142 research outputs found
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
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
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
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
, 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
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