VERTICO. VII. Environmental Quenching Caused by the Suppression of Molecular Gas Content and Star Formation Efficiency in Virgo Cluster Galaxies

VERTICO is a completed Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 7 Large Program that uses ALMA's Morita array to map the distribution and kinematics of CO(2–1) across 51 Virgo Cluster galaxies on subkiloparsec scales. Here we briefly describe the VERTICO sample, CO(2–1) data products, and derivation of the molecular gas surface densities. For more details, we refer the reader to Brown et al. (2021). From here on, we use "CO" to explicitly refer to the CO(2–1) transition. Other transitions are noted accordingly.

All resolved data used in this work are smoothed to the smallest common beam diameter of 9'' with a pixel scale of 4''. The 9'' beam corresponds to a physical size of 720 pc at the distance of Virgo (16.5 Mpc, 1'' ≈ 80 pc; Mei et al. 2007). Matching the beam size across all galaxies ensures that each resolution element covers approximately the same physical area of the disk, and the approximate Nyquist sampling with the pixel scale reduces the effect of beam oversampling contributing to trends in our analysis. Given the consistent physical scale of each pixel, we use the words "pixel" and "region" interchangeably to improve readability.

The molecular gas surface density maps, in units of solar masses per square parsec, are derived assuming a constant CO(1–0) conversion factor of ${\alpha }_{{\rm{C}}{\rm{O}}}=4.35\,{M}_{\odot }\,{{\rm{p}}{\rm{c}}}^{-2}\,{({\rm{K}}\,{\rm{k}}{\rm{m}}\,{{\rm{s}}}^{-1})}^{-1}$ and CO(1–0)-to-CO(2–1) line luminosity ratio (R₂₁) of 0.8 (Brown et al. 2021). These maps include the 36% contribution of helium in addition to H₂ and are corrected for projection effects using the optical r-band inclination.

From the VERTICO sample of 49 galaxies detected in CO, we selected all galaxies with an optical r-band inclination ≤80° and pixels with a signal-to-noise ratio ≥2. Further, we remove the three lowest stellar mass galaxies in our sample (NGC 4299, NGC 4532, and NGC 4561) since these galaxies have low metallicity and their molecular gas component is likely underestimated by the constant α_CO used in this paper. We also exclude NGC 4321 from our sample as the resolving beam of the ACA CO data is ≈105. This selection yields a final sample of 15,401 pixels drawn from 33 galaxies covering a range in projected cluster-centric distances between ∼0.2 r₂₀₀ and ∼2.5 r₂₀₀ (r₂₀₀ = 1.55 Mpc; McLaughlin 1999; Brown et al. 2021).

HERACLES is a survey of CO(2–1) emission in 48 nearby field galaxies (2 < D(Mpc) < 25) spanning a comparable range in global stellar mass and specific star formation rate (sSFR) to VERTICO (10^8.5 < M_⋆(M_⊙) < 10¹¹ and 10^−11.5 < sSFR(yr⁻¹) < 10^−9.2, respectively). The survey's public data cubes have an angular beam diameter of 13'' in 5 km s⁻¹ wide channels. We refer the reader to Leroy et al. (2009) for further details on the survey design and data products.

The HERACLES targets span a range of distances so the physical scales probed by the data's common 13'' beam varies considerably. Therefore, for the comparison to VERTICO, we select the 10 closest galaxies (2.9 ≤ D (Mpc) ≤ 10.6) and convolve the cubes with a Gaussian kernel so that the final beam matches the 720 pc physical resolution of the VERTICO 9'' beam. This ensures we are probing the same physical scales with our comparison. We also smooth the data to the same 10 km s⁻¹ velocity resolution as the final VERTICO data cubes. Science ready data products such as flux, gas surface density, and signal-to-noise maps are produced using an identical procedure to the VERTICO data products described in Brown et al. (2021). For this work we apply the selection criteria described in Section 2.1, yielding a final field control sample of 10,817 pixels from drawn from 10 HERACLES survey galaxies.

