Aims. We study the chemical evolution of the disks of the Milky Way (MW) and of Andromeda (M 31), to identify the common properties and differences between the two major galaxies of the Local Group. Methods. We use a large set of observational data for M 31, including observations of the star formation rate (SFR) and gas profiles, as well as stellar metallicity distributions along its disk. When expressed in terms of the corresponding disk scale lengths, we show that the observed radial profiles of MW and M 31 exhibit interesting similarities, suggesting the possibility of a description within a common framework. Results. We find that the profiles of stars, gas fraction, and metallicity of the two galaxies, as well as most of their global properties, are well described by our model, provided that the star formation efficiency in M 31 disk is twice as high as in the MW. We show that the star formation rate profile of M 31 cannot be described by any form of the Kennicutt-Schmidt law (KS Law) for star formation. We propose that these discrepancies are caused by the fact that M 31 has an active star formation history in the recent past, consistent with the hypotheses of a "head-on" collision with the neighboring galaxy (most probably M 32) about 200 Myr ago. Conclusions. The MW has most probably experienced quiescent secular evolution, making possible a fairly successful description with a simple model. If M 31 is more typical of spiral galaxies, more complex models, involving galaxy interactions, will be required for the description of spirals.

We use a sample of 800 galaxies with H I mass measurements from the HyperLeda catalogue and optical photometry from the fourth data release of the Sloan Digital Sky Survey (SDSS) to calibrate a new photometric estimator of the H I-to-stellar-mass ratio for nearby galaxies. Our estimator, which is motivated by the Kennicutt-Schmidt star formation law, is log 10 (G H I /S) = −1.732 38(g − r) + 0.215 182μ i − 4.084 51, where μ i is the i-band surface brightness and g − r is the optical colour estimated from the g-and r-band Petrosian apparent magnitudes. This estimator has a scatter of σ = 0.31 dex in log (G H I /S), compared to σ ∼ 0.4 dex for previous estimators that were based on colour alone. We investigate whether the residuals in our estimate of log (G H I /S) depend in a systematic way on a variety of different galaxy properties. We find no effect as a function of stellar mass or 4000 Å break strength, but there is a systematic effect as a function of the concentration index of the light. We then apply our estimator to a sample of 10 5 emission-line galaxies in the SDSS Data Release 4 (DR4) and derive an estimate of the H I mass function, which is in excellent agreement with recent results from H I blind surveys. Finally, we re-examine the well-known relation between gas-phase metallicity and stellar mass, and ask whether there is a dependence on H I-to-stellar-mass ratio, as predicted by chemical evolution models. We do find that gas-poor galaxies are more metal rich at fixed stellar mass. We compare our results with the semi-analytic models of De Lucia & Blaizot, which include supernova feedback, as well as the cosmological infall of gas.

Based on a simple model of the chemical evolution of the Milky Way disk, we investigate the disk oxygen abundance gradient and its time evolution. Two star formation rates (SFRs) are considered, one is the classical Kennicutt-Schmidt law (Ψ = 0.25Σ 1.4 gas , hereafter C-KS law), another is the modified Kennicutt law (Ψ = αΣ 1.4 gas (V /r), hereafter M-KS law). In both cases, the model can produce some amount of abundance gradient, and the gradient is steeper in the early epoch of disk evolution. However, we find that when C-KS law is adopted, the classical chemical evolution model, which assumes a radial dependent infall time scale, cannot produce a sufficiently steep present-day abundance gradient. This problem disappears if we introduce a disk formation time scale, which means that at early times, infalling gas cools down onto the inner disk only, while the outer disk forms later. This kind of model, however, will predict a very steep gradient in the past. When the M-KS law is adopted, the model can properly predict both the current abundance gradient and its time evolution, matching recent observations from planetary nebulae and open clusters along the Milky Way disk. Our best model also predicts that outer disk (artificially defined as the disk with R g ≥ 8kpc) has a steeper gradient than the inner disk. The observed outer disk gradients from Cepheids, open clusters and young stars show quite controversial results. There are also some hints from Cepheids that the outer disk abundance gradient may have a bimodal distribution. More data is needed in order to clarify the outer disk gradient problem. Our model calculations show that for an individual Milky Way-type galaxy, a better description of the local star formation is the modified KS law.

