UN GOBIERNO SERIO, EFICAZ Y HONRADO NO MILAGROSO.

•

’RASS-SDSS Galaxy CLuster Surevy. I. The Catalog and the correlation of X-ray and

optical properties.’, Popesso, P., B¨ohringer, H., Brinkmann, J., Voges, W.; York, D.

G, 2004, A&A, 423, 449

Abstract:’For a detailed comparison of the appearance of cluster of galaxies in X-rays

and in the optical, we have compiled a comprehensive database of X-ray and optical

properties of a sample of clusters based on the largest available X-ray and optical sur-

veys: the ROSAT All Sky Survey (RASS) and the Sloan Digital Sky Survey (SDSS).

The X-ray galaxy clusters of this RASS-SDSS catalog cover a wide range of masses,

from groups of 10

12.5

_M

to massive clusters of 10

15

Min the redshift range 0.002-0.45.

The RASS-SDSS sample comprises all the X-ray selected objects already observed by

the Sloan Digital Sky Survey (114 clusters). For each system we have uniformly de-

termined the X-ray (luminosity in the ROSAT band, bolometric luminosity, center

coordinates) and optical properties (Schechter luminosity function parameters, lumi-

nosity, central galaxy density, core, total and half-light radii). For a subsample of 53

clusters we have also compiled the temperatures and the iron abundances from the lit-

erature. The total optical luminosity can be determined with a typical uncertainty of

20% independent of the choice of local or global background subtraction. We searched

for parameters which provide the best correlation between the X-ray luminosity and

the optical properties and found that the z band luminosity determined within a cluster

aperture of 0.5 Mpc h

−1

70

provides the best correlation, with a scatter of about 60-70%.

The scatter decreases to less than 40% if the correlation is limited to the bright X-ray

clusters. The resulting correlation of

L

X

and

L

op

in the z and i bands shows a loga-

rithmic slope of 0.38, a value not consistent with the assumption of a constant

M/L.

Consistency is found, however, for an M/L increasing with luminosity as suggested by

other observations. We also investigated the correlation between

L

op

and the X-ray

temperature, obtaining the same result.’

•

’RASS-SDSS Galaxy Clusters Survey. II. A unified picture of the Cluster Luminosity

Function.’,Popesso, P., B¨ohringer, H., Romaniello, M., Voges, W., 2005, A&A, 433,

415

Abstract:’We constructed the composite luminosity function (LF) of clusters of galaxies

in the five SDSS photometric bands u,g,r,i and z from the RASS-SDSS galaxy cluster

catalog. Background and foreground galaxies are subtracted using both a local and a

global background correction to take in account the presence of large scale structures

and variations from field to field, respectively. The composite LF clearly shows two

components: a bright-end LF with a classical slope of -1.25 in each photometric band,

and a faint-end LF steeper (−2.1

≤

α

≤ −1.6) in the dwarf galaxy region. The

observed upturn of the faint galaxies has a location ranging from -16 +5log(h) in the

g band to -18.5 +5log(h) in the z band. To study the universality of the cluster LF we

compare the individual cluster LFs with the composite luminosity function. We notice

that, in agreement with the composite LF, a single Schechter component is not a good

fit for the majority of the clusters. We fit a Schechter function to the bright-end of

the individual clusters LFs in the magnitude region brighter than the observed upturn

of the dwarf galaxies. We observe that the distributions of the derived parameters is

close to a Gaussian around the value of the composite bright-end LF parameters with

a dispersion compatible with the statistical errors. We conclude that the bright-end of

the galaxy clusters is universal. To study the behavior of the individual faint-end LF

we define the Dwarf to Giant galaxy Ratio (DGR) of the single clusters. We notice

that the distribution of DGR has a spread much larger than the statistical errors.

Our conclusion is that the cluster luminosity function is not universal since the cluster

faint-end, differently from the bright-end, varies from cluster to cluster.’