This section briefly describes the production of 9'' SFR and stellar mass surface density maps for both the VERTICO and HERACLES galaxies. A fuller description of this procedure can be found in Jiménez-Donaire et al. (2023) and Villanueva et al. (2022) for the SFR and stellar mass surface density maps, respectively.

All maps are constructed using Galaxy Evolution Explorer (GALEX) and Wide-field Infrared Survey Explorer (WISE) photometry following the procedure laid out in Leroy et al. (2019). The final 9'' resolutions match the beam size of the VERTICO CO products. SFR surface density maps are constructed from a combination of GALEX near-ultraviolet (NUV) and WISE3 photometry at 9''. We are required to use WISE3 as our obscured tracer at 9'' resolution as the WISE4 beam is too coarse. Stellar mass surface density maps are derived from WISE1 photometry. We determine the local mass-to-light ratio (at 3.4 μm) using the WISE3-to-WISE1 color as an sSFR-like proxy and following the calibrations in Leroy et al. (2019). The WISE1 images are then combined with the derived mass-to-light ratios to produce stellar mass surface density maps. As with the molecular gas, the stellar and SFR surface densities are corrected for inclination effects by a factor of cos(i).

Figure 1 shows the distributions of the resolved physical properties for the 15,401 pixels in the VERTICO analysis sample and 10,817 pixels in the HERACLES field control sample. From left to right, the properties shown are the molecular gas surface density (Σ_mol), stellar surface density (Σ_⋆), SFR surface density (Σ_SFR), and sSFR (=Σ_SFR/Σ_⋆). The distribution of pixels belonging to the VERTICO cluster galaxy sample extend to slightly lower molecular gas surface and SFR surface densities. There is an excess in cluster pixels above Σ_⋆ ∼ 10⁸ M_⊙ kpc⁻² with respect to the field sample, although the shapes of the distributions are similar, which may be driven by an excess in central stellar features such as bars and bulges in VERTICO. The combination of lower SFRs and high stellar surface densities results in the significant skew of the VERTICO pixels to lower sSFRs. The next section explores whether the differences between the field and cluster samples are the result of environmental processes or internal secular evolution or a mix of both.

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The rSFMS, rMGMS, and rKSR for the VERTICO H i–normal (blue) and H i–poor (red) subsamples as well as the HERACLES field sample (gray) are shown in the top-left, bottom-left, and bottom-right panels of Figure 2, respectively. The corresponding best-fit relations for the H i–normal and H i–poor subsamples are shown by the solid lines in each plot. The fits are calculated using the LTSFIT Python package described in Cappellari et al. (2013, their Section 3.2) which accounts for measurement uncertainties in both axes to determine the best-fit parameters and scatter. The fit parameters for each relation are provided in the Appendix.

The rSFMS (top right) and rMGMS (bottom left) in the H i–normal cluster galaxies closely follow the field relations although there is a small deviation to lower gas densities at low stellar mass density in the rMGMS. This shows that where there is little effect on the H i content, the influence of environment on the molecular gas and subsequent star formation activity is also minimal. The rMGMS offset at the lowest stellar densities may be an indicator of environmental effects beginning to take hold in the galaxy outskirts and we return to this point with Figure 5. On the other hand, the H i–poor galaxies are clearly offset to lower molecular gas and SFR densities at fixed stellar density with respect to the field relations. Thus, in galaxies where the global H i content is reduced with respect to the field, there is also a significant reduction in the molecular densities and, consequently, star formation activity.

The best-fit rSFMS relation for H i–poor galaxies is 0.62 dex below the H i–normal relation, demonstrating that the processes that are reducing the global H i content are also responsible for quenching star formation within the truncation radius (Watts et al. 2023) and on the scale of individual pixels (∼720 pc).

The rMGMS fit for the H i–poor galaxies is offset below the H i–normal relation by 0.38 dex. Interestingly, the slopes of the H i–poor and H i–normal rMGMSs agree well (0.94 ± 0.013 and 0.94 ± 0.010, respectively) with the H i–normal relation but is steeper than the field relation (0.79 ± 0.011). One reason for this is the lack of molecular gas at low stellar surface densities (Σ_⋆ ≲ 10^7.5 M_⊙ kpc⁻²) in H i–poor galaxies, which implies that the impact of environment on the molecular gas disk is greatest in the low gas and stellar density regimes. In the context of the rSFMS analysis, the difference in molecular gas content for the H i–normal and H i–poor subsamples suggests that the link between global H i deficiency and subkiloparsec quenching is at least in part driven by a reduction in the molecular gas density.

