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NASA NTRS1990

Variable stars in the Pegasus dwarf galaxy (DDO 216)

Observations obtained over a period of five years of the resolved stars in the Pegasus dwarf irregular galaxy (DDO 216) have been searched for variable stars. Thirty-one variables were found, and periods established for 12. Two of these variable stars are clearly eclipsing variables, seven are very likely Cepheid variables, and the remaining three are probable Cepheids. The period-luminosity relation for the Cepheids indicates a distance modulus for Pegasus of m - M = 26.22 + or - 0.20. This places Pegasus very near the zero-velocity surface of the Local Group.

Hoessel, J. G.↗

NASA NTRS1990

Deep narrow band imagery of the diffuse ISM in M33

Very deep narrow band images were obtained for several fields in the local group spiral galaxy M33 using a wide field reimaging Charge Coupled Device (CCD) camera on the 1.5 m telescope at Palomar Observatory. The reimaging system uses a 306 mm collimator and a 58 mm camera lens to put a 16 minute by 16 minute field onto a Texas Instruments 800 x 800 pixel CCD at a resolution of 1.2 arcseconds pixel (-1). The overall system is f/1.65. Images were obtained in the light of H alpha (S II) lambda lambda 6717, 6731, (O III) lambda 5007, and line-free continuum bands 100A wide, centered at 6450A and 5100A. Assuming a distance of 600 kpc to M33 (Humphreys 1980, Ap. J., 241, 587), this corresponds to a linear scale of 3.5 pc pixel (-1), and a field size of 2.8 kpc x 2.8 kpc. Researchers discuss the H alpha imagery of a field centered approx. equal to 8 minutes NE of the nucleus, including the supergiant HII region complex NGC 604. Two 2000 second H alpha images and two 300 second red continuum images were obtained of two slightly offset fields. The fields were offset to allow for discrimination between real emission and possible artifacts in the images. All images were resampled to align them with one of the H alpha frames. The continuum images were normalized to the line images using the results of aperture photometry on a grid of stars in the field, then the rescaled continuum data were directly subtracted from the line data.

Hester, J. Jeff↗

NASA NTRS1990

Star formation in proto dwarf galaxies

The effects of the onset of star formation on the residual gas in primordial low-mass Local-Group dwarf spheroidal galaxies is studied by a series of hydrodynamical simulations. The models have concentrated on the effect of photoionization. The results indicate that photoionization in the presence of a moderate gas density gradient can eject most of the residual gas on a time scale of a few 10 to the 7th power years. High central gas density combined with inefficient star formation, however, may prevent mass ejection. The effect of supernova explosions is discussed briefly.

Noriega-Crespo, A.↗

NASA NTRS1990

High spatial resolution studies of galaxies in the far IR: Observations with the KAO, and the promise of SOFIA

NASA, in collaboration with the West German Science Ministry (BMFT), plans a larger airborne telescope as a successor to the Kuipper Airborne Observatory (KAO) that will achieve these goals. The Stratospheric Observatory for Infrared Astronomy (SOFIA) is entering the final stages of Phase B review with targeted new start early in the next decade. SOFIA is a 2.7 m diameter telescope that is carried in a Boeing 747SP. In addition to having 3 times the spatial resolution of the KAO, and 10 times the light gathering power, it will incorporate improvements over the KAO in lower optical emissivity and better telescope tracking stability. The thin primary mirror will equilibrate quickly to ambient temperature at an altitude which, accompanied by airflow improvements across the telescope cavity, will result in better image quality. The sensitivity of SOFIA will allow us to see a large number of typical bright galactic HII regions in local group galaxies. The spatial resolution of 8 seconds (full width half maximum Airy disk) at 100 microns will allow these regions to be measured independently, if they are distributed similarly to those in our own galaxy. At this spatial resolution, the disks of normal galaxies will be easily resolved out to distances of several hundred Mpc. This portion of space includes many of the superluminous galaxies discovered by the Infrared Astronomy Satellite (IRAS), and this spatial scale is relevant for studies of the morphology of regions of interaction among the majority of these galaxies that are members of colliding pairs.

