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RPS in Galaxy Clusters Analysis

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Jellyfish: spectroscopic study of ram-pressure stripping in massive galaxy clusters*


We continue our exploration of ram-pressure stripping (RPS) in massive galaxy clusters at z>0.3 by assessing the spectroscopic properties of RPS candidates selected previously based on their morphological appearance in Hubble Space Telescope images. We confirm cluster membership for 55 of our candidates, thereby tripling the number of RPS candidates known at z>0.2. Although many of these systems are too faint and too distant for the kind of in-depth investigation required to unambiguously confirm or refute the presence of RPS, the ensemble properties of our sample are consistent with increased star formation, and many of the selected galaxies exhibit visible debris trails. Specifically, about two thirds of all galaxies exhibit line emission ([OII]λ3727A˚ , Hβ, and, where observationally accessible, Hα) consistent with ro- bust star-formation rates that significantly exceed those expected for systems on the galaxy main sequence. We find no significant dependence of either the presence of line emission or the inferred star-formation rate on the relaxation state of the host cluster.

Although we caution that our sample may contain not only galaxies undergoing RPS by the diffuse intra-cluster medium (ICM), but also minor mergers located at the low-density cluster outskirts and merely projected onto the cluster cores, we expect our results to facilitate and inform realistic process models of the stripping process by providing the first statistically significant sample of RPS candidates in truly massive clusters. While extremely rapid removal of the intrastellar medium is not ruled out by our findings, extended periods of triggered star formation are clearly an integral component of the physics of ICM-galaxy interaction in massive clusters.


Spiral and elliptical galaxies are both commonly observed in the universe but inhabit (and dominate) very different environments. The inverse correlation between spiral fraction and density of the environment has long been established based on both galaxy mor- phology and colour (Dressler 1980; Baldry et al. 2006) and is so pronounced as to suggest causation. Since the preponderance of red, elliptical galaxies is not limited to the densest environments (i.e., the cores of massive galaxy clusters) but is notable already in groups of galaxies (Blanton & Moustakas 2009), several phys- ical mechanisms may be responsible for the observed segregation of galaxy types and appear to be have been at work for several Gyr, as evinced by the steady increase in the dominance of ellipticals in clusters from z∼1.5 to the present day (Scoville et al. 2013).

* Most of the data presented herein were obtained at the W.M. Keck Ob- servatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aero- nautics and Space Administration. The observatory was made possible by the generous finical support of the W.M. Keck Foundation.

Although simulations have indicated that elliptical galaxies can form directly through spherical collapse of dark-matter halos in high-density environments (e.g. Navarro & Benz 1991), it is widely accepted that transformations of galaxies from late to early types are central to the creation of the Hubble sequence. These occur in a range of environments and, most likely, over a range of character- istic timescales. While slow-acting gradual effects such as galaxy harassment (Moore et al. 1996, 1998) are bound to contribute, more violent interactions have been shown to be highly effective in turning disk galaxies into spheroids. In modestly dense environ- ments with commensurately modest relative galaxy velocities, i.e., in galaxy groups and at the outskirts of more massive galaxy clus- ters, galaxy mergers as predicted by Holmberg (1941) and explored in numerical simulations (e.g., Toomre & Toomre 1972; Barnes & Hernquist 1992, 1996; Mihos & Hernquist 1996) can create a wide range of remnants, including spheroidal galaxies (Toomre 1977; Hammer et al. 2009). By contrast, at the extreme opposite end of the density range where galaxies move too fast to have a signif- icant cross section for merging, ram-pressure stripping (RPS) by the diffuse intra-cluster medium (ICM) has been predicted (Gunn & Gott 1972), simulated (e.g., Farouki & Shapiro 1980; Vollmer et  al. 2001; Roediger & Hensler 2005; Domainko et al. 2006; Kron- berger et al. 2008; Bekki 2009; Tonnesen & Bryan 2010), and ob- served across a wide range of wavelengths. Numerous studies have established that RPS is capable of rapidly displacing and removing gas from spirals falling into galaxy clusters (e.g., White et al. 1991; Rangarajan et al. 1995; Veilleux et al. 1999; Vollmer et al. 2008; Sun et al. 2010).

We here present new results from an observational study de- signed to identify and characterise RPS events in massive clusters at intermediate redshift. Our project is motivated by the fact that, while RPS has been well studied in the local Universe (e.g., Sun et al. 2006; Sun, Donahue & Voit 2007; Merluzzi et al. 2013; Fuma- galli et al. 2014; Poggianti et al. 2016), work at higher redshift has advanced more slowly, due to the obvious challenges in attaining commensurate signal and spatial resolution (but see Poggianti et al. 2004; Cortese et al. 2007; Moran et al. 2007; Owers et al. 2012). It is only at z>0.2, however, that the volume probed by any clus- ter survey becomes large enough to contain a significant number of truly massive clusters (systems more massive than Coma), i.e., clusters that allow us to study RPS over the full range of environ- ments, from the only mildly overdense cluster outskirts to extreme densities in the core regions that are never reached in local clusters like Virgo.

