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  • 1
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    Is the stellar initial mass function universal? (2016)
    Oxford University Press
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    Publication Date: 2016-03-31
    Description: Ignacio Ferreras , Francesco La Barbera and Alexandre Vazdekis take a close look at one of the key ingredients in modelling star formation.
    Print ISSN: 1366-8781
    Electronic ISSN: 1468-4004
    Topics: Geosciences , Physics
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 2
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    Light versus dark in strong-lens galaxies: dark matter haloes that are rounder than their stars (2015)
    Oxford University Press
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    Publication Date: 2015-12-20
    Description: We measure the projected density profile, shape and alignment of the stellar and dark matter mass distribution in 11 strong-lens galaxies. We find that the projected dark matter density profile – under the assumption of a Chabrier stellar initial mass function – shows significant variation from galaxy to galaxy. Those with an outermost image beyond ~10 kpc are very well fit by a projected Navarro–Frenk–White (NFW) profile; those with images within 10 kpc appear to be more concentrated than NFW, as expected if their dark haloes contract due to baryonic cooling. We find that over several half-light radii, the dark matter haloes of these lenses are rounder than their stellar mass distributions. While the haloes are never more elliptical than e dm = 0.2, their stars can extend to e * 〉 0.2. Galaxies with high dark matter ellipticity and weak external shear show strong alignment between light and dark; those with strong shear ( 0.1) can be highly misaligned. This is reassuring since isolated misaligned galaxies are expected to be unstable. Our results provide a new constraint on galaxy formation models. For a given cosmology, these must explain the origin of both very round dark matter haloes and misaligned strong-lens systems.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Further evidence for a time-dependent initial mass function in massive early-type galaxies (2015)
    Oxford University Press
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    Publication Date: 2015-02-04
    Description: Spectroscopic analyses of gravity-sensitive line strengths give growing evidence towards an excess of low-mass stars in massive early-type galaxies (ETGs). Such a scenario requires a bottom-heavy initial mass function (IMF). However, strong constraints can be imposed if we take into account galactic chemical enrichment. We extend the analysis of Weidner et al. and consider the functional form of bottom-heavy IMFs used in recent works, where the high-mass end slope is kept fixed to the Salpeter value, and a free parameter is introduced to describe the slope at stellar masses below some pivot mass scale ( M  〈  M P  = 0.5 M ). We find that no such time-independent parametrization is capable to reproduce the full set of constraints in the stellar populations of massive ETGs – resting on the assumption that the analysis of gravity-sensitive line strengths leads to a mass fraction at birth in stars with mass M  〈 0.5 M above 60 per cent. Most notably, the large amount of metal-poor gas locked in low-mass stars during the early, strong phases of star formation results in average stellar metallicities [ M /H]  –0.6, well below the solar value. The conclusions are unchanged if either the low-mass end cutoff, or the pivot mass are left as free parameters, strengthening the case for a time-dependent IMF.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    Variations of the dust properties of M82 with galactocentric distance (2015)
    Oxford University Press
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    Publication Date: 2015-07-16
    Description: We use near ultraviolet and optical photometry to investigate the dust properties in the nearby starburst galaxy M82. By combining imaging from the Swift /UVOT instrument and optical data from the Sloan Digital Sky Survey, we derive the extinction curve parametrized by the standard R V factor, and the strength of the NUV 2175 Å feature – quantified by a parameter B – out to projected galactocentric distances of 4 kpc. Our analysis is robust against possible degeneracies from the properties of the underlying stellar populations. Both B and R V correlate with galactocentric distance, revealing a systematic trend of the dust properties. Our results confirm previous findings that dust in M82 is better fitted by a Milky Way standard extinction curve, in contrast to a Calzetti law. We find a strong correlation between R V and B , towards a stronger NUV bump in regions with higher R V , possibly reflecting a distribution with larger dust grain sizes. The data we use were taken before SN2014J, and therefore can be used to probe the properties of the interstellar medium before the event. Our R V values around the position of the supernova are significantly higher than recent measurements post-SN2014J ( R V   1.4). This result is consistent with a significant change in the dust properties after the supernova event, either from disruption of large grains or from the contribution by an intrinsic circumstellar component. Intrinsic variations among supernovae not accounted for could also give rise to this mismatch.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Radial constraints on the initial mass function from TiO features and Wing-Ford band in early-type galaxies (2016)