Lester, Dan F.↗

NASA NTRS1991

The core of the nearby S0 galaxy NGC 7457 imaged with the HST planetary camera

A brief analysis is presented of images of the nearby S0 galaxy NGC 7457 obtained with the HST Planetary Camera. While the galaxy remains unresolved with the HST, the images reveal that any core most likely has r(c) less than 0.052 arcsec. The light distribution is consistent with a gamma = -1.0 power law inward to the resolution limit, with a possible stellar nucleus with luminosity of 10 million solar. This result represents the first observation outside the Local Group of a galaxy nucleus at this spatial resolution, and it suggests that such small, high surface brightness cores may be common.

Lauer, Tod R.↗

NASA NTRS1991

An X-ray image of the violent interstellar medium in 30 Doradus

A detailed analysis of the X-ray emission from the largest H II region complex in the Local Group, 30 Dor, is presented. Applying a new maximum entropy deconvolution algorithm to the Einstein Observatory data, reveals striking correlations among the X-ray, radio, and optical morphologies of the region, with X-ray-emitting bubbles filling cavities surrounded by H-alpha shells and coextensive diffuse X-ray and radio continuum emission from throughout the region. The total X-ray luminosity in the 0.16-3.5 keV band from an area within 160 pc of the central cluster R136 is about 2 x 10 to the 37th ergs/sec.

Wang, Q.↗

NASA NTRS1990

The all-sky X-ray foreground - Outstanding issues

An all-sky X-ray foreground survey can be used to determine how supermassive black holes trace the underlying mass distribution responsible for the particular velocity of the local group. The dipole moment for the cosmic X-ray background is compared to that for the all-sky extragalactic foreground of discrete sources, and the survey of AGNs is determined to be an effective method for discerning pronounced large-scale foreground anisotropies. The required sensitivity for an all-sky survey of this type is found to be 3.3 x 10 to the -13th ergs/sq cm/s which would allow an unbiased AGN survey.

Boldt, Elihu↗

NASA NTRS1991

On the derivation of selection functions from redshift survey data

A previously unrecognized effect is described in the derivation of luminosity functions and selection functions from existing redshift survey data, due to binning of quoted magnitudes and diameters. Corrections are made for this effect in the Center for Astrophysics (CfA) and Southern Sky (SSRS) Redshift Surveys. The correction makes subtle but systematic changes in the derived density fields of the CfA survey, especially within 2000 km/s of the Local Group. The effect on the density field of the SSRS survey is negligible.

Strauss, Michael A.↗

NASA NTRS1993

Wide-field direct CCD observations supporting the Astro-1 Space Shuttle mission's Ultraviolet Imaging Telescope

Wide field direct CCD observations are being obtained to support and complement the vacuum-ultraviolet (VUV) images provided by Astro's Ultraviolet Imaging Telescope (UIT) during a Space Shuttle flight in December 1990. Because of the wide variety of projects addressed by UIT, the fields observed include (1) galactic supernova remnants such as the Cygnus Loop and globular clusters such as Omega Cen and M79; (2) the Magellanic Clouds, M33, M81, and other galaxies in the Local Group; and (3) rich clusters of galaxies, principally the Perseus cluster and Abell 1367. Ground-based observations have been obtained for virtually all of the Astro-1 UIT fields. The optical images allow identification of individual UV sources in each field and provide the long baseline in wavelength necessary for accurate analysis of UV-bright sources. To facilitate use of our optical images for analysis of UIT data and other projects, we plan to archive them, with the UIT images, at the National Space Science Data Center (NSSDC), where they will be universally accessible via anonymous FTP. The UIT, one of three telescopes comprising the Astro spacecraft, is a 38-cm f/9 Ritchey-Chretien telescope on which high quantum efficiency, solar-blind image tubes are used to record VUV images on photographic film. Five filters with passbands centered between 1250A and 2500A provide both VUV colors and a measurement of extinction via the 2200A dust feature. The resulting calibrated VUV pictures are 40 arcminutes in diameter at 2.5 arcseconds resolution. The capabilities of UIT, therefore, complement HST's WFPC: the latter has 40 times greater collecting area, while UIT's usable field has 170 times WFPC's field area.

