ALMA Evidence for Ram Pressure Compression and Stripping of Molecular Gas in the Virgo Cluster Galaxy NGC 4402

NGC 4402 is one of the nearest and clearest examples of a spiral galaxy experiencing ram pressure stripping. It is close enough that one can study with high spatial and kinematic resolution the effects of RPS on gas in the disk. NGC 4402 is located in the Virgo cluster at a distance of ∼17 Mpc (Mei et al. 2007), and ∼0.4 Mpc in projected distance from the cluster center, as indicated in Figure 1. It may be a member of the merging subcluster located around the giant elliptical M86 (Böhringer et al. 1994). Images of the galaxy at different wavelengths shown in Figure 2 reveal a highly inclined (i = 80 deg), 0.3L* Sc galaxy. Infrared images at 3.6, 8, and 24 μm (Figures 2(d)–(f)) show a spiral arm on the western side of the galaxy, but more irregular structure on the east. Tracers of young stars, e.g., 24 μm, Hα, and far-UV (FUV; Figures 2(b), (c), and (f)) show emission truncated at a radius of 75'' (6 kpc), indicating that star formation has been quenched beyond here.

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Crowl et al. (2005) established NGC 4402 as being actively ram pressure stripped, via the detection of a prominent radio continuum tail. They also identified several large filaments of dust in the galaxy that protrude into the otherwise stripped zone, and use the morphology of these features to estimate the projected wind direction. Furthermore, Crowl et al. (2005) also found the stellar disk of NGC 4402 to be undisturbed, ruling out any strong influence from tidal interactions on the structure of the galaxy. Some of the effects of the RPS on the ISM of the galaxy were characterized through analysis of the dust extinction features with HST (Abramson & Kenney 2014; Abramson et al. 2016). The authors found dense clouds of dust, comprising ∼1% of the pre-stripped ISM mass, that appeared to be the only surviving ISM in the mostly stripped, H i deficient southern zone of the galaxy.

The galaxy was also observed in H i with the VLA by Crowl et al. (2005), as part of the survey of Virgo galaxies in Atomic gas (VIVA) presented in Chung et al. (2009). The galaxy is moderately H i deficient, with a deficiency parameter of 0.61, which corresponds to about 25% of the typical H i content for a galaxy in a low-density environment (Giovanelli & Haynes (1983)). We reproduce the VIVA H i map in Figure 2(a) to show that the H i appears to be compressed and truncated on the leading side of the galaxy at about the same radius as the star formation is truncated (Crowl et al. 2005; Chung et al. 2009). NGC 4402 also has a modest H i tail that can be seen on the trailing side. HST images of the galaxy shown in Figure 2(a) shows a dust plume rising above the disk on the leading (eastern) side. While the H i appears approximately coextensive with the dust, due to the low resolution (15'' ∼ 1.2 kpc) of the VLA H i observations, it is not possible to tell how much is in the disk and how much is extraplanar, or to begin to resolve any of the disk ISM features affected by ram pressure.

Star formation tracers (Hα, 24 μm, and particularly FUV) show a local peak at the leading edge of the disk in the east, as seen in Figure 2. It is possible this is star formation which has been triggered by ram pressure (see further discussion in Section 6.4).

NGC 4402 was previously observed in CO(1−0) by Kenney & Young (1989), in which the authors found a roughly normal CO content, but an asymmetric gas distribution, with stronger emission on the eastern side. The galaxy was recently observed in CO(2−1) by Lee et al. (2017) with the Submillimeter Array (SMA) at a resolution of 7'' × 4''. The authors compared their observations with H i maps and found that, like the H i, CO and Hα appeared to be compressed on the southern side of the disk and more extended to the north, consistent with ram pressure from the southeast.

Our ALMA observations reveal a greater spatial extent of CO emission, and much more detail related to the ram pressure interaction than could be seen with the SMA data of Lee et al. (2017) as we have greater resolution and sensitivity.

