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1.
Somnath Bharadwaj Biman B. Nath Shiv K. Sethi 《Journal of Astrophysics and Astronomy》2001,22(1):21-34
The redshifted 1420 MHz emission from the HI in unresolved damped Lyman-α clouds at high z will appear as a background radiation
in low frequency radio observations. This holds the possibility of a new tool for studying the universe at high-z, using the mean brightness temperature to probe the HI content and its fluctuations to probe power spectrum. Existing estimates
of the HI density atz−3 imply a mean brightness temperature of 1 mK at 320 MHz. The cross-correlation between the temperature fluctuation across
different frequencies and sight lines is predicted to vary from 10−7 K2 to 10−8 K2 over intervals corresponding to spatial scales from 10 Mpc to 40 Mpc for some of the currently favoured cosmological models.
Comparing this with the expected sensitivity of the GMRT, we find that this can be detected with ∼ 10 hrs of integration,
provided we can distinguish it from the galactic and extragalactic foregrounds which will swamp this signal. We discuss a
strategy based on the very distinct spectral properties of the foregrounds as against the HI emission, possibly allowing the
removal of the foregrounds from the observed maps. 相似文献
2.
3.
Rekhesh Mohan K. S. Dwarakanath G. Srinivasan Jayaram N. Chengalur 《Journal of Astrophysics and Astronomy》2001,22(1):35-50
Nearby interstellar clouds with high (|ν|≥10km s−1) random velocities although easily detected in NaI and CaII lines have hitherto not been detected (in emission or absorption)
in the HI 21cm line. We describe here deep Giant Metrewave Radio Telescope (GMRT) HI absorption observations toward radio
sources with small angular separation from bright O and B stars whose spectra reveal the presence of intervening high random
velocity CaII absorbing clouds. In 5 out of the 14 directions searched we detect HI 21cm absorption features from these clouds.
The mean optical depth of these detections is ∼0.09 and FWHM is ∼10km s−1, consistent with absorption arising from CNM clouds. 相似文献
4.
High sensitivity observations of radio halos in galaxy clusters at frequencies ν ≤ 330 MHz are still relatively rare, and very little is known compared to the classical 1.4 GHz images. The few radio halos
imaged down to 150–240 MHz show a considerable spread in size, morphology and spectral properties. All clusters belonging
to the GMRT Radio Halo Survey with detected or candidate cluster-scale diffuse emission have been imaged at 325 MHz with the
GMRT. Few of them were also observed with the GMRT at 240 MHz and 150 MHz. For A 1682, imaging is particularly challenging
due to the presence of strong and extended radio galaxies at the center. Our data analysis suggests that thew radio galaxies
are superposed to very low surface brightness radio emission extended on the cluster scale, which we present here. 相似文献
5.
The GMRTHI 21 cm-line observations of galaxies in the Eridanus group are presented. The Eridanus group, at a distance of ≈ 23 Mpc, is
a loose group of ≈200 galaxies. The group extends to more than 10 Mpc in projection. The velocity dispersion of the galaxies
in the group is ≈240 km s−1. The galaxies are clustered into different sub-groups. The overall population mix of the group is 30% (E + S0) and 70% (Sp
+ Irr). The observations of 57 Eridanus galaxies were carried out with the GMRT for ≈ 200 h. HI emission was detected from
31 galaxies. The channel rms of ≈ 1 mJy beam−1 was achieved for most of the image-cubes made with 4 h of data. The corresponding HI column density sensitivity (3σ) is ≈
1 × 1020 cm−2 for a velocity-width of ≈ 13.4 km s−1. The 3σ detection limit of HI mass is ≈ 1.2 X 107 Mpd for a line-width of 50 km s−1. Total HI images, HI velocity fields, global HI line profiles, HI mass surface densities, HI disk parameters and HI rotation
curves are presented. The velocity fields are analysed separately for the approaching and the receding sides of the galaxies.
