Statistical Properties of the IntraCluster Light from SDSS Image Stacking

The presence of a diffuse stellar component in galaxy clusters has been established by a number of observational works in recent years. In this contribution I summarize our results (Zibetti et al. 2005) obtained by stacking SDSS images of 683 clusters, selected with the maxBCG algorithm at 0.2< z <0.3. Thanks to our large sample and the advantages of image stacking applied to SDSS images, we are able to measure the systematic properties of the intracluster light (ICL) with very high accuracy. We find that the average surface brightness of the ICL ranges between 26 and 32 mag/arcsec^2, and constantly declines from 70 kpc cluster-centric distance (i.e. distance from the BCG) to 700 kpc. The fraction of diffuse light over the total light (including galaxies), monotonically declines from ~50 to <~5% over the same range of distances, thus showing that the ICL is more easily produced close to the bottom of a cluster's potential well. Clusters lacking a bright BCG hardly build up a large amount of intracluster stellar component. The link between the growth of the BCG and the ICL is also suggested by the strong degree of alignment between these two components which is observed in clusters where the BCG displays a significant elongation. With the additional fact that the colors of the ICL are consistent with those of galaxies, all this appears to be evidence for IC stars being stripped from galaxies that suffer very strong tidal interactions in the center of clusters and eventually merge into the BCG. Our measurements also show that IC stars are a minor component of a cluster's baryonic budget, representing only ~10% of the total optical emission within 500 kpc. Finally, we discuss some open issues that emerge from a comparison of the present results with other observations and recent theoretical modeling.Comment: 10 pages, 3 figures, to appear in the Proceedings of the IAUS 244 "Dark Galaxies & Lost Baryons", J. I. Davies & M. J. Disney, eds., Cardiff 15-29 June 200

A multiscale approach to environment

Physical processes influencing the properties of galaxies can be traced by the dependence and evolution of galaxy properties on their environment. A detailed understanding of this dependence can only be gained through comparison of observations with models, with an appropriate quantification of the rich parameter space describing the environment of the galaxy. We present a new, multiscale parameterization of galaxy environment which retains an observationally motivated simplicity whilst utilizing the information present on different scales. We examine how the distribution of galaxy (u-r) colours in the Sloan Digital Sky Survey (SDSS), parameterized using a double gaussian (red plus blue peak) fit, depends upon multiscale density. This allows us to probe the detailed dependence of galaxy properties on environment in a way which is independent of the halo model. Nonetheless, cross-correlation with the group catalogue constructed by Yang et al, 2007 shows that galaxy properties trace environment on different scales in a way which mimics that expected within the halo model. This provides independent support for the existence of virialized haloes, and important additional clues to the role played by environment in the evolution of the galaxy population. This work is described in full by Wilman et al., 2010, MNRAS, acceptedComment: A brief summary of the work presented by Wilman et al., 2010, MNRAS, accepted; LaTeX, 4 pages, 2 figures. To appear in "Hunting for the Dark: The Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P. Debattista & C.C. Popescu, AIP Conference Serie

NIR observations of dEs in the Virgo cluster: a structural continuity with giant Ellipticals

The structural properties of a sample of 50 dEs in the Virgo cluster are here derived from Near InfraRed (NIR, H-band 1.65 micron) surface photometry and analysed. One-dimensional surface brightness (SB) profiles are extracted using elliptical isophote fitting. They are characterised by means of structural parameters, namely the half light radius R_e, the average surface brightness within R_e (mu_e), and a concentration index (c_31). We show that typical dEs have close-to-exponential NIR SB distributions. The relations between dEs and giant ellipticals (Es) are investigated by comparing the NIR structural parameters of 273 Es in nearby clusters. Further analysis is conducted using the optical-NIR colour B-H and by studying the relationships between structural and dynamical parameters (fundamental plane) for the two classes of galaxies. The transition between the two regimes is smooth and no dichotomy is seen.Comment: 2 pages, 2 figures, use iauc.cls. Poster presentation to appear in the proceedings of "IAU Colloquium 198 - Near-Field Cosmology with Dwarf Elliptical Galaxies", Les Diablerets 14-18 March 2005, B. Binggeli and H. Jerjen eds. GOLDMiNe website at http://goldmine.mib.infn.it

Diffuse light in z~0.25 galaxy clusters: constraining tidal damage and the faint end of the Luminosity Function

