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Star formation in the massive cluster merger Abell 2744

We present a comprehensive study of star-forming (SF) galaxies in the HST Frontier Field recent cluster merger A2744 (z=0.308). Wide-field, ultraviolet-infrared (UV-IR) imaging enables a direct constraint of the total star formation rate (SFR) for 53 cluster galaxies, with SFR{UV+IR}=343+/-10 Msun/yr. Within the central 4 arcmin (1.1 Mpc) radius, the integrated SFR is complete, yielding a total SFR{UV+IR}=201+/-9 Msun/yr. Focussing on obscured star formation, this core region exhibits a total SFR{IR}=138+/-8 Msun/yr, a mass-normalised SFR{IR} of Sigma{SFR}=11.2+/-0.7 Msun/yr per 10^14 Msun and a fraction of IR-detected SF galaxies f{SF}=0.080(+0.010,-0.037). Overall, the cluster population at z~0.3 exhibits significant intrinsic scatter in IR properties (total SFR{IR}, Tdust distribution) apparently unrelated to the dynamical state: A2744 is noticeably different to the merging Bullet cluster, but similar to several relaxed clusters. However, in A2744 we identify a trail of SF sources including jellyfish galaxies with substantial unobscured SF due to extreme stripping (SFR{UV}/SFR{IR} up to 3.3). The orientation of the trail, and of material stripped from constituent galaxies, indicates that the passing shock front of the cluster merger was the trigger. Constraints on star formation from both IR and UV are crucial for understanding galaxy evolution within the densest environments.

In relaxed clusters, galaxies in the core are generally quiescent (although c.f. brightest cluster galaxies, Rawle et al. 2012a), and star formation is concentrated in the periphery (Fadda et al. 2008). The core-passage phase of a massive cluster merger may produce a shock front moving through the ICM that could initiate further starbursts in central galaxies (e.g. Bekki & Couch 2003) or curtail star formation via extreme ram-pressure stripping (e.g. Fujita et al. 1999). The relative significance of these processes is not currently well constrained (Owen et al. 2005;Johnston-Hollitt et al. 2008;Rawle et al. 2012b;Owers et al. 2012).
An understanding of the influence of cluster mergers requires a comprehensive view of star formation in cluster galaxies. The local Universe lacks recent massive mergers for a detailed analysis. The Bullet cluster is a famous example at intermediate redshift (z ∼ 0.3), presenting a massive supersonic merger in the plane of the sky. IR analysis shows a marginal increase in total SFR compared to relaxed clusters at similar redshift, but no obvious individual enhancement in the vicinity of the shock front (Chung et al. 2010;Rawle et al. 2010Rawle et al. , 2012b. However, IR data only probes obscured star formation, which may miss activity in galaxies undergoing gas (and therefore dust; Cortese et al. 2010) stripping. Unfortunately, wide-field UV imaging (tracing unobscured star formation) is non-existent for the Bullet cluster due in part to a GALEX bright star constraint.
In this paper we explore star formation in Abell 2744 (00 h 14 m 19 s , -30 • 23'19", z = 0.308), also known as AC118 or "Pandora's Cluster". A2744 includes a recent supersonic merger approximately in the plane of the sky, thus offering an alternative laboratory to probe the effect of cluster-scale shocks on galaxy formation. Analysis of A2744 benefits from UV-IR coverage and many 100s of cluster galaxy spectroscopic redshifts, which led to the cluster's selection as the first HST Frontier Field (HFF). 1 Early optical studies agreed with the predicted blue galaxy fraction from the Butcher & Oemler (1984) effect (Couch & Sharples 1987). The merger scenario was not proposed until discovery of a radio relic (Giovannini et al. 1999) and a non-gaussian velocity distribution (Girardi et al. 2001). Chandra X-ray imaging then helped identify two remnant cores from a recent, ∼3:1 mass ratio, 'bullet'like merger (Kempner & David 2004;Boschin et al. 2006;Braglia et al. 2009;Owers et al. 2011;Merten et al. 2011). Debris stripped from the more massive northern core is found between the remnants, while the outskirts comprise the mixed, de-coupled halos of the pre-merger systems. A third component lies to the northwest, originally characterised as a pre-infall sub-cluster (Kempner & David 2004;Boschin et al. 2006), but recently re-interpreted as a poststripped system moving north-northeast (Owers et al. 2011) or northwest (Merten et al. 2011). Two large-scale filaments extend to the south and northwest (Braglia et al. 2009).
We explore the cluster galaxy (UV+IR) SFRs in the context of this substructure. Section 2 overviews the observations and Section 3 derives SFRs. Section 4 discusses star formation properties of individual galaxy and the integrated cluster. Section 5 summarises the results. We adopt standard cosmological parameters (ΩM,ΩΛ,h) = (0.3,0.7,0.7). At the cluster redshift, 1 arcmin corresponds to 0.27 Mpc.

