COSMOS-DASH: The Evolution of the Galaxy Size-Mass Relation Since z~3
  from new Wide Field WFC3 Imaging Combined with CANDELS/3DHST
  We present COSMOS-Drift And SHift (DASH), a Hubble Space Telescope WFC3
imaging survey of the COSMOS field in the H_160 filter. The survey comprises
456 individual WFC3 pointings corresponding to an area of 0.49 deg^2 (0.66
deg^2 when including archival data) and reaches a 5 point-source limit of H_160
=25.1 (0".3 aperture). COSMOS-DASH is the widest HST/WFC3 imaging survey in
H_160 filter, tripling the extragalactic survey area in the near-infrared at
HST resolution. We make the reduced H_160 mosaic available to the community. We
use this dataset to measure the sizes of 162 galaxies with log(M_star/M_sun) >
11.3 at 1.5 < z < 3.0, and augment this sample with 748 galaxies at 0.1 < z <
1.5 using archival ACS imaging. We find that the median size of galaxies in
this mass range changes with redshift as r_eff = (10.4+/-0.4)(1
+z)^(0.65+/-0.05) kpc. Separating the galaxies into star forming and quiescent
galaxies using their restframe U-V and V-J colors, we find no statistical
difference between the median sizes of the most massive star-forming and
quiescent galaxies at z = 2.5: they are 4.9+/-0.9 kpc and 4.3 +/-0.3 kpc
respectively. However, we do find a significant difference in the S`ersic index
between the two samples, such that massive quiescent galaxies have higher
central densities than star forming galaxies. We extend the size-mass analysis
to lower masses by combining it with the 3D-HST/CANDELS sample of van der Wel
et al. (2014), and derive empirical relations between size, mass, and redshift.
Fitting a relation of the form r_eff = A m_star^a, m_star = M_star/5x10^10
M_sun and r_eff in kpc, we find log A = -0.25 log (1 + z) + 0.79 and a = -0.13
log(1 + z) + 0.27. We also provide relations for the subsamples of star forming
and quiescent galaxies. Our results confirm previous studies that were based on
smaller samples or ground-based imaging.
