Colour gradients of high-redshift Early-Type Galaxies from hydrodynamical monolithic models

We analyze the evolution of colour gradients predicted by the hydrodynamical models of early type galaxies (ETGs) in Pipino et al. (2008), which reproduce fairly well the chemical abundance pattern and the metallicity gradients of local ETGs. We convert the star formation (SF) and metal content into colours by means of stellar population synthetic model and investigate the role of different physical ingredients, as the initial gas distribution and content, and eps_SF, i.e. the normalization of SF rate. From the comparison with high redshift data, a full agreement with optical rest-frame observations at z < 1 is found, for models with low eps_SF, whereas some discrepancies emerge at 1 < z < 2, despite our models reproduce quite well the data scatter at these redshifts. To reconcile the prediction of these high eps_SF systems with the shallower colour gradients observed at lower z we suggest intervention of 1-2 dry mergers. We suggest that future studies should explore the impact of wet galaxy mergings, interactions with environment, dust content and a variation of the Initial Mass Function from the galactic centers to the peripheries.Comment: 13 pages, 7 figures, 1 table, accepted for publication on MNRA

The Effect of Stellar Rotation on Colour-Magnitude Diagrams: On the apparent presence of multiple populations in intermediate age stellar clusters

A significant number of intermediate age clusters (1-2 Gyr) in the Magellanic Clouds appear to have multiple stellar populations within them, derived from bi-modal or extended main sequence turn offs. If this is interpreted as an age spread, the multiple populations are separated by a few hundred Myr, which would call into question the long held notion that clusters are simple stellar populations. Here we show that stellar rotation in stars with masses between 1.2-1.7 Msun can mimic the effect of a double or multiple population, whereas in actuality only a single population exists. The two main causes of the spread near the turn-off are the effects of stellar rotation on the structure of the star and the inclination angle of the star relative to the observer. Both effects change the observed effective temperature, hence colour, and flux of the star. In order to match observations, the required rotation rates are 20-50% of the critical rotation, which are consistent with observed rotation rates of similar mass stars in the Galaxy. We provide scaling relations which can be applied to non-rotating isochrones in order to mimic the effects of rotation. Finally, we note that rotation is unlikely to be the cause of the multiple stellar populations observed in old globular clusters, as low mass stars (<1 Msun) are not expected to be rapid rotators.Comment: 5 pages, 1 figure, MNRAS letters, in pres

Constraining globular cluster formation through studies of young massive clusters - V. ALMA observations of clusters in the Antennae

Some formation scenarios that have been put forward to explain multiple populations within Globular Clusters (GCs) require that the young massive cluster have large reservoirs of cold gas within them, which is necessary to form future generations of stars. In this paper we use deep observations taken with Atacama Large Millimeter/sub-millimeter Array (ALMA) to assess the amount of molecular gas within 3 young (50 − 200 Myr) massive (� 106 M⊙) clusters in the Antennae galaxies. No significant CO(3–2) emission was found associated with any of the three clusters. We place upper limits for the molecular gas within these clusters of � 1 × 105 M⊙ (or < 9% of the current stellar mass). We briefly review different scenarios that propose multiple episodes of star formation and discuss some of their assumptions and implications. Our results are in tension with the predictions of GC formation scenarios that expect large reservoirs of cool gas within young massive clusters at these ages

The impact of chorionicity on pregnancy outcome and neurodevelopment at 2 years old among twins born preterm: the EPIPAGE-2 cohort study

