1,977 research outputs found
Comparison of the properties of two fossil groups of galaxies with the normal group NGC 6034 based on multiband imaging and optical spectroscopy
We collected multiband imaging and spectroscopy for two fossil groups (RX
J1119.7+2126 and 1RXS J235814.4+150524) and one normal group (NGC 6034). We
computed photometric redshifts in the central zones of each group, combining
previous data with the SDSS five-band data. For each group we investigated the
red sequence (RS) of the color-magnitude relation and computed the luminosity
functions, stellar population ages and distributions of the group members.
Spectroscopy allowed us to investigate the large-scale surroundings of these
groups and the substructure levels in 1RXS J235814.4+150524 and NGC 6034. The
large-scale environment of 1RXS J235814.4+150524 is poor, though its galaxy
density map shows a clear signature of the surrounding cosmic web. RX
J1119.7+2126 appears to be very isolated, while the cosmic environment of NGC
6034 is very rich. At the group scale, 1RXS J235814.4+150524 shows no
substructure. Galaxies with recent stellar populations seem preferentially
located in the group outskirts. A RS is discernable for all three groups in a
color-magnitude diagram. The luminosity functions based on photometric redshift
selection and on statistical background subtraction have comparable shapes, and
agree with the few points obtained from spectroscopic redshifts. These
luminosity functions show the expected dip between first and second brightest
galaxies for the fossil groups only. Their shape is also regular and relatively
flat at faint magnitudes down to the completeness level for RX J1119.7+2126 and
NGC 6034, while there is a clear lack of faint galaxies for 1RXS
J235814.4+150524. RX J1119.7+2126 is definitely classified as a fossil group;
1RXS J235814.4+150524 also has properties very close to those of a fossil
group, while we confirm that NGC 6034 is a normal group.Comment: Accepted in A&A, english-improved, 5 jpeg figures, and shortened
abstrac
Forming Clusters of Galaxies as the Origin of Unidentified GeV Gamma-Ray Sources
Over half of GeV gamma-ray sources observed by the EGRET experiment have not
yet been identified as known astronomical objects. There is an isotropic
component of such unidentified sources, whose number is about 60 in the whole
sky. Here we calculate the expected number of dynamically forming clusters of
galaxies emitting gamma-rays by high energy electrons accelerated in the shock
wave when they form, in the framework of the standard theory of structure
formation. We find that a few tens of such forming clusters should be
detectable by EGRET and hence a considerable fraction of the isotropic
unidentified sources can be accounted for, if about 5% of the shock energy is
going into electron acceleration. We argue that these clusters are very
difficult to detect in x-ray or optical surveys compared with the conventional
clusters, because of their extended angular size of about 1 degree. Hence they
define a new population of ``gamma-ray clusters''. If this hypothesis is true,
the next generation gamma-ray telescopes such as GLAST will detect more than a
few thousands of gamma-ray clusters. It would provide a new tracer of
dynamically evolving structures in the universe, in contrast to the x-ray
clusters as a tracer of hydrodynamically stabilized systems. We also derive the
strength of magnetic field required for the extragalactic gamma-ray background
by structure formation to extend up to 100 GeV as observed, that is about
10^{-5} of the shock-heated baryon energy density.Comment: Accepted by ApJ after minor revisions. Received May 9, Accepted
August 3. 8 pages including 2 figure
An optical view of the filament region of Abell 85
We compare the distribution of optically and Halpha (Ha) selected galaxies in
the Southern half of the nearby Abell 85 (A85) cluster with the recently
discovered X-ray filament (XRF). We search for galaxies where star formation
(SF) may have been triggered by interactions with intracluster gas or tidal
pressure due to the cluster potential when entering the cluster. Our analysis
is based on images obtained with CFHT MegaPrime/MegaCam (1x1 deg2 field) in
four bands (ugri) and ESO 2.2mWFI (38'x36' field) in a narrow band filter
corresponding to the redshifted Halpha (Ha) line and in a broad R-band filter.
