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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
Astrophysics of Galaxies · Submitted 7 Jan 2019
abstract compiled · 2026-09-23 · no language-model review
The authors' main claim 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.
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The abstract, sentence by sentence
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, s\'ersic 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 $M_{*}>10^{11}M_{\odot}$ 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 $M_{*}$, $z$ and $r_{e}$ are driven by the evolution in the s\'ersic 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 s\'ersic index correlates more weakly with flattening for star forming galaxies than for quiescent galaxies.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | 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 $q$. | contribution 0.67 | 0.96 | 0.03 | asserted 0.86 |
| s1 | We separate the sample into bins of redshift, stellar-mass, s\'ersic index, size, and UVJ determined star-forming state to discover the most important drivers of the median $q$ ($q_{med}$). | method 0.95 | 0.96 | 0.02 | asserted 0.78 |
| s2 | Quiescent galaxies at $z<1$ and $M_{*}>10^{11}M_{\odot}$ are rounder than those at lower masses, consistent with the hypothesis that they have grown significantly through dry merging. | result 0.97 | 0.84 | 0.12 | asserted 0.69 |
| s3 | 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. | result 0.96 | 0.91 | 0.57 | asserted 0.67 |
| s4 | We find that in quiescent galaxies, correlations with $q_{med}$ and $M_{*}$, $z$ and $r_{e}$ are driven by the evolution in the s\'ersic index ($n$), consistent with the growing accumulation of minor mergers at lower redshift. | result 0.71 | 0.96 | 0.17 | asserted 0.99 |
| s5 | Interestingly, $n$ does not drive these trends fully in star-forming galaxies. | result 0.99 | 0.91 | 0.04 | asserted 1.00 |
| s6 | Instead, the strongest predictor of $q$ in star-forming galaxies is the effective radius, where larger galaxies are flatter. | result 1.00 | 0.94 | 0.10 | asserted 1.00 |
| s7 | 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 s\'ersic index correlates more weakly with flattening for star forming galaxies than for quiescent galaxies. | interpretation 1.00 | 0.63 | 0.20 | hedged possibility 1.00 |
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The paper
Original 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, s\'ersic 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 $M_{*}>10^{11}M_{\odot}$ 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 $M_{*}$, $z$ and $r_{e}$ are driven by the evolution in the s\'ersic 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 s\'ersic index correlates more weakly with flattening for star forming galaxies than for quiescent galaxies.
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