Galaxy Zoo: Evidence for Diverse Star Formation Histories through the Green Valley (1501.05955v1)

Abstract: Does galaxy evolution proceed through the green valley via multiple pathways or as a single population? Motivated by recent results highlighting radically different evolutionary pathways between early- and late-type galaxies, we present results from a simple Bayesian approach to this problem wherein we model the star formation history (SFH) of a galaxy with two parameters, [t, \tau] and compare the predicted and observed optical and near-ultraviolet colours. We use a novel method to investigate the morphological differences between the most probable SFHs for both disc-like and smooth-like populations of galaxies, by using a sample of 126,316 galaxies (0.01 < z < 0.25) with probabilistic estimates of morphology from Galaxy Zoo. We find a clear difference between the quenching timescales preferred by smooth- and disc-like galaxies, with three possible routes through the green valley dominated by smooth- (rapid timescales, attributed to major mergers), intermediate- (intermediate timescales, attributed to minor mergers and galaxy interactions) and disc-like (slow timescales, attributed to secular evolution) galaxies. We hypothesise that morphological changes occur in systems which have undergone quenching with an exponential timescale \tau < 1.5 Gyr, in order for the evolution of galaxies in the green valley to match the ratio of smooth to disc galaxies observed in the red sequence. These rapid timescales are instrumental in the formation of the red sequence at earlier times; however we find that galaxies currently passing through the green valley typically do so at intermediate timescales.

• The paper demonstrates that galaxies in the green valley experience diverse quenching timescales, indicating multiple evolutionary pathways from the blue cloud to the red sequence.
• The researchers apply a Bayesian model combining Galaxy Zoo morphological classifications with SDSS and GALEX data to trace star formation histories.
• The paper shows that smooth galaxies quench rapidly while disc galaxies evolve more slowly, highlighting the influence of mergers and secular processes on galaxy evolution.

An Analysis of Star Formation Histories in the Green Valley: Insights from Galaxy Zoo

The paper "Galaxy Zoo: Evidence for Diverse Star Formation Histories through the Green Valley" by Smethurst et al. investigates the evolutionary pathways of galaxies as they transition from the blue cloud to the red sequence, particularly focusing on the intermediate green valley region of the color-magnitude diagram. Utilizing data from the Galaxy Zoo project, which relies on citizen scientists for morphological classification, the authors apply a Bayesian methodology to model the star formation histories (SFHs) of galaxies in terms of quenching onset time and quenching timescale. Through this analysis, the paper seeks to understand whether these transitions occur through multiple evolutionary pathways or follow a singular trajectory.

Methodology and Approach

The authors employ a simple Bayesian statistical approach, wherein galaxies are modeled using an exponentially declining star formation rate (SFR) characterized by parameters $t_q$ (quenching onset time) and $\tau$ (quenching timescale). By integrating the Galaxy Zoo morphological classifications with optical and near-ultraviolet (NUV) data from the Sloan Digital Sky Survey (SDSS) and the Galaxy Evolution Explorer (GALEX), the paper samples over 126,000 galaxies. Through these methods, the paper explores the potential morphological differences in SFHs between disc-like and smooth-like galaxy populations, examining specifically how these histories correlate with their passage through the green valley.

Key Findings

The research identifies that star formation quenching occurs over a continuum of timescales, revealing diverse pathways through the green valley:

1. Rapid Quenching in Smooth Galaxies: The paper finds that rapid quenching timescales (characterized by $\tau < 1.0$ Gyr) are more prevalent in smooth galaxies compared to disc galaxies, suggesting that major mergers, potentially with effective black hole feedback, might be a primary driver of this process. This rapid cessation of star formation likely contributes to morphological transformations, moving galaxies from the blue cloud to the red sequence rapidly.
2. Intermediate Quenching for Diverse Morphologies: The evidence points to significant intermediate quenching (with timescales of $1.0 < \tau < 2.0$ Gyr) across both smooth and disc morphologies. The paper suggests that these timescales may reflect a combination of mechanisms, including minor mergers and interactions within galaxy clusters.
3. Slow Evolution in Disc Galaxies: For disc galaxies, quenching more often occurs over slower timescales ($\tau > 2.0$ Gyr), hinting at secular processes such as internal dynamics and bar formation that deplete star-forming gas over extended periods. These findings align with the observational presence of red spirals, which maintain their morphological identity while gradually quenching star formation.
4. Observing Galactic Evolution: The analysis shows that galaxies currently observed in the green valley primarily undergo intermediate quenching, contrasting with past epochs where rapid quenching was more common. This indicates not merely a shift in quenching mechanisms over cosmic time but also in the morphological composition of galaxies undergoing quenching.

Implications and Future Prospects

The paper by Smethurst et al. provides nuanced insights into galaxy evolution by emphasizing morphological dependencies in SFHs and the diverse routes through the green valley. With the Bayesian method applied, the paper unveils the significance of intermediate timescales, which were previously underappreciated.

Practically, these findings challenge the traditional dichotomy of galaxy evolution, suggesting a more varied set of evolutionary processes at play. Theoretically, this lays the foundation for more advanced modeling of galaxy formation and evolution, taking into account the intricate interplay of internal mechanisms and external interactions.

Looking ahead, the extension of this analysis to higher redshift ranges or incorporating more comprehensive multi-wavelength datasets could yield further insights. Additionally, exploring variations within morphological sub-classes (e.g., barred versus non-barred spirals) can further dissect the complexity of galaxy evolution.

In summary, this paper provides a detailed account of galaxy evolution in the green valley, proposing a framework that accommodates the observed heterogeneity in morphological and quenching behavior. This offers a refined perspective on galaxy populations and presents a pathway for more targeted studies in galaxy formation and evolution.