In this paper we examine the spectroscopic properties of galaxies tentatively identified as undergoing RPS in massive galaxy clusters at z>0.3. All clusters considered for this work were iden- tified by their X-ray emission and optically confirmed in the course of the Massive Cluster Survey (MACS; Ebeling et al. 2001, 2007, 2010; Mann & Ebeling 2012). Potential stripping events were se- lected based on the morphology of galaxies in images of MACS cluster cores obtained with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST) (see Repp & Ebeling, in preparation, for an overview of this dataset). In Ebel- ing et al. (2014, hereafter E14) we presented a first sample of six textbook cases of RPS identified visually in these data and, ow- ing to their appearance, referred to as "jellyfish" (Fig. 1). Our sec- ond paper (McPartland et al. 2016, hereafter M16) defined a cus- tomized set of morphological selection criteria used to compile a larger sample of 223 potential RPS candidates and examined the spatial distribution and apparent projected direction of motion of the most plausible candidates. In this third paper, we present, dis- cuss, and interpret the results of extensive spectroscopic follow-up observations of the M16 sample.

Our paper is organised as follows: After a brief introduction in §1, §2 describes the setup and execution of our spectroscopic follow-up observations of RPS candidates, the data reduction, as well as our criteria to assess cluster membership for any given galaxy. In §3 we derive fundamental spectral properties of the con-firmed cluster members, infer star-formation rates, and estimate their stellar mass. §4 compares the properties of RPS candidates with those of the general population of star-forming galaxies, dis- cusses physical triggering mechanisms, and investigates correla- tions between the star-formation rate of RPS candidates and the relaxation state of the host cluster. We summarise our findings in Â§5.

Throughout this paper we adopt the concordance ΛCDM cos-mology, characterised by Ωm= 0.3, ΩΛ = 0.7, and H0 = 70 km s−1 Mpc−1.

Figure1.HST/ACS snapshot image of MACSJ0451-JFG1, a textbook case of ram-pressure stripping from the E14 sample. The red and yellow arrows mark the inferred direction of motion in the plane of the sky and the di- rection to the cluster centre, respectively. Note that the tell-tale "jellyfish" morphology of this z=0.43 galaxy is readily discernible only thanks to the superb resolution of HST/ACS. (Reproduced from E14)


The targets of our spectroscopic follow-up observations were drawn from the set of 223 galaxies tentatively identified by M16 as undergoing ram-pressure stripping. We refer to M16 for a detailed discussion of the morphological criteria applied to select these can- didates from a master catalogue of over 15,000 galaxies detected in short HST/ACS exposures in the F606W and F814W bands of 63 MACS clusters in the redshift range of 0.3<z<0.7. A comprehen- sive description of the HST observations used by M16 is provided by Repp & Ebeling (in preparation).

Since most of the RPS candidates from the list of 223 were targeted by us in spectroscopic observations of MACS clusters that supported several complementary research projects, compromises had to be made in the design of the observations. In order to max- imise scientific returns, clusters that feature large numbers of tar- gets for each of the different projects were given priority, resulting in a bias in favour of clusters with multiple RPS candidates. In ad- dition, the simultaneous focus on many targets made it impossible to optimise the orientation of individual slits or even the position angle of the entire mask for the study of RPS candidates.

  1. Keck/DEIMOS observations

All spectroscopic data for this work were obtained with the Deep Imaging Multi-Object Spectrograph (DEIMOS; Faber et al. 2003) on the Keck II 10m telescope on Maunakea. All multi-object spec- troscopy (MOS) masks used 1//-wide slits of at least 8//length, i.e., long enough to allow sky subtraction from in-slit data. Spectra were obtained using the 600 l/mm Zerodur grating set to a central wavelength of 6300AËš ; the GG455 blocking filter was employed to prevent second-order contamination at λ>9000AËš . Exposure times ranged from 3Ã-600 to 3Ã-1200 seconds. The seeing during these observations was typically 0.8//. All data were reduced with the


DEIMOS DEEP2 pipeline (Cooper et al. 2012; Newman et al. 2013), creating sky-subtracted and wavelength-calibrated one- and two-dimensional spectra. Redshifts were determined from the one- dimensional spectra using elements of the SpecPro software pack- age (Masters & Capak 2011).

Overall 110 RPS candidates were observed in 26 MACS clus-ters.

  1. Cluster membership

We establish (likely) cluster membership by comparing the differ- ence between an RPS candidate's redshift and the systemic redshift of the cluster with the cluster velocity dispersion. The latter is com- puted from all galaxy redshifts measured for the respective cluster in the course of the extensive spectroscopic follow-up work per- formed by the MACS team; a description of the underlying data and of the procedure employed to determine robust velocity dispersions for MACS clusters is provided by Repp & Ebeling (in preparation).

