galaxies (LSBs) differ from ordinary galaxies simply because they form in haloes with large spin parameters. We compute star formation rates using the Schmidt law, assuming the same gas infall dependence on surface density as used in models of the Milky Way. We build stellar population models, predicting colours, spectra and chemical abundances. We compare our predictions with observed values of metallicity and colours for LSBs, and find excellent agreement with all observables. In particular, integrated colours, colour gradients, surface brightness and metallicity match very well to the observed values of LSBs for models with ages larger than 7 Gyr and high values (λ > 0.05) for the spin parameter of the haloes. We also compute the global star formation rate (SFR) in the Universe due to LSBs, and show that it has a flatter evolution with redshift than the corresponding SFR for normal discs. We furthermore compare the evolution in redshift of [ Zn / H ] for our models to those observed in damped Lyman α systems by Pettini et al. and show that damped Lyman α system abundances are consistent with the predicted abundances at different radii for LSBs. Finally, we show how the required late redshift of collapse of the halo may constrain the power spectrum of fluctuations.     

Dynamical stability and evolution of the discs of Sc galaxies

We examine the local stability of galactic discs against axisymmetric density perturbations with special attention to the different dynamics of the stellar and gaseous components. In particular, the discs of the Milky Way and of NGC 6946 are studied. The Milky Way is shown to be stable, whereas the inner parts of NGC 6946, a typical Sc galaxy from the Kennicutt sample, are dynamically unstable. The ensuing dynamical evolution of the composite disc is studied by numerical simulations. The evolution is so fierce that the stellar disc heats up dynamically on a short time-scale to such a high degree, which seems to contradict the morphological appearance of the galaxy. The star formation rate required to cool the disc dynamically is estimated. Even if the star formation rate in NGC 6946 is at present high enough to meet this requirement, it is argued that the discs of Sc galaxies cannot sustain such a high star formation rate for extended periods.     

A cosmological study of the star formation history in the solar neighbourhood

We use a cosmological galactic evolutionary approach to model the Milky Way. A detailed treatment of the mass aggregation and dynamical history of the growing dark halo is included, together with a self-consistent physical treatment for the star formation processes within the growing galactic disc. This allows us to calculate the temporal evolution of star and gas surface densities at all galactic radii, in particular, the star formation history (SFH) at the solar radius. A large range of cosmological mass aggregation histories (MAHs) is capable of producing a galaxy with the present-day properties of the Milky Way. The resulting SFHs for the solar neighbourhood bracket the available observational data for this feature, the most probable MAH yielding the optimal comparison with these observations. We also find that the rotation curve for our Galaxy implies the presence of a constant density core in its dark-matter halo.     

Origin and structure of the Galactic disc(s)

We examine the chemical and dynamical structure in the solar neighbourhood of a model Galaxy that is the endpoint of a simulation of the chemical evolution of the Milky Way in the presence of radial mixing of stars and gas. Although the simulation's star formation rate declines monotonically from its unique peak and no merger or tidal event ever takes place, the model replicates all known properties of a thick disc, as well as matching special features of the local stellar population such as a metal-poor extension of the thin disc that has high rotational velocity. We divide the disc by chemistry and relate this dissection to observationally more convenient kinematic selection criteria. We conclude that the observed chemistry of the Galactic disc does not provide convincing evidence for a violent origin of the thick disc, as has been widely claimed.     

Phase-space density constraints on the formation of bulges from discs

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.     

Chemo-spectrophotometric evolution of spiral galaxies – V. Properties of galactic discs at high redshift

We explore the implications for the high-redshift universe of 'state-of-the-art' models for the chemical and spectrophotometric evolution of spiral galaxies. The models are based on simple 'scaling relations' for discs, obtained in the framework of cold dark matter models for galaxy formation, and were 'calibrated' so as to reproduce the properties of the Milky Way and of nearby discs (at redshift z ∼0) . In this paper, we compare the predictions of our 'hybrid' approach to galaxy evolution to observations at moderate and high redshift. We find that the models are in fairly good agreement with observations up to z ∼1 , while some problems appear at higher redshift (provided there is no selection bias in the data); these discrepancies may suggest that galaxy mergers (not considered in this work) played a non-negligible role at z >1 . We also predict the existence of a 'universal' correlation between abundance gradients and disc scalelengths, independent of redshift.     

