644 research outputs found
Paired and interacting galaxies: Conference summary
The author gives a summary of the conference proceedings. The conference began with the presentation of the basic data sets on pairs, groups, and interacting galaxies with the latter being further discussed with respect to both global properties and properties of the galactic nuclei. Then followed the theory, modelling and interpretation using analytic techniques, simulations and general modelling for spirals and ellipticals, starbursts and active galactic nuclei. Before the conference the author wrote down the three questions concerning pairs, groups and interacting galaxies that he hoped would be answered at the meeting: (1) How do they form, including the role of initial conditions, the importance of subclustering, the evolution of groups to compact groups, and the fate of compact groups; (2) How do they evolve, including issues such as relevant timescales, the role of halos and the problem of overmerging, the triggering and enhancement of star formation and activity in the galactic nuclei, and the relative importance of dwarf versus giant encounters; and (3) Are they important, including the frequency of pairs and interactions, whether merging and interactions are very important aspects of the life of a normal galaxy at formation, during its evolution, in forming bars, shells, rings, bulges, etc., and in the formation and evolution of active galaxies? Where possible he focuses on these three central issues in the summary
The disk-halo connection and the nature of the interstellar medium
Some results on the nature of the interstellar medium that are specifically concerned with the disk-halo interaction are discussed. Over the last five years or so it has become clear that the supernovae rate in our Galaxy is spatially clumped and the consequences of such clumping are superbubbles and supershells fed by tens or hundreds of supernovae per shell. These objects evolve and expand rapidly and soon break out of the disk of the Galaxy, feeding the halo with very significant mass, energy, and momentum. As cooling occurs, gas will rain down onto the disk of the Galaxy completing the cycle. The basic flow of physical quantities from disk to halo and vice versa are discussed. Some of the many implications are noted including aspects of dynamo theory, quasar absorption lines, the theory of galactic coronae, and the nature of the x ray background. The essential difference here with the McKee-Ostriker (1977) theory is that the filling factor of the hot gas in the disk is significantly less than unity
Dissecting the IRX - dust attenuation relation: exploring the physical origin of observed variations in galaxies
The use of ultraviolet (UV) emission as a tracer of galaxy star-formation
rate (SFR) is hampered by dust obscuration. The empirical relationship between
UV slope, , and the ratio between far-infrared and UV luminosity, IRX,
is commonly employed to account for obscured UV emission. We present a simple
model that explores the physical origin of variations in the IRX - dust
attenuation relation. A relative increase in FUV attenuation compared to NUV
attenuation and an increasing stellar population age cause variations towards
red UV slopes for a fixed IRX. Dust geometry effects (turbulence, dust screen
with holes, mixing of stars within the dust screen, two-component dust model)
cause variations towards blue UV slopes. Poor photometric sampling of the UV
spectrum causes additional observational variations. We provide an analytic
approximation for the IRX - relation invoking a subset of the explored
physical processes (dust type, stellar population age, turbulence). We discuss
observed variations in the IRX - relation for local (sub-galactic
scales) and high-redshift (normal and dusty star-forming galaxies, galaxies
during the epoch of reionization) galaxies in the context of the physical
processes explored in our model. High spatial resolution imaging of the UV and
sub-mm emission of galaxies can constrain the IRX - dust attenuation
relation for different galaxy types at different epochs, where different
processes causing variations may dominate. These constraints will allow the use
of the IRX - relation to estimate intrinsic SFRs of galaxies, despite
the lack of a universal relation.Comment: Accepted for publication in MNRA
Obscuring Material around Seyfert Nuclei with Starbursts
The structure of obscuring matter in the environment of active galactic
nuclei with associated nuclear starbursts is investigated using 3-D
hydrodynamical simulations. Simple analytical estimates suggest that the
obscuring matter with energy feedback from supernovae has a torus-like
structure with a radius of several tens of parsecs and a scale height of about
10 pc. These estimates are confirmed by the fully non-linear numerical
simulations, in which the multi-phase inhomogeneous interstellar matter and its
interaction with the supernovae are consistently followed. The globally stable,
torus-like structure is highly inhomogeneous and turbulent. To achieve the high
column densities (> 10^{24} cm^{-2}) as suggested by observations of some
Seyfert 2 galaxies with nuclear starbursts, the viewing angle should be larger
than about 70 degree from the pole-on for a 10^8 solar mass massive black hole.
