The Tully-Fisher relation and its evolution with redshift in cosmological simulations of disc galaxy formation

We present predictions on the evolution of the Tully-Fisher (TF) relation with redshift, based on cosmological N-body/hydrodynamical simulations of disc galaxy formation and evolution. The simulations invoke star formation and stellar feedback, chemical evolution with non-instantaneous recycling, metallicity dependent radiative cooling and effects of a meta-galactic UV field, including simplified radiative transfer. At z=0, the simulated and empirical TF relations are offset by about 0.4 magnitudes (1 sigma) in the B and I bands. The origin of these offsets is somewhat unclear, but it may not necessarily be a problem of the simulations only. As to evolution, we find a brightening of the TF relation between z=0 and z=1 of about 0.85 mag in rest-frame B band, with a non-evolving slope. The brightening we predict is intermediate between the (still quite discrepant) observational estimates. This evolution is primarily a luminosity effect, while the stellar mass TF relation shows negligible evolution. The individual galaxies do gain stellar mass between z=1 and z=0, by a 50-100%; but they also correspondingly increase their characteristic circular speed. As a consequence, individually they mainly evolve ALONG the stellar mass TF relation, while the relation as such does not show any significant evolution

Mass-to-Light ratio, Initial Mass Function and chemical evolution in disc galaxies

Cosmological simulations of disc galaxy formation, when compared to the observed Tully-Fisher relation, suggest a low Mass-to-Light (M/L) ratio for the stellar component in spirals. We show that a number of "bottom-light" Initial Mass Functions (IMFs) suggested independently in literature, do imply M/L ratios as low as required, at least for late type spirals (Sbc/Sc). However the typical M/L ratio, and correspondingly the zero-point of the Tully-Fisher relation, is expected to vary considerably with Hubble type. Bottom-light IMFs tend to have a metal production in excess of what is tipically estimated for spiral galaxies. Suitable tuning of the IMF slope and mass limits, post-supernova fallback of metals onto black holes or metal outflows must then be invoked, to reproduce the observed chemical properties of disc galaxies.Comment: 4 pages, 6 colour figures. To appear on PASA (refereed proceedings of Galactic ChemoDynamics V, B.K. Gibson and D. Kawata eds.

On the Mass-to-Light ratio and the Initial Mass Function in disc galaxies

A low mass-to-light ratio for the stellar component of spiral galaxies (M/L<1 in the I-band) is advocated by various dynamical arguments and by recent cosmological simulations of the formation of these systems. We discuss this possibility by means of chemo-photometric models for galactic discs, adopting different Initial Mass Functions. We show that a number of "bottom--light" IMFs (namely, with less mass locked in low-mass stars than the standard Salpeter IMF), suggested independently in recent literature, do imply M/L ratios as low as mentioned above, at least for late type spirals (Sbc/Sc). This conclusion still holds when the bulge contribution to mass and light is included. We also predict the typical stellar M/L ratio, and correspondingly the zero-point of the Tully-Fisher relation, to vary considerably with Hubble type (about 0.5-0.7 mag in the red bands, from Sa to Sc type). For some of the bottom-light IMFs considered, the efficiency of metal production tends to exceed what is typically estimated for spiral galaxies. Suitable tuning of the IMF mass limits, post-supernova fallback of metals onto black holes or metal outflows must then be invoked, to reproduce the observed chemical properties of disc galaxies. In the appendix we provide M/L-colour relations to estimate the stellar M/L ratio of a galaxy on the base of its colours, for several IMFs.Comment: 32 pages, 28 figures, version in press on MNRAS. With respect to previous electronic version: extended discussion of model results in new Sect 7.6, 7.7, 7.8; discussion of B/D ratios in K-band in new Sect. 8.2; Appendix B added with M/L vs. colour relations from our model

Simulating galaxy clusters -- I. Thermal and chemical properties of the intra-cluster medium

We have performed a series of N-body/hydrodynamical (TreeSPH) simulations of clusters and groups of galaxies, selected from cosmological N-body simulations within a $\Lambda$CDM framework: these objects have been re-simulated at higher resolution to $z$=0, in order to follow also the dynamical, thermal and chemical input on to the ICM from stellar populations within galaxies. The simulations include metal dependent radiative cooling, star formation according to different IMFs, energy feedback as strong starburst-driven galactic super-winds, chemical evolution with non-instantaneous recycling of gas and heavy elements, effects of a meta-galactic UV field and thermal conduction in the ICM. In this Paper I of a series of three, we derive results, mainly at $z=0$, on the temperature and entropy profiles of the ICM, its X-ray luminosity, the cluster cold components (cold fraction as well as mass--to--light ratio) and the metal distribution between ICM and stars. In general, models with efficient super-winds, along with a top-heavy stellar IMF, are able to reproduce fairly well the observed $L_X-T$ relation, the entropy profiles and the cold fraction. Observed radial ICM temperature profiles can be matched, except for the gradual decline in temperature inside of $r\sim$~0.1$R_{\rm{vir}}$. Metal enrichment of the ICM gives rise to somewhat steep inner iron gradients; yet, the global level of enrichment compares well to observational estimates after correcting for the stars formed at late times at the base of the cooling flows; also the metal partition between stars and ICM gets into good agreement with observations.Comment: 23 pages, 20 colour figures; final version accepte