In this paper we present a detailed study of the molecular gas distribution and kinematics of NGC 4402 at 1'' × 2'' (80 × 160 pc) resolution, focusing on the eastern (leading) side since that is the region most affected by ram pressure. In Section 2, we present technical details of our observations. In Section 3, we present the data and provide a general description of the overall CO distribution and kinematics. In Section 4, we present our velocity model of the galaxy, and with this we identify noncircular motions in the data that are presumably due to ram pressure. In Section 5, we analyze the kinematics, masses, densities, spatial extents, and other key properties of interesting ISM features sculpted by ram pressure at the leading side. In Section 6, we discuss the implications of our findings, and the predicted evolution of the ISM as a result of ram pressure, via compression, star formation, and stripping. Finally, in Section 7 we present a summary of our results.

We plan to present ALMA CO(3−2) observations and discuss line ratios as well as the western side of the galaxy in Paper II.

Our ALMA observations reveal the molecular gas distribution in the central few kiloparsecs of the galaxy NGC 4402. The CO(2−1) moment 0 map in Figures 3(b) and (c) shows that the central kiloparsec has three CO peaks of similar brightness. One of these is close to the nucleus, the other two symmetrically straddle the nucleus, resembling the twin peaks CO distribution commonly observed in the center of barred galaxies (Kenney et al. 1992). In addition, the kinematic minor axis (Figure 3(d)) is tilted with respect to the photometric minor axis within r = 0.4 kpc, as is typically found in barred galaxies. The CO intensity map (Figures 3(b) and (c)) appears to show a strong spiral arm in the west, which aligns with the stellar pattern seen at 3.6 μm in Figure 2, and a significantly weaker one in the east. The CO intensity drops off sharply at a radius of r = 60'' = 4.8 kpc in the east, and r = 65'' = 5.2 kpc in the west, although some weak CO features are detected beyond this in the west out to 95''.

The CO distribution shows evidence for ram pressure acting from the southeast, consistent with the estimated projected ram pressure wind direction of ∼−45° for NGC 4402, based on dust filament orientation and the radio continuum morphology (Crowl et al. 2005; Murphy et al. 2009; Abramson & Kenney 2014). The CO distribution is more compressed to the south than the north, as was also noted by Lee et al. (2017). We also detect a (stripped) extraplanar CO plume at the eastern (leading side) edge of the gas disk in the new ALMA data, a feature first noted in the dust by Abramson & Kenney (2014), but not detected in molecular gas in the shallower SMA data presented by Lee et al. (2017). Beyond r = 55'' in the east, all of the CO emission is part of the plume and on the north side of the major axis.

Comparison of the two sides of the galaxy reveals clear evidence of disturbed kinematics. In an undisturbed galaxy, both the moment 1 velocity map, and the position–velocity diagram (PVD) along the major axis, should appear symmetric about the minor axis, apart from any bar or spiral arm streaming motions. In NGC 4402 this is not the case, as is highlighted in the PVD shown in Figure 4. The PVD we present encompasses the entire north–south extent of the CO(2−1) in the galaxy (width of ∼33''), mirrored about the central velocity of the galaxy, 230 km s⁻¹. In contours, we show the west side of the galaxy mirrored onto the east side, to highlight the asymmetric kinematics. The upper envelope of the velocity on the two sides of the galaxy looks relatively symmetric inside r = 25'', including both the steeply rising circumnuclear component inside r = 5'', and the region beyond, from r = 5''–25''. Thus, we are confident that the central velocity we chose for folding is correct.