•

’RASS-SDSS Galaxy Cluster Survey. III. Scaling relations of galaxy clusters.’, Popesso,

P., B¨ohringer, H., Romaniello, M., Voges, W., 2005, A&A, 433, 431

Abstract:’We use the RASS-SDSS galaxy cluster sample to compare the quality of

optical and X-ray luminosities as predictors of other cluster properties such as their

masses, temperatures, and velocity dispersions. We use the SDSS spectroscopic data

to estimate the velocity dispersions and the virial masses of a subsample of 69 clusters

within

r500

and

r200. The ASCA temperature of the intra-cluster medium,

T

X

, is

retrieved from the literature for a subsample of 49 clusters. For this subsample we

estimate the cluster masses also by using the mass-temperature relation. We show

that the optical luminosity,

L

op

, correlates with the cluster mass much better than the

X-ray luminosity,

L

X

.

L

op

can be used to estimate the cluster mass with an accuracy

of 40% while

L

X

can predict the mass only with a 55% accuracy. We show that

correcting

L

X

for the effect of a cool core at the center of a cluster, lowers the scatter

of theL

X

−M

relation only by 3%. We find that the scatter observed in theL

op

−L

X

relation is determined by the scatter of the

L

X

−M

relation. The mass-to-light ratio

in the SDSSiband clearly increases with the cluster mass with a slope 0.2±0.08. The

optical and X-ray luminosities correlate in excellent way with bothT

X

and

σ

V

with an

orthogonal scatter of 20% in both relations. Moreover,

L

op

and

L

X

can predict with

the same accuracy both variables. We conclude that the cluster optical luminosity is

a key cluster parameter since it can give important information about fundamental

cluster properties such as the mass, the velocity dispersion, and the temperature of the

intra-cluster medium.’

•

’RASS-SDSS Galaxy Cluater survey. IV. An ubiquitous dwarf galaxy population in

clusters.’, Popesso, P., Biviano, A., B¨ohringer, H., Romaniello, M., 2006, A&A, 445,

29

Abstract:’We analyze the Luminosity Functions (LFs) of a subsample of 69 clusters

from the RASS-SDSS galaxy cluster catalog. When calculated within the cluster phys-

ical sizes, given byr200

orr500, all the cluster LFs appear to have the same shape, well

fitted by a composite of two Schechter functions with a marked upturn and a steepening

at the faint-end. Previously reported cluster-to-cluster variations of the LF faint-end

slope are due to the use of a metric cluster aperture for computing the LF of clusters

of different masses.

We determine the composite LF for early- and late-type galaxies, where the typing is

based on the galaxyu−r

colors. The late-type LF is well fitted by a single Schechter

function with a steep slope (α

=

−2.0 in the

r

band, within

r200). The early-type

LF instead cannot be fitted by a single Schechter function, and a composite of two

Schechter functions is needed. The faint-end upturn of the global cluster LF is due

to the early-type cluster galaxies. The shape of the bright-end tail of the early-type

LF does not seem to depend upon the local galaxy density or the distance from the

cluster center. The late-type LF shows a significant variation only very near the cluster

center. On the other hand, the faint-end tail of the early-type LF shows a significant

and continuous variation with the environment.

We provide evidence that the process responsible for creating the excess population

of dwarf early type galaxies in clusters is a threshold process that occurs when the

density exceeds

∼500 times the critical density of the Universe.

We interpret our results in the context of the ’harassment’ scenario, where faint early-

type cluster galaxies are predicted to be the descendants of tidally-stripped late-type

galaxies.’

•

,RASS-SDSS Galaxy Cluster survey. V. The Abell X-ray Underlumious Clusters.’, P.

Popesso, A. Biviano, H. B¨ohringer, M. Romaniello, 2007, A&A, 461, 397

Abstract:’In this paper we consider a large sample of optically selected clusters, in order

to elucidate the physical reasons for the existence of X-ray underluminous clusters.

For this purpose we analyzed the correlations of the X-ray and optical properties of

a sample of 137 spectroscopically confirmed Abell clusters in the SDSS database. We

searched for the X-ray counterpart of each cluster in the ROSAT All Sky Survey. We

find that 40% of our clusters have a marginal X-ray detection or remain undetected in

