We use a sample of 8298 galaxies observed as part of the HST H _ { 160 } -band GOODS NICMOS Survey ( GNS ) to construct the galaxy stellar mass function both as a function of redshift and stellar mass up to z = 3.5 .
Our mass functions are constructed within the redshift range z = 1 - 3.5 and consist of galaxies with stellar masses of M _ { * } = 10 ^ { 12 } M _ { \odot } down to nearly dwarf galaxy masses of M _ { * } = 10 ^ { 8.5 } M _ { \odot } in the lowest redshift bin .
We discover that a significant fraction of all massive M _ { * } > 10 ^ { 11 } M _ { \odot } galaxies are in place up to the highest redshifts we probe , with a decreasing fraction of lower mass galaxies present at all redshifts .
This is an example of ‘ galaxy mass downsizing ’ , and is the result of massive galaxies forming before lower mass ones , and not just simply ending their star formation earlier as in traditional downsizing scenarios , whose effect is seen at z < 1.5 .
By fitting Schechter functions to our mass functions we find that the faint end slope ranges from \alpha = -1.36 to -1.73 , which is significantly steeper than what is found in previous investigations of the mass function at high redshift .
We demonstrate that this steeper mass function better matches the stellar mass added due to star formation , thereby alleviating some of the mismatch between these two measures of the evolution of galaxy mass .
We furthermore examine the stellar mass function divided into blue/red systems , as well as for star forming and non-star forming galaxies .
We find a similar mass downsizing present for both blue/red and star-forming/non-star forming galaxies , and further find that red galaxies dominate at the high mass end of the mass function , but that the low mass galaxies are mostly all blue , and therefore blue galaxies are creating the steep mass functions observed at z > 2 .
We furthermore show that , although there is a downsizing such that high mass galaxies are nearer their z = 0 values at high redshift , this turns over at masses M _ { * } \sim 10 ^ { 10 } M _ { \odot } , such that the lowest mass galaxies are more common than galaxies at slight higher masses , creating a ‘ dip ’ in the observed galaxy mass function .
We argue that the galaxy assembly process may be driven by different mechanisms at low and high masses , and that the efficiency of the galaxy formation process is lowest at masses M _ { * } \sim 10 ^ { 10 } M _ { \odot } at 1 < z < 3 .
Finally , we calculate the integrated stellar mass density for the total , blue and red populations .
We find the integrated stellar mass density of the total and blue galaxy population is consistent with being constant over z = 1 - 2 , while the red population shows an increase in integrated stellar mass density over the same redshift range .