Low mass galaxy clusters and galaxy morphology ...

Asociación Argentina de Astronomía BAAA, Vol. 54, 2011 J.J. Clariá, P. Benaglia, R. Barbá, A.E. Piatti & F.A. Bareilles, eds.

PRESENTACIÓN ORAL

Low mass galaxy clusters and galaxy morphology evolution J. L. Nilo Castellón1,2 ; Y. Órdenes2 , F. Ramos2 , M. V. Alonso1,3 , H. Cuevas2 , D. García Lambas1,3 & A. Ramírez2 (1) IATE-CONICET, Laprida 922, Córdoba, Argentina (2) Departamento de Física, Universidad de La Serena, Chile (3) OAC-UNC, Laprida 854, Córdoba, Argentina Abstract. We present preliminary results about the galaxy morphology evolution in three low mass galaxy clusters: RX J0533.9-5809 ([VMF98]046, z ∼ 0.198), RX J1204.3-0350 ([VMF98]113, z ∼ 0.261) and RX J0533.8-5746 ([VMF98]045, z ∼ 0.295). Full photometric catalogues were created using SExtractor v2.8.0. Also, photometric redshifts (zphot ) were obtained for all the object classified as galaxies, using the ANNz code. Color-Magnitude Diagrams (CMD) were generated for those galaxies classified as cluster members. Clear Red Cluster Sequences (RCS) with a median slopes of ∼ -0.03 are observed for all the tree clusters. Based on the RCS best fit, a blue and a red population of galaxies were defined, observing that the color distribution of the cluster [VMF98]045 is well fitted by a double Gaussian function (χ2 ∼0.2), while the clusters [VMF98]046 and [VMF98]113 presents a third population between the blue and red peak distributions. These preliminary results would show the existence of a possible transition population between the blue and the red population in these low mass galaxy clusters at low redshifts. Resumen. Presentamos resultados preliminares respecto a la evolución morfológica de galaxias en tres cúmulos de galaxias de baja masa: RX J0533.95809 ([VMF98]046, z∼0.198) y RX J1204.3-0350 ([VMF98]113, z∼0.261) y RX J0533.8-5746 ([VMF98]045, z∼0.295). Completos catálogos fotométricos fueron creados usando SExtractor v(2.8.0). También, se obtuvieron redshifts fotométricos para todos los objetos clasificados como galaxias, usando el código ANNz. Diagramas Color-Magnitud (CMD) fueron generados para aquellas galaxias clasificadas como miembros de los cúmulos. Claras Secuencias Rojas de galaxias (RCS), con una pendiente media de ∼ -0.03 son observadas para los tres cúmulos. Basados en el mejor ajuste de la RCS, poblaciones rojas y azules de galaxias fueron creadas, observando que la distribución de color del cúmulo [VMF98]045 es bien ajustada por una función gausiana doble (χ2 ∼0.2), mientras que los cúmulos [VMF98]046 y [VMF98]113 presentan una tercera población entre los picos de las distribuciones rojas y azules. Estos resultados preliminares, podrían mostrar la existencia de una posible po-

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blación de galaxias en transición entre las poblaciones azules a rojas en estos cúmulos de galaxias de baja masa, a bajo redshift. 1.

Introduction

The morphology content of galaxy clusters at intermediate redshifts differs significantly from those nearby clusters (Dressler et al. 1997). Galaxy properties, as morphology, luminosity, mass, age, have been widely studied both in the nearby universe and at higher redshifts, but mostly in massive systems. Low mass systems were not systematically nor deeply studied, and, as pointed out by Poggianti et al.(2009), they should show that morphological evolution does not occur exclusively in massive clusters, being actually more conspicuous in low mass clusters. In this proceeding, we study the color bimodality evolution of three low X-ray luminosity, low mass galaxy clusters at different redshifts. The results are part of an ongoing project, that study the galaxy morphology evolution in low-mass galaxy clusters. 2.