OBSERVATIONS
This comprehensive study of star formation requires spectroscopic redshifts, well-sampled infrared SEDs (indicating dust-obscured star formation) and photometry in UV bands (tracing unobscured young stellar populations).

Herschel observations
A2744 was targeted by the Herschel Space Observatory (Pilbratt et al. 2010) as part of the "Herschel Lensing Survey" (HLS; Egami et al. 2010). PACS images at 100 and 160 µm (Poglitsch et al. 2010) were produced from calibrated time-stream data, taken directly from the Herschel Science Archive. 2 Map-making followed the Generalised Least Squares method of Unimap (Piazzo et al. 2012). For SPIRE (250, 350, 500 µm;Griffin et al. 2010), images were created via the standard reduction pipeline in HIPE v9, plus a median baseline removal and the fastDestriper module.
The all-sky WISE (Wright et al. 2010) catalogue 4 contains fluxes at 3.4, 4.6, 12, 22 µm for several cluster members (5-σ sensitivity of 0.25, 0.35, 3.0, 18.0 mJy), which is particular useful outside Spitzer coverage. Source blending within the 12 arcsec beam of WISE at 22 µm is a major problem, so fluxes in this band are disregarded for SED fitting.
U -band (365 nm) imaging is from the ESO2.2m/WFI, with raw data downloaded from the ESO archive. 6 Rereduction used an updated version of Alambic (Vandame 2002). The average seeing was ∼1.5 arcsec FWHM and the final image has a 5-σ sensitivity mAB∼21.5 mag.
UV observation originates from the Galaxy Evolution Explorer (GALEX ) All Sky Imaging Survey (AIS). 7 The SPIRE footprint is located within the overlap of two 500 s tiles, yielding a near-UV band (N U V ; 2267 nm) magnitude limit of mAB = 22.5 mag. The PSF FWHM is 5 arcsec.
The 298 foreground galaxies with spectroscopic redshifts are used as a field sample in Section 4.1.3, and are analysed identically to the cluster sample. Background galaxies are excluded due to increased uncertainties from the lensing effect.

Multi-band catalogues
Band-merged catalogues were produced within the Rainbow Cosmological Surveys Database framework. 8 Sourcematching between spectroscopic, UV, optical and near-IR (NEWFIRM, WISE, IRAC) catalogues follow a trivial nearest-counterpart algorithm. The deep IRAC data allows excellent astrometric co-alignment of the MIPS image via bright isolated 24 µm sources. Despite the 6 arcsec FWHM PSF, every MIPS source is robustly associated with a single counterpart at shorter wavelengths. Increased source blending in Herschel bands calls for the use of a simultaneous PSF-fitting photometry technique. We use the master Spitzer catalogue as priors where available, and revert to direct detection in SPIRE beyond the Spitzer coverage.
Integrated cluster properties (Section 4.3) are taken from a central 4 arcmin-radius (1.1 Mpc) region, where UV, IR and spectroscopic coverage is most complete. The area encompasses 73 sources detected in at least two Herschel bands, of which 37 have a spectroscopic redshift, with 20 confirmed cluster members. The deep 24 µm imaging probes marginally fainter dust than Herschel, so three MIPS-detected (but Herschel -undetected) cluster members are also included. There are 46 N U V -detected sources, of which 42 have a spectroscopic redshift, with 14 galaxies in the cluster. In all, this central region contains 27 starforming cluster galaxies (i.e. detected in UV and/or IR).
At larger radii, IR-bright cluster members are restricted to directly-detected SPIRE sources matched to the optical via NEWFIRM and WISE data only. We are very conservative in confirming counterparts without full Spitzer and PACS coverage, which significantly impacts completeness. Seven secure Herschel -detected spectroscopic cluster members are identified, two of which are aided by IRAC coverage. We also locate one additional MIPS/IRAC-detected cluster member without SPIRE flux. Spectroscopy confirms that 24 GALEX sources beyond the 4 arcmin radius are cluster members. In total, the outer region has 26 confirmed star-forming cluster members. 8 https://rainbowx.fis.ucm.es

