The stellar populations of spiral galaxies

We have used a large sample of low-inclination spiral galaxies with radially resolved optical and near-infrared photometry to investigate trends in star formation history with radius as a function of galaxy structural parameters. A maximum-likelihood method was used to match all the available photometry of our sample to the colours predicted by stellar population synthesis models. The use of simplistic star formation histories, uncertainties in the stellar population models and considering the importance of dust all compromise the absolute ages and metallicities derived in this work; however, our conclusions are robust in a relative sense. We find that most spiral galaxies have stellar population gradients, in the sense that their inner regions are older and more metal rich than their outer regions. Our main conclusion is that the surface density of a galaxy drives its star formation history, perhaps through a local density dependence in the star formation law. The mass of a galaxy is a less important parameter; the age of a galaxy is relatively unaffected by its mass; however, the metallicity of galaxies depends on both surface density and mass. This suggests that galaxy‐mass-dependent feedback is an important process in the chemical evolution of galaxies. In addition, there is significant cosmic scatter suggesting that mass and density may not be the only parameters affecting the star formation history of a galaxy.     

Exploring the evolution of spiral galaxies

We have constructed a family of simple models for spiral galaxy evolution to allow us to investigate observational trends in star formation history with galaxy parameters. The models are used to generate broad-band colours from which ages and metallicities are derived in the same way as the data. We generate a grid of model galaxies and select only those that lie in regions of parameter space covered by the sample. The data are consistent with the proposition that the star formation history of a region within a galaxy depends primarily on the local surface density of the gas but that one or two additional ingredients are required to explain the observational data fully. The observed age gradients appear steeper than those produced by the density dependent star formation law, indicating that the star formation law or infall history must vary with galactocentric radius. Furthermore, the metallicity–magnitude and age–magnitude correlations are not reproduced by a local density dependence alone. These correlations require one or both of the following: (i) a combination of mass dependent infall and metal enriched outflow, or (ii) a mass dependent galaxy formation epoch. Distinguishing these possibilities on the basis of current data is extremely difficult.     

An analysis of 900 rotation curves of southern sky spiral galaxies: Do rotation curves fall into discrete classes?

One of the largest rotation curve data bases of spiral galaxies currently available is that provided by Persic& Salucci (1995; hereafter, PS) which has been derived by them from unreduced rotation curve data of 965 southern sky spirals obtained by Mathewson, Ford& Buchhorn (1992; hereafter, MFB). Of the original sample of 965 galaxies, the observations on 900 were considered by PS to be good enough for rotation curve studies, and the present analysis concerns itself with these 900 rotation curves. The analysis is performed within the context of the hypothesis that velocity fields within spiral discs can be described by generalized power-laws. Rotation curve data was found to impose an extremely strong and detailed correlation between the free parameters of the power-law model, and this correlation accounts for virtually all the variation in the pivotal diagram. In the process, the analysis reveals completely unexpected structure which indicates that rotation curves can be partitioned into well-defined discrete subclasses.     

Rotation curves of luminous spiral galaxies

We have investigated the stellar light distribution and the rotation curves of high‐luminosity spiral galaxies in the local Universe. The sample contains 30 high‐quality extended Hα and H I rotation curves. The stellar disk scale‐length of these objects was measured or taken from the literature. We find that in the outermost parts of the stellar disks of these massive objects, the rotation curves agree with the Universal Rotation Curve (Salucci et al. 2007), however a few rotation curves of the sample show a divergence (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characteristics of clockwise and counterclockwise spiral galaxies

We compared measurements of 126 501 spiral galaxies to test whether the photometry of galaxies that rotate clockwise is different from the photometry of galaxies that rotate counterclockwise for the purpose of testing whether there is a link between photometry and spin direction of galaxies. The rotation directionality of the galaxies was determined by converting the galaxy image to its radial intensity plot, and then the galaxies in each 30° RA sector were separated into clockwise and counterclockwise rotating galaxies. The mean and standard deviation of SDSS DR7 photometric attributes of clockwise and counterclockwise rotating galaxies were then compared. The results show no significant difference between galaxies that rotate clockwise and galaxies that rotate counterclockwise. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spurs and feathering in spiral galaxies

