141 research outputs found
X-ray/optical classification of cluster mergers and the evolution of the cluster merger fraction
We present the results of a simple but robust morphological classification of
a statis- tically complete sample of 108 of the most X-ray luminous clusters at
0.15 < z < 0.7 observed with Chandra. Our aims are to (a) identify the most
disturbed massive clusters to be used as gravitational lenses for studies of
the distant universe and as probes of particle acceleration mechanisms
resulting in non-thermal radio emission, (b) find cluster mergers featuring
subcluster trajectories that make them suitable for quantitative analyses of
cluster collisions, and (c) constrain the evolution with redshift of the
cluster merger fraction. Finally, (d) this paper represents the third public
release of clusters from the MACS sample, adding 24 clusters to the 46
published previously. To classify clusters by degree of relaxation, we use the
projected offset of the brightest cluster galaxy from the peak (or the global
centroid) of the X-ray emission as a measure of the segregation between the
intracluster gas and dark matter. Regarding (a), we identify ten complex
systems likely to have undergone multiple merger events in the recent past.
Regarding (b), we identify eleven systems likely to be post-collision, binary,
head-on mergers (BHOMs), as well as another six mergers that are possible BHOMs
but probably harder to interpret because of non-negligible impact parameters
and merger axes closer to our line of sight. Regarding (c), we find a highly
significant increase with redshift in the fraction of morphologically disturbed
clusters starting at z \sim 0.4, in spite of a detection bias in our sample
against very disturbed systems at high redshift. A larger sample of clusters
with high-quality X-ray data in particular at high redshift will be needed to
trace the evolutionary history of cluster growth and relaxation closer to the
primary epoch of cluster formation z \sim 1.Comment: Accepted to MNRA
An X-ray/optical study of the complex dynamics of the core of the massive intermediate-redshift cluster MACSJ0717.5+3745
Using CHANDRA, we investigate the spatial temperature distribution of the
intracluster medium (ICM) within 700 kpc of the center of the massive merging
cluster MACSJ0717.5+3745 at z=0.55. Combining the X-ray evidence with
information about the distribution and velocities of the cluster galaxies near
the core provides us with a snapshot of the three-dimensional geometry and
dynamics of one of the most complex cluster studied to date. We find
MACSJ0717.5+3745 to be an active triple merger with ICM temperatures exceeding
20 keV. Although radial velocity information and X-ray/optical offsets indicate
that all three mergers proceed along distinctly different directions, the
partial alignment of the merger axes points to a common origin in the
large-scale filament south-east of the cluster core. Clear decrements in the
ICM temperature observed near two of these subclusters identify the respective
X-ray surface brightness peaks as remnants of cool cores; the compactness and
low temperature of 5.7 keV of one of these features suggest that the respective
merger, a high-velocity collision at 3,000 km/s, is still in its very early
stages. Looking beyond the triple merger, we find the large-scale filament to
not only provide a spatial as well as temporal arrow for the interpretation of
the dynamics of the merger events near the cluster core; we also find
tantalizing, if circumstantial, evidence for direct, large-scale heating of the
ICM by contiguous infall of low-density gas from the filament.Comment: accepted by ApJL, 2 color figures, fix the typo of one author's nam
An X-ray/optical study of the geometry and dynamics of MACS J0140.0-0555, a massive post-collision cluster merger
We investigate the physical properties, geometry and dynamics of the massive
cluster merger MACS J0140.0-0555 (z=0.451) using X-ray and optical diagnostics.