    Oxford University Press
    Details
    Publication Date: 2016-02-06
    Description: At present, the main challenge to the interpretation of variations in gravity-sensitive line strengths as driven by a non-universal initial mass function (IMF) lies in understanding the effect of the other population parameters. Most notably, [α/Fe]-enhanced populations or even departures in the individual element abundances with respect to the solar-scaled ratio may lead to similar observational results. We combine various TiO-based, IMF-sensitive indicators in the optical and NIR spectral windows, along with the FeH-based Wing–Ford band to break this degeneracy. We obtain a significant radial trend of the IMF slope in XSG1, a massive early-type galaxy (ETG), with velocity dispersion ~ 300 km s –1 , observed with the Very Large Telescope/X-shooter instrument. In addition, we constrain – for the first time – both the shape and normalization of the IMF, using only a stellar population analysis. We robustly rule out a single power law to describe the IMF, whereas a power law tapered off to a constant value at low masses (defined as a bimodal IMF) is consistent with all the observational spectroscopic data and with the stellar M / L constraints based on the Jeans anisotropic modelling method. The IMF in XSG1 is bottom-heavy in the central regions (corresponding to a bimodal IMF slope b ~ 3, or a mass normalization mismatch parameter α ~ 2), changing towards a standard Milky Way-like IMF ( b ~ 1.3; α ~ 1) at around one half of the effective radius. This result, combined with previous observations of local IMF variations in massive ETGs, reflects the varying processes underlying the formation of the central core and the outer regions in this type of galaxies.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    The fate of high-redshift massive compact galaxies (2016)
    Oxford University Press
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    Publication Date: 2016-02-07
    Description: Massive high-redshift quiescent compact galaxies (nicknamed red nuggets ) have been traditionally connected to present-day elliptical galaxies, often overlooking the relationships that they may have with other galaxy types. We use large bulge–disc decomposition catalogues based on the Sloan Digital Sky Survey to check the hypothesis that red nuggets have survived as compact cores embedded inside the haloes or discs of present-day massive galaxies. In this study, we designate a compact core as the bulge component that satisfies a prescribed compactness criterion. Photometric and dynamic mass–size and mass–density relations are used to show that, in the inner regions of galaxies at z ~ 0.1, there are abundant compact cores matching the peculiar properties of the red nuggets, an abundance comparable to that of red nuggets at z ~ 1.5. Furthermore, the morphology distribution of the present-day galaxies hosting compact cores is used to demonstrate that, in addition to the standard channel connecting red nuggets with elliptical galaxies, a comparable fraction of red nuggets might have ended up embedded in discs. This result generalizes the inside-out formation scenario; present-day massive galaxies can begin as dense spheroidal cores (red nuggets), around which either a spheroidal halo or a disc is formed later.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    The (galaxy-wide) IMF in giant elliptical galaxies: from top to bottom (2013)
    Oxford University Press
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    Publication Date: 2013-10-08
    Description: Recent evidence based independently on spectral line strengths and dynamical modelling point towards a non-universal stellar initial mass function (IMF), probably implying an excess of low-mass stars in elliptical galaxies with a high velocity dispersion. Here, we show that a time-independent bottom-heavy IMF is compatible neither with the observed metal-rich populations found in giant ellipticals nor with the number of stellar remnants observed within these systems. We suggest a two-stage formation scenario involving a time-dependent IMF to reconcile these observational constraints. In this model, an early strong starbursting stage with a top-heavy IMF is followed by a more prolonged stage with a bottom-heavy IMF. Such model is physically motivated by the fact that a sustained high star formation will bring the interstellar medium to a state of pressure, temperature and turbulence that can drastically alter the fragmentation of the gaseous component into small clumps, promoting the formation of low-mass stars. This toy model is in good agreement with the different observational constrains on massive elliptical galaxies, such as age, metallicity, α-enhancement, mass-to-light ratio or the mass fraction of the stellar component in low-mass stars.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    A panchromatic analysis of starburst galaxy M82: probing the dust properties (2014)
    Oxford University Press
    Details
    Publication Date: 2014-04-02
