E. Ricciardelli

Systematic variation of the stellar initial mass function with velocity dispersion in early-type galaxies

An essential component of galaxy formation theory is the stellarinitial mass function (IMF) that describes the parent distribution ofstellar mass in star-forming regions. We present observational evidence in a sample of early-type galaxies (ETGs) of a tight correlation between central velocity dispersion and the strength of several absorption features sensitive to the presence of low-mass stars. Our samplecomprises ~40 000 ETGs from the Spheroids Panchromatic Investigation in Different Environmental Regions survey (z ∼ 0.1). The data-extracted from the Sloan Digital Sky Survey-are combined, rejecting both noisy data, and spectra with contamination from telluric lines, resulting in a set of 18 stacked spectra at high signal-to-noise ratio (S/N ≳ 400a-1). A combined analysis of IMF-sensitive line strengths and spectral fitting is performed with the latest state-of-the-art population synthesis models (an extended version of the MILES models). Asignificant trend is found between IMF slope and velocity dispersion, towards an excess of low-mass stars in the most massive galaxies. Although we emphasize that accurate values of the IMF slope will requirea detailed analysis of chemical composition (such as [α/Fe] or even individual element abundance ratios), the observed trends suggest thatlow-mass ETGs are better fitted by a Kroupa-like IMF, whereas massivegalaxies require bottom-heavy IMFs, exceeding the Salpeter slope at σ ≳ 200 km s-1.

SPIDER VIII – constraints on the stellar initial mass function of early-type galaxies from a variety of spectral features

We perform a spectroscopic study to constrain the stellar initial mass function (IMF) by using a large sample of 24 781 early-type galaxies from the Sloan Digital Sky Survey-based Spheroids Panchromatic Investigation in Different Environmental Regions survey. Clear evidence is found of a trend between IMF and central velocity dispersion (σ0), evolving from a standard Kroupa/Chabrier IMF at σ0 ~ 100 km s-1 towards a more bottom-heavy IMF with increasing σ0, becoming steeper than the Salpeter function at σ0 > 220 km s-1. We analyse a variety of spectral indices, combining gravity-sensitive features, with age- and metallicitysensitive indices, and we also consider the effect of non-solar abundance variations. The indices, corrected to solar scale by means of semi-empirical correlations, are fitted simultaneously with the (nearly solar-scaled) extended MILES (MIUSCAT) stellar population models. Similar conclusions are reached when analysing the spectra with a hybrid approach, combining constraints from direct spectral fitting in the optical with those from IMF-sensitive indices. Our analysis suggests that σ0, rather than [a/Fe], drives the variation of the IMF. Although our analysis cannot discriminate between a single power-law (unimodal) IMF and a low-mass (<0.5Mȯ) tapered (bimodal) IMF, robust constraints can be inferred for the fraction in lowmass stars at birth. This fraction (by mass) is found to increase from ~20 per cent at σ0 ~ 100 km s-1, up to ~80 per cent at σ0 ~ 300 km s-1. However, additional constraints can be provided with stellar mass-to-light (M/L) ratios: unimodal models predict M/L significantly larger than dynamical M/L, across the whole σ0 range, whereas a bimodal IMF is compatible. Our results are robust against individual abundance variations. No significant variation is found in Na and Ca in addition to the expected change from the correlation between [s-1/Fe] and σ0.

Evolutionary stellar population synthesis with MILES – II. Scaled-solar and α-enhanced models

We are indebted to the Padova group for making their isochrone calculations available to us. We are grateful to A. de C. Milone for providing us with his [Mg/Fe] estimates for the stars of the MILES library before publication. We also are grateful to H.-S. Kim, J. Cho, R. M. Sharples and S.-J. Yoon, who provided us with GGC line-strength measurements prior to publication. We are grateful to B. Barbuy, J. L. Cervantes, I. G. de la Rosa, M. Koleva, R. Peletier and A. Sansom for very useful discussions. We thank the referee H.-C. Lee for a careful reading of the manuscript and for very valuable clarifications that helped us to improve the manuscript. We also would like to thank J. A. Perez Prieto for helping us in the construction of the web page for the models. The MILES library was observed at the INT on the island of La Palma, operated by the Isaac Newton Group at the Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. This research has made an extensive use of the SIMBAD data base and VizieR catalogue access tool (both operated at CDS, Strasbourg, France), the NASAs Astrophysics Data System Article Service. We made use of data retrieved from the Sloan Digital Sky Survey archives ( http://www.sdss.org ). Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society and the Higher Education Funding Council for England. PC acknowledges the financial support by FAPESP via project 2008/58406-4 and fellowship 2009/09465-0. JFB acknowledges support from the Ramon y Cajal programme by the Spanish Ministry of Economy and Competitiveness (MINECO). This work has been supported by the Programa Nacional de Astronomia y Astrofisica of MINECO, under grants AYA2013-48226-C3-1-P, AYA2013-48226-C3-2-P, AYA2013-48226-C3-3-P and by the Generalitat Valenciana under grant PROMETEOII2014-069.

MIUSCAT: extended MILES spectral coverage – II. Constraints from optical photometry

In the present work we show a comprehensive comparison of our new stellar population synthesis MIUSCAT models with photometric data of globular clusters and early-type galaxies. The models compare remarkably well with the colours of Milky Way globular clusters in the optical range. Likewise, the colours of M31 globular clusters can also be explained by the models by assuming younger ages then their Galactic counterparts. When compared with quiescent galaxies we reproduce the colour evolution at intermediate redshift. On the other hand we find that the colour relations of nearby early-type galaxies are still a challenge for present-day stellar population synthesis models. We investigate a number of possible explanations and establish the importance of alpha-enhanced models to bring down the discrepancy with observations.

On the universality of void density profiles

The massive exploitation of cosmic voids for precision cosmology in the upcoming dark energy experiments, requires a robust understanding of their internal structure, particularly of their density profile. We show that the void density profile is insensitive to the void radius both in a catalogue of observed voids and in voids from a large cosmological simulation. However, the observed and simulated voids display remarkably different profile shapes, with the former having much steeper profiles than the latter. Sparsity can not be the main reason for this discrepancy, as we demonstrate that the profile can be recovered with reasonable accuracy even with very sparse samples of tracers. On the other hand, the observed profile shows a significant dependence on the galaxy sample used to trace the matter distribution. Samples including low-mass galaxies lead to shallower profiles with respect to the samples where only massive galaxies are used, as faint galaxies live closer to the void centre. We argue that galaxies are biased tracers when used to probe the matter distribution within voids.

Fully cosmological virtual massive galaxies at z = 0: kinematical, morphological and stellar population characterization

We present the results of a numerical adaptive mesh refinement hydrodynamical and N-body simulation in a

The structure of cosmic voids in a ΛCDM Universe

\Lambda CDM

The star formation activity in cosmic voids

cosmology. We focus on the analysis of the main properties of massive galaxies (

Large-scale jets from active galactic nuclei as a source of intracluster medium heating: cavities and shocks

M_* > 10^{11}\,M_{\odot}

Shards: a global view of the star formation activity at z ~ 0.84 and z ~ 1.23

) at