The starlight coming from the intergalactic space in galaxy clusters and groups witnesses the violent tidal interactions that galaxies experience in these dense environments. Such interactions may be (at least partly) responsible for the transformation of normal star-forming galaxies into passive dwarf ellipticals (dEs). In this contribution we present the first systematic study of the IntraCluster Light (ICL) for a statistically representative sample (Zibetti et al. 2005), which comprises 683 clusters selected between z=0.2 and 0.3 from ~1500 deg^2 in the SDSS. Their ICL is studied by stacking the images in the g-, r-, and i-band after masking out all galaxies and polluting sources. In this way a very uniform background illumination is obtained, that allows us to measure surface brightnesses as faint as 31 mag/arcsec^2 and to trace the ICL out to 700 kpc from the central galaxy. We find that the local fraction of light contributed by intracluster stars rapidly decreases as a function of the clustercentric distance, from ~40% at 100 kpc to ~5% at 500 kpc. By comparing the distribution and colours of the ICL and of the clusters galaxies, we find indication that the main source of ICL are the stars stripped from galaxies that plunge deeply into the cluster potential well along radial orbits. Thus, if dEs are the remnants of these stripped progenitors we should expect similar radial orbital anisotropies and correlations between the dE luminosity function and the amount of ICL in different clusters. The diffuse emission we measure is contaminated by faint unresolved galaxies: this makes our flux estimate depend to some extent on the assumed luminosity function, but, on the other hand, allows us to constrain the number of faint galaxies. Our present results disfavour steep (alpha<-1.35) faint-end powerlaw slopes.Comment: 4 pages, 2 figures, use iauc.cls. Oral presentation to appear in the proceedings of "IAU Colloquium 198 - Near-Field Cosmology with Dwarf Elliptical Galaxies", Les Diablerets 14-18 March 2005, B. Binggeli and H. Jerjen ed

Halos around edge-on disk galaxies in the SDSS

We present a statistical analysis of halo emission for a sample of 1047 edge-on disk galaxies imaged in five bands by the Sloan Digital Sky Survey (SDSS). Stacking the homogeneously rescaled images of the galaxies, we can measure surface brightnesses as deep as mu_r~31 mag/arcsec^2. The results strongly support the almost ubiquitous presence of stellar halos around disk galaxies, whose spatial distribution is well described by a power-law $\rho\propto r^{-3}$, in a moderately flattened spheroid (c/a~0.6). The colour estimates in g-r and r-i, although uncertain, give a clear indication for extremely red stellar populations, hinting at old ages and/or non-negligible metal enrichment. These results support the idea of halos being assembled via early merging of satellite galaxies.Comment: 15 pages, 9 figures. Accepted for publication on MNRAS. Version with full resolution images available at http://www.mpa-garching.mpg.de/~zibetti/papers/halos_edgeon.pd

More than just halo mass: Modelling how the red galaxy fraction depends on multiscale density in a HOD framework

The fraction of galaxies with red colours depends sensitively on environment, and on the way in which environment is measured. To distinguish competing theories for the quenching of star formation, a robust and complete description of environment is required, to be applied to a large sample of galaxies. The environment of galaxies can be described using the density field of neighbours on multiple scales - the multiscale density field. We are using the Millennium simulation and a simple HOD prescription which describes the multiscale density field of Sloan Digital Sky Survey DR7 galaxies to investigate the dependence of the fraction of red galaxies on the environment. Using a volume limited sample where we have sufficient galaxies in narrow density bins, we have more dynamic range in halo mass and density for satellite galaxies than for central galaxies. Therefore we model the red fraction of central galaxies as a constant while we use a functional form to describe the red fraction of satellites as a function of halo mass which allows us to distinguish a sharp from a gradual transition. While it is clear that the data can only be explained by a gradual transition, an analysis of the multiscale density field on different scales suggests that colour segregation within the haloes is needed to explain the results. We also rule out a sharp transition for central galaxies, within the halo mass range sampled.Comment: 24 pages, 21 figures, accepted for publication by MNRA

Constraining the photometric properties of MgII absorbing galaxies with the SDSS

Using a sample of nearly 700 quasars with strong (W_0(2796)>0.8 Angstrom) MgII absorption lines detected in the Early Data Release of the SDSS, we demonstrate the feasibility of measuring the photometric properties of the absorber systems by stacking SDSS imaging data. As MgII lines can be observed in the range 0.37<z_abs<2.2, the absorbing galaxies are in general not identified in SDSS images, but they produce systematic light excesses around QSOs which can be detected with a statistical analysis. In this Letter we present a 6-sigma detection of this effect over the whole sample in i-band, rising to 9.4-sigma for a low-redshift subsample with 0.37<z_abs<=0.82. We use a control sample of QSOs without strong MgII absorption lines to quantify and remove systematics with typical 10-20% accuracy. The signal varies as expected as a function of absorber redshift. For the low z_abs subsample we can reliably estimate the average luminosities per MgII absorber system in the g, r, and i bands and find them to be compatible with a few-hundred-Myr old stellar population of M_r ~ -21 in the rest frame. Colors are also consistent with typical absorbing galaxies resembling local Sb-c spirals. Our technique does not require any spectroscopic follow-up and does not suffer from confusion with other galaxies arising along the line-of-sight. It will be applied to larger samples and other line species in upcoming studies.Comment: Accepted on ApJ Letters, 5 pages, 2 figure