X-rays. These clusters appear too X-ray faint on average for their mass as determined

by velocity dispersion; i.e. they do not follow the scaling relation between X-ray

luminosity and virial mass traced by the other clusters. On the other hand, they do

follow the general scaling relation between optical luminosity and virial mass. We

refer to these clusters as the X-ray-underluminous Abell clusters (AXU clusters, for

short) and designate as ‘normal’ the X-ray detected Abell systems. We separately

examined the distributions and properties of the galaxy populations of the normal and

the AXU clusters. The AXU clusters are characterized by leptokurtic (more centrally

concentrated than a Gaussian) velocity distribution of their member galaxies in the

outskirts (1.5

< r/r200

≤

3.5), as expected for the systems in accretion. In addition,

the AXU clusters have a higher fraction of blue galaxies in the external region and show

a marginally significant paucity of galaxies at the center. Our results seem to support

the interpretation that the AXU clusters are systems in formation undergoing a phase

of mass accretion. Their low X-ray luminosity should be due to the still accreting

intracluster gas or to an ongoing merging process.’

•

’RASS-SDSS Galaxy Cluster survey. VI. The dependence of the cluster SFR on the

cluster global properties.’, P. Popesso, A. Biviano, M. Romaniello, H. B¨ohringer, 2007,

A&A, 461, 411

Abstract: ,To quantify the relationships between star formation in cluster galaxies and

global cluster properties. Using a subsample of 79 nearby clusters from the RASS-SDSS

galaxy cluster catalogue of Popesso et al. (2005a), we perform a regression analysis

between the cluster integrated star formation rate (ΣSF R) the cluster total stellar mass

(M

?

), the fractions of star forming (f

SF

) and blue (f

b

) galaxies and other cluster global

properties, namely its richness (N

gal

, i.e. the total number of cluster members within

the cluster virial radius, corrected for incompleteness), velocity dispersion (σ

v

), virial

mass (M200), and X-ray luminosity (L

X

). All cluster global quantities are corrected for

projection effects before the analysis. GalaxySF Rs and stellar masses are taken from

the catalog of Brinchmann et al. (2004), which is based on SDSS spectra. We only

consider galaxies with

M

r

≤ −20.25 in our analysis, and exclude AGNs. We find that

both ΣSF R

and

M

?

are correlated with all the cluster global quantities. A partial

correlation analysis show that all the correlations are induced by the fundamental one

between ΣSF R

and

N

gal

, hence there is no evidence that the cluster properties affect

the mean

SF R

orM

?

per galaxy. The relations between ΣSF R

and

M

?

, on one side,

and both

N

gal

and

M200, on the other side, are linear, i.e. we see no evidence that

different clusters have different

SF R

or different

M

?

per galaxy and per unit mass.

The fraction

f

SF

does not depend on any cluster property considered, while

f

b

does

depend on

L

X

. We note that a significant fraction of star-forming cluster galaxies

are red (∼

25% of the whole cluster galaxy population). We conclude that the global

cluster properties are unable to affect the

SF

properties of cluster galaxies, but the

presence of the X-ray luminous intra-cluster medium can affect their colors, perhaps

through the ram-pressure stripping mechanism.’

•

’RASS-SDSS Galaxy Cluster survey. VII. On the Cluster Mass to Light ratio and

Halo Occupation Number.’ P. Popesso, A. Biviano, M. Romaniello, H. B¨ohringer,

2007, astro-ph/0606260, A&A in press

Abstract:’We explore the mass-to-light ratio in galaxy clusters and its relation to the

cluster mass. We study the relations among the optical luminosity (L

op

), the cluster

mass (M200) and the number of cluster galaxies within

r200

(N

gal

) in a sample of 217

galaxy clusters with confirmed 3D overdensity. We correct for projection effect, by

determining the galaxy surface number density profile in our cluster sample. This is

best fitted by a cored King profile in low and intermediate mass systems. The core

radius decreases with cluster mass, and, for the highest mass clusters, the profile is

better represented by a generalized King profile or a cuspy Navarro, Frenk & White

profile. We find a very tight proportionality betweenL

op

andN

gal

, which, in turn, links

the cluster mass-to-light ratio to the Halo Occupation DistributionN

gal

vs.

M200. After

correcting for projection effects, the slope of theL

op

−M200

and

N

gal

−M200

relations

is found to be 0.92±0.03, close, but still significantly less than unity. We show that

the non-linearity of these relations cannot be explained by variations of the galaxy

luminosity distributions and of the galaxy M/L with the cluster mass. We suggest

that the nonlinear relation between number of galaxies and cluster mass reflects an

underlying nonlinear relation between number of subhaloes and halo mass.

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