The evolving slope of the stellar mass function at 0.6 ≤ z < 4.5 from deep WFC3 data

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 sigma), as well as deep K-S (as faint as 25.5 at 5 sigma) 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-* similar or equal to 7.6 x 10(9) M-circle dot at z similar to 3. Although the area used is relatively small (similar to 33 arcmin(2)), 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 alpha = -1.44 +/- 0.03 at z similar to 0.8 to alpha = -1.86 +/- 0.16 at z similar to 3, although indications exist that it does not steepen further between z similar to 3 and z similar to 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 similar to 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 similar to 3, as well as a dearth of massive galaxies at z similar to 4 with respect to the data, and that the predicted SMD is generally overestimated at z less than or similar to 2.