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NASA NTRS1985

Carbon monoxide isotope ratios in galactic centers and disks

For each of the six galaxies whose mm-wavelength emission of CO and C-13O were observed from both the center and the disk, and the ratio of integrated intensities P(CO)/P(C-13O) is noted to vary markedly from galactic center to galactic center, from galactic disk to galactic disk, and from point to point within a given galaxy. The observed integrated intensity ratios vary from 3 to 16, indicating that the use of a constant P(CO) to sigma(H2) conversion to derive H2 surface densities from extragalactic CO observations can produce major errors.

Rickard, L. J.↗
NASA NTRS1985

The neutral-gas disk around the galactic center

The mapping of far-infrared C(+) and O(0) fine-structure lines and the first detection of far-infrared CO and OH rotational line emission and OH absorption toward the Galactic center are reported. These measurments are compared with far-infrared and radio continuum maps, with recent observations of the 12.8 micron forbidden Ne II line, and with measurements of CO 2.6 mm emission and H I 21 cm absorption. The new observations clearly show that there is a 10 pc diameter neutral gas disk or torus of several times 10,000 solar masses surrounding the inner ionized cavity. The disk has an inner radius of 1.7 pc, is inclined by about 20 deg to the Galactic plane, and may be quite thin. The atomic gas in the neutral disk is dense and fills less than 10 percent of the volume. The disk rotates about the Galactic center about an axis remarkably similar to that of the Galactic rotation at much larger scales.

Genzel, R.↗
NASA NTRS1985

A wide latitude CO survey of molecular clouds in the III galactic quadrant

Molecular clouds in the third galactic quadrant from 180-280 galactic longitude were surveyed in the 115.271 GHz line of CO at an angular resolution of 0.5 deg. In galactic latitude the survey covers from b = -5 to +5 deg, with a few extensions to higher latitudes. A preliminary analysis of the data allowed the determination of distances and masses of at least 30 molecular clouds related to the Perseus arm and to the Cygnus, or outer, arm. The studied clouds are located between 2.5 kpc and 10.7 kpc from the sun with most of the masses between 30,000-700,000 solar masses, and only three up to two million solar masses. The most interesting result comes from the distribution of the clouds distance from the galactic plane that indicates a warp in the arms to which these clouds belong.

May, J.↗
NASA NTRS1986

The galactic corona

The IUE observations and theories involving the gaseous galactic corona of the Milky Way are reviewed. Observational topics emphasized include results relevant to the z distribution, kinematics, and ionic and elemental abundances of the gas. The theories for the support, ionization, and kinematics of the galactic halo appear to require a merging of the ideas associated with the galactic fountain models and the photoionized halo models. The Si IV found in the galactic halo is likely mostly produced by photoionization while the N V is probably mostly produced by collisional ionization in cooling fountain gas; C IV may represent an intermediate case with important contributions from photoionization and from collisional ionization.

Savage, Blair D.↗
NASA NTRS1987

The origin of the diffuse galactic IR/submm emission: Revisited after IRAS

Balloon observations are compared with Infrared Astronomy Satellite observations. There was good agreement for the longitudinal profiles. However, the dust emission observed by IRAS, contrary to the balloon observations which show dust emission only within the absolute value of b is equal to or less than 3 degrees, extends all the way to the galactic pole. The model fits were repeated using more recent parameters for the distribution of interstellar matter in the galactic disk and central region. The IR luminosities are derived for the revised galactic distance scale of solar radius - 8.5 Kpc. A total IR luminosity of 1.2 E10 solar luminosity is obtained, which is about one third of the estimated stellar luminosity of the Galaxy. The dust emission spectrum lambdaI(sub lambda) attains it maximum at 100 microns. A secondary maximum in the dust emission spectrum occurs at 10 microns, which contains 15% of the total IR luminosity of the Galaxy. The galactic dust emission spectrum was compared with the dust emission spectra of external IRAS galaxies. The warm dust luminosity relates to the present OB star formation rate, while flux densities observed at longer submm wavelengths are dominated by cold dust emission and thus can be used to estimate gas masses.

