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Register a new userThe rest-frame optical sizes of massive galaxies with suppressed star formation at $zsim4$
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We present the rest-frame optical sizes of massive quiescent galaxies (QGs) at $zsim4$ measured at $K$-band with the Infrared Camera and Spectrograph (IRCS) and AO188 on the Subaru telescope. Based on a deep multi-wavelength catalog in the Subaru XMM-Newton Deep Survey Field (SXDS), covering a wide wavelength range from the $u$-band to the IRAC $8.0mu m$ over 0.7 deg$^2$, we evaluate photometric redshift to identify massive ($M_{star}sim10^{11} M_odot$) galaxies with suppressed star formation. These galaxies show a prominent 4000$rm AA$ break feature at $zsim4$, suggestive of an evolved stellar population. We then conduct follow-up $K$-band imaging with adaptive optics for the five brightest galaxies ($K_{AB,total}=22.5sim23.4$). Compared to lower redshift ones, QGs at $zsim4$ have smaller physical sizes of effective radii $r_{eff}=0.2$ to $1.8$ kpc. The mean size measured by stacking the four brightest objects is $r_{eff}=0.7rm kpc$. This is the first measurement of the rest-frame optical sizes of QGs at $zsim4$. We evaluate the robustness of our size measurements using simulations and find that our size estimates are reasonably accurate with an expected systematic bias of $sim0.2$ kpc. If we account for the stellar mass evolution, massive QGs at $zsim4$ are likely to evolve into the most massive galaxies today. We find their size evolution with cosmic time in a form of $log(r_e/{rm kpc})= -0.44+1.77 log(t/rm Gyr)$. Their size growth is proportional to the square of stellar mass, indicating the size-stellar mass growth driven by minor dry mergers.
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We study the rest-frame ultra-violet sizes of massive (~0.8 x 10^11 M_Sun) galaxies at 3.4<z<4.2, selected from the FourStar Galaxy Evolution Survey (ZFOURGE), by fitting single Sersic profiles to HST/WFC3/F160W images from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS). Massive quiescent galaxies are very compact, with a median circularized half-light radius r_e = 0.63 +/- 0.18 kpc. Removing 5/16 (31%) sources with signs of AGN activity does not change the result. Star-forming galaxies have r_e = 2.0 +/- 0.60 kpc, 3.2 +/- 1.3 x larger than quiescent galaxies. Quiescent galaxies at z~4 are on average 6.0 +- 0.17 x smaller than at z~0 and 1.9 +/- 0.7 x smaller than at z~2. Star-forming galaxies of the same stellar mass are 2.4 +/- 0.7 x smaller than at z~0. Overall, the size evolution at 0<z<4 is well described by a powerlaw, with r_e = 5.08 +/- 0.28 (1+z)^(-1.44+/-0.08) kpc for quiescent and r_e = 6.02 +/- 0.28 (1+z)^(-0.72+/-0.05) kpc for star-forming galaxies. Compact star-forming galaxies are rare in our sample: we find only 1/14 (7%) with r_e / (M / 10^11 M_Sun)^0.75 < 1.5, whereas 13/16 (81%) of the quiescent galaxies is compact. The number density of compact quiescent galaxies at z~4 is 1.8 +/- 0.8 x 10^-5 Mpc^-3 and increases rapidly, by >5 x, between 2<z<4. The paucity of compact star-forming galaxies at z~4 and their large rest-frame ultra-violet median sizes suggest that the formation phase of compact cores is very short and/or highly dust obscured.
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