A. Zurita

Study of the stellar line-strength indices and kinematics along bars - I. Bar age and metallicity gradients

This is the first paper of a series aimed to understand the formation and evolution of bars in early-type spirals and their influence in the evolution of the galaxy. Optical long-slit spectra along the major axis of the bar of a sample of 20 galaxies are analyzed. Line-strength indices in the bar region are measured to derive stellar mean-age and metallicity distributions along the bars using stellar population models. We find three different types of bars according to their metallicity and age distribution along the radius: 1) Bars with negative metallicity gradients. They show mean young/intermediate population (< 2 Gyr), and have amongst the lowest stellar maximum central velocity dispersion of the sample. 2) Bars with null metallicity gradients. These galaxies tend to have negative age gradients. 3) Bars with positive metallicity gradients. These galaxies are predominantly those with higher velocity dispersion and older mean population. We found no significant correlation between the age and metallicity distribution, and bar/galaxy parameters such as the AGN presence, size or the bar strength. From the kinematics, we find that all the galaxies show a disk-like central component. The results from the metallicity and age gradients indicate that most galaxies with high central stellar velocity dispersion host bars that could have been formed more than 3 Gyrs ago, while galaxies with lower central velocity dispersions show a wider distribution in their population and age gradients. A few bars show characteristics compatible with having been formed less than <2 Gy ago. These results place strong constrains to models of bar formation and evolution. The disk-like central components also show the important role played by bars in the secular evolution of the central structure.

The internal dynamical equilibrium of H II regions: A statistical study

We present an analysis of the integrated Hα emission line profiles for the H  region population of the spiral galaxies NGC 1530, NGC 6951 and NGC 3359. We show that ∼70% of the line profiles show two or three Gaussian components. The relations between the luminosity (log LHα) and non-thermal line width (log σnt )f or the H regions of the three galaxies are studied and compared with the relation found taken all the H  regions of the three galaxies as a single distribution. In all of these distributions we find a lower envelope in log σnt. A clearer envelope in σnt is found when only those H  regions with σnt >σ s(13 km s −1 ) are considered, where σs is a canonical estimate of the sound speed in the interestellar medium. The linear fit for the envelope is log LHα = (36.8 ± 0.7) + (2.0 ± 0.5) log σnt where the Hα luminosity of the region is taken directly from a photometric H  region catalogue. When the Hα luminosity used instead is that fraction of the H  region luminosity, corresponding to the principal velocity component, i.e. to the turbulent non-expanding contribution, the linear fit is log LHα = (36.8 ± 0.6) + (2.0 ± 0.5) log σnt, i.e. unchanged but slightly tighter. The masses of the H  regions on the envelope using the virial theorem and the mass estimates from the Hα luminosity are comparable, which offers evidence that the H  regions on the envelope are virialized systems, while the remaining regions, the majority, are not in virial equilibrium.

Evolution of Structure in Late-type Spiral Galaxies I: Ionized Gas Kinematics in NGC 628

Aims. We study two dimensional Fabry-Perot interferometric observations of the nearby face-on late-type spiral galaxy, NGC 628, in order to analyse the ionized gas component of the interstellar medium. Covering the galaxy out to a radius larger than 12 kpc, and with a spatial sampling of 1. ′′ 6, we aim to investigate the large-scale dynamics as well as feedback from individual Hii regions into their surrounding medium. Methods. The observed Hα emission distribution and kinematics are compared with auxiliary data from molecular and atomic gas observations, which display many similarities. We decompose the observed line-of-sight velocities into rotational a nd higher-order harmonic components, and study the role of gravitational perturbations along with that of external triggers which can disturb the kinematics and morphology of NGC 628. We calculate radial profiles of the emission-line velocity dispersion which we use to study the role of feedback from individual Hii regions. Results. We verify the presence of an inner rapidly rotating disc-like component in NGC 628, which we interpret as caused by slow secular evolution of the large-scale spiral arms and oval st ructure. In combination with auxiliary data, we find indicat ion for that gas is falling in from the outer parts towards the central region s, where a nuclear ring has formed at the location of the inner Lindblad resonance radius of an an m = 2 perturbation. Complementary continuum subtracted narrow band images in Hα have been used to identify 376 Hii regions with calibrated luminosities ⋆ . The mean velocity dispersion for the ionized gas (even when excluding pixels belonging to Hii regions) is almost constant out to 12 kpc, although it varies from 14 to 20 km s −1 , with a steady decline in the outer parts. Conclusions. We have found kinematic signatures of radial motions caused by an m = 2 perturbation. Such a perturbation may well be responsible for the inflow of material forming the nuclear ring and the inner rapidly rotating disc-like structure. Th e latter, in turn, could help build a pseudo-bulge in NGC 628. The current paper demonstrates a number of tools that we have developed for building a solid frame work for studying the evolution of structure in spiral galaxies using two dimensional kinematic observations.

New light on the formation and evolution of bars - Trends in the stellar line-strength indices distribution inside the bar region

Aims. Our aim is to study the stellar content of the bar region to constrain its formation and evolution. Methods. Line-strength indices in the bar region of a sample of 6 barred galaxies were employed to derive age and metallicity gradients along the bars using stellar population models. Results. We find clear radial gradients in the line-strength indices for all the galaxies. We find positive gradients within the bar region in the metal indices in four of the six galaxies and opposite trends in the other two. These two galaxies are classified as SAB, and they present exponential bar light profiles. For all the galaxies, we find a positive gradient in the Balmer indices. There is a clear correlation between the position of morphological features inside the bar region with changes in the slope and value of the indices, which indicate changes in the stellar populations, when using stellar population analysis. Therefore, it seems that the bar regions show a gradient in both age and metallicity, changing radially to younger and more meta-rich populations for all the galaxies except for the two with exponential profiles.

