Search

We quantify the stellar rotation of galaxies by computing the $\lambda_{R}$ parameter, a proxy for the stellar angular momentum in a sample of 106 galaxies with redshift 0.1 $<$ z $<$ 0.8 and stellar masses from $\sim$10$^{7.5}$ to 10$^{11.8}$ M$_{\odot}$. The sample is located in the CANDELS/GOODS-S and COSMOS fields, and it was observed by various MUSE surveys. We create stellar velocity and velocity dispersion maps using a full-spectrum fitting technique, covering spatially $\sim$2$R_{e}$ for the galaxies. We study the impact of the atmospheric seeing on the spin parameter and apply corrections when pertinent. Through the analysis of the $\lambda_{R}-\epsilon$ diagram, we notice that the fraction of round and massive galaxies increases with redshift. We lack galaxies with $\lambda_{R}$ < 0.1 in the sample and we find only one potential, but uncertain, low-mass slow rotator at z $\sim0.3$. Moreover, we do not see an evident evolution or trend in the stellar angular momentum with redshift. We characterize the sample environment using two indicators: a local estimator based on the Voronoi tesselation method, and a global estimator derived by the use of the Friends-of-Friends algorithm. We find no correlation between the environment and $\lambda_{R}$ given that we are not probing dense regions or massive galaxy structures. We also analyze the kinematic maps of the sample finding that about 40$\%$ of galaxies are consistent with being regular rotators, having rotating stellar discs with flat velocity dispersion maps, while $\sim20\%$ of galaxies have complex velocity maps and can be identified as non-regular rotators in spite of their $\lambda_{R}$ values. For the remaining galaxies the classification is uncertain. As we lack galaxies with $\lambda_{R}$< 0.1, we are not able to identify when galaxies become slow rotators within the surveyed environments, area and redshift range., Comment: 26 pages, 18 figures, full abstract on the paper

We introduce the MUSE gAlaxy Groups in COSMOS (MAGIC) survey, which was built to study the impact of environment on galaxy evolution over the last 8 Gyr. It consists of 17 MUSE fields targeting 14 massive structures at intermediate redshift ($0.321.5$. The spectroscopic redshift completeness is high: in the redshift range of [OII] emitters ($0.25 \le z < 1.5$), where most of the groups are found, it globally reaches a maximum of 80% down to $z_{app}^{++}=25.9$, and locally decreases from $\sim 100$% to $\sim50$% in magnitude bins from $z_{app}^{++}=23-24$ to $z_{app}^{++}=25.5$. We find that the fraction of quiescent galaxies increases with local density and with the time spent in groups. A morphological dichotomy is also found between bulge-dominated quiescent and disk-dominated star-forming galaxies. As environment gets denser, the peak of the stellar mass distribution shifts towards $M_*>10^{10}~M_\odot$, and the fraction of galaxies with $M_*<10^9~M_\odot$ decreases significantly, even for star-forming galaxies. We also highlight peculiar features such as close groups, extended nebulae, and a gravitational arc. Our results suggest that galaxies are preprocessed in groups of increasing mass before entering rich groups and clusters. We publicly release two catalogs containing the properties of galaxies and groups, respectively., Comment: 27 pages, 22 figures, accepted for publication in A&A

The evolution of galaxies is influenced by many physical processes which may vary depending on their environment. We combine Hubble Space Telescope (HST) and Multi-Unit Spectroscopic Explorer (MUSE) data of galaxies at 0.25

The neutral intergalactic medium above redshift 6 is opaque to ionizing radiation, therefore one needs indirect measurements of the escape fraction of ionizing photons from galaxies of this epoch. Low-ionization state absorption lines are a common feature in the spectrum of galaxies, showing a diversity of strengths and shapes. Since these lines indicate the presence of neutral gas in front of the stars, they have been proposed to carry information on the escape of ionizing radiation from galaxies. We study which processes are responsible for the shape of the absorption lines, to better understand their origin. We then explore whether the absorption lines can be used to predict the escape fractions. Using a radiation-hydrodynamical zoom-in simulation and the radiative transfer code RASCAS, we generate mock CII 1334 and LyB lines of a virtual galaxy at redshift 3 as seen from many directions of observation. We also compute the escape fraction of ionizing photons in those directions and look for correlations between the lines and the escape fractions. We find that the resulting mock absorption lines are comparable to observations and that the lines and the escape fractions vary strongly depending on the direction of observation. Gas velocity and dust always affect the absorption profile significantly. We find no strong correlations between observable LyB or CII 1334 and the escape fraction. After correcting the continuum for attenuation by dust to recover the intrinsic continuum, the residual flux of CII 1334 correlates well with the escape fraction for directions with a dust corrected residual flux larger than 30%. For other directions, the relations have a strong dispersion, and the residual flux overestimates the escape fraction for most cases. Concerning LyB, the residual flux after dust correction does not correlate with the escape fraction but can be used as a lower limit. (abridged), Comment: 25 pages, 22 figures, accepted for publication in Astronomy and Astrophysics. New title, shorter abstract, corrected typos and url for Krome website

