RPS in Galaxy Clusters Analysis

RPS in Galaxy Clusters Analysis jellyfishA spectroscopicalal rentofram-pressurestrippinginmassivegalax dots*ABSTRACTWe continue our exploration of ram-pressure stripping (RPS) in massive galax practice bundlings at z0.3 by assessing the spectroscopic properties of RPS candidates granted previously base on their morphological appearance in Hubble Space Telescope flicks. We confirm b either(a) membership for 55 of our candidates, thereby tripling the number of RPS candidates known at z0.2. Although galore(postnominal) of these systems argon withal faint and too distant for the kind of in-depth investigation required to unambiguously confirm or refute the presence of RPS, the t by ensemble properties of our ingest be consistent with increased star formation, and many of the selected galaxies let on visible debris trails. Specific completelyy, ab unwrap two thirds of all galaxies exhibit breeze emission (OII3727A , H, and, where data-basedly accessible, H) consistent with ro - bust star-formation rates that epoch-makingly exceed those expected for systems on the extragalactic nebula 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 army clustering.Although we caution that our sample may contain not yet galaxies undergoing RPS by the diffuse intra-cluster middling (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 earthy process models of the stripping process by providing the first statistically significant sample of RPS candidates in truly massive clusters. magic spell extremely fast removal of the intrastellar medium is not ruled out by our findings, all-inclusive periods of triggered star formation are clearly an integral component of the natural philosophy of ICM-galaxy interaction in massive clusters.INTRODUCTIONSpiral and elliptical galaxies are some(prenominal) comm sole(prenominal) detect in the universe but survive (and dominate) very antithetic environments. The inverse correlation between spiral ingredient 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 prevalence of red, elliptical galaxies is not limited to the densest environments (i.e., the cores of massive galaxy clusters) but is famous 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 lap up for several Gyr, as evinced by the steady increase in the dictum of ellipticals inclusters from z1.5 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 partnershi p among the California pioneer of Technology, the University of California, and the National Aero- nautics and Space Administration. The observatory was do possible by the handsome finical support of the W.M. Keck Foundation.Although simulations have indicated that elliptical galaxies can form without delay 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 lately to early types are central to the creation of the Hubble sequence. These occur in a wave of environments and, most in all likelihood, over a range of character- istic timescales. While slow-acting procrastinating effects such as galaxy harassment (Moore et al. 1996, 1998) are give to contribute, more violent interactions have been shown to be highly effective in turning disk galaxies into spheroids. In modestly dense environ- ments with satisfactoryly modest sexual congress 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 fabricate 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 go into galaxy clusters (e.g., sporty et al. 1991 Rangarajan et al. 1995 Veilleux et a l. 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 healthful 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 z0.2, however, that the the great unwashed 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 chuck up the sponge us to study RPS over the full range of environ- ments, from the only softly overdense cluster outskirts to extr eme 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 z0.3. all(a) clusters considered for this work were iden- tified by their X-ray emission and ocularly 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) on base the Hubble Space Telescope (HST) (see Repp Ebeling, in preparation, for an overview of this data do). 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 use to compile a larger sample of 223 potential RPS candidates and examined the spatial distribution and apparent projected armorial bearing of motion of the most plausible candidates. In this third paper, we present, dis- cuss, and interpret the results of prolonged 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 spectroscopicfollow-up observations of RPS candidates, the data reduction, as well as our criteria to assess cluster membership for any givengalaxy. In 3 we derive entire spectral properties of the con-firmed cluster members, infer star-formation rates, and estimate their stellar mass. 4 compares the properties of RPS candidateswith those of the cosmopolitan population of star-forming galaxies, dis- cusses physical triggering mechanisms, and investigates correla- tions between the star-formation rate of RPS candi dates and the relaxation state of the host cluster. We summarise our findings in5.Throughout this paper we adopt the concordance CDM cos-mology, characterised by m= 0.3, = 0.7, and H0 = 70 km s1 Mpc1. paradigm1.HST/ACS snapshot image of MACSJ0451-JFG1, a textbook case of ram-pressure stripping from the E14 sample. The red and yellow arrows musical score the inferred direction of motion in the plane of the pitch and the di- rection to the cluster centre, respectively. tone 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)SPECTROSCOPIC OBSERVATIONS AND DATA REDUCTIONThe 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 apply to select these can- didates from a master catalogue of over 15,000 galax ies detected in unmindful HST/ACS exposures in the F606W and F814W bands of 63 MACS clusters in the redshift range of 0.30.7. 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 purchase order to max- imise scientific returns, clusters that gas 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 focal point on many targets made it impossible to optimise the orientation of individual(a) snatchs or even the position angle of the entire mask for the study of RPS candidates.Keck/DEIMOS observations exclusively spectroscopic data for this work were obtai ned 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 peckers of at least 8//length, i.e., long enough to allow sky subtraction from in- whoreson data. Spectra were obtained utilize the 600 l/mm Zerodur grating set to a central wavelength of 6300A the GG455 blocking filter was employed to prevent second-order contamination at 9000A . Exposure timesranged from 3-600 to 3-1200 seconds. The seeing during theseobservations was typically 0.8//. All data were reduced with theDEIMOS DEEP2 melodic phrase (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.Cluster membershipWe establish (likely) cluster membership by canvass the differ- ence between an RPS candidates 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 all-encompassing spectroscopic follow-up work per- formed by the MACS team a description of the underlying data and of the procedure employed to determine half-hardy 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 deep down 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 l ikely cluster members based on their radial velocity within the comoving cluster rest frame.Spectral corrections and immingle calibrationThe 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 legal instrumental setup and the pre- cision of the dispersion solution. The determination of line coursees and, in particular, line-flux ratios across a significant wavelength range, however, require flux-calibrated spectra. In addition, flux helpless during the data-reduction process (due to CCD defects, non- optimal definition of spectral apertures, and, importantly, the finite lolly width) inescapably 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 embarrassing for multi-object spec- trographs (e specially 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 problematic 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 tie 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 twist 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 DEIMO S slit (Fig. 2). The re- sulting linear calibration, illustrated in Fig. 3, achieves two goals itFigure2.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 ( honey oil) 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.80300025006020004015001000205000040005000600070008000900010000wavelength (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) ingenuously corrects for wavelength-dependent variations in the to- tal throughput of our observationa l 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 covert 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 batten downs conformity and comparability between line fluxes (and de- rived properties like star-formation rates) obtained in studies using different instrumental setups and observational strategies.PHYSICAL PROPERTIES OF RPS CANDIDATESStellar massIn 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 F606Wand F814W used for their original selection b y M16, or from the optical spectroscopy within the 5000 9000A range described in segment 2, photometry across a wider spectral range that extends into the near-infrared (NIR) regime is well meet 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 guide of HSTs 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 authentic pri- marily for the determin ation of photometric redshifts of galaxies in the COSMOS field.Emission-line fluxes and star-formation rates3.2.1 ExtinctioncorrectionDISCUSSIONBPT diagramRPS candidates and the galaxy main sequenceProperties of the host clustersCONCLUSIONS ACKNOWLEDGEMENTSWe 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. 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