Chemical abundances in M31 from HII regions

We have obtained multislit spectroscopic observations from 3700 to 9200 A with Low Resolution Imaging Spectrometer at the Keck I telescope for 31 H ii regions in the disc of the Andromeda galaxy (M31), spanning a range in galactocentric distance from 3.9 to 16.1 kpc. In nine H ii regions we measure one or several auroral lines ([O iii] λ4363, [N ii] λ5755, [S iii] λ6312, [O ii] λ7325) from which we determine the electron temperature (Te) of the gas and derive chemical abundances using the direct Te-based method. We analyse, for the first time in M31, abundance trends with galactocentric radius from the direct method, and find that the Ne/O, Ar/O, N/O and S/O abundance ratios are consistent with a constant value across the M31 disc, while the O/H abundance ratio shows a weak gradient. We have combined our data with all spectroscopic observations of H ii regions in M31 available in the literature, yielding a sample of 85 H ii regions spanning distances from 3.9 to 24.7 kpc (0.19–1.2 R25) from the galaxy centre. We have tested a number of empirical calibrations of strong emission line ratios. We find that the slope of the oxygen abundance gradient in M31 is −0.023 ± 0.002 dex kpc−1, and that the central oxygen abundance is in the range 12+log(O/H)   8.71–8.91 dex (i.e. between 1.05 and 1.66 times the solar value, for 12+log(O/H)⊙ = 8.69), depending on the calibration adopted. The H ii region oxygen abundances are compared with the results from other metallicity indicators (supergiant stars and planetary nebulae). The comparison shows that H ii region O/H abundances are systematically ∼0.3 dex below the stellar ones. This discrepancy is discussed in terms of oxygen depletion on to dust grains and possible biases affecting Te-based oxygen abundances at high metallicity.

Where are the stars of the bar of NGC 1530 forming

Aims. NGC 1530 has one of the strongest bars ever observed and recent star formation sites are distributed across its bar. Our ai m is to study the photometric properties of the bar and its Hii regions, to elucidate the conditions under which Hii regions form and their spatial relation to the principal dynamical features of the bar. Methods. We obtained BVRK s and Hphotometry of the Hii regions of the bar in NGC 1530. Broad-band integrated colours and Hequivalent widths were carefully measured and analysed as a function of position with respect to the main dust-lanes of the bar. Results. We measured differences in the Hequivalent widths of the Hii regions that are located in the trailing and leading sides of the bar dust-lane. The possible factors that could produce this difference, such as (Nii) contamination, Lyman-continuum-photon dust-extinction, escape of ionizing radiation, metallici ty, IMF and age, were carefully analysed. Age differences were confirmed to be the most plausible explanation. This implies that Hii regions that are located further away from the bar dust-lane in its leading side, downstream from the main bar dust-lane, are older than the rest by∼ 1.5− 2.5 Myr. In addition, a clear spatial correlation has been found between: location of Hii regions, dust spurs on the trailing side of the bar dust-lane , and the loci of maximum velocity gradients parallel to the bar major axis (possibly tracing g as flow towards the main bar dust-lane).These results suppor t the hypothesis that massive stars are forming on the trailing side of the bar dust-lane, and age as they cross the bar, on a timescale that i s compatible with the bar dynamics timescale.

Central enhancement of the nitrogen-to-oxygen abundance ratio in barred galaxies

Bar-induced gas inflows towards the galaxy centres are recognized as a key agent for the secular evolution of galaxies. One immediate consequence is the accumulation of gas in the centre of galaxies where it can form stars and alter the chemical and physical properties. We use a sample of nearby face--on disc galaxies with available SDSS spectra to study whether the properties of the ionised gas in the central parts (radii 10^10 M_sun) or galaxies with total stellar mass above ~ 10^10.8 M_sun. In conclusion, we find observational evidence that the presence of a galactic bar affects the central ionised gas properties of disc galaxies, where the most striking effect is an enhancement in the N/O abundance ratio, which can be qualitatively interpreted as due to a different origin or evolutionary processes for less and more massive bulges, with the gaseous phase of the former having currently a closer relation with bars.

On the morphology of dust lanes in galactic bars

The aim of our study is to use dynamical simulations to explore the influence of two important dynamical bar parameters, bar strength and bar pattern speed, on the shape of the bar dust lanes. To quantify the shape of the dust lanes we have developed a new systematic method to measure the dust lane curvature. Previous numerical simulations have compared the curvature of bar dust lanes with the bar strength, predicting a relation between both parameters which has been supported by observational studies but with a large spread. We take into account the bar pattern speed to explore, simultaneously, the effect of both parameters on the dust lane shape. To that end, we separate our galactic bars in fast bars

Multiwavelength study of the star-formation in the bar of NGC 2903

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Observational constraints to boxy/peanut bulge formation time

, obtaining, as previous simulations, an inverse relation between the dust lane curvature and the bar strength for fast bars. For the first time, we extend the study to slow bars, finding a constant curvature as a function of the bar strength. As a result, we conclude that weak bars with straight dust lanes are candidates for slow bars. Finally, we have analysed a pilot sample of ten S