analysis we do here. We know that Balmer emission lines can be severely aected by the absorption of the underlying stellar component (e.g., Martín-Manjón et al.
2008,2010) and this would aect the derived values of the reddening coecient, the line uxes and indices and, as a consequence, the SFH and gas-phase metallicity.
This eect is expected to be negligible after the correction, allowing a reliable study of the galaxies' gas chemical abundances.
4.2 The large-scale structure of the Hercules Superclus- ter
In this work we consider the HSC region, comprising 22.5 deg2as dened byBarmby
& Huchra (1998): 239.0<R.A.<243.6 deg, 14.35<DEC.<19.34 deg, in a redshift range 0.0283 < z <0.0483. In this region, we nd 1379 galaxies with SDSS-DR8 spectroscopic data. In Figure 4.1 we give the 2D position map of all the galaxies in the eld, color coded to their radial velocities. The overposed squares indicate regions of 1.6×1.6 deg2 around the center of each cluster in this region: A2151, A2152, and A2147. Based on the cluster velocity dispersions derived byBarmby &
Huchra(1998) (705, 715, 821 km s−1, for A2151, A2152, and A2147 respectively), these square regions would correspond to a region ∼ 4R200 across (from Equation 2.1), around each cluster center.
In Figure 4.1 we see that, in projection, the4R200 square regions of A2151 and A2147 are separated, while the region of A2152 overlaps with the other two. In the 3D space, however, A2152 is located in the background, as indicated by the higher value of its mean redshift, illustrated by the red/orange points that dominate in this region in Figure4.1. Previous works (Barmby & Huchra 1998;Tarenghi et al. 1980) have pointed out the diculty to unequivocally assign the galaxies to each of the three clusters. However, our large sample of galaxies with redshift (1379) put us in an optimal position to perform this classication.
For this purpose we perform a 3D consideration, taking into account the distri- bution of galaxies in the position and velocity space. Initially we consider galaxies within the square regions indicated in Figure4.1, forming three groups. In Figure 4.2 we plot the velocity distributions of the galaxies of each group, corresponding to each one of the three regions. The velocity distribution of a separate dynamical entity is composed of a single Gaussian, and we see that a well behaved velocity distribution appears around the mean cluster velocity for each cluster region. Par- ticularly, in the region of A2152, two separated distributions appear, indicating that two dynamically separated systems are projected on the sky.
We perform Gaussian ts to the velocity distributions of the three regions and we assign to each cluster the galaxies that have a radial velocity withinvclus±3σv, where vclus is the mean cluster velocity and σv is the dispersion for each cluster given by the ts. The radial velocity range considered for each cluster is indicated by crosses on the distributions of Figure4.2. Under these constraints, no galaxy is
102 Chapter 4. Star-formation and chemical history in the large-scale structure of the Hercules Supercluster
Figure 4.1: 2D position map of all the galaxies in the Hercules supercluster eld, color coded to their radial velocities. Square zones indicate regions ∼4R200 across around the center of each cluster: A2151 (blue), A2152 (yellow), A2147 (red).
assigned to two clusters simultaneously. The majority of the galaxies that form the left peak of the velocity distribution in zone of A2152 (overploted in the histogram in red), have been assigned either to A2147 or to A2151. The galaxies of the HSC that are not assigned in neither of the three clusters are labelled DISP standing for dispersed component.
In Table4.1 we give the name of the cluster (column 1), the coordinates of the cluster center (columns 2, 3), the scale at the distance of each cluster (column 4), the mean radial velocity (column 5) and velocity dispersion (column 6) derived in this work, as given by the Gaussian t. Column 7 and 8 give the R200 and cluster mass derived using Equations 2.1 and 2.2. In column 9 we give the total X-ray luminosities taken by Ledlow et al.(2003).
We note that the velocity dispersion we derive for A2152 is signicantly dierent
4.2. The large-scale structure of the Hercules Supercluster 103
Table4.1:PropertiesoftheclustersintheHSC ClusterR.A.DEC.ScalevclusσvR200MclLX J2000.0J2000.0Mpcdeg−1 kms−1 kms−1 Mpc(M¯)(1043 ergs−1 ) (1)(2)(3)(4)(5)(6)(7)(8)(9) A2151160515.01744552.76109728011.376.1×1014 1.60±0.10 A2152160522.41626553.08132553590.615.5×1013 0.28±0.07 A2147160217.21553432.65107416281.072.9×1014 3.00±0.12 Note.Column1:clustername;column2:rightascensionoftheclustercenterasgiveninNED(Abelletal.1989)inhours, minutes,seconds;column3:declinationoftheclustercenterasgiveninNED(Abelletal.1989)indegrees,minutes,seconds; column4:clusterscaleasgiveninNED;column5:meanclustervelocityvclusderivedbytheGaussiant;column6:σvderivedby theGaussiant;column7:R200;column8:clustermass;column8:ROSATtotalX-rayluminosityinunits1043 ergs−1 (Ledlow etal.2003).
104 Chapter 4. Star-formation and chemical history in the large-scale structure of the Hercules Supercluster
Figure 4.2: Velocity distributions for the galaxies located in the three regions indi- cated in Figure 4.1. In each region a well behaved Gaussian velocity distribution appears around the mean radial velocity for each cluster. We perform Gaussian ts to dene the mean cluster velocity and velocity dispersion. In the region of A2152, the left part of the distribution is principally due to galaxies that are assigned to A2147 or A2151 (overploted in red). We assign to each cluster the galaxies with radial velocity within vclus±3σv, where vclus is the mean cluster velocity and σv is the dispersion for each cluster given by the ts. The crosses indicate the range vclus±3σv.
from the values derived previously (see e.g. Barmby & Huchra 1998); however, the sample of galaxies used by these authors was very limited, and possibly, in their considerations, A2152 was biased by galaxies belonging to A2151 and A2147. The value derived here implies that A2152 is a cluster of much lower mass, in agreement with the low level of X-ray emission of this cluster.