F. Calura

Metallicity gradients in disks - Do galaxies form inside-out?

Aims. We examine radial and vertical metallicity gradients using a suite of disk galaxy hydrodynamical simulations, supplemented with two classic chemical evolution approaches. We determine the rate of change of gradient slope and reconcile the differences existing between extant models and observations within the canonical “inside-out” disk growth paradigm. Methods. A suite of 25 cosmological disks is used to examine the evolution of metallicity gradients; this consists of 19 galaxies selected from the RaDES (Ramses Disk Environment Study) sample, realised with the adaptive mesh refinement code ramses ,i ncluding eight drawn from the “field” and six from “loose group” environments. Four disks are selected from the MUGS (McMaster Unbiased Galaxy Simulations) sample, generated with the smoothed particle hydrodynamics (SPH) code gasoline. 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. Results. We first show that generically flatter gradients are observed at redshift zero when comparing older stars with those forming today, consistent with expectations of kinematically hot simulations, but counter to that observed in the Milky Way. The vertical abundance gradients at ∼1−3 disk scalelengths are comparable to those observed in the thick disk of the Milky Way, but significantly shallower than those seen in the thin disk. Most importantly, 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. Conclusions. 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.

Dynamical properties of AMAZE and LSD galaxies from gas kinematics and the Tully-Fisher relation at z ∼3

We present a SINFONI integral-field kinematical study of 33 galaxies at z ∼ 3 from the AMAZE and LSD projects, which are aimed at studying metallicity and dynamics of high-redshift galaxies. The number of galaxies analyzed in this paper constitutes a significant improvement over existing data in the literature, and this is the first time that a dynamical analysis is obtained for a relatively large sample of galaxies at z ∼ 3. Eleven galaxies show ordered rotational motions (∼30% of the sample). In these cases we estimate dynamical masses by modeling the gas kinematics with rotating disks and exponential mass distributions. We find dynamical masses .

The dust content of high-z submillimeter galaxies revealed by Herschel

We use deep observations taken with the Photodetector Array Camera and Spectrometer (PACS), on board the Herschel satellite as part of the PACS evolutionary probe (PEP) guaranteed project along with submm ground-based observations to measure the dust mass of a sample of high-z submillimeter galaxies (SMGs). We investigate their dust content relative to their stellar and gas masses, and compare them with local star-forming galaxies. High-z SMGs are dust rich, i.e. they have higher dust-to-stellar mass ratios compared to local spiral galaxies (by a factor of 30) and also compared to local ultraluminous infrared galaxies (ULIRGs, by a factor of 6). This indicates that the large masses of gas typically hosted in SMGs have already been highly enriched with metals and dust. Indeed, for those SMGs whose gas mass is measured, we infer dust-to-gas ratios similar or higher than local spirals and ULIRGs. However, similarly to other strongly star-forming galaxies in the local Universe and at high-z ,S MGs are characterized by gas metalicities lower (by a factor of a few) than local spirals, as inferred from their optical nebular lines, which are generally ascribed to infall of metal-poor gas. This is in contrast with the large dust content inferred from the far-IR and submm data. In short, the metalicity inferred from the dust mass is much higher (by more than an order of magnitude) than that inferred from the optical nebular lines. We discuss the possible explanations of this discrepancy and the possible implications for the investigation of the metalicity evolution at high-z.

The chemical evolution of elliptical galaxies with stellar and QSO dust production

Context. The presence of dust strongly affects the way we see galaxies and also the chemical abundances we measure in gas. It is therefore important to study the chemical evolution of galaxies by taking into account dust evolution. Aims. We aim at performing a detailed study of abundance ratios of high redshift objects and their dust properties. We focus on Lyman-Break galaxies (LBGs) and Quasar (QSO) hosts as likely progenitors of low- and high-mass present-day elliptical galaxies, respectively. Methods. We have adopted a chemical evolution model for elliptical galaxies taking into account the dust production from low and intermediate mass stars, supernovae Ia, supernovae II, QSOs and both dust destruction and accretion processes. By means of such a model we have followed the chemical evolution of ellipticals of different baryonic masses. Our model complies with chemical downsizing. Results. We made predictions for the abundance ratios versus metallicity trends for models of differing masses that can be used to constrain the star formation rate, initial mass function and dust mass in observed galaxies. We predict the existence of a high redshift dust mass-stellar mass relationship. We have found a good agreement with the properties of LBGs if we assume that they formed at redshift z = 2–4. In particular, a non-negligible amount of dust is needed to explain the observed abundance pattern. We studied the QSO SDSS J114816, one of the most distant QSO ever observed (z = 6.4), and we have been able to reproduce the amount of dust measured in this object. The dust is clearly due to the production from supernovae and the most massive AGB stars as well as from the grain growth in the interstellar medium. The QSO dust is likely to dominate only in the very central regions of the galaxies and during the early development of the galactic wind.