We have compiled a sample of 2728 nearby (z < 0.08) elliptical galaxies with photometry in the g, r , i, z bands from the Sloan Digital Sky Survey (SDSS) and J, H, K photometry from the Two-Micron All-Sky Survey (2MASS). Stellar masses, stellar velocity dispersions and structural parameters such as sizes and surface mass densities are also available for these objects. In order to correct the aperture mismatch between SDSS and 2MASS, we correct the SDSS magnitudes to the isophotal circular radius where the 2MASS magnitudes are measured. We compare the correlations between optical, optical-infrared and infrared colours and galaxy luminosity, stellar mass, velocity dispersion and surface mass density. We find that all galaxy colours correlate more strongly with stellar mass and velocity dispersion than with any other structural parameter. The dispersion about these two relations is also smaller. We also study the correlations between a variety of stellar absorption-line indices and the same set of galaxy parameters and we reach very similar conclusions. Finally, we analyse correlations between absorption-line indices and colour. Our results suggest that the optical colours of elliptical galaxies are sensitive to a combination of age, metallicity and α-enhancement, while the optical-infrared colours are sensitive to metallicity and to α-enhancement, but are somewhat less sensitive to age.

We construct a parametrized model to explore the main properties of the star-formation history of M33. We assume that the disc originates and grows by primordial gas infall and adopt a simple form of gas accretion rate with one free parameter, the infall time-scale. We also include the contribution of the gas outflow process. A major update of the model is that we adopt a molecular-hydrogen-correlated star-formation law and calculate the evolution of the atomic and molecular gas separately. Comparisons between the model predictions and observational data show that the model predictions are very sensitive to the adopted infall time-scale, while the gas-outflow process mainly influences the metallicity profile. A model adopting a moderate outflow rate and an inside-out formation scenario can be in good agreement with most of the observed constraints of the M33 disc. We also compare model predictions based on a molecular-hydrogen-correlated star-formation law and that based on the Kennicutt starformation law. Our results imply that the molecular-hydrogen-correlated star-formation law should be preferred to describe the evolution of the M33 disc, especially the radial distributions of both the cold gas and the stellar population.

We present a detailed study of the Galaxy Evolution Explorer's photometric catalogs with special focus on the statistical properties of the All-sky and Medium Imaging Surveys. We introduce the concept of primaries to resolve the issue of multiple detections and follow a geometric approach to define clean catalogs with well-understood selection functions. We cross-identify the GALEX sources (GR2+3) with Sloan Digital Sky Survey (DR6) observations, which indirectly provides an invaluable insight about the astrometric model of the UV sources and allows us to revise the band merging strategy. We derive the formal description of the GALEX footprints as well as their intersections with the SDSS coverage along with analytic calculations of their areal coverage. The crossmatch catalogs are made available for the public. We conclude by illustrating the implementation of typical selection criteria in SQL for catalog subsets geared toward statistical analyses, e.g., correlation and luminosity function studies.

In the present paper, we introduce a two-component model of the Galactic disk to investigate its chemical evolution. The formation of the thick and thin disks occur in two main accretion episodes with both infall rates to be Gaussian. Both the pre-thin and post-thin scenarios for the formation of the Galactic disk are considered.The best-fitting is obtained through χ 2 -test between the models and the new observed metallicity distribution function of G dwarfs in the solar neighbourhood (Hou et al 1998).Our results show that post-thin disk scenario for the formation of the Galactic disk should be preferred. Still, other comparison between model predictions and observations are given.

We introduce a simple model to explore the star formation histories of disk galaxies. We assume that the disk origins and grows by continuous gas infall. The gas infall rate is parametrized by the Gaussian formula with one free parameter: infall-peak time t p . The Kennicutt star formation law is adopted to describe how much cold gas turns into stars. The gas outflow process is also considered in our model. We find that, at given galactic stellar mass M * , model adopting late infall-peak time t p results in blue colors, low metallicity, high specific star formation rate and high gas fraction, while gas outflow rate mainly influences the gas-phase metallicity and star formation efficiency mainly influences the gas fraction. Motivated by the local observed scaling relations, we construct a mass-dependent model by assuming low mass galaxy has later infall-peak time t p and larger gas outflow rate than massive systems. It is shown that this model can be in agreement with not only the local observations, but also the observed correlations between specific star formation rate and galactic stellar mass SF R/M * ∼ M * at intermediate redshift z < 1. Comparison between the Gaussian-infall model and exponential-infall model is also presented. It shows that the exponential-infall model predicts higher star formation rate at early stage and lower star formation rate later than that of Gaussian-infall. Our results suggest that the Gaussian infall rate may be more reasonable to describe the gas cooling process than the exponential infall rate, especially for low-mass systems.