The LFs are estimated by statistically subtracting a reference field.
Background contamination is minimized by cutting out galaxies redder than the
observed red sequence in the g-i vs. i colour-magnitude diagram. The galaxy
distribution shows a significantly flattened cluster, whose principal axis is
slightly offset from the XRF. The analysis of the broad band LFs shows that the
filament region is well populated. The filament is also independently detected
as a gravitationally bound structure by the Serna & Gerbal hierarchical method.
101 galaxies are detected in Ha, among which 23 have spectroscopic redshifts in
the cluster, 2 have spectroscopic redshifts higher than the cluster and 58 have
photometric redshifts that tend to indicate that they are background
objects.The 23 galaxies with spectroscopic redshifts in the cluster are mostly
concentrated in the South part of the cluster and along the filament. We find a
number of galaxies showing evidence for SF in the XRF, and all our results are
consistent with the previous hypothesis that the XRF in A85 is a
gravitationally bound structure made of groups falling on to the main cluster.Comment: Accepted in A&A. 39 pages, 107 figures. Full resolution images
available at ftp://ftp.iap.fr/pub/from_users/gam/A85
Critical and Near-Critical Branching Processes
Scale-free dynamics in physical and biological systems can arise from a
variety of causes. Here, we explore a branching process which leads to such
dynamics. We find conditions for the appearance of power laws and study
quantitatively what happens to these power laws when such conditions are
violated. From a branching process model, we predict the behavior of two
systems which seem to exhibit near scale-free behavior--rank-frequency
distributions of number of subtaxa in biology, and abundance distributions of
genotypes in an artificial life system. In the light of these, we discuss
distributions of avalanche sizes in the Bak-Tang-Wiesenfeld sandpile model.Comment: 9 pages LaTex with 10 PS figures. v.1 of this paper contains results
from non-critical sandpile simulations that were excised from the published
versio
GHASP: an H kinematic survey of spiral galaxies - X. Surface photometry, decompositions and the Tully-Fisher relation in the Rc-band
We present Rc-band surface photometry for 170 of the 203 galaxies in GHASP,
Gassendi H-Alpha survey of SPirals, a sample of late-type galaxies for which
high-resolution Fabry-Perot H{\alpha} maps have previously been obtained. Our
data set is constructed by new Rc-band observations taken at the Observatoire
de Haute-Provence (OHP), supplemented with Sloan Digital Sky Survey (SDSS)
archival data, obtained with the purpose of deriving homogeneous photometric
profiles and parameters. Our results include Rc-band surface brightness
profiles for 170 galaxies and profiles for 108 of these objects. We
catalogue several parameters of general interest for further reference, such as
total magnitude, effective radius and isophotal parameters -- magnitude,
position angle, ellipticity and inclination. We also perform a structural
decomposition of the surface brightness profiles using a multi-component method
in order to separate disks from bulges and bars, and to observe the main
scaling relations involving luminosities, sizes and maximum velocities.