The distribution of metals in cosmological hydrodynamical simulations of dwarf disc galaxies

We examine the chemical properties of five cosmological hydrodynamical simulations of an M33-like disc galaxy which have been shown previously 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 Project, in which stellar feedback is tuned to match the stellar mass–halo mass relationship. Each realization 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 surrounding interstellar medium, (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 supernovae remnants, (d) the treatment of metal diffusion and (e) varying the initial mass function. Our analysis focusses on the resulting stellar metallicity distribution functions (MDFs) in each simulated (analogous) ‘solar neighbourhood’ (2–3 disc scalelengths from the galactic centre) and central ‘bulge’ region. We compare and contrast the simulated MDFs’ skewness, kurtosis and dispersion (inter-quartile, inter-decile, inter-centile and inter-tenthpercentile regions) with that of the empirical solar neighbourhood MDF and Local Group dwarf galxies. We find that the MDFs of the simulated discs are more negatively skewed, with higher kurtosis, than those observed locally in the Milky Way and Local Group dwarfs. We can trace this difference to the simulations’ very tight and correlated age–metallicity relations (compared with that of the Milky Way’s solar neighbourhood), suggesting that these relations �

Cosmic evolution of the C iv in high-resolution hydrodynamic simulations

We investigate the properties of triply ionized carbon (Cxa0iv) in the intergalactic medium (IGM) using a set of high resolution and large box size cosmological hydrodynamic simulations of a Lambda cold dark matter (ΛCDM) model. We rely on a modification of the publicly available TreeSPH code gadget-2 that self-consistently follows the metal enrichment mechanism by means of a detailed chemical evolution model. We focus on several numerical implementations of galactic feedback: galactic winds in the energy-driven and momentum-driven prescriptions, galactic winds hydrodynamically coupled to the surrounding gas and active galactic nuclei (AGNs) powered by gas accretion on to massive black holes. Furthermore, our results are compared to a run in which galactic feedback is not present and we also explore different initial stellar mass function. After having addressed some global properties of the simulated volume like the impact on the star formation rate and the content in carbon and Cxa0iv, we extract mock IGM transmission spectra in neutral hydrogen (Hxa0i) and Cxa0iv and perform Voigt profile fitting. The results are then compared with high-resolution quasar (QSO) spectra obtained with the Ultraviolet Echelle Spectrograph (UVES) at the Very Large Telescope (VLT) and the High Resolution Echelle Spectrograph (HIRES) at Keck. We find that feedback has little impact on statistics related to the neutral hydrogen, while Cxa0iv is more affected by galactic winds and/or AGN feedback. The feedback schemes investigated have a different strength and redshift evolution with a general tendency for AGN feedback to be more effective at lower redshift than galactic winds. When the same analysis is performed over observed and simulated Cxa0iv lines, we find a reasonably good agreement between data and simulations over the column density range NCxa0IV= 1012.5-15xa0cm−2. Also the Cxa0iv linewidth distribution appears to be in agreement with the observed values, while the Hxa0i Doppler parameters, bHxa0I, are in general too large, showing that the diffuse cosmic web is heated more than what is inferred by observations. The simulation without feedback fails in reproducing the Cxa0iv systems at high column densities at all redshift, while the AGN feedback case agrees with observations only at z < 3, when this form of feedback is particularly effective. We also present scatter plots in the b–N and in the NCxa0IV–NHxa0I planes, showing that there is rough agreement between observations and simulations only when feedback is taken into account. n n n nAlthough it seems difficult to constrain the nature and the strength of galactic feedback using the present framework and find a unique model that fits all the observations, these simulations offer the perspective of understanding the galaxy–IGM interplay and how metals produced by stars can reach the low-density IGM.

