Newly-quenched galaxies as the cause for the apparent evolution in average size of the population (1302.5115v2)

Abstract: Abridged. We use COSMOS to study in a self-consistent way the change in the number densities of quenched early-type galaxies (Q-ETGs) of a given size over the interval 0.2 < z < 1.0 to study the claimed size evolution of these galaxies. At 10^10.5<Mgalaxy<10^11 Msun, we see no change in the number density of compact Q-ETGs, while at >10^11 Msun we find a decrease by 30%. In both mass bins, the increase of the median sizes of Q-ETGs with time is primarily caused by the addition to the size function of larger and more diffuse Q-ETGs. At all masses, compact Q-ETGs become systematically redder towards later epochs, with a (U-V) difference consistent with passive evolution of their stellar populations, indicating that they are a population that does not appreciably evolve in size. At all epochs, the larger Q-ETGs (at least in the lower mass bin) have average rest-frame colors systematically bluer than those of the more compact Q-ETGs, suggesting that the former are younger than the latter. The idea that new, large, Q-ETGs are responsible for the observed growth in the median size of the population at a given mass is supported by the sizes and number of the star-forming galaxies that are expected to be progenitors of the new Q-ETGs over the same period. In the low mass bin, the new Q-ETG have 30% smaller sizes than their star-forming progenitors. This is likely due to the fading of their disks after they cease star-formation. Comparison with higher z shows that the median size of newly-quenched galaxies roughly scales, at constant mass, as (1+z)^-1. The dominant cause of the size evolution seen in the Q-ETG population is thus that the average sizes of individual Q-ETGs scale with the average density of the Universe at the time when they were quenched, with subsequent size changes in individual objects through eg merging of secondary importance, especially at masses <10^11 Msun.

Overview of "Newly-quenched galaxies as the cause for the apparent evolution in average size of the population"

The paper by Carollo et al. effectively re-examines the widely discussed phenomenon of the apparent size evolution in quenched early-type galaxies (Q-ETGs) from redshift $z \sim 1$ to the present day. Utilizing the extensive COSMOS survey data, the authors challenge the traditional narrative of significant physical growth in individual galaxies being the primary driver of this observed change in median sizes. Instead, they propose that the increase in average sizes of Q-ETGs is predominantly a result of newly-quenched galaxies with larger sizes contributing to the population, rather than significant growth of pre-existing compact galaxies.

Key Findings

1. Stability of Compact Q-ETGs: In the stellar mass range of $10^{10.5}<M_{galaxy}<10^{11} M_\odot$, the paper finds no significant change in the number density of compact Q-ETGs from $z=1$ to $z=0.2$. This indicates that, once formed, these compact galaxies do not experience substantial size evolution individually. In the higher mass range ($M_{galaxy} > 10^{11} M_\odot$), a slight decrease of about 30% in their number density suggests some minor growth or merging activity, but this is not the dominant trend.
2. Emergence of Large Q-ETGs: The increase in the average size of the Q-ETG population is primarily due to a substantial increase in number density of larger Q-ETGs over time. In both mass bins, the observed growth factor of larger Q-ETGs is pronounced, with a notable shift towards larger radii, which significantly impacts the population's median size metrics.
3. Evolutionary Drivers: The paper posits that the dominant cause of the size evolution in the Q-ETG population is the continual quenching of star-forming galaxies at progressively larger sizes as the universe evolves. This shift in the size distribution of newly-quenched galaxies is consistent with an environment-dependent scaling of size, approximately following a $(1+z)^{-1}$ relation.

Implications and Future Directions

• Quenching and Morphological Evolution: The results underscore the significance of quenching processes in shaping the morphological properties of galaxies. The fading of disk components in star-forming galaxies upon quenching could explain the apparent reduction in size as they transition to Q-ETGs.
• Constraints on Minor Merging Models: The data challenge minor merging as a dominant mechanism for size growth in Q-ETGs, which, while contributing to some evolution particularly at higher masses, does not account for the observed increase in median size across the broader population. The consistent number density of the most compact systems further constrains the extent of size growth attributable to mergers.
• Aging of Stellar Populations: The paper highlights a color-age relationship, where compact Q-ETGs exhibit redder colors over cosmic time, consistent with passive aging rather than rapid structural changes.

Conclusion

Carollo et al. advance a compelling argument that the evolution in average size of Q-ETGs is predominantly driven by the addition of larger newly-quenched galaxies rather than the growth of existing small ones. This perspective challenges existing paradigms, suggesting a reinterpretation of galaxy evolution dynamics where size distributions are more reflective of quenching timelines aligned with cosmic density fluctuations. Future investigations will likely explore detailed mechanisms driving these quenching processes and refine models to better align with the empirical evidence provided by this paper.