of the objects observed spectroscopically. 1R is a language and environment for statistical computing and graphics, designed by Robert. Gentleman and Ross ...
Asociaci´ on Argentina de Astronom´ıa BAAA, Vol. 53, 2010 J.J. Clari´ a, M.V. Alonso, A.E. Piatti & F.A. Bareilles, eds.
´ MURAL PRESENTACION
Witnessing the Formation of Galaxy Clusters: Photometric and Spectroscopic Evidence of Sub-Structures in RX J1124,3-1700 ([VMF98]102) at z∼ 0.41 J. L. Nilo Castell´ on1,2 , M. V. Alonso1,3 , H. Cuevas2 , E.R. Carrasco4 , D. 1,3 Garc´ıa Lambas & A. Ram´ırez2 (1) Instituto de Astronom´ıa Te´ orica y Experimental, (IATE-CONICET), Laprida 922, C´ ordoba, Argentina. (2) Departamento de F´ısica, Facultad de Ciencias, Universidad de La Serena, Cisternas 1200, La Serena, Chile. (3) Observatorio Astron´ omico de C´ ordoba (OAC-UNC), Laprida 854, C´ ordoba, Argentina. (4) Gemini Southern Operation Center, Casilla 603, La Serena, Chile. Abstract. We present a photometric and spectroscopic study of the poor cluster RX J1124,0-1700 ([VMF98]102) based on GMOS/Gemini South observations. The cluster shows a complex morphology, with a mean redshift of 0,409 and a velocity dispertion of 674,94 km/s. Using deep photometry in the r′ band, we find evidence of overdensities in the projected galaxy distribution, showing at least three sub-structures in the cluster neighborhoods. We obtained spectral data of two of them (North and South sub-structure), showing a separation of about 2000 km/s in the cluster rest of frame. These evidences suggest that the cluster [VMF98]102 is an example of a poor cluster in process of accreting substructures to become a rich cluster. Resumen. Presentamos un estudio fotom´etrico y espectrosc´ opico del c´ umulo pobre RX J1124,0-1700 ([VMF98]102) basados en observaciones realizadas en GMOS/Gemini South. El c´ umulos muestra una morfolog´ıa compleja, con un corrimiento al rojo promedio de 0,409 y una dispersi´ on de velocidades de 674,94 km/s. Usando fotometr´ıa profunda en la banda r′ , encontramos evidencias de sobredendidades en la distribuci´on proyectada de galax´ıas, mostrando al menos 3 subestructuras en las vecindades del c´ umulo. Obtuvimos informaci´ on espectrosc´ opica de 2 de ellos (subestructuras Norte y Sur) mostrando una separaci´on de 2.000 km/s aproximadamente. Estas evidencias sugieren que el c´ umulo [VMF98]102 ser´ıa un ejemplo de c´ umulo pobre en proceso de acreci´ on de subestructuras para convertirse en un c´ umulo rico.
1.
Introduction
The morphology content of galaxy clusters at intermediate redshifts differs significantly from those nearby clusters (Dressler et al. 1997). Analyzing clusters 83
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at higher redshifts and their connection to present day cluster properties is a subject of debate. Galaxy properties, as morphology, luminosity, mass, age, etc. and intracluster medium are expected to be strongly related to cluster assembly. Massive systems have been widely studied both in the nearby universe and at higher redshifts. Intermediate mass systems were not systematically nor deeply studied. They are very important to understand the galaxy evolution in the hierarquical model. Deep photometric and spectroscopic studies of these intermediate-mass systems at different redshifts would allow us to disentangle the different mechanisms involve in their evolution as ram pressure, galaxy interactions, etc. To try to understand the cluster assembly and evolution, we selected the intermediate-mass low X-ray galaxy cluster RX J1124,0-1700 ([VMF98]102) from the 160 Square Degree ROSAT Cluster Survey (Vikhlinin et al. 1998). This intermediate-mass system was selected to test the different formation mechanisms. As pointed out by Poggianti et al. (2009), these low mass systems, should show that morphological evolution does not occur exclusively in massive clusters, being actually more conspicuous in low mass clusters.
