共查询到20条相似文献，搜索用时 859 毫秒
1.
P. M. S. Namboodiri K. S. Sastry K. S. V. V. S. Narasimhan 《Astrophysics and Space Science》2006,301(1-4):3-9
Numerical simulations are performed to study the tidal effects of non-merging rapid head-on collision between a disk galaxy
and a spherical galaxy. The disk consists of three components – a disk, a bulge and a halo – and the spherical galaxy is a
Plummer model. The galaxies have the same dimensions with different mass ratios viz., 2, 1 and 0.5. They move in a rectilinear
orbit with a relative velocity of 1000 km s−1.
None of the simulations leads to the merger of the galaxies by tidal capture. The results of our simulations indicate that
although tidal effects are sensitive to both the mass ratio and the inclination of the disk to the orbital plane, it is the
mass ratio which is more important in producing tidal damage to the less massive galaxy. The spherical galaxy undergoes considerable
tidal effects if the mass of the disk is same or larger. On the other hand the collisions in which the mass of the spherical
galaxy is more, result in the formation of a ring structure after the closest approach and the structure disappears by the
end of the simulations. 相似文献
2.
Gerhard Hensler Daniel Tschöke Dominik Bomans Alessandro Boselli 《Astrophysics and Space Science》2003,284(2):467-470
Observations of the Virgo Cluster galaxy NGC 4569 in soft X-rays and in Hα reveal both an asymmetrically distributed hot gaseous
halo and a giant filament of diffuse emission, respectively, extending from the same side of the disk, the latter one to at
least 8.8 kpc. A deep longslit spectrum along the filament shows velocities systematically different from the systemic velocity
of NGC 4569. With the inclination of the disk one can determine the Hα spur as a giant outflow in NGC 4569.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
3.
4.
Nikos Prantzos 《Astrophysics and Space Science》2003,284(2):675-684
We develop a detailed model of the Milky Way (a `prototypical' disk galaxy) and extend it to other disks with the help of
some simple scaling relations, obtained in the framework of Cold Dark Matter models. This phenomenological (`hybrid') approach
to the study of disk galaxy evolution allows us to reproduce successfully a large number of observed properties of disk galaxies
in the local Universe and up to redshift z ∼ 1. The important conclusion is that, on average, massive disks have formed the bulk of their stars earlier than their lower
mass counterparts: the `star formation hierarchy' has been apparently opposite to the `dark matter assembly' hierarchy. It
is not yet clear whether `feedback' (as used in semi-analytical models of galaxy evolution) can explain that discrepancy.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
5.
Jonathan Braine P.-A. Duc U. Lisenfeld V. Charmandaris O. Vallejo S. Leon E. Brinks 《Astrophysics and Space Science》2002,281(1-2):407-408
We investigate the process of galaxy formation as can be observed in the only currently forming galaxies - the so-called Tidal
Dwarf Galaxies, hereafter TDGs - through observations of the molecular gas detected via its CO (Carbon Monoxide) emission.