Cox, P.↗
NASA NTRS1987

Survey of the galactic disk from 1 = -150 deg to 1 = 82 deg in the submillimeter range

The first almost complete survey of the galactic disk from 1 = -150 deg to 1 = 82 deg in the submillimeter range (effective wavelength = 380 microns), performed with the AGLAE balloon-borne instrument modified to include a submillimeter channel, is reported. The instrumentation and observational procedures are described, as are the signal processing and calibration. The results are presented as a profile of the submillimeter brightness of the galactic disk displayed as a function of the galactic longitude. This profile exhibits diffuse emission all along the disk with bright peaks associated with resolved sources. The averaged galactic spectrum is in agreement with a temperature distribution of the interstellar cold dust.

Caux, Emmanuel↗
NASA NTRS1987

Star formation around active galactic nuclei

Active galactic nuclei (Seyfert nuclei and their relatives) and intense star formation can both deliver substantial amounts of energy to the vicinity of a galactic nucleus. Many luminous nuclei have energetics dominated by one of these mechanisms, but detailed observations show that some have a mixture. Seeing both phenomena at once raises several interesting questions: (1) Is this a general property of some kinds of nuclei? How many AGNs have surround starbursts, and vice versa? (2) As in 1, how many undiscovered AGNs or starbursts are hidden by a more luminous instance of the other? (3) Does one cause the other, and by what means, or do both reflect common influences such as potential well shape or level of gas flow? (4) Can surrounding star formation tell us anything about the central active nuclei, such as lifetimes, kinetic energy output, or mechanical disturbance of the ISM? These are important points in the understanding of activity and star formation in galactic nuclei. Unfortunately, the observational ways of addressing them are as yet not well formulated. Some preliminary studies are reported, aimed at clarifying the issues involved in study of the relationships between stellar and nonstellar excitement in galactic nuclei.

Keel, William C.↗
NASA NTRS1988

SMM detection of diffuse Galactic 511 keV annihilation radiation

Observations of the 511 keV annihilation line from the vicinity of the Galactic center from October to February for 1980/1981, 1981/1982, 1982/1983, 1984/1985, and 1985/1986 are presented. The measurements were made with the gamma-ray spectrometer on the SMM. The design of the instrument and some of its properties used in the analysis are described, and the methods used for accumulating, fitting, and analyzing the data are outlined. It is shown how the Galactic 511 keV line was separated from the intense and variable background observed in orbit. The SMM observations are compared with previous measurements of annihilation radiation from the Galactic center region, and the astrophysical implications are discussed. It is argued that most of the measurements made to date suggest the presence of an extended Galactic source of annihilation radiation.

Share, G. H.↗
NASA NTRS1988

The properties of the gaseous Galactic corona

The properties of the Galactic halo are examined as a bases for studying the phenomena observed in QSO spectra which might represent absorption by the halos and/or disks of other intervening spiral galaxies. Studies concerning the distribution of the gas in the Galactic halo are reviewed, including H I 21-cm emission line studies, optical and UV absorption and emission line studies, and radio studies. Consideration is given to the ionization characteristics, elemental abundances, kinematics, and origin of the gas in the Galactic halo. The approximate absorption line strengths expected for the sight line through the Galactic disk and halo at the position of the sun are listed and the absorption line system is compared to those seen in the spectra of QSOs.

