Ruixiang Chang

Origin and Evolution of the Abundance Gradient Along the Milky Way Disk

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 (Psi = 0.25 Sigma(1.4)(gas), hereafter C-KS law), another is the modified Kennicutt law (Psi = alpha Sigma(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 radially dependent infall timescale, cannot produce a sufficiently steep present-day abundance gradient. This problem disappears if we introduce a disk formation timescale, 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) >= 8 kpc) 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 M-KS law.

Colour–magnitude relations of late-type galaxies

We use a large sample of galaxies drawn from the Sloan Digital Sky Survey (SDSS) and Two-Micron All-Sky Survey (2MASS) to present colour-magnitude relations (CMRs) for late-type galaxies in both optical and optical-infrared (optical-IR) bands. A sample from SDSS Data Release 4 (DR4) is selected to investigate the optical properties. Optical-IR colours are estimated from a position matched sample of DR4 and the 2MASS, in which the photometric aperture mismatch between these two surveys is carefully corrected. It is shown that, after correcting the dust attenuation, the optical colours for faint galaxies (i.e.M-r > -21) have a very weak correlation with the luminosity, whereas the optical colours for bright galaxies (i.e. M-r < -21) are redder than those for more-luminous galaxies. All (optical, optical-IR and IR) colours show similar but stronger correlations with stellar mass than with absolute magnitude. The optical colours correlate more strongly with stellar mass surface density than with stellar mass, whereas optical-IR and IR colours show stronger correlations with stellar mass. By comparing the observed colours of our sample galaxies with the colours predicted by stellar population synthesis model, we find that massive late-type galaxies have older and higher-metallicity stellar population than that of less-massive galaxies. This suggests that CMRs for late-type galaxies are trends defined by the combination of stellar mean age and metallicity. Moreover, our results suggest that the stellar mean metallicity of late-type galaxy is mainly determined by its stellar mass, whereas the star formation history is mainly regulated by the stellar mass surface density.

The evolution and star-formation history of M33

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 star-formation 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.

The star formation history of low-mass disk galaxies: A case study of NGC 300

Context. Since NGC300 is a bulgeless, isolated low-mass galaxy and it has not experienced radial migration during its evolution history, it can be treated as an ideal laboratory to test the simple galactic chemical evolution model.

THE MORPHOLOGICAL-DEPENDENT TULLY-FISHER RELATION OF SPIRAL GALAXIES

The Tully-Fisher relation of spiral galaxies shows notable dependence on morphological types, with earlier type spirals having systematically lower luminosity at fixed maximum rotation velocity V max. This decrement of luminosity is more significant in shorter wavelengths. By modeling the rotation curve and stellar population of different morphological-type spiral galaxies in combination, we find that the V max of spiral galaxies is weakly dependent on the morphological type, whereas the difference of the stellar population originating from the bulge-disk composition effect mainly account for the morphological type dependence of the Tully-Fisher relation.

Color gradients of spiral disks in the Sloan Digital Sky Survey

We investigate the radial color gradients of galactic disks using a sample of ~ 20 000 face-on spiral galaxies selected from the fourth data release of the Sloan Digital Sky Survey (SDSS-DR4).We combine galaxies with similar concentrations, sizes and luminosities to construct composite galaxies, and then measure their color profiles by stacking the azimuthally averaged radial color profiles of all the member galaxies. Except for the smallest galaxies ( R 50 3 kpc), almost all galaxies show negative disk color gradients with mean g − r gradient mag kpc − 1 and r − z gradient mag kpc − 1 . The disk color gradients are independent of the morphological types of galaxies and strongly dependent on the disk surface brightness μ d , with lower surface brightness galactic disks having steeper color gradients.We quantify the intrinsic correlation between color gradients and surface brightness as G gr = − 0 . 011 μ d + 0 . 233 and G rz = − 0 . 015 μ d + 0 . 324 . These quantified correlations provide tight observational constraints on the formation and evolution models of spiral galaxies.

Isolated Galaxies versus Interacting Pairs with MaNGA

We present preliminary results of the spectral analysis on the radial distributions of the star formation history in both a galaxy merger and a spiral isolated galaxy observed with MaNGA. We find that the central part of the isolated galaxy is composed by older stellar population (~2 Gyr) than in the outskirts (~7 Gyr). Also, the time-scale is gradually larger from 1 Gyr in the inner part to 3 Gyr in the outer regions of the galaxy. In the case of the merger, the stellar population in the central region is older than in the tails, presenting a longer time-scale in comparison to central part in the isolated galaxy. Our results are in agreement with a scenario where spiral galaxies are built from inside-out. In the case of the merger, we find evidence that interactions enhance star formation in the central part of the galaxy.

The role of environment on the star formation history of disc galaxies

NGC 2403, NGC 300 and M33 are three nearby pure-disc galaxies with similar stellar mass in different environments; they are benchmarks for understanding late-type spiral galaxies in different environments. The chemical evolution and growth of their discs are investigated by using the simple chemical evolution model, in which their discs are assumed to originate and grow through the accretion of the primordial gas, and the gas outflow process is also taken into account. Through a comparative study of the best-fitting model-predicted star formation histories for them, we hope to derive a picture of the local environment on the evolution and star formation histories of galaxies and whether or not the isolated galaxies follow similar evolution history. Our results show that these three galaxies accumulated more than 50 per cent of their stellar mass at z

Why do disk galaxies present a common gas-phase metallicity gradient?

CALIFA data show that isolated disk galaxies present a common gas-phase metallicity gradient, with a characteristic slope of −0.1dex/re between 0.3 and 2 disk effective radius re (Sanchez et al. 2014). Here we construct a simple model to investigate which processes regulate the formation and evolution. Similar to our previous models (Chang et al. 2012), here we also adopt a Gaussian formula of the gas infall rate fin(t) = A 2πσ e −(t−tp )2 /2σ 2 , where the infall-peak time tp is a free parameter, A is a normalized constant and we fixed σ = 3Gyr. We adopt the classical Schmidt star formation (SF) law as Ψ = νΣgas .