共查询到20条相似文献,搜索用时 31 毫秒
1.
Susanta K. Bisoi C. H. Ishwara-Chandra S. K. Sirothia P. Janardhan 《Journal of Astrophysics and Astronomy》2011,32(4):613-614
High red-shift radio galaxies are best searched at low radio frequencies, due to its steep radio spectra. Here we present
preliminary results from our programme to search for high red-shift radio galaxies to ∼10 to 100 times fainter than the known
population till date. We have extracted ultra-steep spectrum (USS) samples from deep 150 MHz Giant Meter-wave Radio Telescope
(GMRT) observations from one of the three well-studied DEEP2 fields to this effect. From correlating these radio sources with
respect to the high-frequency catalogues such as VLA, FIRST and NVSS at 1.4 GHz, we find ∼100 steep spectrum (spectral index,
α > 1) radio sources, which are good candidates for high red-shift radio galaxies. 相似文献
2.
Using the method of two-dimensional spectroscopy, we have investigated the kinematics and distribution of the gas and stars
at the center of the early-type spiral galaxy NGC 7177 with a mediumscale bar as well as the change in the mean age of the
stellar population along the radius. A classical picture of radial gas inflow to the galactic center along the shock fronts
delineated by dust concentration at the leading edges of the bar has been revealed. The gas inflow is observed down to a radius
R = 1″.5−2″, where the gas flows at the inner Lindblad resonance concentrate in an azimuthally highly inhomogeneous nuclear
star formation ring. The bar in NGC 7177 is shown to be thick in z coordinate—basically, it has already turned into a pseudo-bulge as a result of secular dynamical evolution. The mean stellar
age inside the star formation ring, in the galactic nucleus, is old, ∼10 Gyr.Outside, at a distance R = 6″−8″ from the nucleus, the mean age of the stellar population is ∼2 Gyr. If we agree that the bar in NGC 7177 is old,
then, obviously, the star formation ring has migrated radially inward in the last 1–2 Gyr, in accordance with the predictions
of some dynamical models. 相似文献
3.
D. Pérez-Ramírez J. H. Knapen † R. F. Peletier S. Laine R. Doyon † D. Nadeau † 《Monthly notices of the Royal Astronomical Society》2000,317(2):234-248
We present subarcsecond-resolution, ground-based, near-infrared images of the central regions of a sample of 12 barred galaxies with circumnuclear star formation activity, which is organized in ring-like regions typically 1 kpc in diameter. We also present Hubble Space Telescope near-infrared images of 10 of our sample galaxies, and compare them with our ground-based data. Although our sample galaxies were selected for the presence of circumnuclear star formation activity, our broad-band near-infrared images are heterogeneous, showing a substantial amount of small-scale structure in some galaxies, and practically none in others. We argue that, where it exists, this structure is caused by young stars, which also cause the characteristic bumps or changes in slope in the radial profiles of ellipticity, major axis position angle, surface brightness and colour at the radius of the circumnuclear ring in most of our sample galaxies. In seven out of 10 HST images, star formation in the nuclear ring is clearly visible as a large number of small emitting regions, organized into spiral arm fragments, which are accompanied by dust lanes. Near-infrared colour index maps show much more clearly the location of dust lanes and, in certain cases, regions of star formation than single broad-band images. Circumnuclear spiral structure thus outlined appears to be common in barred spiral galaxies with circumnuclear star formation. 相似文献
4.
GianLuca Israel 《Astrophysics and Space Science》2007,308(1-4):25-31
On 27th December 2004 SGR 1806–20, one of the most active Soft γ-ray Repeaters (SGRs), displayed an extremely rare event, also known as giant flare, during which up to 1047 ergs were released in the ∼1–1000 keV range in less than 1 s. Before and after the giant flare we carried out IR observations
by using adaptive optics (NAOS-CONICA) mounted on VLT which provided images of unprecedented quality (FWHM better than 0.1″).
We discovered the likely IR counterpart to SGR 1806–20 based on positional coincidence with the VLA uncertainty region and
flux variability of a factor of about 2 correlated with that at higher energies.
