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Register a new userThe Bright End of the Colour-Magnitude Relation
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We investigate the origin of the colour-magnitude relation (CMR) followed by early-type cluster galaxies by using a combination of cosmological N-body simulations of cluster of galaxies and a semi-analytic model of galaxy formation (Lagos, Cora & Padilla 2008). Results show good agreement between the general trend of the simulated and observed CMR. However, in many clusters, the most luminous galaxies depart from the linear fit to observed data displaying almost constant colours. With the aim of understanding this behaviour, we analyze the dependence with redshift of the stellar mass contributed to each galaxy by different processes, i.e., quiescent star formation, and starburst during major/minor and wet/dry mergers, and disk instability events. The evolution of the metallicity of the stellar component, contributed by each of these processes, is also investigated. We find that the major contribution of stellar mass at low redshift is due to minor dry merger events, being the metallicity of the stellar mass accreted during this process quite low. Thus, minor dry merger events seem to increase the mass of the more luminous galaxies without changing their colours.
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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 $zapprox 2$, is mainly driven by minor and major dry mergers. Since the most luminous galaxies have a narrow spread in ages ($1.0times 10^{10}$ yr $<t<1.2times 10^{10}$ 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.
We investigate the development of the colour-magnitude re- lation (CMR) of cluster galaxies. This study is carried out using a semi- analytic model of galaxy formation and evolution coupled to a sample of simulated galaxy clusters of different masses, reinforcing the conclusions reached by Jimenez et al. (2009). We compare both simulated and obeserved CMRs in different colour-magnitude planes, finding a very good agreement in all cases. This indicates that model parameters are correctly tuned, giving accurate values of the main properties of galaxies for further use in our study. In the present work, we perform a statistical analysis of the relative contribution to the stellar mass and metallicity of galaxies along the CMR by the different processes involved in their formation and evolution (i.e. quiescent star formation, disc instability events and galaxy mergers). Our results show that a mix of minor and major dry mergers at low redshifts is relevant in the evolution of the most luminous galaxies in the CMR. These processes contribute with low metallicity stars to the remnant galaxies, thus increasing the galaxy masses without significantly altering their colours. These results are found for all simulated clusters, supporting the idea of the universality of the CMR in agreement with observational results.
Although the optical colour-magnitude diagram of galaxies allows one to select red sequence objects, neither can it be used for galaxy classification without additional observational data such as spectra or high-resolution images, nor to identify blue galaxies at unknown redshifts. We show that adding the near ultraviolet colour to the optical CMD reveals a tight relation in the three-dimensional colour-colour-magnitude space smoothly continuing from the blue cloud to the red sequence. We found that 98 per cent of 225,000 low-redshift (Z<0.27) galaxies follow a smooth surface g-r=F(M,NUV-r) with a standard deviation of 0.03-0.07 mag making it the tightest known galaxy photometric relation. There is a strong correlation between morphological types and integrated NUV-r colours. Rare galaxy classes such as E+A or tidally stripped systems become outliers that occupy distinct regions in the 3D parameter space. Using stellar population models for galaxies with different SFHs, we show that (a) the (NUV-r, g-r) distribution is formed by objects having constant and exponentially declining SFR with different characteristic timescales; (b) colour evolution for exponentially declining models goes along the relation suggesting its weak evolution up-to a redshift of 0.9; (c) galaxies with truncated SFHs have very short transition phase offset from the relation thus explaining the rareness of E+A galaxies. This relation can be used as a powerful galaxy classification tool when morphology remains unresolved. Its mathematical consequence is the photometric redshift estimates from 3 broad-band photometric points. This approach works better than most existing photometric redshift techniques applied to multi-colour datasets. Therefore, the relation can be used as an efficient selection technique for galaxies at intermediate redshifts (0.3<Z<0.8) using optical imaging surveys.
In this work, we study the evolution with redshift of the colour-magnitude relation (CMR) of early-type galaxies. This evolution is analyzed through cosmological numerical simulations from z = 2 to z = 0. The preliminary results shown here represent the starting point of a study aimed at identifying the processes that originated the observed CMR of early-type galaxies at z = 0.
We study the colour-magnitude relation (CMR) for a sample of 172 morphologically-classified E/S0 cluster galaxies from the ESO Distant Cluster Survey (EDisCS) at 0.4<z<0.8. The intrinsic colour scatter about the CMR is very small (0.076) in rest-frame U-V. Only 7% of the galaxies are significantly bluer than the CMR. The scarcity of blue S0s indicates that, if they are the descendants of spirals, these were already red when they became S0s. We observe no dependence of the CMR scatter with redshift or cluster velocity dispersion. This implies that by the time cluster E/S0s achieve their morphology, the vast majority have already joined the red sequence. We estimate the galaxy formation redshift z_F for each cluster and find that it does not depend on the cluster velocity dispersion. However, z_F increases weakly with cluster redshift. This trend becomes clearer when including higher-z clusters from the literature, suggesting that, at any given z, in order to have a population of fully-formed E and S0s they needed to have formed most of their stars 2-4 Gyr prior to observation. In other words, the galaxies that already have early-type (ET) morphologies also have reasonably-old stellar populations. This is partly a manifestation of the progenitor bias, but also a consequence of the fact that the vast majority of the ETs in clusters (in particular the massive ones) were already red by the time they achieved their morphology. E and S0 galaxies exhibit very similar colour scatter, implying similar stellar population ages. We also find that fainter ETs finished forming their stars later, consistent with the cluster red sequence being built over time and the brightest galaxies reaching the red sequence earlier than fainter ones. Finally, we find that the ET cluster galaxies must have had their star formation truncated over an extended period of at least 1 Gyr. [abridged]
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