Savage, Blair D.↗
NASA NTRS1989

The nature of the annihilation radiation and gamma-ray continuum from the Galactic center region

Analysis of the observations of the time-dependent, electron-positron annihilation line radiation and gamma-ray continuum emission from the region of the Galactic center shows that ther are two components to the emission: (1) a variable, compact source of 511 keV annihilation line and higher energy continuum emission active from about 1977 through 1979 at or near the Galactic center; and (2) a steady, diffuse interstellar source of 511 keV annihilation line and orthopositronium continuum emission, having a galactic longitudinal distribution consistent with that of the more than 70 MeV gamma rays or the molecular (CO) gas. It is found that the continuum spectrum of the compact source shows no evidence for orthopositronium annihilation. It is suggested that all of the three-photon orthopositronium continuum observed so far originates from the diffuse interstellar component. The continuum spectrum of the compact source is very similar to that of the black-hole candidate Cygnus X-1, supporting our earlier arguments that this object, at or near the Galactic center, is a black hole of stellar mass.

Lingenfelter, Richard E.↗
NASA NTRS1988

The age of the galactic disk

The galactic disk is a dissipative structure and must, therefore be younger than the halo if galaxy formation generally proceeds by collapse. Just how much younger the oldest stars in the galactic disk are than the oldest halo stars remains an open question. A fast collapse (on a time scale no longer than the rotation period of the extended protogalaxy) permits an age gap of the order of approximately 10 to the 9th power years. A slow collapse, governed by the cooling rate of the partially pressure supported falling gas that formed into what is now the thick stellar disk, permits a longer age gap, claimed by some to be as long as 6 Gyr. Early methods of age dating the oldest components of the disk contain implicit assumptions concerning the details of the age-metallicity relation for stars in the solar neighborhood. The discovery that this relation for open clusters outside the solar circle is different that in the solar neighborhood (Geisler 1987), complicates the earlier arguments. The oldest stars in the galactic disk are at least as old as NGC 188. The new data by Janes on NGC 6791, shown first at this conference, suggest a disk age of at least 12.5 Gyr, as do data near the main sequence termination point of metal rich, high proper motion stars of low orbital eccentricity. Hence, a case can still be made that the oldest part of the galactic thick disk is similar in age to the halo globular clusters, if their ages are the same as 47 Tuc.

Sandage, Allan↗
NASA NTRS1990

The onset of galactic winds in early-type galaxies

Researchers report on a program using Einstein x ray observations of the x ray spectra and surface brightness profiles (or extents) of a large sample of early-type (elliptical and SO) galaxies for which the goal is to determine the critical optical luminosity for which galactic winds are important. For galaxies in which the x ray emission is dominated by hydrostatic coronae, the x ray spectra will be relatively soft (characterized by a temperature of approx. 10 to the 7th power K), while for galaxies with a galactic wind, the emission will be dominated by the spectrally harder discrete sources (since the x ray emission from the wind is essentially negligible). In this new sample of 180 galaxies, there are 28 early type galaxies with sufficient counts to obtain a spectrum with the Einstein Image Proportional Counter (IPC). This sample more than doubles the total number of early-type galaxies in earlier compilations (Forman, Jones, and Tucker 1985; Canizares et al. 1987). The new spectral observations will help determine the critical optical luminosity for the onset of galactic winds which is important for understanding the chemical evolution of galaxies and of the intergalactic medium. The implications of galactic winds for the heavy element enrichment and energy content of the intracluster medium are discussed.

Forman, W.↗
NASA NTRS1991

VLA and VLBI angular broadening measurements - The distribution of interstellar scattering at low Galactic latitudes

Dual-frequency VLA and multifrequency VLBI observations of a number of sources in the Galactic plane have been made and used to estimate the amount of interstellar scattering in the Galactic plane as manifested by angular broadening of the observed sources. Some objects that may be affected by interstellar scattering are identified. The observed scattering in the Galactic plane decreases with increasing Galactic longitude, reaching a minimum at l roughly 60 deg and then becomes prominent again in the Cygnus region. The magnitude of scattering in the interarm region between l = 50-70 deg is comparable to that predicted by a model calculation of a 'smooth' component of interstellar turbulence. Thus, the more intense 'clumps' of interstellar turbulence may be mainly confined to the spiral arms. A computer simulation suggests that a complex distribution of clumps exists in the 'clumped' component, and that the form of the clump distribution as well as the density of clumps must change with increasing Galactocentric distance.