IR SEDs and SFRIR
The characteristic dust temperature (T dust ) of the IR component is calculated via the best-fitting single-temperature modified blackbody. Dust temperature is degenerate with the emissivity index β (e.g. Blain, Barnard & Chapman 2003), so we assume β = 1.5. Using β = 2.0 instead would systematically decrease T dust by ∼10%.
Chandra imaging reveals one X-ray point source associated with an IR-bright cluster galaxy (HLS001427-302344). The X-ray luminosity (LX ∼ 10 41 erg s −1 ) suggests the presence of an active galactic nucleus (AGN), and optical spectroscopy confirms a low luminosity Seyfert 1 (Owers et al. 2012). Hence for this galaxy, we find the best-fitting sum of a Rieke et al. template and the mean low-luminosity AGN from Mullaney et al. (2011). The AGN dominates 24 µm flux (∼75%), but has negligible influence in the Herschel bands.
For sources detected by MIPS but not Herschel, we extrapolate SFRIR directly from the 24 µm flux using the Rieke et al. formula at z = 0.308. 10 T dust remains unconstrained.
All uncertainties are calculated by performing 1000 Monte Carlo simulations based on the estimated flux errors.

SFRUV
The unobscured SFR (SFRUV) is derived directly from the UV luminosity following Daddi et al. (2004). 11 For z = 0.3 cluster members we use the GALEX N U V band to approximate rest frame 150 µm. The mean N U V -band sensitivity within the SPIRE footprint corresponds to a SFRUV limit of SFRUV∼1 M ⊙ yr −1 .

SFRUV+IR
Total SFR (SFRUV+IR) is simply the sum of the two SFRs (no extinction corrections are applied). However, for galaxies undetected in either UV or IR, the unknown SFR is estimated as follows. For UV-bright galaxies undetected at 24 µm or longer wavelengths, we derive the obscured SFRIR from the internal extinction. The extinction is estimated via the UV slope (observed N U V -U ∼ rest frame 150-230 µm), following the general trend of the Local Volume Legacy survey from Dale et al. (2009) (their Figure 13). Given the intrinsic scatter, uncertainties are ∼50%. The largest estimated SFRIR is for GLX001339-303015 (7.3±3.3 M ⊙ yr −1 ), which lies outside of the southwestern extremity of the SPIRE coverage. For all galaxies, the estimated SFRIR is consistent with the IR non-detections.
Estimating SFRUV for IR-bright galaxies undetected in N U V (and typically also undetected in U ), is more challenging. However, the detection limit indicates that SFRUV < 1 M ⊙ yr −1 , so we simply assume that SFRUV+IR=SFRIR + 0.5 M ⊙ yr −1 . Generally in these cases, SFRUV constitutes <20% of the total.

Stellar mass
Galaxy stellar mass (M * ) for all spectroscopic cluster member galaxies is estimated from IRAC (or WISE, outside IRAC coverage) 3.6 and 4.5 µm fluxes, following the relation of Eskew, Zaritsky & Meidt (2012) at z = 0.308. 12 Stellar mass is unconstrained for 22/447 cluster members (six within the GALEX -detected SF catalogue) as they are beyond IRAC imaging and are undetected by WISE.