Contrasting the chemical evolution of the Milky Way and Andromeda

The chemical evolution history of a galaxy hides clues about how it formed and has been changing through time. We have studied the chemical evolution history of the Milky Way (MW) and Andromeda (M31) to find which are common features in the chemical evolution of disc galaxies as well as which are galaxy-dependent. We use a semi-analytic multizone chemical evolution model. Such models have succeeded in explaining the mean trends of the observed chemical properties in these two Local Group spiral galaxies with similar mass and morphology. Our results suggest that while the evolution of the MW and M31 shares general similarities, differences in the formation history are required to explain the observations in detail. In particular, we found that the observed higher metallicity in the M31 halo can be explained by either (i) a higher halo star formation efficiency (SFE), or (ii) a larger reservoir of infalling halo gas with a longer halo formation phase. These two different pictures would lead to (i) a higher [O/Fe] at low metallicities, or (ii) younger stellar populations in the M31 halo, respectively. Both pictures result in a more massive stellar halo in M31, which suggests a possible correlation between the halo metallicity and its stellar mass.     

History of Star Formation and Chemical Enrichment in the Milky Way Disk

Based on a physical treatment of the star formation law similar to that given by Efstathiou, we have improved our two-component chemical evolution model for the Milky Way disk. Two gas infall rates are compared, one exponential, one Gaussian. It is shown that the star formation law adopted in this paper depends more strongly on the gas surface density than that in Chang et al. It has large effects on the history of star formation and gas evolution of the whole disk. In the solar neighborhood, the history of chemical evolution and star formation is not sensitive to whether the infall rate is Gaussian or exponential. For the same infall time scale, both forms predict the same behavior for the current properties of the Galactic disk. The model predictions do depend on whether or not the infall time scale varies with the radius, but current available observations cannot decide which case is the more realistic. Our results also show that it would be inadequate to describe the gradient evolution along the Gala     

The evolution of barium and europium in local dwarf spheroidal galaxies

By means of a detailed chemical evolution model, we follow the evolution of barium (Ba) and europium (Eu) in four Local Group Dwarf Spheroidal (dSph) galaxies, in order to set constraints on the nucleosynthesis of these elements and on the evolution of this type of galaxies compared with the Milky Way. The model, which is able to reproduce several observed abundance ratios and the present-day total mass and gas mass content of these galaxies, adopts up-to-date nucleosynthesis and takes into account the role played by supernovae (SNe) of different types (II, Ia) allowing us to follow in detail the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca, Fe, Ba and Eu). By assuming that Ba is a neutron-capture element produced in low-mass asymptotic giant branch stars by s-process but also in massive stars (in the mass range 10–30 M_⊙) by r-process, during the explosive event of SNe of Type II, and that Eu is a pure r-process element synthesized in massive stars also in the range of masses 10–30 M_⊙, we are able to reproduce the observed [Ba/Fe] and [Eu/Fe] as functions of [Fe/H] in all four galaxies studied. We confirm also the important role played by the very low star formation (SF) efficiencies (ν= 0.005–0.5 Gyr⁻¹) and by the intense galactic winds (6–13 times the star formation rate) in the evolution of these galaxies. These low SF efficiencies (compared to the one for the Milky Way disc) adopted for the dSph galaxies are the main reason for the differences between the trends of [Ba/Fe] and [Eu/Fe] predicted and observed in these galaxies and in the metal-poor stars of our Galaxy. Finally, we provide predictions for Sagittarius galaxy for which data of only two stars are available.     

老年疏散星团的金属丰度及银盘金属丰度梯度演化研究

疏散星团是探究银河系结构与演化的良好示踪体,一直以来颇受关注.之前关于疏散星团的研究中,仅有一小部分疏散星团有金属丰度参数,而且,金属丰度的测量,是基于不同质量的观测数据,采用了不同的方法.收集了一个年龄大于2 Gyr的老年疏散星团样本,通过整理这些星团成员星的金属丰度数据,一方面,以星团NGC 2682为例,对比了不同光谱巡天项目给出的星团成员星金属丰度的系统差异;另一方面,计算了星团成员星金属丰度的平均值和中位值,作为该疏散星团的金属丰度推荐值.此外,还利用该样本探究了银盘径向金属丰度梯度随时间的演化,结果表明,早期银盘有着更加陡峭的径向金属丰度梯度,随着演化时间的增加,银盘径向金属丰度梯度逐渐趋于平缓,为银盘化学演化模型提供了更加严格的观测约束.     

