525 research outputs found
Nuclear star cluster formation in energy-space
In a virialized stellar system, the mean-square velocity is a direct tracer
of the energy per unit mass of the system. Here, we exploit this to estimate
and compare root-mean-square velocities for a large sample of nuclear star
clusters and their host (late- or early-type) galaxies. Traditional
observables, such as the radial surface brightness and second-order velocity
moment profiles, are subject to short-term variations due to individual
episodes of matter infall and/or star formation. The total mass, energy and
angular momentum, on the other hand, are approximately conserved. Thus, the
total energy and angular momentum more directly probe the formation of galaxies
and their nuclear star clusters, by offering access to more fundamental
properties of the nuclear cluster-galaxy system than traditional observables.
We find that there is a strong correlation, in fact a near equality, between
the root-mean-square velocity of a nuclear star cluster and that of its host.
Thus, the energy per unit mass of a nuclear star cluster is always comparable
to that of its host galaxy. We interpret this as evidence that nuclear star
clusters do not form independently of their host galaxies, but rather that
their formation and subsequent evolution are coupled. We discuss how our
results can potentially be used to offer a clear and observationally testable
prediction to distinguish between the different nuclear star cluster formation
scenarios, and/or quantify their relative contributions.Comment: 12 pages, 3 figures, 4 tables; accepted for publication in MNRA
Анализ систем оценки информационно коммуникационных компетенций
The article considers the methods of development of information-analytical system of formation of information-communicative competence of students through the portal of "Electronic University of information technology"
Explaining two circumnuclear star forming rings in NGC5248
The distribution of gas in the central kiloparsec of a galaxy has a
dynamically rapid evolution. Nonaxisymmetries in the gravitational potential of
the galactic disk, such as a large scale stellar bar or spiral, can lead to
significant radial motion of gaseous material from larger radii to the central
region. The large influx of gas and the subsequent star formation keep the
central region constantly changing. However, the ability of gas to reach the
nucleus proper to fuel an AGN phase is not guaranteed. Gas inflow can be halted
at a circumnuclear star forming ring several hundred parsec away. The nearby
galaxy NGC5248 is especially interesting in this sense since it is said to host
2 circumnuclear star forming rings at 100pc and 370pc from its quiescent
nucleus. Here we present new subarcsecond PdBI+30m CO(2-1) emission line
observations of the central region. For the first time the molecular gas
distribution at the smallest stellar ring is resolved into a gas ring,
consistent with the presence of a quiescent nucleus. However, the molecular gas
shows no ring structure at the larger ring. We combine analyses of the gaseous
and stellar content in the central kiloparsec of this galaxy to understand the
gas distribution and dynamics of this star forming central region. We discuss
the probability of two scenarios leading to the current observations, given our
full understanding of this system, and discuss whether there are really two
circumnuclear star forming rings in this galaxy.Comment: Accepted for publication in A&A, 14pages + long tabl
Graph-Based Approach to the Edit Distance Cryptanalysis of Irregularly Clocked Linear Feedback Shift Registers
This paper proposes a speed-up of a known-plaintext attack on some stream ciphers
based on Linear Feedback Shift Registers (LFSRs). The algorithm consists of two basic steps:
first, to guess the initial seed value of one of the LFSRs, and then to use the resulting binary
sequence in order to deduce useful information about the cipher parameters. In particular, the
proposed divide-and-conquer attack is based on a combination of graph-based techniques with
edit distance concepts. While the original edit distance attack requires the exhaustive search over
the set of all possible initial states of the involved LFSR, this work presents a new heuristic optimization
that avoids the evaluation of an important number of initial states through the identification
of the most promising branches of the search graph. The strongest aspects of the proposal
are the facts that the obtained results from the attack are absolutely deterministic, and that many
inconsistent initial states of the target LFSRs are recognized and avoided during search.This work was supported by the Spanish Ministry of Science and Innovation and European
FEDER Fund under Project TIN2008-02236/TSI as well as by CDTI (Spain)and the companies INDRA, Unin Fenosa, Tecnobit, Visual Tool, Brainstorm, SAC and
Technosafe under Project Cenit-HESPERIA.Peer reviewe
Chandra Observations of the Nuclear Star Cluster and Ultraluminous X-ray Sources in NGC 2139
We report Chandra observations of the Scd galaxy NGC 2139, which is known to
host a recently formed (10^7.6 yrs) nuclear star cluster. The star cluster is
undetected in X-rays, with an upper bound on 0.5-7 keV luminosity of L_X < 7.1
x 10^37 erg/s. This bound implies a bolometric accretion luminosity <0.3
percent of the Eddington luminosity for a black hole with the mass
(approximately 3400 M_sun) expected from extrapolation of the M-sigma relation.
The lack of X-ray emission indicates that a black hole, if present, is not
undergoing significant accretion at the current time. While the central cluster
is undetected, the data reveal a substantial population of bright X-ray point
sources elsewhere in this galaxy, with eight qualifying as ultraluminous X-ray
sources with L_X > 10^39 erg/s. We use archival Hubble Space Telescope images
to identify candidate optical counterparts for seven Chandra sources, which in
most cases have optical luminosities and spatial profiles consistent with star
clusters. Compared with other galaxies, the number of luminous X-ray sources in
NGC 2139 is larger by a factor of 4 - 10 than expected based on its present
star formation rate and stellar mass. This finding can be understood if NGC
2139 has concluded a burst of star formation in the recent past, and suggests
that this galaxy could be important for testing the use of X-ray source
populations as a chronometer of star formation history.Comment: 17 pages, 3 figures, accepted for publication in A
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