Jellyfish: Ram Pressure Stripping As a Diagnostic Tool in Studies of Cluster Collisions^{∗ †}

Observations of A1758N with HST's Advanced Camera for Surveys (ACS; Ford et al. 1998) in the F435W, F606W, and F814W filters were performed for GO-12253 (PI: Clowe) in 2011 December for total exposure times of 2536, 2544, and 5000 s, respectively. Two observations of the quoted durations were necessary in each filter to cover both components of this merging cluster. Additional shallow coverage at near-infrared wavelengths, again for both of the subclusters, was added by program GO-14096 (PI: Coe) through short exposures (656 s, 331 s, 431 s, and 1056 s) with the Wide Field Camera 3 (WFC3; Kimble et al. 2008) in 2016 April and June in the F105W, F125, F140W, and F160W filters, respectively, as part of the RELICS program (Coe et al. 2019). We use the resulting high-level science products publicly available from the MAST archive; a color image of A1758N based on the cited HST/ACS observations is shown in Figure 2.

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Extensive groundbased spectroscopy of galaxies in the A1758N field was performed by Boschin et al. (2012) and Monteiro-Oliveira et al. (2017). Blissfully unaware of this earlier work, we observed A1758N with the DEIMOS spectrograph on the Keck II 10 m telescope in poor conditions in 2018 July. Three multiobject spectroscopy masks were designed to obtain low-resolution spectra of presumed cluster members, potential strong-lensing features, and RPS candidates (see Section 5.2.1 for details of the target selection). All slits were 1'' wide; the instrumental setup combined the 600 l/mm grating (set to a central wavelength of 6300 Å) with the GG455 blocking filter to suppress second-order contributions at λらむだ > 9000 Å.

Although these observations were later found to have duplicated a significant number of measurements already reported in the literature, they corrected one erroneous literature redshift, added 23 new spectra (and redshifts), and allowed us to test for systematic biases as discussed in Section 5.

A1758 was observed with the Chandra X-ray Observatory's ACIS-S detector in 2001 (Sequence Number 800152; PI: David) for 58 ks, and with ACIS-I in 2012 September and October (Sequence Number 801177; PI: David) for a total exposure time of 148 ks. All observations were performed in Very Faint mode. We reprocessed and merged all ACIS-I observations using CIAO 4.8, and then adaptively smoothed the emission in the 0.5–7 keV band to 3σしぐま significance using the Asmooth algorithm (Ebeling et al. 2006). The resulting isointensity X-ray surface brightness contours are shown in Figure 2.

The X-ray properties of both A1758S and A1758N as determined from Chandra and XMM-Newton observations, as well as their significance for the interpretation of the extensive merging activity in this system, are discussed by David & Kempner (2004) and Durret et al. (2011).

Diffuse radio emission from A1758N was first detected in the NVSS and WENSS surveys (Kempner & Sarazin 2001); investigations conducted with the Very Large Array at 1.4 GHz, the GMRT at 325 MHz, and LOFAR at 144 MHz resolve a giant radio halo extending beyond the X-ray emission and across the full extent of Figure 2 (Giovannini et al. 2009; Venturi et al. 2013; Botteon et al. 2018). No radio relics associated with A1758N were detected.

We use the photometric data provided as a high-level science product by the HST MAST archive at https://archive.stsci.edu/missions/hlsp/relics/abell1758/catalogs/, obtained with SExtractor (Bertin & Arnouts 1996) in dual-image mode with the F606W image as the detection band and the settings employed for all data acquired for GO-14096.

All spectroscopic data, gathered by us with Keck II/DEIMOS as described in Section 3.2, were reduced using a modified version of the DEEP2 pipeline (Cooper et al. 2012; Newman et al. 2013). In the following we describe the selection of galaxy targets and the determination of cluster membership from the resulting redshifts.

5.2.1. Target Selection

The automatically generated SExtractor source catalog referred to in Section 5.1 contains a significant number of false detections. We excluded 1092 spurious sources at the field edges and then limited the remaining catalog to objects classified as galaxies and meeting the criterion m_F814W < 28. After visual scrutiny of the brightest sources in this list, we removed a further 22 objects with m_F814W < 24 as obvious stars. A color–magnitude diagram of the resulting galaxy sample is shown in Figure 3. Highlighted are galaxies selected by us for follow-up spectroscopy either because of their disturbed morphology in the HST image (this subset includes both RPS candidates and potential strong-lensing features), or because their location on the cluster red sequence (clearly visible in Figure 3) makes them likely cluster members.

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In the selection of RPS candidates we followed the prescription provided by Ebeling et al. (2014), which considers three primary morphological indicators, namely (1) signs of unilateral external forces, (2) brightness or color gradients suggesting triggered star formation, and (3) the presence of debris trails. Similar criteria have been used by other authors to select RPS candidates (e.g., Poggianti et al. 2016).

