High Redshift Massive Quiescent Galaxies are as Flat as Star Forming Galaxies: The Flattening of Galaxies and the Correlation with Structural Properties in CANDELS/3D-HST

Abstract

We investigate the median flattening of galaxies at $0.2<z<4.0$ in all five CANDELS/3D-HST fields via the apparent axis ratio $q$. We separate the sample into bins of redshift, stellar-mass, sérsic index, size, and UVJ determined star-forming state to discover the most important drivers of the median $q$ ($q_{med}$). Quiescent galaxies at $z<1$ and $M_{\ast }>10^{11}{M}_{\odot }$ are rounder than those at lower masses, consistent with the hypothesis that they have grown significantly through dry merging. The massive quiescent galaxies at higher redshift become flatter, and are as flat as star forming massive galaxies at $2.5<z<3.5$, consistent with formation through direct transformations or wet mergers. We find that in quiescent galaxies, correlations with $q_{med}$ and $M_{\ast }$, $z$ and $r_e$ are driven by the evolution in the sérsic index ($n$), consistent with the growing accumulation of minor mergers at lower redshift. Interestingly, $n$ does not drive these trends fully in star-forming galaxies. Instead, the strongest predictor of $q$ in star-forming galaxies is the effective radius, where larger galaxies are flatter. Our findings suggest that $q_{med}$ is tracing bulge-to-total ($B/T$) galaxy ratio which would explain why smaller/more massive star-forming galaxies are rounder than their extended/less massive analogues, although it is unclear why sérsic index correlates more weakly with flattening for star forming galaxies than for quiescent galaxies.

13 pages, 11 figures, accepted to ApJ

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Cited by in corpus (7)

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• The kinematics of massive quiescent galaxies at $1.4<z<2.1$: dark matter fractions, IMF variation, and the relation to local early-type galaxies
• Compact, bulge dominated structures of spectroscopically confirmed quiescent galaxies at z~3
• An exquisitely deep view of quenching galaxies through the gravitational lens: Stellar population, morphology, and ionized gas
• Dust attenuation, dust content and geometry of star-forming galaxies
• Galaxy populations in the most distant SPT-SZ clusters -- II. Galaxy structural properties in massive clusters at 1.4<z<1.7
• 3D intrinsic shapes of quiescent galaxies in observations and simulations