The H i–poor rKSR (bottom right) is, on average, 0.24 dex below the fit to the H i–normal galaxies. This lower SFE in regions belonging to H i–deficient galaxies is also shown by both Villanueva et al. (2022) and Jiménez-Donaire et al. (2023). The fact that the combined subsample offsets in rMGMS and rKSR match the observed offset in rSFMS is reflective of the fact that the locations of regions in the rSFMS plane are fundamentally a consequence of their position with respect to the rMGMS and rKSR (e.g., Lin et al. 2019; Ellison et al. 2020; Baker et al. 2022) and this, consequently, holds true for the best-fit relations as well.

The rKSR slopes for the H i–normal and H i–poor subsamples are very similar (0.84 ± 0.006 versus 0.82 ± 0.007), but flatter than is observed for the entire sample (0.97 ± 0.07; Jiménez-Donaire et al. 2023). This is likely a consequence of a nonlinear relationship between H i deficiency and molecular gas density, meaning the offset to lower SFR density is greatest for regions with high gas density in H i–poor galaxies.

Although it is clear that a linear fit is not the best descriptor of the data in each relationship, our goal here is simply to provide a straightforward quantification of the offset between the H i–poor and H i–normal populations.

To quantify further the observed differences between regions belonging to H i–normal and H i–poor galaxies, we calculate the offset of each pixel from the median rSFMS, rMGMS, and rKSR of the full sample. These offset quantities are termed ΔΣ_SFR, Δf_mol, and ΔSFE, respectively.

We define ΔΣ_SFR for each pixel following Ellison et al. (2020), using the relevant (VERTICO or HERACLES) sample itself to select a set of star-forming control pixels (sSFR ≥ 10^−10.5 yr⁻¹) that are matched within a narrow range of stellar surface density (±0.1 dex) to the target pixel. We then calculate ${\rm{\Delta }}{{\rm{\Sigma }}}_{\mathrm{SFR}}={{\rm{\Sigma }}}_{\mathrm{SFR},\mathrm{pixel}}-{\widetilde{{\rm{\Sigma }}}}_{\mathrm{SFR},\mathrm{control}}({{\rm{\Sigma }}}_{\star })$, where Σ_SFR,pixel is the SFR surface density of the pixel and ${\widetilde{{\rm{\Sigma }}}}_{\mathrm{SFR},\mathrm{control}}({{\rm{\Sigma }}}_{\star })$ is the median SFR surface density of the control sample. The sSFR threshold value was chosen to maximize the difference between the pixel sSFR distributions at fixed stellar density, i.e., regions with Σ_⋆ < 10⁸ M_⊙ kpc⁻² tend to have sSFR > 10^−10.5 yr⁻¹ and vice versa.

The result is that the median rSFMS is defined as ΔΣ_SFR = 0, with positive and negative ΔΣ_SFR values for regions that are more star forming or more quenched with respect to this reference point. The normalization of this relation means that the median value of ΔΣ_SFR for all VERTICO pixels is −0.23 dex or, in other words, the median VERTICO pixel is ∼40% less star forming than the median rSFMS.