The Lyman α forest flux probability distribution at z>3

We present a measurement of the Lyman α flux probability distribution function (PDF) obtained from a set of eight high-resolution quasar spectra with emission redshifts in the range 3.3 ≤z≤ 3.8. We carefully study the effect of metal absorption lines on the shape of the PDF. Metals have a larger impact on the PDF measurements at lower redshift, where there are relatively fewer Lyman α absorption lines. This may be explained by an increase in the number of metal lines that are blended with Lyman α absorption lines towards higher redshift, but may also be due to the presence of fewer metals in the intergalactic medium (IGM) at earlier times. We also provide a new measurement of the redshift evolution of the effective optical depth, τeff, at 2.8 ≤z≤ 3.6, and find no evidence for a deviation from a power-law evolution in the logxa0(τeff)–logxa0(1 +z) plane. The flux PDF measurements are furthermore of interest for studies of the thermal state of the IGM at z≃ 3. By comparing the PDF to state-of-the-art cosmological hydrodynamical simulations, we place constraints on the temperature of the IGM and compare our results with previous measurements of the PDF at lower redshift. At redshift z= 3, our new PDF measurements are consistent with an isothermal temperature–density relation, T=T0Δγ− 1, with a temperature at the mean density of T0= 19xa0250 ± 4800xa0K and a slope γ= 0.90 ± 0.21 (1σ uncertainties). In comparison, joint constraints with existing lower redshift PDF measurements at z < 3 favour an inverted temperature–density relation with T0= 17xa0900 ± 3500xa0K and γ= 0.70 ± 0.12, in broad agreement with previous analyses.

The stellar metallicity distribution of disc galaxies and bulges in cosmological simulations

By means of high-resolution cosmological hydrodynamical simulations of Milky Way (MW) 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 subcomponents, 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 MW. 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 MW analogue, 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.

RAMSES-CH: a new chemodynamical code for cosmological simulations

We present a new chemodynamical code – RAMSES-CH – for use in simulating the self-consistent evolution of chemical and hydrodynamical properties of galaxies within a fully cosmological framework. We build upon the adaptive mesh refinement code RAMSES, which includes a treatment of self-gravity, hydrodynamics, star formation, radiative cooling and supernova feedback, to trace the dominant isotopes of C, N, O, Ne, Mg, Si and Fe. We include the contribution of Type Ia and Type II supernovae, in addition to low- and intermediate-mass asymptotic giant branch stars, relaxing the instantaneous recycling approximation. The new chemical evolution modules are highly flexible and portable, lending themselves to ready exploration of variations in the underpinning stellar and nuclear physics. We apply RAMSES-CH

The cold gas content of bulgeless dwarf galaxies

We present an analysis of the neutral hydrogen (H i) properties of a fully cosmological hydrodynamical dwarf galaxy, run with varying simulation parameters. As reported by Governato et al., the high-resolution, high star formation density threshold version of this galaxy is the first simulation to result in the successful reproduction of a (dwarf) spiral galaxy without any associated stellar bulge. We have set out to compare in detail the H i distribution and kinematics of this simulated bulgeless disc with what is observed in a sample of nearby dwarfs. To do so, we extracted the radial gas density profiles, velocity dispersion (e.g. velocity ellipsoid and turbulence) and the power spectrum of structure within the cold interstellar medium (ISM) from the simulations. The highest resolution dwarf, when using a high-density star formation threshold comparable to densities of giant molecular clouds, possesses bulk characteristics consistent with those observed in nature, though the cold gas is not as radially extended as that observed in nearby dwarfs, resulting in somewhat excessive surface densities. The lines-of-sight velocity dispersion radial profiles have values that are in good agreement with the observed dwarf galaxies, but due to the fact that only the streaming velocities of particles are tracked, a correction to include the thermal velocities can lead to profiles that are quite flat. The ISM power spectra of the simulations appear to possess more power on smaller spatial scales than that of the Small Magellanic Cloud. We conclude that unavoidable limitations remain due to the unresolved physics of star formation and feedback within parsec-scale molecular clouds.