Smithsonian Astrophysical Observatory

About: Smithsonian Astrophysical Observatory is a facility organization based out in Cambridge, Massachusetts, United States. It is known for research contribution in the topics: Galaxy & Stars. The organization has 1665 authors who have published 3622 publications receiving 132183 citations. The organization is also known as: SAO.

High-resolution imaging of photodissociation regions in ngc 6334

Abstract: We have used the SPIREX telescope to conduct a wide-—eld thermal infrared imaging study of the star formation complex NGC 6334 in the southern Galactic plane. We imaged a 30@ region along the main star-forming ridge of NGC 6334 with pixel scale through broadband —lters for L (3.5 km) and 0A.6 M (4.8 km) and through narrowband —lters for the v \ 1¨0 Q-branch (2.42 km), polycyclic aromatic H 2 hydrocarbon (PAH) (3.3 km), and Bra (4.05 km) lines. The images reveal the spectacular, complex struc- ture of the photodissociation regions (PDRs) that pervade the region, with enhanced line emission around each of the seven sites of massive star formation along the ridge. Bubbles and loops of PAH emission, typically 1¨1.5 pc across, have been carved out of the parent molecular cloud by the intense UV radiation from the massive stars and surround H II regions (seen in Bra) typically 0.2¨0.3 pc across. The PAH emission regions coincide with both (C II) 158 km line emission, indicating that the PAHs are excited in PDR gas, and extensive emission, which therefore must be —uorescent. However, the tex- H 2 tures of the emission regions in PAH and are diUerent. This is attributable to variations in the physi- H 2 cal environment in which the gas is excited. Several compact reddened objects are observed; these are likely to be massive protostars. Subject headings: infrared: generalinfrared: ISM: lines and bandsISM: molecules ¨ ISM: structurestars: formationtelescopes

Uhuru observations of Hercules X-1 during the low state of the 35-day cycle.

Abstract: X-ray emission from Her X-1 has been observed during the 23-day low state of the 35-day cycle. During this time, all the activity normally present during the high state was seen, including regular eclipses, absorption 'dips,' and 1.24-s pulsations. The present observations are compared with the models of Her X-1 derived by Roberts (1974), Gerend and Boynton (1976), Katz (1973), and Petterson (1975). It is suggested that a hot optically thin corona along both faces of the accretion disk can produce the unpulsed component of emission observed during both the high- and low-intensity states.

The Fornax Deep Survey (FDS) with VST - VI. Optical properties of the dwarf galaxies in the Fornax cluster

Abstract: Context . Dwarf galaxies are the most common type of galaxies in galaxy clusters. Due to their low mass, they are more vulnerable to environmental effects than massive galaxies, and are thus optimal for studying the effects of the environment on galaxy evolution. By comparing the properties of dwarf galaxies with different masses, morphological types, and cluster-centric distances we can obtain information about the physical processes in clusters that play a role in the evolution of these objects and shape their properties. The Fornax Deep Survey Dwarf galaxy Catalog (FDSDC) includes 564 dwarf galaxies in the Fornax cluster and the in-falling Fornax A subgroup. This sample allows us to perform a robust statistical analysis of the structural and stellar population differences in the range of galactic environments within the Fornax cluster.Aims . By comparing our results with works concerning other clusters and the theoretical knowledge of the environmental processes taking place in galaxy clusters, we aim to understand the main mechanisms transforming galaxies in the Fornax cluster.Methods . We have exploited the FDSDC to study how the number density of galaxies, galaxy colors and structure change as a function of the cluster-centric distance, used as a proxy for the galactic environment and in-fall time. We also used deprojection methods to transform the observed shape and density distributions of the galaxies into the intrinsic physical values. These measurements are then compared with predictions of simple theoretical models of the effects of harassment and ram pressure stripping on galaxy structure. We used stellar population models to estimate the stellar masses, metallicities and ages of the dwarf galaxies. We compared the properties of the dwarf galaxies in Fornax with those in the other galaxy clusters with different masses.Results . We present the standard scaling relations for dwarf galaxies, which are the size-luminosity, Sersic n -magnitude and color-magnitude relations. New in this paper is that we find a different behavior for the bright dwarfs (−18.5 mag r ′ r ′ > −16 mag): While considering galaxies in the same magnitude-bins, we find that, while for fainter dwarfs the g ′−r ′ color is redder for lower surface brightness objects (as expected from fading stellar populations), for brighter dwarfs the color is redder for the higher surface brightness and higher Sersic n objects. The trend of the bright dwarfs might be explained by those galaxies being affected by harassment and by slower quenching of star formation in their inner parts. As the fraction of early-type dwarfs with respect to late-types increases toward the central parts of the cluster, the color-surface brightness trends are also manifested in the cluster-centric trends, confirming that it is indeed the environment that changes the galaxies. We also estimate the strength of the ram-pressure stripping, tidal disruption, and harassment in the Fornax cluster, and find that our observations are consistent with the theoretically expected ranges of galaxy properties where each of those mechanisms dominate. We furthermore find that the luminosity function, color–magnitude relation, and axis-ratio distribution of the dwarfs in the center of the Fornax cluster are similar to those in the center of the Virgo cluster. This indicates that in spite of the fact that the Virgo is six times more massive, their central dwarf galaxy populations appear similar in the relations studied by us.

