Where are the missing cosmic metals ?

The majority of the heavy elements produced by stars 2 billion years after the Big Bang (redshift z~3) are presently undetected at those epochs. We propose a solution to this cosmic `missing metals' problem in which such elements are stored in gaseous halos produced by supernova explosions around star-forming galaxies. By using data from the ESO/VLT Large Program, we find that:(i) only 5%-9% of the produced metals reside in the cold phase, the rest being found in the hot (log T=5.8-6.4) phase; (ii) 1%-6% (3%-30%) of the observed CIV (OVI) is in the hot phase. We conclude that at z~3 more than 90% of the metals produced during the star forming history can be placed in a hot phase of the IGM, without violating any observational constraint. The observed galaxy mass-metallicity relation, and the intergalactic medium and intracluster medium metallicity evolution are also naturally explained by this hypothesis.Comment: 9 pages, 2 figures, ApJ Letters, in pres

Hot Atmospheres, Cold Gas, AGN Feedback and the Evolution of Early Type Galaxies: a Topical Perspective

Most galaxies comparable to or larger than the mass of the Milky Way host hot, X-ray emitting atmospheres, and many such galaxies are radio sources. Hot atmospheres and radio jets and lobes are the ingredients of radio-mechanical active galactic nucleus (AGN) feedback. While a consensus has emerged that such feedback suppresses cooling of hot cluster atmospheres, less attention has been paid to massive galaxies where similar mechanisms are at play. Observation indicates that the atmospheres of elliptical and S0 galaxies were accreted externally during the process of galaxy assembly and augmented significantly by stellar mass loss. Their atmospheres have entropy and cooling time profiles that are remarkably similar to those of central cluster galaxies. About half display filamentary or disky nebulae of cool and cold gas, much of which has likely cooled from the hot atmospheres. We review the observational and theoretical perspectives on thermal instabilities in galactic atmospheres and the evidence that AGN heating is able to roughly balance the atmospheric cooling. Such heating and cooling may be regulating star formation in all massive spheroids at late times.Comment: Final versio

Clues for the origin of the fundamental metallicity relations. I: The hierarchical building up of the structure

We analyse the evolutionary history of galaxies formed in a hierarchical scenario consistent with the concordance $\Lambda$-CDM model focusing on the study of the relation between their chemical and dynamical properties. Our simulations consistently describe the formation of the structure and its chemical enrichment within a cosmological context. Our results indicate that the luminosity-metallicity (LZR) and the stellar mass-metallicity (MZR) relations are naturally generated in a hierarchical scenario. Both relations are found to evolve with redshift. In the case of the MZR, the estimated evolution is weaker than that deduced from observational works by approximately 0.10 dex. We also determine a characteristic stellar mass, $M_c \approx 3 \times 10^{10} M_{\odot}$, which segregates the simulated galaxy population into two distinctive groups and which remains unchanged since $z\sim 3$, with a very weak evolution of its metallicity content. The value and role played by $M_c$ is consistent with the characteristic mass estimated from the SDSS galaxy survey by Kauffmann et al. (2004). Our findings suggest that systems with stellar masses smaller than $M_c$ are responsible for the evolution of this relation at least from $z\approx 3$. Larger systems are stellar dominated and have formed more than 50 per cent of their stars at $z \ge 2$, showing very weak evolution since this epoch. We also found bimodal metallicity and age distributions from $z\sim3$, which reflects the existence of two different galaxy populations. Although SN feedback may affect the properties of galaxies and help to shape the MZR, it is unlikely that it will significantly modify $M_c$ since, from $z=3$ this stellar mass is found in systems with circular velocities larger than 100 \kms.Comment: 17 pages, 13 figures. Minor changes to match accepted version. Accepted October 3 MNRA

Fingerprints of the Hierarchical Building up of the Structure on the Mass-Metallicity Relation

We study the mass-metallicity relation of galactic systems with stellar masses larger than 10^9 Mo in Lambda-CDM scenarios by using chemical hydrodynamical simulations. We find that this relation arises naturally as a consequence of the formation of the structure in a hierarchical scenario. The hierarchical building up of the structure determines a characteristic stellar mass at M_c ~10^10.2 Moh^-1 which exhibits approximately solar metallicities from z ~ 3 to z=0. This characteristic mass separates galactic systems in two groups with massive ones forming most of their stars and metals at high redshift. We find evolution in the zero point and slope of the mass-metallicity relation driven mainly by the low mass systems which exhibit the larger variations in the chemical properties. Although stellar mass and circular velocity are directly related, the correlation between circular velocity and metallicity shows a larger evolution with redshift making this relation more appropriate to confront models and observations. The dispersion found in both relations is a function of the stellar mass and reflects the different dynamical history of evolution of the systems.Comment: 4 pages, 4 figures. Accepted MNRAS Letter

The Milky Way and Andromeda galaxies in a constrained hydrodynamical simulation: morphological evolution

We study the two main constituent galaxies of a constrained simulation of the Local Group as candidates for the Milky Way (MW) and Andromeda (M31). We focus on the formation of the stellar discs and its relation to the formation of the group as a rich system with two massive galaxies, and investigate the effects of mergers and accretion as drivers of morphological transformations. We use a state-of-the-art hydrodynamical code which includes star formation, feedback and chemical enrichment to carry out our study. We run two simulations, where we include or neglect the effects of radiation pressure from stars, to investigate the impact of this process on the morphologies and star formation rates of the simulated galaxies. We find that the simulated M31 and MW have different formation histories, even though both inhabit, at z=0, the same environment. These differences directly translate into and explain variations in their star formation rates, in-situ fractions and final morphologies. The M31 candidate has an active merger history, as a result of which its stellar disc is unable to survive unaffected until the present time. In contrast, the MW candidate has a smoother history with no major mergers at late times, and forms a disc that grows steadily; at z=0 the simulated MW has an extended, rotationally-supported disc which is dominant over the bulge. Our two feedback implementations predict similar evolution of the galaxies and their discs, although some variations are detected, the most important of which is the formation time of the discs: in the model with weaker/stronger feedback the discs form earlier/later. In summary, by comparing the formation histories of the two galaxies, we conclude that the particular merger/accretion history of a galaxy rather than its environment at the LG-scales is the main driver of the formation and subsequent growth or destruction of galaxy discs.Comment: 12 pages, 7 figures, accepted for publication in A&

The Effect of Environment on Milky Way-mass galaxies in a Constrained Simulation of the Local Group

In this letter we present, for the first time, a study of star formation rate, gas fraction and galaxy morphology of a constrained simulation of the Milky Way (MW) and Andromeda (M31) galaxies, compared to other MW-mass galaxies. By combining with unconstrained simulations we cover a sufficient volume to compare these galaxies environmental densities ranging from the field to that of the Local Group (LG). This is particularly relevant as it has been shown that, quite generally, galaxy properties depend intimately upon their environment, most prominently when galaxies in clusters are compared to those in the field. For galaxies in loose groups such as the LG, however, environmental effects have been less clear. We consider the galaxy's environmental density in spheres of 1200 kpc (comoving) and find that whilst environment does not appear to directly affect morphology, there is a positive trend with star formation rates. This enhancement in star formation occurs systematically for galaxies in higher density environments, regardless whether they are part of the LG or in filaments. Our simulations suggest that the richer environment at Mpc-scales may help replenish the star-forming gas, allowing higher specific star formation rates in galaxies such as the MW.Comment: 6 pages, 4 figures, accepted to ApJ