A Photometric System for Metallicity Mapping in Galaxies

A new photometric system suitable for deep, precise and quick metallicity mapping in galaxies is proposed. We find a linear correlation between our metallicity index and the Mg₂ index for stellar, globular-cluster, and early-type galaxy spectra, and model spectral energy distributions of the simple stellar populations. This revised version was published online in July 2006 with corrections to the Cover Date.     

The stellar content of a prototype double barred galaxy

High spatial resolution visible and NIR observations of the bar of NGC 5850, a prototype of double barred spirals, together with visible intermediate and high dispersion spectra along the primary bar, are being used, with the aid of simple stellar population synthesis models, to investigate the mean age and metallicity of the different stellar components of the central part of the galaxy. The determination of stellar ages and metallicities would constrain theoretical scenarios for secondary bar formation and the evolution of barred spirals. Unfortunately, we cannot obtain a good fit with simple stellar populations (SSPs) to the spectral indices, so it can not give us, by now, insight into the mean stellar age and metallicity of the real populations in the central region of the galaxy. These preliminary results show a relatively old primary bar with metallicity about solar, although absolute values must be taken with care. The nucleus has a young stellar component, and is very dusty. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spectrophotometric investigations of the blue compact dwarf galaxy Mrk 35

Preliminary results from a detailed spectrophotometric analysis of the blue compact dwarf galaxy (BCD) Mrk 35 are presented. We have performed deep UBVRI broad-band and H_α narrow-band optical observations, near-infrared (JHK _s) imaging and long-slit spectroscopy of the galaxy. Mrk 35 is composed of a very young starburst population distributed in a bar-like structure, placed on top of an underlying, older stellar host galaxy. Using predictions of evolutionary synthesis models, we estimate the ages of both the starburst regions and the underlying stellar component. This revised version was published online in August 2006 with corrections to the Cover Date.     

A catalogue and analysis of local galaxy ages and metallicities

We have assembled a catalogue of relative ages, metallicities and abundance ratios for about 150 local galaxies in field, group and cluster environments. The galaxies span morphological types from cD and ellipticals, to late-type spirals. Ages and metallicities were estimated from high-quality published spectral line indices using Worthey & Ottaviani (1997) single stellar population evolutionary models.
The identification of galaxy age as a fourth parameter in the fundamental plane ( Forbes, Ponman & Brown 1998 ) is confirmed by our larger sample of ages. We investigate trends between age and metallicity, and with other physical parameters of the galaxies, such as ellipticity, luminosity and kinematic anisotropy. We demonstrate the existence of a galaxy age–metallicity relation similar to that seen for local galactic disc stars, whereby young galaxies have high metallicity, while old galaxies span a large range in metallicities.
We also investigate the influence of environment and morphology on the galaxy age and metallicity, especially the predictions made by semi-analytic hierarchical clustering models (HCM). We confirm that non-cluster ellipticals are indeed younger on average than cluster ellipticals as predicted by the HCM models. However we also find a trend for the more luminous galaxies to have a higher [Mg/Fe] ratio than the lower luminosity galaxies, which is opposite to the expectation from HCM models.     

The Stellar Populations of the Bars of Barred Spirals Through Evolutionary Synthesis: First Results

As part of an ongoing project aimed at studying the age and metallicity gradients of the stellar populations along the bars of a sample of barred spirals of different morphological types, we present our first results on NGC 4314 (SBa). We have obtained optical and NIR colours and spectral indices along the bar and we interpret some of these results here and discuss their uncertainties on the basis of single stellar population models. In a preliminary analysis, we constrain the limits for the age and metallicity of the nucleus and two selected regions in the star formation ring of NGC 4314, characterizing both as metal rich (Z<Z _solar) stellar populations, and finding a difference in the mean luminosity-weighted age of at least ∼ 3–4 Gyr. This revised version was published online in July 2006 with corrections to the Cover Date.     