5.2.2. Redshift Measurements and Cluster Membership

The DEIMOS spectra were flux calibrated by first dividing the observed spectra by the spectrograph's response function for the grating, blocking filter, and central wavelength used during our observations (see Section 3.2 for details) and then scaling the integrated signal within the ACS/F606W passband such that it matches the observed HST photometry in the same filter, i.e.,

$$\begin{eqnarray}&&f(\lambda )=\displaystyle \frac{{f}_{\lambda ,{\rm{F}}606{\rm{W}}}}{\int s(\lambda ){T}_{{\rm{F}}606{\rm{W}}}(\lambda )d\lambda \,/\,2235.5\,\mathring{\rm A} }\,s(\lambda ),\end{eqnarray} \tag{ 1 }$$

where f_{λらむだ,F606W} is the flux density derived from the galaxy's isophotal magnitude in the ACS/F606W image, and f(λらむだ) and s(λらむだ) are the flux-calibrated and the response-corrected observed spectrum, respectively. T_F606W(λらむだ) and 2235.5 Å are the effective throughput and bandwidth (cumulative throughput width, CTW95) of the ACS/F606W filter, respectively. For all spectra that fully contain both the F606W and F814W bandpasses we computed the flux calibration factor of Equation (1) for either filter and found the results to be consistent within 6%.

We subsequently corrected the spectra for interstellar extinction in the Milky Way following Seaton (1979) and adopting the $E(B-V)$ reddening coefficient of 0.0118 measured by Schlafly & Finkbeiner (2011) in the direction of our cluster target.

Note that this calibration procedure implicitly assumes that the spectrum recorded within each DEIMOS slit is representative of that of the entire galaxy.

We measured (net) emission-line fluxes for the thermally excited Hαあるふぁ and Hβべーた lines, as well as for the collisionally excited [N ii] and [O iii] lines, from the calibrated DEIMOS spectra, where possible. The resulting ratios [N ii]λらむだ6583/Hαあるふぁ and [O iii]λらむだ5007/Hβべーた will be used as diagnostics in Section 6.3.

Star formation rates (SFRs) for our RPS candidates were derived from the Hαあるふぁ emission-line luminosity, using the scaling relation

$$\begin{eqnarray*}&&\mathrm{SFR}[{M}_{\odot }\,{\mathrm{yr}}^{-1}]=9.9\,\times \,{10}^{-42}L({\rm{H}}\alpha )\,[\mathrm{erg}\,{{\rm{s}}}^{-1}]\end{eqnarray*}$$

(Kennicutt 1998), where L(Hαあるふぁ) is the net luminosity of the Hαあるふぁ emission line, corrected for intrinsic extinction as determined from the Balmer decrement following Calzetti et al. (2000).

Finally, we obtained stellar masses with the SED fitting package Prospector (Leja et al. 2017), using both the HST photometry (ACS and WFC3 where available) and the calibrated DEIMOS spectrum as observational constraints. Like the DEIMOS spectra (see Section 5.3), the HST photometry was also corrected for Galactic extinction.

Of the 15 galaxies targeted by us with DEIMOS because of their disturbed optical morphology, two were found to be foreground galaxies, and five are potentially gravitationally lensed background objects at z = 0.6–1.3; the remaining eight are cluster members. All of the latter show emission lines, as do four additional cluster members, three of which were targeted because of their red-sequence color. The DEIMOS spectra of the sample of 12 line emitters are shown in Figure 4, their HST images are presented in Figure 5, and their location within the cluster is indicated in Figure 6.

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The 159 spectroscopically confirmed cluster members in the combined data set yield a cluster redshift of z = 0.2785 and a velocity dispersion of $\sigma ={1553}_{-100}^{+80}$ km s⁻¹. While the redshift distribution of A1758N's galaxy population (presented in Figure 7) shows no sign of bimodality, splitting the galaxy sample, as viewed in projection on the sky, at the midpoint between the two BCGs results in redshift distributions that differ at the 3σしぐま confidence level according to a two-sided Kolmogorov–Smirnov test, suggesting a significant, albeit small, line-of-sight velocity of two subclusters relative to each other. Since this simplistic split of the overall galaxy population based on projected position in the sky does not cleanly separate the populations of the two subclusters, regardless of the orientation of the merger axis, the observed velocity difference is merely a qualitative indication of relative motions along our line of sight. Complex velocity substructure was also reported by Boschin et al. (2012) in a study based on 92 cluster members which, however, did not find a significant difference in the systemic redshifts of the two subclusters of A1758N, consistent with the small radial velocity difference of 180 km s⁻¹ between the BCGs (see Figure 7).

