اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدHoles within galaxies: the egg or the hen?
40
0
0.0
(
0
)
تأليف
R. Caimmi
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
Unsustained matter distributions unescapely collapse unless fragmentation and centrifugal or pressure support take place. Starting from the above evidence, supermassive compact objects at the centre of large-mass galaxies are conceived as the end-product of the gravitational collapse of local density maxima around which overdensities are located. At the beginning of evolution, local density maxima are idealized as homogeneous peaks, while the surrounding envelopes are described by a power-law density profile. The dependence of the density profile on a second parameter, chosen to be the ratio between peak and total mass, is analysed. Overdensity evolution is discussed in the context of quintessence cosmological models and further investigation is devoted to a special case with the aim to describe the central collapse. An empirical relation between hole and dark halo mass is translated into a dependence of the fractional hole mass on the overdensity mass. Computations are performed up to the end of central collapse, and density profiles of related configurations are determined together with additional parameters. The central collapse is completed in early times, no longer than a few hundredths of Gyr, which implies hole formation when proto-haloes, proto-bulges, and proto-disks are still expanding or contracting. No appreciable change in evolution is found with regard to different mean peak heights related to equal masses. On the other hand, it is recognized that homogeneous peaks collapse ``faster with respect to surroundings envelopes, in low-mass than in large-mass overdensities. In conclusion, it is inferred that gravitational collapse of homogeneous peaks within overdensities may be a viable mechanism for hole generation.
قيم البحث
اقرأ أيضاً
Scaling theory generates transferable (even universal) algebraic and geometrical relations between the amino acid sequences and the aggregation functions of the three titled radically different proteins. In addition to the two hydropathicity scales a
nd beta strand scales used in earlier p53 work, a second beta strand Hot Spot scale is shown to yield very accurate results for oligomerization of p53, the tumor suppressor. These algebraic and geometrical relations could be caused topologically by the dominance of protein-protein aggregation by interactions in a membrane catalytic surface layer.
A great flourish of interest in the development of new high-strength and high-toughness materials is taking place in contemporary materials science, with the aim of surpassing the mechanical properties of commercial high-performance fibres. Recently,
macroscopic buckypapers, nanotube bundles and graphene sheets have been manufactured. While their macroscopic strength remains 1-2 orders of magnitude lower than their theoretical strength, and is thus comparable to that of current commercial fibres, recent progress has been made in significantly increasing toughness. In particular, researchers have produced extremely tough nanotube fibres with toughness modulus values of up to 570 J/g, 870 J/g and very recently, including graphene, reaching 970 J/g, thus well surpassing that of spider silk (170 J/g, with a record for a giant riverine orb spider of 390 J/g and Kevlar (80 J/g). In this letter, thanks to a new paradigm based on structural mechanics rather than on materials science, we present the Egg of Columbus for making fibres with unprecedented toughness: a slider, in the simplest form just a knot, is introduced as smart frictional element to dissipate energy and in general to reshape the fibre constitutive law, showing evidence of a previously hidden toughness, strictly related to the specific strength of the material. The result is a nearly perfectly plastic constitutive law, with a shape mimicking that of spider silk. The proof of concept is experimentally realized making the world toughest fibre, increasing the toughness modulus of a commercial Endumax fibre from 44 J/g up to 1070 J/g. The maximal achievable toughness is expected for graphene, with an ideal value of 100000 J/g.
We consider constraints on primordial black holes (PBHs) in the mass range $( 10^{-18}text{-}10^{15} ),M_{odot}$ if the dark matter (DM) comprises weakly interacting massive particles (WIMPs) which form halos around them and generate $gamma$-rays by
annihilations. We first study the formation of the halos and find that their density profile prior to WIMP annihilations evolves to a characteristic power-law form. Because of the wide range of PBH masses considered, our analysis forges an interesting link between previous approaches to this problem. We then consider the effect of the WIMP annihilations on the halo profile and the associated generation of $gamma$-rays. The observed extragalactic $gamma$-ray background implies that the PBH DM fraction is $f^{}_{rm PBH} lesssim 2 times 10^{-9},( m_{chi} / {rm TeV} )^{1.1}$ in the mass range $2 times 10^{-12},M_{odot},( m_{chi} / {rm TeV} )^{-3.2} lesssim M lesssim 5 times 10^{12},M_{odot},( m_{chi} / {rm TeV} )^{1.1}$, where $m_{chi}$ and $M$ are the WIMP and PBH masses, respectively. This limit is independent of $M$ and therefore applies for any PBH mass function. For $M lesssim 2times 10^{-12},M_{odot},( m_{chi}/ {rm TeV} )^{-3.2}$, the constraint on $f^{}_{rm PBH}$ is a decreasing function of $M$ and PBHs could still make a significant DM contribution at very low masses. We also consider constraints on WIMPs if the DM is mostly PBHs. If the merging black holes recently discovered by LIGO/Virgo are of primordial origin, this would rule out the standard WIMP DM scenario. More generally, the WIMP DM fraction cannot exceed $10^{-4}$ for $M > 10^{-9},M_{odot}$ and $m_{chi} > 10,$GeV. There is a region of parameter space, with $M lesssim 10^{-11},M_{odot}$ and $m_{chi} lesssim 100,$GeV, in which WIMPs and PBHs can both provide some but not all of the DM, so that one requires a third DM candidate.
We present new optical STIS HST spectroscopic observations of the jets of the proto-planetary nebula Hen 3-1475. The excitation conditions of the knots of Hen 3-1475 are derived from the observed optical spectra, confirming that the knots are shock e
xcited. The shocked spectra are qualitatively reproduced by simple ``3/2D bow shock models. We present a set of bow shock models devoted to planetary nebulae, and discuss the effects of the pre-ionization conditions, the bow shock velocity, the bow shock shape and the chemical abundances on the predicted spectra. To explore the reliability of the ``3/2D bow shock models, we also compare the observed spectra of other three proto-planetary nebulae (M 1-92, M 2-56 and CRL 618) to the predicted spectra.
We present a catalog of 1921 spectroscopic redshifts measured in the fields of the massive galaxy clusters MACSJ0416.1--2403 ($z=0.397$), MACSJ0717.5+3745 ($z=0.546$), and MACSJ1149.5+2223 ($z=0.544$), i.e., three of the four clusters selected by STS
cI as the targets of the Frontier Fields (FF) initiative for studies of the distant Universe via gravitational lensing. Compiled in the course of the MACS project (Massive Cluster Survey) that detected the FF clusters, this catalog is provided to the community for three purposes: (1) to allow the identification of cluster members for studies of the galaxy population of these extreme systems, (2) to facilitate the removal of unlensed galaxies and thus reduce shear dilution in weak-lensing analyses, and (3) to improve the calibration of photometric redshifts based on both ground- and spacebased observations of the FF clusters.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