2.
The Data
The cluster was observed both, in photometry and spectroscopy, using the Gemini Multi-Object Spectrograph (Hook et al. 2004, hereafter GMOS) at the Gemini South telescope during the system verification process. The photometry was obtained only in the r′ band. We use SExtractor v2.5.0 (Bertin & Arnouts, 1996) to detect objects and to obtain relevant photometric parameters. We adopt the MAG AUTO as the object total magnitudes. Figure 1 shows the magnitud distribution for all the galaxies in the cluster field of view. The histogram shows that our limit magnitud reach to r′ =26,5 mag. We selected objects with r′ < 23 mag (Mr′ = -18,16) to be observed with GMOS in the MOS mode. We used one mask to observe 38 objects of which 16 were confirmed as galaxy cluster members. Our procedure to measure radial velocities consist in inspect the spectra to search for obvious absortion and/or emission features. Then, two IRAF routines were used: RVIDLINE and FXCOR both, within the RV package. RVIDLINE was used in galaxies with clear emission lines, while FXCOR was applied in earlytype galaxies. Right pannel of Figure 2 shows clearly foreground and background structures while left pannel shows the radial velocity distribution of the cluster. Using the 3 sigma-clipping iteration we identified redshift positions of the cluster members. Therefore, the mean redshift of the cluster is 0,409, with a velocity dispertion of 674,94 km/s. There is a background cluster identified in the velocity space (7 galaxies) at 0,440 with a velocity dispertion of 673,68 km/s, but is not considered in this analysis (Figure 2 right).
Sub-structures in Cluster RX J1124,3-1700 3.
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Galaxy projected Distribution Map
Figure 3 shows the r′ band image in the neighborhoods of the studied cluster (left pannel) and the galaxy projected distribution (right pannel) generated with the lattice package in the R software environment1 . Black contours represent objects within the magnitude limits while red contours the spectroscopic sample. Three overdensities are clearly identified: the North sub-structure located at 1 arcmin of the center image; the South one at 2 arcmin of the center and the third one, 2,5 arcmin in the south-west. We have spectroscopic data for the North and South sub-structures. The first one seems to be composed of galaxies with lower redshifts (z∼0,406) than the South sub-structure (z∼0,413). The difference in radial velocities of the two sub-structures is about 2100 [km/s].
4.
Final Comments
The main goal of this project is to determine the spatial and velocity structures in the cluster and eventually determine the mechanisms that contribute to the morphological galaxy evolution in these clusters. This work is part of an ongoing project to study intermediate-mass low X-ray galaxy clusters. In this work we present a preliminary photometric and spectroscopic analysis of the low X-ray luminosity galaxy cluster [VMF98]102. We find at least two sub-structures in the cluster velocity space with mean redshifts of 0,406 and 0,413. These evidences suggest that [VMF98]102 is a cluster in process of accreting sub-structures to become a rich cluster.
Figura 1. Total magnitude distribution in the r′ band. The long-dashed (short-dashed) line indicates the limiting magnitude of the photometric (spectroscopic) sample. Sticks on the upper part of the box represents r′ magnitude of the objects observed spectroscopically.
1
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Figura 2. Left: Redshift distribution of the observed spectra. The shaded histograms indicate the cluster location in the redshift space. Right: Radial Velocity distribution. Sticks on the upper part of the box indicates individual velocities for those objects observed spectroscopically.
Figura 3. Left: 5′ × 5′ arcmin2 of the r′ band image of the cluster [VMF98]102. Right: galaxy projected distribution in the cluster neighborhoods.
Referencias Bertin, E. & Arnouts, S., 1996, A&AS, 117, 393 Dressler, A. et al., 1997, MNRAS, 283, 1388 Poggianti, B. M. et al.,2009,ApJ,697,137 Hook, I. M. et al., 2004,PASP,116,425 Vikhlinin A., et al., 1998, ApJ,502,558