Optical morphology of SF galaxies
High resolution optical data can help differentiate evolutionary mechanisms. For example, close companions and looping trails indicate galaxy-galaxy interaction, while isolated disturbance suggest a role for the cluster potential. HST /ACS observations, including recently obtained HST Frontier Fields data, cover nineteen star-forming cluster galaxies ( Figure 2). We classify the morphologies of these objects to aid discussion in later sections.
Several galaxies exhibit multiple star-forming trails, blue knots and asymmetric morphologies consistent with 'jellyfish' galaxies in the local Universe (first class, Figure 2). Three (MIP001417-302303, GLX001426-302413, HLS001427-302344) were previously reported by Owers et al. (2012) (their F1228, F0237, F0083 respectively). A fourth, HLS001428-302334, is located very close to the Owers et al. jellyfish with a similar optical colour and disturbed morphology, but was previously ignored due to the ACS image edge. Finally, GLX001354-302212 is a newly identified jellyfish (with a >20 kpc blue tail) to the west of the cluster in the HFF parallel field.
HLS001427-302344 is the most spectacular jellyfish, with a large number of blue tentacles, the highest total SFR (SFRUV+IR = 34.2 ± 1.3 M ⊙ yr −1 ) in the cluster and an unusually large ratio of unobscured-to-obscured star formation (SFRUV/SFRIR∼3.3) for such an active galaxy. Only one other IR-detected galaxy (HLS001430-302433) exhibits SFRUV>SFRIR (SFRUV/SFRIR∼1.8), located <0.25 Mpc (in the plane of the sky) from the group of three jellyfish, but unfortunately beyond HST coverage so the morphology is unknown. Four of five confirmed jellyfish are located together near the cluster centre, where cluster in-fall is unlikely to be the driving mechanism. We discuss the location of these galaxies, and the orientation of their blue tails, in Section 4.2.
Spiral is the predominant morphology for star forming cluster members (11/19), exhibiting a wide range of arm/disc contrast and arm tightness. By eye, we identify a distinct morphological sub-set of spirals with optically-red cores, broad dusty (not blue) arms and no sign of recent galaxy interaction (second class, Figure 2). This group of five galaxies includes ∼50% of the star formation (and all of the LIRGs) covered by ACS imaging and have a mean SFRUV/SFRIR∼0.16. They are likely to represent the as-yet unstripped in-falling spiral population. The other six spirals exhibit lower SFRs and a higher mean SFRUV/SFRIR∼0.72.
The remaining sources have non-spiral morphologies and low total SFRs ( 4 M ⊙ yr −1 ). HLS001420-302116 has a long, faint tidal loop (extending ∼25 kpc to the southeast) suggestive of a past galaxy encounter rather than disturbance driven by the cluster potential. HLS001415-302149 is an edge-on disc galaxy, while MIPS001430-302210 appears to be a smooth, compact early-type.

Specific SFR (sSFR)
SFR per unit stellar mass (specific SFR; sSFR), quantifies the efficiency of star formation, which generally increases with redshift (e.g. Pérez-González et al. 2005). At fixed redshift, sSFR decreases with increasing stellar mass (i.e. less massive galaxies are relatively more efficient; Pérez-González et al. 2008), providing a main sequence (MS) of galaxies forming stars via the 'normal' spatially extended mode.
We derive sSFR from SFRUV+IR/M * , and hence uncertainties in sSFR and M * are highly anti-correlated along the lines of constant SFR. The majority of SF cluster galaxies in A2744 are located on the MS (Figure 3). The best-fitting linear relation to the cluster sources yields a steeper relation than Pérez-González et al., but incompleteness below SFR 2 M ⊙ yr −1 is increasingly important at lower M * . Here, we primarily employ Figure 3 to differentiate modes of star formation. Starbursts, a highly efficient mode of concentrated star formation, are often defined as those with sSFRs at least twice that of the intrinsic mass-dependent MS (e.g. Elbaz et al. 2011). The jellyfish galaxy HLS001427-302344 is starbursting, presumably observed near peak efficiency as SFRUV/SFRIR suggests that the dusty gas reservoir has been almost completely stripped and exhausted. HLS001430-302433, the only other IR-bright galaxy with SFRUV/SFRIR > 1 (Section 4.1.1), also has a starburst-like sSFR.
The dusty, red-core spirals are also located in the star- burst region of Figure 3, exhibiting a mean sSFR four times that of the MS ( sSFR/sSFRMS = 4.2 ± 0.6). The remaining confirmed ('further') spirals are on the main sequence ( sSFR/sSFRMS = 1.0 ± 0.3), although we note that sSFR could be underestimated if the SFR of localised starbursts is counter-balanced by large-scale quenching throughout a galaxy disc (e.g. Merluzzi et al. 2013).

