2,468 research outputs found
Emission Lines in X-ray Spectra of Clusters of Galaxies
Emission lines in X-ray spectra of clusters of galaxies reveal the presence
of heavy elements in the diffuse hot plasma (the Intra Cluster Medium, or ICM)
in virial equilibrium in the dark matter potential well. The relatively simple
physical state of the ICM allows us to estimate, with good accuracy, its
thermodynamical properties and chemical abundances. These measures put strong
constraints on the interaction processes between the galaxies and the
surrounding medium, and have significant impact on models of galaxy formation
as well. This field is rapidly evolving thanks to the X-ray satellites Chandra
and XMM-Newton. Among the most relevant progresses in the last years, we
briefly discuss the nature of cool cores and the measure of the Iron abundance
in high redshift clusters. Future X-ray missions with bolometers promise to
provide a substantial step forward to a more comprehensive understanding of the
complex physics of the ICM.Comment: 8 pages, 3 figures, Proceedings of the VI Serbian Conference on
Spectral Line Shapes in Astrophysics, Sremski Karlovci, Serbia June 11-15
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The evolution of the spatially-resolved metal abundance in galaxy clusters up to z=1.4
We present the combined analysis of the metal content of 83 objects in the
redshift range 0.09-1.39, and spatially-resolved in the 3 bins (0-0.15,
0.15-0.4, >0.4) R500, as obtained with similar analysis using XMM-Newton data
in Leccardi & Molendi (2008) and Baldi et al. (2012). We use the pseudo-entropy
ratio to separate the Cool-Core (CC) cluster population, where the central gas
density tends to be relatively higher, cooler and more metal rich, from the
Non-Cool-Core systems. The average, redshift-independent, metal abundance
measured in the 3 radial bins decrease moving outwards, with a mean metallicity
in the core that is even 3 (two) times higher than the value of 0.16 times the
solar abundance in Anders & Grevesse (1989) estimated at r>0.4 R500 in CC (NCC)
objects. We find that the values of the emission-weighted metallicity are
well-fitted by the relation at given radius. A
significant scatter, intrinsic to the observed distribution and of the order of
0.05-0.15, is observed below 0.4 R500. The nominal best-fit value of
is significantly different from zero in the inner cluster regions () and in CC clusters only. These results are confirmed also with a
bootstrap analysis, which provides a still significant negative evolution in
the core of CC systems (P>99.9 per cent). No redshift-evolution is observed
when regions above the core (r > 0.15 R500) are considered. A reasonable good
fit of both the radial and redshift dependence is provided from the functional
form , with in CC clusters
and for NCC systems. Our results
represent the most extensive study of the spatially-resolved metal distribution
in the cluster plasma as function of redshift.Comment: 5 pages. Research Note accepted for publication in A&
Polarimetry and photometry of the peculiar main-belt object 7968 = 133P/Elst-Pizarro
133P/Elst-Pizarro is an object that has been described as either an active
asteroid or a cometary object in the main asteroid belt. Here we present a
photometric and polarimetric study of this object in an attempt to infer
additional information about its origin.
With the FORS1 instrument of the ESO VLT, we have performed during the 2007
apparition of 133P/Elst-Pizarro quasi-simultaneous photometry and polarimetry
of its nucleus at nine epochs in the phase angle range 0 - 20 deg. For each
observing epoch, we also combined all available frames to obtain a deep image
of the object, to seek signatures of weak cometary activity. Polarimetric data
were analysed by means of a novel physical interference modelling.
The object brightness was found to be highly variable over timescales <1h, a
result fully consistent with previous studies. Using the albedo-polarization
relationships for asteroids and our photometric results, we found for our
target an albedo of about 0.06-0.07 and a mean radius of about 1.6 km.
Throughout the observing epochs, our deep imaging of the comet detects a tail
and an anti-tail. Their temporal variations are consistent with an activity
profile starting around mid May 2007 of minimum duration of four months. Our
images show marginal evidence of a coma around the nucleus. The overall light
scattering behaviour (photometry and polarimetry) resembles most closely that
of F-type asteroids.Comment: Accepted by Astronomy and Astrophysic
A Chandra archival study of the temperature and metal abundance profiles in hot Galaxy Clusters at 0.1 < z < 0.3
We present the analysis of the temperature and metallicity profiles of 12
galaxy clusters in the redshift range 0.1--0.3 selected from the Chandra
archive with at least ~20,000 net ACIS counts and kT>6 keV. We divide the
sample between 7 Cooling-Core (CC) and 5 Non-Cooling-Core (NCC) clusters
according to their central cooling time. We find that single power-laws can
describe properly both the temperature and metallicity profiles at radii larger
than 0.1 r_180 in both CC and NCC systems, showing the NCC objects steeper
profiles outwards. A significant deviation is only present in the inner 0.1
r_180. We perform a comparison of our sample with the De Grandi & Molendi
BeppoSAX sample of local CC and NCC clusters, finding a complete agreement in
the CC cluster profile and a marginally higher value (at ~1sigma) in the inner
regions of the NCC clusters. The slope of the power-law describing kT(r) within
0.1 r_180 correlates strongly with the ratio between the cooling time and the
age of the Universe at the cluster redshift, being the slope >0 and
tau_c/tau_age<=0.6 in CC systems.Comment: 12 pages, 6 figures, Accepted for publication by the Astrophysical
Journa
Evolution in the iron abundance of the ICM
We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies
at , which cover a temperature range of keV. Our analysis
is aimed at measuring the iron abundance in the ICM out to the highest redshift
probed to date. We find that the emission-weighted iron abundance measured
within in clusters below 5 keV is, on average, a factor of
higher than in hotter clusters, following , which confirms the trend seen in local samples. We made use of
combined spectral analysis performed over five redshift bins at
to estimate the average emission weighted iron abundance. We find a constant
average iron abundance as a function of redshift,
but only for clusters at . The emission-weighted iron abundance is
significantly higher () in the redshift range
, approaching the value measured locally in the inner radii for a mix of cool-core and non cool-core clusters in the
redshift range . The decrease in with can be
parametrized by a power law of the form . The observed
evolution implies that the average iron content of the ICM at the present epoch
is a factor of larger than at . We confirm that the ICM is
already significantly enriched () at a look-back time
of 9 Gyr. Our data provide significant constraints on the time scales and
physical processes that drive the chemical enrichment of the ICM.Comment: 4 pages, 4 figures, to appear in the Proceedings of "The Extreme
Universe in the Suzaku Era", Dicember 2006, Kyoto (Japan
Where does the gas fueling star formation in BCGs originate?