The resolved red giant branches of E/S0 galaxies

Formation paradigms for massive galaxies have long centered around two antipodal hypotheses – the monolithic-collapse and the accretion/merger scenarios. Empirical data on the stellar contents of galaxy halos is crucial in order to develop galaxy formation and assembly scenarios which have their root in observations, rather than in numerical simulations. The Hubble Space Telescope (HST) has enabled us to study directly individual stars in the nearby E/S0 galaxies Cen A, NGC 3115, NGC 5102, and NGC 404. We here present and discuss HST single-star photometry in V and I bands. Using color-magnitude diagrams and stellar luminosity functions, we gauge the galaxies' stellar contents. This can be done at more than one position in the halo, but data with deeper limiting magnitudes are desired to quantify the variation of metallicity with galactocentric radius. We here compare the color distributions of red giant stars with stellar isochrones, and we intercompare the galaxies' halo populations, noting that their total absolute V magnitudes cover the range from about –21.5 to –17.5. In the future, we plan to model the stellar metallicity distributions with the aim to constrain chemical enrichment scenarios, a step towards unravelling the evolutionary history of elliptical and lenticular galaxies. This revised version was published online in August 2006 with corrections to the Cover Date.     

The evolutionary history of early-type galaxies as derived from the fundamental plane

The fundamental plane (FP) scaling relations and their evolution are a powerful tool for studying the global properties of early-type galaxies and their evolutionary history. The form of the FP, as derived by surveys in the local Universe at wavelengths ranging from the U to the K band, cannot be explained by metallicity variations alone among early-type galaxies; systematic variations in age, dark matter content, or homology breaking are required. A large-scale study of early-type galaxies at 0.1 < z < 0.6demonstrates that the SB intercept of the FP, the rest frame (U-V) colour, and the absorption line strengths all evolve passively, thereby implying a high mean formation redshift for the stellar content. The slope of the FP evolves with redshift, which is broadly consistent with systematic age effects occurring along the early-type galaxy sequence. The implication that the least luminous early-type galaxies formed later than the luminous galaxies is discussed in the context of the evolution of thecolour–magnitude relation, the Butcher–Oemler effect and hierarchical galaxy formation models. This revised version was published online in July 2006 with corrections to the Cover Date.     

The formation of a disk galaxy within a slowly growing dark halo

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.     

Effects of Binary Interactions on the Early-type Galaxy Chemical Evolution and Gas Cooling Functiontwo

In the galaxy parameter fitting by means of stellar population synthesis, it is found that compared with the evolutionary population synthesis (EPS) model without binary interactions, the stellar age and metallicity of a galaxy derived from the EPS model with binary interactions are larger. But, we are still unclear how the binary interactions affect the galaxy evolution. For the early-type galaxies with the UV-excess phenomenon, there are two main-stream explanations: recent star formation (RSF) and binary interactions. In this study, we obtain the mass return rate and chemical yield for the stellar populations with and without binary interactions. In combination with the galaxy chemical evolution and photoionization models, we study the effects of binary interactions on the chemical evolution and metallicity evolution for the early-type galaxies with the UV-excess phenomenon under the two formation mechanisms. We find that the inclusion of binary interactions can raise the ejected mass, metallicity, alpha element, and accelerate the gas cooling. These can reasonably explain the conclusions made by the EPS models. Moreover, we find that the gas cooling is more efficient under the UV-excess formation mechanism by the binary interactions rather than the RSF, and the ratio of element abundance is different for the two mechanisms, which can be further used to distinguish these two mechanisms.     

基于深度学习的LAMOST星系星族参数测量

星系的光谱包含其内部恒星的年龄和金属丰度等信息, 从观测光谱数据中测量这些信息对于深入了解星系的形成和演化至关重要. LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope)巡天发布了大量的星系光谱, 这些高维光谱与它们的物理参数之间存在着高度的非线性关系. 而深度学习适合于处理多维、海量的非线性数据, 因此基于深度学习技术构建了一个8个卷积层$+$4个池化层$+$1个全连接层的卷积神经网络, 对LAMOST Data Release 7 (DR7)星系的年龄和金属丰度进行自动估计. 实验结果表明, 使用卷积神经网络通过星系光谱预测的星族参数与传统方法基本一致, 误差在0.18dex以内, 并且随着光谱信噪比的增大, 预测误差越来越小. 实验还对比了卷积神经网络与随机森林回归模型、深度神经网络的参数测量结果, 结果表明卷积神经网络的结果优于其他两种回归模型.     