These objects are formed of material torn off of the outer parts of a spiral disk due to tidal forces in a collision between
two massive galaxies. Molecular gas is a key element in the galaxy formation process, providing the link between a cloud of
gas and a bona fide galaxy. We have detected CO in 8 TDGs (Braine, Lisenfeld, Duc and Leon, 2000: Nature
403, 867; Braine, Duc, Lisenfeld, Charmandaris, Vallejo, Leon and Brinks: 2001, A&A
378, 51), with an overall detection rate of 80%, showing that molecular gas is abundant in TDGs, up to a few 108
M
⊙. The CO emission coincides both spatially and kinematically with the HI emission, indicating that the molecular gas forms
from the atomic hydrogen where the HI column density is high. A possible trend of more evolved TDGs having greater molecular
gas masses is observed, in accord with the transformation of HI into H2. Although TDGs share many of the properties of small irregulars, their CO luminosity is much greater (factor ∼ 100) than
that of standard dwarf galaxies of comparable luminosity. This is most likely a consequence of the higher metallicity (≳sim
1/3 solar) of TDGs which makes CO a good tracer of molecular gas. This allows us to study star formation in environments ordinarily
inaccessible due to the extreme difficulty of measuring the molecular gas mass. The star formation efficiency, measured by
the CO luminosity per Hα flux, is the same in TDGs and full-sized spirals. CO is likely the best tracer of the dynamics of
these objects because some fraction of the HI near the TDGs may be part of the tidal tail and not bound to the TDG. Although
uncertainties are large for individual objects, as the geometry is unknown, our sample is now of eight detected objects and
we find that the ‘dynamical’ masses of TDGs, estimated from the CO line widths, seem not to be greater than the ‘visible’
masses (HI + H2 + a stellar component). Although higher spatial resolution CO (and HI) observations would help reduce the uncertainties,
we find that TDGs require no dark matter, which would make them the only galaxy-sized systems where this is the case. Dark
matter in spirals should then be in a halo and not a rotating disk. Most dwarf galaxies are dark matter-rich, implying that
they are not of tidal origin. We provide strong evidence that TDGs are self-gravitating entities, implying that we are witnessing the
ensemble of processes in galaxy formation: concentration of large amounts of gas in a bound object, condensation of the gas,
which is atomic at this point, to form molecular gas and the subsequent star formation from the dense molecular component.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
6.
We discuss the properties of the galaxies in about 60 rich, nearby clusters, using kinematic data from the ESO Nearby Abell
Cluster Survey, combined with new imaging data. The images were used to classify the galaxies, and to recalibrate the galaxy
types derived from the ENACS spectra; this yields galaxy-type estimates for about4800 galaxies. For about 1200 galaxies, a
bulge/disk decomposition could be made, which yields sizes and luminosities of bulges and disks. From the projected radial
distances and relative l.o.s.-velocities we derived the galaxy ensembles with significantly different phase-space distributions.
We find that galaxies in and outside substructure must be distinguished. The morphological composition of the substructures
appears to vary with projected radius. Outside substructures, 4 galaxy ensembles must be defined: viz. the brighest Es, the
other early-type galaxies, the early spirals(Sa-Sb), and the late spirals (including the emission-line galaxies). We also
study the morphology-density relation, and we find that the segregation of the late spirals is driven mostly by global factors,
while the segregation of Es, S0s and early spirals is driven mostly by local density. The properties of early spirals and
S0ssupport the picture in which early spirals transform into S0s, while the properties of the late spirals do not support
such a relation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
F.P. Israel 《Astronomy and Astrophysics Review》1998,8(4):237-278
Summary. At a distance of 3.4 Mpc, NGC 5128 (Centaurus A) is by far the nearest active radio galaxy. It is often considered to be
the prototype Fanaroff-Riley Class I ‘low-luminosity’ radio galaxy, and as such it plays an important role in our understanding
of a major class of active galaxies. Its proximity has spawned numerous detailed investigations of its properties, yielding
unrivalled but still incomplete knowledge of its structure and dynamics.
The massive elliptical host galaxy is moderately triaxial and contains a thin, strongly warped disk rich in dust, atomic and
molecular gas and luminous young stars. Its globular cluster ensemble has a bimodal distribution of metallicities. Deep optical
images reveal faint major axis extensions as well as a system of filaments and shells. These and other characteristics are
generally regarded as strong evidence that NGC 5128 has experienced a major merging events at least once in its past.
The galaxy has a very compact, subparsec nucleus exhibiting noticeable intensity variations at radio and X-ray wavelengths,
probably powered by accretion events. The central object may be a black hole of moderate mass. Towards the nucleus, rich absorption
spectra of atomic hydrogen and various molecular species suggest the presence of significant amounts of material falling into
the nucleus, presumably ‘feeding the monster’. Emanating from the nucleus are linear radio/X-ray jets, becoming subrelativistic
at a few parsec from the nucleus. At about 5 kpc from the nucleus, the jets expand into plumes. Huge radio lobes extend beyond
the plumes out to to 250 kpc. A compact circumnuclear disk with a central cavity surrounds the nucleus. Its plane, although
at an angle to the minor axis of the galaxy, is perpendicular to the inner jets. The jet-collimating mechanism, probably connected
to the circumnuclear disk, appears to precess on timescales of order a few times 10 years.
