The ALMaQUEST Survey XVII: Unveiling Multiple Quenching Pathways in Green Valley Galaxies via Molecular Gas and Quenching Timescale Analyses

TL;DR

This work investigates how green valley galaxies quench by combining spatially resolved molecular gas and star-formation data from ALMaQUEST with MaNGA stellar-population analyses. By measuring per-spaxel offsets in star-formation efficiency ($\Delta$SFE) and molecular-gas fraction ($\Delta f_{\rm gas}$) relative to star-forming main-sequence relations, the authors classify GV galaxies into $f_{\rm gas}$-driven, SFE-driven, or mixed quenching modes and link these modes to quenching timescales $\tau_{decay}$ derived from integrated spectra fitted with a double-power-law SFH. The results show substantial galaxy-to-galaxy variation in quenching modes, with fixed $\alpha_{CO}$ yielding roughly 36% $f_{\rm gas}$-driven, 39% SFE-driven, and 25% mixed GV galaxies; a metallicity-dependent $\alpha_{CO}$ shifts the balance toward more $f_{\rm gas}$-driven systems. Most GV galaxies (about 75%) have $\tau_{decay}<1$ Gyr, indicating predominantly rapid quenching and challenging slow-quenching scenarios like starvation; SFE-driven quenching is closely associated with short timescales regardless of $\alpha_{CO}$, while $f_{\rm gas}$-driven quenching spans a broader range. The findings support a multi-channel quenching picture in GV galaxies, with implications for disentangling intrinsic versus environmental quenching mechanisms and motivating larger samples for statistical robustness.

Abstract

Statistically, green valley (GV) galaxies exhibit lower molecular gas fractions ($f_{gas}$) and reduced star formation efficiency (SFE) compared to star-forming galaxies. However, it remains unclear whether quenching is primarily driven by one factor or results from a combination of mechanisms in individual GV galaxies. In this study, we address this question by examining the spatial distributions of star formation and molecular gas in 28 GVs selected from the ALMaQUEST survey and additional literature samples. For each galaxy, we identify regions with suppressed specific star formation rate (sSFR) and measure $Δf_{gas}$ and $Δ$SFE-offsets from the resolved scaling relations of the star-forming main sequence galaxies. By comparing the fraction of regions with negative $Δf_{gas}$ and $Δ$SFE, we classify 35.7$\pm$13.2\% (57.1$\pm$17.9\%) of GV galaxies as $f_{gas}$-driven, 39.3$\pm$14.0\% (39.3$\pm$14.0\%) as SFE-driven, and 25.0$\pm$10.6\% (3.6$\pm$3.6\%) as mixed mode when adopting a fixed (variable) CO-to-$\rm H_{2}$ conversion factor ($α_{CO}$). These results indicate that GVs undergo quenching through multiple pathways. As sSFR decreases from the main sequence to the green valley, we observe a transition toward predominantly SFE-driven quenching, possibly linked to internal processes such as morphological quenching or AGN activity. We further estimate the quenching timescale ($τ_{decay}$), defined as the time from the peak SFR to 1/e (approximately 37\%) of its value, using integrated MaNGA spectra. SFE-driven quenching is typically associated with short $τ_{decay}$ , while $f_{gas}$-driven quenching shows a broader range. Overall, 75\% of GVs exhibit $τ_{decay}$ shorter than 1 Gyr, suggesting that quenching in most GVs proceeds rapidly, challenging purely slow-quenching scenarios like starvation.

Figures (10)

• Figure 1: The global specific star formation rate (sSFR) vs. stellar mass ($\rm~M_{*}$) of 71 galaxies used in this work and 4656 MaNGA galaxies (black dots) from SDSS DR15. The global measurements are taken from the Pipe3D san16asan16b value-added catalog san18 in the SDSS DR15 release aqu19. Colored symbols represent various sub-populations, including the ALMaQUEST survey lin20, ALMaQUEST-Mergers tho22, and post-starburst galaxies ott22. The red lines denote the dividing line defining the MS and GV subsamples used in this work.
• Figure 2: An example (MaNGA object with 'plateifu' ID = 9195-3702) of the integrated spectrum (top panel, grey), the model fitting (top panel, red) and the SFH (bottom panel). The vertical light grey stripes in the top panel indicate the positions of masked sky lines. The black line and the grey shaded areas in the bottom panel show the 50th, 16th, and 84th percentiles of the posteriors. In the bottom panel, the two dashed vertical lines indicate the look-back times corresponding to the peak SFR (right) and the point at which the SFR declines to 1/e of its peak value (left).
• Figure 3: $\Delta$SFE versus $\Delta$$f_{\rm gas}$ for spaxels with $\Delta$sSFR $<$ 0 for 28 GV galaxies (i.e., global sSFR below $10^{-10.5}$yr$^{-1}$). The black one-to-one line in each panel corresponds to equal contribution, above (below) which is $f_{\rm gas}$ (SFE) driven. The background color denotes the classified gas quenching mode (orange: SFE driven; blue: $f_{\rm gas}$ driven; green: mixed). The percentages of spaxels lying above and below the one-to-one line are indicated on either side of the dividing line.
• Figure 4: Quenching timescale ($\tau_{decay}$) as a function of global sSFR. The black curve represents the median $\tau_{decay}$ values in bins of Log(sSFR). Uncertainties smaller than the symbol size are not visible in the plot. Galaxies with sSFR $<$$10^{-10.5}$yr$^{-1}$ (indicated by the green background) define the GV population analyzed in this study.
• Figure 5: Quenching timescale ($\tau_{decay}$) as a function of global sSFR but for GV galaxies (i.e., sSFR below $10^{-10.5}$yr$^{-1}$) only. Colors denote different gas quenching modes (blue: gas-driven; orange: SFE-driven, green: mixed). Uncertainties smaller than the symbol size are not visible in the plot. Left: $\rm M_{H_{2}}$ is computed using a fixed $\alpha_{\mathrm{CO}}$. Right: $\rm M_{H_{2}}$ is computed using a metallicity-dependent $\alpha_{\mathrm{CO}}$.