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There are notable differences in the two sides of the galaxy beyond r = 25'', consistent with the direct pushing of molecular gas by ram pressure. On the east side of the galaxy, upon which the ram pressure would be strongest, the isovelocity contours in the moment 1 map of the galaxy (Figure 3(d)) appear to straighten (constant R.A.) from r = 25''–55'' as opposed to the more typical "V-shaped" isovelocity contours on the west side (this was also noted with the SMA by Lee et al. 2017). These V-shaped isovelocity contours are what one would expect to observe in an undisturbed galaxy dominated by rotational motion. In addition, the spiral arm density waves appear to be stronger in the west, as observed in the total CO intensity map and the isovelocity contour map, Figures 3(c) and (d). The east side appears to have overall lower velocity dispersion than the west side (Figure 3(e)). The PVD in Figure 4 shows more prominent low velocity components near peaks in the spiral arm, suggesting that the lower velocity components may be associated with spiral arm streaming motions, although it may also be ram pressure accelerated gas. Some of the outermost gas (r > 60'') in the west is clearly morphologically and kinematically disturbed, probably from ram pressure, and there are other interesting features in the west, but these will be discussed in detail in Paper II.

The two sides of the galaxy begin to clearly diverge in their average velocity around r = 30''. The velocities of the upper envelope of the PVD on the western side continue to gradually increase with the same slope as r = 10''–25'', and the isovelocity contours of the moment 1 map are consistent with a pattern of mostly circular motions. The velocities on the eastern side rise much more sharply, and the isovelocity pattern shows clear noncircular motions, as shown in Figure 3(d). At r = 55'', the CO surface brightness peaks at its highest value outside of the circumnuclear region, 0.1 Jy km s⁻¹ (∼75 M_⊙ pc⁻²). The combination of disturbed kinematics in the direction of the ram pressure and high gas surface density strongly suggests compression of this region of the disk due to the ram pressure. We discuss the evidence for this and implications for the ISM evolution in this region in Section 6.3.

Beyond r = 60'' on the east side we observe the most extreme velocity divergence relative to the less disturbed western side of the galaxy. This is in the region of the dust plume, shown in Figure 8. The plume gas has a significant increase in velocity with radial distance from the galaxy center. At the northeastern end of the plume, the gas reaches its most extreme velocity, ∼60 km s⁻¹ higher than any other molecular gas in the galaxy. Ram pressure is the only plausible explanation that could account for both the morphology and kinematics of this feature. The stellar body of the galaxy is symmetric and regular, with no suggestion of any gravitational disturbance (Crowl et al. 2005) such as would be expected from a tidal interaction. Furthermore, the effects of ram pressure are expected to be stronger on the eastern side, since this is the leading side of the ram pressure interaction. The plume gas is redshifted with respect to the galaxy, which is the velocity offset expected for stripped gas, as the galaxy is moving toward us (blueshifted) with respect to the cluster.

As our data shows strong evidence for the direct stripping of molecular gas from the disk of NGC 4402 via ram pressure, we wish to estimate the strength of ram pressure and the opposing restoring force from the disk around the stripped region. By estimating the ICM gas density ${\rho }_{\mathrm{ICM}}$, the relative velocity of the galaxy with respect to the cluster v, and the restoring force per unit mass $d{{\rm{\Phi }}}_{g}/{dz}$, we can use the Gunn & Gott (1972) stripping criteria ${\rho }_{\mathrm{ICM}}{v}^{2}\geqslant {{\rm{\Sigma }}}_{\mathrm{ISM}}\tfrac{d{\rm{\Phi }}}{{dz}}$ to estimate the surface density of gas ${{\rm{\Sigma }}}_{\mathrm{ISM}}$ that would be stripped, and compare it to our estimates for the surface densities of features in Table 1. The projected distance of NGC 4402 from the cluster center is 0.4 Mpc, and its 3D distance would be larger than this. From measurements of the X-ray surface brightness profile of Virgo from Schindler et al. (1999), at the 3D cluster radius of 0.4 Mpc, the density is estimated to be ${\rho }_{\mathrm{ICM}}=3\times {10}^{-28}$ g cm⁻³, and further out it would be less. However NGC 4402 is located only about 50 kpc in projected distance from the elliptical M86, and may be a member of the M86 subcluster, which has its own local peak in X-ray surface brightness and ICM density. Schindler et al. (1999) did not model the M86 region, but from comparison of the X-Ray surface brightness near NGC 4402 and at other locations at the same projected radius, we estimate the ICM density near NGC 4402 could be up to two times higher than the above estimate. Thus, we adopt an ICM density in the range of 3 × 10⁻²⁸ g cm⁻³, up to a possible maximum of 6 × 10⁻²⁸ g cm⁻³.