Moreover, by analysing the Rossi-XTE/PCA data we have discovered rapid Quasi-Periodic Oscillations (QPOs) in the pulsating
tail of the 27th December 2004 giant flare of SGR 1806–20. QPOs at ∼92.5 Hz are detected in a 50 s interval starting 170 s
after the onset of the giant flare. These QPOs appear to be associated with increased emission by a relatively hard unpulsed
component and are seen only over phases of the 7.56 s spin period pulsations away from the main peak. QPOs at ∼18 and ∼30 Hz
are also detected ∼200–300 s after the onset of the giant flare. This is the first time that QPOs are unambiguously detected
in the flux of a Soft Gamma-ray Repeater, or any other isolated neutron star. We interpret the highest QPOs in terms of the
coupling of toroidal seismic modes with Alfvén waves propagating along magnetospheric field lines. The lowest frequency QPO
might instead provide indirect evidence on the strength of the internal magnetic field of the neutron star.
相似文献
5.
The submillimeter (submm) extragalactic background light (EBL) traces the integrated star formation history throughout the
cosmic time. Deep blank-field 850 μm and 1.4 GHz surveys and optical follow-up have been only able to determine the redshift
of ∼20% of the submm EBL. The majority (80%) of the submm EBL is still below the confusion and sensitivity limits of current
submm and radio instruments. We break through these limits with stacking analyses on our deep 850 μm image in the GOODS-N
and find that the submm EBL mostly comes from galaxies at redshifts around 1.0. This redshift is much lower than the redshift
of z=2–3 previously implied from radio identified submm sources. This result significantly decreases the number of high redshift
galaxies that may be seen by ALMA. 相似文献
6.
C. L. Carilli F. Walter R. Wang A. Wootten K. Menten F. Bertoldi E. Schinnerer P. Cox A. Beelen A. Omont 《Astrophysics and Space Science》2008,313(1-3):307-311
We discuss observations of the first galaxies, within cosmic reionization, at centimeter and millimeter wavelengths. We present
a summary of current observations of the host galaxies of the most distant QSOs (z∼6). These observations reveal the gas, dust, and star formation in the host galaxies on kpc-scales. These data imply an enriched
ISM in the QSO host galaxies within 1 Gyr of the big bang, and are consistent with models of coeval supermassive black hole
and spheroidal galaxy formation in major mergers at high redshift. Current instruments are limited to studying truly pathologic
objects at these redshifts, meaning hyper-luminous infrared galaxies (L
FIR
∼1013
L
⊙). ALMA will provide the one to two orders of magnitude improvement in millimeter astronomy required to study normal star
forming galaxies (i.e. Ly-α emitters) at z∼6. ALMA will reveal, at sub-kpc spatial resolution, the thermal gas and dust—the fundamental fuel for star formation—in galaxies
into cosmic reionization. 相似文献
7.
Alfonso Aragón-Salamanca Bo Milvang-Jensen George Hau Inger Jørgensen Jens Hjorth 《Astrophysics and Space Science》2002,281(1-2):339-342
We are carrying out a programme to measure the evolution of the stellar and dynamical masses and M/L ratios for a sizeable sample of morphologically-classified disk galaxies in rich galaxy clusters at 0.2 < z < 0.9. Using FORS2 at the VLT we are obtaining rotation curves for the cluster spirals so that their Tully-Fisher relation
can be studied as a function of redshift and compared with that of field spirals. We already have rotation curves for ∼ 10
cluster spirals at z = 0.83, and 25 field spirals at lower redshifts and we plan to increase this sample by one order of magnitude. We present
here the first results of our study, and discuss the implications of our data in the context of current ideas and models of
galaxy formation and evolution.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
8.
S. Lourenso A. Vazdekis R.F. Peletier J.E. Beckman 《Astrophysics and Space Science》2001,276(2-4):651-658
As part of an ongoing project aimed at studying the age and metallicity gradients of the stellar populations along the bars
of a sample of barred spirals of different morphological types, we present our first results on NGC 4314 (SBa). We have obtained
optical and NIR colours and spectral indices along the bar and we interpret some of these results here and discuss their uncertainties
on the basis of single stellar population models. In a preliminary analysis, we constrain the limits for the age and metallicity
of the nucleus and two selected regions in the star formation ring of NGC 4314, characterizing both as metal rich (Z<Z
solar) stellar populations, and finding a difference in the mean luminosity-weighted age of at least ∼ 3–4 Gyr.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
By systematically searching regions around planetary nebulae (PNe) for signs of interactions of their precursors’ wind with
ambient matter we found a number of huge IRAS dust structures. Some of them may be chance projections, but a few appear to
be real, like those around NGC 6826 and NGC 2899. In the case of NGC 6826 we noticed a giant (∼2°) bipolar dust emission,
whose axis is along the proper motion of the central star. The PN itself is offset in the direction of motion both as to the
center of this ∼30 pc large dust structure and to the center of a similarly large new Hα nebula. NGC 2899 was found in the center of a 14×11 pc quadrupolar cavity, whose directions of axes coincide with the directions
of the main axes of the optical PN. In both cases, the formation of these structures appears to have commenced in the asymptotic
giant branch (AGB) phase. 相似文献
10.
