A Search for the Hidden Baryonic Component of the Universe
Carmen P. PadillaPadilla-Torres1, Ricardo GénovaGénova-Santos1, Carlos M. Gutiérrez1, Robert Juncosa1, Rafael Rebolo1,2, Robert A. Watson3.
1
Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Canary Islands, Spain.
2 Consejo Superior de Investigaciones Científicas, Spain
3 Jodrell Bank Observatory, University of Manchester, Cheshire, SK11 9DL, UK
ABSTRACT
Recent observations with the Very Small Array (VSA) in the region of the Corona Borealis Supercluster show several decrements with
amplitudes too large to be produced by primordial CMB anisotropies. These decrements are not related to known clusters of galaxies.
They could be the result of the interaction of CMB photons with a possible baryonic component in the form of warm/hot gas at large
scale (Sunyaev-Zeldovich effect, SZE). Here, we present preliminary results on an optical survey for galaxies in these regions. A high
galaxy density might be interpreted as evidence for a large amount of gas at supercluster scales.
Introduction
One of the most relevant parameters in the current description of the Universe
is the mean baryonic density. This has been estimated from abundances of
primordial light elements (Burles, Nollet, & Turner, 2001), or from
observations of the Ly-α forest in QSO spectra (Rauch et al.,1997). More
recently, estimates have been obtained from the power spectra of the cosmic
microwave background (CMB) fluctuations (see e.g. Bennet et al.. 2003, Rebolo
et al. 2004). All these methods give consistent values. However, studies of the
baryon budget in the present Universe at z=0 (Fukugita et al. 1998) give
values lower by a factor 2. Several explanations have been given to account for
the above discrepancy; for instance Cen & Ostriker (1999) proposed that an
important fraction of these missing baryons could be distributed in large-scale
sheet-like structures and filaments connecting clusters of galaxies, in the form of
a warm/hot diffuse gas component with temperatures in the range 105 K
<T<107 K. The most natural approach to detect such component would be in the
soft X-rays band; however this detection is difficult due to the presence of
several galactic foregrounds, and extragalactic contributions of groups of
galaxies and AGNs. In the last years, several intents have been made to detect
this gas by studying the correlation between the observerd soft X-rays
structures and overdensities in the distribution of galaxies (Scharf et al.,2000;
Zappacosta et al., 2004). Génova-Santos et al. (2004) have proposed another
approach to detect this elusive component through the effect on the CMB
photons, via the so-called
Sunyaev-Zel’dovich effect (SZE) (Sunyev &
Zel’dovich 1970) in superclusters of galaxies.
Fig1.-(Left) Source subtracted VSA mosaic core of the CrB-SC, built up
from 9 pointings (Génova-Santos et al. 2004). (Right) Spatial
distribution of Abell and Zwicky clusters in that region. The Abell
clusters labelled with blue letter belong to the Crb-SC. It is also
indicated the centers of the SCL158 (CrB) and SCL157 supercluster, as
reported by Einasto et al. (2001). Red dots indicate galaxies which
according to their position and redshift are members of any of the known
clusters in this field.. The red squares indicate the region observed with
INT and analyzed in this work. The arrows show the correspondence
between these and the two large decrements found in the VSA data..
CrB-CB1
field
CrB-CB2
field
The Very Small Array
The Very Small Array (VSA) is an interferometer working at 34 GHz sited at
the Teide Observatory in Tenerife, whose primary goal has been the detection of
CMB primary anisotropies (Scott et al. 2003). The instrument has a resolution
~10 arcmin and a sensitivity ~3 Jy s1/2 beam-1. These properties make it
appropriate for the detection of possible SZE effects associated to large scale
structures.
Génova-Santos et al. (2004) have conducted observatinos with VSA in the region
of the Corona Borealis supercluster (Einasto et al. 2001) combining several
pointings to obtain a mosaic covering a total field of 23 square degrees (see
Figures 1). The data show the detection of SZE associated with the cluster
A2069. In addition, the data show two conspicous decrements with amplitudes
-72.4±11.9 mJy (CrB-H) and -106.3±12.9 mJy. (CrB-B). Apart of these features
the amplitudes and sizes of the rest of fluctuations are compatibles with the
CMB primordial anisotrophies.
These two large decrements are not related to known clusters. Génova-Santos
et al. 2004 (in preparation) argue that it is not likely these are spots associated to
primordial CMB fluctuations or to previously undetected clusters of galaxies.
Fig. 2.- Observations with INT of the CrB-CB1 and CrB-CB2 fields where two
large decrements of unknown origin were detected with VSA at 34 GHz. Each of
the fields cover ~0.3 square degrees in the sky and was observed in the Ugriz’
bands.