Several deep observations such as those carried out at the Canada-France-Hawaii Telescope (CFHT) have revealed prominent Low Surface Brightness (LSB) fine structures that change the apparent morphology of galaxies. Previous photometry surveys have developed observational techniques which exploit the diffuse light detected in the external regions of galaxies. In these studies the outer perturbations have been identified and classified like tidal tails, stellar streams, and shells. These structures are tracers of interacting and merging events and they keep a memory of the mass assembly of galaxies. Cosmological numerical simulations are needed to estimate their visibility time-scale (among other properties) in order to reconstruct the past merger history of galaxies. In the present work, we analyze a hydrodynamical cosmological simulation to build a comprehensive interpretation of the properties of fine structures. We make a census of several types of LSB fine structures by visual inspection of individual snapshots at various time. We reconstruct the evolution of the number of fine structures detected around an early-type galaxy and we compare with the merger history of the galaxy. We find that most of fine structures are associated with major and intermediate mass merger events. Their survival time scale ranges between 0.7 and 4 Gyr. Shells and streams remain visible for a longer time than tidal tails. These estimates of survival times provide clues to interpret the shape and frequency of fine structures observed in deep images in terms of mass assembly. We find that the detectability of stellar streams is the most sensitive to the surface brightness limit. We see 2-3 times more streams with a surface brightness cut of 33 mag arcsec$^{-2}$ than with 29 mag arcsec$^{-2}$. The detection of shells display a strong dependence on the projection angle., Comment: 16 pages, 14 figures, accepted in A&A (v2 after language editing)

We present a study of the [OIII]5007/[OII]3727 (O32) ratios of star-forming galaxies drawn from MUSE data spanning a redshift range 0.284. Here we aim to understand the properties and the occurrences of galaxies with such high line ratios. Combining data from several MUSE GTO programmes, we select a population of star-forming galaxies with bright emission lines, from which we draw 406 galaxies for our analysis based on their position in the z-dependent star formation rate (SFR) - stellar mass (M*) plane. Out of this sample 15 are identified as extreme oxygen emitters based on their O32 ratios (3.7%) and 104 galaxies have O32>1 (26%). Our analysis shows no significant correlation between M*, SFR, and the distance from the SFR-M* relation with O32. We find a decrease in the fraction of galaxies with O32>1 with increasing M*, however, this is most likely a result of the relationship between O32 and metallicity, rather than between O32 and M*. We draw a comparison sample of local analogues with ~0.03 from SDSS, and find similar incidence rates for this sample. In order to investigate the evolution in the fraction of high O32 emitters with redshift, we bin the sample into three redshift subsamples of equal number, but find no evidence for a dependence on redshift. Furthermore, we compare the observed line ratios with those predicted by nebular models with no LyC escape and find that most of the extreme oxygen emitters can be reproduced by low metallicity models. The remaining galaxies are likely LyC emitter candidates. Finally, based on a comparison between electron temperature estimates from the [OIII4363]/[OIII]5007 ratio of the extreme oxygen emitters and nebular models, we argue that the galaxies with the most extreme O32 ratios have young light-weighted ages., Comment: 17 pages, 23 figures, accepted for publication in A&A

We provide, for the first time, robust observational constraints on the galaxy major merger fraction up to $z\approx 6$ using spectroscopic close pair counts. Deep Multi Unit Spectroscopic Explorer (MUSE) observations in the Hubble Ultra Deep Field (HUDF) and Hubble Deep Field South (HDF-S) are used to identify 113 secure close pairs of galaxies among a parent sample of 1801 galaxies spread over a large redshift range ($0.2

