660 research outputs found
Preliminary galaxy extraction from DENIS images
The extragalactic applications of NIR surveys are summarized with a focus on
the ability to map the interstellar extinction of our Galaxy. Very preliminary
extraction of galaxies on a set of 180 consecutive images is presented, and the
results illustrate some of the pitfalls in attempting an homogeneous extraction
of galaxies from these wide-angle and shallow surveys.Comment: Invited talk at "The Impact of Large-Scale Near-IR Sky Surveys",
meeting held in Tenerife, Spain, April 1996. 10 pages LaTeX with style file
and 4 PS files include
Star Formation in Violent and Normal Evolutionary Phases
Mergers of massive gas-rich galaxies trigger violent starbursts that - over
timescales of Myr and regions kpc - form massive and compact
star clusters comparable in mass and radii to Galactic globular clusters. The
star formation efficiency is higher by 1 - 2 orders of magnitude in these
bursts than in undisturbed spirals, irregulars or even BCDs. We ask the
question if star formation in these extreme regimes is just a scaled-up version
of the normal star formation mode of if the formation of globular clusters
reveals fundamentally different conditions.Comment: 4 pages To appear in The Evolution of Galaxies. II. Basic building
blocks, eds. M. Sauvage, G. Stasinska, L. Vigroux, D. Schaerer, S. Madde
THE AGE, MASS, and SIZE DISTRIBUTIONS of STAR CLUSTERS in M51
We present a new catalog of 3816 compact star clusters in the grand design spiral galaxy M51 based on observations taken with the Hubble Space Telescope. The age distribution of the clusters declines starting at very young ages, and can be represented by a power law, , with . No significant changes in the shape of the age distribution at different masses is observed. The mass function of the clusters younger than can also be described by a power law, , with . We compare these distributions with the predictions from various cluster disruption models, and find that they are consistent with models where clusters disrupt approximately independent of their initial mass, but not with models where lower mass clusters are disrupted earlier than their higher mass counterparts. We find that the half-light radii of clusters more massive than and with ages between 100 and 400 are larger by a factor of ≈3-4 than their counterparts that are younger than 107 years old, suggesting that the clusters physically expand during their early life.R. C. is grateful for support from NSF through CAREER award 0847467 and from NASA through grant GO-10501-01-A from STScI, which is operated by AURA, Inc., under NASA contract NAS5-26555
Secular Evolution of Galaxy Morphologies
Today we have numerous evidences that spirals evolve dynamically through
various secular or episodic processes, such as bar formation and destruction,
bulge growth and mergers, sometimes over much shorter periods than the standard
galaxy age of 10-15 Gyr. This, coupled to the known properties of the Hubble
sequence, leads to a unique sense of evolution: from Sm to Sa. Linking this to
the known mass components provides new indications on the nature of dark matter
in galaxies. The existence of large amounts of yet undetected dark gas appears
as the most natural option. Bounds on the amount of dark stars can be given
since their formation is mostly irreversible and requires obviously a same
amount of gas.Comment: 8 pages, Latex2e, crckapb.sty macros, 1 Postscript figure, replaced
with TeX source; To be published in the proceeedings of the "Dust-Morphology"
conference, Johannesburg, 22-26 January, 1996, D. Block (ed.), (Kluwer
Dordrecht
Rotation Curves of Spiral Galaxies
Rotation curves of spiral galaxies are the major tool for determining the
distribution of mass in spiral galaxies. They provide fundamental information
for understanding the dynamics, evolution and formation of spiral galaxies. We
describe various methods to derive rotation curves, and review the results
obtained. We discuss the basic characteristics of observed rotation curves in
relation to various galaxy properties, such as Hubble type, structure,
activity, and environment.Comment: 40 pages, 6 gif figures; Ann. Rev. Astron. Astrophys. Vol. 39, p.137,
200
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SUPERNOVA REMNANTS AND THE INTERSTELLAR MEDIUM OF M83: IMAGING AND PHOTOMETRY WITH THE WIDE FIELD CAMERA 3 ON THE HUBBLE SPACE TELESCOPE
We present Wide Field Camera 3 images taken with the Hubble Space Telescope within a single field in the southern grand design star-forming galaxy M83. Based on their size, morphology, and photometry in continuum-subtracted Hα, [S II], Hβ, [O III], and [O II] filters, we have identified 60 supernova remnant (SNR) candidates, as well as a handful of young ejecta-dominated candidates. A catalog of these remnants, their sizes and, where possible, their Hα fluxes are given. Radiative ages and pre-shock densities are derived from those SNRs that have good photometry. The ages lie in the range 2.62 \u3c log (τrad/yr) \u3c 5.0, and the pre-shock densities at the blast wave range over 0.56 \u3c n 0/cm-3 \u3c 1680. Two populations of SNRs have been discovered. These divide into a nuclear and spiral arm group and an inter-arm population. We infer an arm to inter-arm density contrast of 4. The surface flux in diffuse X-rays is correlated with the inferred pre-shock density, indicating that the warm interstellar medium (ISM) is pressurized by the hot X-ray plasma. We also find that the ISM in the nuclear region of M83 is characterized by a very high porosity and pressure, and infer an SNR rate of 1 per 70-150 yr for the nuclear (R \u3c 300 pc) region. On the basis of the number of SNRs detected and their radiative ages, we infer that the lower mass of Type II SNe in M83 is M min = 16+7 –5 M . Finally, we give evidence for the likely detection of the remnant of the historical supernova, SN1968L
Secular evolution versus hierarchical merging: galaxy evolution along the Hubble sequence, in the field and rich environments
In the current galaxy formation scenarios, two physical phenomena are invoked
to build disk galaxies: hierarchical mergers and more quiescent external gas
accretion, coming from intergalactic filaments. Although both are thought to
play a role, their relative importance is not known precisely. Here we consider
the constraints on these scenarios brought by the observation-deduced star
formation history on the one hand, and observed dynamics of galaxies on the
other hand: the high frequency of bars and spirals, the high frequency of
perturbations such as lopsidedness, warps, or polar rings.