Due to the inhomogeneous internal structure of the torus, the observed column
density is sensitive to the line-of-sight, and it fluctuates by a factor of
order 100. The covering fraction for N > 10^{23} cm^{-2} is about 0.4. The
average accretion rate toward R < 1 pc is 0.4 solar mass/yr, which is boosted
to twice that in the model without the energy feedback.Comment: ApJL in press (4 pages, 3 figures) A gziped ps file with high
resolution figures is available at http://th.nao.ac.jp/~wada/AGN
The Turbulent Interstellar Medium: Generalizing to a Scale-dependent Phase Continuum
We discuss the likely sources of turbulence in the ISM and explicitly
calculate the detailed grand source function for the conventional sources of
turbulence from supernovae, superbubbles, stellar winds and HII regions. We
find that the turbulent pumping due to the grand source function is broad band,
consequently expanding the inertial range of the cascade. We investigate the
general properties of the turbulent spectrum using a simple approach based on a
spectral transfer equation derived from the hydrodynamic Kovasznay
approximation. There are two major findings in this paper: I. The turbulent
pressure is calculated from the grand source function II. With the scale
dependent energy dissipation from a turbulent cascade the multi-phase medium
concept can be generalized to a more natural continuum description where
density and temperature are functions of scale. Approximate power-law behavior
is seen over a large dynamic range. We discuss some of the implications of
models for the ISM, particularly addressing the energy balance of the warm
neutral phase.Comment: 26 pages, aasms LaTeX, 5 figure
The Global Structure and Evolution of a Self-Gravitating Multi-phase Interstellar Medium in a Galactic Disk
Using high resolution, two-dimensional hydrodynamical simulations, we
investigate the evolution of a self-gravitating multi-phase interstellar medium
in the central kiloparsec region of a galactic disk. We find that a
gravitationally and thermally unstable disk evolves, in a self-stabilizing
manner, into a globally quasi-stable disk that consists of cold (T < 100 K),
dense clumps and filaments surrounded by hot (T > 10^4 K), diffuse medium. The
quasi-stationary, filamentary structure of the cold gas is remarkable. The hot
gas, characterized by low-density holes and voids, is produced by shock
heating. The shocks derive their energy from differential rotation and
gravitational perturbations due to the formation of cold dense clumps. In the
quasi-stable phase where cold and dense clouds are formed, the effective
stability parameter, Q, has a value in the range 2-5. The dynamic range of our
multi-phase calculations is 10^6 - 10^7 in both density and temperature. Phase
diagrams for this turbulent medium are analyzed and discussed.Comment: 10 pages, 3 figures, ApJ Letters in press (vol. 516
The ROSAT Deep Cluster Survey: the X-ray Luminosity Function out to z=0.8
We present the X-ray Luminosity Function (XLF) of the ROSAT Deep Cluster
Survey (RDCS) sample over the redshift range 0.05-0.8. Our results are derived
from a complete flux-limited subsample of 70 galaxy clusters, representing the
brightest half of the total sample, which have been spectroscopically
identified down to the flux limit of 4*10^{-14} erg/cm^2/s (0.5-2.0 keV) and
have been selected via a serendipitous search in ROSAT-PSPC pointed
observations. The redshift baseline is large enough that evolutionary effects
can be studied within the sample. The local XLF (z < 0.25) is found to be in
excellent agreement with previous determinations using the ROSAT All-Sky Survey
data. The XLF at higher redshifts, when combined with the deepest number counts
constructed to date (f>2*10^{-14} arg/cm^2/s), reveal no significant evolution
at least out to z=0.8, over a luminosity range 2*10^{42}-3*10^{44} erg/s in the
[0.5-2 keV] band. These findings extend the study of cluster evolution to the
highest redshifts and the faintest fluxes probed so far in X-ray surveys. They
complement and do not necessarily conflict with those of the Einstein Extended
Medium Sensitivity Survey, leaving the possibility of negative evolution of the
brightest end of the XLF at high redshifts.Comment: 12 pages, 4 figures, LaTeX (aasms4.sty). To appear in ApJ Letter
Active Beam Shaping System and Method Using Sequential Deformable Mirrors
An active optical beam shaping system includes a first deformable mirror arranged to at least partially intercept an entrance beam of light and to provide a first reflected beam of light, a second deformable mirror arranged to at least partially intercept the first reflected beam of light from the first deformable mirror and to provide a second reflected beam of light, and a signal processing and control system configured to communicate with the first and second deformable mirrors. The first deformable mirror, the second deformable mirror and the signal processing and control system together provide a large amplitude light modulation range to provide an actively shaped optical beam
- …