Fig 5.- A composition showing the region observed with INT, the
positions of all the galaxies detected (black) and the galaxie members
(red) of possible large scale structure according to the Voronoi’s
tessellations methods.
Table 2.- A summary of
the positions and sizes of
overdensities in both
fields like the figure 5
which
presents
the
different
Voronoi's
tessellations predict. Yet,
we need to confirm if
any of them are known
clusters
in
others
catalogues,
for
this
moment all of them are
unknown.
It is left as an open possibility that the spots are caused by SZ effect associated to
warm/hot diffuse gas at supercluster scales
Goal
We decided to conduct observations in these two regions were strong CMB
decrements were detected in the optical and near-infrared bands to search for
the galaxy population in this region.
Observations
The first observations were done in Ugriz’ filters in April 2004. We used the
Wide Field Camera (WFC) at the Isaac Newton Telescope (INT) in Roque de
Los Muchachos Observatory (ORM) . The typical seeing was ~1.2 arcsec. A
summary of the observations is presented in the table below.
Fig. 3.- Density of galaxies by square arc minute and magnitude in the CrB-CB1
and CrB-CB2 fields (left and right respectively). The apparent holes in the
distribution of galaxies are due to the presence of saturated stars. The different
colours correspond to the four CCDs of the WFC at the INT. The black lines are
the combination of the results in the four CCDs.
Recently, Hernández-Monteagudo et al. (2004) have performed a
pixel to pixel correlation analysis between the first year WMAP data
and the 2MASS infrared galaxy catalogue to search for possible
evidence of extended SZE. They found a temperature decrement of 35±7 µK in the 2048 pixels of 7' with the highest projected galaxy
density (area of ~26 squares degrees). The 1024 densest pixels (13
square degrees) give a signal of -35±9 µK. 15 of these pixels are
located in the region of CrB-SC observed with VSA; these are
represented in Figure 1 with crosses. One of these pixels correspond
to the cluster A2069, and another is close to decrement B.
The observations were reduced using IRAF and a standard procedure which
involves bias subtraction, flat-field correction, fringing and co-addition of
exposures. The nights were not completely photometric and then the calibration
presented here have an uncertainty ~0.1 mag. Several methods to improve this
calibration are in progress.
Catalogues of objects were built using the software Sextractor (Bertin &
Arnouts 1996). We discarded stellar-like objects and those with unaccurate
photometry according to Sextractor, Under these restrictions we have detected
14995 objects in the g, r and i bands.
Detection of large structures in the optical an near
infrared
In addition to the X-ray and SZE searches, several methods have been
traditionnally used to detect clusters of galaxies (and in genereal large scale
structures) in optical and infrared bands. The common basis of all these
methods is the detection of overdensities in the observed spatial distribution of
galaxies. Some of the frequently used are:
Box counting: (Lidman & Peterson 1996) This is a very simple method which
counts objects in boxes of a given size over the image. The boxes that present a
density of objects significatively larger than the statistical mean value enlosed
candidates to be clusters of galaxies.
•Matched filter: (Postman et al. 1996; Kepner et al.1999; Kawasaki et al. 1998).
The projected density of galaxies is convolved with a 2D function with the
profile expected for a cluster of galaxies. This method was used to obtain the
Palomar Distant Cluster Survey (PDCS), (Postman & Lubin 1996) and the EIS
Cluster catalogue (Olsen et al.1999).
•Voroni galaxy cluster finder: (Ramella et al. 2000). This is based on
Voronoi’s tessellation to estimate the local density of galaxies and to identify
cluster as significant density fluctuations above the background. Differently
from the previous methods, here it is not assumed a given shape of the
structures.
•The sequence of elliptical galaxies ( Gladders & Yee 2000). It is based on
the detection of the tight color-magnitude relation followed by the elliptical
galaxies in a cluster.
Fig. 4.- Colour-magnitude diagrams of the galaxies in the CB1 (left) and CB2
(right) fields respectively obtained with the INT observations analyzed here. These
diagrams give an indication of the sensitivity of the observations. According to this
diagram and the expected colors of real galaxies (e.g. Gladders & Yee 2000) it
would be possible to detect a reasonable number of elliptical galaxies in a cluster up
to redshift ~0.8.
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Summary
-We have conducted optical observations in several bands (limiting
magnitude ~24 in g) of two fields in the Corona Borealis supercluster where
decrements with amplitudes ~-100 mJy have recently been detected with
the VSA interferometer.
-We have applied Voronoi techniques to detect density enhancements in the
distribution of galaxies in these fields and present preliminary identification
of possible structures (size less than 1 arcmin) in the galaxy distribution
which are not related to previously known clusters of galaxies in the Corona
Borealis supercluster.
-For the future, we plan to improve and extend this analysis using several
other techniques to study the 3-D distribution of objects, and complement
this with spectroscopy of selected candidates.