The bright end of the colour-magnitude relation of cluster galaxies

We investigate the physical processes involved in the development of the red sequence (RS) of cluster galaxies by using a combination of cosmological N-body simulations of clusters of galaxies and a semi-analytic model of galaxy formation. Results show good agreement between the general trend of the simulated RS and the observed colour-magnitude relation (CMR) of early-type galaxies in different magnitude planes. However, in many clusters, the most luminous galaxies (MR ∼ MV ∼ MT1 ≲ −20) depart from the linear fit to observed data, as traced by less luminous ones, displaying almost constant colours. With the aim of understanding this particular behaviour of galaxies in the bright end of the RS, we analyse the dependence with redshift of the fraction of stellar mass contributed to each galaxy by different processes, i.e. quiescent star formation, and starbursts triggered by disc instability and merger events. We find that the evolution of galaxies in the bright end since z ≈ 2 is mainly driven by minor and major dry mergers, while minor and major wet mergers are relevant in determining the properties of less luminous galaxies. Since the most luminous galaxies have a narrow spread in ages (1.0 × 1010 yr 10 yr), their metallicities are the main factor that affects their colours. Their mean iron abundances are close to the solar value and have already been reached at z ≈ 1. This fact is consistent with several observational evidence that favour a scenario in which both the slope and scatter of the CMR are in place since z ≈ 1.2. Galaxies in the bright end reach an upper limit in metallicity as a result of the competition of the mass of stars and metals provided by the star formation occurring in the galaxies themselves and by the accretion of merging satellites. Star formation activity in massive galaxies (stellar mass M★ ≳ 1010M⊙) that takes place at low redshifts contributes with stellar components of high metallicity, but the fraction of stellar mass contributed since z ≈ 1 is negligible with respect to the total mass of the galaxy at z= 0. On the other hand, mergers contribute with a larger fraction of stellar mass (≈10-20 per cent), but the metallicity of the accreted satellites is lower by ≈0.2 dex than the mean metallicity of galaxies they merge with. The effect of dry mergers is to increase the mass of galaxies in the bright end, without significantly altering their metallicities. Hence, very luminous galaxies present similar colours that are bluer than those expected if recent star formation activity were higher, thus giving rise to a break in the RS. These results are found for simulated clusters with different virial masses (1014 - 1015h-1M⊙), supporting the idea of the universality of the CMR in agreement with observational results.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Ram pressure stripping in a galaxy formation model - I. A novel numerical approach

We develop a new numerical approach to describe the action of ram pressure stripping (RPS) within a semi-analytic model of galaxy formation and evolution which works in combination with non-radiative hydrodynamical simulations of galaxy clusters. The new feature in our method is the use of the gas particles to obtain the kinematical and thermodynamical properties of the intragroup and intracluster medium (ICM). This allows a self-consistent estimation of the RPS experienced by satellite galaxies. We find that the ram pressure in the central regions of clusters increases approximately one order of magnitude between z= 1 and 0, consistent with the increase in the density of the ICM. The mean ram pressure experienced by galaxies within the virial radius increases with decreasing redshift. In clusters with virial masses Mvir≃ 1015 h-1 M⊙, over 50 per cent of satellite galaxies have experienced ram pressures ~10-11 h-2 dyn cm-2 or higher for z≲ 0.5. In smaller clusters (Mvir≃ 1014 h-1 M⊙) the mean ram pressures are approximately one order of magnitude lower at all redshifts. RPS has a strong effect on the cold gas content of galaxies for all cluster masses. At z= 0, over 70 per cent of satellite galaxies within the virial radius are completely depleted of cold gas. For the more massive clusters the fraction of depleted galaxies is already established at z≃ 1, whereas for the smaller clusters this fraction increases appreciably between z= 1 and 0. This indicates that the rate at which the cold gas is stripped depends on the virial mass of the host cluster. Compared to our new approach, the use of an analytic profile to describe the ICM results in an overestimation of the ram pressure larger than 50 per cent for z > 0.5.Facultad de Ciencias Astronómicas y Geofísica

Calibration of semi-analytic models of galaxy formation using Particle Swarm Optimization

