We fit model spectral energy distributions to each pixel in 67 nearby (= 0.0057) galaxies using broadband photometry from the Sloan Digital Sky Survey and GALEX. For each galaxy, we compare the stellar mass derived by summing the mass of each pixel to that found from fitting the entire galaxy treated as an unresolved point source. We find that, while the pixel-by-pixel and unresolved masses of galaxies with low specific star formation rates (such as ellipticals and lenticulars) are in rough agreement, the unresolved mass estimate for star-forming galaxies is systematically lower then the measurement from spatially-resolved photometry. The discrepancy is strongly correlated with sSFR, with the highest sSFRs in our sample having masses underestimated by 25% (0.12 dex) when treated as point sources. We found a simple relation to statistically correct mass estimates derived from unresolved broad-band SED fitting to the resolved mass estimates: m resolved = m unresolved /(−0.057 log (sSF R) + 0.34) where sSFR is in units of yr −1 . We study the effect of varying spatial resolution by degrading the image resolution of the largest images and find a sharp decrease in the pixel-bypixel mass estimate at a physical scale of approximately 3 kpc, which is comparable to spiral arm widths. The effects we observe are consistent with the "outshining" idea which posits that the youngest stellar populations mask more massive, older -and thus fainter -stellar populations. Although the presence of strong dust lanes can also lead to a drastic difference between resolved and unresolved mass estimates (up to 45% or 0.3 dex) for any individual galaxy, we found that resolving dust does not affect mass estimates on average. The strong correlation between mass discrepancy and sSFR is thus most likely due to the outshining systematic bias.

We perform spatially-resolved, pixel-by-pixel SED fitting on galaxies up to z ∼ 2.5 in the Hubble Extreme Deep Field (XDF). Comparing stellar mass estimates from spatially resolved and spatially unresolved photometry we find that unresolved masses can be systematically underestimated by factors of up to 5. The ratio of the unresolved to resolved mass measurement depends on the galaxy's specific star formation rate (sSFR): at low sSFRs the bias is small, but above sSFR ∼ 10 −9.5 yr −1 the discrepancy increases rapidly such that galaxies with sSFRs ∼ 10 −8 yr −1 have unresolved mass estimates of only one half to one fifth of the resolved value. This result indicates that stellar masses estimated from spatially-unresolved datasets need to be systematically corrected, in some cases by large amounts, and we provide an analytic prescription for applying this correction. We show that correcting stellar mass measurements for this bias changes the normalization and slope of the star-forming main sequence and reduces its intrinsic width; most dramatically, correcting for the mass bias increases the stellar mass density of the Universe at high redshift and can resolve the long-standing discrepancy between the directly-measured cosmic star formation rate density at z > ∼ 1 and that inferred from stellar mass densities ("the missing mass problem").

We introduce the largest to date survey of massive quiescent galaxies at redshift z∼1.6. With these data, which cover 27.6 deg 2 , we can find significant numbers of very rare objects such as ultra-massive quiescent galaxies that populate the extreme massive end of the galaxy mass function, or dense environments that are likely to become present-day massive galaxy clusters. In this paper, the first in a series, we apply our gzK s adaptation of the BzK technique to select our z∼1.6 galaxy catalog and then study the quiescent galaxy stellar mass function with good statistics over M ∼ 10 10.2 -10 11.7 M -a factor of 30 in mass -including 60 ultra-massive z∼1.6 quiescent galaxies with M > 10 11.5 M . We find that the stellar mass function of quiescent galaxies at z∼1.6 is well represented by the Schechter function over this large mass range. This suggests that the mass quenching mechanism observed at lower redshifts must have already been well established by this epoch, and that it is likely due to a single physical mechanism over a wide range of mass. This close adherence to the Schechter shape also suggests that neither merging nor gravitational lensing significantly affect the observed quenched population. Finally, comparing measurements of M * parameters for quiescent and star-forming populations (ours and from the literature), we find hints of an offset (M * SF > M * PE ), that could suggest that the efficiency of the quenching process evolves with time.