102 research outputs found
Tidally Compressed Gas in Centers of Early Type and Ultraluminous Galaxies
In this paper we propose that the compressive tidal field in the centers of
flat-core early type galaxies and ultraluminous galaxies compresses molecular
clouds producing dense gas obseved in the centers of these galaxies. The effect
of galactic tidal fields is usually considered disruptive in the literature.
However, for some galaxies, the mass profile flattens towards the center and
the resulting galactic tidal field is not disruptive but instead it is
compressive within the flat-core region. We have used the virial theorem to
determine the minimum density of a molecular cloud to be stable and
gravitationally bound within the tidally compressive region of a galaxy. We
have applied the mechanism to determine the mean molecular cloud densities in
the centers of a sample of flat-core, early-type galaxies and ultraluminous
galaxies.Comment: 18 latex pages and uses aaspp4.sty, accepted for publication in
Astrophysical Journa
Extended HI spiral structure and the figure rotation of triaxial dark halos
The HI disk of the blue compact dwarf (BCD) galaxy NGC 2915 extends to
22 optical scalelengths and shows spiral arms reaching far beyond the optical
component. None of the previous theories for spiral structure provide likely
explanations for these very extended spiral arms. Our numerical simulations
first demonstrate that such large spiral arms can form in an extended gas disk
embedded in a massive triaxial dark matter halo with slow figure rotation,
through the strong gravitational torque of the rotating halo. We then show that
the detailed morphological properties of the developed spirals and rings depend
strongly on the pattern speed of the figure rotation, the shape of the triaxial
halo, and the inclination of the disk with respect to the plane including the
triaxial halo's long and middle axes. These results strongly suggest that the
dark matter halo of NGC 2915 is triaxial and has figure rotation. Based on
these results, we also suggest that dynamical effects of triaxial halos with
figure rotation are important in various aspect of galaxy formation and
evolution, such as formation of polar ring galaxies, excitation of
non-axisymmetric structures in low surface-brightness galaxies, and gas fueling
to the central starburst regions of BCDs.Comment: 13 pages 2 figures (fig.2 = jpg format), accepted by ApJ
A New System of Parallel Isolated Nonthermal Filaments Near the Galactic Center: Evidence for a Local Magnetic Field Gradient
We report the discovery of a system of isolated nonthermal filaments
approximately 0.5 deg. northwest (75 pc in projection) of Sgr A. Unlike other
isolated nonthermal filaments which show subfilamentation, braiding of
subfilaments, and flaring at their ends, these filaments are simple linear
structures and more closely resemble the parallel bundled filaments in the
Galactic center radio arc. However, the most unusual feature of these filaments
is that the 20/90 cm spectral index uniformly decreases as a function of
length, in contrast to all other nonthermal filaments in the Galactic center.
This spectral gradient may not be due to simple particle aging but could be
explained by a curved electron energy spectrum embedded in a diverging magnetic
field. If so, the scale of the magnetic gradient is not consistent with a large
scale magnetic field centered on Sgr A* suggesting that this filament system is
tracing a local magnetic field.Comment: 10 pages, AASTeX 5.01 LaTeX2e; 7 figures in 9 PostScript files;
scheduled for publication in the 2001 December 10, v. 563 issue of Ap
The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI: Final data release of the combined LDS and IAR surveys with improved stray-radiation corrections
We present the final data release of observations of lambda 21-cm emission
from Galactic neutral hydrogen over the entire sky, merging the
Leiden/Dwingeloo Survey (LDS: Hartmann & Burton, 1997) of the sky north of
delta = -30 deg with the Instituto Argentino de Radioastronomia Survey (IAR:
Arnal et al., 2000, and Bajaja et al., 2005) of the sky south of delta = -25
deg. The angular resolution of the combined material is HPBW ~ 0.6 deg. The LSR
velocity coverage spans the interval -450 km/s to +400 km/s, at a resolution of
1.3 km/s. The data were corrected for stray radiation at the Institute for
Radioastronomy of the University of Bonn, refining the original correction
applied to the LDS. The rms brightness-temperature noise of the merged database
is 0.07 - 0.09 K. Residual errors in the profile wings due to defects in the
correction for stray radiation are for most of the data below a level of 20 -
40 mK. It would be necessary to construct a telescope with a main beam
efficiency of eta_{MB} > 99% to achieve the same accuracy. The merged and
refined material entering the LAB Survey of Galactic HI is intended to be a