Resolved maps of stellar mass and SED of galaxies from optical/NIR imaging and SPS models

We report on the method developed by Zibetti, Charlot and Rix (2009) to construct resolved stellar mass maps of galaxies from optical and NIR imaging. Accurate pixel-by-pixel colour information (specifically g-i and i-H) is converted into stellar mass-to-light ratios with typical accuracy of 30%, based on median likelihoods derived from a Monte Carlo library of 50,000 stellar population synthesis models that include dust and updated TP-AGB phase prescriptions. Hence, surface mass densities are computed. In a pilot study, we analyze 9 galaxies spanning a broad range of morphologies. Among the main results, we find that: i) galaxies appear much smoother in stellar mass maps than at any optical or NIR wavelength; ii) total stellar mass estimates based on unresolved photometry are biased low with respect to the integral of resolved stellar mass maps, by up to 40%, due to dust obscured regions being under-represented in global colours; iii) within a galaxy, on local scales colours correlate with surface stellar mass density; iv) the slope and tightness of this correlation reflect/depend on the morphology of the galaxy.Comment: 4 pages, 2 figures, to appear in the proceedings of the IAU Symposium 262 "Stellar Populations: Planning for the Next Decade", Charlot & Bruzual ed

Diffuse stellar components in galaxies and galaxy clusters

In currently favoured scenarios, the Universe evolves from a highly homogeneous phase, emerging from the hot Big Bang, to the present-day state, which is characterised by a wealth of hierarchically organised structures, spanning several orders of magnitudes in size: galaxies, clusters of galaxies, superclusters, walls and filaments. Structures are formed via gravitational instability and grow hierarchically: the smallest ``haloes'' collapse first and then grow by accreting mass from other haloes or by merging with other similar structures. Gravitational and dynamical interactions, like mergers, accretions, tidal distortions and disruptions thus play a fundamental role in shaping galaxies and galaxy clusters. As a natural by-product of these interactions, stars, originally located within galaxies, are ejected into the space surrounding galaxies and into intracluster space, giving rise to diffuse stellar components. The study of these components can reveal important details of galaxy and cluster formation, and are therefore of great interest. Observations in this field are severely hampered by the extremely low surface brightness that has to be measured, corresponding to less than 1/1,000 of the typical surface brightness of the sky. So far, this has prevented observing large statistical samples of stellar haloes and intracluster stellar populations. The statistical characterisation of stellar haloes and of the intracluster light is the objective of this thesis. In order to do this, I have developed a new method in which a large number (approx. 1,000) of relatively shallow images of homogeneous objects are stacked to produce an extremely deep average image. Systematic effects that arise from contaminating sources and instrumental biases in the observation of individual objects are cancelled out by taking the average of many different observations and by adopting a conservative masking of the polluting sources. The large image database required for this technique has been provided by the Sloan Digital Sky Survey, the largest optical survey of the Universe ever undertaken, which will eventually cover approximately 10,000 square degrees of sky with 5-band photometry and spectroscopy. In the first part of the thesis I have studied the stellar haloes of disc galaxies. From the stacking of 1043 galaxies I have been able to infer the almost ubiquitous presence of this component around discs, thus demonstrating that haloes are essential ingredients of galaxy evolution. On average, stellar haloes have power-law profiles, consistent with those of the Milky Way and M31. Their shape is moderately flattened. The average halo colours hint at old and fairly metal-enriched stars. However, a puzzling emission excess in the redmost bands has been measured that cannot be explained by any stellar emission, but suggests the presence of ionised gas. These results have been confirmed by the analysis of a galaxy, which has been observed by the Hubble Space Telescope in its deepest exposure (the Ultra Deep Field). The second part of the thesis is devoted to a study of the intracluster light (ICL) from the stacking of a sample of 683 clusters of galaxies in the redshift range 0.2--0.3. The average contribution of the ICL to the total light of a cluster is 17.5 +- 2.0% within 700 kpc from the cluster centre. The ICL is significantly more centrally concentrated than the light in galaxies, consistent with the idea that the ICL is formed via tidal stripping and disruption of galaxies that plunge deep into the cluster potential. The colours of the ICL are consistent with those of the cluster galaxies, thus indicating that the intracluster stars stem from the same population as the stars in galaxies. The amount of ICL correlates more strongly with the luminosity of the central galaxy of the cluster than with cluster richness. Furthermore, the ICL aligns more strongly with the central galaxy than with the larger scale galaxy distribution in the cluster. This strongly suggest that the mechanism of formation of the ICL is strongly coupled with the process of growth of the cluster central galaxy