This review summarizes the present state of knowledge of NGC 5128 and its associated radio source Centaurus A. Underlying
physical processes are outside its scope: they are briefly referred to, but not discussed.
Received 30 December 1997 相似文献
8.
We study the spatial orientation of 5 169 galaxies that have radial velocity 3 000 to 5 000 km s−1. The ‘position angle–inclination’ method is used to find the spin vector and the projections of spin vector of the galaxy
rotation axes. The spatial isotropic distribution is assumed to examine the non-random effects. For this, we have performed
chi-square, Fourier, and auto-correlation tests. We found a random alignment of spin vectors of total galaxies with respect
to the equatorial coordinate system. The spin vector projections of total galaxies is found to be oriented tangentially with
respect to the equatorial center. The spiral galaxies show a similar orientation as shown by the total sample. Five subsamples
of barred spiral (late-type) galaxies show a preferred alignment. However, early-type barred spirals show a random orientation.
A weak morphological dependence is noticed in the subsamples of late type barred spirals. A comparison with the previous works
and the possible explanation of the results will be presented. 相似文献
9.
Markus Samland 《Astrophysics and Space Science》2003,284(2):841-844
I present a model for the formation and evolution of a massive disk galaxy, within a growing dark halo whose mass evolves
according to cosmological simulations of structure formation. The galactic evolution is simulated with a new three-dimensional
chemo-dynamical code, including dark matter, stars and a multi-phase ISM. We follow the evolution from redshift z= 4.85 until the present epoch. The energy release by massive stars and supernovae prevents a rapid collapse of the baryonic
matter and delays the maximum star formation until redshift z ≈ 1. The galaxy forms radially from inside-out and vertically from top-to-bottom. Correspondingly, the inner halo is the
oldest component, followed by the outer halo, the bar/bulge, the thick and the thin disk. The bulge in the model consists
of at least two stellar subpopulations, an early collapse population and a population that formed later in the bar.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
10.
S. García-Burillo F. Combes J. Graciá-Carpio A. Usero M. Guélin 《Astrophysics and Space Science》2008,313(1-3):261-265
High-resolution CO maps are an essential tool to search for observational evidence of AGN fueling in galaxy nuclei. While
their capabilities will be surpassed by ALMA, current mm-interferometers can already provide relevant information on scales
which are critical for the process of angular momentum transfer in fueling the AGN. In this context we present the latest
results issued from the NUclei of GAlaxies (NUGA) project, a high-resolution (0.5′′–1′′) CO survey of low luminosity AGNs conducted with the IRAM Plateau de Bure interferometer (PdBI). The use of more specific
molecular tracers of dense gas can probe the feedback influence of activity on the chemistry and energy balance in the interstellar
medium of nearby galaxies, a prerequisite to understanding how feedback operate at higher redshift galaxies. We discuss the
results obtained in an ongoing study devoted to probe the feedback of activity from nearby Seyferts to high-redshift QSO. 相似文献
11.
Observations on the 6-m telescope in the Hα line and in the continuum are reported for 10 dwarf companions of the galaxy M31:
And I, And II, And III, And V, And IX, And X, Cass dSph, Peg DSph, NGC147, NGC221, and one irregular dwarf galaxy in the background,
And IV. All the observed companions of M31 have current star formation rates (SFR) on the order of or less than 10−6 M⊙ /yr. On a “star formation rate-neutral hydrogen mass” diagram for galaxies in the local volume, the dwarf spheroidal companions
of Andromeda lie in the region of extremely low values for these parameters.
__________
Translated from Astrofizika, Vol. 49, No. 3, pp. 337–350 (August 2006). 相似文献
12.