Data, images and catalogues

The clusters were selected from the catalogue of 200 objects serendipitously detected in the ROSAT PSPC Pointed Observations (Vikhlinin et al. 1998) and the revised version by Mullis et al.(2003) with spectroscopic redshifts. The catalogue contains galaxy clusters with X-ray luminosities ranging to very poor groups (1042 erg s−1 ) to poor clusters (∼ 5 × 1044 erg s−1 ) in the redshift regime of 0.015 to 0.7. We create a sub-sample composed by galaxy clusters with X-ray emission lower than LX ∼20×1043 erg s−1 , redshifts higher than 0.15 and 60 arcsec in angular core-radius. We select the clusters RX J0533.9-5809 ([VMF98]046, z∼0.198), RX J1204.30350 ([VMF98]113, z∼0.261) and RX J0533.8-5746 ([VMF98]045, z∼0.295) to be observed in the CTIO Victor Blanco 4m telescope at the Cerro Tololo Observatory. All the clusters were observed under good photometric conditions, with a median seeing of about 0.6 arcsec. We use the MOSAIC-II instrument using B, V, R and I filters, cover an optical range between ∼ 4000 to 8500 Å. We used SExtractor v2.8.0 (Bertin & Arnouts, 1996) to determinate the main astrometric and photometric properties of the objects detected in our images, using the R image as reference. We realize the Photometry in the other bands using SExtractor in dual-image mode. This allows us to eliminate bad detections, spurious objects and high redshift objects detected in the R band images, and minimize the errors in the final catalogues. 3.

Color-Magnitud diagram and sample definition

We generate Colour-Magnitude Diagrams (CMD) for all the objects classified as galaxies. We use (V-R) color in order to observe the 4000Å break at the clusters redshift. A clear Red Cluster Sequence (RCS) can be observed for all the three clusters (Figure 1). As Gladders et al (1998) pointed out, the RCS can be parameterized using a simple linear fit. We perform several restrictions to define the RCS sample, as distance

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to the cluster center, galaxy morphological type, and magnitudes. Finnaly, we use the ANNz code to obtain photometric redshifts and define real cluster members. Using a least-square regression fit and a 1-σ clipping iteration, we obtain median slopes of -0.03 for the RCS. We define those galaxies with colours over and below 1σ the linear fit as the Red Sample (RS), and those 1σ below the RCS fit, as the Blue Sample (BS).

Figure 1. (V-R) Color-Magnitude Diagrams por all the three clusters. Open-red circles correspond to the called Red Galaxy Sample. Open-Blue triangles indicate the Blue Sample. Dashed lines indicate the best fit (section 3). Continues lines shows 1-σ over and bellow the best RCS fit. Right histogram indicate the color distribution for both, the Red and the Blue Sample. Bottom histogram the magnitude distribution for these samples.

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Analysis and final remarks

We use the R software environment1 to analyze the color bimodality distribution. We separate the RS and the BS in the 3 clusters and fit them Gaussian functions. To determinate if the distribution is well defined by this fit, we use 4 main statistical values: Skewness, Kurtosis, χ2 and the P-Value (P)2 . A perfect Gaussian fit has Skewness and Kurtosis values of 0, a χ2 lower than 1, and high P-Value. Finally, Quantile-Quantile Plots (Q-Q plot) are used to check whether or not the sample is well fited by a Gaussian distribution. The RS in the three clusters shows low χ2 (∼0.2) an a high P values (∼0.9). Also, low skewness and kurtosis values, and good lineal fit in the Q-Q plots (Figure 2), indicate that the Red Sample for the three clusters is well fitted by a normal distribution. Different scenario show the Blue Sample. While in clusters [VMF98]046 and [VMF98]113, the distribution is not well fited by a normal distribution (χ2 ∼2.5 and P∼0.2), [VMF98]045 have values of 0.22 and 8.95 for the χ2 and P-Value respectivelly. This indicate that the BS is well fited by a Gaussian distribution only in the cluster at z∼0.295. In the clusters [VMF98]046 and [VMF98]113, we separate those galaxies between 1 and 2σ bellow the RCS fit, and a new sample is created with those galaxies below 2σ 1

R is a language and environment for statistical computing and graphics, designed by Robert Gentleman and Ross Ihaka of the Statistics Department of the University of Auckland. http://www.r-project.org/.

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The P-Value is defined as the probability of obtaining a test statistic at least as extreme as the one that was actually observed, assuming that the null hypothesis is true.

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the RCS fit. we call to this new sample the Blue Corrected Sample (BCS). These new samples presents perfect Gaussian distributions, with χ2 values lower than 0.6 and P near to 1 (Figure 2). These results would show that a green galaxy population between the blue and red galaxies, appears in this redshift regime (0.2