We also quantify the molecular gas content and SFE by calculating the pixel offsets from the rMGMS and rKSR (Δf_mol and ΔSFE, respectively). Briefly, ${\rm{\Delta }}{f}_{\mathrm{mol}}={{\rm{\Sigma }}}_{\mathrm{mol},\mathrm{pixel}}\,-{\widetilde{{\rm{\Sigma }}}}_{\mathrm{mol},\mathrm{control}}({{\rm{\Sigma }}}_{\star })$, where Σ_mol,pixel is the gas surface density of the pixel and ${\widetilde{{\rm{\Sigma }}}}_{\mathrm{mol},\mathrm{control}}({{\rm{\Sigma }}}_{\star })$ is the median gas surface density of all pixels within ±0.1 dex in stellar surface density. Similarly, ${\rm{\Delta }}{SFE}={{\rm{\Sigma }}}_{\mathrm{SFR},\mathrm{pixel}}-{\widetilde{{\rm{\Sigma }}}}_{\mathrm{SFR},\mathrm{control}}({{\rm{\Sigma }}}_{\mathrm{mol}})$, where Σ_SFR,pixel is the SFR surface density of the pixel and ${\widetilde{{\rm{\Sigma }}}}_{\mathrm{SFR},\mathrm{control}}({{\rm{\Sigma }}}_{\mathrm{mol}})$ is the median SFR surface density of all pixels within ±0.1 dex in molecular gas surface density. The only difference from the calculation of ΔΣ_SFR is that we do not impose an sSFR cut on the control samples for ΔSFE and Δf_mol. Thus, regions with positive (negative) Δf_mol values are gas normal (poor) with respect to the median rMGMS, and regions where ΔSFE is positive (negative) are forming stars more (less) efficiently than the median rKSR.

For the regions plotted in Figure 2, the median ΔΣ_SFR values for the H i–normal and H i–poor subsamples are 0 and −0.54 dex, respectively. This tells us, perhaps unsurprisingly, that our H i–normal sample tends to follow the median rSFMS for the entire population. Of course, this is mainly by construction given that we only include star-forming pixels when defining the ΔΣ_SFR control samples. Interestingly, pixels in the H i–poor subsample are a factor of ∼3.5 less star forming than the rSFMS.

We take this analysis further by using the rMGMS to compare the Δf_mol values for our two subsamples. The median Δf_mol value for pixels in H i–normal galaxies is 0.01 dex. On the other hand, we find that pixels belonging to the H i–poor subsample have less molecular gas, with a median Δf_mol of −0.39 dex. The fact that the difference in median ΔΣ_SFR between the H i–normal and H i–poor galaxies is significantly larger than for Δf_mol tells us that suppression of molecular gas content is not the whole picture.

The regions belonging to the two subsamples also bracket the median rKSR, with pixels belonging to H i–normal and H i–poor galaxies having median ΔSFE values of 0 and −0.22 dex, respectively. Again, this clearly shows that by selecting pixels based upon their host galaxy's H i deficiency, we are selecting subsamples of pixels with significantly different SFEs. Although trivial, it is important to note that the combined difference in the mean values of Δf_mol and ΔSFE values (0.6 dex) between the H i–normal and H i–poor subsamples closely matches the difference in ΔΣ_SFR of 0.54 dex. In fact, the residual 0.06 dex is well within the typical pixel uncertainties of these quantities of 0.1 dex.

It is clear from Figure 2 that the H i–poor galaxies do not cover the same range in stellar mass surface density as either the HERACLES or H i–normal galaxies in VERTICO. While this difference is controlled for when calculating Δf_mol and ΔΣ_SFR, it is not when computing ΔSFE. Thus, it is important to check that the observed differences in the rKSR for the H i–normal and H i–poor galaxies are not caused by stellar surface density selection effects. Figure 3 shows the median ΔSFE values of pixels belonging to H i–normal, H i–poor, and HERACLES galaxies at fixed stellar mass surface density. In each bin, the median ΔSFE for regions in the H i–poor galaxies is lower than for regions in the H i–normal and field galaxies. Interestingly, the difference between H i normal and H i poor is largest at lower Σ_⋆, where one would expect environmental mechanisms to be most effective. Figure 3 demonstrates that the offset of the H i–poor rKSR to lower SFEs shown in Figure 2 is not caused by different stellar mass density distributions between the two samples.

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In an effort to understand the physics driving these trends, we now look to confirm the connection between environmental mechanisms and H i deficiency in our sample and connect the influence of these mechanisms on the gas–star formation cycle back to the rSFMS, rMGMS, and rKSR.