P. Papaderos Y.I. Izotov K.G. Noeske L.M. Cairós N.G. Guseva T.X. Thuan K.J. Fricke 《Astrophysics and Space Science》2003,284(2):619-622
Using HST and ground-based optical and NIR imaging data, we investigate whether the blue compact dwarf (BCD) galaxy I Zw 18
possesses an extended low-surface brightness (LSB) old stellar population underlying its star-forming (SF) regions. We show
that the exponential intensity decrease observed in the filamentary LSB envelope of the BCD out to 18″ (1.3 kpc at the adopted
distance of 15 Mpc) is not due to an evolved stellar disc, but rather due to extended ionized gas emission. Broad-band images
reveal, after subtraction of nebular line emission, a compact stellar LSB component extending slightly beyond the SF regions.
This stellar host, being blue over a radius range of 5 exponential scale lengths and showing little colour contrast to the
SF component, differs strikingly from the red LSB host of standard BCDs. This fact, in connection with the blue colours of
component I Zw 18 C (see discussion in Papaderos et al. 2002), suggests that most of the stellar mass in I Zw 18 has formed
within the last 0.5 Gyr. Furthermore, we show that the exponential intensity fall-off in the filamentary ionized envelope
of I Zw 18 is not particular to this system but a common property of the ionized halo of many SF dwarf galaxies on galactocentric
distances of several kpc. In the absence of an appreciable underlying stellar background, extended ionized gas emission dominates
in the periphery of I Zw 18, superficially resembling an exponential stellar disc on optical surface brightness profiles.
The case of I Zw 18 suggests caution in the search of more distant young galaxy candidates. Intense SF activity in the early
phase of dwarf galaxy formation may result in an extended ionized gas halo which can be mistaken for an evolved stellar disc
by studying only its exponential surface brightness profile.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
13.
This is a study of optical emission lines of the nucleus of the Seyfert galaxy NGC 3227. This work is based on 53 spectrograms
obtained with the 6-m telescope at the brightness maximum of this galactic nucleus on January 12–15, 1977. A set of profiles
of the hydrogen Hα, and Hβ lines and the equivalent widths (EWλ) of six emission lines and blends averaged over series of observations are presented. Broadening of the hydrogen line profiles
was observed over the three days. At a level of 0.50 times the peak brightness the hydrogen line profiles were broadened by
12%, 35%, and 44%, respectively. The broadening of the Hβ line was accompanied by a reduction in its equivalent width (EW
;
). It is suggested that a flare was observed in the nucleus of the galaxy on a time scale of 3 days, possibly caused to a
shock wave in long-lived flows from the galaxy nucleus.
__________
Translated from Astrofizika, Vol. 49, No. 4, pp. 499–510 (November 2006). 相似文献
14.
Results from a spectrophotometric study of 21 galaxies with UV excess are presented. The half widths (FWHM) and equivalent widths of observed spectrum lines of these galaxies, as well as the relative intensities of the emission lines observed in the spectrum of the galaxy Kaz243, are determined. It is conjectured that the latter galaxy has the properties of an Sy2 type galaxy. The electron densities and masses of the gaseous components are found for 15 galaxies, along with the masses of 8 galaxies for which the ratio M/L has been calculated. It is shown that the spectral structures of these galaxies do not depend on whether they are members of physical systems or are isolated. 相似文献
15.
We have modelled, for the cases of Milky Way and M31, the effects on the galactic discs, of the arrival at high velocity (≥150
km s−1) of giant HI clouds, with masses of up to 108M⊙. Predictions are compared with the detailed structure of the observed rotation curves for these two galaxies. The model
explains the rises and falls observed at large distances from the centre of each galaxy, distributed with a degree of regularity
in radius, in terms of a specific type of perturbations driven by the infall of the high velocity clouds (HVC's) arriving
from the intracluster medium of the Local Group. The underlying rotation curve is explained conventionally via the distribution
of the baryonic and dark matter components of the galaxy in question. This scenario, though tested here on the two major Local
Group objects, is in principle applicable to galaxies undergoing minor mergers with subgalactic mass gas clouds. 相似文献
16.
