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Groups of Galaxies at Intermediate Redshift

Galaxy groups are key tracers of galaxy evolution, cluster evolution, and structure formation, yet they are difficult to study at even moderate redshift. We have undertaken a project to observe a flux-limited sample of intermediate-redshift (0.1<z<0.5) group candidates identified by the XBootes Chandra survey. When complete, this project will nearly triple the current number of groups with measured temperatures in this redshift range. Here we present deep Suzaku/XIS and Chandra/ACIS follow-up observations of the first 10 targets in this project; all are confirmed sources of diffuse, thermal emission with derived temperatures and luminosities indicative of rich groups/poor clusters. By exploiting the multi-wavelength coverage of the XBootes/NOAO Deep Wide Field Survey (NDWFS) field, we aim to (1) constrain non-gravitational effects that alter the energetics of the intragroup medium, and (2) understand the physical connection between the X-ray and optical properties of groups. We discuss the properties of the current group sample in the context of observed cluster scaling relations and group and cluster evolution and outline the future plans for this project.


Motivation
Galaxy groups are vital to our understanding of structure formation, cluster evolution, and galaxy evolution. In the local universe, 50%-70% of all galaxies are found in groups (Tully 1987). Under the standard picture of hierarchical structure formation, groups merge to form clusters. Since group masses are relatively low, they are ideal sites in which to observe non-gravitational processes that affect the energetics of the plasma in both groups and clusters (e.g. Balogh 2006). Interactions among group galaxies, as well as interactions between individual galaxies and the group gravitational potential and intragroup medium (IGM), can alter galaxy properties substantially (Mulchaey 2000, Rasmussen et al. 2006).
Groups are difficult to study at even moderate redshifts (z > 0.1) because the galaxy overdensity is low and because X-ray luminosities are modest (L X ∼ 10 41 -10 43 erg s −1 ). The XBootes Chandra survey (Murray et al. 2005) provides a powerful opportunity for systematic study of more distant groups. The 9.3 deg 2 survey region is almost fully covered by deep optical and near-IR imaging from the NOAO Deep Wide-Field survey (NDWFS; Jannuzi & Dey 1999), by the Spitzer/IRAC Shallow Survey (Eisenhardt et al. 2004), and by optical spectroscopy of over 20,000 galaxies from the AGN and Galaxy Evo-lution Survey (AGES) and ongoing MMT observations.
We have undertaken a project to observe a flux-limited sample of intermediate-redshift (0.1 < z < 0.5) group candidates identified by the XBootes Chandra survey (Kenter et al. 2005). By exploiting the multi-wavelength coverage of the XBootes/NDWFS field, we aim to constrain non-gravitational effects that alter the energetics of the IGM, and to understand the connection between the X-ray and optical properties of groups. Here we present deep Suzaku/XIS and Chandra/ACIS follow-up observations of the first targets in this project.

Sample Selection
Of the 43 extended X-ray sources identified by the XBootes survey (see Figure 1), 27 exceed our flux threshold of 2×10 −14 erg s −1 cm −2 . For 17 of these sources, we have confirmed the presence of a bound group of galaxies with MMT spectroscopy. The brightest 13 targets of the total sample have been observed (10 groups) or awarded time (3 groups) with either Suzaku or Chandra, depending on the presence of contaminating X-ray point sources. These targets are listed in Table 1. We continue to obtain redshift information for the remaining sample, and additional groups will be proposed for observation in future cycles of Suzaku and Chandra. Due to the size and depth of the XBootes survey, this sample of groups fills a niche not covered by other surveys. In general, our groups are poorer/cooler than the clusters found in the larger but shallower ROSAT 400d survey (Burenin et al. 2007). They should be richer than those detected with XMM in the COSMOS survey (Finoguenov et al. 2007). When complete, this project will triple the current number of intermediate-redshift groups with measured temperatures.