The line of sight (LOS) velocity of NGC 4402 with respect to the cluster is ${v}_{\mathrm{gal}}-{v}_{\mathrm{Virgo}}=847$ km s⁻¹, and the 3D velocity would be larger than this. The orbital velocity of a typical Virgo galaxy at a 3D radius of 0.4 Mpc is predicted to be ∼1300 km s⁻¹ (Binggeli et al. 1993), although the maximum could be as high as ∼1800 km s⁻¹ (Vollmer et al. 2001). Thus we adopt a relative velocity range of 1300–1800 km s⁻¹. If NGC 4402 were not a member of the M86 subcluster, but happened to be falling through it, NGC 4402 could be moving at the higher end of the velocity estimate of 1800 km s⁻¹ and experiencing the higher estimated ICM density (6 × 10⁻²⁸ g cm⁻³) associated with the M86 subcluster. Therefore, in summary, the ram pressure at NGC 4402 is predicted to be in the range of ${P}_{\mathrm{ram}}=\rho {v}^{2}\sim 5\times {10}^{-12}$ to $2\times {10}^{-11}$ dyne cm⁻².

We estimate the gravitational restoring force per unit mass $\tfrac{d{\rm{\Phi }}}{{dz}}$ as approximately equal to ${V}_{\mathrm{rot}}^{2}{R}^{-1}$, as done in Lee et al. (2017). At the radial distance of the plume, r ∼ 60'' = 4.8 kpc, the circular velocity predicted from our DiskFit model is 119 km s⁻¹, so ${V}_{\mathrm{rot}}^{2}{R}^{-1}=9.6\times {10}^{-14}$ km s⁻². Using the Gunn & Gott formula, we estimate the surface density of gas that would be stripped under these conditions as ${{\rm{\Sigma }}}_{\mathrm{ISM}}={\rho }_{\mathrm{ICM}}{v}^{2}{V}_{\mathrm{rot}}^{-2}R\approx 2.5$ M_⊙ pc⁻² for the lowest estimated density and velocity, and ${{\rm{\Sigma }}}_{\mathrm{ISM}}\approx 10$ M_⊙ pc⁻² for the upper limit.

The small clouds we have labeled as "decoupled clouds" (1–3) are located right below the plume, and are spatially and kinematically distinct from the plume, with velocities consistent with normal rotation, and thus, they appear to not be accelerated by ram pressure. In these respects, these clouds are consistent with denser clouds that decouple from surrounding lower density gas that is stripped. With surface densities of 14–28 M_⊙ pc⁻², these clouds have higher surface densities than the surface densities of 2.5–10 M_⊙ pc⁻² expected to be stripped from r ∼ 5 kpc in NGC 4402. Thus, these clouds are consistent with the expectations of clouds that have resisted stripping because of high surface densities.

The observed surface density of the plume gas is similar to that of the decoupled clouds, which at first might seem inconsistent with the observation that the plume gas is being directly stripped. However, the relevant surface density to compare with these theoretical expectations for stripping is the gas surface density in the wind direction. This is different from the observed (plane of sky) surface density, since the ram pressure has a component in the plane of the sky (and perpendicular to the disk plane) as well as along the LOS.

If we assume that the gas features have a similar depth along the LOS as they do on the sky, then the plume gas would have a depth of ∼1 kpc (∼5 beams) and the smaller clouds would have a depth of ∼200 pc (∼1 beam), a difference of a factor of ∼5. Thus, the angular correction factor would be ∼5 times larger for the plume than the decoupled clouds, and the surface density of the plume in the wind direction could be ∼2–5 times less than the observed surface on the sky, depending on the precise wind angle.