Characteristic dust temperature (T dust )
In the merging Bullet cluster (z = 0.296), Rawle et al. (2012b) measured high characteristic dust temperatures for several sub-LIRGs, interpreted as a post-stripped population with cooler, outer gas/dust removed. A lack of similar sources in low density environments or a relaxed control cluster (MS2137, z = 0.313) indicated a stripping mechanism driven by the cluster merger. A2744 offers an opportunity to investigate further. The unusually warm Bullet cluster sources are immediately obvious in the LIR-T dust diagram (Figure 4), which exhibits a very shallow correlation below the sharp upturn at LIR ∼ 2 × 10 11 L ⊙ . A2744 contains no similar warm galaxies, with the highest measured temperature <28 K. While the T dust distribution for the low IR luminosity sample in the Bullet cluster ({mean,rms} = 31.4, 6.6 K) exhibits a tail to warm temperatures, the peaky distribution of A2744 closely resembles that of MS2137 ({mean,rms} = 23.8, 2.8 K; 24.8, 3.1 K respectively). The T dust measurements for galaxies in A2744 and the Bullet cluster follow identical procedures with the same modified blackbody parameterisation (β = 1.5). The mean uncertainty on the temperature is 2.5 K in A2744 and 2.0 K for the Bullet cluster. Physically, the similarity between the   Figure 8), with 1 σ uncertainty shown by green shading. The steeper relation in A2744 is caused by the SFR limit becoming increasingly restrictive at the lowest masses. Galaxies with at least twice the sSFR of the fiducial main sequence (those above the dashed green line) can be considered as starbursts (Elbaz et al. 2011). Symbols indicate optical morphology, where HST imaging is available.
log 10 (L IR L sun )  . A2744 is ∼20% less massive, and has a larger X-ray cooling time and higher central entropy, indicating a more disruptive merger (Hudson et al. 2010). The unusual sources in the Bullet cluster reside at local densities which are not atypical for A2744. Further examples of sources with unusual T dust are required to constrain their origin.

