12 research outputs found
The Formation of the First Massive Black Holes
Supermassive black holes (SMBHs) are common in local galactic nuclei, and
SMBHs as massive as several billion solar masses already exist at redshift z=6.
These earliest SMBHs may grow by the combination of radiation-pressure-limited
accretion and mergers of stellar-mass seed BHs, left behind by the first
generation of metal-free stars, or may be formed by more rapid direct collapse
of gas in rare special environments where dense gas can accumulate without
first fragmenting into stars. This chapter offers a review of these two
competing scenarios, as well as some more exotic alternative ideas. It also
briefly discusses how the different models may be distinguished in the future
by observations with JWST, (e)LISA and other instruments.Comment: 47 pages with 306 references; this review is a chapter in "The First
Galaxies - Theoretical Predictions and Observational Clues", Springer
Astrophysics and Space Science Library, Eds. T. Wiklind, V. Bromm & B.
Mobasher, in pres
Host response mechanisms in periodontal diseases
Periodontal diseases usually refer to common inflammatory disorders known as gingivitis and periodontitis, which are caused by a pathogenic microbiota in the subgingival biofilm, including Porphyromonas gingivalis, Aggregatibacter actinomycetemcomitans, Tannerella forsythia and Treponema denticola that trigger innate, inflammatory, and adaptive immune responses. These processes result in the destruction of the tissues surrounding and supporting the teeth, and eventually in tissue, bone and finally, tooth loss. The innate immune response constitutes a homeostatic system, which is the first line of defense, and is able to recognize invading microorganisms as non-self, triggering immune responses to eliminate them. In addition to the innate immunity, adaptive immunity cells and characteristic cytokines have been described as important players in the periodontal disease pathogenesis scenario, with a special attention to CD4+ T-cells (T-helper cells). Interestingly, the T cell-mediated adaptive immunity development is highly dependent on innate immunity-associated antigen presenting cells, which after antigen capture undergo into a maturation process and migrate towards the lymph nodes, where they produce distinct patterns of cytokines that will contribute to the subsequent polarization and activation of specific T CD4+ lymphocytes. Skeletal homeostasis depends on a dynamic balance between the activities of the bone-forming osteoblasts (OBLs) and bone-resorbing osteoclasts (OCLs). This balance is tightly controlled by various regulatory systems, such as the endocrine system, and is influenced by the immune system, an osteoimmunological regulation depending on lymphocyte- and macrophage-derived cytokines. All these cytokines and inflammatory mediators are capable of acting alone or in concert, to stimulate periodontal breakdown and collagen destruction via tissue-derived matrix metalloproteinases, a characterization of the progression of periodontitis as a stage that presents a significantly host immune and inflammatory response to the microbial challenge that determine of susceptibility to develop the destructive/progressive periodontitis under the influence of multiple behavioral, environmental and genetic factors
Active galactic nuclei: what’s in a name?
Active Galactic Nuclei (AGN) are energetic astrophysical sources powered by
accretion onto supermassive black holes in galaxies, and present unique
observational signatures that cover the full electromagnetic spectrum over more
than twenty orders of magnitude in frequency. The rich phenomenology of AGN has
resulted in a large number of different "flavours" in the literature that now
comprise a complex and confusing AGN "zoo". It is increasingly clear that these
classifications are only partially related to intrinsic differences between
AGN, and primarily reflect variations in a relatively small number of
astrophysical parameters as well the method by which each class of AGN is
selected. Taken together, observations in different electromagnetic bands as
well as variations over time provide complementary windows on the physics of
different sub-structures in the AGN. In this review, we present an overview of
AGN multi-wavelength properties with the aim of painting their "big picture"
through observations in each electromagnetic band from radio to gamma-rays as
well as AGN variability. We address what we can learn from each observational
method, the impact of selection effects, the physics behind the emission at
each wavelength, and the potential for future studies. To conclude we use these
observations to piece together the basic architecture of AGN, discuss our
current understanding of unification models, and highlight some open questions
that present opportunities for future observational and theoretical progress.Comment: Accepted for publication in Astronomy & Astrophysics Review, 56
pages, 25 figure