On the Contraction of Protostellar Clouds with Different Metallicities

We examine the thermal and chemical evolution of gravitationally collapsing protostellar clouds with metallicity 0≤Z/Z _⊙≤1.During the first collapse stage, the temperatures are higher for lower metallicity clouds. However, in the course of the adiabatic contraction of transient cores, the evolutionary trajectories of the clouds converge to a curve that is determined only by fundamental physical constants. The trajectories coincide each other thereafter. The size of the stellar core at formation is the same regardless of metallicity. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Morphological Evolution of Galaxy Satellites

We study the evolution of galaxy satellites with high resolutionN-body simulations. Satellites are modelled as replicas of typical low and high surface brightness galaxies (LSBs and HSBs). Encounters on high eccentricity orbits (as typical in hierarchical models of galaxy formation) strip LSBs of most of their stars and tend to decrease their surface brightness. In contrast, bar instability in HSBs leads to substantial loss of angular momentum of the stellar component and to an increase in central surface brightness. In both cases the remnant resembles a spheroidal galaxy with an exponential surface brightness profile. A simple modelling of colour evolution and interaction-driven star formation gives M/L ratios for the remnants that are roughly consistent with observations. These results suggest an evolutionary scenario for the dwarf galaxies in our Local Group, faint dSphs being the descendants of LSBs and brighter dSphs/dEs being the final state of HSB satellites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evolutionary history of the elliptical galaxy NGC 1052

We have obtained Keck spectra for 16 globular clusters (GCs) associated with the merger remnant elliptical NGC 1052, as well as a long-slit spectrum of the galaxy. We derive ages, metallicities and abundance ratios from simple stellar population models using the recently published methods of Proctor & Sansom , applied to extragalactic GCs for the first time. A number of GCs indicate the presence of strong blue horizontal branches that are not fully accounted for in the current stellar population models. We find all of the GCs to be ∼13 Gyr old according to simple stellar populations, with a large range of metallicities. From the galaxy spectrum we find NGC 1052 to have a luminosity-weighted central age of ∼2 Gyr and metallicity of  [Fe/H]∼+0.6  . No strong gradients in either age or metallicity were found to the maximum radius measured  (0.3  r _e≃ 1 kpc)  . However, we do find a strong radial gradient in α-element abundance, which reaches a very high central value. The young central starburst age is consistent with the age inferred from the H  i tidal tails and infalling gas of ∼1 Gyr. Thus, although NGC 1052 shows substantial evidence for a recent merger and an associated starburst, it appears that the merger did not induce the formation of new GCs, perhaps suggesting that little recent star formation occurred. This interpretation is consistent with 'frosting' models for early-type galaxy formation.     

A comparison of chemical and chemodynamical models

Representative results from a comparison of the chemical evolution of spherical collapse models without and with a intercloud medium are presented. The hot metal-rich gas distributes quickly the metals produced in supernovae throughout the galaxy, thus leading to a more homogeneous chemical evolution and flatter metallicity gradients in the gas and the stars. The stellar population is somewhat less concentrated towards the centre. The strong outflow results in a substantial loss of metals from the galaxy to its surroundings, and a lower effective yield in the galaxy. This revised version was published online in August 2006 with corrections to the Cover Date.     

The evolution of disk galaxies in clusters

We are carrying out a programme to measure the evolution of the stellar and dynamical masses and M/L ratios for a sizeable sample of morphologically-classified disk galaxies in rich galaxy clusters at 0.2 < z < 0.9. Using FORS2 at the VLT we are obtaining rotation curves for the cluster spirals so that their Tully-Fisher relation can be studied as a function of redshift and compared with that of field spirals. We already have rotation curves for ∼ 10 cluster spirals at z = 0.83, and 25 field spirals at lower redshifts and we plan to increase this sample by one order of magnitude. We present here the first results of our study, and discuss the implications of our data in the context of current ideas and models of galaxy formation and evolution. This revised version was published online in September 2006 with corrections to the Cover Date.     