We present smoothed particle hydrodynamic (SPH) simulations of the response of gas discs to a spiral potential. These simulations show that the commonly observed spurs and feathering in spiral galaxies can be understood as being due to structures present in the spiral arms that are sheared by the divergent orbits in a spiral potential. Thus, dense molecular cloud-like structures generate the perpendicular spurs as they leave the spiral arms. Subsequent feathering occurs as spurs are further sheared into weaker parallel structures as they approach the next spiral passage. Self-gravity of the gas is not included in these simulations, stressing that these features are purely due to the hydrodynamics in spiral shocks. Instead, a necessary condition for this mechanism to work is that the gas need be relatively cold (1000 K or less) in order that the shock is sufficient to generate structure in the spiral arms, and such structure is not subsequently smoothed by the gas pressure.     

The disc mass of spiral galaxies

We derive the disc masses of 18 spiral galaxies of different luminosity and Hubble type, both by mass modelling their rotation curves and by fitting their spectral energy distribution with spectrophotometric models. The good agreement of the estimates obtained from these two different methods allows us to quantify the reliability of their performance and to derive very accurate stellar mass-to-light ratio versus colour (and stellar mass) relationships.     

On the question of radio emission of spiral galaxies in groups of galaxies

It has been shown that the radio emission properties of spiral galaxies, if the other conditions are the same, are determined rather by the presence of the close neighbours than by space density of galaxies around them. The rate of occurence of radio sources and their radio luminosities among the spiral members of groups of galaxies depend on the projected seperation between them and their nearest neighbour. The shorter this seperation the higher the probability of radio emission.     

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.     

The structure of spiral galaxies — II. Near-infrared properties of spiral arms

We have imaged a sample of 45 face-on spiral galaxies in the K band, to determine the morphology of the old stellar population, which dominates the mass in the disc. The K -band images of the spiral galaxies have been used to calculate different characteristics of the underlying density perturbation such as arm strengths, profiles and cross-sections, and spiral pitch angles. Contrary to expectations, no correlation was found between arm pitch angle and Hubble type, and combined with previous results this leads us to conclude that the morphology of the old stellar population bears little resemblance to the optical morphology used to classify galaxies. The arm properties of our galaxies seem inconsistent with predictions from the simplest density wave theories, and some observations, such as variations in pitch angle within galaxies, seem hard to reconcile even with more complex modal theories. Bars have no detectable effect on arm strengths for the present sample. We have also obtained B -band images of three of the galaxies. For these galaxies we have measured arm cross-sections and strengths, to investigate the effects of disc density perturbations on star formation in spiral discs. We find that B -band arms lead K -band arms and are narrower than K -band arms, apparently supporting predictions made by the large-scale shock scenario, although the effects of dust on B -band images may contribute towards these results.     

The optical polarization of spiral galaxies

Scattering of starlight by dust, molecules and electrons in spiral galaxies will produce a modification of the direct intensity and a polarization in the observed light. We treat the case where the distribution of scatterers can be considered to be optically thin, and derive semi-analytic expressions for the resolved intensity and polarized intensity for Thomson, Rayleigh, and more general scattering mechanisms. These expressions are applied to a parametric model for spiral galaxies. It is further shown that in the case of Thomson and Rayleigh scattering, and when scatterers and stars are distributed with rotational symmetry, the total polarized flux depends on the inclination, i , of the galactic axis to the line of sight according to a simple sin²  i law. This generalizes the well-known result for point-like and spherical light sources. By using a method based on spherical harmonics, we generalize this law for more general mechanisms, and show that to good approximation, the sin²  i law still holds for the class of models considered.