arXiv subjects

Stefano Andreon

Publications and source records attributed to Stefano Andreon.

24 records · Page 2Linked to original sources

arXiv2014

Spectroscopic Confirmation of the Rich z=1.80 Galaxy Cluster JKCS 041 Using the WFC3 Grism: Environmental Trends in the Ages and Structure of Quiescent Galaxies

We present Hubble Space Telescope imaging and grism spectroscopy in the field of the distant galaxy cluster JKCS 041 using the Wide Field Camera 3. We confirm that JKCS 041 is a rich cluster and derive a redshift z=1.80 via the spectroscopic identification of 19 member galaxies, of which 15 are quiescent. These are centered upon diffuse X-ray emission seen by the Chandra observatory. As JKCS 041 is the most distant known cluster with such a large, spectroscopically confirmed quiescent population, it provides a unique opportunity to study the effect of the environment on galaxy properties at early epochs. We construct high-quality composite spectra of the quiescent cluster members that reveal prominent Balmer and metallic absorption lines. Using these, we measure the mean stellar ages in two bins of stellar mass. The quiescent cluster members' ages agree remarkably closely with that inferred by Whitaker et al. for similarly selected samples in the field, supporting the idea that the cluster environment is more efficient at truncating star formation while not having a strong effect on the mean epoch of quenching. We find some evidence (90% confidence) for a lower fraction of disk-like quiescent systems in JKCS 041 compared to a sample of coeval field galaxies drawn from the CANDELS survey. Taking this into account, we do not detect a significant difference between the mass-radius relations of the quiescent JKCS 041 members and our z~1.8 field sample. Finally, we demonstrate how differences in the morphological mixture of quenched systems can complicate measures of the environmental dependence of size growth.

astro-ph.CO↗

arXiv2002

Neural Networks and Photometric Redshifts

We present a neural network based approach to the determination of photometric redshift. The method was tested on the Sloan Digital Sky Survey Early Data Release (SDSS-EDR) reaching an accuracy comparable and, in some cases, better than SED template fitting techniques. Different neural networks architecture have been tested and the combination of a Multi Layer Perceptron with 1 hidden layer (22 neurons) operated in a Bayesian framework, with a Self Organizing Map used to estimate the accuracy of the results, turned out to be the most effective. In the best experiment, the implemented network reached an accuracy of 0.020 (interquartile error) in the range 0<zphot<0.3, and of 0.022 in the range 0<zphot<0.5.

astro-ph↗

arXiv1998

Composite luminosity function of cluster galaxies

We constructed the composite Luminosity Function (LF) of cluster galaxies in the g,r and i bands from the photometry of a mixed (Abell and X-ray selected) sample of the cores of 65 clusters, ranging in redshift from 0.05 to 0.25. The composite LF has been obtained from complete samples of ~2200 galaxies in the magnitude range -23<M<-17.5 (-18 in i). Cluster membership has been determined on the basis of color-color plots for each cluster and the resulting outlier counts have been checked against field counts in the same bands. We find that the galaxy density of the environment determines the shape of the LF, in the sense that bright galaxies are more abundant in dense clusters.

astro-ph↗

arXiv1998

Is there a deficit of S0s at intermediate redshift?

Two contradictory results on the evolution of SO galaxies now exist in the recent literature; either S0s are old ($z_{formation}>2$) and are evoloving passively or most of them formed at $z<0.5$, as implied by the deficit of S0s in intermediate redshift ($z\sim0.5$) clusters. The resolution of this controversy may be that the apparent deficit of S0s has been derived from a quantity -- the E to S0 ratio -- which is prone to morphological classification errors. Once all sources of error are taken into account, the E to S0 ratios of clusters at different redshifts are fully compatible, and no additional creation of S0s at $z<0.5$ is required by the data. Furthermore, there is no deficit at all of S0s in the intermediate redshift cluster for which we have morphological types of very high quality, and thus derive an E to S0 ratio with a small error.

astro-ph↗

arXiv1997

Two-Color Surface Photometry of Brightest Cluster Members

The Gunn g, r and i CCD images of a representative sample of 17 Brightest Cluster Galaxies (BCM) have been analyzed in order to derive surface brightness and color profiles, together with geometrical parameters like eccentricity and position angle. The sample includes both X-ray and optically selected clusters, ranging in redshift from z=0.049 to z=0.191. We find that BCMs are substantially well described by de Vaucouleurs' law out to radii of $\sim 60-80$ kpc, and that color gradients are generally absent. Only in two cases we find a surface brightness excess with respect to the $r^{1/4}$ law, which for A150 is coupled with a change in the $g-r$ color. The rest frame colors of BCMs do not show any intrinsic dispersion. By parametrizing the environment with the local galaxy number density, we find that it is correlated with the BCM extension, i.e. BCMs with larger effective radii are found in denser environments.

astro-ph↗

arXiv1996

Structural classification of galaxies in clusters

The traditional method of morphological classification, by visual inspection of images of uniform quality and by reference to standards for each type, is critically examined. The rate of agreement among traditional morphologists on the morphological type of galaxies is estimated from published classification works, and is estimated at about 20 %, when galaxies are classified into three bins (E, S0, S+Irr). The advantages of the quantitative method of structural classification for classifying galaxies in clusters are outlined. This method is based on the isophotal analysis of galaxy images, and on the examination of quantitative structural parameters derived from this analysis, such as the profiles of luminosity, ellipticity and deviations from ellipticity of the galaxy. The structural and traditional methods are compared on a complete sample of 190 galaxies in the Coma cluster. The morphological types derived by both methods agree to within 15 or 20 %, the same rate as among traditional morphologists alone, thus showing that our morphological classes do coincide with the traditional ones. The galaxies with discrepant types are mostly faint (mag_B > 16.0), or have features typical of spirals, but which have not been detected, noticed or taken into account by traditional morphologists. The rate of agreement is also good for galaxies in a distant cluster (z ~0.4). The structural method, gives highly reproducible results, never reached by traditional estimations of the morphological type. Thus, the morphological types obtained with this method should be preferred. The advantages of the method are further demonstrated by new results on the properties of galaxies in clusters.

astro-ph↗