2.4 Galaxy Group sample
2.4.1 X-ray analysis and group sample selection
As explained in details in Section 2.1, all the blank fields considered in our analysis are observed extensively in the X-ray with ChandraandXM M−N ewton. Taking advantage of the better spatial resolution of Chandra, we use the point source catalogs derived from the Chandra maps of each field to remove point sources in the XM M −N ewton image at lower spatial resolution. This is done by convolving the Chandra PSF with the
Figure 2.11: Comparison between SF RIR and SF RSED for the ECDFS and GOODS-
S. The upper left panel shows all the sources with IR detection in the field. The same distribution is represented as gray dots in the following panels, where we show all the sources for different redshift bin with blue dots : 0<z≤0.5, 0.5<z≤1, and 1<z≤1.6, respectively from left to right and top to bottom. In all panels the dashed line represents the one to one relation.
XM M −N ewton PSF as explained in Finoguenov et al. (2009). The “residual” image,
free of point sources, is then used to identify extended emission. Groups and clusters are selected as extended emission with at least 4σ significance with respect to the background (see Finoguenov et al. (2009) for further detail on the precise definition of background and, thus, detection significance level). Finoguenov et al. (2009) and Finoguenov et al. (in prep.) assigned a redshift to each systems on the basis of spectroscopic redshift, when available, or photometric redshift. The X-ray luminosity LXis estimated within R2002 after
2R∆(where ∆ = 500,200) is the radius at which the density of a cluster is equal to ∆ times the critical
density of the universe (ρc) and M∆is defined as M∆= (4π/3)∆ρcR3
Figure 2.12: Comparison betweenSF RIRandSF RU V for the ECDFS. The upper left panel
shows all the sources with IR detection and for which we have data coverage at 1500 ˚A rest-frame. As in Fig. 2.11, the same distribution of the upper left panel is represented as gray dots in the following panels, which represent all the sources for different redshift bin with blue dots: 0<z≤0.5, 0.5<z≤1, and 1<z≤1.6, respectively from left to right and top to bottom. In all panels the dashed line represents the one to one relation. taking into account the possible missed flux through the use of the beta-model. The X-ray masses M200, within R200, are estimated based on the measured LX and its errors, using
the scaling relation of Leauthaud et al. (2010) through the equation:
hM200E(z)i M0 =A hLXE(z)−1i LX,0 α (2.4) where E(z) ≡ pΩm(1 +z)3+ Ωλ is the Hubble parameter evolution for a flat metric,
M0 = 1013.7 h−721 M and LX,0 = 1042.7 h−722 erg s
−1. The intrinsic scatter in this relation is
Figure 2.13: Histograms of SF RSED −SF RIR (on the left) and SF RU V −SF RIR (on
the right) residuals. The different colors correspond to the same redshift bins used for Fig. 2.11. All the histograms peak around 0, meaning that our approximation is robust. We measure a scatter of 0.73 dex (0.76 dex) for the whole range of redshifts, 0.74 dex (0.99 dex) for 0<z≤0.5, 0.63 dex (0.79 dex) for 0.5<z≤1, and 0.68 dex (0.73 dex) for 1<z≤1.6 for theSF RSED−SF RIR (SF RU V −SF RIR) relation.
with the measurement of LX. We use, then, the L-T relation to compute the temperature,
which we used for the computation of the k-correction.
The X-ray group catalogs derived with this approach comprise 277 detections in the COSMOS field and 50 detections in the ECDFS (Fig. 2.14). However, in order to study the galaxy population belonging to each X-ray group, we need to locate precisely in redshift each detection and to identify a reasonable number of spectroscopic members (at least 10 as shown in Biviano et al. 2006) to derive velocity dispersion and possibly the dynamical mass. This process was done in Popesso et al. (2012) for the 277 COSMOS groups and in Wilman et al (in prep.) for the ECDFS X-ray groups. In both cases, we classify as insecure X-ray groups, those showing more than one peak in the spectroscopic redshift distribution along the line of sight and within 3 times R200 from the X-ray group center and those with
a clear presence of a close companion. Indeed, in the former case the redshift associated is doubtful, and in the latter case a close companion can strongly bias the estimate of the velocity dispersion and membership. In addition, we require to identify at least 10 members. Our selection criteria lead to a final number of 28 groups in the COSMOS field and 22 in the ECDFS (see Popesso et al. 2012 and Wilman et al. in prep. for further details). We also impose a velocity dispersion cut at σ < 1200 km/s to define a clear group catalog and to avoid contamination by massive clusters, whose galaxy population could follow a different evolutionary path, as shown in Popesso et al. (2012) This velocity dispersion cut exclude only two systems of the COSMOS group sample. We add also other 3 structures: 2 in the GOODS-N field and one in the GOODS-S field. Of the GOODS- N systems, one is X-ray detected at z = 1.02 (Elbaz et al. 2007), and one system at
Figure 2.14: X-ray detected groups in ECDFS after a wavelet+PSF reconstruction. The units of the color bar are in count s−1 pxl−1.