These data will be used to study the HI and the radio continuum properties, the Tully-Fisher relations, the dark matter halos,
and the kinematical and HI lopsidedness in galaxies. 相似文献
6.
Rekhesh Mohan K. S. Dwarakanath G. Srinivasan 《Journal of Astrophysics and Astronomy》2004,25(3-4):185-201
We have carried out a sensitive high-latitude (|b| > 15°) HI 21 cm-line absorption survey towards 102 sources using the GMRT. With a 3σ detection limit in optical depth of
∼ 0.01, this is the most sensitive HI absorption survey. We detected 126 absorption features most of which also have corresponding
HI emission features in the Leiden Dwingeloo Survey of Galactic neutral Hydrogen. The histogram of random velocities of the
absorption features is well-fit by two Gaussians centered at V1sr ∼ 0 km s−1 with velocity dispersions of 7.6 ± 0.3 km s−1 and 21 ± 4 km s−1 respectively. About 20% of the HI absorption features form the larger velocity dispersion component. The HI absorption features
forming the narrow Gaussian have a mean optical depth of 0.20 ± 0.19, a mean HI column density of (1.46 ± 1.03) × 1020 cm−2, and a mean spin temperature of 121 ± 69 K. These HI concentrations can be identified with the standard HI clouds in the
cold neutral medium of the Galaxy. The HI absorption features forming the wider Gaussian have a mean optical depth of 0.04
± 0.02, a mean HI column density of (4.3 ± 3.4) × 1019 cm−2, and a mean spin temperature of 125 ± 82 K. The HI column densities of these fast clouds decrease with their increasing random
velocities. These fast clouds can be identified with a population of clouds detected so far only in optical absorption and
in HI emission lines with a similar velocity dispersion. This population of fast clouds is likely to be in the lower Galactic
Halo. 相似文献
7.
The Eridanus galaxies follow the well-known radio—FIR correlation. The majority (70%) of these galaxies have their star formation
rates below that of the Milky Way. The galaxies that have a significant excess of radio emission are identified as low luminosity
AGNs based on their radio morphologies obtained from the GMRT observations. There are no powerful AGNs (L
20cm > 1023 W Hz−1) in the group. The two most far-infrared and radio luminous galaxies in the group have optical and HI morphologies suggestive
of recent tidal interactions. The Eridanus group also has two far-infrared luminous but radio-deficient galaxies. It is believed
that these galaxies are observed within a few Myr of the onset of an intense star formation episode after being quiescent
for at least a 100 Myr. The upper end of the radio luminosity distribution of the Eridanus galaxies (L
20cm ∼ 1022 W Hz−1) is consistent with that of the field galaxies, other groups, and late-type galaxies in nearby clusters. 相似文献
8.
K. S. Dwarakanath Siddharth Malu Ruta Kale 《Journal of Astrophysics and Astronomy》2011,32(4):529-532
We have discovered a giant radio halo in the massive merging cluster MACSJ0417.5-1154. This cluster, at a redshift of 0.443,
is one of the most X-ray luminous galaxy cluster in the MAssive Cluster Survey (MACS) with an X-ray luminosity in the 0.1–2.4
keV band of 2.9×1045 erg s − 1. Recent observations from GMRT at 230 and 610 MHz have revealed a radio halo of ∼ 1.2 × 0.3 Mpc2 in extent. This halo is elongated along the North-West, similar to the morphology of the X-ray emission from Chandra. The
1400 MHz radio luminosity (L
r) of the halo is ∼2 × 1025 W Hz − 1, in good agreement with the value expected from the L
x − L
r correlation for cluster halos. 相似文献
9.
Using the recently completed Giant Meterwave Radio Telescope, we have detected the HI 21 cm-line absorption from the peculiar
galaxy C153 in the galaxy cluster Abell 2125. The HI absorption is at a redshift of 0.2533, with a peak optical depth of 0.36.
