Evidence for intermediate-age stellar populations in early-type galaxies from K-band spectroscopy

The study of stellar populations in early-type galaxies in different environments is a powerful tool for constraining their star formation histories. This study has been traditionally restricted to the optical range, where dwarfs around the turn-off and stars at the base of the RGB dominate the integrated light at all ages. The near-infrared spectral range is especially interesting since in the presence of an intermediate-age population, AGB stars are the main contributors. In this letter, we measure the near-infrared indices NaI and D$_{\rm CO}$ for a sample of 12 early-type galaxies in low density environments and compare them with the Fornax galaxy sample presented by Silva et al. (2008). The analysis of these indices in combination with Lick/IDS indices in the optical range reveals i) the NaI index is a metallicity indicator as good as C4668 in the optical range, and ii) D$_{\rm CO}$ is a tracer of intermediate-age stellar populations. We find that low-mass galaxies in low density environments show higher NaI and D$_{\rm CO}$ than those located in Fornax cluster, which points towards a late stage of star formation for the galaxies in less dense environments, in agreement with results from other studies using independent methods.Comment: 7 pages, 3 figures, accepted for publication in ApJ

The cosmic evolution of the spatially-resolved star formation rate and stellar mass of the CALIFA survey

We investigate the cosmic evolution of the absolute and specific star formation rate (SFR, sSFR) of galaxies as derived from a spatially-resolved study of the stellar populations in a set of 366 nearby galaxies from the CALIFA survey. The analysis combines GALEX and SDSS images with the 4000 break, H_beta, and [MgFe] indices measured from the datacubes, to constrain parametric models for the SFH, which are then used to study the cosmic evolution of the star formation rate density (SFRD), the sSFR, the main sequence of star formation (MSSF), and the stellar mass density (SMD). A delayed-tau model, provides the best results, in good agreement with those obtained from cosmological surveys. Our main results from this model are: a) The time since the onset of the star formation is larger in the inner regions than in the outer ones, while tau is similar or smaller in the inner than in the outer regions. b) The sSFR declines rapidly as the Universe evolves, and faster for early than for late type galaxies, and for the inner than for the outer regions of galaxies. c) SFRD and SMD agree well with results from cosmological surveys. At z< 0.5, most star formation takes place in the outer regions of late spiral galaxies, while at z>2 the inner regions of the progenitors of the current E and S0 are the major contributors to SFRD. d) The inner regions of galaxies are the major contributor to SMD at z> 0.5, growing their mass faster than the outer regions, with a lookback time at 50% SMD of 9 and 6 Gyr for the inner and outer regions. e) The MSSF follows a power-law at high redshift, with the slope evolving with time, but always being sub-linear. f) In agreement with galaxy surveys at different redshifts, the average SFH of CALIFA galaxies indicates that galaxies grow their mass mainly in a mode that is well represented by a delayed-tau model, with the peak at z~2 and an e-folding time of 3.9 Gyr.Comment: 23 pages, 16 figures, 6 tables, accepted for publication in Astronomy & Astrophysics. *Abridged abstract

An updated MILES stellar library and stellar population models

(Aims) We present a number of improvements to the MILES library and stellar population models. We correct some small errors in the radial velocities of the stars, measure the spectral resolution of the library and models more accurately, and give a better absolute flux calibration of the models. (Methods) We use cross-correlation techniques to correct the radial velocities of the offset stars and the penalised pixel-fitting method, together with different sets of stellar templates, to re-assess the spectral resolution of the MILES stellar library and models. We have also re-calibrated the zero-point flux level of the models using a new calibration scheme. (Results) The end result is an even more homogeneously calibrated stellar library than the originally released one, with a measured spectral resolution of ~2.5\AA, almost constant with wavelength, for both the MILES stellar library and models. Furthermore, the new absolute flux calibration for the spectra excellently agrees with predictions based on independent photometric libraries. (Conclusions) This improved version of the MILES library and models (version 9.1) is available at the project's website (http://miles.iac.es).Comment: 7 pages, 8 figures. Accepted for publication in Astronomy & Astrophysics (Research Note

Stellar population gradients in Fornax Cluster S0 galaxies: connecting bulge and disk evolution

We present absorption-line index gradients for a sample of S0 galaxies in the Fornax Cluster. The sample has been selected to span a wide range in galaxy mass, and the deep VLT-FORS2 spectroscopy allows us to explore the stellar populations all the way to the outer disk-dominated regions of these galaxies. We find that globally, in both bulges and disks, star formation ceased earliest in the most massive systems, as a further manifestation of downsizing. However, within many galaxies, we find an age gradient which indicates that star formation ended first in the outermost regions. Metallicity gradients, when detected, are always negative such that the galaxy centres are more metal-rich. This finding fits with a picture in which star formation continued in the central regions, with enriched material, after it had stopped in the outskirts. Age and metallicity gradients are correlated, suggesting that large differences in star formation history between the inner and outer parts of S0 galaxies yield large differences in their chemical enrichment. In agreement with previous results, we conclude that the radial variations in the stellar populations of S0 galaxies are compatible with the hypothesis that these galaxies are the descendants of spiral galaxies whose star formation has ceased. With the addition of radial gradient information, we are able to show that this shutdown of star formation occurred from the outside inward, with the later star formation in the central regions offering a plausible mechanism for enhancing the bulge light in these systems, as the transformation to more bulge-dominated S0 galaxies requires.Comment: 9 pages, 4 figures and Appendix, accepted for publication in MNRA

Stellar populations of early-type galaxies in different environments III. Line-strength gradients

