Missing Stellar Mass in SED Fitting: Spatially Unresolved Photometry can
  Underestimate Galaxy Masses
  We fit model spectral energy distributions to each pixel in 67 nearby
(<z>=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}/(-0.057log(sSFR) + 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-by-pixel 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.