The full width at half minimum of the absorption line is 100 km s−1. The estimated column density of atomic Hydrogen is 0.7×1022(T
s
/100) cm−2. The HI absorption is redshifted by ∼400km s−1 compared to the [OIII] emission line from this system. We attribute this to an infalling cold gas or to an out-flowing ionised
gas, or to a combination of both as a consequence of tidal interactions of C153 with either a cluster galaxy or the cluster
potential. 相似文献
10.
X.Z. Zhang S.J. Qian L.A. Higgs T.L. Landecker X.J. Wu 《Astrophysics and Space Science》2002,279(4):355-366
Radio observational results at 232 MHz and multifrequency studies of supernova remnant (SNR) HB21 are presented. Its integrated
flux density at 232 MHz is about 390 ± 30 Jy. Both the integrated spectral index and the spatial variations of spectral index
of the remnant were calculated by combining the new map at 232 MHz with previously published maps made at 408, 1420, 2695,
and 4750 MHz. The SNR has an integrated spectral index of about α = -0.43(S
ν ∝ να) between 232 and 4750 MHz. In general the spectral index varies from –0.5 in southeast and west regions of the remnant to
–0.3 in the central region and near the northwest edge. The new data of 232 MHz reveals that there is interaction between
the remnant and the surrounding gas along the east edge of the remnant which causes the spectrum flattening at low frequency,
while the very good agreement between the structure of X-ray emission and the central flat spectrum area suggests that the
existence of thermal emission is the reason of spectrum flattening in the area.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
Robert Braun 《New Astronomy Reviews》2004,48(11-12):1271
Only in recent years has the realization emerged that galaxies do not dominate the universal baryon budget but are merely the brightest pearls of an underlying cosmic web. Although the gas in these inter-galactic filaments is moderately to highly ionized, QSO absorption lines have shown that the surface area increases dramatically in going down to lower HI column densities. The first image of the cosmic web in HI emission has just been made of the Local Group filament connecting M31 and M33. The corresponding HI distribution function is in very good agreement with that of the QSO absorption lines, confirming the 30-fold increase in surface area expected between 1019 and 1017 cm−2. The critical observational challenge is crossing the “HI desert”, the range of log(NHI) from about 19.5 down to 18, over which photo-ionization by the intergalactic radiation field produces an exponential decline in the neutral fraction from essentially unity down to a few percent. Nature is kinder again to the HI observer below log(NHI) = 18, where the neutral fraction decreases only very slowly with log(NHI). With the SKA, we can begin the systematic study of the cosmic web beyond the Local Group. With moderate integration times, the necessary resolution and sensitivity can be achieved out to distances beyond the Virgo cluster. When combined with targeted optical and UV absorption line observations, the total baryonic masses and enrichment histories of the cosmic web will be determined over the complete range of environmental over-densities. 相似文献
12.
Sk. Saiyad Ali Somnath Bharadwaj Jayaram N. Chengalur 《Monthly notices of the Royal Astronomical Society》2008,385(4):2166-2174
Foreground subtraction is the biggest challenge for future redshifted 21-cm observations to probe reionization. We use a short Giant Meter Wave Radio Telescope (GMRT) observation at 153 MHz to characterize the statistical properties of the background radiation across ∼1° to subarcmin angular scales, and across a frequency band of 5 MHz with 62.5 kHz resolution. The statistic we use is the visibility correlation function, or equivalently the angular power spectrum C l . We present the results obtained from using relatively unsophisticated, conventional data calibration procedures. We find that even fairly simple-minded calibration allows one to estimate the visibility correlation function at a given frequency V 2 ( U , 0) . From our observations, we find that V 2 ( U , 0) is consistent with foreground model predictions at all angular scales except the largest ones probed by our observations where the model predictions are somewhat in excess. On the other hand, the visibility correlation between different frequencies κ( U , Δν) seems to be much more sensitive to calibration errors. We find a rapid decline in κ( U , Δν) , in contrast with the prediction of less than 1 per cent variation across 2.5 MHz. In this case, however, it seems likely that a substantial part of the discrepancy may be due to limitations of data reduction procedures. 相似文献
13.
