The Intrinsic Scatter Along The Main Sequence of Star-Forming Galaxies
  at z ~ 0.7
  A sample of 12614 star-forming galaxies (SFGs) with stellar mass >10^9.5
M_sun between 0.6<z<0.8 from COSMOS is selected to study the intrinsic scatter
of the correlation between star formation rate (SFR) and stellar mass. We
derive SFR from ultraviolet (UV) and infrared (IR) luminosities. A stacking
technique is adopted to measure IR emission for galaxies undetected at 24
micron. We confirm that the slope of the mass-SFR relation is close to unity.
We examine the distributions of specific SFRs (SSFRs) in four equally spaced
mass bins from 10^9.5 M_sun to 10^11.5 M_sun. Different models are used to
constrain the scatter of SSFR for lower mass galaxies that are mostly
undetected at 24 micron. The SFR scatter is dominated by the scatter of UV
luminosity and gradually that of IR luminosity at increasing stellar mass. We
derive SSFR dispersions of 0.18, 0.21, 0.26 and 0.31 dex with a typical
measurement uncertainty of <~ 0.01 dex for the four mass bins. Interestingly,
the scatter of the mass-SFR relation seems not constant in the sense that the
scatter in SSFR is smaller for SFGs of stellar mass <10^10.5 M_sun. If
confirmed, this suggests that the physical processes governing star formation
become systematically less violent for less massive galaxies. The SSFR
distribution for SFGs with intermediate mass 10^10-10^10.5 M_sun is
characterized by a prominent excess of intense starbursts in comparison with
other mass bins. We argue that this feature reflects that both violent (e.g.,
major/minor mergers) and quiescent processes are important in regulating star
formation in this intermediate mass regime.
