HST/WFC3 Confirmation of the Inside-Out Growth of Massive Galaxies at 0<z<2 and Identification of their Star Forming Progenitors at z~3 (1208.0341v2)

Abstract: We study the structural evolution of massive galaxies by linking progenitors and descendants at a constant cumulative number density of n_c=1.4x10^{-4} Mpc^{-3} to z~3. Structural parameters were measured by fitting Sersic profiles to high resolution CANDELS HST WFC3 J_{125} and H_{160} imaging in the UKIDSS-UDS at 1<z<3 and ACS I_{814} imaging in COSMOS at 0.25<z<1. At a given redshift, we selected the HST band that most closely samples a common rest-frame wavelength so as to minimize systematics from color gradients in galaxies. At fixed n_c, galaxies grow in stellar mass by a factor of ~3 from z~3 to z~0. The size evolution is complex: galaxies appear roughly constant in size from z~3 to z~2 and then grow rapidly to lower redshifts. The evolution in the surface mass density profiles indicates that most of the mass at r<2 kpc was in place by z~2, and that most of the new mass growth occurred at larger radii. This inside-out mass growth is therefore responsible for the larger sizes and higher Sersic indices of the descendants toward low redshift. At z<2, the effective radius evolves with the stellar mass as r_e M^{2.0}, consistent with scenarios that find dissipationless minor mergers to be a key driver of size evolution. The progenitors at z~3 were likely star-forming disks with r_e~2 kpc, based on their low Sersic index of n~1, low median axis ratio of b/a~0.52, and typical location in the star-forming region of the U-V versus V-J diagram. By z~1.5, many of these star-forming disks disappeared, giving rise to compact quiescent galaxies. Toward lower redshifts, these galaxies continued to assemble mass at larger radii and became the local ellipticals that dominate the high mass end of the mass function at the present epoch.

Structural Evolution and Inside-Out Growth of Massive Galaxies

This paper presents a comprehensive paper of massive galaxy evolution, focusing on structural changes from redshift $z \sim 3$ to $z \sim 0$. Utilizing high-resolution imaging data from the Hubble Space Telescope (HST), the researchers examined the progression in size, mass distribution, and morphological features across different epochs. They employed a method based on selecting galaxy samples at a constant cumulative number density, rather than traditional mass selections, to trace the evolutionary pathway of typical galaxies over cosmic time from progenitors to descendants.

Key Findings

1. Inside-Out Growth: The paper finds that massive galaxies undergo an inside-out growth, where most of the stellar mass accumulation occurs at larger radii from $z \sim 2$ onward. By $z \sim 2$, significant mass was already concentrated within 2 kpc, and further growth primarily involved the addition of mass to the outer regions. This process results in an increase in the effective radius by a factor of three to four, consistent with minor merger events contributing to this structural development.
2. Size and Sersic Index Evolution: From $z \sim 3$ to $z \sim 0$, the effective radius of these galaxies increases markedly below redshift $z = 2$. The evolution follows $r_e \propto M^{2.0}$, which aligns with scenarios emphasizing dissipationless minor mergers driving size evolution. The Sersic index also increases substantially, suggesting a transition from disk-like star-forming progenitors at high redshifts to elliptical-like quiescent systems at lower redshifts.
3. Star Forming Progenitors: At $z \sim 3$, progenitors of massive galaxies were predominantly star-forming disks with low Sersic indices and small sizes. Their subsequent evolution includes transitioning to compact quiescent galaxies by $z \sim 1.5$, followed by further mass assimilation into local elliptical systems.
4. Quiescent and Star-Forming Fractions: The proportion of quiescent galaxies increases dramatically over time, from about 23% at $z \sim 2.75$ to 89% at $z \sim 0.375$. Star formation activities are progressively reduced as galaxies settle into their quiescent states.

Implications

The insights gained from this paper emphasize a model of galaxy growth primarily influenced by minor mergers, while highlighting the complexity and non-linearity of structural evolution. Understanding this transformation suppresses conventional models that entirely rely on major mergers or simplistic mass-accretion scenarios. These findings have profound implications for comprehending galaxy morphology and evolution at different nodes in cosmic timelines.

Future Directions

Further research could expand on the nuances of the minor merger processes and explore these dynamics at even higher redshifts. Such endeavors would benefit from broader directed use of imaging and spectroscopic data, potentially adjusting cumulative density-selection methodologies for exploring other galaxy types or sizes.

In summary, this paper offers critical perspectives on galaxy growth patterns that contribute to the broader understanding of cosmic evolution and bridge gaps between theoretical predictions and observed characteristics of massive galaxies.