Cluster substructure and star formation
We now consider the location of cluster star formation (Figures 1 and 5). Owers et al. (2011) used a Kaye's Mixture Model algorithm to assign galaxies to sub-components. Although 25% were allotted to the two central remnant cores, only one of those is detected in the UV or IR: HLS001420-302116 (northern remnant). HST imaging shows that the galaxy has a faint tidal loop, suggesting harassment, but no obvious near neighbour or dual-nucleus, arguing against a recent interaction. Galaxies within the remnant cores appear wholly undisturbed by the cluster merger, and most SF cluster members are located in the post-merger halo. The two remnant cores exhibit a mutual ∆z = 0.014, but redshift is not a true spatial dimension, combining distance from the observer and peculiar velocity. The southern remnant core (the 'bullet') is travelling away from the observer (v los ∼ 2400 km s −1 ; Owers et al. 2011) and is closer to the northern remnant core than Figure 5 may suggest. A trail of SF galaxies (including four jellyfish and three LIRGtype red-core spirals) traces the expected past trajectory of the bullet, clearly visible in the central panel of Figure   5. The galaxies are unlikely to be debris stripped from the supersonic southern remnant itself, as they are mostly blueshifted with respect to the northern remnant. Rather, we suggest that many of these galaxies were located within the halo of the pre-merger northern cluster, and were encountered by the shock front associated with the passage of the bullet.
Morphological characteristics such as the jellyfish blue tails originate from the stripping of material by extreme ram pressure. The potency of the mechanism will be a function of intrinsic galaxy properties (mass, SFR) and disc/shock orientation. Three of the jellyfish within the trail highlighted above (GLX001426-302413, HLS001427-302344, HLS001428-302334) exhibit blue tails pointing in the direction of the dynamic axis of the cluster merger (Figure 2). This orientation is perpendicular to the radial line of the cluster and therefore inconsistent with an in-fall scenario, in which tails would tend to point away from the cluster centre (e.g. Smith et al. 2010). Furthermore, the merger core-passage phase occurred 120-150 Myr ago (Merten et al. 2011), which corresponds well with the stellar population age of the young stars in the jellyfish (∼100 Myr ago; Owers et al. 2011). We conclude that at least these three galaxies, located very close to the past trajectory of the bullet-like remnant core, have been stripped by the passing shock front.
The blue tail of the fourth central jellyfish (MIP001417-302303) is orientated in a different direction. Pointing towards the cluster centre, it is inconsistent both with the shock front motion and with stripping during in-fall first pass. Noting the proximity of the nominal cluster centre (<150 kpc) we suggest that the galaxy is observed moving outwards soon after the first pass. The final jellyfish (GLX001354-302212) is located away from the primary merger, surrounded by several other SF galaxies. The proximity of the filament suggests that these galaxies may be a small in-falling group, with the jellyfish stripped by an increasingly dense ICM. However, the blue tail points perpendicular to the radial line, towards the northeast ( Figure  2). This orientation is consistent with the proposed northeastern motion of the northwestern interloper (Owers et al. 2011), raising the intriguing alternative scenario that the clump of star forming galaxies may be associated with the passage of this third sub-component.
At larger radii (>4 arcmin), ∼70% of the cluster galaxies (by number or stellar mass) reside within the southwestern half of the cluster. This is not a consequence of spectroscopic coverage, which is symmetric about the cluster core, but rather the primary evidence for the existence of largescale filaments interfacing with the cluster to the south and northwest (Braglia et al. 2009). This southwestern half contains ∼95% of known cluster SFR beyond 4 arcmin, and the enhancement is not due to IR/UV coverage (Figure 1). The filaments themselves account for ∼65% of the SFR (from 40% of the stellar mass) at large radii, including 3 LIRGs (SFRIR = 12.9, 15.0, 16.7 M ⊙ yr −1 ). This overabundance of activity is consistent with preferential location of peripheral star formation in filaments (e.g. Fadda et al. 2008).
Generally, the distribution of the SF population appears typical of cluster environments. However, the location of at least three extreme jellyfish galaxies suggests an evolutionary path connected with the passage of cluster merger in-  Figure 5. Declination versus redshift (∆cz with respect to z = 0.308) in three RA slices. The central slice spans 4 arcmin around the nominal cluster RA, thereby containing the primary northsouth merger. Star-forming galaxies are represented by symbols, as in Figure 3 (black have SFR UV+IR > 3 M ⊙ yr −1 ; otherwise grey). All cluster galaxies with a spectroscopic redshift are shown by blue dots. Cluster substructure is marked via green regions analogous to Figure 1. The estimated dynamic axis of the southern remnant is marked in orange.
duced shock fronts. In the next section we examine whether these transformations significantly affect the global star formation characteristics of cluster.

