Galaxies Probing Galaxies in PRIMUS - II. The Coherence Scale of the
  Cool Circumgalactic Medium
  The circumgalactic medium (CGM) close to ~L* star-forming galaxies hosts
strong MgII 2796 absorption (with equivalent width W_2796>0.1 Ang) with a
near-unity covering fraction. To characterize the spatial coherence of this
absorption, we analyze the W_2796 distribution in the CGM of 27 star-forming
galaxies detected in deep spectroscopy of bright background (b/g) galaxies
first presented in Rubin et al. (2018). The sample foreground (f/g) systems
have redshifts 0.35<z<0.8 and stellar masses 9.1<log M_*/M_sun<11.1, and the
b/g galaxies provide spatially-extended probes with half-light radii 1.0
kpc<R_eff<7.9 kpc at projected distances R_perp<50 kpc. Our analysis also draws
on literature W_2796 values measured in b/g QSO spectroscopy probing the halos
of f/g galaxies with a similar range in M_* at z ~ 0.25. By making the
assumptions that (1) samples of like galaxies exhibit similar circumgalactic
W_2796 distributions; and that (2) the quantity log W_2796 has a Gaussian
distribution with a dispersion that is constant with M_* and R_perp, we use
this QSO-galaxy pair sample to construct a model for the log W_2796
distribution in the CGM. We then demonstrate the dependence of this
distribution on the ratio of the surface area of the b/g probe to the projected
absorber surface area (x_A=A_G/A_A), finding that distributions which assume
x_A>=15 are statistically inconsistent with that observed toward our b/g
galaxies at a 95% confidence level. This limit, in combination with the b/g
galaxy sizes, requires that the length scale over which W_2796 does not vary
(the "coherence scale" of MgII absorption) is l_A>1.9 kpc. This novel
constraint on the morphology of cool, photoionized structures in the inner CGM
suggests that either these structures each extend over kiloparsec scales, or
that the numbers and velocity dispersion of these structures are spatially
correlated over the same scales.
