2012A&A...538A..33S

We used Early Release Science (ERS) observations taken with the Wide Field Camera 3 (WFC3) in the GOODS-S field to study the galaxy stellar mass function (GSMF) at 0.6≤z<4.5. Deep WFC3 near-IR data (for Y as faint as 27.3, J and H as faint as 27.4 AB mag at 5 σ), as well as deep K_S (as faint as 25.5 at 5 σ) Hawk-I band data, provide an exquisite data set with which determine in an unprecedented way the low-mass end of the GSMF, allowing an accurate probe of masses as low as M_*≃7.6x10⁹M_☉ at z∼3. Although the area used is relatively small (∼ 33arcmin²), we found generally good agreement with previous studies on the entire mass range. Our results show that the slope of the faint-end increases with redshift, from α=-1.44±0.03 at z∼0.8 to α=-1.86±0.16 at z∼3, although indications exist that it does not steepen further between z∼3 and z∼4. This result is insensitive to any uncertainty in the M^* parameter. The steepness of the GSMF faint-end solves the well-known disagreement between the stellar mass density (SMD) and the integrated star-formation history at z>2. However, we confirm that there appears to be an excess of integrated star formation with respect to the SMD at z<2, by a factor of ∼2-3. Our comparison of the observations with theoretical predictions shows that the models forecast a greater abundance of low mass galaxies, at least up to z∼3, as well as a dearth of massive galaxies at z∼4 with respect to the data, and that the predicted SMD is generally overestimated at z≲2.