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Intra-Cluster Light at the Frontier: Abell 2744

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Intra-Cluster Light at the Frontier: Abell 2744 Draft version August 19, 2018 Preprint typeset using LATEX style emulateapj v. 5/2/11 INTRA-CLUSTER LIGHT AT THE FRONTIER: ABELL 2744 Mireia Montes1,2 and Ignacio Trujillo1,2 1Instituto de Astrof´ısica de Canarias,c/ V´ıa L´actea s/n, E38205 - La Laguna, Tenerife, Spain and 2Departamento de Astrof´ısica, Universidad de La Laguna, E38205 La Laguna, Tenerife, Spain Draft version August 19, 2018 ABSTRACT The ultra-deep multiwavelength HST Frontier Fields coverage of the Abell Cluster 2744 is used to derive the stellar population properties of its intra-cluster light (ICL). The restframe colors of the ICL of this intermediate redshift (z =0.3064) massive cluster are bluer (g − r =0.68 ± 0.04; i − J = 0.56±0.01) than those found in the stellar populations of its main galaxy members (g−r =0.83±0.01; i−J =0.75±0.01). Based on these colors, we derive the following mean metallicity Z =0.018±0.007 for the ICL. The ICL age is 6 ± 3 Gyr younger than the average age of the most massive galaxies of the cluster. The fraction of stellar mass in the ICL component comprises at least 6% of the total stellar mass of the galaxy cluster. Our data is consistent with a scenario where the bulk of the ICL of Abell 2744 has been formed relatively recently (z < 1). The stellar population properties of the ICL suggest that this diffuse component is mainly the result of the disruption of infalling galaxies with 10 similar characteristics in mass (M⋆ ∼ 3 × 10 M⊙) and metallicity than our own Milky Way. The amount of ICL mass in the central part of the cluster (< 400 kpc) is equivalent to the disruption of 4 − 6 Milky Way-type galaxies. Subject headings: galaxies: clusters: individual (Abell 2744) — galaxies: evolution — galaxies: pho- tometry — galaxies: halos 1. INTRODUCTION that the bulk of the ICL is produced by the most mas- ∼ 10−11 A substantial fraction of stars in clusters are not grav- sive (M⋆ 10 M⊙) satellites as they fall into the itationally bound to any particular galaxy. These stars cluster core (e.g. Murante et al. 2007; Purcell et al. 2007; constitute the so-called intra-cluster light. The ICL Martel et al. 2012; Contini et al. 2014). If this scenario is distributed around the central galaxy of the clus- is correct, the ages and metallicities of the intracluster ter and extends to several hundred kpc away from the population will be highly dependent on the progenitor cluster center (e.g. Murante et al. 2004; Zibetti et al. galaxies from which they were stripped. Therefore, the 2005). This diffuse light is thought to form primarily ICL is expected to have a metallicity that is similar to by the tidal stripping of stars from galaxies which inter- that of these massive satellites. The goal of this paper is act and merge during the hierarchical accretion history to explore this question in detail and characterize, for the of the cluster (e.g. Gregg & West. 1998; Rudick et al. first time, the age and metallicity of the ICL of a massive 2006; Conroy et al. 2007; Contini et al. 2014). There- cluster at radial distances R> 50 kpc with unprecedented fore, the characterization of the ICL provides a direct accuracy. way of determining the assembly mechanisms occurring We achieve our goal taking advantage of the deepest inside galaxy clusters. In this sense, the ICL is the signa- optical and near-infrared images ever taken by the Hub- ble Space Telescope (HST ) of the Abell Cluster 2744. ture of how violent the assembly of the cluster has been 1 through its cosmic history. For that reason, it is abso- The Hubble Frontier Fields (HFF) project represents lutely key determining how and when the ICL formed. the largest investment of HST time for deep observa- However, the identification of this light observationally tions of galaxy clusters. Abell 2744 is the first cluster arXiv:1405.2070v3 [astro-ph.CO] 26 Aug 2014 remains difficult and uncertain. Indeed, the typical sur- observed as part of the HFF program. This structure, 2 at z =0.3064 (Owers et al. 2011), is a rich cluster (virial face brightness of the ICL is µV & 26.5 mag/arcsec (e.g. 15 Mihos et al. 2005; Zibetti et al. 2005; Rudick et al. 2006) mass of ∼ 7 × 10 M⊙ within R < 3.7 Mpc) undergo- and it is contaminated by foreground and background ing a major merger as evidenced by its complex internal galaxies. Moreover, it is difficult to dissociate between structure (Boschin et al. 2006). Therefore, this cluster the ICL and the brightest central galaxy surface