Fey, A. L.↗
NASA NTRS1991

Gamma ray constraints on the Galactic supernova rate

We perform Monte Carlo simulations of the expected gamma ray signatures of Galactic supernovae of all types to estimate the significance of the lack of a gamma ray signal due to supernovae occurring during the last millenium. Using recent estimates of the nuclear yields, we determine mean Galactic supernova rates consistent with the historic supernova record and the gamma ray limits. Another objective of these calculations of Galactic supernova histories is their application to surveys of diffuse Galactic gamma ray line emission.

Hartmann, D.↗
NASA NTRS1992

On the galactic origin of gamma ray bursts

If gamma ray bursters are in the galactic disk, then the average value of the V/Vmax parameter cannot be lower than 0.4, no matter what is the source luminosity function or what is the disk thickness. The value of 0.4 is reached only asymptotically when the range of observations is much larger than the disk scale height, and almost all sources are within a very thin galactic equatorial belt. This distribution is in a direct conflict with the BATSE results as presented. If the bursters have a galactic halo distribution similar to that of any other known class of objects, then a strong dipole moment in their angular distribution should be seen, i.e., a strong concentration towards the galactic center. To make the dipole moment unmeasurable, the hypothetical burster's halo must have a core radius of at least 14 kpc (a 3 sigma limit) to be consistent with the BATSE results as presented.

Paczynski, Bohdan↗
NASA NTRS1992

Gamma ray constraints on the galactic supernova rate

Monte Carlo simulations of the expected gamma-ray signatures of galactic supernovae of all types are performed in order to estimate the significance of the lack of a gamma-ray signal due to supernovae occurring during the last millenium. Using recent estimates of nuclear yields, we determine galactic supernova rates consistent with the historic supernova record and the gamma-ray limits. Another objective of these calculations of galactic supernova histories is their application to surveys of diffuse galactic gamma-ray line emission.

Hartmann, D.↗
NASA NTRS1992

The Jodrell bank 'C' pulsar survey - A survey of the northern Galactic plane for rapidly rotating pulsars

Nearly 1500 sq deg of the northern Galactic plane were surveyed for rapidly rotating pulsars using the 76-m Lovell telescope. The sampling rate was such that there was sufficient nominal sensitivity to detect pulsars with periods as short as 0.6 ms. To overcome the effects of dispersion, scattering, and galactic background radiation, the high frequency of 1420 MHz was used to survey the inner Galaxy, while lower frequencies of 928 and 610 MHz were used for regions further from the Galactic center. PSR 1937 + 21 was detected in the survey, but no new pulsars were discovered. This is consistent with the results of other recent surveys and suggests that rapidly rotating, high-luminosity pulsars are not very common in the Galactic disk.

Biggs, J. D.↗
NASA NTRS1991

Observations of the Galactic center 511 keV line

A review of observations of the positron annihilation 511 keV line from the Galactic center and plane is presented. Emphasis is given to the new data from balloon observations in 1988 and 1989. The positron annihilation source in the direction of the center was found to be turned on again in 1988 October after being observed in a quiescent state since the early 1980s. There is evidence for variability between May and October in 1988. The new observations give the first data in which the line is spectrally resolved and in which the Galactic center and plane components of the annihilation radiation are independently measured. These data support the two-component model of the emission with a variable point source near the center and a steady-state source distributed along the galactic plane. Comparing measurements of narrow and wide field instruments in the 1980s gives a flux of about 8 x 10 exp -4 photons/sq cm/s for the point source in its high state and a flux of about 1.2 x 10 exp-3 photons/sq cm/s/rad for the galactic plane component with a relatively flat distribution over the central radian. The line width for the point source component is about 3 keV FWHM.

Gehrels, Neil↗