Title
How do galaxies accrete their mass? Quiescent and star-forming massive  
galaxies at high redshift.

ABSTRACT

I will first provide a broad overview of what we know (and what we  
don't) about the evolution of galaxies in the first 3-4 Gyrs of the  
history of Universe.
In particular, I will show that recent surveys have shown that massive  
galaxies have undergone a major evolution during the epoch  
corresponding to redshift range 2-4, assembling a significant fraction  
of their stellar mass in this epoch, but indicating also that  
significant star-formation rate occurred in galaxies also much  
earlier, i.e. at z>>6.

 I will then focus on our recent results, combining two pieces of  
evidence:
- The results of the analysis of the 24mu MIPS data of the GOODS-S  
field, combined with UV-IR deep images, that allow to trace star  
formation (or the lack of) in high redshift galaxies without biases  
due to dust extinction. I will show  in particular the resulting   
fraction of quiescent and "red&dead" galaxies as a function of  
redshft, and  the evolution of the specific star formation rate as a  
function of redshift and stellar mass.
- The results of an ESO LP dedicated to the search of galaxies at z  
>6.5. I will briefly discuss the strategy and the results obtained so  
far, and the comparison with recent WFC3 data.

The scenario emerging from these data will be compared with recent  
predictions of theoretical models, to discuss the validity of their  
physical ingredients.