Engineering topics

Cuzzi, Jeff C.

Publications and source records attributed to Cuzzi, Jeff C..

3 records

NASA NTRS1996

The Response of Disks to Oscillatory Modes in Galaxies

Recent studies suggest that galaxies can oscillate in normal modes with essentially no damping over a Hubble time. These modes may play an important role in the structure and evolution of disk/halo systems. Motivated by the possibility that normal mode oscillations exist in real galaxies, we are investigating the response of galactic disks to halo oscillations. The goal of these investigations is to ascertain whether or not observational signatures exist for such oscillations. Our approach is to perform numerical experiments on the response of a self-gravitating disk to a time-varying halo potential. We assume that a significant fraction of the mass in a galaxy is in a dark halo. The halo oscillates and the luminous disk material responds to these oscillations. Preliminary results are reported for disks embedded in a radially oscillating gravitational potential. The equilibrium initial disk is represented by an exponential density profile. Considerable care was taken to build an initial disk model that was "stable" over long time scales. A control experiment was run with the disk in a static halo potential. The disk responds to the time-varying potential by developing a ring structure, which forms and disappears during each halo oscillation cycle. The density of stars becomes depressed in an annular region at the radius where the disk epicycle frequency is equal to the halo oscillation period. This pattern of response persists over time periods approaching a Hubble time. In the oscillating potential, a bar develops in the inner disk. This bar is absent when the halo remains static. Specific targets of this study include the implications for large-scale disk structure, the gas dynamical response of the interstellar medium in such systems, and the inflow of material into the central regions of the galaxy.

Smith, Bruce F.↗

NASA NTRS1996

Observational Consequences of Galactic Oscillations

The normal mode oscillations of galaxies we reported recently are large enough to produce observational consequences. The two lowest order modes were both very strong and long lived: the fundamental is a "breathing" mode in which the entire galaxy expands and contracts homologously, while the second radial mode is spherically symmetrical and has one radial node. A sheet of neutral hydrogen in an oscillating galaxy partakes of the general oscillations, and the resulting velocity fields would normally be interpreted in terms of warps or oval distortions. More interestingly, the second radial mode might be a dynamical driver for bulges in early-type spiral galaxies. If bulges result from a dynamical process within the galaxy, there might be other observable properties driven by the same dynamical process. The hole often reported in neutral hydrogen in early type spiral galaxies is one such candidate. It has sometimes been reported to be about the same size as the bulge. We will discuss evidence from velocity fields concerning the fundamental mode and from the coincidence of bulge and HI hole sizes as evidence for the second mode.

Miller, R. H.↗

NASA NTRS1996

Cooling of the Interstellar Medium

This talk will review the various heating and cooling processes in the interstellar medium. The most important heating processes include the photoelectric effect on dust grains and PAH molecules. Cooling of the gas in the interstellar medium is dominated by emission in the far infrared fine-structure lines of OI, CII, SiII, and CI, and the rotational transitions of CO. Which of these heating and cooling processes dominates in a given region depends on its physical and especially chemical conditions, which in turn depend themselves on the interstellar UV radiation field. As a result of the interplay of these processes, the interstellar medium is organized in various structures with distinctly different physical conditions (i.e., temperature, density, and degree of ionization). The dominant cooling lines of the neutral interstellar medium have been studied using the Kuiper Airborne Observatory, balloon-borne instruments, and recently space-based missions (IRTS, ISO). These observations have concentrated on dense regions illuminated by strong UV fields from nearby stars, so-called PhotoDissociation Regions (PDRs). These observations allow us to study in detail the interaction of UV photons observations allow us to study in detail the interaction of UV photons and the interstellar gas and dust. Detailed theoretical models have been developed for PDRs. These will be reviewed and compared to the observations. Recent space based missions have measured the dominant cooling line of the galaxy, the [CII] 158 micron line. These observations and their implications will be reviewed.

Tielens, A. G. G. M.↗