MrK86: A 'Not-So-Dark' Galaxy

We have studied the velocity field of the Blue Compact Dwarf galaxy Mrk86 using an Hα Fabry-Perot image and 14 long-slit optical spectra. From the ionized gas velocities measured we have found that Mrk86 follows a solid-body rotation law with a central angular velocity of 34 ± 5 km s^-1 kpc^-1. The comparison of the modeled rotation curve with our velocity data indicates a clear dominance of the stellar mass component over the dark matter. Related with the galaxy most intense star forming knot, a very strong gradient steeping is observed. Finally, we give kinematic evidence for the Mrk86-C expanding bubble. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spatially resolved kinematics and stellar populations of brightest cluster and group galaxies

We present an examination of the kinematics and stellar populations of a sample of three brightest group galaxies (BGGs) and three brightest cluster galaxies (BCGs) in X-ray groups and clusters. We have obtained high signal-to-noise ratio Gemini/Gemini South Multi-Object Spectrograph (GMOS) long-slit spectra of these galaxies and use Lick indices to determine ages, metallicities and α-element abundance ratios out to at least their effective radii. We find that the BGGs and BCGs have very uniform masses, central ages and central metallicities. Examining the radial dependence of their stellar populations, we find no significant velocity dispersion, age, or α-enhancement gradients. However, we find a wide range of metallicity gradients, suggesting a variety of formation mechanisms. The range of metallicity gradients observed is surprising, given the homogeneous environment these galaxies probe and their uniform central stellar populations. However, our results are inconsistent with any single model of galaxy formation and emphasize the need for more theoretical understanding of both the origins of metallicity gradients and galaxy formation itself. We postulate two possible physical causes for the different formation mechanisms.     

Galactic evolution of 7LI: observational clues for models

Different stellar sources may have contributed to the ⁷Li enrichmentof the Galaxy: type-II supernovae, novae, and AGB stars. In the latter case, the interplay between the Hot Bottom Burning (HBB) process (via the Cameron-Fowler mechanism) and a very high mass-loss rate before the evolution off the AGB (the so-called ‘superwind’ phase), can lead to a significant production of ⁷Li from low- and intermediate-mass AGB stars (Travaglio et al., 2001). We have now undertaken an observational campaign aimed at constraining our stellar and Galactic models, with a twofold goal: (i) to assemble a compilation of high-resolution spectra of Galactic, unevolved (i.e. dwarfs), warm(spectral type F) stars, in a selected metallicity range (-1.0 ≤>[Fe/H] ≤ -0.3), using the ESO 1.5m telescope and the FEROS spectrograph; (ii) to carry out a Li survey among a sample of selected AGB stars, to investigate the possible correlation between⁷Li abundance (when detected) and mass-loss rate. This revised version was published online in September 2006 with corrections to the Cover Date.     

The Nearby Field Galaxy Survey

We report on our observing programme to obtain integrated spectrophotometry, intermediate- and high-resolution major-axis spectra, and UBR surface photometry of a representative sample of ∼ 200galaxies in the nearby field. The main goal of this programme is to provide a comparison sample for high-redshift studies and to study the variation in star formation rates (SFRs), star formation history (SFH),excitation, metallicity, and internal kinematics over a wide range of galaxy luminosities and morphological types. In particular, we extend the work of Kennicutt (1992) to lower-luminosity systems. We present the main results of our analysis so far. In these proceedings, we condense the two atlases presented of 1) images and radial surface brightness profiles and colour profiles, and 2) images and integrated spectra into several example images, profiles and spectra, showing the general trends observed. For the original atlases we refer the reader to the electronic distribution on CDROM or as available on the Web at http://www.astro.rug.nl/∼nfgs/. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemodynamical gas flow cycles and their influence on the chemical evolution of dwarf irregular galaxies

Here we investigate an exemplary chemodynamical evolutionary simulation of a dwarf irregular galaxy. By means of this model we demonstrate the existence of three gas mixing cycles: 1) An inner local cycle mixing the metals produced in stars locally, and 2) an outer galactic cycle on which hot gas is driven out of the galaxy by multiple supernovae type II and mixes on a short timescale with the available cold gas. 3) Only a small fraction of the metals leaves the galactic gravitational field and follows the global cycle with the intergalactic matter. The large-scale mixing results in a temporary depletion of supernova ejected metals. We will discuss this delayed recycling and its influence on the chemical evolution, especially on the nitrogen over oxygen ratio which is increased temporarily. The results presented here are also relevant for less sophisticated analytical approaches and chemical evolutionary models of galaxies which have to parameterize the metal loss through outflow. This revised version was published online in August 2006 with corrections to the Cover Date.