2,930 research outputs found
Shock heating of the merging galaxy cluster A521
A521 is an interacting galaxy cluster located at z=0.247, hosting a low
frequency radio halo connected to an eastern radio relic. Previous Chandra
observations hinted at the presence of an X-ray brightness edge at the position
of the relic, which may be a shock front. We analyze a deep observation of A521
recently performed with XMM-Newton in order to probe the cluster structure up
to the outermost regions covered by the radio emission. The cluster atmosphere
exhibits various brightness and temperature anisotropies. In particular, two
cluster cores appear to be separated by two cold fronts. We find two shock
fronts, one that was suggested by Chandra and that is propagating to the east,
and another to the southwestern cluster outskirt. The two main interacting
clusters appear to be separated by a shock heated region, which exhibits a
spatial correlation with the radio halo. The outer edge of the radio relic
coincides spatially with a shock front, suggesting this shock is responsible
for the generation of cosmic ray electrons in the relic. The propagation
direction and Mach number of the shock front derived from the gas density jump,
M = 2.4 +/- 0.2, are consistent with expectations from the radio spectral
index, under the assumption of Fermi I acceleration mechanism
Selecting background galaxies in weak-lensing analysis of galaxy clusters
In this paper, we present a new method to select the faint, background
galaxies used to derive the mass of galaxy clusters by weak lensing.
The method is based on the simultaneous analysis of the shear signal, that
should be consistent with zero for the foreground, unlensed galaxies, and of
the colors of the galaxies: photometric data from the COSMic evOlution Survey
are used to train the color selection. In order to validate this methodology,
we test it against a set of state-of-the-art image simulations of mock galaxy
clusters in different redshift [] and mass
[] ranges, mimicking medium-deep multicolor
imaging observations (e.g. SUBARU, LBT).
The performance of our method in terms of contamination by unlensed sources
is comparable to a selection based on photometric redshifts, which however
requires a good spectral coverage and is thus much more observationally
demanding. The application of our method to simulations gives an average ratio
between estimated and true masses of . As a further test,
we finally apply our method to real data, and compare our results with other
weak lensing mass estimates in the literature: for this purpose we choose the
cluster Abell 2219 (), for which multi-band (BVRi) data are publicly
available.Comment: MNRAS, Accepted 2016 February 2
One Actor, Multiple Roles: The Performances of Cryptochrome in Drosophila
Cryptochromes (CRYs) are flavoproteins that are sensitive to blue light, first identified in Arabidopsis and then in Drosophila and mice. They are evolutionarily conserved and play fundamental roles in the circadian clock of living organisms, enabling them to adapt to the daily 24-h cycles. The role of CRYs in circadian clocks differs among different species: in plants, they have a blue light-sensing activity whereas in mammals they act as light-independent transcriptional repressors within the circadian clock. These two different functions are accomplished by two principal types of CRYs, the light-sensitive plant/insect type 1 CRY and the mammalian type 2 CRY acting as a negative autoregulator in the molecular circadian clockwork. Drosophila melanogaster possesses just one CRY, belonging to type 1 CRYs. Nevertheless, this single CRY appears to have different functions, specific to different organs, tissues, and even subset of cells in which it is expressed. In this review, we will dissect the multiple roles of this single CRY in Drosophila, focusing on the regulatory mechanisms that make its pleiotropy possible
Non-hydrostatic gas in the core of the relaxed galaxy cluster A1795
Chandra data on A1795 reveal a mild edge-shaped discontinuity in the gas
density and temperature in the southern sector of the cluster at r=60/h kpc.
The gas inside the edge is 1.3-1.5 times denser and cooler than outside, while
the pressure is continuous, indicating that this is a "cold front", the surface
of contact between two moving gases. The continuity of the pressure indicates
that the current relative velocity of the gases is near zero, making the edge
appear to be in hydrostatic equilibrium. However, a total mass profile derived
from the data in this sector under the equilibrium assumption, exhibits an
unphysical jump by a factor of 2, with the mass inside the edge being lower. We
propose that the cooler gas is "sloshing" in the cluster gravitational
potential well and is now near the point of maximum displacement, where it has
zero velocity but nonzero centripetal acceleration. The distribution of this
non-hydrostatic gas should reflect the reduced gravity force in the
accelerating reference frame, resulting in the apparent mass discontinuity.
Assuming that the gas outside the edge is hydrostatic, the acceleration of the
moving gas can be estimated from the mass jump, a ~ 800 h km/s/(10^8 yr). The
gravitational potential energy of this gas that is available for dissipation is
about half of its current thermal energy. The length of the cool filament
extending from the cD galaxy (Fabian et al.) may give the amplitude of the gas
sloshing, 30-40/h kpc. Such gas bulk motion might be caused by a disturbance of
the central gravitational potential by past subcluster infall.Comment: Minor text clarifications to correspond to published version. 5
pages, 1 figure in color, uses emulateapj.sty. ApJ Letters in pres
Frozen to death? -- Detection of comet Hale-Bopp at 30.7 AU
Comet Hale--Bopp (C/1995 O1) has been the single most significant comet
encountered by modern astronomy, still having displayed significant activity at
25.7 AU solar distance in late 2007. It is a puzzling question when and where
this activity will finally cease. Here we present new observations with the ESO
2.2m telescope at La Silla to check the activity of Hale--Bopp at 30.7 AU solar
distance. On 2010-12-04, 26 CCD images were taken with 180 s exposure times for
photometry and morphology. The comet was detected in R and had a total
brightness of 23.3+-0.2 mag, referring to an absolute brightness of
R(1,1,0)=8.3. The profile of the coma was star-like at a seeing of 1.9",
without any evidence of a coma or tail extending farther than 2.5" (=55,000 km
in projection) and exceeding 26.5 mag/arcs^2 surface brightness. The measured
total brightness corresponds to a relative total reflecting surface, a_RC, of
485 km^2, nine times less than three years before. The calculated a_RC value
would imply a nucleus with 60--65 km radius assuming 4% albedo. This size
estimate is in significant contradiction with the previous results scattering
around 35 km. Therefore we suggest that the comet may still be in a low-level
activity, despite the lack of a prominent coma. Alternatively, if the nucleus
is already dormant, the albedo should be as high as 13%, assuming a radius of
35 km. With this observation, Hale--Bopp has been the most distant comet ever
observed, far beyond the orbit of Neptune.Comment: 4 pages, accepted by A&
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