We present line-strength gradients for 22 spectral indices measured in a sample of 82 early-type galaxies in different environments,including the high-density core of the Coma cluster, the Virgo cluster,poor groups,and field galaxies. We derive age and metallicity gradients, and compare the mean values with the predictions of different galaxy formation models. We explore the behaviour of individual chemical species by deriving the metallicity gradient with different indicators.We find that the strength of the metallicity gradient inferred from stellar population models depends on the specific Lick index employed. In particular, metallicity gradients obtained with CN2 and C4668 combined with Hb are steeper than when measured using Ca4227 or Fe4383. The correlation of the metallicity gradients with other parameters also depends on the specific index employed. If the metallicity gradient is obtained using CN2 and Mgb then it correlates with the central age of the galaxies. On the contrary, if Fe4383 or Ca4227 are used, the metallicity gradient correlates with the velocity dispersion gradient.This may suggests that several mechanism have helped to set the age and metallicity gradients in early-type galaxies. While we do not find any correlation between the metallicity gradient and the central velocity dispersion for galaxies in low-density environments, we find a marginal correlation between the metallicity gradient and the mass for galaxies in the centre of the Coma cluster. We also find a trend for which galaxies in denser environments show a steeper metallicity gradient than galaxies in less dense environments.We interpret these results in light of the different models to explain the differences between galaxies as a function of environment

Medium-resolution Isaac Newton Telescope library of empirical spectra

A new stellar library developed for stellar population synthesis modelling is presented. The library consists of 985 stars spanning a large range in atmospheric parameters. The spectra were obtained at the 2.5-m Isaac Newton Telescope and cover the range λλ 3525-7500 Å at 2.3 Å (full width at half-maximum) spectral resolution. The spectral resolution, spectral-type coverage, flux-calibration accuracy and number of stars represent a substantial improvement over previous libraries used in population-synthesis model

Stellar velocity dispersion of Luminous Compact Galaxies at intermediate redshift

We present the stellar velocity dispersion measurements for 5 Luminous Compact Galaxies (LCGs) at z=0.5-0.7. These galaxies are vigorously forming stars with average SFR $\sim$ 40 M$_{\odot}$/yr. We find that their velocity dispersions range from $\sim137\ \rm{km/s}$ to $260\ \rm{km/s}$, while their stellar masses range between $4\times 10^{9}$ and $10^{11}$ M$_{\odot}$. If these LCGs evolve passively after this major burst of star formation, their masses and velocity dispersions, as well as their evolved colours and luminosities are most consistent with the values characteristic of early-type spiral galaxies today.Comment: 6 pages, 3 figure

Dynamic model of wind energy conversion systems with variable speed synchronous generator and full-size power converter for large-scale power system stability studies

Se inscribe dentro de la línea de investigación de Integración en red de sistemas eólicos. Se presenta un modelo informático en plataforma PSS/E para sistemas de generación eólica de velocidad variable del tipo Síncrono con Convertidor de plena potencia, capaz de simular el comportamuiento dinámico de estos sistemas en estudios de estabilidad del sistema eléctric

Processing and mechanical properties of novel biodegradable poly-lactic acid/Zn 3D printed scaffolds for application in tissue regeneration

The feasibility to manufacture scaffolds of poly-lactic acid reinforced with Zn particles by fused filament fabrication is demonstrated for the first time. Filaments of 2.85 mm in diameter of PLA reinforced with different weight fractions of $\mu$m-sized Zn - 1 wt. \% Mg alloy particles (in the range 3.5 to 17.5 wt. \%) were manufactured by a double extrusion in method in which standard extrusion is followed by a precision extrusion in a filament-maker machine. Filaments with constant diameter, negligible porosity and a homogeneous reinforcement distribution were obtained for Zn weight fractions of up to 10.5\%. It was found that the presence of Zn particles led to limited changes in the physico-chemical properties of the PLA that did not affect the window temperature for 3D printing nor the melt flow index. Thus, porous scaffolds could be manufactured by fused filament fabrication at 190\textdegree C with poly-lactic acid/Zn composites containing 3.5 and 7 wt. \% of Zn and at 170\textdegree C when the Zn content was 10.5 wt. \% with excellent dimensional accuracy and mechanical properties

Insights on the stellar mass-metallicity relation from the CALIFA survey

We use spatially and temporally resolved maps of stellar population properties of 300 galaxies from the CALIFA integral field survey to investigate how the stellar metallicity (Z*) relates to the total stellar mass (M*) and the local mass surface density ($\mu$*) in both spheroidal and disk dominated galaxies. The galaxies are shown to follow a clear stellar mass-metallicity relation (MZR) over the whole 10$^9$ to 10$^{12}$ M$_{\odot}$ range. This relation is steeper than the one derived from nebular abundances, which is similar to the flatter stellar MZR derived when we consider only young stars. We also find a strong relation between the local values of $\mu$* and Z* (the $\mu$ZR), betraying the influence of local factors in determining Z*. This shows that both local ($\mu$*-driven) and global (M*-driven) processes are important in determining the metallicity in galaxies. We find that the overall balance between local and global effects varies with the location within a galaxy. In disks, $\mu$* regulates Z*, producing a strong $\mu$ZR whose amplitude is modulated by M*. In spheroids it is M* who dominates the physics of star formation and chemical enrichment, with $\mu$* playing a minor, secondary role. These findings agree with our previous analysis of the star formation histories of CALIFA galaxies, which showed that mean stellar ages are mainly governed by surface density in galaxy disks and by total mass in spheroids.Comment: 6 pages, 3 figures, accepted for publication in ApJ