Although it is possible that some of the galaxies for which we rule out cluster membership are in fact still undergoing RPS within their local environment in the fore- or background of the respective MACS cluster, the majority of such non-cluster members are more likely to owe their disturbed optical morphology (and thus their selection in M16) to merger events or to gravitational lensing. In the following, we thus limit the term "RPS candidates" to galaxies classified as likely cluster members based on their radial velocity within the comoving cluster rest frame.

  1. Spectral corrections and flux calibration

The reduced spectra created with the DEEP2 pipeline are wave- length-calibrated and thus allow redshift measurements that are ac- curate to within the limits set by the instrumental setup and the pre- cision of the dispersion solution. The determination of line fluxes and, in particular, line-flux ratios across a significant wavelength range, however, require flux-calibrated spectra. In addition, flux lost during the data-reduction process (due to CCD defects, non- optimal definition of spectral apertures, and, importantly, the finite slit width) needs to be recovered, if the measured line fluxes are to be interpreted as characteristics of the observed galaxy as a whole. Whereas corrections for missing flux are fairly straightforward to apply, flux calibration is notoriously difficult for multi-object spec- trographs (especially when the respective observations were not performed at the parallactic angle), owing to spatial variations in the instrument response across the field of view covered by the slit mask.

Before flux calibration is performed, we visually inspect the two-dimensional spectra of all RPS candidates classified as likely cluster members. We manually mask out the spectral traces of non- target sources falling serendipitously into a slit, fill in bad detector columns, and re-extract the target spectra within an aperture that maximizes the object flux at all wavelengths.

We then resort to external means to calibrate these spectra by tying the latter at two wavelengths to the photometry obtained for the respective galaxy with HST/ACS in the F606W and F814W passbands. To this end, we convolve the HST images in these two filters with a Gaussian whose full width at half maximum is matched to the average seeing during our DEIMOS observations and then integrate the flux within the DEIMOS slit (Fig. 2). The re- sulting linear calibration, illustrated in Fig. 3, achieves two goals: it

Figure2.Example of the procedure applied to obtain accurate absolute photometry for the flux entering a slit on our MOS masks. Left: HST/ACS image of an RPS candidate in the F606W filter; overlaid are isophotal flux contours (green) and the slit as positioned during the DEIMOS observation. Right: As left, but convolved with a Gaussian kernel that mimics the seeing of the groundbased observation and rotated to align the slit with the image axes.












wavelength (A)

Figure 3.DEIMOS spectrum of one of our RPS candidates before and af- ter flux calibration and slit size correction. The green and red lines show the throughput (in arbitrary units) of the ACS/F606W and F814W filters, respectively, used to anchor the flux calibration.

(1) crudely corrects for wavelength-dependent variations in the to- tal throughput of our observational setup; and (2) extrapolates the spectrum actually observed through the slit to the spectrum of the entire galaxy. Note that the validity of the latter correction rests on the implicit assumption that the spectrum recorded within the slit is representative of that of the galaxy as a whole. Although this as- sumption is not necessarily well justified, it is widely applied and ensures consistency and comparability between line fluxes (and de- rived properties like star-formation rates) obtained in studies using different instrumental setups and observational strategies.

    1. Stellar mass

In order to establish the locus of our RPS candidates within the general population of star-forming galaxies, we need to ensure that comparisons are made only between galaxies of comparable stellar mass. While the stellar mass of galaxies in our sample cannot re- liably be determined from only the HST/ACS data in the F606W and F814W used for their original selection by M16, or from the optical spectroscopy within the ∼5000 −9000AËš range described in Section 2, photometry across a wider spectral range that extends into the near-infrared (NIR) regime is well suited to constrain the spectral-energy distribution (SED) of galaxies and thus their stel- lar masses. For a significant fraction (QUANTIFY) of our cluster fields, the required data are available thanks to imaging observa- tions of MACS clusters with the NIR channel of HST's Wide-Field Camera 3 (WFC3) performed for the CLASH project (Postman et al. 2012) and the MACS SNAPshot programs GO-12188 and -12884 (PI: Ebeling) described in Repp & Ebeling (in preparation). The resulting photometry in the XXX passbands (CLASH) for 15 of our RPS candidates, and in the F606W, F814W, F110W, and F140W filters (SNAPshot programs) for an additional 17 galaxies, is fit with synthetic spectral templates using LePhare (Arnouts et al. 1999; Ilbert et al. 2006), an SED modeling code developed pri- marily for the determination of photometric redshifts of galaxies in the COSMOS field.

  1. Emission-line fluxes and star-formation rates

3.2.1  Extinctioncorrection

    1. BPT diagram
    2. RPS candidates and the galaxy main sequence
    3. Properties of the host clusters

We thank Elke Roediger for helpful discussions on the latest in nu- merical simulations of ram-pressure stripping and how to further constrain them via imaging and spectroscopic observations. Most of the data presented herein were obtained at the W. M. Keck Ob- servatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The observa- tory was made possible by the generous finical support of the W.



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