arXiv:0806.0376v1 [astro-ph] 2 Jun 2008 RXJ1648.7+6109: Witnessing the Formation of a Massive Group/Poor Cluster and its Brightest Galaxy Tesla E. Jeltema1, John S. Mulchaey2, and Lori M. Lubin3 ABSTRACT Using deep Chandra and optical spectroscopic observations, we investigate an intriguing, young massive group, RXJ1648.7+6109, at z = 0.376, and we combine these observations with previous measurements to fit the scaling rela- tions of intermediate-redshift groups and poor clusters. RXJ1648 appears to be in an early stage of formation while it follows X-ray scaling relations, its X- ray emission is highly elongated and it lacks a central, dominant BCG. Instead, RXJ1648 contains a central string of seven bright galaxies, which have a smaller velocity dispersion, are on average brighter, and have less star formation (lower EW([OII]) and EW(H��)) than other group galaxies. The 4-5 brightest galaxies in this string should sink to the center and merge through dynamical friction by z = 0, forming a BCG consistent with a system of RXJ1648���s mass even if 5-50% of the light is lost to an intracluster light component (ICL). The LX ��� TX relation for intermediate-redshift groups/poor clusters is very similar to the low- redshift cluster relation and consistent with the low-redshift group relation. In contrast, the LX ��� ��v and ��v ��� TX relations reveal that intermediate-redshift groups/poor clusters have significantly lower velocity dispersions for their X-ray properties compared to low-redshift systems, however the intermediate-redshift relations are currently limited to a small range in luminosity. Subject headings: galaxies: clusters: general ��� X-rays: galaxies:clusters ��� galax- ies: clusters: individual (RXJ1648.7+6109) ��� galaxies: evolution 1Morrison Fellow, UCO/Lick Observatories, 1156 High St., Santa Cruz, CA 95064 tesla@ucolick.org 2The Observatories of the Carnegie Institution of Washington, 813 Santa Barbara St., Pasadena, CA 91101 3Department of Physics, University of California at Davis, One Shields Ave., Davis, CA 95616

��� 2 ��� 1. INTRODUCTION Groups and poor clusters of galaxies are the building blocks of larger scale structures, and they are particularly important environments for diagnosing the origin of the non- gravitational heating of the intracluster medium (ICM)(Balogh et al. 2006) and for galaxy evolution (Zabludoff & Mulchaey 1998). For example, at low redshift non-gravitational heating appears to be proportionally more important in the group regime, leading to a steepening of the LX ��� T relation for groups versus clusters and excess group entropy above self-similar expectations (e.g. Helsdon & Ponman 2000 Ponman et al. 2003). The evolution in group X-ray scaling relations with redshift can constrain models of non-gravitational heating (Balogh et al. 2006). In addition, most galaxies in the local universe lie in groups (e.g. Turner & Gott 1976), making them important environments in the study of galaxy evolution. We have only recently begun to study the evolution of this important environment with redshift (e.g. Mulchaey et al. 2006 Jeltema et al. 2006,2007 Willis et al. 2005 Pacaud et al. 2007 Wilman et al. 2005a,b Gerke et al. 2007) and very few groups/poor clusters at even moderate redshifts (z 0.2) have both significant X-ray and optical data (Mulchaey et al. 2006 Jeltema et al. 2006,2007 Willis et al. 2005 Gastaldello et al. 2007). Current X-ray observations at intermediate-redshifts (0.2 z 0.6) are limited to systems with kT ��� 1 keV or above, and therefore probe a transitional regime of galaxy associations, which we refer to as massive groups or poor clusters, between what are typically referred to as clusters and lower mass groups of galaxies. Our group has undertaken a program to study in depth X-ray luminous intermediate- redshift groups/poor clusters selected from the ROSAT Deep Cluster Survey (Rosati et al. 1998). Spectroscopic confirmation and HST imaging of a sample of nine groups was presented in Mulchaey et al. (2006 Paper I). Jeltema et al. (2006 Paper II) investigated the X-ray properties of six of these groups based on observations taken with XMM-Newton, and Jeltema et al. (2007 Paper III) presented deeper spectroscopic follow-up of seven groups with the Gemini-North and Keck telescopes. One of the most interesting results of this study was the discovery that unlike X-ray luminous groups at low redshift, most of these systems either lacked a dominant central galaxy or contained multi-component brightest galaxies indicative of a recent merger (Papers I and III). In other ways these intermediate- redshift groups are similar to low-redshift groups and clusters they have X-ray luminosities and temperatures consistent with the low-redshift LX ��� TX relation (Paper II), and they contain high fractions of elliptical galaxies and low fractions of star-forming galaxies (Paper III). These observations are consistent with a picture in which BCGs form/grow late with respect to the collapse of the system itself (De Lucia & Blaizot 2007 Dubinski 1998). In

��� 3 ��� fact, the groups in our sample appear to span a range of evolutionary states. Of the six groups with XMM-Newton observations, two lack a brightest galaxy at the X-ray center and have elongated X-ray emission, two have bright central galaxies composed of three or more luminous nuclei but round X-ray emission, and two have bright galaxies at the X-ray center with no galaxies of similar luminosity nearby. In this paper, we investigate in detail one particularly intriguing system in our sample, RXJ1648.7+6109 at z = 0.376. Based on previous observations, this massive group appears to be in an early state of evolution. Instead of a single, dominant galaxy it contains a string of seven galaxies within 200 kpc of the X-ray center, five of which are brighter than L��� (Paper III). RXJ1648���s X-ray emission appears to be elongated, but the XMM-Newton observation was almost completely contaminated by background flares leading to large uncertainties in its X-ray temperature and spatial distribution (Paper II). Here we report on a new ��� 100 ks Chandra observation of RXJ1648, which allows us to determine its average X-ray properties and spatial structure (��3.1). Combining these new X-ray measurements with the other groups in our sample and intermediate-redshift groups/poor clusters from the literature, we derive the LX ���TX, LX �����v, and ��v ���TX relations for poor clusters and groups at intermediate redshifts (��3.3). In addition, we investigate in greater detail the galaxy properties in RXJ1648, including the magnitude, star formation, and velocity distributions of the member galaxies (��3.2) and the future formation of a BCG through the mergers of the bright galaxies in the central string (��4.1). Throughout the paper, we assume a cosmology of H0 = 70h70 km s���1 Mpc���1, ���m = 0.27, and �� = 0.73. 2. DATA REDUCTION 2.1. X-ray Data RXJ1648.7+6109 was observed twice with ACIS-S in August 2007 for 49.7 ks (ObsID 7903) and 47.6 ks (ObsID 8472). The data were prepared using CIAO 3.4.1 and CALDB version 3.4.1 following the standard data processing. We chose to reprocess the data starting from the level 1 file, including re-detecting hot pixels and afterglow events using the latest tools, applying the newest gain file, and destreaking the S4 chip. We also applied the CTI and time-dependent gain corrections. We kept only events with ASCA grades of 0, 2, 3, 4, and 6 and a status of zero to filter out particle background, bad pixel, cosmic ray afterglow and other ���bad��� events1. Since the data were taken in VFAINT (VF) mode, the additional 1Chandra Proposers��� Observatory Guide http://cxc.harvard.edu/proposer/POG/, section ���ACIS���