We have conducted a two-layered spectroscopic survey (1'x1' ultra deep and 3'x3' deep regions) in the Hubble Ultra Deep Field (HUDF) with the Multi Unit Spectroscopic Explorer (MUSE). The combination of a large field of view, high sensitivity, and wide wavelength coverage provides an order of magnitude improvement in spectroscopically confirmed redshifts in the HUDF; i.e., 1206 secure spectroscopic redshifts for HST continuum selected objects, which corresponds to 15% of the total (7904). The redshift distribution extends well beyond z>3 and to HST/F775W magnitudes as faint as ~30 mag (AB, 1-sigma). In addition, 132 secure redshifts were obtained for sources with no HST counterparts that were discovered in the MUSE data cubes by a blind search for emission-line features. In total, we present 1338 high quality redshifts, which is a factor of eight increase compared with the previously known spectroscopic redshifts in the same field. We assessed redshifts mainly with the spectral features [OII] at z<1.5 (473 objects) and Lya at 2.9~20% up to 28-29 mag and ~27 mag, respectively. We used the determined redshifts to test continuum color selection (dropout) diagrams of high-z galaxies. The selection condition for F336W dropouts successfully captures ~80% of the targeted z~2.7 galaxies. However, for higher redshift selections (F435W, F606W, and F775W dropouts), the success rates decrease to ~20-40%. We empirically redefine the selection boundaries to make an attempt to improve them to ~60%. The revised boundaries allow bluer colors that capture Lya emitters with high Lya equivalent widths falling in the broadbands used for the color-color selection. Along with this paper, we release the redshift and line flux catalog., Comment: 26 pages, 26 figures, 4 tables, accepted for publication in A&A (MUSE UDF Series Paper II), the redshift catalogs will be available at the CDS

(Abridged) We make use of the deepest VLT/MUSE observations performed so far on the Hubble Deep Field South (HDFS) to characterize the low-mass (< $10^{10}$M$_\odot$) galaxy population at intermediate redshift. We identify a sample of 28 spatially-resolved emission-line galaxies in the deep (27h integration time) MUSE data cube, spread over a redshift interval of 0.2 < z < 1.4. The public HST images and multi-band photometry over the HDFS are used to constrain the stellar mass and star formation rate (SFR) of the galaxies and to perform a morphological analysis. We derive the resolved ionized gas properties of these galaxies from the MUSE data and model the disk (both in 2D and with GalPaK$^{\rm 3D}$) to retrieve their intrinsic gas kinematics. We build a sample of resolved emission-line galaxies of much lower stellar mass and SFR (by $\sim$1-2 orders of magnitude) than previous 3D spectroscopic surveys. Most of the spatially-resolved MUSE-HDFS galaxies have gas kinematics consistent with disk-like rotation, but about 20% have velocity dispersions larger than the rotation velocities, and 30% are part of a close pair and/or show clear signs of recent gravitational interactions. In the high-mass regime, the MUSE-HDFS galaxies follow the Tully-Fisher relation defined from previous surveys in a similar redshift range. This scaling relation extends also to lower masses/velocities but with a higher dispersion. The MUSE-HDFS galaxies follow the scaling relations defined in the local universe between the specific angular momentum and the stellar mass. However, we find that intermediate-redshift star-forming galaxies fill a continuum transition from the spiral to elliptical local scaling relations, according to the dynamical state of the gas. This indicates that some galaxies may lose their angular momentum and become dispersion-dominated prior to becoming passive., Comment: 26 pages. Published in A&A

We explore the properties of high-redshift Lyman-alpha emitters (LAE), and their link with the Lyman-Break galaxy population (LBG), using a semi-analytic model of galaxy formation that takes into account resonant scattering of Lya photons in gas outflows. We can reasonably reproduce the abundances of LAEs and LBGs from redshift 3 to 7, as well as most UV LFs of LAEs. The stronger dust attenuation for (resonant) Lya photons compared to UV continuum photons in bright LBGs provides a natural interpretation to the increase of the LAE fraction in LBG samples, X_LAE, towards fainter magnitudes. The redshift evolution of X_LAE seems however very sensitive to UV magnitudes limits and EW cuts. In spite of the apparent good match between the statistical properties predicted by the model and the observations, we find that the tail of the Lya equivalent width distribution (EW > 100 A) cannot be explained by our model, and we need to invoke additional mechanisms. We find that LAEs and LBGs span a very similar dynamical range, but bright LAEs are about 4 times rarer than LBGs in massive halos. Moreover, massive halos mainly contain weak LAEs in our model, which might introduce a bias towards low-mass halos in surveys which select sources with high EW cuts. Overall, our results are consistent with the idea that LAEs and LBGs make a very similar galaxy population. Their apparent differences seem mainly due to EW selections, UV detection limits, and a decreasing Lya-to-UV escape fraction ratio in high SFR galaxies., Comment: accepted in MNRAS