OBJECTIVE To compare the short‐ and mid‐term outcomes of preterm twins by chorionicity of pregnancy. DESIGN Prospective nationwide population‐based EPIPAGE‐2 cohort study. SETTING 546 maternity units in France, between March and December 2011. POPULATION A total of 1700 twin neonates born between 24 and 34 weeks of gestation. METHODS The association of chorionicity with outcomes was analysed using multivariate regression models. MAIN OUTCOME MEASURES First, survival at 2‐year corrected age with or without neurosensory impairment, and second, perinatal, short‐, and mid‐term outcomes (survival at discharge, survival at discharge without severe morbidity) were described and compared by chorionicity. RESULTS In the EPIPAGE 2 cohort, 1700 preterm births were included (850 twin pregnancies). In all, 1220 (71.8%) were from dichorionic (DC) pregnancies and 480 from monochorionic (MC) pregnancies. MC pregnancies had three times more medical terminations than DC pregnancies (1.67 versus 0.51%, P < 0.001), whereas there were three times more stillbirths in MC than in DC pregnancies (10.09 versus 3.78%, P < 0.001). Both twins were alive at birth in 86.6% of DC pregnancies compared with 80.0% among MC pregnancies (P = 0.008). No significant difference according to chorionicity was found regarding neonatal deaths and morbidities. Likewise, for children born earlier than 32 weeks, the 2‐year follow‐up neurodevelopmental results were not significantly different between DC and MC twins. CONCLUSIONS This study confirms that MC pregnancies have a higher risk of adverse outcomes. However, the outcomes among preterm twins admitted to neonatal intensive care units are similar irrespective of chorionicity

X-ray observations of three young, early-type galaxies

Massive haloes of hot plasma exist around some, but not all elliptical galaxies. There is evidence that this is related to the age of the galaxy. In this paper, new X-ray observations are presented for three early-type galaxies that show evidence of youth, in order to investigate their X-ray components and properties. NGC 5363 and NGC 2865 were found to have X-ray emission dominated by purely discrete stellar sources. Limits are set on the mass distribution in one of the galaxies observed with XMM–Newton, NGC 4382, which contains significant hot gas. We detect the X-ray emission in NGC 4382 out to 4re. The mass-to-light ratio is consistent with a stellar origin in the inner regions but rises steadily to values indicative of some dark matter by 4re. These results are set in context with other data drawn from the literature, for galaxies with ages estimated from dynamical or spectroscopic indicators. Ages obtained from optical spectroscopy represent central luminosity-weighted stellar ages. We examine the X-ray evolution with age, normalized by B- and K-band luminosities. Low values of Log(LX/LB) and Log(LX/LK) are found for all galaxies with ages between 1 and 4 Gyr. Luminous X-ray emission only appears in older galaxies. This suggests that the interstellar medium is removed and then it takes several gigayears for hot gas haloes to build up, following a merger. A possible mechanism for gas expulsion might be associated with feedback from an active nucleus triggered during a merger

Chemical evolution of local galaxies in a hierarchical model

We investigate the chemical properties of local galaxies within a cosmological framework in the hierarchical picture of galaxy formation. To this aim, we use a hierarchical semi-analytic model which includes the contribution from (i) low and intermediate mass stars, (ii) type Ia Supernovae (SNe) and (iii) massive stars. - Abridged - We compare our predictions with available observations in the Milky Way (MW), in local dwarf galaxies and in local ellipticals. For Milky-Way-like galaxies, we can successfully reproduce the [O-Fe] vs [Fe/H] relation observed in disc stars and the stellar metallicity distribution (SMD). For dwarf galaxies, the stellar metallicity vs mass relation is reproduced by assuming that a substantial fraction of the heavy elements is lost through metal-enhanced outflows and a type Ia SN realization probability lower than the one of MW-like galaxies. - Abridged - In ellipticals, the observations indicate higher [alpha/Fe] values in larger galaxies. - Abridged - Our results computed with a standard Salpeter initial mass function (IMF) indicate a flat [alpha/Fe]-sigma relation. However, we suggest a possible solution to this problem and show how, by assuming a star formation-dependent IMF with a slope x=1.35 in systems with star formation rates < 100 M_sun/yr and slightly flatter (i.e. with x=1) in object with stronger star formation, the observed correlation between [alpha/Fe] and sigma can be accounted for on a large velocity dispersion range. Fundamental roles are played also by interaction-triggered starbursts and AGN.Comment: MNRAS, accepted, 27 pages, 22 figure

