1,053 research outputs found
Starbursts and their dynamics
Detailed mechanisms associated with dynamical process occurring in starburst galaxies are considered including the role of bars, waves, mergers, sinking satellites, self gravitating gas and bulge heating. The current understanding of starburst galaxies both observational and theoretical is placed in the context of theories of galaxy formations, Hubble sequence evolution, starbursts and activity, and the nature of quasar absorption lines
Two-Phase Cooling Flows with Magnetic Reconnection
Motivated by the observations of high Faraday rotation measures measured in
cooling flow clusters we propose a model relevant to plasmas with comparable
thermal and magnetic pressures. Magnetic field reconnection may play a major
role in changing the topology of the magnetic field in the central cooling flow
regions. The effect of the topology change is that cool flux loops can
reconnect to hot flux loops that are connected to the overall thermal reservoir
of the cluster. There can be a rapid recycling of mass between hot and cold
phases on a time scale of 3 x 10^8-10^9 yr which may reduce the inferred inflow
and mass condensation rates by at least an order of magnitude. A central
multiphase medium is a direct consequence of such a model. Throughout the
cooling flow the filling factor of the hot loops (T > 2 x 10^7 K) is of order
unity. The filling factor of the cool loops (T < 2 x 10^7 K) is 0.1-1% with a
corresponding mass fraction of cold phase of 1-10%. A crucial parameter is the
coherence length of the field relative to the cooling radius and the
distribution of field energy with scale. When the cooling radius is greater
than the field coherence length then cooling flows proceed as usual. When the
coherence length is greater than the central cooling radius, however, the
thermal energy of the reservoir can be tapped and the mass condensation rates
may be very significantly reduced. Three additional conditions must be
satisfied: I. Cold loops must be able to fall at least as far as the mean
distance between hot loops in a cooling time; II. Loops must enter an
evaporative phase on reconnecting; and III. A sufficient number of hot loops
penetrate the cold phase region to power the radiative losses.Comment: 16 pages, uses AAS macro aasm
The connection between X-ray Clusters and Star Formation
The properties of X-ray clusters of galaxies can be well understood in terms
of a competition between shock heating and adiabatic compression. Strong shocks
are expected to be important for massive clusters, while adiabatic compression
is dominant for small clusters and groups. The scale of the shock/adiabatic
transition is marked by a change of slope of the L-T relation and in the global
properties of the emitting plasma. This scale is connected to star formation
processes. Two quantities are crucial: the average energy injected in the IGM
from stars and SNe, and the epoch of the energy injection. We show how these
quantities can be synthesized in terms of specific entropy, which ultimately
determines the X-ray emission from groups and clusters.Comment: 4 pages, including 2 figures, LaTex2e. To be published in the
Proceedings of the ``VLT Opening Symposium'', Antofagasta (Chile), 1-4 March
1999. Typos changed in eq. (3
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
The disk-halo interface in edge-on spirals
We are studying the disk-halo interface in several edge-on spiral galaxies through extensive imagery in H(alpha) and other emission lines from Diffuse Ionized Gas (DIG), also referred to as the Warm Ionized Medium (WIM). In addition, for the nearby Sc galaxy NGC4631 we have obtained x-ray observations with ROSAT, to map the distribution of hot (10(exp 6) - 10(exp 7)) gas in the disk and halo. Here we present initial results for two late-type spirals, NGC4244 and NGC4631
- …