We present a method to constrain galaxy parameters directly from three-dimensional data cubes. The algorithm compares directly the data with a parametric model mapped in $x,y,\lambda$ coordinates. It uses the spectral lines-spread function (LSF) and the spatial point-spread function (PSF) to generate a three-dimensional kernel whose characteristics are instrument specific or user generated. The algorithm returns the intrinsic modeled properties along with both an `intrinsic' model data cube and the modeled galaxy convolved with the 3D-kernel. The algorithm uses a Markov Chain Monte Carlo (MCMC) approach with a nontraditional proposal distribution in order to efficiently probe the parameter space. We demonstrate the robustness of the algorithm using 1728 mock galaxies and galaxies generated from hydrodynamical simulations in various seeing conditions from 0.6" to 1.2". We find that the algorithm can recover the morphological parameters (inclination, position angle) to within 10% and the kinematic parameters (maximum rotation velocity) to within 20%, irrespectively of the PSF in seeing (up to 1.2") provided that the maximum signal-to-noise ratio (SNR) is greater than $\sim3$ pixel$^{-1}$ and that the ratio of the galaxy half-light radius to seeing radius is greater than about 1.5. One can use such an algorithm to constrain simultaneously the kinematics and morphological parameters of (nonmerging) galaxies observed in nonoptimal seeing conditions. The algorithm can also be used on adaptive-optics (AO) data or on high-quality, high-SNR data to look for nonaxisymmetric structures in the residuals., Comment: 16 pages, 10 figures, accepted to publication in AJ, revised version after proofs corrections. Algorithm available at http://galpak.irap.omp.eu

We observed the Hubble Deep Field South with the new panoramic integral field spectrograph MUSE that we built and just commissioned at the VLT. The data cube resulting from 27 hours of integration covers one arcmin^2 field of view at an unprecedented depth with a 1 sigma emission line surface brightness limit of 1x$10^{-19}$ erg/s/cm$^2$/arcsec$^2$ and contains ~90,000 spectra. We present the combined and calibrated data cube, and we perform a first-pass analysis of the sources detected in the HDF-S imaging. We measured the redshifts of 189 sources up to a magnitude F814W = 29.5, increasing by more than an order of magnitude the number of known spectroscopic redshifts in this field. We also discovered 26 Lya emitting galaxies which are not detected in the HST WFPC2 deep broad band images. The intermediate spectral resolution of 2.3{\AA} allows us to separate resolved asymmetric Lya emitters, [O II] emitters, and C III] emitters and the large instantaneous wavelength range of 4500{\AA} helps to identify single emission lines. We also show how the three dimensional information of MUSE helps to resolve sources which are confused at ground-based image quality. Overall, secure identifications are provided for 83% of the 227 emission line sources detected in the MUSE data cube and for 32% of the 586 sources identified in the HST catalog of Casertano et al 2000. The overall redshift distribution is fairly flat to z=6.3, with a reduction between z=1.5 to 2.9, in the well-known redshift desert. The field of view of MUSE also allowed us to detect 17 groups within the field. We checked that the number counts of [O II] and Ly-a emitters are roughly consistent with predictions from the literature. Using two examples we demonstrate that MUSE is able to provide exquisite spatially resolved spectroscopic information on intermediate redshift galaxies present in the field., Comment: 21 pages, 20 figures. Note that the catalogues, data cube and associated spectra will be released upon acceptance of the paper

The relation between the stellar mass and the star formation rate characterizes how the instantaneous star formation is determined by the galaxy past star formation history and by the growth of the dark matter structures. We deconstruct the M-SFR plane by measuring the specific SFR functions in several stellar mass bins from z=0.2 out to z=1.4. Our analysis is primary based on a MIPS 24$\mu m$ selected catalogue combining the COSMOS and GOODS surveys. We estimate the SFR by combining mid- and far-infrared data for 20500 galaxies. The sSFR functions are derived in four stellar mass bins within the range 9.5