Hintzen, Paul↗

NASA NTRS1993

Is a local bar a good place to find a companion? The near infrared morphology of Maffei 2

Maffei 2 is one of the closest large spiral galaxies lying just beyond the Local Group. It would probably be one of the most heavily studied galaxies in the sky were it not for the approximately 5 magnitudes of visual extinction resulting from its position behind the Galactic plane. It is the site of a burst of nuclear star formation indicated by strong infrared and radii continuum emission. Interferometric maps of CO-12 and CO-13 emission indicate that star formation is associated with a barlike structure consisting of arms of molecular gas that extend from within approximately 50 pc of the dynamical center out to a radius of at least 500 pc. HI maps have shown the galaxy to have an angular extent of approximately 15 feet and a neutral gas mass typical of a large spiral galaxy.

Hurt, Robert L.↗

NASA NTRS1993

The vertical disk structure of the edge-on spiral galaxy NGC 3079

NGC 3079 is an edge-on SB(s)c galaxy at a redshift of 1225 km/s relative to the Local Group. Earlier researchers found a spectacular 'figure-eight' radio structure aligned along the minor axis of the galaxy, centered on the nucleus, and extending 3 kpc above and below the plane. The geometry of this structure and the evidence of unusually high nuclear gas velocities suggest that a wind-type outflow from the nucleus is taking place. The disk of NGC 3079 is also remarkable: it is extremely rich in H 2 regions and is the only unambiguous example of a galaxy outside M31 and our own Galaxy to exhibit 'Heiles-like' shells. Other researchers have also identified a nebulosity with a ragged X-shaped morphology formed by a system of lumpy filaments with individual lengths of 3 - 5 kpc. They suggest that this material is ambient halo gas entrained into the boundary layers of the nuclear outflow. The complex structure of the line emission in NGC 3079 makes this object an ideal target for an imaging spectroscopic study. The present paper reports the preliminary results of such a study.

Veilleux, S.↗

NASA NTRS1992

Tuning the Cepheid distance scale

Ongoing observational programs (both from the ground and space) will provide a significantly larger sample of galaxies with well-studied Cepheids both within the Local Group and in more distant galaxies. Recent efforts in the calibration of the Cepheid distance scale utilizing Cepheids in star clusters in the Galaxy and in the Magellanic Clouds are described. Some of the significant advantages of utilizing LMC Cepheids in particular are emphasized, and the current status of the field is summarized.

Mateo, Mario↗

NASA NTRS1992

RR Lyrae stars as standard candles

Recent developments in our understanding of RR Lyrae variables are discussed, with special emphasis on their role as distance indicators. The dependence of RR Lyrae luminosity on metallicity and horizontal-branch (HB) morphology is now well understood with many pieces of supporting evidence. This allows the determination of distances to several Local Group galaxies (LMC, M31, and M33) by constructing HB population models that reproduce the observed luminosity functions of RR Lyraes in these galaxies. The new RR Lyrae distances to M31 and M33 are some 15-37 percent larger than those adopted by de Vaucouleurs (1986) and by Aaronson et al. (1986), while they are in reasonable agreements with the recent results based on multicolor CCD photometry of classical Cepheids. This indicates that the large value of Hubble constant (90-100 km/s/Mpc) suggested by these authors may be due, at least in part, to the errors in distances to nearby calibrating galaxies.