general resource useful to a wide range of studies of the physical and
structural characteristices of the Galactic interstellar environment. The LAB
Survey is the most sensitive Milky Way HI survey to date, with the most
extensive coverage both spatially and kinematically.Comment: 9 pages, 5 figures, accepted for publication by Astronomy &
Astrophysic
Estructura fina en nubes de alta velocidad positiva
Se presentan los resultados observacionales de dos complejos de HI: HVC 287.5 + 225 + 240 y HVC 273 + 9.5 + 247. Los mismos fueron observados con una resolución angular de 35' y una resolución en velocidad de 2 km/seg.Asociación Argentina de Astronomí
The Star Clusters in the Irregular Galaxy NGC 4449
We examine the star clusters in the irregular galaxy NGC 4449. We use a
near-infrared spectrum and broad-band images taken with the HST to place a
limit of 8--15 Myrs on the age of the bright central ojbect in NGC 4449. Its
luminosity and size suggest that it is comparable to young super star clusters.
However, there is a peculiar nucleated-bar structure at the center of this star
cluster, and we suggest that this structure is debris from the interaction that
has produced the counter-rotating gas systems and extended gas streamers in the
galaxy.
From the images we identify 60 other candidate compact star clusters in NGC
4449. Fourteen of these could be background elliptical galaxies or old globular
star clusters. Of the star clusters, three, in addition to the central object,
are potentially super star clusters, and many others are comparable to the
populous clusters found in the LMC. The star clusters span a large range in
ages with no obvious peak in cluster formation that might be attributed to the
interaction that the galaxy has experienced.Comment: To be published in PASP, Feb. 2001; also attainable from
ftp.lowell.edu, cd pub/dah/n4449pape
The interstellar medium surrounding the Scorpius-Centaurus association revisited
Aims: We want to make a large-scale study of the morphology, kinematics, and origin of the H I, which surrounds the Sco-Cen association. Methods. We combine our high-sensitivity southern H I survey with the Leiden/Dwingeloo Survey, considering l = 240◦−400◦, b = −60◦ to +60◦, and radial velocities of V = −41.8 to +40.8 km s−1. We point out the main H I branches surrounding the association and derive their kinematics. Kinematical H I-maps were compared with spatial maps of interstellar (IS) Na I from the literature. Upper limits for distances d were derived from optical IS absorption components from the literature. Models of expanding spherical H I shells were fitted around each stellar subgroup. Results: The expanding ring of H I associated with the Gould Belt (GB) is very prominent. At each l, its radial velocity shifts ∼−7 km s−1 within an interval Δb ∼ 10◦−25◦. On the sky, the shifts occur within a narrow stripe extending between l, b ∼ 250◦, −18◦ and 400◦, +50◦. The ranges of distances and radial thicknesses of most H I branches are nearly 70−160 pc and 40−90 pc, respectively. The shell-models fit the main branches. Interactions between the shells built a large expanding bubble with a transverse diameter of nearly 250 pc around the association. The near face is approaching with a mean velocity V¯ ∼ −6.6 km s−1 at d ≤ 76 pc from the Sun, covering about 102◦ × 65◦ (l, b), forming an “H I-wall”. There are streamers at V¯ ∼ −15 to −35 km s−1, as well as gas presumably overshot into Galactic Quadrant II. The receding gas is more tenuous. Conclusions: The association is traversing the ring since a time comparable to its age producing a significant disturbance on the expanding GB-ring of gas. The latter was almost totally shocked by the association, northerly of the stripe of velocity shifts. Southerly there are large amounts of preshocked gas, as well as smaller more localized shocked regions. Hot gas within the bubble produces most of the 1/4 keV radiation detected toward it by ROSAT. The total mass of the GB gas embedding the Sco-Cen association is estimated at Mt ∼ 368 000 M ± 60%, including ∼34 000 M of associated H2 and 30% of He. About 39% of the embedding gas was shocked by the association. At |b|≥ 35◦, a comparison of the observations with test points moving on ballistic orbits is consistent with the formation of the Sco-Cen association within the gas ring of the GB and the presumable explosive origin of the latter. The rotation of the ring is assumed.Fil: Pöppel, W, G. L.. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; ArgentinaFil: Bajaja, E.. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; ArgentinaFil: Arnal, Edmundo Marcelo. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; ArgentinaFil: Morras, Ricardo. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; Argentin
Is NGC 3108 transforming itself from an early to late type galaxy -- an astronomical hermaphrodite?