We trace the formation and advection of several elements within a cosmological adaptive mesh refinement simulation of an L* galaxy. We use nine realisations of the same initial conditions with different stellar Initial Mass Functions (IMFs), mass limits for type-II and type-Ia supernovae (SNII, SNIa) and stellar lifetimes to constrain these sub-grid phenomena. Our code includes self-gravity, hydrodynamics, star formation, radiative cooling and feedback from multiple sources within a cosmological framework. Under our assumptions of nucleosynthesis we find that SNII with progenitor masses of up to 100 Msun are required to match low metallicity gas oxygen abundances. Tardy SNIa are necessary to reproduce the classical chemical evolution knee in [O/Fe]-[Fe/H]: more prompt SNIa delayed time distributions do not reproduce this feature. Within our framework of hydrodynamical mixing of metals and galaxy mergers we find that chemical evolution is sensitive to the shape of the IMF and that there exists a degeneracy with the mass range of SNII. We look at the abundance plane and present the properties of different regions of the plot, noting the distinct chemical properties of satellites and a series of nested discs that have greater velocity dispersions, are more alpha-rich and metal poor with age., Comment: 20 pages, 9 figures, accepted for publication in MNRAS

We present a set of hydrodynamical/Nbody controlled simulations of isolated gas rich galaxies that self-consistently include SN feedback and a detailed chemical evolution model, both tested in cosmological simulations. The initial conditions are motivated by the observed star forming galaxies at z ~ 2-3. We find that the presence of a multiphase interstellar media in our models promotes the growth of disc instability favouring the formation of clumps which in general, are not easily disrupted on timescales compared to the migration time. We show that stellar clumps migrate towards the central region and contribute to form a classical-like bulge with a Sersic index, n > 2. Our physically-motivated Supernova feedback has a mild influence on clump survival and evolution, partially limiting the mass growth of clumps as the energy released per Supernova event is increased, with the consequent flattening of the bulge profile. This regulation does not prevent the building of a classical-like bulge even for the most energetic feedback tested. Our Supernova feedback model is able to establish a self-regulated star formation, producing mass-loaded outflows and stellar age spreads comparable to observations. We find that the bulge formation by clumps may coexit with other channels of bulge assembly such as bar and mergers. Our results suggest that galactic bulges could be interpreted as composite systems with structural components and stellar populations storing archaeological information of the dynamical history of their galaxy., Comment: Accepted for publication in MNRAS - Aug. 20, 2013

We test the validity of comparing simulated field disk galaxies with the empirical properties of systems situated within environments more comparable to loose groups, including the Milky Way's Local Group. Cosmological simulations of Milky Way-mass galaxies have been realised in two different environment samples: in the field and in environments with similar properties to the Local Group. Apart from the environments of the galaxies, the samples are kept as homogeneous as possible with equivalent ranges in last major merger time, halo mass and halo spin. Comparison of these two samples allow for systematic differences in the simulations to be identified. Metallicity gradients, disk scale lengths, colours, magnitudes and age-velocity dispersion relations are studied for each galaxy in the suite and the strength of the link between these and environment of the galaxies is studied. The bulge-to-disk ratio of the galaxies show that these galaxies are less spheroid dominated than many other simulated galaxies in literature with the majority of both samples being disk dominated. We find that secular evolution and mergers dominate the spread of morphologies and metallicity gradients with no visible differences between the two environment samples. In contrast with this consistency in the two samples there is tentative evidence for a systematic difference in the velocity dispersion-age relations of galaxies in the different environments. Loose group galaxies appear to have more discrete steps in their velocity dispersion-age relations. We conclude that at the current resolution of cosmological galaxy simulations field environment galaxies are sufficiently similar to those in loose groups to be acceptable proxies for comparison with the Milky Way provided that a similar assembly history is considered., Comment: 16 pages, 11 figures, abstract abridged for arXiv. Accepted for publication in Astronomy & Astrophysics