Jesper Sommer-Larsen Martin Götz Laura Portinari 《Astrophysics and Space Science》2002,281(1-2):519-524
We have performed TreeSPH simulations of galaxy formation in a standard ΛCDM cosmology, including effects of star formation,
energetic stellar feedback processes and a meta-galactic UV field, and obtain a mix of disk, lenticular and elliptical galaxies.
The disk galaxies are deficient in angular momentum by only about a factor of two compared to observed disk galaxies. The
stellar disks have approximately exponential surface density profiles, and those of the bulges range from exponential to r
1/4, as observed. The bulge-to-disk ratios of the disk galaxies are consistent with observations and likewise are their integrated
B-V colours, which have been calculated using stellar population synthesis techniques. Furthermore, we can match the observed
I-band Tully-Fisher (TF) relation, provided that the mass-to-light ratio of disk galaxies is (M/L
I) ≃ 0.6–0.7. The ellipticals and lenticulars have approximately r
1/4 stellar surface density profiles, are dominated by non-disklike kinematics and flattened due to non-isotropic stellar velocity
distributions, again consistent with observations.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
11.
DDO 68 (UGC 5340) is the second most metal-poor star-forming galaxy (12 + log(O/H) = 7.14). Its peculiar optical morphology
and its HI distribution and kinematics are indicative of a merger origin. We use the u, g, r, and i photometry based on the SDSS images of DDO 68 to estimate its stellar population ages. The Hα images of DDO 68 were used to select several representative regions without nebular emission. The derived colors were analyzed
by comparison with the PEGASE2 evolutionary tracks for various star formation (SF) scenarios, including the two extreme cases:
(i) an instantaneous starburst and (ii) continuous SF with a constant rate. The (u − g) and (g − r) colors for all of the selected regions are consistent with the scenario of several “instantaneous” SF episodes with ages
between ∼0.05 and ∼1 Gyr. The total mass of the visible stars in DDO 68 was estimated by comparing the colors and fluxes of
the observed stellar subsystems with PEGASE2 models to be ∼2.4 × 107
M
⊙. This accounts for ∼6% of the total baryonic mass of the galaxy. All of the available data are consistent with the fact that
DDO 68 is a very rare candidate for young galaxies. The bulk of its stars were formed during the recent (with the first encounter
∼1 Gyr ago) merger of two very gas-rich disks. DDO 68 is closest in its properties to cosmologically young low-mass galaxies.
This article was submitted by the authors in English. 相似文献
12.
R. Guzmán 《Astrophysics and Space Science》1998,263(1-4):127-130
We study the nature of faint blue compact galaxies (BCGs) at redshifts z ∼ 0.2 - 1.3 using Keck and HST. Despite being very
luminous (LB ∼ L*), most distant BCGs have masses M ∼ 1010M⊙, i.e., they are dwarf stellar systems. The majority of these galaxies have colors, sizes, surface brightnesses, luminosities,
velocity widths, excitations, star formation rates (SFR), and mass-to-light ratios characteristic of the most luminous nearby
HII galaxies. The more massive BCGs form a more heterogeneous class of evolved starburst, similar to local disk starburst
galaxies. Without additional star formation, HII-like BCGs will most likely fade to resemble today's spheroidal galaxies such
as NGC 205.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
C. D. Wilson G. R. Petitpas D. Iono A. Peck M. Krips B. E. Warren A. J. Baker M. S. Yun Y. Pihlstrom C. Mihos S. Matsushita M. Juvela P. T. P. Ho T. J. Cox L. Armus 《Astrophysics and Space Science》2008,313(1-3):297-302
Luminous and Ultraluminous infrared galaxies (ULIRGs) contain the most intense regions of star formation in the local universe.