We interpret the differences seen in Figure 2 as evidence that environmental processes are responsible for the systematic quenching of Virgo galaxies on subkiloparsec scales. However, this assumes that H i deficiency is an effective quantitative measure of environmental influence on VERTICO galaxies. The large body of work that underpins this assumption is detailed in the Introduction and we can also demonstrate this explicitly for our sample.

Yoon et al. (2017) provide classifications of environmental influence for the VERTICO targets based on a comparison of H i morphology with their orbital history. These authors identify distinct RPS stages (early, active, and post), as well as two other classes for unperturbed and anemic (i.e., low surface density) H i disks. Yoon et al. (2017) suggest that the anemic galaxies are likely undergoing starvation and/or thermal evaporation. While this may be true, our view is that making this distinction implies that ram pressure is not at play, which is unlikely since these galaxies have been in the cluster for more than one pericenter passage and have comparable H i extents to the other classes at fixed H i deficiency. Additionally, unlike the other classes, the anemic disks are also only present at high stellar mass (Yoon et al. 2017). Thus, we suggest that anemic disks are a case where environmental processes such as RPS have removed the H i in the outskirts, but not all the H i in the inner regions (perhaps due the large stellar mass of the galaxy), and there is no significant accretion of new gas onto to the disk.

Figure 4 uses the data of Yoon et al. (2017) to illustrate the dependence of H i deficiency upon the gas stripping histories of galaxies previously identified by those authors. The H i content of galaxies is systematically depleted as galaxies transition from unperturbed disks through the early, active-, and post-RPS stages. The anemic disks are also very H i deficient, although generally not to the extent of the highly truncated post-RPS class.

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In Figure 5, the panels from left to right show the distribution of ΔΣ_SFR, Δf_mol, and ΔSFE pixel values as a function of host galaxy H i–identified evolutionary stage. We include the HERACLES galaxies as our field sample in addition to the environmental classes identified by Yoon et al. (2017) and shown in Figure 4. The distributions are separated into pixels from the inner and outer disk based up on whether they fall inside or outside of the galaxy's 90% molecular gas mass–radius relation published by Brown et al. (2021; r_90,mol in their Table 3). The median value of each distribution is shown by the dashed lines, while the dotted lines show the 25th and 75th percentiles, respectively.

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Local SFR decreases as a function of RPS stage. This quenching is most apparent in the outer disk where the median values of ΔΣ_SFR in the early, active-, and post-RPS phases are 0.18, −0.18, and −0.65 dex, respectively. In comparison, in the inner disk during these same phases, the median values of ΔΣ_SFR are 0.08, −0.07 dex, and −0.12 dex, respectively. The observed star formation quenching as galaxies are stripped is primarily driven by decreases in the molecular gas content, with a secondary dependence on SFE. As with star formation activity, the effects of RPS on molecular gas are most pronounced in the outer disks where galaxies in the early, active-, and post-RPS stages have median values of Δf_mol of 0.31, −0.11, and −0.47 dex, respectively (inner disk median Δf_mol = 0.07, −0.06, and −0.24 dex, respectively). There is also a significant decrease in the SFE of the outer disks during RPS, although considerably less than for molecular gas. The median values of ΔSFE in the outer disks of the early, active-, and post-RPS phases are −0.01, −0.12, and −0.33 dex, respectively (inner disk median ΔSFE = 0.01, −0.07, and −0.17 dex, respectively). In summary, by the time the H i disk of a galaxy is fully stripped ("post RPS") its molecular gas content has reduced by a factor of 1.7 (0.24 dex) and 3 (0.47 dex) in the inner and outer regions, respectively. This drives quenching in both radial bins that is compounded by the simultaneous reduction in the SFE (factors of 1.5 and 2.1, respectively).

The star formation activity of anemic disks is quenched to a similar extent to the post-RPS phase. However, unlike galaxies in the RPS phases, anemic galaxies are quenched across the disk with a median ΔΣ_SFR of −0.6 and −0.56 dex in the inner and outer regions, respectively. This is caused by the suppression of molecular gas content and SFE throughout the galaxy (median Δf_mol = −0.50 and −0.49 dex and median ΔSFE = −0.22 and −0.22 dex, for inner and outer disks, respectively).