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A clear physical division within the sample of emission-line galaxies becomes apparent when we examine star formation as a function of stellar mass. As shown in Figure 8, all of the eight galaxies targeted because of morphological signs of RPS in the HST images (see Figure 5) lie well above even the outermost confines of the so-called main sequence (e.g., Lee et al. 2015), most of them dramatically so. By contrast, the three emission-line galaxies appearing morphologically undisturbed and featuring colors consistent with the cluster red sequence show no elevated star formation³ (and neither does g1, observed as a "filler" on our DEIMOS masks).

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Used as diagnostics in the BPT diagram (Figure 9), the emission-line ratios [N ii]λらむだ6583/Hαあるふぁ and [O iii]λらむだ5007/Hβべーた place all of the galaxies of our emission-line subsample (see Figures 4–6) within the star formation regime (Kewley et al. 2001). Dividing this regime further, following Kauffmann et al. (2003), we find the two most massive galaxies targeted because of their disturbed morphology (JFG1, discussed in detail by Kalita & Ebeling (2019), and d05, see Table 1 and Figure 8) to lie in the "composite" region occupied by galaxies that show both star formation and additional nuclear activity. Note that not all galaxies from Table 1 and Figure 8 are shown in Figure 8 since some lack measurable intensities in the required emission lines.

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Table 1.  Properties of RPS Candidates

Name	log(M⋆ ${M}_{\odot }^{-1}$)	SFR	Δでるたvrad
		(M⊙ yr−1)	(km s−1)
JFG1	10.87 ± 0.02	47.9	−936
d1	8.47 ± 0.06	5.6	−1933
d2	9.50 ± 0.03	5.9	−1355
d3	8.39 ± 0.04	6.0	−739
d4	9.83 ± 0.06	15.1	−1174
d5	10.10 ± 0.05	16.6	−1410
d6	8.88 ± 0.02	4.8	−2257
d7	9.78 ± 0.01	13.8	1320
g1	9.47 ± 0.01	1.6	1370
r1	10.46 ± 0.10	>1.1a	−1885
r2	10.62 ± 0.12	12.3	−2409
r3	10.02 ± 0.06	6.0	−1381

Notes. Stellar mass, star formation rate, and radial velocity relative to the (in projection) closest BCG of all galaxies in our DEIMOS sample that show Hαあるふぁ emission (see Figure 5). Galaxy coordinates and redshifts can be found in the Appendix. We assign a name to the only indisputable case of RPS and denote the provenance of all other galaxies in our line-emitter sample by a leading "d" (for Disturbed), "r" (for red sequence), and "g" (for other galaxies).

^aLower limit since negligible Hβべーた emission prevents computation of the Balmer decrement and hence correction for dust extinction.

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Figures 8 and 9 demonstrate that all eight RPS candidates observed by us are undergoing a period of intense star formation. In addition, the location of the most massive galaxies among our targets in the composite region of the BPT diagram (Figure 9) is consistent with modest nuclear activity, in agreement with interplay and possibly a causal link between RPS and AGN as pointed out by Poggianti et al. (2017) and George et al. (2019). A caveat regarding classifications based on the BPT diagram is in order though: our RPS candidate d07 (just outside the composite region in Figure 9) is found to be an X-ray bright point source in the archival Chandra data (Figure 2), which unambiguously identifies this galaxy as an AGN. Nonetheless, the combination of these systems' disturbed morphology and extreme SFRs strongly supports our initial RPS hypothesis for most of our targets, in particular, since galaxy mergers, the only other plausible physical process that could explain the observational evidence, are highly improbable due to the very low cross-section for galaxy collisions at the high peculiar velocities encountered in massive clusters.

While the spatial distribution (as projected onto the plane of the sky) of our eight RPS galaxies does not exhibit any obvious pattern, their projected direction of motion, as deduced from their debris trails (where discernible), shows a clear tendency to point toward the NW, as indicated in Figure 6 (six out of eight,⁴ a ratio that has a probability of 10.9% of occurring by chance in an isotropic distribution). Moreover, all of them also feature high radial velocities relative to both BCGs, and for most of them (seven out of eight) these peculiar velocities are negative (Table 1 and Figure 7), indicative of a rapid motion toward us. The probability of this distribution occurring by chance in a radial velocity distribution centered on the systemic cluster redshift is 3.1%. Since the radial and transverse velocity components are statistically independent, the combined probability of the observed distribution of velocity vectors being a coincidence is 3 × 10⁻³. While this number (which formally corresponds to 2.9σしぐま significance) ought to be taken with a grain of salt, primarily because of the subjective interpretation of the orientation of the potential debris trails, the observed distribution of velocities for the sample of emission-line galaxies is clearly far from isotropic. The implications of this result are discussed in Section 7.3. We further note that, since at least four of the RPS candidates (JFG1, d1, d2, and d3) feature clearly visible and credible debris trails, the significant (but unknown) velocity component in the plane of the sky of these systems makes their total three-dimensional velocities in the cluster rest frame even higher than the observed radial velocities of 700–2000 km s⁻¹. Moreover, the observed large radial peculiar velocities imply that the debris trails of these systems are likely to be much longer in three dimensions than the observed, projected lengths of 10–30 kpc (see Figure 5), rendering these galaxies potentially extreme cases of RPS akin to ESO 137-001 in the nearby universe (Sun et al. 2007; Fumagalli et al. 2014). Although unambiguous evidence of ongoing RPS events is lacking (or less compelling) for the remaining four candidates, the location of all eight galaxies in the SFR versus M_* plane (Figure 8) is indicative of vigorous starbursts that are consistent with, but greatly exceed, SFR enhancements found for RPS events in less massive clusters (Vulcani et al. 2018).

