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The Rise and Fall of Stellar Disks in Massive Galaxies


Massive galaxies at higher redshift, z > 2, show different characteristics than their local counterparts. They are compact and most likely have a disk. Understanding the evolutionary path of these massive galaxies can give us some clues on how the universe has been behaving in the last 10 billion years. How well can we measure the bulge and disk properties of these systems? We perform two sets of comprehensive simulations in order to systematically quantify the effects of non-homology in structures and the methods employed.

For the first set of simulations, by accurately capturing the detailed substructures of nearby elliptical galaxies and then rescaling their sizes and signal-to-noise to mimic galaxies at different redshifts, we confirm that the massive quiescent galaxies at z ≈ 2 are significantly more compact intrinsically than their local counterparts. Their observed compactness is not a result of missing faint outer light due to systematic errors in modeling.

For the second set of simulations, we employ empirical scaling relations to produce realistic-looking two-component local galaxies with a uniform and wide range of bulge-to-total ratios (B/T), and then rescale them to mimic the signal-to-noise ratios and sizes of observed galaxies at z ≈ 2. This provides the first set of simulations for which we can examine the robustness of two-component decomposition of compact disk galaxies at different B/T. We can measure B/T accurately without imposing any constraints on the light profile shape of the bulge, but, due to the small angular sizes of bulges at high redshift, their detailed properties can only be recovered for galaxies with B/T \gax\ 0.2. The disk component, by contrast, can be measured with little difficulty.

Next, we trace back the evolution of local massive galaxies but performing detailed morphological analysis: namely, single Sérsic fitting and bulge+disk decomposition. CANDELS images and catalogues offer an ideal dataset for this study. We analyze about 250 massive galaxies selected from all available CANDELS fields (COSMOS, UDS, EGS, GOODS-South and GOODS-North). We confirm that both star-forming and quiescent galaxies have increased their sizes significantly between 0 < z < 2.5. The Sérsic index of quiescent galaxies have increased over time (from n ≈ 2.5 to n > 4) while for star-forming galaxies, it stays almost the intact (n ≈ 2.5). The quiescent galaxie have become rounder. The bulge+disk decompositions reveal that massive galaxies at higher redshift are more disk dominated and by z ≈ 0.5, massive quiescent galaxies begin to resemble local elliptical galaxies. Star-forming galaxies have lower B/T at each redshift bin and their fraction decreases at lower redshifts. Bulges of star-forming and quiescent galaxies follow different evolutionary trajectories, while their disks evolve similarly. I show that major mergers, along with minor mergers, have played a crucial role in the significant size increase of high-z galaxies and the destruction of their massive and large-scale disks.

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