A common feature of hierarchical galaxy formation models is the process of
"inverse" morphological transformation: a bulge dominated galaxy accretes a gas
disk, dramatically reducing the system's bulge-to-disk mass ratio. During their
formation, present day galaxies may execute many such cycles across the Hubble
diagram. A good candidate for such a "hermaphrodite" galaxy is NGC 3108: a
dust-lane early-type galaxy which has a large amount of HI gas distributed in a
large scale disk. We present narrow band H_alpha and R-band imaging, and
compare the results with the HI distribution. The emission is in two
components: a nuclear bar and an extended disk component which coincides with
the HI distribution. This suggests that a stellar disk is currently being
formed out of the HI gas. The spatial distributions of the H_alpha and HI
emission and the HII regions are consistent with a barred spiral structure,
extending some 20 kpc in radius. We measure an extinction- corrected SFR of
0.42 Msun/yr. The luminosity function of the HII regions is similar to other
spiral galaxies, with a power law index of -2.1, suggesting that the star
formation mechanism is similar to other spiral galaxies. We measured the
current disk mass and find that it is too massive to have been formed by the
current SFR over the last few Gyr. It is likely that the SFR in NGC 3108 was
higher in the past. With the current SFR, the disk in NGC 3108 will grow to be
~6.2x10^9 Msun in stellar mass within the next 5.5 Gyr. While this is
substantial, the disk will be insignificant compared with the large bulge mass:
the final stellar mass disk-to-bulge ratio will be ~0.02. NGC 3108 will fail to
transform into anything resembling a spiral without a boost in the SFR and
additional supply of gas.Comment: 9 pages, 3 figures, accepted for publication in MNRA
Observaciones de CO en una región altamente polarizada de M31
El objetivo de este trabajo ha sido la correlación entre las observaciones de alta resolución en el continuo, en la longitud de onda de 20 cm, efectuadas con el VLA, y las observaciones de la transición J =1 —► 0 de la molécula ¹²CO , en la longitud de onda de 2,6 mm, efectuadas con el radiotelescopio IRAM de 30 m en Pico Veleta (España), de una región de NGC 224 (M31). En esta región, ubicada a aproximadamente X = 12' Sur e Y = 10' Oeste del centro de la galaxia, la polarización de la emisión continua muestra variaciones bruscas e intensas. Además, se encuentran aquí: una radiofuente discreta intensa, nubes de polvo, la asociación OB66, HI con alta densidad columnar a lo largo de los brazos y un agujero en el HI asociado a un shell de Hα (P354). La línea de CO fue observada a lo largo de tres líneas rectas, una de 6', a lo largo del brazo, y dos perpendiculares a la anterior y centradas en ella, de 4'de extensión, sobre puntos espaciados en 11". Los resultados de las comparaciones efectuadas indican que existe una correlación marcada entre la potencia total del continuo extendido y el gas, y una anticorrelación entre éste y el grado de polarización. Cuantitativamente: Tₐ ≃ N_gas^(2.5±0.6), p ≃ N_gas^(-2.0±0.8), donde Tₐ es la intensidad total de la emisión en el continuo, p el porcentaje de polarización y N_gas =Nₕᵢ + 2Nₕ₂.Asociación Argentina de Astronomí
Identificación óptica de radiofuentes
Se informa del estado actual del trabajo de identificación óptica de radiofuentes, realizado en base a un catalogo de Bolton y colaboradores, preparado con el radiotelescopio de Parkes, Australia. Se describe también un método simplificado y suficientemente preciso para determinar posiciones sobre las placas.Asociación Argentina de Astronomí
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