MIGRATION OF SMALL MOONS IN SATURN's RINGS

Abstract: The motions of small moons through Saturn's rings provide excellent tests of radial migration models. In theory, torque exchange between these moons and ring particles leads to radial drift. We predict that moons with Hill radii r {sub H} {approx} 2-24 km should migrate through the A ring in 1000 yr. In this size range, moons orbiting in an empty gap or in a full ring eventually migrate at the same rate. Smaller moons or moonlets-such as the propellers-are trapped by diffusion of disk material into corotating orbits, creating inertial drag. Larger moons-such as Pan or Atlas-do not migrate because of their own inertia. Fast migration of 2-24 km moons should eliminate intermediate-size bodies from the A ring and may be responsible for the observed large-radius cutoff of r {sub H} {approx} 1-2 km in the size distribution of the A ring's propeller moonlets. Although the presence of Daphnis (r {sub H} Almost-Equal-To 5 km) inside the Keeler gap challenges this scenario, numerical simulations demonstrate that orbital resonances and stirring by distant, larger moons (e.g., Mimas) may be important factors. For Daphnis, stirring by distant moons seems the most promising mechanism to halt fast migration. Alternatively, Daphnis may be amore » recent addition to the ring that is settling into a low inclination orbit in {approx}10{sup 3} yr prior to a phase of rapid migration. We provide predictions of observational constraints required to discriminate among possible scenarios for Daphnis.« less

K-Band Properties of Well-Sampled Groups of Galaxies

Abstract: We use a sample of 55 groups and six clusters of galaxies ranging in mass from 7 × 1011 to 1.5 × 1015 M⊙ to examine the correlation of the Ks-band luminosity with mass discovered by Lin and coauthors in 2003. We use the Two-Micron All-Sky Survey catalog and published redshifts to construct complete magnitude-limited redshift surveys of the groups. From these surveys we explore the IR photometric properties of groups members, including their IR color distribution and luminosity function. Although we find no significant difference between the group Ks luminosity function and the general field, there is a difference between the color distribution of luminous group members and their counterparts (generally background) in the field. There is a significant population of luminous galaxies with H - Ks 0.35, which are rarely, if ever, members of the groups in our sample. The most luminous galaxies that populate the groups have a very narrow range of IR color. Over the entire mass range covered by our sample, the Ks luminosity increases with mass as L ∝ M0.64±0.06, implying that the mass-to-light ratio in the Ks band increases with mass. The agreement between this result and earlier investigations of essentially nonoverlapping sets of systems shows that this window in galaxy formation and evolution is insensitive to the selection of the systems and to the details of the mass and luminosity computations.