Because molecular gas is the fuel for current and future star formation, the physical properties and distribution of the warm,
dense molecular gas are key components for understanding the processes and timescales controlling star formation in these
merger and merger remnant galaxies. We present new results from a legacy project on the Submillimeter Array which is producing
high resolution images of a representative sample of galaxies with log L
FIR
>11.4 and D<200 Mpc. 相似文献
14.
Edward P. J. Van den Heuvel 《Journal of Astrophysics and Astronomy》1984,5(3):209-233
The peculiar combination of a relatively short pulse period and a relatively weak surface dipole magnetic field strength of
binary radio pulsars finds a consistent explanation in terms of (i) decay of the surface dipole component of neutron-star
magnetic fields on a timescale of (2–5) × 106 yr, in combination with (ii) spin-up of the rotation of the neutron star during a subsequent mass-transfer phase.
The four known binary radio pulsars appear to fall into two different categories. Two of them, PSR 0655 + 64 and PSR 1913
+ 16, have short orbital periods (<25 h) and high mass functions, indicating companion masses 0.7M⊙ (∼1 (± 0.3) M⊙ and 1.4 M⊙, respectively). The other two, PSR 0820 + 02 and PSR 1953 + 29, have long orbital periods (117d),
nearly circular orbits, and low, almost identical mass functions of about 3×10-3 M⊙, suggesting companion masses of about 0.3M⊙. It is pointed out that these two classes of systems are expected to be formed by the later evolution of binaries consisting
of a neutron star and a normal companion star, in which the companion was (considerably) more massive than the neutron star,
or less massive than the neutron star, respectively. In the first case the companion of the neutron star in the final system
will be a massive white dwarf, in a circular orbit, or a neutron star in an eccentric orbit. In the second case the final
companion to the neutron star will be a low-mass (∼ 0.3 M⊙) helium white dwarf in a wide and nearly circular orbit.
In systems of the second type the neutron star was most probably formed by the accretion-induced collapse of a white dwarf.
This explains in a natural way why PSR 1953 + 29 has a millisecond rotation period and PSR 0820 + 02 has not.
Among the binary models proposed for the formation of the 1.5-millisecond pulsar, the only ones that appear to be viable are
those in which the companion disappeared by coalescence with the neutron star. In such models the companion may have been
a red dwarf of mass 0.03M⊙, a neutron star, or a massive (>0.7M⊙) white dwarf. Only in the last-mentioned case is a position of the pulsar close to the galactic plane a natural consequence.
In the first-mentioned case the progenitor system most probably was a cataclysmic-variable binary in which the white dwarf
collapsed by accretion. 相似文献
15.
Stellar abundance pattern of n-capture elements such as barium is used as a powerful tool to infer how the star formation proceeded in dwarf spheroidal
(dSph) galaxies. It is found that the abundance correlation of barium with iron in stars belonging to dSph galaxies orbiting
the Milky Way, i.e., Draco, Sextans, and Ursa Minor have a feature similar to that in Galactic metal-poor stars. The common
feature of these two correlations can be realized by our in homogeneous chemical evolution model based on the supernova-driven
star formation scenario if dSph stars formed from gas with a velocity dispersion of ∼ 26 km s-1. This velocity dispersion together with the stellar luminosities strongly suggest that dark matter dominated dSph galaxies.
The tidal force of the Milky Way links this velocity dispersion with the currently observed value ≲ 10 km s-1 by stripping the dark matter in dSph galaxies. As a result, the total mass of each dSph galaxy is found to have been originally
∼ 25 times larger than at present. In this model, supernovae immediately after the end of the star formation can expel the
remaining gas over the gravitational potential of the dSph galaxy.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
16.
The causes of variability of line profiles in the spectra of O supergiants are analyzed. It is suggested that the main cause
of the variability is the motion in the atmosphere of dense clumps of matter (inhomogeneities or clouds) along the Une of
sight between star and observer. The profiles of C IV and Si IV UV resonance lines in the spectra of bright OB supergiants
are calculated for spherically symmetric atmospheres and for atmospheres with inhomogeneity along the line of sight. The dependence
of the line profiles on the distance of the inhomogeneity from the center of the star is investigated. It is shown that the
formation and evolution with time of discrete absorption components (DACs) in the profiles of C IV and Si IV UV resonance
lines can be explained within the framework of the proposed model of variability of line profiles. The parameters of the inhomogeneities
moving in the atmosphere to produce DACs are estimated.