We examine the chemical properties of 5 cosmological hydrodynamical simulations of an M33-like disc galaxy which have been shown to be consistent with the morphological characteristics and bulk scaling relations expected of late-type spirals. These simulations are part of the Making Galaxies In a Cosmological Context (MaGICC) Project, in which stellar feedback is tuned to match the stellar mass -- halo mass relationship. Each realisation employed identical initial conditions and assembly histories, but differed from one another in their underlying baryonic physics prescriptions, including (a) the efficiency with which each supernova energy couples to the ISM, (b) the impact of feedback associated with massive star radiation pressure, (c) the role of the minimum shut-off time for radiative cooling of Type II SNe remnants, (d) the treatment of metal diffusion, and (e) varying the IMF. Our analysis focusses on the resulting stellar metallicity distribution functions (MDFs) in each simulated (analogous) `solar neighbourhood' and central `bulge' region. We compare the simulated MDFs' skewness, kurtosis, and dispersion (inter-quartile, inter-decile, inter-centile, and inter-tenth-percentile regions) with that of the empirical solar neighbourhood MDF and Local Group dwarfs. We find that the MDFs of the simulated discs are more negatively skewed, with higher kurtosis, than those observed locally. We can trace this difference to the simulations' tight and correlated age-metallicity relations (compared with that of the Milky Way), suggesting that these relations within `dwarf' discs might be steeper than in L* discs and/or the degree of stellar orbital re-distribution and migration inferred locally has not been captured in their entirety, at the resolution of our simulations. The important role of metal diffusion in ameliorating the over-production of extremely metal-poor stars is highlighted., Comment: Accepted for publication in MNRAS; 12 pages; 7 figures

By means of high-resolution cosmological hydrodynamical simulations of Milky Way-like disc galaxies, we conduct an analysis of the associated stellar metallicity distribution functions (MDFs). After undertaking a kinematic decomposition of each simulation into spheroid and disc sub-components, we compare the predicted MDFs to those observed in the solar neighbourhood and the Galactic bulge. The effects of the star formation density threshold are visible in the star formation histories, which show a modulation in their behaviour driven by the threshold. The derived MDFs show median metallicities lower by 0.2-0.3 dex than the MDF observed locally in the disc and in the Galactic bulge. Possible reasons for this apparent discrepancy include the use of low stellar yields and/or centrally-concentrated star formation. The dispersions are larger than the one of the observed MDF; this could be due to simulated discs being kinematically hotter relative to the Milky Way. The fraction of low metallicity stars is largely overestimated, visible from the more negatively skewed MDF with respect to the observational sample. For our fiducial Milky Way analog, we study the metallicity distribution of the stars born "in situ" relative to those formed via accretion (from disrupted satellites), and demonstrate that this low-metallicity tail to the MDF is populated primarily by accreted stars. Enhanced supernova and stellar radiation energy feedback to the surrounding interstellar media of these pre-disrupted satellites is suggested as an important regulator of the MDF skewness., Comment: 20 pages, 14 figures, MNRAS, accepted

We examine radial and vertical metallicity gradients using a suite of disk galaxy simulations, supplemented with two classic chemical evolution approaches. We determine the rate of change of gradient and reconcile differences between extant models and observations within the `inside-out' disk growth paradigm. A sample of 25 disks is used, consisting of 19 from our RaDES (Ramses Disk Environment Study) sample, realised with the adaptive mesh refinement code RAMSES. Four disks are selected from the MUGS (McMaster Unbiased Galaxy Simulations) sample, generated with the smoothed particle hydrodynamics (SPH) code GASOLINE, alongside disks from Rahimi et al. (GCD+) and Kobayashi & Nakasato (GRAPE-SPH). Two chemical evolution models of inside-out disk growth were employed to contrast the temporal evolution of their radial gradients with those of the simulations. We find that systematic differences exist between the predicted evolution of radial abundance gradients in the RaDES and chemical evolution models, compared with the MUGS sample; specifically, the MUGS simulations are systematically steeper at high-redshift, and present much more rapid evolution in their gradients. We find that the majority of the models predict radial gradients today which are consistent with those observed in late-type disks, but they evolve to this self-similarity in different fashions, despite each adhering to classical `inside-out' growth. We find that radial dependence of the efficiency with which stars form as a function of time drives the differences seen in the gradients; systematic differences in the sub-grid physics between the various codes are responsible for setting these gradients. Recent, albeit limited, data at redshift z=1.5 are consistent with the steeper gradients seen in our SPH sample, suggesting a modest revision of the classical chemical evolution models may be required., Comment: Accepted for publication in A&A; 12 pages; 5 figures (several typos corrected and author affiliations updated)