Featuring two galaxy overdensities separated by about 250 kpc in projection on
the sky, and a single peak in the X-ray surface brightness distribution located
between them, MACS J0140.0-0555 shows the tell-tale X-ray/optical morphology of
a binary, post-collision merger. Our spectral analysis of the X-ray emission,
as measured by our Chandra ACIS-I observation of the system, finds the
intra-cluster medium to be close to isothermal (~8.5 keV) with no clear signs
of cool cores or shock fronts. Spectroscopic follow-up of galaxies in the field
of MACS J0140.0-0555 yields a velocity dispersion of 875 (+70/-100) km/s
(n_z=66) and no significant evidence of bimodality or substructure along the
line of sight. In addition, the difference in radial velocity between the
brightest cluster galaxies of the two sub-clusters of 144+/-25 km/s is small
compared to typical collision velocities of several 1000 km/s. A strongly
lensed background galaxy at z=0.873 (which features variable X-ray emission
from an active nucleus) provides the main constraint on the mass distribution
of the system. We measure M(<75 kpc) = (5.6+/- 0.5)*10^13 M_sun for the
north-western cluster component and a much less certain estimate of
(1.5-3)*10^13 M_sun for the south-eastern subcluster. These values are in good
agreement with our X-ray mass estimates which yield a total mass of MACS
J0140.0-0555 of M(<r_500) ~ (6.8-9.1)*10^14 M_sun. ......Comment: 11 pages, 8 figures, and 2 tables. Accepted for publication in MNRA
The Origin of the Spatial Distribution of X-ray luminous AGN in Massive Galaxy Clusters
We study the spatial distribution of a 95% complete sample of 508 X-ray point
sources (XPS) detected in the 0.5-2.0 keV band in Chandra ACIS-I observations
of 51 massive galaxy clusters found in the MACS survey. Covering the redshift
range z=0.3-0.7, our cluster sample is statistically complete and comprises all
MACS clusters with X-ray luminosities in excess of 4.5 x 10^44 erg/s (0.1-2.4
keV, h_0=0.7, LCDM). Also studied are 20 control fields that do not contain
clusters. We find the XPS surface density, computed in the cluster restframe,
to exhibit a pronounced excess within 3.5 Mpc of the cluster centers. The
excess, believed to be caused by AGN in the cluster, is significant at the 8.0
sigma confidence level compared to the XPS density observed at the field edges.
No significant central excess is found in the control fields. To investigate
the physical origin of the AGN excess, we study the radial AGN density profile
for a subset of 24 virialized clusters. We find a pronounced central spike
(r<0.5 Mpc), followed by a depletion region at about 1.5 Mpc, and a broad
secondary excess centered at approximately the virial radius of the host
clusters (~2.5 Mpc). We present evidence that the central AGN excess reflects
increased nuclear activity triggered by close encounters between infalling
galaxies and the giant cD-type elliptical occupying the very cluster center. By
contrast, the secondary excess at the cluster-field interface is likely due to
black holes being fueled by galaxy mergers. In-depth spectroscopic and
photometric follow-up observations of the optical counterparts of the XPS in a
subset of our sample are being conducted to confirm this picture.Comment: ApJ Letters, accepted (4 pages, 3 figures, uses emulateapj
Jellyfish: The origin and distribution of extreme ram-pressure stripping events in massive galaxy clusters
We investigate the observational signatures and physical origin of ram-pressure stripping (RPS) in 63 massive galaxy clusters at z = 0.3–0.7, based on images obtained with the Hubble Space Telescope. Using a training set of a dozen ‘jellyfish’ galaxies identified earlier in the same imaging data, we define morphological criteria to select 211 additional, less obvious cases of RPS. Spectroscopic follow-up observations of 124 candidates so far confirmed 53 as cluster members. For the brightest and most favourably aligned systems, we visually derive estimates of the projected direction of motion based on the orientation of apparent compression shocks and debris trails. Our findings suggest that the onset of these events occurs primarily at large distances from the cluster core (>400 kpc), and that the trajectories of the affected galaxies feature high-impact parameters. Simple models show that such trajectories are highly improbable for galaxy infall along filaments but common for infall at high velocities, even after observational biases are accounted for, provided the duration of the resulting RPS events is ≲500 Myr. We thus tentatively conclude that extreme RPS events are preferentially triggered by cluster mergers, an interpretation that is supported by the disturbed dynamical state of many of the host clusters. This hypothesis implies that extreme RPS might occur also near the cores of merging poor clusters or even merging groups of galaxies. Finally, we present nine additional ‘jellyfish” galaxies at z > 0.3 discovered by us, thereby doubling the number of such systems known at intermediate redshift
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