8,988 research outputs found
Triclosan: An Instructive Tale
The Food and Drug Administration (FDA) recently released a final rule to ban triclosan and 18 other antimicrobial chemicals from soaps. We applaud this rule specifically because of the associated risks that triclosan poses to the spread of antibiotic resistance throughout the environment. This persistent chemical constantly stresses bacteria to adapt, and behavior that promotes antibiotic resistance needs to be stopped immediately when the benefits are null
Mechanical Feedback from Active Galactic Nuclei in Galaxies, Groups, and Clusters
The radiative cooling timescales at the centers of hot atmospheres
surrounding elliptical galaxies, groups, and clusters are much shorter than
their ages. Therefore, hot atmospheres are expected to cool and to form stars.
Cold gas and star formation are observed in central cluster galaxies but at
levels below those expected from an unimpeded cooling flow. X-ray observations
have shown that wholesale cooling is being offset by mechanical heating from
radio active galactic nuclei. Feedback is widely considered to be an important
and perhaps unavoidable consequence of the evolution of galaxies and
supermassive black holes. We show that cooling X-ray atmospheres and the
ensuing star formation and nuclear activity are probably coupled to a
self-regulated feedback loop. While the energetics are now reasonably well
understood, other aspects of feedback are not. We highlight the problems of
atmospheric heating and transport processes, accretion, and nuclear activity,
and we discuss the potential role of black hole spin. We discuss X-ray imagery
showing that the chemical elements produced by central galaxies are being
dispersed on large scales by outflows launched from the vicinity of
supermassive black holes. Finally, we comment on the growing evidence for
mechanical heating of distant cluster atmospheres by radio jets and its
potential consequences for the excess entropy in hot halos and a possible
decline in the number of distant cooling flows.Comment: Accepted for publication in New Journal of Physics Focus Issue on
Clusters of Galaxie
Time-Series Ensemble Photometry and the Search for Variable Stars in the Open Cluster M11
This work presents the first large-scale photometric variability survey of
the intermediate age (~200 Myr) open cluster M11. Thirteen nights of data over
two observing seasons were analyzed (using crowded field and ensemble
photometry techniques) to obtain high relative precision photometry. In this
study we focus on the detection of candidate member variable stars for
follow-up studies. A total of 39 variable stars were detected and can be
categorized as follows: 1 irregular (probably pulsating) variable, 6 delta
Scuti variables, 14 detached eclipsing binary systems, 17 W UMa variables, and
1 unidentified/candidate variable. While previous proper motion studies allow
for cluster membership determination for the brightest stars, we find that
membership determination is significantly hampered below V=15,R=15.5 by the
large population of field stars overlapping the cluster MS. Of the brightest
detected variables that have a high likelihood of cluster membership, we find
five systems where further work could help constrain theoretical stellar
models, including one potential W UMa member of this young cluster.Comment: 38 pages, 13 figures, accepted for December 2005 AJ, high-resolution
version available upon reques
Active galactic nucleus feedback in clusters of galaxies
Observations made during the last ten years with the Chandra X-ray
Observatory have shed much light on the cooling gas in the centers of clusters
of galaxies and the role of active galactic nucleus (AGN) heating. Cooling of
the hot intracluster medium in cluster centers can feed the supermassive black
holes found in the nuclei of the dominant cluster galaxies leading to AGN
outbursts which can reheat the gas, suppressing cooling and large amounts of
star formation. AGN heating can come in the form of shocks, buoyantly rising
bubbles that have been inflated by radio lobes, and the dissipation of sound
waves.Comment: Refereed review article published in Chandra's First Decade of
Discovery Special Feature edition of the Proceedings of the National Academy
of Science
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