The impact of selection criteria on the properties of green valley galaxies

TL;DR

The paper addresses how eight commonly used green valley selection criteria bias the identification and derived properties of green valley galaxies at $z<0.1$ by applying them to SDSS optical and GALEX UV data. It systematically compares colour-based, SFR-based, sSFR-based, and Gaussian-fitting criteria, assessing differences in stellar mass, SFR, sSFR, luminosity, morphology, and spectroscopy between optical and UV selections, and evaluates extinction effects on colour-based methods. Key findings show that colour-based selections, especially without Gaussian fitting, yield the strongest optical–UV differences, while Gaussian fitting reduces subjectivity but remains sensitive to extinction and sample range; sSFR- and SFR–M$_{*}$-based criteria are more stable and recommended when available. The work highlights that green valley galaxies are a diverse population at different evolutionary stages, and that combining optical and UV selections provides a more complete view of quenching processes, with practical guidance for interpreting past studies and designing future analyses.

Abstract

Context: The bi-modality in the distribution of galaxies usually obtained from colour-colour or colour-stellar mass diagrams has been studied to show the difference between the galaxies in the blue cloud and in the red sequence and to define the green valley region. As a transition region, the green valley galaxies can give clues about morphological transformation of galaxies from late- to early-types, and therefore the selection of green valley is of fundamental importance. Aims: In this work, for the first time, we evaluate the selection effects of the most used green valley selection criteria. The aim is to understand how these criteria affect the identification of green valley galaxies, their properties, and their impact on galaxy evolution studies. Methods: Using the SDSS optical and GALEX ultraviolet data at redshift z < 0.1, we selected the eight most commonly used criteria based on colours, specific star formation rate, and star formation rate vs. stellar mass. We then studied the properties of the green valley galaxies (their stellar mass, star formation rate, specific star formation rate, intrinsic brightness, morphological and pectroscopic types) for each selection criterion. Results: We found that when using different criteria, we select different types of galaxies. UV-optical colour-based criteria tend to select more massive galaxies, with lower star formation rates, with higher fractions of composite and elliptical galaxies, than when using pure optical colours. Our results also show that the colour-based criteria are the most sensitive to galaxy properties, rapidly changing the selection of green valley galaxies. Conclusions: Whenever possible, we suggest avoiding the green valley colour-based selection and using other methods or a combination of several, such as the star formation rate vs. stellar mass or specific star formation rate.

Figures (17)

• Figure 1: Distribution of g - r rest-frame colour for the optical sample. The two black dashed lines define the green valley region, with the red sequence peak (on the left) and the blue cloud peak (on the right).
• Figure 2: Distribution of the NUV - r colour for the UV sample. The green valley is defined between the two dashed lines.
• Figure 3: SFR-M$_\ast$ diagram for an optical sample of galaxies. The green valley is defined between the two dashed lines. These lines were determined by plotting the density contours in the galaxy distribution.
• Figure 4: SFR versus stellar mass for the UV sample. The green valley is defined between the two dashed lines.
• Figure 5: sSFR distribution for the optical sample. The green valley lies between the two vertical dashed lines.