Further evidence that the plume gas may be significantly extended along the LOS comes from its large line width compared to the decoupled clouds. While the plume and the decoupled clouds have similar total surface densities, the line widths in the plume are much larger than those of the decoupled clouds, so the surface density per velocity interval, i.e., the "spectral" surface density (Table 1), is 2–4 times lower in the plume than in the decoupled clouds.

This difference in spectral surface density suggests that the plume gas is more diffuse, with a lower intrinsic volume density, than the decoupled clouds. In the Milky Way galaxy, Roman-Duval et al. (2016) find that diffuse molecular gas has a lower spectral surface density, on average than dense, GMC associated gas. Given its large line width, the plume may contain multiple clouds superimposed, such that the surface density in the wind direction for the plume is plausibly ∼2–4 times lower than the observed surface density, or 6–12 M_⊙ pc⁻². This would mean the plume has a lower surface density in the wind direction than the decoupled clouds, and is likely consistent with having a surface density in the range of 2.5–10 M_⊙ pc⁻² that could be stripped from r ∼ 5 kpc in NGC 4402.

We note that our estimates of molecular gas surface densities are based on a standard CO–H₂ relationship (X-factor, or X_CO), and we have no observational constraints on the CO–H₂ relationship in the plume or decoupled clouds of NGC 4402. It will be of interest to measure other molecular lines in the future to constrain X_CO.

Our calculation of the maximum density of ISM gas that could be stripped is less than the lower end of the measured density of GMCs in the outer regions of the Milky Way (Heyer & Dame 2015), suggesting that ram pressure is not likely to directly strip GMCs. On the other hand, it is probably strong enough to strip more diffuse molecular gas, which can comprise a significant fraction of the total molecular gas in a galaxy. In the Milky Way, Roman-Duval et al. (2016) estimate that "diffuse" molecular gas, as identified from a non-detection in ¹³CO, comprises 25% of the total molecular gas, although the fraction increases with galactocentric radius up to 50% at a radius of 15 kpc. Conversely, this means that ∼50%–75% of the molecular gas is "dense," and likely associated with GMCs. However, we observe that on the leading side of NGC 4402 only 12% of the CO is in denser decoupled clouds below the plume, that are dense enough they appear to be kinematically unaffected by ram pressure. 88% of the CO appears to be undergoing stripping in the plume, and we have measured that it has a lower spectral surface density than that of the DCs, and thus is likely diffuse. A near 90:10 ratio of diffuse to GMC gas seems incompatible with the fraction of dense gas we would expect in this region based on the findings of Roman-Duval et al. (2016). And furthermore, the presence of significant ongoing star formation in this part of NGC 4402 indicates that much of the molecular gas in this region must have relatively recently been part of a GMC.

If ram pressure is too weak to strip GMCs directly, they must be disrupted by some other effect on a timescale shorter than the ram pressure stripping timescale. We estimate this to be the time that the ISM spends on the leading quadrant of the galaxy, where it is not shielded by any intervening ISM, and thus experiences close to the maximum ram pressure. We estimate the time to accelerate gas at the end of the plume as $t=2\ast \cot (\theta )\ast {L}_{\mathrm{sky}}/{v}_{\mathrm{LOS}}$, where L_sky and v_LOS are the observed projected components of length and velocity, and θ is the angle of the ram pressure wind with respect to the LOS (see Kenney et al. (2014) for a derivation of this formula). This assumes constant acceleration starting from zero velocity. The angle of the ram pressure wind with respect to the LOS is not known, but is probably in the range 30°–60° based on the likely total velocity of galaxies in Virgo (Vollmer et al. 2001), corresponding to a range of 0.58–1.73 for the cotangent factor. For ${L}_{\mathrm{sky}}=1\,\mathrm{kpc}$, v_LOS = 60 km s⁻¹, and a −45° wind-LOS angle, t = 3.2 × 10⁷ yr, or 32 Myr. Based on our rotation curve, we estimate the orbital period of NGC 4402 to be ∼250 Myr at the radial distance of the plume. Therefore, gas in the plume has likely been experiencing strongly effects of RPS for ∼15% of the orbital time, consistent with gas on the leading side having been exclusively accelerated from the leading side.