A high galactic latitude HI 21 cm-line absorption survey using the GMRT: I. Observations and spectra
Rekhesh Mohan K. S. Dwarakanath G. Srinivasan 《Journal of Astrophysics and Astronomy》2004,25(3-4):143-183
We have used the Giant Meterwave Radio Telescope (GMRT) to measure the Galactic HI 21-cm line absorption towards 102 extragalactic
radio continuum sources, located at high (|b| > 15°) Galactic latitudes. The Declination coverage of the present survey is δ}> - 45°. With a mean rms optical depth of ∼ 0.003, this is the most sensitive Galactic HI 21-cm line absorption survey to
date. To supplement the absorption data, we have extracted the HI 21-cm line emission profiles towards these 102 lines of
sight from the Leiden Dwingeloo Survey of Galactic neutral hydrogen. We have carried out a Gaussian fitting analysis to identify
the discrete absorption and emission components in these profiles. In this paper, we present the spectra and the components.
A subsequent paper will discuss the interpretation of these results. 相似文献
14.
O. V. Abramova 《Astronomy Letters》2012,38(4):222-230
Observations of neutral hydrogen in spiral galaxies reveal a sharp cutoff in the radial density profile at some distance from
the center. Using 22 galaxies with known HI distributions as an example, we discuss the question of whether this effect can
be associated exclusively with external ionizing radiation, as is commonly assumed. We show that before the surface density
reaches δ
HI ≤ 0.5 M
⊙ pc−2 (the same for galaxies of different types), it is hard to expect the gas to be fully ionized by background radiation. For
two of 13 galaxies with a sharp drop in the HI profile, the “steepening” can actually be caused by ionization. At the same
time, for the remaining galaxies, the observed cutoff in the radial HI profile is closer to the center than if it was a consequence
of ionization by background radiation and, therefore, it should be caused by other factors. 相似文献
15.
Prospects for Detecting the 326.5 MHz Redshifted 21-cm HI Signal with the Ooty Radio Telescope (ORT)
Observations of the redshifted 21-cm HI fluctuations promise to be an important probe of the post-reionization era (z≤ 6). In this paper we calculate the expected signal and foregrounds for the upgraded Ooty Radio Telescope (ORT) which operates at frequency ν o = 326.5 MHz which corresponds to redshift z = 3.35. Assuming that the visibilities contain only the HI signal and system noise, we show that a 3 σ detection of the HI signal (~1 mK) is possible at angular scales 11′ to 3° with ≈1000 h of observation. Foreground removal is one of the major challenges for a statistical detection of the redshifted 21 cm HI signal. We assess the contribution of different foregrounds and find that the 326.5 MHz sky is dominated by the extragalactic point sources at the angular scales of our interest. The expected total foregrounds are 104?105 times higher than the HI signal. 相似文献
16.
We analyze the influence of neutrino helicity conversion, ν
L → ν
R, on the neutrino flux from a supernova attributable to the interaction of the Dirac neutrino magnetic moment with a magnetic
field.We show that if the neutrino has a magnetic moment in the interval 10−13μB < μν < 10−12μB and provided that a magnetic field of ∼1013–1014 G exists in the supernova envelope, a peculiar kind of time evolution of the neutrino signal from the supernova attributable
to the resonance transition ν
L → ν
R in the magnetic field of the envelope can appear. 相似文献
17.
We present the results of a search for carbon recombination lines in the Galaxy at 34.5 MHz (C575α) made using the dipole
array at Gauribidanur near Bangalore. Observations made towards 32 directions resulted in detections of lines, in absorption
at nine positions. Followup observations at 328 MHz (C272α) using the Ooty Radio Telescope detected these lines in emission.