Observations and Spectral Modeling
We detect extended soft X-ray emission from all 10 observed targets, with 0.5-2 keV fluxes ranging from 3-21 ×10 −14 erg s −1 cm −2 . For some sources this is nearly five times the measured XBootes snapshot flux (given in Table 1), although this is expected from the deeper exposures. Sample images from Suzaku and Chandra are shown in Figures 2 and 3, with the exposure-corrected X-ray image overlaid on a combined NDWFS B W ,I and Spitzer/IRAC 3.6 µm image. The Suzaku image in Figure 2 contains X-ray point sources; the Chandra image in Figure 3 has been smoothed after point source removal.
The Suzaku/XIS group spectra were extracted from circular apertures of 1 Mpc radius at the group redshift. Point sources brighter than 10 −14 erg s −1 cm −2 were masked out. Due to vignetting and non-uniform OBF contamination, the X-ray background was fit simultaneously to a region outside the group aperture. Detector background was corrected using the accumulated Suzaku night Earth background data. The Chandra/ACIS spectra were extracted from a similar aperture, with a nearby region used as the background. Point sources were masked by hand.  (Kenter et al. 2005). The 13 confirmed groups in our current sample are identified in white, the remaining 30 extended X-ray sources are circled in green. The image has been smoothed, so some bright point sources appear extended. A handful of fields suffered from high background, leading to the patchy appearance.
The diffuse group emission was modeled using the APEC plasma code with variable temperature and abundance. For all groups, the best-fit kT ranges from 0.7-2.5 keV with abundances of 0.1-0.7 solar. The range of temperatures is illustrated in Figure 4.  galactic/AGN feedback, radiative cooling). Evolution in scaling relations at group (rather than cluster) scales is a powerful diagnostic because these non-gravitational effects are more important at smaller mass scales. A small number of groups have been observed at intermediate redshift with XMM-Newton (Willis et al. 2005, Jeltema et al. 2006, and they show little if any evolution in the L X -T X scaling relation (see Figure 4). A number of other groups at z > 0.15 have measured T X (Bauer et al. 2002, Grant et al. 2004, Fassnacht et al. 2007, bringing the total to 16, excluding this work. The large intrinsic scatter in the L X -T X relation requires a large sample of groups to distinguish between various models. Our full sample will triple the number of groups with measured T X in this redshift range. The first 10 groups have properties consistent with the observed L X -T X relation at z ∼ 0 (see Figure 4). They lie in a region on the faint end of the cluster population and bright end of the typical group population, similar to the XMM-Newton group samples.
The XBootes group L X -T X relation shows a hint of flattening at the low kT end, however systematic effects prevent us from concluding this at the present. No spatial analysis has been completed to extrapolate the group X-ray fluxes out to r 500 . We expect the aperture correction to be less than 50%, and this will increase the L X systematically. On the other hand, the Suzaku data suffer from AGN contamination due to the poor spatial resolution. This correction will decrease L X for the six groups observed with Suzaku.

Future Work: Evolution of the Group X-Ray/Optical
Connection Most studies of the properties of groups beyond the local universe rely on X-ray selected samples, since finding the extended X-ray emission indicative of a virialized system is not difficult in surveys with sufficient depth. On cluster scales, it is clear that there are systematic differences in the X-ray properties depending on sample selection method (e.g. Donahue et al. 2001, Lubin et al. 2004, Barkhouse et al. 2006). On group scales (kT < 3 keV), only half of optically-selected groups at low redshift are seen to produce X-ray emission (Mulchaey 2000, Rasmussen et al. 2006). This could be due to incomplete gravitational collapse, a shallower potential (and lower X-ray temperature) than expected, or simply a lack of IGM gas, perhaps as a result of feedback from group galaxies to the IGM (Rasmussen et al. 2006). These differences likely result from the details of group formation and the interplay between galaxies and the IGM, but they have been very difficult to understand because we lack appropriate X-ray and optical data on representative group samples beyond the local universe.
We have begun an optical search for groups using AGES and additional MMT spectroscopic galaxy red- Fig. 4. L X -T X relation for our 10 groups, plotted with low-z group and cluster samples and a sample of 11 other intermediate-z groups with measured T X . The XBootes systems lie in a region of rich groups and poor clusters. No aperture correction has been performed on the XBootes group emission, therefore an additional systematic error of < 50% is possible in L X,bol .
shifts. With follow-up data of the full X-ray sample, we will directly compare the properties of optically-selected and X-ray-selected groups, and we will be able to directly compare groups at intermediate redshift to those at z ∼ 0. This will enhance larger surveys such as the 400d cluster survey (Burenin et al. 2007), which is not sensitive to groups of this mass in this redshift range. It will also complement the deeper COSMOS survey (Finoguenov et al. 2007), which is sensitive to lower mass groups but due to its smaller field detects few groups in this mass range. In addition to probing the X-ray properties of optically-selected groups, the multi-wavelength coverage of this Bootes field opens the door for additional studies. We will be able to constrain the group velocity dispersion and compare to X-ray mass estimators. With the existing deep, multi-band imaging, we will also investigate the role of environment (local galaxy density, early-type fraction, brightest group galaxy) in determining group X-ray properties (X-ray luminosity, gas temperature).

Acknowledgements
We thank the conference organizers for arranging an enjoyable and stimulating meeting in a fantastic lo-cale. EDM acknowledges support from NASA grant NNX08AZ64G.