z = 0.85 lies close to the Chandra CCD chip gap and it is not X-ray detected (Bauer et al. 2008). These two systems are necessary to populate thez ∼1 redshift bin as shown later. We consider also a “super-group” or large scale structure spectroscopically confirmed at z ∼1.6 by Kurk et al. (2009). We devote the next section to a detailed description of this structure.
Superstructure at z ∼1.6
The structure atz ∼1.6 was first identified by Kurk et al. (2009) as a significant overdensity of galaxies at that redshift in the GOODS-S field. Popesso et al. (2012) identify 76 members dynamically related to the system using all secure spectroscopic redshift in GOODS-S and GMASS. The structure shows a clear overdensity of galaxies within R200 = 0.51 Mpc
Figure 2.15: Signal-to-Noise map in a logarithmic scale of the region of the structure studied in Kurk et al. (2009). The color bar indicates the significance inσ units and black contours highlight the 4σ level. We plot the position of the member galaxies with small red circles while the dashed cyan circle shows the position of the super-group with its radius representing R200 of the structure. The white circles identify 6 groups (Finoguenov et al.
in prep.) which could be associated to the LSS of the structure studied in Kurk et al. (2009). Their radii represent R200 of each group and are centered on the density peaks of
Kurk et al. (2009).
estimated via optical analysis, and an elongation in the galaxy density distribution towards the South, consistent with the findings of Kurk et al. (2009) based on GMASS redshift only. Kurk et al. (2009) identify also a clear red sequence of galaxies with absorption line spectra. So far only an upper limit on the X-ray emission of this structure was available. Indeed, Trevese et al. (2010) measure an upper limit of ∼ 1043 erg/s based on the 4Ms observation of the CDFS (Bauer et al. 2008), consistent with a mass of ∼ 1014 M
. This
mass estimate is in agreement with the optical mass estimates of Kurk et al. (2009) and Popesso et al. (2012), who report a mass of 1014and 1.5×1014Mand a velocity dispersion
between 400-500 km/s, respectively. The missed secure X-ray detection of this structure suggests that it should be a non virialized structure in the process of formation.
Applying the same technique used for the COSMOS and ECDFS X-ray maps to the more recent CDFS 4 Ms observation, leads to find few X-ray emissions possibly associated to the superstructure at z ∼ 1.6 (Finoguenov et al. in prep.). Fig. 2.15 shows the X-ray signal-to-noise map (obtained from the CDFS map after removal of point sources) of the region covered by the structure. Finoguenov et al. (in prep.) identify 6 extended X-ray emission with a significance larger than 3σ. The area within a preliminary rough estimate of R200 of each group is shown by the white circles, while the dashed cyan circle represents
the R200 = 0.51 Mpc of the structure as studied in Popesso et al. (2012). The galaxies
associated to the structure according the analysis of Popesso et al. (2012) are shown by the red circles. There is consistency between the X-ray group candidate of Finoguenov et al. (in prep) and the galaxy density distribution. However, as shown in Fig. 2.15, the region of the structure is highly confused due to the presence of other three structures in foreground at lower redshift, that might contaminate and bias the estimate of the X-ray luminosity of these z = 1.6 group candidates.
All these results indicate that the z = 1.6 structure of the GOODS-S field is likely a “super-group” in phase of formation. Thus, it provides information about a stage of the group and cluster formation not studied before at such high redshift. Given the uncertain- ties of the X-ray detection in such confused region, we keep as structure parameters and membership the ones derived via the dynamical analysis of Popesso et al. (2012).