bright- represents an excellent target for studying the forma- ness profile (e.g. Gonzalez et al. 2005; Krick et al. 2007). tion of the ICL. The ages and metallicities of the ICL Although the properties of the stellar populations of are studied in detail taking advantage of the inclusion the ICL provide a vital tool to understand its forma- of very deep near-infrared (NIR) data to break the age- tion, little is known about their characteristics. The metallicity degeneracy (e.g. Anders et al. 2004). ICL is reported to be metal-poor in some studies (e.g. Throughout this work, we adopt a standard cosmo- Durrell et al. 2002; Williams et al. 2007) while other logical model with the following parameters: H0=70 km −1 −1 studies show super-solar metallicities (e.g. Krick et al. s Mpc , Ωm=0.3 and ΩΛ=0.7. At z = 0.3064, this 2006). Simulations predict that the ICL forms very late corresponds to a spatial scale of 4.52 kpc/arcsec. (z < 1; e.g. Murante et al. 2007; Contini et al. 2014) and email:[email protected] 1 http://www.stsci.edu/hst/campaigns/frontier-fields 2 M. Montes & I. Trujillo 2. DATA background regions are chosen to avoid the contamina- tion of the light of the cluster. With this aim we have The data used in this work are based on the first release 2 0.7 selected areas with µJ > 28 mag/arcsec , ρ < 10 of the HFF program and include all the NIR WFC3 data 2 (ID13495, PI: J. Lotz and ID13386, PI: S. Rodney) of M⊙/pc and R> 230 kpc (see Section 2.2). We have ex- Abell 2744. The ACS images were taken from the HST plored whether the light distribution in the background archive as part of the program: ID11689 (PI: R. Dupke) area follows a normal distribution. A Kolmogorov- and consist of six orbits in F435W and five orbits each in Smirnov test indicates that a gaussian distribution is F606W and F814W. NIR observations include imaging in compatible with the data (the null hypothesis is not re- four filters F105W, F125W, F140W, F160W based on 24, jected with any statistical significance). We do not find 12, 10, and 24 orbits, respectively. The data were directly any evidence supporting that our background distribu- retrieved from the archive2. The images were reduced tions present gradients. by the HFF team following standard HST procedures To estimate the surface brightness limits, we calcu- × 2 both for the ACS and WFC3 data3. For both cameras, lated the r.m.s of the images on boxes of 3 3 arcsec × 2 flat fields are claimed to be accurate to better than 1% (equivalent to 10 10 kpc at the cluster redshift). The across the detector. A few of the HFF observations in surface brightness limits we provide correspond to 3σ ± the IR exhibit a time-variable sky background signal due detections. These limits are: 29.31 0.02 (F435W), ± ± ± to time variable atmospheric emission. This exposures 29.60 0.10 (F606W), 29.13 0.07 (F814W), 30.32 0.08 ± ± were corrected from this variable emission and included (F105W), 29.97 0.09 (F125W), 30.01 0.05 (F140W) into the final mosaics (Koekemoer 2014, priv. comm.). and 30.05 ± 0.03 (F160W) mag/arcsec2. Although the The mosaics we used consist on drizzled science images HFF data is sky substracted, in order to have a bet- with pixel size 0′′. 06. These mosaics were build using the ter sky determination, we have used the above regions package Astrodrizzle4. In the case of the WFC3, this to reestimate the background of each of the images and pixel size is closer to one half of an original pixel. While subtracted (added) it to the whole image. working with subpixel dithered data has several advan- Finally, for the analysis of the ICL, it is crucial to iden- tages as helping to the comic ray rejection step, improv- tify the galaxies that are members of the cluster and get ing the resolution and lowering the correlated noise, it rid of background and foreground sources. For this pur- also reduces the sensitivity to low surface brightness fea- pose, we build a redshift mask of the cluster image. The tures. This last problem, however, can be avoided by mask was constructed using the spectroscopic redshift combining several pixels later on as we have done in our catalog of Owers et al. (2011) and adding all the photo- work (see next section). metric redshifts available in NED5. To assign a redshift The 0.4′′diameter aperture depth at 5σ of each im- value to the pixels of the images, first we run sextrac- age is: 27.4 (F435W), 28.0 (F606W), 27.1 (F814W), tor in the F160W image to identify the area that cor- 28.6 (F105W), 28.5 (F125W), 28.7 (F140W) and 28.2 respond to each object, using a detection threshold at a (F160W) mag (Laporte et al.
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