Figure 5 shows that for galaxies following the classic RPS sequence, environment acts to decrease gas content and SFE, especially in the outskirts, resulting in the outside-in quenching of star formation. On the other hand, there is a comparable decrease in gas content and SFE for the anemic disks of high-mass galaxies, but the effect is found in both the inner and outer disk.

Even in galaxies with morphologically unperturbed H i disks, we see suppression of local star formation in the outer disks (median ΔΣ_SFR = −0.15 dex). This may be the result of "preprocessing" of galaxies in the group environment or outermost regions of the cluster (Fujita 2004). This appears to be driven solely by reduced gas content (median Δf_mol = −0.34 dex), with SFEs typical of the field (median ΔSFE = −0.02 dex).

We also see enhanced star formation activity driven by increased gas densities with respect to the field in the earliest RPS stage, with the effect again most prominent in the outer disk. The median Δf_mol in the outer disks of these galaxies is 0.31 dex, while the inner disk exhibits a typical field value of 0.07 dex. Although the number of galaxies in the stage is small (four), this result agrees with a picture from other surveys where ram pressure initially increases gas content before quenching (e.g., Moretti et al. 2020a; Cramer et al. 2020; Troncoso-Iribarren et al. 2020; Roberts et al. 2022). At face value we appear to contradict the result of Watts et al. (2023) who find no evidence for environment-driven increases in molecular gas densities in VERTICO galaxies. However, Watts et al. (2023) are comparing much coarser subsets of the VERTICO galaxies divided into three bins of H i deficiency. Figure 5 demonstrates that the increase in gas densities due to ram pressure is only apparent in the earliest stage of stripping identified by the Yoon et al. (2017) morphology and phase-space analysis. Furthermore, the effect in the inner regions is relatively subtle, only becoming prominent in the outskirts of the galaxy gas disk which are not explicitly examined by Watts et al. (2023). Our result is also in agreement with Roberts et al. (2023) who observe increased gas densities on the leading half (with respect to their trailing half) of VERTICO galaxies undergoing RPS. Finally, we also see that in the field galaxies, it is the inner disk that has reduced molecular gas content and star formation activity, which may be indicative of secular processes regulating the star formation cycle in these galaxies.

This analysis suggests that the environmental mechanisms causing global H i deficiency are also systemically and significantly reducing molecular gas content and SFE, and consequently star formation activity, on subkiloparsec scales in cluster galaxies. The influence of RPS increases as the stripping processes proceed, occurring preferentially in the disk outskirts. The star formation cycle of galaxies with anemic H i disks, likely caused by starvation of the gas supply, is impacted to a similar extent as the post-RPS galaxies. However, the effect of starvation is commensurate in both the inner and outer disk. In the next section, we discuss the physical causes of H i deficiency in Virgo Cluster galaxies, their connection to molecular gas content, and the local and global star formation cycles.

To refocus this discussion on the bigger picture of how environment regulates the star formation cycle in VERTICO galaxies, we now place our results in the context of the four previous VERTICO science papers described in Section 1: Zabel et al. (2022, hereafetr Z22), Villanueva et al. (2022, hereafter V22), Jiménez-Donaire et al. (2023, hereafter JD23), and Watts et al. (2023, hereafter W23).

Z22, V22, and W23 all find evidence for the systematic decrease in molecular gas content in H i–deficient galaxies that we see in this work. W23 shows that this environment-driven suppression occurs across the galaxy disk, both inside and outside of the truncation radius. Along with Z22, they also demonstrate that low-density molecular gas is preferentially affected. While gas content is suppressed across the disk, Z22 and V22 use radial analyses to show that molecular gas disks in VERTICO are more centrally concentrated than in field galaxies, with galaxies impacted by their environment showing steeper and/or truncated gas density profiles. These results clearly align with the offset of the H i–poor rMGMS to lower gas densities (Figure 2) and the reduction of gas content as a function of environment and location within the disk (Figure 5). Taken together, this demonstrates that environmental processes have a predominantly destructive effect on molecular gas reservoirs throughout the disk, with the low-density gas in the outskirts of galaxies affected first and foremost.