In order to simulate evolution of a large-scale magnetic field in a barred galaxy possessing a gaseous halo we apply a three-dimensional
(3D) MHD numerical model. We solve a induction equation using a time-dependent velocity field of molecular gas resulting from
self-consistent 3D N-body simulations of a galactic disk. The gaseous halo rotates differentially co-rotating with the disk.
In our model we introduce the dynamo process causing the amplification of the magnetic field as well as the formation of field
structures high above the galactic disk. The simulated magnetic fields are used to construct the models of a high-frequency
(Faraday rotation-free) polarized radio emission that accounts for effects of projection and limited resolution, and is thus
suitable for direct comparison with observations. We found that the resultant magnetic field correctly reproduces the observed
structures of polarization B-vectors, forming coherent patterns well aligned with spiral arms and with the bar. The process
initializing a wave-like behavior of the magnetic field, which efficiently forms magnetic maxima between the spiral arms,
is demonstrated.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
17.
The results of a spectral study of the galaxy Kaz 17 are presented. 150 lines are identified in its spectrum, of which 21
are in emission while the rest are absorption lines. These lines belong to the following elements and ions:..... and FeI.
Roughly 56% of the absorption lines belong to FeI. Of the absorption lines, 10 belonging to TiI and FeI are isolated. Because
of the closeness of their wavelengths, the rest merge and form separate absorption bands. 10 lines, including 8 emission lines,
are unidentified in the spectrum of Kaz 17. In all, 160 lines were observed in the spectrum of Kaz 17. In terms of the structure
of the emission lines, Kaz 17 can be assigned to type Sy1, and in terms of the absorption lines, it more closely resembles
a normal galaxy.
__________
Translated from Astrofizika, Vol. 49, No. 4, pp. 541–551 (November 2006). 相似文献
18.
We present a 3d code for the dynamical evolution of a multi-phase interstellar medium (ISM) coupled to stars via star formation
(SF) and feedback processes. The multi-phase ISM consists of clouds (sticky particles) and diffuse gas (SPH): exchange of
matter, energy and momentum is achieved by drag (due to ram pressure) and condensation or evaporation processes. The cycle
of matter is completed by SF and feedback by SNe and PNe. A SF scheme based on a variable SF efficiency as proposed by Elmegreen
and Efremov (1997) is presented. For a Milky Way type galaxy we get a SF rate of ∼1 M⊙ yr-1 with an average SF efficiency of ∼5%.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
19.
S. Baes-Fischlmair W.W. Zeilinger J.-C. Vega-Beltran J.E. Beckman 《Astrophysics and Space Science》2003,284(2):735-738
It is assumed that the two-fold disc-wide symmetry of spirals is caused by density waves, but also the potential of a bar
component may have a significant influence on structural properties. The strength of the bar component appears to be anti-correlated
with the degree of symmetry of star-forming regions in the spiral arms (Rozas et al., 1998). We present new results of R and
Hα surface photometry of a sample of bright barred spirals. A photometric decompositon of the galaxy components is carried
out in order to make a more accurate measurement of the strength of the bar and its interrelation to gas and stars in the
disc.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
20.
Results are presented from a detailed spectrophotometric study of a unique system of physically coupled galaxies whose spectra
contain identical emission lines. This system consists of four galaxies, three of which have a double or multiple structure
in their central portions and are, by definition, galaxies with two or multiple nuclei. Two of these objects are the galaxies
with an ultraviolet excess (UVE), Mark 261 and Mark 262, while one is a galaxy made up of two identical starlike nuclei that
are referred to as the “twin objects.” In the DSS2 charts, the latter show up as two adjacent stars without any surroundings.
However, there are two condensations between these starlike nuclei. The fourth object has a triplet structure. It is shown
here that all these objects have the same emission (line) spectra and red shifts. All the physical characteristics that can
be determined from the spectra are determined: red shifts, relative intensities of emission lines, their equivalent widths,
distances to the galaxies, etc. It is concluded that the members of this system have a common origin, more or less consistent
with Ambartsumyan's idea of the fragmentation of an isolated, high density body.
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Translated from Astrofizika, Vol. 48, No. 4, pp. 503–514 (November 2005). 相似文献