For estimating this timescale, we also consider the lower and upper limit, the upper limit being for θ = 30°, t = 5.7 × 10⁷ yr, and the lower limit, for θ = 60° is t = 1.9 × 10⁷ yr. Because we detect no clouds with densities like those of GMCs in this region, GMCs must be disrupted, and prevented from reforming, on a timescale below ∼20–60 Myr. One possible explanation for the lack of GMCs detected in the stripped zone is disruption by stellar feedback, then stripping of the resulting disrupted gas by ram pressure preventing the normal reforming via cooling and gravitational collapse of GMC gas. This hypothesis is supported by results from Kruijssen et al. (2019), in which they find evidence for the dispersal of GMCs in NGC 300 on short timescales of ∼1.5 Myr after the formation of massive stars due to photoionization and stellar winds, limiting the average lifetime of GMCs to ∼10 Myr.

The compression from ram pressure could increase the fraction of the ISM that becomes GMCs, which then form stars. This may synchronizes and concentrate the formation of GMCs in a similar manner to how spiral arms act on the gas (Vogel et al. 1988). The resulting concentrated star formation and its feedback results in the synchronized destruction of the GMCs, the remnants of which would be less dense than the original GMC, and thus easier to strip.

Throughout this paper we have referred to "direct stripping," by which we mean that gas that was molecular in the disk is stripped as molecular gas and remains molecular on the way out. The alternative to this would be "indirect stripping," by which molecular clouds in the disk, instead of being directly pushed by ram pressure, have their gas revert to a lower density atomic or ionized state as a result of star formation feedback or natural evolution, and then have the gas stripped. In this scenario, molecular gas we detect in the plume would have been formed in situ from this stripped atomic or ionized gas after some period of time.

Our evidence supports that GMCs are not directly stripped, but that their gas is somehow effectively removed within a short time after the surrounding gas is stripped. But we also find our evidence is consistent with direct stripping of diffuse molecular gas. Most of the molecular gas is gone in the outer radial zone (4–5 kpc) on the leading side of the disk, and there is stripped molecular gas a short distance downstream in the plume. The likely surface density of this gas is comparable to that of diffuse molecular gas in spiral galaxies, and is probably low enough to be directly stripped, based on our estimates of the likely ram pressure at NGC 4402 in Section 6.1. We cannot prove that this gas began as molecular gas in the disk, but that is the simplest explanation.

If this gas was stripped when it was not molecular, then the molecules form in situ very quickly after stripping. The molecular plume starts within ∼2'' = 200 pc of the decoupled clouds. In Section 6.2 we calculate an acceleration time of 32 Myr for gas to reach the end of the plume at ∼1 kpc, and this corresponds to an acceleration time of ∼6 Myr for the start of the plume (the portion closest to the disk midplane) at ∼200 pc. Thus, in situ formation of molecular gas would have to happen on a timescale of a few Myr.

For most purposes related to galaxy evolution, whether or not the gas remains molecular the whole time does not matter since most of the molecular gas is gone in this radial zone of the disk, and there is stripped molecular gas a short distance downstream from this zone.