Translated from Astrofizika, Vol. 41, No. 3, pp. 423–441, July–September, 1998. 相似文献
17.
A statistical study of the dependence of the star formation rate in the nuclear regions of 39 Kazarian galaxies on the integral
parameters of these galaxies is carried out on the basis of spectra from SDSS DR6. The value of SFR/kpc2 for our sample lies in the range 0.013÷2.04M
⨀ year−1kpc−2 (with the maximum value of 2.04 corresponding to the Kaz 98 (merger)). It is found that the surface density of the rate of
star formation correlates positively with the bar structure parameter and EW(Hα), and that, for spiral galaxies of early morphological types, SFR/kpc2 is greater than for the later types. It is shown that the color B-R for the galaxies and the color (u − g)
nucl
for the nuclear region correlate positively with the total absorption A(Hα) in the Ha line for the nuclear region. The average value of A(Hα) for our samples is found to be A(Hα)=1.3±0.09 magnitudes.
Translated from Astrofizika, Vol. 52, No. 2, pp. 211–224 (May 2009). 相似文献
18.
We present the first modern systematic study of local brightest cluster galaxy (BCG) profiles that extends to radii beyond
200h
-1kpc. Measuring the surface brightness profiles of BCGs out to large radii is critical for understanding the processes driving
their formation. The form of the profiles yields information about the current dynamical state, constrains the accretion history
of these galaxies, and places limits on any radially symmetric component of intracluster light. The observational challenges
associated with CCD photometry at low surface brightness levels have until now precluded such an analysis for a statistical
sample of BCGs. Utilizing drift-scan data and new techniques that we have developed, we extend upon previous work by modelling
the profiles for a sample of 31 clusters at z≃ that span a wide range in mass and dynamical state. We find that the BCGs in our sample generally are best fit using two-component
models consisting of inner and outer Sersic profiles. In this proceeding we present the preliminary results of our analysis
and discuss implications for current models of BCG formation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
We present narrow band AAO/UKST Hα images and medium and low resolution optical spectra of a nebula shell putatively associated with the Wolf-Rayet star WR 60.
We also present the first identification of this shell in the radio regime at 843 MHz and at 4850 MHz from the Sydney University
Molonglo Sky Survey (SUMSS), and from the Parkes-MIT-NRAO (PMN) survey respectively. This radio emission closely follows the
optical emission. The optical spectra from the shell exhibits the typical shock excitation signatures sometimes seen in Wolf-Rayet
stellar ejecta but also common to supernova remnants. A key finding however, is that the WR 60 star, is not, in fact, anywhere
near the geometrical centre of the putative arcuate nebula ejecta as had been previously stated. This was due to an erroneous
positional identification for the star in the literature which we now correct. This new identification calls into serious
question any association of the nebula with WR 60 as such nebula are usually quite well centred on the WR stars themselves.
We now propose that this fact combined with our new optical spectra, deeper Hα imaging and newly identified radio structures actually imply that the WR 60 nebula should be reclassified as an unassociated
new supernova remnant which we designate G310.5+0.8. 相似文献
20.
We describe technical aspects of an astrometric and photometric survey of the North Celestial Cap (NCC), from the Pole (δ=90°) to δ=80°, in support of the TAUVEX mission. This region, at galactic latitudes from ∼17° to ∼37°, has poor coverage in modern
CCD-based surveys. The observations are performed with the Wise Observatory one-meter reflector and with a new mosaic CCD
camera (LAIWO) that images in the Johnson–Cousins R and I bands a one-square-degree field with sub-arcsec pixels. The images are treated using IRAF and SExtractor to produce a final
catalogue of sources. The astrometry, based on the USNO-A2.0 catalogue, is good to ∼1 arcsec and the photometry is good to
∼0.1 mag for point sources brighter than R=20.0 or I=19.1 mag. The limiting magnitudes of the survey, defined at photometric errors smaller than 0.15 mag, are 20.6 mag (R) and 19.6 (I). We separate stars from non-stellar objects based on the object shapes in the R and I bands, attempting to reproduce the SDSS star/galaxy dichotomy. The completeness test indicates that the catalogue is complete
to the limiting magnitudes. 相似文献