As shown in this work, environmental impact on the amount of molecular gas available for star formation is not the only factor. V22 show that the SFE of molecular gas within the stellar radius decreases with increasing environmental perturbation. JD23 complete a more detailed investigation of the rKSR than in this work and are the first to report the lower SFEs in H i–deficient galaxies with respect to H i–normal cluster galaxies, with the latter following the relationship found in field galaxies. As well as the longer depletion times in galaxies undergoing environmental transformation, JD23 also find significant galaxy-to-galaxy and region-to-region variations in the SFE, suggesting there is no single rKSR that can be used to describe all cluster galaxies accurately. We do note that this galaxy-to-galaxy diversity in the SFE is also found in field galaxies (e.g., Ellison et al. 2021; Casasola et al. 2022; Saintonge & Catinella 2022). So, while the correlation with stripping stage and lower SFE in the outer disks in Figure 5 suggests environmental processes are at play, we have not yet precisely determined the extent to which the cluster environment is responsible for decreasing the efficiency with which galaxies convert their molecular gas into stars.

We do, however, demonstrate that environment generally acts to reduce both the molecular gas content and SFE throughout the disks of Virgo galaxies, albeit preferentially in the outer regions for galaxies undergoing stripping. In an effort to identify the particular environmental process(es) responsible for these effects, we use existing H i morphological classifications to show that the observed reduction in gas content, SFE, and activity in H i–poor VERTICO galaxies increases with RPS stage. We also investigate the effect that the lack of gas accretion after parts of the disk have been stripped (i.e., starvation) has on the star formation cycle in the remaining disk. We demonstrate that the gas content and SFE of starved galaxies are negatively influenced to the same degree as stripped galaxies, although the reduction is uniform throughout the remaining disk, which suggests ongoing star formation is primarily responsible.

In support of a picture where ram pressure strongly regulates the star formation cycle of most H i–poor VERTICO galaxies, V22 show that the ratio of molecular to atomic gas (R_mol) increases within the stellar radius with the level of H i truncation and/or asymmetry. In the same vein, W23 observe a decrease in molecular gas content and increases in R_mol that cannot be driven by changes in the ISM physical conditions alone and invoke stripping of the molecular gas to explain their results. The RPS scenario is also entirely consistent with the preferential suppression of low-density gas in the outskirts seen in our work, as well as the steep and truncated gas density profiles in H i–deficient galaxies (Z22; W23; V22).

In agreement with previous work focusing on local versus global quenching (e.g., Bluck et al. 2020, 2022), our results and those of JD23 show that in Virgo the two go hand in hand. Indeed, JD23 report that H i–deficient VERTICO galaxies are offset below the global SFMS for field galaxies by 0.6 dex. In our work, we show subkiloparsec regions belonging to H i–poor galaxies are, on average, a factor of ∼3.5 (∼0.54 dex) less star forming than their counterparts in H i–normal systems. Given that H i deficiency broadly traces stripping stage in our sample (Figure 4), this shows that RPS quenches star formation on both local and global scales in VERTICO galaxies. The analysis of Figure 5 suggest that stripping acts from the outside-in to suppress gas content and SFE throughout the disk.

Lastly, we note that we find evidence for the elevation of star formation activity driven by increased molecular gas densities at fixed stellar mass in the earliest stripping stage, especially in the outskirts. In combination with Roberts et al. (2023), who find gas compression on the leading half of RPS VERTICO galaxies, these results suggest that although ram pressure is ultimately a global quenching process, its initial effect is to induce local star formation activity driven by systematic gas compression prior to removal. Neither this work nor Roberts et al. (2023) find evidence for increased SFE due to RPS and, while JD23 find azimuthal variations in the SFE in two VERTICO galaxies, only one of these (NGC 4654) has a clear signature of an enhanced SFE on the leading edge. One such instance in the entire 49 CO-detected galaxies in VERTICO supports our conclusion that this scenario is the exception rather than the rule, at least at the sensitivity and resolution of our observations.