Our high resolution ALMA observations show the molecular gas undergoing both compression and stripping as a result of ram pressure. We suggest that these represent two different evolutionary stages of the ISM under ram pressure. The compressed region (see Figure 8) shows the first stage of major RPS effects. While gas in this region does not reach noncircular velocities near that of the plume, Figure 7 shows that it is kinematically offset by ∼20 km s⁻¹ in the direction of the ram pressure from the predicted rotation velocity at its location. The ISM is compressed, which likely results in both direct concentration of molecular gas and/or enhanced formation of molecular gas via compression of the atomic gas, as an increase in relative gas and dust density could lead to more H₂ formation on dust grains. This increase is enough to make the center of the compressed region the brightest peak in CO surface brightness outside the circumnuclear region of the galaxy, corresponding to a peak surface density of ∼75 M_⊙ pc⁻². It is possible this triggers intense star formation in the compressed region, as is seen on the leading side of a number of other galaxies under ram pressure (Koopmann & Kenney 2004; Vollmer et al. 2012b; Lee & Jang 2016; Vulcani et al. 2018; Roberts & Parker 2020). Both FUV and Hα observations (Figures 1(b) and (c) and 9) of the leading side of NGC 4402 show spatially correlated strong peaks in emission. The FUV image shows by far the strongest emission at the leading, eastern edge. The ram pressure wind has probably blown away some of the surrounding gas and dust in this star-forming region, so that fewer of the UV photons are absorbed by gas and dust. Furthermore, the removal of gas and dust in this region means that tracers of star formation like Hα and far-infrared may underestimate the star formation rate in this region, as the material that normally emits in this range (gas in H ii regions and dust) is removed. Moreover, it is possible that the typical relation of Hα-SFR may underpredict the true star formation rate (SFR) in this region. Strong ram pressure stripping of the gas and dust around H ii regions may remove this material, which would otherwise absorb some fraction of the Lyman continuum ionizing photons from young massive stars. In the ram pressure stripped jellyfish galaxy JW100, Poggianti et al. (2019) found that the escape fraction of Lyman continuum photons around H ii regions in the tail was as high as ∼52%, while the average escape fraction estimated for H ii regions in the GAs Stripping Phenomena in galaxies with MUSE (GASP) sample was ∼18%. We believe it is likely, given the FUV peak and the local peaks in Hα and 24 μm, that the leading side of the galaxy has recently undergone strong star formation.

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We find evidence in the plume region for an earlier evolutionary stage of compressed gas and triggered star formation. Figure 9 shows contours of Hα emission superimposed on the HST and ALMA CO(2−1) images. Within the bottom part of the plume, there is an Hα ridge that angles upward from the disk and is offset from the major axis in the direction of ram pressure. There are a number of visible blue stars in the region of the disk midplane cleared of dust just south of the Hα ridge, indicating recent star formation. The gas that formed the young stars powering the H ii regions in the plume detected in Hα was likely displaced from the disk midplane by ram pressure. It is likely that the compressed region, indicated in Figure 9, is at an earlier evolutionary stage of this process. From the PVD shown in Figure 7 we can see that the compressed region has been kinematically disturbed by ram pressure, but there has not yet been sufficient time and acceleration for it to have caused a significant spatial offset. In several more Myr, the acceleration this gas has received will likely cause it to be spatially offset from the disk midplane, like the offset observed for the H ii regions in the plume region. Thus, the stripped zone appears to be consistent with being the expected later evolutionary stage of the compressed zone.

In regions of intense star formation, triggered by ram pressure compression, stellar feedback could have a significant effect on local stripping efficiency. Stellar feedback would blow gas out from the region, both accelerating it toward the escape velocity (∼300 km s⁻¹ at this radius for NGC 4402) and lowering the density of the ISM, and thus requiring less ram pressure to eventually strip the gas. Stellar feedback may also lead to the formation of holes in the ISM, allowing RPS to ablate the densest regions (Quilis et al. 2000). In the high resolution gas simulations by Tonnesen & Bryan (2010, 2012), the authors found ram pressure of P_ram = 6.4 × 10⁻¹² dyne cm⁻² (very similar to our estimate of the ram pressure experienced by NGC 4402) was too weak to strip gas with the density of typical GMCs directly, but did find gradual stripping via hydrodynamical ablation of the envelopes surrounding the dense, GMC-like clumps. However, the authors notably did not include stellar feedback in these simulations. As we find that any GMCs that avoid the initial stripping do not survive for long, the inclusion of proper prescriptions for stellar feedback in simulations may be key to accurate predictions for the fate of GMCs during stripping events.