Chemo-spectrophotometric evolution of spiral galaxies – II. Main properties of present-day disc galaxies

We study the chemical and spectrophotometric evolution of galactic discs with detailed models calibrated on the Milky Way and using simple scaling relations, based on currently popular semi-analytic models of galaxy formation. We compare our results with a large body of observational data on present-day galactic discs, including disc sizes and central surface brightness, Tully–Fisher relations in various wavelength bands, colour–colour and colour–magnitude relations, gas fractions versus magnitudes and colours and abundances versus local and integrated properties, as well as spectra for different galactic rotational velocities. Despite the extremely simple nature of our models, we find satisfactory agreement with all those observables, provided that the time-scale for star formation in low-mass discs is longer than for more massive ones. This assumption is apparently in contradiction with the standard picture of hierarchical cosmology. We find, however, that it is extremely successful in reproducing major features of present-day discs, like the change in the slope of the Tully–Fisher relation with wavelength, the fact that more massive galaxies are on average 'redder' than low-mass ones (a generic problem of standard hierarchical models) and the metallicity–luminosity relation for spirals. It is concluded that, on a purely empirical basis, this new picture is at least as successful as the standard one. Observations at high redshifts could help to distinguish between the two possibilities.     

Chemo-spectrophotometric evolution of spiral galaxies – III. Abundance and colour gradients in discs

We study the relations between luminosity and chemical-abundance profiles of spiral galaxies, using detailed models for the chemical and spectrophotometric evolution of galactic discs. The models are 'calibrated' on the Milky Way disc and are successfully extended to other discs with the help of simple 'scaling' relations, obtained in the framework of semi-analytic models of galaxy formation. We find that our models exhibit oxygen abundance gradients that increase in absolute value with decreasing disc luminosity (when expressed in dex kpc⁻¹) and are independent of disc luminosity (when expressed in dex scalelength⁻¹), both in agreement with observations. We notice an important strong correlation between abundance gradient and disc scalelength. These results support the idea of 'homologous evolution' of galactic discs.     

On the link between disc-galaxies and Damped Lyman-α systems

We model the chemical and spectrophotometric evolution of the Milky Way, spiral galaxies and Low Surface Brightness Galaxies in a consistent way. Some consequences with respect to the nature of Damped Lyman-α systems (DLAs) are investigated. This revised version was published online in September 2006 with corrections to the Cover Date.     

Cosmological origin of the lowest metallicity halo stars

We explore the predictions of the standard hierarchical clustering scenario of galaxy formation, regarding the numbers and metallicities of PopIII stars that are likely to be found within our Galaxy today. By PopIII we refer to stars formed at large redshift ( z >4), with low metallicities ([ Z /Z_⊙]<−2.5) and in small systems (total mass ≲ 2×10⁸ M_⊙) that are extremely sensitive to stellar feedback, and which through a prescribed merging history end up becoming part of the Milky Way today. An analytic, extended Press–Schechter formalism is used to obtain the mass functions of haloes which will host PopIII stars at a given redshift, and which will end up in Milky Way sized systems today. Each of these is modelled as a mini-galaxy, with a detailed treatment of the dark halo structure, angular momentum distribution, final gas temperature and disc instabilities, all of which determine the fraction of the baryons that are subject to star formation. The use of new primordial metallicity stellar evolutionary models allows us to trace the history of the stars formed, and give accurate estimates of their expected numbers today and their location in L /L_⊙ versus T /K Hertzsprung–Russell (HR) diagrams. A first comparison with observational data suggests that the initial mass function (IMF) of the first stars was increasingly high-mass weighted towards high redshifts, levelling off at z ≳9 at a characteristic stellar mass scale m _s=10–15 M_⊙.     

Young massive star clusters in M51

A search for young massive star clusters (YMCs) in the nearby face-on spiral galaxy M51 (NGC 5194) has been carried out using UBV CCD images from the prime focus camera on the Lick 3-m Shane telescope. The YMC population is found to be quite rich with a specific U -band luminosity     consistent with the high current star formation rate (SFR) of this galaxy. The brightest clusters have     far brighter than any young clusters currently known in the Milky Way and even surpassing the luminosity of the R136 cluster in the 30 Dor complex in the Large Magellanic Cloud. A few of the YMCs are examined on archive images from the Wide Field Planetary Camera (WF/PC2) on board the Hubble Space Telescope ( HST ), confirming their cluster nature and providing estimates of their effective radii of 2–3 pc. The number of YMCs in M51 is compatible with extrapolation of a power-law luminosity function with exponent ∼−2 from a Milky Way-like population of open clusters. Both the SFR and T _L ( U ) of M51 are similar to those of other cluster-rich spiral galaxies like NGC 1313 and M83.     