We present a fast and accurate method to select an optimal set of parameters in semi-analytic models of galaxy formation and evolution (SAMs). Our approach compares the results of a model against a set of observables applying a stochastic technique called Particle Swarm Optimization (PSO), a self-learning algorithm for localizing regions of maximum likelihood in multidimensional spaces that outperforms traditional sampling methods in terms of computational cost. We apply the PSO technique to the SAG semi-analytic model combined with merger trees extracted from a standard $\Lambda$CDM N-body simulation. The calibration is performed using a combination of observed galaxy properties as constraints, including the local stellar mass function and the black hole to bulge mass relation. We test the ability of the PSO algorithm to find the best set of free parameters of the model by comparing the results with those obtained using a MCMC exploration. Both methods find the same maximum likelihood region, however the PSO method requires one order of magnitude less evaluations. This new approach allows a fast estimation of the best-fitting parameter set in multidimensional spaces, providing a practical tool to test the consequences of including other astrophysical processes in SAMs.Comment: 11 pages, 4 figures, 1 table. Accepted for publication in ApJ. Comments are welcom

Ram pressure profiles in galaxy groups and clusters

Using a hybrid method which combines non-radiative hydrodynamical simulations with a semi-analytic model of galaxy formation, we determine the ram pressure as a function of halocentric distance experienced by galaxies in haloes with virial masses 12.5 <= log (M_200 h/M_Sun) < 15.35, for redshifts 0 <= z <= 3. The ram pressure is calculated with a self-consistent method which uses the simulation gas particles to obtain the properties of the intergalactic medium. The ram pressure profiles obtained can be well described by beta profile models, with parameters that depend on redshift and halo virial mass in a simple fashion. The fitting formulae provided here will prove useful to include ram pressure effects into semi-analytic models based on methods which lack gas physics, such as dark matter-only simulations or the Press-Schechter formalism.Comment: 7 pages, 6 figures. Accepted for publication in MNRA

The bright end of the colour-magnitude relation of cluster galaxies

We investigate the physical processes involved in the development of the red sequence (RS) of cluster galaxies by using a combination of cosmological N-body simulations of clusters of galaxies and a semi-analytic model of galaxy formation. Results show good agreement between the general trend of the simulated RS and the observed colour-magnitude relation (CMR) of early-type galaxies in different magnitude planes. However, in many clusters, the most luminous galaxies (MR ∼ MV ∼ MT1 ≲ −20) depart from the linear fit to observed data, as traced by less luminous ones, displaying almost constant colours. With the aim of understanding this particular behaviour of galaxies in the bright end of the RS, we analyse the dependence with redshift of the fraction of stellar mass contributed to each galaxy by different processes, i.e. quiescent star formation, and starbursts triggered by disc instability and merger events. We find that the evolution of galaxies in the bright end since z ≈ 2 is mainly driven by minor and major dry mergers, while minor and major wet mergers are relevant in determining the properties of less luminous galaxies. Since the most luminous galaxies have a narrow spread in ages (1.0 × 1010 yr 10 yr), their metallicities are the main factor that affects their colours. Their mean iron abundances are close to the solar value and have already been reached at z ≈ 1. This fact is consistent with several observational evidence that favour a scenario in which both the slope and scatter of the CMR are in place since z ≈ 1.2. Galaxies in the bright end reach an upper limit in metallicity as a result of the competition of the mass of stars and metals provided by the star formation occurring in the galaxies themselves and by the accretion of merging satellites. Star formation activity in massive galaxies (stellar mass M★ ≳ 1010M⊙) that takes place at low redshifts contributes with stellar components of high metallicity, but the fraction of stellar mass contributed since z ≈ 1 is negligible with respect to the total mass of the galaxy at z= 0. On the other hand, mergers contribute with a larger fraction of stellar mass (≈10-20 per cent), but the metallicity of the accreted satellites is lower by ≈0.2 dex than the mean metallicity of galaxies they merge with. The effect of dry mergers is to increase the mass of galaxies in the bright end, without significantly altering their metallicities. Hence, very luminous galaxies present similar colours that are bluer than those expected if recent star formation activity were higher, thus giving rise to a break in the RS. These results are found for simulated clusters with different virial masses (1014 - 1015h-1M⊙), supporting the idea of the universality of the CMR in agreement with observational results.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Ram pressure stripping in a galaxy formation model - I. A novel numerical approach