The mass assembly of galaxies leaves various imprints on their surroundings, such as shells, streams and tidal tails. The frequency and properties of these fine structures depend on the mechanism driving the mass assembly: e.g. a monolithic collapse, rapid cold-gas accretion followed by violent disk instabilities, minor mergers or major dry / wet mergers. Therefore, by studying the outskirts of galaxies, one can learn about their main formation mechanism. I present here our on-going work to characterize the outskirts of Early-Type Galaxies (ETGs), which are powerful probes at low redshift of the hierarchical mass assembly of galaxies. This work relies on ultra-deep optical images obtained at CFHT with the wide-field of view MegaCam camera of field and cluster ETGs obtained as part of the Atlas-3D and NGVS projects. State of the art numerical simulations are used to interpret the data. The images reveal a wealth of unknown faint structures at levels as faint as 29 mag arcsec-2 in the g-band. Initial results for two galaxies are presented here., Comment: 4 pages, 3 figures, proceedings of the IAU Symposium 277, tracing the ancestry of galaxies

We focus on an analysis of galaxies of high stellar mass and low metallicity. We cross-correlated the Millenium Galaxy Catalogue (MGC) and the Sloan Digital Sky Survey (SDSS) galaxy catalogue to provide a sample of MGC objects with high resolution imaging and both spectroscopic and photometric information available in the SDSS database. For each galaxy in our sample, we conducted a systematic morphological analysis by visual inspection of MGC images using their luminosity contours. The galaxies are classified as either disturbed or undisturbed objects. We divide the sample into three metallicity regions, within wich we compare the properties of disturbed and undisturbed objects. We find that the fraction of galaxies that are strongly disturbed, indicative of being merger remnants, is higher when lower metallicity objects are considered. The three bins analysed consist of approximatively 15%, 20%, and 50% disturbed galaxies (for high, medium, and low metallicity, respectively). Moreover, the ratio of the disturbed to undisturbed relative distributions of the population age indicator, Dn(4000), in the low metallicity bin, indicates that the disturbed objects have substantially younger stellar populations than their undisturbed counterparts. In addition, we find that an analysis of colour distributions provides similar results, showing that low metallicity galaxies with a disturbed morphology are bluer than those that are undisturbed. The bluer colours and younger populations of the low metallicity, morphologically disturbed objects suggest that they have experienced a recent merger with an associated enhanced star formation rate. [abridged], Comment: Astronomy & Astrophysics, in press

We study the mass-metallicity relation of galaxies in pairs and in isolation taken from the SDSS-DR4 using the stellar masses and oxygen abundances derived by Tremonti et al. (2004). Close galaxy pairs, defined by projected separation r_p < 25kpc/h and radial velocity Delta_V < 350 km/s, are morphologically classified according to the strength of the interaction signs. We find that only for pairs showing signs of strong interactions, the mass-metallicity relation differs significantly from that of galaxies in isolation. In such pairs, the mean gas-phase oxygen abundances of galaxies with low stellar masses (Mstar ~< 10^9 Msun/h) exhibit an excess of 0.2 dex. Conversely, at larger masses (Mstar >~ 10^10 Msun/h) galaxies have a systematically lower metallicity, although with a smaller difference (-0.05 dex). Similar trends are obtained if g-band magnitudes are used instead of stellar masses. In minor interactions, we find that the less massive member is systematically enriched, while a galaxy in interaction with a comparable stellar mass companion shows a metallicity decrement with respect to galaxies in isolation. We argue that metal-rich starbursts triggered by a more massive component, and inflows of low metallicity gas induced by comparable or less massive companion galaxies, provide a natural scenario to explain our findings., Comment: Accepted for publication in MNRAS Letters

We report about the fact that the stellar population that is born in the gas inflowing towards the central regions can be vertically unstable leading to a B/PS feature remarkably bluer that the surrounding bulge. Using new chemodynamical simulations we show that this young population does not remain as flat as the gaseous nuclear disc and buckles out of the plane to form a new boxy bulge. We show that such a young B/PS bulge can be detected in colour maps., Comment: 2 pages, 5 figures, to appear in IAU Symposium 245, Formation and Evolution of Galaxy Bulges, M. Bureau, E. Athanassoula, and B. Barbuy (eds.), Oxford, 16-20 July 2007