The number density and mass density of star-forming and quiescent galaxies at 0.4 < z < 2.2 (1104.2595v1)

Abstract: We study the build-up of the bimodal galaxy population using the NEWFIRM Medium-Band Survey, which provides excellent redshifts and well-sampled spectral energy distributions of ~27,000 galaxies with K<22.8 at 0.4 < z < 2.2. We first show that star-forming galaxies and quiescent galaxies can be robustly separated with a two-color criterion over this entire redshift range. We then study the evolution of the number density and mass density of quiescent and star-forming galaxies, extending the results of the COMBO-17, DEEP2, and other surveys to z=2.2. The mass density of quiescent galaxies with M > 3 10^11 solar masses increases by a factor of ~10 from z=2 to the present day, whereas the mass density in star-forming galaxies is flat or decreases over the same time period. Modest mass growth by a factor of 2 of individual quiescent galaxies can explain roughly half of the strong density evolution at M>10^11 solar masses, due to the steepness of the exponential tail of the mass function. The rest of the density evolution of massive, quiescent galaxies is likely due to transformation (e.g. quenching) of the massive star-forming population, a conclusion which is consistent with the density evolution we observe for the star-forming galaxies themselves, which is flat or decreasing with cosmic time. Modest mass growth does not explain the evolution of less massive quiescent galaxies (~10^10.5 solar masses), which show a similarly steep increase in their number densities. The less massive quiescent galaxies are therefore continuously formed by transforming galaxies from the star-forming population

• The paper robustly distinguishes star-forming from quiescent galaxies using a two-color criterion and precise NEWFIRM survey data.
• The paper finds a tenfold increase in quiescent galaxy mass density above 3 × 10^10 M⊙ from z ~2 to present, while star-forming galaxies show flat or declining trends.
• The paper attributes these trends to quenching processes and minor mergers, highlighting their critical roles in galaxy evolution.

Evolution of Galaxy Number and Mass Densities from Redshift 0.4 to 2.2

This paper investigates the evolution of galaxies by examining the distribution of star-forming and quiescent galaxies over a redshift range of 0.4 to 2.2. Utilizing the NEWFIRM Medium-Band Survey, which provides precise redshift estimates and well-sampled spectral energy distributions for approximately 27,000 galaxies, the paper expands upon previous findings from the COMBO-17 and DEEP2 surveys.

Key Findings:

1. Separation of Galaxy Types: The paper uses a two-color criterion to distinguish between star-forming and quiescent galaxies robustly across the studied redshift range, effectively addressing the challenge of dust-reddened star-forming galaxies masquerading as quiescent systems.
2. Mass Density Evolution: A significant increase in the mass density of quiescent galaxies, by a factor of about 10 from z~2 to present-day, is observed for masses above 3 × 10^10 M_\odot. In contrast, the mass density for star-forming galaxies remains flat or declines over the same period.
3. Role of Quenching and Mergers: The paper attributes the evolution in the population of massive, quiescent galaxies primarily to the transformation of star-forming galaxies into quiescent ones through processes such as quenching. Moreover, modest mass growth, potentially due to minor mergers, is suggested to explain about half of the observed density evolution for galaxies with masses greater than 10^11 M_\odot.
4. Trends in Less Massive Galaxies: For less massive galaxies (~10^10.5 M_\odot), there isn't sufficient mass growth to explain their number density evolution purely through mergers. Instead, continuous transformation from the star-forming population is proposed as the dominant process.

Implications and Future Directions:

• Galaxy Formation Models: The observed trends challenge models that rely solely on star formation within the quiescent population to account for mass density changes. They highlight the importance of galaxy transformations and mergers in the cosmic evolution of the galaxy population.
• Further Observational Studies: Future studies should focus on obtaining more precise measurements of galaxy properties, including dynamics, to reduce the uncertainties related to systematic mass estimation errors—critical for understanding the mechanisms of galaxy quenching and the role of minor mergers.
• Intermediate-Redshift Discoveries: Extending these analyses to intermediate redshifts could unearth significant insights into the transition phases and environmental impacts on galaxy evolution, linking observations from the early universe to the present-day structures.

This research significantly contributes to our understanding of galaxy evolution over cosmic time, especially the role of galaxy mergers and the transformation processes from star-forming to quiescent states, enhancing our comprehension of the assembly and transformation of galaxy populations.