Dynamical evolution of galactic disks driven by interaction with a satellite

Dynamical evolution of galactic disks driven by interaction with satellite galaxies, particularly the problem of the disk warping and thickening is studied numerically. One of the main purpose of the study is to resolve the long standing problem of the origin of the disk warping. A possible cause of the warp is interaction with a satellite galaxy. In the case of the Milky Way, the LMC has been considered as the candidate. Some linear analysis have already given a positive result, but one had to wait for a fully self-consistent simulation as a proof. I have accomplished the numerical simulations with a million particles, by introducing a hybrid algorithm, SCF-TREE. Those simulations give us quantitative estimates for the Milky Way system. We have found an example in which large warp amplitudes are developed. We also found that the warp amplitudes depend on the halo distribution. Among our three models, the most massive and spherical halo is preferable for the observable warp excitation. This revised version was published online in August 2006 with corrections to the Cover Date.     

A grid of chemical evolution models as a tool to interpret spiral and irregular galaxies data

We present a generalization of the multiphase chemical evolution model (CEM) applied to a wide set of theoretical galaxies with different masses and evolutionary rates. This generalized set of models has been computed using the so-called universal rotation curve from Persic, Salucci & Steel to calculate the radial mass distribution of 44 theoretical protogalaxies. This distribution is a fundamental input which, besides its own effect on the galaxy evolution, defines the characteristic collapse time-scale or gas infall rate on to the disc. We have adopted 10 sets of values, between 0 and 1, for the molecular cloud and star formation efficiencies, as corresponding to their probability nature, for each one of the radial distributions of total mass. Thus, we have constructed a biparametric grid of models, depending on those efficiency sets and on the rotation velocity, whose results are valid in principle for any spiral or irregular galaxy. The model results provide the time-evolution of different regions of the disc and the halo along galactocentric distance, measured by the gas (atomic and molecular) and stellar masses, the star formation rate (SFR) and chemical abundances of 14 elements, for a total of 440 models. This grid may be used to estimate the evolution of a given galaxy for which only present time information, such as radial distributions of elemental abundances, gas densities and/or star formation, which are the usual observational constraints of chemical evolution models (CEMs), is available.     

Chemo-spectrophotometric evolution of spiral galaxies – IV. Star formation efficiency and effective ages of spirals

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.     

Chemical evolution models of local dwarf spheroidal galaxies

We calculate chemical evolution models for four dwarf spheroidal (dSph) satellites of the Milky Way (Carina, Ursa Minor, Leo I and Leo II) for which reliable non-parametric star formation histories have been derived. In this way, the independently-obtained star formation histories are used to constrain the evolution of the systems we are treating. This allows us to obtain robust inferences on the history of such crucial parameters of galactic evolution as gas infall, gas outflows and global metallicities for these systems. We can then trace the metallicity and abundance ratios of the stars formed, the gas present at any time within the systems and the details of gas ejection, of relevance to enrichment of the galaxies environment. We find that galaxies showing one single burst of star formation (Ursa Minor and Leo II) require a dark halo slightly larger that the current estimates for their tidal radii, or the presence of a metal-rich selective wind that might carry away much of the energy output of their supernovae before this might have interacted and heated the gas content, for the gas to be retained until the observed stellar populations have formed. Systems showing extended star formation histories (Carina and Leo I), however, are consistent with the idea that their tidally-limited dark haloes provide the necessary gravitational potential wells to retain their gas. The complex time structure of the star formation in these systems remains difficult to understand. Observations of detailed abundance ratios for Ursa Minor strongly suggest that the star formation history of this galaxy might in fact resemble the complex picture presented by Carina or Leo I, but localized at a very early epoch.     

The collision between the Milky Way and Andromeda

We use an N -body/hydrodynamic simulation to forecast the future encounter between the Milky Way and the Andromeda galaxies, given present observational constraints on their relative distance, relative velocity, and masses. Allowing for a comparable amount of diffuse mass to fill the volume of the Local Group, we find that the two galaxies are likely to collide in a few billion years – within the Sun's lifetime. During the interaction, there is a chance that the Sun will be pulled away from its present orbital radius and reside in an extended tidal tail. The likelihood for this outcome increases as the merger progresses, and there is a remote possibility that our Sun will be more tightly bound to Andromeda than to the Milky Way before the final merger. Eventually, after the merger has completed, the Sun is most likely to be scattered to the outer halo and reside at much larger radii (>30 kpc). The density profiles of the stars, gas and dark matter in the merger product resemble those of elliptical galaxies. Our Local Group model therefore provides a prototype progenitor of late-forming elliptical galaxies.