The observed strong bias in favor of negative peculiar velocities highlighted in Section 7.2 (see also Table 1) is noteworthy, since it effectively rules out two possible scenarios regarding the dynamics and origin of the RPS population in A1758N, both of which should result in a largely symmetric velocity distribution. They are (a) isotropic infall of late-type galaxies from the surrounding field, and (b) stripping of gas-rich cluster members caused by the high velocities created by a merger proceeding in, or very close to, the plane of the sky (the geometry assumed by all previous studies of this system, including the simulations by Machado et al. 2015 and Monteiro-Oliveira et al. 2017).

Although the lopsided distribution and high amplitudes of radial velocities of our RPS candidates heavily disfavor the specific merger geometry adopted historically for A1758N, the observational evidence nonetheless strongly supports a causal link between RPS and merger events in clusters. Previous studies found two opposing effects of such a link: Stroe et al. (2015, 2017) and Ruggiero et al. (2019) report evidence of RPS triggered by merger-induced shocks, while other studies (Pranger et al. 2014; Deshev et al. 2017) find the fraction of star-forming galaxies in mergers reduced compared to nonmerging clusters, possibly after a preceding short starburst phase. Although seemingly in conflict with each other at face value, these findings might be reconciled as part of a bigger picture in which RPS events in mergers first trigger an initial burst of star formation, and then reduce or completely quench star formation as the supply of atomic and molecular gas is either exhausted or removed from the affected galaxies. The prominence and duration of either phase are, however, likely to depend on details of the merging systems, such as mass, time since first core passage, and collision geometry. In the following section we describe how a combination of both of these effects can explain the phase-space distribution of RPS candidates in the A1758N merger.

For the majority of clusters, it is primarily the infall of gas-rich galaxies from the surroundings that gives rise to RPS events. In the absence of pronounced filaments, the resulting velocity distribution is expected to be approximately isotropic, an expectation that clearly is not met by A1758N. Since galaxy infall from the field is unavoidable for a cluster as massive as A1758N, the resulting isotropic RPS distribution is either not well enough sampled to contribute discernibly to our small sample,⁵ or the A1758N collision is sufficiently advanced for merger-driven shocks to have quenched star formation in the pre-merger RPS population (Deshev et al. 2017). The latter possibility finds additional support in the findings by Haines et al. (2009) who report the presence of a significant and distinct population of passive spiral galaxies in their infrared study of this system.

While the absence of an isotropic velocity distribution in the RPS population of A1758N thus may (but need not) be a direct consequence of the system's merger history, the cause of the specific, observed directional velocity bias is almost certainly tied to the three-dimensional geometry and environment of the ongoing collision, more specifically to the only preferred direction in this system: the merger axis. Since late-type (gas-rich) galaxies are rare in massive clusters, the population undergoing RPS in A1758N must originate from a much less dense environment, suggesting infall along a filament (rather than isotropically from the general field). However, as the orientation of large-scale filaments determines and indicates the direction along which clusters accrete matter at the vertices of the cosmic web, the direction of the bulk flow of the RPS population of A1758N also marks the most probable orientation of the merger axis. It follows that the latter must be strongly inclined with respect to the plane of the sky, with the feeding filament extending behind and to the SE of the cluster center, as illustrated in Figure 10.

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In this scenario, A1758N is not merging in the plane of the sky (as assumed so far in the literature based on the argument that the absence of a significant difference in redshift between the subclusters is evidence of them moving perpendicular to our line of sight) but, in a geometry not unlike that of MACSJ0553.4−3342 (Ebeling et al. 2017) along a greatly inclined axis, rendering the observed projected distance between the BCGs of 750 kpc a severe underestimate of their true three-dimensional separation (Figure 10). In this scenario, the lack of a pronounced difference in radial velocity between the subclusters and their BCG represents strong evidence of the merger being observed near turnaround.