Age as a Major Factor in the Onset of Multiple Populations in Stellar Clusters

It is now well established that globular clusters (GCs) exhibit star-to-star light-element abundance variations (known as multiple stellar populations, MPs). Such chemical anomalies have been found in (nearly) all the ancient GCs (more than 10 Gyr old) of our Galaxy and its close companions, but so far no model for the origin of MPs is able to reproduce all the relevant observations. To gain new insights into this phenomenon, we have undertaken a photometric Hubble Space Telescope survey to study clusters with masses comparable to that of old GCs, where MPs have been identified, but with significantly younger ages. Nine clusters in the Magellanic Clouds with ages between $\sim$ 1.5-11 Gyr have been targeted in this survey. We confirm the presence of multiple populations in all clusters older than 6 Gyr and we add NGC 1978 to the group of clusters for which MPs have been identified. With an age of $\sim$ 2 Gyr, NGC 1978 is the youngest cluster known to host chemical abundance spreads found to date. We do not detect evident star-to-star variations for slightly younger massive clusters ($\sim$ 1.7 Gyr), thus pointing towards an unexpected age dependence for the onset of multiple populations. This discovery suggests that the formation of MPs is not restricted to the early Universe and that GCs and young massive clusters share common formation and evolutionary processes

A model of spectral galaxy evolution including the effects of nebular emission

This paper presents a new spectral evolutionary model of galaxies, properly taking the effects of nebular emission and pre-main sequence evolution into account. The impact of these features in different photometric filters is evaluated, along with the influence that variations in the physical conditions of the gas may have on broadband colours, line ratios and equivalent widths. Inclusion of nebular emission is demonstrated to radically change the predicted ultraviolet, optical and near-infrared colours during active star formation. Pre-main sequence evolution is also seen to give a non-negligible contribution to the luminosity in the near-infrared during the first few millions years of evolution and should not be omitted when very young systems are being modelled. Finally, we present a comparison of our predictions to observations and two other recent codes of evolutionary synthesis.Comment: 13 pages, 10 figures, accepted for publication in A&

NGC 1399: a complex dynamical case

Evidence for a disturbed velocity structure in the outer regions of the galaxy NGC1399 comes from a re-analysis of the planetary nebulae data from Arnaboldi et al. (1994). We find a strong rotation along a P.A.=140 degrees for R<=140'' which is followed by a rapid drop off at larger radii, where the velocity dispersion starts to increase. This kinematical behavior can be interpreted as an indication for an interaction scenario. Interaction is advocated in previous analysis of the halo regions of this system, based on different dynamical tracers such as globular clusters and X-rays, but in all these studies the mass distribution is derived under the equilibrium hypothesis, which is not appropriate when an interaction takes place. Here we attempt a non-equilibrium dynamical analysis of NGC1399: with a simple model and under the impulse approximation, we show that the observed kinematics is consistent with an energy injection caused by a flyby encounter of NGC 1399 with the nearby system NGC 1404. In this approach, we find a mass-to-light ratio, M/L_B=26 within R=400'', which is about half of that requested when equilibrium is assumed, i.e. M/L_B=56.Comment: 10 pages, 15 figures, accepted for pubblication in A&

The morphology of the sub-giant branch and red clump reveal no sign of age spreads in intermediate-age clusters

A recent surprise in stellar cluster research, made possible through the precision of Hubble Space Telescope photometry, was that some intermediate-age (1–2 Gyr) clusters in the Large and Small Magellanic Clouds have main-sequence turn-off (MSTO) widths that are significantly broader than would be expected for a simple stellar population (SSP). One interpretation of these extended MSTOs (eMSTOs) is that age spreads of the order of ∼500 Myr exist within the clusters, radically redefining our view of stellar clusters, which are traditionally thought of as single-age, single-metallicity stellar populations. Here we test this interpretation by studying other regions of the CMD that should also be affected by such large age spreads, namely the width of the sub-giant branch (SGB) and the red clump (RC). We study two massive clusters in the LMC that display the eMSTO phenomenon (NGC 1806 and NGC 1846) and show that both have SGB and RC morphologies that are in conflict with expectations if large age spreads exist within the clusters. We conclude that the SGB and RC widths are inconsistent with extended star formation histories within these clusters, hence age spreads are not likely to be the cause of the eMSTO phenomenon. Our results are in agreement with recent studies that also have cast doubt on whether large age spreads can exist in massive clusters; namely the failure to find age spreads in young massive clusters, a lack of gas/dust detected within massive clusters, and homogeneous abundances within clusters that exhibit the eMSTO phenomenon