The structure of spiral galaxies: radial profiles in stellar Mass-to-Light ratio and the Dark Matter distribution

The colour and metallicity gradients observed in spiral galaxies suggest that the mass-to-light ratio (M*/L) of the stellar disc is a function of radius. This is indeed predicted by chemo-photometric models of galactic discs. We investigate the distribution of luminous and dark matter in spiral galaxies, taking into account the radial dependence of the stellar M*/L - which is usually assumed to be constant in studies of the mass structure. From the chemo-photometric models of Portinari et al. (2004) and in agreement with the observed radial profiles of galaxy colours, we derive the typical average M*/L profile of the stellar discs of spiral galaxies. We compute the corresponding Variable Mass-to-Light (VML) stellar surface density profile and then the VML disc contribution to the circular velocity. We use the latter, combined with a well studied dark matter velocity profile, to mass model the co-added rotation curves of Persic et al. (1996). (See http://www.facebook.com/home.php#!/group.php?gid=310260450630 to retrieve extra information on the kinematics of galaxies) By investigating rotation curves in the framework of VML stellar discs, we confirm, to a first approximation, the scenario obtained with the constant M*/L assumption: a dark matter halo with a shallow core, an inner baryon-dominated region and a larger proportion of Dark Matter in smaller objects. However, the resulting size of the the dark halo core and of the inner baryon dominance region are somewhat smaller. The stronger role that VML discs have in the innermost regions is important to constrain the galaxy mass structure in both Lambda Cold Dark Matter and MOND scenarios.Comment: 12 pages, 10 figures, in press on Astronomy and Astrophysics. Minor text revisions to match published version. Reference added to web-link to http://www.facebook.com/home.php#!/group.php?gid=31026045063

The Formation of Fossil Galaxy Groups in the hierarchical Universe

We use a set of twelve high-resolution N-body/hydrodynamical simulations in the $\Lambda$CDM cosmology to investigate the origin and formation rate of fossil groups (FGs), which are X-ray bright galaxy groups dominated by a large elliptical galaxy, with the second brightest galaxy being at least two magnitudes fainter. The simulations invoke star formation, chemical evolution with non-instantaneous recycling, metal dependent radiative cooling, strong star burst driven galactic super winds, effects of a meta-galactic UV field and full stellar population synthesis. We find an interesting correlation between the magnitude gap between the first and second brightest galaxy and the formation time of the group. It is found that FGs have assembled half of their final dark matter mass already at z\ga1, and subsequently typically grow by minor merging only, wheras non-FGs on average form later. The early assembly of FGs leaves sufficient time for galaxies of $L \sim L_*$ to merge into the central one by dynamical friction, resulting in the large magnitude gap at $z=0$. A fraction of 33$\pm$16% of the groups simulated are found to be fossil, whereas the observational estimate is $\sim$10-20%. The FGs are found to be X-ray over-luminous relative to non-FGs of the same optical luminosity, in qualitative agreement with observations. Finally, from a dynamical friction analysis is found that only because infall of $L \sim L_*$ galaxies happens along filaments with small impact parameters do FGs exist at all.Comment: 4 pages, 3 figures, one figure removed. Accepted for publication in ApJ Lette

Effects of radial flows on the chemical evolution of the Milky Way disk

The majority of chemical evolution models assume that the Galactic disk forms by means of infall of gas and divide the disk into several independent rings without exchange of matter between them. However, if gas infall is important, radial gas flows should be taken into account as a dynamical consequence of infall. The aim of this paper is to test the effect of radial gas flows on detailed chemical evolution models (one-infall and two-infall) for the Milky Way disk with different prescriptions for the infall law and star formation rate. We found, that with a gas radial inflow of constant speed the metallicity gradient tends to steepen. Taking into account a constant time scale for the infall rate along the Galaxy disk and radial flows with a constant speed, we obtained a too flat gradient, at variance with data, implying that an inside-out formation and/or a variable gas flow speed are required. To reproduce the observed gradients the gas flow should increase in modulus with the galactocentric distance, both in the one-infall and two-infall models. However, the inside-out disk formation coupled with a threshold in the gas density (only in the two-infall model) for star formation and/or a variable efficiency of star formation with galactocentric distance can also reproduce the observed gradients without radial flows. We showed that the radial flows can be the most important process in reproducing abundance gradients but only with a variable gas speed. Finally, one should consider that uncertainties in the data concerning gradients prevent us to draw firm conclusions. Future more detailed data will help to ascertain whether the radial flows are a necessary ingredient in the formation and evolution of the Galactic disk and disks in general.Comment: Accepted by A&A; 11 pages, 16 figure