Engineering topics

Benjamin, Robert A.

Publications and source records attributed to Benjamin, Robert A..

5 records

DOE OSTI2020

The Effect of Bars on the Ionized ISM: Optical Emission Lines from Milky Way Analogs

Gas interior to the bar of the Milky Way has recently been shown to be the closest example of a low-ionization (nuclear) emission region (LI(N)ER) in the universe. To better understand the nature of this gas, a sample of face-on galaxies with integral field spectroscopy is used to study the ionized gas conditions of 240 barred and 250 nonbarred galaxies, focusing on those that are most similar to the Milky Way. Strong optical line emission of [N ii] λ6584, Hα, [O iii] λ5007, and Hβ are used to diagnose the dominant ionization mechanisms of gas across galaxies and the Galaxy via Baldwin-Phillips-Terlevich diagrams. Barred galaxies show a strong suppression of star formation and an increase in composite and LI(N)ER-like spectra in their inner regions when compared with similar nonbarred counterparts. This effect is lessened in galaxies of very low (log 10 (M * /M ⊙ ) ≲ 10.4) or very high (log 10 (M * /M ⊙ ) ≳ 11.1) total stellar mass. Bar masks from Galaxy Zoo:3D show the bar's nonaxisymmetric effect on the ionized gas and help predict the face-on distribution of ionized gas conditions near the bar of the Milky Way.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2000

Vertical Shear of the Galactic Interstellar Medium

The detection of UV absorption, 21 cm, H alpha and other diffuse optical emission lines from gas up to ten kiloparsecs above the plane of the Milky Way and other galaxies provides the first, opportunity to probe the rotational properties of the ionized "atmospheres" of galaxies. This rotation has implications for our understanding of the Galactic gravitational potential, angular momentum transport in the Galactic disk, and the maintenance of a Galactic dynamo. The available evidence indicates that gas rotates nearly cylindrically up to a few kiloparsecs. This is in contrast to the expectation that there should be a significant gradient in rotation speed as a function of height assuming a reasonable mass model for the Galaxy. For example, for a vertical cut at galactocentric radius R = 5 kpc in NGC 891 by Rand, the rotation speed is observed to drop by approximately 30 kilometers per second from z = 1 to 5 kpc and is expected to drop by 80 kilometers per second. Magnetic tension forces may resolve this discrepancy. Other possibilities will be examined in the near future.

Benjamin, Robert A.↗

NASA NTRS1993

The absorption and emission spectrum of radiative cooling galactic fountain gas

We have calculated the time-dependent, nonequilibrium thermal and ionization history of gas cooling radiatively from 10(exp 6) K in a one-dimensional, planar, steady-state flow model of the galactic fountain, including the effects of radiative transfer. Our previous optically thin calculations explored the effects of photoionization on such a flow and demonstrated that self-ionization was sufficient to cause the flow to match the observed galactic halo column densities of C 4, Si 4, and N 5 and UV emission from C 4 and O 3 in the constant density (isochoric) limit, which corresponded to cooling regions homogeneous on scales D less than or approximately equal to 1 kpc. Our new calculations which take full account of radiative transfer confirm the importance of self-ionization in enabling such a flow to match the data but allow a much larger range for cooling region sizes, i.e. D(sub 0) greater than or approximately equal to 15 pc. For an initial flow velocity v(sub 0) approximately equal to 100 km/s, comparable to the sound speed of a 10(exp 6) K gas, the initial density is found to be n(sub h,0) is approximately 2 x 10(exp -2) cm(exp -3), in reasonable agreement with other observation estimates, and D(sub 0) is approximately equal to 40 pc. We also compare predicted H(alpha) fluxes, UV line emission, and broadband x-ray fluxes with observed values. One dimensional numerical hydrodynamical calculations including the effects of radiative cooling are also presented.

Benjamin, Robert A.↗

NASA NTRS1991

New results concerning the Galactic fountain

The time-dependent nonequilibrium thermal and ionization history is traced of hot optically-thin radiatively-cooling gas in a 1D planar steady-state flow model of the Galactic fountain. The calculation of such a flow which neglects photoionization is found to be unable to simultaneously reproduce the observed C IV, Si IV, and N V column densities through Galactic halo gas. It is shown that, when photoionization is taken into account, a fountain flow can match the observations of both UV absorption and emission lines for a range of ionizing fluxes which depends on the characteristic size of the cooling regions within the flow. What levels of the external photoionizing flux, such as that contributed by Galactic starlight, supernova remnants, and the metagalactic radiation background, can produce agreement with observational results are determined. The ionizing radiation emitted by the cooling gas itself is found to be sufficient to cause the flow to match the observed column densities and line emission.

Shapiro, Paul R.↗

NASA NTRS1990

Near-infrared spectroscopy of classical novae in the coronal phase

Spectroscopic observations of V1819 Cyg, V827 Her, and V2214 Oph, obtained at 1-2.5 microns using a 32-channel InSb array detector at the Cassegrain focus of the 2.7-m telescope at McDonald Observatory during 1986-1989, are reported. The data are presented in extensive tables and graphs and characterized in detail, with a focus on the nature of the 'coronal' emission. The ratios of Br-gamma (2.167 microns) and forbidden Si VI (1.960 microns) are found to be consistent with coronal gas temperatures of about 10,000 K and with photoionization as the primary mechanism of origin for the coronal species. Severe constraints on theoretical models involving shock heating and collisional ionization are inferred.

Benjamin, Robert A.↗