We investigate the relationship between X-ray cooling and star formation in
brightest cluster galaxies (BCGs). We present an X-ray spectral analysis of the
inner regions, 10-40 kpc, of six nearby cool core clusters (z<0.35) observed
with Chandra ACIS. This sample is selected on the basis of the high star
formation rate (SFR) observed in the BCGs. We restrict our search for cooling
gas to regions that are roughly cospatial with the starburst. We fit single-
and multi-temperature mkcflow models to constrain the amount of isobarically
cooling intracluster medium (ICM). We find that in all clusters, below a
threshold temperature ranging between 0.9 and 3 keV, only upper limits can be
obtained. In four out of six objects, the upper limits are significantly below
the SFR and in two, namely A1835 and A1068, they are less than a tenth of the
SFR. Our results suggests that a number of mechanisms conspire to hide the
cooling signature in our spectra. In a few systems the lack of a cooling
signature may be attributed to a relatively long delay time between the X-ray
cooling and the star burst. However, for A1835 and A1068, where the X-ray
cooling time is shorter than the timescale of the starburst, a possible
explanation is that the region where gas cools out of the X-ray phase extends
to very large radii, likely beyond the core of these systems.Comment: to appear in A&
Photometry and polarimetry of the nucleus of comet 2P/Encke
Broadband imaging photometry, and broadband and narrowband linear polarimetry
was measured for the nucleus of 2P/Encke over the phase-angle range 4 - 28 deg.
An analysis of the point spread function of the comet reveals only weak coma
activity, corresponding to a dust production of the order of 0.05 kg/s. The
nucleus displays a color independent photometric phase function of almost
linear slope. The absolute R filter magnitude at zero phase angle is 15.05 +/-
0.05, and corresponds to an equivalent radius for the nucleus of 2.43 +/- 0.06
km (for an adopted albedo of 0.047). The nucleus color V - R is 0.47 +/- 0.07,
suggesting a spectral slope of 11 +/- 8 %/100nm. The phase function of linear
polarimetry in the V and R filters shows a widely color independent linear
increase with phase angle (0.12 +/- 0.02%/deg). We find discrepancies in the
photometric and polarimetric parameters between 2P/Encke and other minor bodies
in the solar system, which may indicate significant differences in the surface
material properties and light-scattering behavior of the bodies.
The linear polarimetric phase function of 2P/Encke presented here is the
first ever measured for a cometary nucleus, and its analysis encourages future
studies of cometary nuclei in order to characterize the light-scattering
behavior of comets on firm empirical grounds and provide suitable input to a
comprehensive modeling of the light scattering by cometary surfaces.Comment: Accepted by A&
Tracing the evolution in the iron content of the ICM
We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies
at z>0.3, which cover a temperature range of 3>kT>15 keV. Our analysis is aimed
at measuring the iron abundance in the ICM out to the highest redshift probed
to date. We find that the emission-weighted iron abundance measured within
(0.15-0.3)R_vir in clusters below 5 keV is, on average, a factor of ~2 higher
than in hotter clusters, following Z(T)~0.88T^-(0.47)Z_o, which confirms the
trend seen in local samples. We made use of combined spectral analysis
performed over five redshift bins at 0.3>z>1.3 to estimate the average emission
weighted iron abundance. We find a constant average iron abundance Z_Fe~0.25Z_o
as a function of redshift, but only for clusters at z>0.5. The
emission-weighted iron abundance is significantly higher (Z_Fe~0.4Z_o) in the
redshift range z~0.3-0.5, approaching the value measured locally in the inner
0.15R_vir radii for a mix of cool-core and non cool-core clusters in the
redshift range 0.1<z<0.3. The decrease in Z_Fe with redshift can be
parametrized by a power law of the form ~(1+z)^(-1.25). The observed evolution
implies that the average iron content of the ICM at the present epoch is a
factor of ~2 larger than at z=1.2. We confirm that the ICM is already
significantly enriched (Z_Fe~0.25Z_o) at a look-back time of 9 Gyr. Our data
provide significant constraints on the time scales and physical processes that
drive the chemical enrichment of the ICM.Comment: 6 pages, 6 figures, to appear in the Proceedings of "Heating vs.
Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching
(Germany
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