3D-HST+CANDELS: The Evolution of the Galaxy Size-Mass Distribution since $z=3$ (1404.2844v2)

Abstract: Spectroscopic + photometric redshifts, stellar mass estimates, and rest-frame colors from the 3D-HST survey are combined with structural parameter measurements from CANDELS imaging to determine the galaxy size-mass distribution over the redshift range 0<z\<3. Separating early- and late-type galaxies on the basis of star-formation activity, we confirm that early-type galaxies are on average smaller than late-type galaxies at all redshifts, and find a significantly different rate of average size evolution at fixed galaxy mass, with fast evolution for the early-type population, R_eff ~ (1+z)^-1.48, and moderate evolution for the late-type population, R_eff ~ (1+z)^-0.75. The large sample size and dynamic range in both galaxy mass and redshift, in combination with the high fidelity of our measurements due to the extensive use of spectroscopic data, not only fortify previous results, but also enable us to probe beyond simple average galaxy size measurements. At all redshifts the slope of the size-mass relation is shallow, R_eff ~ M_star^0.22, for late-type galaxies with stellar mass \>3x10^9 M_sol, and steep, R_eff M_star^0.75, for early-type galaxies with stellar mass >2x10^10 M_sol. The intrinsic scatter is <~0.2 dex for all galaxy types and redshifts. For late-type galaxies, the logarithmic size distribution is not symmetric, but skewed toward small sizes: at all redshifts and masses a tail of small late-type galaxies exists that overlaps in size with the early-type galaxy population. The number density of massive (~10^11 M_sol), compact (R_eff < 2 kpc) early-type galaxies increases from z=3 to z=1.5-2 and then strongly decreases at later cosmic times.

• The paper shows early-type galaxies undergo faster size evolution (∝(1+z)⁻¹.48) compared to late-types (∝(1+z)⁻0.75).
• It uses a robust mix of spectroscopic and photometric redshifts with stellar mass estimates to map galaxy structure over cosmic time.
• The study finds a tight intrinsic scatter (<0.2 dex) in the size-mass relation, setting a benchmark for galaxy formation models.

Analysis of the Evolution of the Galaxy Size-Mass Distribution Since $z=3$

The paper conducted by van der Wel et al. presents a detailed analysis of the galaxy size-mass distribution from $z = 3$ to the present, leveraging data from the 3D-HST survey and CANDELS imaging. This comprehensive work seeks to elucidate the evolution of galaxy structural parameters across different types and cosmic epochs, providing insights into galaxy formation and development.

Methodological Approach

The authors employ a combination of spectroscopic and photometric redshifts, as well as stellar mass estimates, to scrutinize the size-mass relationship over a broad redshift range. By categorizing galaxies as early or late types based on their star formation activity, the paper reveals differential growth patterns between these populations. Notably, early-type galaxies demonstrate a significantly faster rate of size evolution, $\propto (1+z)^{-1.48}$, compared to late-type galaxies, $\propto (1+z)^{-0.75}$.

Key Findings

1. Distinct Size-Mass Relations: The paper confirms that early-type galaxies are generally smaller than late-type galaxies at all redshifts, with a steeper size-mass relation. This suggests divergent evolutionary paths and assembly histories for these classes.
2. Redshift-Dependent Evolution: At all redshifts, the slope was found to be shallow ($\propto M_*^{0.22}$) for late-type galaxies and steep ($\propto M_*^{0.75}$) for early-types, with little change in slope over time. This hints at stable processes governing the size-mass relation for both galaxy types across cosmic time.
3. Intrinsic Scatter: The paper finds the intrinsic scatter around the size-mass relation to be less than 0.2 dex, highlighting a tight correlation in the size-mass plane despite inherent morphological diversity.
4. Number Density and Compact Galaxies: Observations indicate an increase in the number density of massive, compact early-type galaxies from $z=3$ to $z=1.5-2$, followed by a decline at later times. This decline supports the premise of significant size growth for individual galaxies over time.

Implications and Future Directions

The differential evolutionary trajectories of early- and late-type galaxies inform our understanding of galaxy assembly and the role of environmental factors like mergers and accretion. The findings imply that early-type galaxies undergo substantial size growth, likely influenced by processes such as minor mergers that accrete material onto the galaxy outskirts.

For late-type galaxies, the slower evolution may be indicative of evolutionary mechanisms tied to angular momentum conservation and the direct impact of dark matter halos on galaxy size.

These results encourage further exploration into how dark matter halo properties correlate with stellar mass growth and size evolution. The future trajectory of this research could benefit from integrating simulations to test hypotheses relating to feedback mechanisms, halo dynamics, and internal galaxy processes.

The comprehensive dataset and analysis methods detailed in this paper serve as a robust framework for future models of galaxy evolution, further enhancing our comprehension of the underlying cosmological factors influencing galaxy structure and formation over billions of years.