Lee, Young-Wook↗

NASA NTRS1990

The all-sky extragalactic X-ray foreground

It is argued that the local gravitational dipole implied by the 50 brightest X-ray clusters of galaxies at z greater than 0.013 considered by Lahav et al. (1989) is relatively small compared with that inferred from the only three clusters at lower redshifts. Recent dipole analysis of the X-ray flux from bright AGN observed with HEAO-1A2 indicates that they are indeed strong tracers of this matter. The implications of this for the very pronounced large-scale foreground anisotropies to be measured via low-redshift AGN resolved in more sensitive all-sky surveys are explored. For the total extragalactic X-ray sky, i.e., including the relatively large Cosmic X-ray Background as well as the contribution of the resolved sources, the dipole moment is small compared to the monopole. Because of the large peculiar velocity of the Local Group, it is found that the currently estimated value for this total dipole moment can be used to set severe constraints on the volume emissivity arising from all X-ray sources within the present epoch.

Boldt, Elihu↗

NASA NTRS1993

The cosmic Mach number - Direct comparisons of observations and models

A robust method is used to derive the cosmic Mach number, M, from existing peculiar velocity data sets. For the IR Tully-Fisher distances of spirals in the Local Supercluster of Aaronson et al. (1979, 1982), M of 1.03 and a characteristic distance from the Local Group of 1464 km/s are found. For the elliptical galaxy sample of Faber et al. (1987, 1988), M of 0.57, with a characteristic distance of 2572 km/s is found. These results are compared with Monte Carlo simulations of the observational realizations drawn from numerical simulations of the universe based on various scenarios, including the standard cold dark matter (CDM) scenario. The effect of velocity bias on the derived Mach number is found to be small. Only 5 percent of the CDM simulations have M as large as or larger than that derived for the Aaronson et al. sample.

Strauss, Michael A.↗

NASA NTRS1991

Star formation in globular clusters and dwarf galaxies and implications for the early evolution of galaxies

Based upon the observed properties of globular clusters and dwarf galaxies in the Local Group, we present important theoretical constraints on star formation in these systems. These constraints indicate that protoglobular cluster clouds had long dormant periods and a brief epoch of violent star formation. Collisions between protocluster clouds triggered fragmentation into individual stars. Most protocluster clouds dispersed into the Galactic halo during the star formation epoch. In contrast, the large spread in stellar metallicity in dwarf galaxies suggests that star formation in their pregenitors was self-regulated: we propose the protocluster clouds formed from thermal instability in the protogalactic clouds and show that a population of massive stars is needed to provide sufficient UV flux to prevent the collapsing protogalactic clouds from fragmenting into individual stars. Based upon these constraints, we propose a unified scenario to describe the early epochs of star formation in the Galactic halo as well as the thick and thin components of the Galactic disk.

Lin, Douglas N. C.↗

NASA NTRS1993

Stellar populations in the dwarf elliptical galaxy NGC 185

The study presents BVRI CCD photometry of about 5300 stars in the central area of the dwarf elliptical galaxy NGC 185 in the Local Group. The color-magnitude diagram shows three distinct stellar populations: a dominant RGB population, AGB stars located above the tip of the RGB stars, and a small number of young stars having blue to yellow colors. The foreground reddening is estimated to be 0.19 +/- 0.03 mag using the (B - V) - (V - I) diagram for the bright foreground stars with good photometry. Surface photometry of the central area of NGC 185 is presented; it shows that the colors become rapidly bluer inside R of about 10 arcsec. Structural parameters indicate that the mass-to-luminosity ratio ranges from 3 to 5.

Lee, Myung G.↗

NASA NTRS1994

Variable stars in the Leo A dwarf galaxy (DDO 69)

Observations of the Leo A dwarf galaxy, obtained over the period from 1980 to 1991 are reported. Forty two separate Charge Coupled Devices (CCD) frames were searched for variable stars. A total of 14 suspected variables were found, 9 had sufficient coverage for period determination, and 5 had Cepheid light curves. Four of these stars fit well on a P-L relation and yield a distance modulus, after correction for Galactic foreground extinction, of m-M = 26.74. This corresponds to a distance of 2.2 Mpc, placing Leo A near the Local Group zero-velocity surface.

Hoessel, John G.↗