arXiv subjects

Richard J. Rand

Publications and source records attributed to Richard J. Rand.

39 records · Page 3Linked to original sources

arXiv2000

Ionization, Kinematics, and Extent of the Diffuse Ionized Gas Halo of NGC 5775

We present key results from deep spectra of the Diffuse Ionized Gas (DIG) halo of the edge-on galaxy NGC 5775. [NII]6583 has been detected up to about z=13 kpc above the plane in one of two vertically oriented long slits -- making this the spiral galaxy with the greatest spectroscopically detected halo extent in emission. Key diagnostic line ratios have been measured up to about z=8 kpc, allowing the source of ionization and physical state to be probed. Ionization by a dilute radiation field from massive stars in the disk can explain some of the line ratio behavior, but departures from this picture are clearly indicated, most strongly by the rise of [OIII]/Halpha with z. Velocities of the gas in both slits approach the systemic velocity of the galaxy at several kpc above the plane. We interpret this trend as a decrease in rotation velocity with z, with essentially no rotation at heights of several kpc. Such a trend was observed in the edge-on galaxy NGC 891, but here much more dramatically. This falloff is presumably due to the gravitational potential changing with z, but will also depend on the hydrodynamic nature of the disk-halo cycling of gas and projection effects. More detailed modeling of the ionization and kinematics of this and other edge-ons will be presented in future papers.

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arXiv2000

Interferometric 12CO Observations of the Central Disk of NGC 4631: An Energetic Molecular Outflow

We present interferometric observations of CO J=1-0 emission in the central regions of the edge-on galaxy NGC 4631, known for its extended gaseous halo and its tidal interactions. Previous single-dish observations revealed that almost all of the CO emission arises from a central ring or bar-like structure of length $\sim$ 4 kpc. We confirm this structure at higher resolution, and find that it is bent at the center, reflecting the overall bend in this galaxy apparent from optical images. The kinematic evidence favors a rigidly rotating ring over a bar. The gaseous halo emission in several tracers is concentrated above and below this molecular structure. To the north of an emission peak at the eastern end of the structure is an extraplanar feature showing filamentary and shell-like properties which we interpret as an energetic molecular outflow. The energies involved are difficult to estimate, but are probably of order 10$^{54}$ ergs or more. The CO concentration in the disk below this structure coincides with a bright HII region complex, a peak of radio emission, and the brightest X-ray feature in the inner disk of the galaxy seen in a ROSAT HRI map, all suggesting intense star formation. A filament of radio continuum emission may also have a footprint in this region of the disk. The origin of the outflow is unclear.

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arXiv1998

Further Spectroscopy of the Diffuse Ionized Gas in NGC 891 and Evidence for a Secondary Source of Ionization

Two long-slit spectra of the diffuse ionized gas in NGC 891 are presented. The first reveals variations parallel to the major axis in emission line ratios in the halo gas at z=700 pc. It is found that filaments of Halpha emission show lower values of [NII]/Halpha, [SII]/Halpha and [OI]/Halpha. Although this result is expected if the filaments represent the walls of evacuated chimneys, it merely reflects a more general correlation of these ratios with Halpha surface brightness along the slit, and may simply arise from radiation dilution effects. Much more interesting information on the nature of this gaseous halo comes from the second observation, which shows the vertical dependence of [NII]/Halpha, [SII]/Halpha, [OI]/Halpha, and [OIII]/Hbeta through the brightest region of the DIG halo. The most surprising result, in complete contradiction to models in which the DIG is ionized by massive stars in the disk, is that [OIII]/Hbeta rises with height above the plane for z>1 kpc (even as [NII]/Halpha, [SII]/Halpha, and [OI]/Halpha are rising, in line with expectations from such models). The run of [SII]/[NII] is also problematic, showing essentially no contrast with z. The [OIII] emission probably arises from shocks, turbulent mixing layers, or some other secondary source of ionization. Composite models in which the line emission comes from a mix of photo-ionized gas and shocks or turbulent mixing layers are considered in diagnostic diagrams, with the result that many aspects of the data can be explained at a qualitative level.

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