All these observations are not easily reproduced in simulations without
important gas accretion. N-body simulations taking into account the mass
exchange between stars and gas through star formation and feedback, can
reproduce the data, only if galaxies double their mass in about 10 Gyr through
gas accretion. Warped and polar ring systems are good tracers of this
accretion, which occurs from cold gas which has not been virialised in the
system's potential. The relative importance of these phenomena are compared
between the field and rich clusters. The respective role of mergers and gas
accretion vary considerably with environment.Comment: 18 pages, 8 figures, review paper to "Penetrating Bars through Masks
of Cosmic Dust: the Hubble Tuning Fork Strikes a New Note", Pilanesberg, ed.
D. Block et al., Kluwe
The formation of massive black holes through collision runaway in dense young star clusters
A luminous X-ray source is associated with a cluster (MGG-11) of young stars
\~200pc from the center of the starburst galaxy M82. The properties of the
X-ray source are best explained by a black hole with a mass of at least
350Msun, which is intermediate between stellar-mass and supermassive black
holes. A nearby but somewhat more massive star cluster (MGG-9) shows no
evidence of such an intermediate mass black hole, raising the issue of just
what physical characteristics of the clusters can account for this difference.
Here we report numerical simulations of the evolution and the motions of stars
within the clusters, where stars are allowed to mergers with each other. We
find that for MGG-11 dynamical friction leads to the massive stars sinking
rapidly to the center of the cluster to participate in a runaway collision,
thereby producing a star of 800-3000Msun, which ultimately collapses to an
black hole of intermediate mass. No such runaway occurs in the cluster MGG-9
because the larger cluster radius leads to a mass-segregation timescale a
factor of five longer than for MGG-11.Comment: Accepted for publication in Nature (Including supplementary
information
Galaxies appear simpler than expected
Galaxies are complex systems the evolution of which apparently results from
the interplay of dynamics, star formation, chemical enrichment, and feedback
from supernova explosions and supermassive black holes. The hierarchical theory
of galaxy formation holds that galaxies are assembled from smaller pieces,
through numerous mergers of cold dark matter. The properties of an individual
galaxy should be controlled by six independent parameters including mass,
angular-momentum, baryon-fraction, age and size, as well as by the accidents of
its recent haphazard merger history. Here we report that a sample of galaxies
that were first detected through their neutral hydrogen radio-frequency
emission, and are thus free of optical selection effects, shows five
independent correlations among six independent observables, despite having a
wide range of properties. This implies that the structure of these galaxies
must be controlled by a single parameter, although we cannot identify this
parameter from our dataset. Such a degree of organisation appears to be at odds
with hierarchical galaxy formation, a central tenet of the cold dark matter
paradigm in cosmology.Comment: 26 pages, 14 figure
Cellular Radiosensitivity: How much better do we understand it?
Purpose: Ionizing radiation exposure gives rise to a variety of lesions in DNA that result in genetic instability and potentially tumorigenesis or cell death. Radiation extends its effects on DNA by direct interaction or by radiolysis of H2O that generates free radicals or aqueous electrons capable of interacting with and causing indirect damage to DNA. While the various lesions arising in DNA after radiation exposure can contribute to the mutagenising effects of this agent, the potentially most damaging lesion is the DNA double strand break (DSB) that contributes to genome instability and/or cell death. Thus in many cases failure to recognise and/or repair this lesion determines the radiosensitivity status of the cell. DNA repair mechanisms including homologous recombination (HR) and non-homologous end-joining (NHEJ) have evolved to protect cells against DNA DSB. Mutations in proteins that constitute these repair pathways are characterised by radiosensitivity and genome instability. Defects in a number of these proteins also give rise to genetic disorders that feature not only genetic instability but also immunodeficiency, cancer predisposition, neurodegeneration and other pathologies.
Conclusions: In the past fifty years our understanding of the cellular response to radiation damage has advanced enormously with insight being gained from a wide range of approaches extending from more basic early studies to the sophisticated approaches used today. In this review we discuss our current understanding of the impact of radiation on the cell and the organism gained from the array of past and present studies and attempt to provide an explanation for what it is that determines the response to radiation
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