Total cluster star formation
We quantify the overall effect of the A2744 merger on the galaxy population by comparing to other clusters at similar redshift. For this purpose, we compute the mass-normalised total SFRIR (ΣSFR) and fraction of SF cluster galaxies (fSF) in each cluster. We include only the central 1.1 Mpc (4 arcmin in A2744), isolating the centrally-located effects of any recent merger and reducing the influence of surrounding infall regions and large-scale structure. 13 The chosen central region of A2744 also benefits from homogeneous UV-IR coverage. Previous studies rarely include unobscured SF, so for a fair comparison we are restricted to IR-detected SF galaxies and SFRIR, although SFRUV is discussed where appropriate. We first explore whether the cluster merger increases the fraction of galaxies with obscured star formation in the total cluster population (fSF). We follow Haines et al. (2013), who consider only galaxies brighter than MK < 13 Mass-dependent scale lengths, such as the virial radius or r 500 , were considered but prove hard to define for unrelaxed cluster mergers. We instead choose to impose a fixed radial limit for all clusters. For comparison, LoCuSS clusters have r 500 =0.8-1.6 Mpc (Haines et al. 2013 Figure 6. Fraction of SF galaxies within R < 1.1 Mpc (f SF ). A2744 (filled blue square) is compared with the Bullet merger (red) and MS2137 (magenta) from Rawle et al. (2012b), and the LoCuSS sample (green; Haines et al. 2013), all recalculated to match our radial limit. The orange lines show the best-fitting evolutionary trend: f SF ∝ (1 + z) n , n = 6.3 +1.7 −1.5 (Haines et al. 2013). In A2744, f SF matches the general cluster population, but for the Bullet, the fraction is enhanced. K * +1.5 mag. 14 Within R < 4 arcmin of A2744 there are 428 K-band sources brighter than this limit. Spectroscopy confirms that 187 are cluster members, while definitively placing 42 outside. Of the remaining 199 sources, 38 have photometric redshifts (Busarello et al. 2002) inconsistent with the cluster (z < 0.2 or z > 0.4), so the cluster population will number between 187 and 348 (=428-42-38). Adopting the Haines et al. definition of star forming (SFRIR 3), A2744 contains 15 SF galaxies, yielding fSF = 0.080 +0.010 −0.037 . Figure 6 shows that A2744 lies on the best-fitting evolutionary trend from the LoCuSS sample, which includes both relaxed and recently disturbed clusters (Sanderson, Edge & Smith 2009;Rawle et al. 2012a), alongside the relaxed MS2137. The merging Bullet cluster exhibits a very weak (∼1 σ) enhancement in fSF (Rawle et al. 2012b). The possibility of a general correlation between cluster morphology and global star formation properties will be explored for the LoCuSS sample by Scott et al. (in preparation).
We now progress to the mass-normalised total SFRIR (ΣSFR). Within a 1.1 Mpc radius, there are 23 IR-detected spectroscopic cluster members in A2744, with a combined SFRIR = 132 ± 7 M ⊙ yr −1 . This underestimates the total cluster obscured SFRIR due to two limits, (1) spectroscopic incompleteness and (2) the IR sensitivity. We account for (1) by examining the 36 Herschel sources without spectroscopic redshifts. Ten have optical photometric redshifts from Busarello et al. (2002), but only two are at zopt ∼ 0.3 (the others are zopt > 0.6). For the remaining 26 sources, we estimate a very approximate "IR photo-z" by fitting a typical greybody (T dust = 30 K; β = 1.5) to the SED. Only one exhibits 0.1 < zIR < 0.6. These three cluster candidates would have SFRIR = 1.8±0.4, 1.8±0.3, 2.4±0.3 M ⊙ yr −1 . Further cluster members with SFRIR 2 M ⊙ yr −1 are unlikely. Limit (2) excludes low-level star formation in e.g. the early-type population (Rawle et al. 2008). We estimate their total SFR by stacking 24 µm flux for 142 spectroscopic cluster members covered by MIPS data but without a formal detection. The stack has a total flux S24 = 3.4 ± 0.1 µJy (SFRIR = 0.07 M ⊙ yr −1 ), which represents a negligible contribution. Finally, neither test above accounts for a population faint in IR and optical bands (i.e. excluded from spectroscopy), such as dwarf galaxies. However, it is unlikely that such galaxies would contribute significantly to the integrated cluster obscured SFR as environmental quenching is highly efficient for dwarves (e.g. Smith et al. 2012). Hence, within R < 1.1 Mpc, the total SFRIR = 138 ± 8 M ⊙ yr −1 . The five LIRGs within this central region contribute 55% of the SFRIR.
For completeness we briefly deviate from the obscured SFR and note that within the central region, total SFRUV = 63 ± 3 M ⊙ yr −1 . The stack of UV-undetected spectroscopic cluster members has a mNUV,AB = 25.8 ± 0.2 mag (SFRUV = 0.05 M ⊙ yr −1 ), confirming that the GALEX sensitivity limit has a negligible effect. Unobscured star formation accounts for 30% of the total SFR, SFRUV+IR = 201 ± 9 M ⊙ yr −1 .
We put these into a wider perspective in Figure 7 by comparing to the 29 LoCuSS clusters, MS2137 (Rawle et al. 2012b, Richard et al. in preparation) and two z ∼ 0.3 − 0.4 clusters from Popesso et al. (2012). 16 A2744 lies on the bestfitting evolutionary trend derived by Popesso et al. (2012) from nine clusters (z ∼ 0.2 − 0.9). Note that A267 is the lowest mass cluster in the LoCuSS sample, which increases the uncertainty in mass considerably, and may dampen the ability of the cluster to quench in-falling galaxies, resulting in the higher SFR. The Bullet cluster has enhanced obscured activity, although previous studies have concluded that individual galaxies immediately adjacent to the shock are unaffected (Chung et al. 2010;Rawle et al. 2010). Furthermore, the relaxed MS2137 and the unrelaxed MS1358 show marginally enhanced and depressed ΣSFR respectively, suggesting that there is significant intrinsic scatter unrelated to the dynamical state of the cluster.
The current data indicates that for z > 0.3 the overall (IR-detected) obscured SFR is not influenced by the occurrence of a recent merger. However, we have already shown that SFRIR may not be a good tracer of the processes triggered by the cluster merger, as at least one jellyfish is known to possess a high unobscured SFR. Figure 7 also displays the effect of including SFRUV for A2744: increasing ΣSFR by ∼50%. With the current comparison data (which excludes SFRUV), it is difficult to ascertain whether A2744 has an unusually high SFRUV, or whether relaxed clusters are similar (systematically increasing ΣSFR).
For A2744, the cluster merger has a negligible, or at least net zero, effect on obscured SFR. The large fraction of unobscured SFR (∼ 1 3 total SFR) in the cluster, together with the known examples of extreme stripping triggered by the merger, suggests that the true effect on total SFR may be more significant.