We develop a new numerical approach to describe the action of ram pressure stripping (RPS) within a semi-analytic model of galaxy formation and evolution which works in combination with non-radiative hydrodynamical simulations of galaxy clusters. The new feature in our method is the use of the gas particles to obtain the kinematical and thermodynamical properties of the intragroup and intracluster medium (ICM). This allows a self-consistent estimation of the RPS experienced by satellite galaxies. We find that the ram pressure in the central regions of clusters increases approximately one order of magnitude between z= 1 and 0, consistent with the increase in the density of the ICM. The mean ram pressure experienced by galaxies within the virial radius increases with decreasing redshift. In clusters with virial masses Mvir≃ 1015 h-1 M⊙, over 50 per cent of satellite galaxies have experienced ram pressures ~10-11 h-2 dyn cm-2 or higher for z≲ 0.5. In smaller clusters (Mvir≃ 1014 h-1 M⊙) the mean ram pressures are approximately one order of magnitude lower at all redshifts. RPS has a strong effect on the cold gas content of galaxies for all cluster masses. At z= 0, over 70 per cent of satellite galaxies within the virial radius are completely depleted of cold gas. For the more massive clusters the fraction of depleted galaxies is already established at z≃ 1, whereas for the smaller clusters this fraction increases appreciably between z= 1 and 0. This indicates that the rate at which the cold gas is stripped depends on the virial mass of the host cluster. Compared to our new approach, the use of an analytic profile to describe the ICM results in an overestimation of the ram pressure larger than 50 per cent for z > 0.5.Facultad de Ciencias Astronómicas y Geofísica

Calibration of semi-analytic models of galaxy formation using particle swarm optimization

We present a fast and accurate method to select an optimal set of parameters in semi-analytic models of galaxy formation and evolution (SAMs). Our approach compares the results of a model against a set of observables applying a stochastic technique called Particle Swarm Optimization (PSO), a self-learning algorithm for localizing regions of maximum likelihood in multidimensional spaces that outperforms traditional sampling methods in terms of computational cost. We apply the PSO technique to the SAG semi-analytic model combined with merger trees extracted from a standard Lambda Cold Dark Matter N-body simulation. The calibration is performed using a combination of observed galaxy properties as constraints, including the local stellar mass function and the black hole to bulge mass relation. We test the ability of the PSO algorithm to find the best set of free parameters of the model by comparing the results with those obtained using a MCMC exploration. Both methods find the same maximum likelihood region, however, the PSO method requires one order of magnitude fewer evaluations. This new approach allows a fast estimation of the best-fitting parameter set in multidimensional spaces, providing a practical tool to test the consequences of including other astrophysical processes in SAMs.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

The bright end of the colour-magnitude relation of cluster galaxies

We investigate the development of the red sequence (RS) of cluster galaxies by using a semi-analytic model of galaxy formation. Results show good agreement between the general trend of the simulated RS and the observed relation in early-type galaxies. However, the most luminous galaxies ($M_V \lesssim -20$) depart from the linear fit to observed data, displaying almost constant colours. We analyze the dependence with redshift of the fraction of stellar mass contributed to each galaxy by different processes (i.e., quiescent star formation, disc instability and mergers), finding that the evolution of the bright end, since $z\approx 2$, is mainly driven by minor and major dry mergers. Since the most luminous galaxies have a narrow spread in ages ($1.0\times 10^{10}$ yr $<t<1.2\times 10^{10}$ yr), their metallicities are the main factor that affects their colours. Galaxies in the bright end reach an upper limit in metallicity as a result of the competition of the mass of stars and metals provided by the star formation within the galaxies and by the accretion of merging satellites. Star formation activity in massive galaxies (M_\star \gtrsim 10^{10} M_{\odot}$) contribute with stellar components of high metallicity, but this fraction of stellar mass is negligible. Mergers contribute with a larger fraction of stellar mass ($\approx 10-20$per cent), but the metallicity of the accreted satellites is lower by$\approx 0.2$ dex than the mean metallicity of galaxies they merge with. The effect of dry mergers is to increase the mass of galaxies in the bright end, without significantly altering their metallicities, and hence,their colours, giving rise to the break in the RS. These results are found for clusters with different virial masses, supporting the idea of the universality of the CMR in agreement with observational results.Comment: 18 pages,7 figures,1 table. Accepted for publication in MNRA