    Description: We combine near-ultraviolet (NUV), optical and IR imaging of the nearby starburst galaxy M82 to explore the properties of the dust both in the interstellar medium of the galaxy and the dust entrained in the superwind. The three NUV filters of Swift -Ultraviolet/Optical Telescope enable us to probe in detail the properties of the extinction curve in the region around the 2175 Å bump. The NUV colour–colour diagram strongly rules out a ‘bump-less’ Calzetti-type law, which can either reflect intrinsic changes in the dust properties or in the star formation history compared to starbursts well represented by such an attenuation law. We emphasize that it is mainly in the NUV region where a standard Milky Way-type law is preferred over the Calzetti law. The age and dust distribution of the stellar populations is consistent with the scenario of an encounter with M81 in the recent 400 Myr. The radial variation of NUV/optical/IR photometry in the galaxy region – including the polycyclic-aromatic-hydrocarbon-dominated emission at 8 μm – confirms the central location of the star formation. The radial gradients of the NUV and optical colours in the superwind region supports the hypothesis that the emission in the wind cone is driven by scattering from dust grains entrained in the ejecta. The observed wavelength dependence, –1.5 , reveals either a grain size distribution n ( a )   a –2.5 , or a flatter distribution with a maximum size cutoff, suggesting that only small grains are entrained in the supernova-driven wind.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    Constraints on the merging channel of massive galaxies since z ~ 1 (2014)
    Oxford University Press
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    Publication Date: 2014-08-25
    Description: We probe the merging channel of massive galaxies over the z  = 0.3–1.3 redshift window by studying close pairs in a sample of 238 galaxies with stellar mass 10 11  M , from the SHARDS (Survey for High-z Absorption Red and Dead Sources) survey. SHARDS provides medium-band photometry equivalent to low-resolution optical spectra ( R  ~ 50), allowing us to obtain extremely accurate photometric redshifts (median | z |/(1 +  z ) ~ 0.55 per cent) and to improve the constraints on the age distribution of the stellar populations. Our data set is volume limited, probing merger progenitors with mass ratios 1:100 (μ M sat / M cen  = 0.01) out to z  = 1.3. A strong correlation is found between the age difference of host and companion galaxy and stellar mass ratio, from negligible age differences in major mergers to age differences ~4 Gyr for 1:100 minor mergers. However, this correlation is simply a reflection of the mass–age trend in the general population. The dominant contributor to the growth of massive galaxies corresponds to mass ratios μ 0.3, followed by a decrease in the fractional mass growth rate linearly proportional to log μ, at least down to μ ~ 0.01, suggesting a decreasing role of mergers involving low-mass companions, especially if dynamical friction time-scales are taken into account. A simple model results in an upper limit for the average mass growth rate of massive galaxies of ( M / M )/ t  ~ 0.08 ± 0.02 Gyr –1 , over the z 1 range, with an ~70 per cent fractional contribution from (major) mergers with μ 0.3. The majority of the stellar mass contributed by mergers does not introduce significantly younger populations, in agreement with the small radial age gradients observed in present-day early-type galaxies.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    Systematic variations of central mass density slopes in early-type galaxies (2014)
    Oxford University Press
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    Publication Date: 2014-09-25
    Description: We study the total density distribution in the central regions (1 effective radius, R e ) of early-type galaxies (ETGs), using data from SPIDER and ATLAS 3D . Our analysis extends the range of galaxy stellar mass ( M * ) probed by gravitational lensing, down to ~ 10 10 M . We model each galaxy with two components (dark matter halo + stars), exploring different assumptions for the dark matter halo profile (i.e. NFW, NFW-contracted, and Burkert profiles), and leaving stellar mass-to-light ( M * / L ) ratios as free fitting parameters to the data. For all plausible halo models, the best-fitting M * / L , normalized to that for a Chabrier initial mass function, increases systematically with galaxy size and mass. For an NFW profile, the slope of the total mass profile is non-universal, independently of several ingredients in the modelling (e.g. halo contraction, anisotropy, and rotation velocity in ETGs). For the most massive ( M *  ~ 10 11.5 M ) or largest ( R e ~ 15 kpc) ETGs, the profile is isothermal in the central regions (~ R e /2), while for the low-mass ( M *  ~ 10 10.2 M ) or smallest ( R e ~ 0.5 kpc) systems, the profile is steeper than isothermal, with slopes similar to those for a constant- M / L profile. For a steeper concentration–mass relation than that expected from simulations, the correlation of density slope with galaxy mass tends to flatten, while correlations with R e and velocity dispersions are more robust. Our results clearly point to a ‘non-homology’ in the total mass distribution of ETGs, which simulations of galaxy formation suggest may be related to a varying role of dissipation with galaxy mass.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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