CONCLUSIONS
This paper presents analysis of multi-wavelength data, including GALEX, Spitzer and Herschel bands, for galaxies within the massive merging HST Frontier Field cluster A2744 (Pandora's Cluster). We determine the overall (unobscured plus obscured) star formation properties for 53 cluster galaxies, with a total SFR, SFRUV+IR = 343 ± 10 M ⊙ yr −1 . From the IR, we find that the cluster contains no ULIRGs, and all eight LIRGs are fainter than LIR < 2 × 10 11 L ⊙ . A2744 contains no sub-LIRG galaxies with unusually warm ( 40 K) dust, suggesting that the presence of such sources in the Bullet cluster (Rawle et al. 2012b) may not be related to the cluster merger.
Within the central 1.1 Mpc (the Herschel Lensing Survey PACS coverage), which encompasses both remnant cores from the primary merger as well as the innermost regions of the mixed halo population, the total SFRIR = 138 ± 8 M ⊙ yr −1 (55% from LIRGs) and SFRUV = 63±3 M ⊙ yr −1 . Comparison with further clusters at similar redshift indicates that the merger has no significant effect on (mass-normalised) total cluster obscured SFR (ΣSFR = 11.2 ± 0.7 M ⊙ yr −1 per 10 14 M ⊙ ) or fraction of (IR-detected) star-forming galaxies (fSF = 0.080 +0.010 −0.037 ). The remnant core galaxy populations exhibit no signs of star formation or disturbance attributable to the cluster merger. However, there is a population of star-forming halo galaxies consistent with morphological transformation due to the passage of the shock front associated with the bulletlike southern sub-cluster. These include at least one extreme, unobscured starburst jellyfish galaxy (SFRUV+IR = 34.2 ± 1.3 M ⊙ yr −1 , SFRUV/SFRIR∼3.3).
In A2744, the merger has a net-zero effect on the bulk obscured star formation properties of the cluster. Generally, total cluster IR dust properties (e.g. total SFRIR and T dust distribution) do not appear to be systematically correlated with the existence of a recent merger; A2744 and the Bullet cluster exhibit significant differences. With the lack of unobscured SFR analysis for relaxed clusters (or the Bullet), it is difficult to ascertain whether SFRUV is unusually enhanced by the merger.
We have demonstrated that future cluster studies, particularly for merging systems where extreme ram pressure may play a significant role, require both UV and IR imaging to adequately constrain the total star formation properties of member galaxies. Table 1. Derived properties for the star-forming galaxies in A2744. Bold rows indicate galaxies within the central 4 arcmin (1.1 Mpc).