The gas metallicity gradient and the star formation activity of disc galaxies

We study oxygen abundance profiles of the gaseous disc components in simulated galaxies in a hierarchical universe. We analyse the disc metallicity gradients in relation to the stellar masses and star formation rates of the simulated galaxies. We find a trend for galaxies with low stellar masses to have steeper metallicity gradients than galaxies with high stellar masses at z ~0. We also detect that the gas-phase metallicity slopes and the specific star formation rate (sSFR) of our simulated disc galaxies are consistent with recently reported observations at z ~0. Simulated galaxies with high stellar masses reproduce the observed relationship at all analysed redshifts and have an increasing contribution of discs with positive metallicity slopes with increasing redshift. Simulated galaxies with low stellar masses a have larger fraction of negative metallicity gradients with increasing redshift. Simulated galaxies with positive or very negative metallicity slopes exhibit disturbed morphologies and/or have a close neighbour. We analyse the evolution of the slope of the oxygen profile and sSFR for a gas-rich galaxy-galaxy encounter, finding that this kind of events could generate either positive and negative gas-phase oxygen profiles depending on their state of evolution. Our results support claims that the determination of reliable metallicity gradients as a function of redshift is a key piece of information to understand galaxy formation and set constrains on the subgrid physics.Comment: 12 pages, 8 figures, accepted MNRA

On the mass assembly of low-mass galaxies in hydrodynamical simulations of structure formation

Cosmological hydrodynamical simulations are studied in order to analyse generic trends for the stellar, baryonic and halo mass assembly of low-mass galaxies (M_* < 3 x 10^10 M_sun) as a function of their present halo mass, in the context of the Lambda-CDM scenario and common subgrid physics schemes. We obtain that smaller galaxies exhibit higher specific star formation rates and higher gas fractions. Although these trends are in rough agreement with observations, the absolute values of these quantities tend to be lower than observed ones since z~2. The simulated galaxy stellar mass fraction increases with halo mass, consistently with semi-empirical inferences. However, the predicted correlation between them shows negligible variations up to high z, while these inferences seem to indicate some evolution. The hot gas mass in z=0 halos is higher than the central galaxy mass by a factor of ~1-1.5 and this factor increases up to ~5-7 at z~2 for the smallest galaxies. The stellar, baryonic and halo evolutionary tracks of simulated galaxies show that smaller galaxies tend to delay their baryonic and stellar mass assembly with respect to the halo one. The Supernova feedback treatment included in this model plays a key role on this behaviour albeit the trend is still weaker than the one inferred from observations. At z>2, the overall properties of simulated galaxies are not in large disagreement with those derived from observations.Comment: 19 pages, 12 figures. Accepted for publication in MNRAS: 6th August 2013. First submitted: 7th July 201

Fingerprints of modified gravity on galaxies in voids

We search for detectable signatures of f(R) gravity and its chameleon screening mechanism in the baryonic and dark matter (DM) properties of simulated void galaxies. The enhancement of the gravitational acceleration can have a meaningful impact on the scaling relations as well as on the halo morphology. The galaxy rotational velocity field (calculated with the velocity of the gas disc and the acceleration fields) deviates from the typical values of the Tully–Fisher Relation in General Relativity (GR). For a given stellar mass, f(R) gravity tends to produce greater maximum velocities. On the other hand, the mass in haloes in f(R) gravity is more concentrated than their counterparts in GR. This trend changes when the concentration is calculated with the dynamical density profile, which takes into account the unscreened outer regions of the halo. Stellar discs interact with the overall potential well in the central regions, modifying the morphology of the screening regions and reshaping them. We find a trend for galaxies with a more dominant stellar disc to deviate further from round screening regions. We find that small haloes are less triaxial and more round in f(R) than their GR counterparts. The difference between halo morphology becomes smaller in f(R) haloes whose inner regions are screened. These results suggest possible observables that could unveil modified gravity effects on galaxies in voids in future cosmological tests of gravity