We determine the Rc-band Tully Fisher relation using maximum velocities
derived solely from H rotation curves for a sample of 80 galaxies,
resulting in a slope of , zero point of and an
estimated intrinsic scatter of . We note that, different from
the TF-relation in the near-infrared derived for the same sample, no change in
the slope of the relation is seen at the low-mass end (for galaxies with
km/s). We suggest that this different behaviour of the Tully
Fisher relation (with the optical relation being described by a single
power-law while the near-infrared by two) may be caused by differences in the
stellar mass to light ratio for galaxies with km/s.Comment: 17 pages, 11 figure
Very deep spectroscopy of the Coma cluster line of sight: exploring new territories
Environmental effects have an important influence on cluster galaxies, but
studies at very faint magnitudes (R>21) are almost exclusively based on
imaging. We present here a very deep spectroscopic survey of galaxies on the
line of sight to Coma, based on redshifts obtained with VLT/VIMOS for 715
galaxies in the unprecedented magnitude range 21<R<23 (absolute magnitude -14
to -12). We confirm the substructures previously identified in Coma by Adami et
al. (2005a), and identify three new ones. We detect many groups behind Coma: a
large structure at z~0.5, the SDSS Great Wall, and a large and very young
structure at z~0.054. These structures account for the mass maps derived from a
recent weak lensing analysis by Gavazzi et al. (2009). The orbits of dwarf
galaxies are probably anisotropic and radial, and could originate from field
galaxies radially falling into the cluster. Spectral characteristics of Coma
dwarf galaxies show that red or absorption line galaxies have larger stellar
masses and are older than blue or emission line galaxies. R<22 galaxies show
less prominent absorption lines than R>22 galaxies. This trend is less clear
for field galaxies, suggesting that part of the faint Coma galaxies could have
been recently injected from the field following the NGC 4911 group infall. We
present a list of five Ultra Compact Dwarf galaxy candidates. We also globally
confirm spectroscopically our previous results on the galaxy luminosity
functions and find that dwarf galaxies follow a red sequence similar to that
drawn by bright galaxies. Dwarf galaxies are very abundant in Coma, and are
partly field galaxies that have fallen onto the cluster along filaments.Comment: Accepted for publication in Astronomy & Astrophysic
Classical analogy for the deflection of flux avalanches by a metallic layer
Sudden avalanches of magnetic flux bursting into a superconducting sample
undergo deflections of their trajectories when encountering a conductive layer
deposited on top of the superconductor. Remarkably, in some cases flux is
totally excluded from the area covered by the conductive layer. We present a
simple classical model that accounts for this behaviour and considers a
magnetic monopole approaching a semi-infinite conductive plane. This model
suggests that magnetic braking is an important mechanism responsible for
avalanche deflection.Comment: 14 pages, 5 figure
On the current correlators in QCD at finite temperature
Current correlators in QCD at a finite temperature are considered from
the viewpoint of operator product expansion. It is stressed that at low the
heat bath must be represented by hadronic, and not quark-gluon states. A
possibility to express the results in terms of -dependent resonance masses
is discussed. It is demonstrated that in order the masses do not move and
the only phenomenon which occurs is a parity and isospin mixing.Comment: 6 pages, TPI-MINN-92/64-
The galaxy luminosity function of the Abell 496 cluster and its spatial variations
We investigate the LF in the very relaxed cluster Abell 496. Our analysis is
based on deep images obtained at CFHT with MegaPrime/MegaCam in four bands
(ugri) covering a 1x1 deg2 region, which is centered on the cluster Abell 496
and extends to near its virial radius. The LFs are estimated by statistically
subtracting a reference field taken as the mean of the 4 Deep fields of the
CFHTLS survey. Background contamination is minimized by cutting out galaxies
redder than the observed Red Sequence in the g-i versus i colour-magnitude
diagram. In Abell 496, the global LFs show a faint-end slope alpha=-1.55+/-0.06
and vary little with observing band. Without colour cuts, the LFs are much
noisier but not significantly steeper. The faint-end slopes show a
statistically significant steepening from alpha=-1.4+/-0.1 in the central
region (extending to half a virial radius) to -1.8+/-0.1 in the Southern
envelope of the cluster. Cosmic variance and uncertain star-galaxy separation
are our main limiting factors in measuring the faint-end of the LFs. The
large-scale environment of Abell 496, probed with the fairly complete 6dFGS
catalogue, shows a statistically significant 36 Mpc long filament at PA=137
deg, but we do not find an enhanced LF along this axis. Our LFs do not display
the large number of dwarf galaxies (alpha ~ -2) inferred by several authors,
whose analyses may suffer from field contamination caused by inexistent or
inadequate colour cuts. Alternatively, different clusters may have different
faint-end slopes, but this is hard to reconcile with the wide range of slopes
found for given clusters and for wide sets of clusters.Comment: Accepted in A&A. 13 pages, 11 figure
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