A VLA D-array observation of one of the positions at 330 MHz yielded no detection implying a lower limit of 10′ for the angular
size of the line forming region.
The longitude-velocity distribution of the observed carbon lines indicate that the line forming regions are located mainly
between 4 kpc and 7 kpc from the Galactic centre. Combining our results with published carbon recombination line data near
76 MHz (Erickson, McConnell & Anantharamaiah 1995), we obtain constraintson the physical parameters of the line forming regions.
We find thatif the angular size of the line forming regions is ≥ 4°, then the range of parameters that fit the data are:T
e
=20–40 K,n
e
∼ 0.1–0.3 cm−3 and pathlengths ∼ 0.07–0.9 pc which may correspond to thin photodissociated regions around molecular clouds. On the other
hand, if the line forming regions are ∼ 2° in extent, then warmer gas (T
e
∼ 60–300 K) with lower electron densities (n
e
∼ 0.03–0.05 cm−3) extending over several tens of parsecs along the line of sight and possibly associated with atomic HI gas can fit the data.
Based on the range of derived parameters, we suggest that the carbon line regions are most likely associated with photo-dissociation
regions. 相似文献
18.
D. A. Schwartz D. E. Harris H. Landt A. Siemiginowska H. L. Marshall J. M. Gelbord E. S. Perlman M. Georganopoulos M. Birkinshaw D. M. Worrall C. C. Cheung L. Stawarz S. G. Jorstad A. P. Marscher Y. Uchiyama C. M. Urry 《Astrophysics and Space Science》2007,311(1-3):341-345
We present a Chandra image of the quasar, jet, and lobes of PKS 1354+195 (=4C 19.44). The radio jet is 18 arcsec long, and appears to be very
straight. The length gives many independent spatial resolution elements in the Chandra image while the straightness implies that the geometrical factors are constant along the jet although their values are uncertain.
We also have 4 frequency radio images with half to one arcsecond angular resolution, and use HST and Spitzer data to study
the broad band spectral energy distributions. The X-ray and radio spectra are both consistent with a spectrum f
ν
∝
ν
−0.7 for the integrated jet. Using that spectral index, the model of inverse Compton scattering of electrons on the cosmic microwave
background (IC/CMB) gives magnetic field strengths and Doppler factors that are relatively constant along the jet. Extended
X-ray emission is evident in the direction of the otherwise unseen counter-jet. X-ray emission continues past the radio jet
to the South, and is detected within both the southern and northern radio lobes. 相似文献
19.
Gangabrata Mukherjee 《Journal of Astrophysics and Astronomy》1986,7(4):259-273
We present the condition of vanishing shear in a spatially homogeneous spacetime in terms of the Ricci rotation co-efficients
corresponding to an orthonormal tetrad (ν
α.
A
η
α) (whereν
α is the unit vector along the time axis and
A
η
α are the three independent reciprocal group vectors). Assuming that the velocity vector can be expanded in the direction ofν
α and any one of the
A
η
α’s it is shown that shear-free motion is possible only in case of some special Bianchi types, and these cases are studied
assuming the velocity vector to be geodetic and that there may be a nonvanishing heat flux term. 相似文献
20.
The Tully-Fisher (TF) or the luminosity-linewidth relations of the galaxies in the Eridanus group are constructed using the
HI rotation curves and the luminosities in the optical and in the near-infrared bands. The slopes of the TF relations (absolute
magnitudevs log2V
flat) are −8.6 ± 1.1, −10.0 ±1.5, −10.7 ±2.1, and −9.7 ±1.3 in the R, J, H, and K bands respectively for galaxies having flat
HI rotation curves. These values of the slopes are consistent with those obtained from studies of other groups and clusters.
The scatter in the TF relations is in the range 0.5-1.1 mag in different bands. This scatter is considerably larger compared
to those observed in other groups and clusters. It is suggested that the larger scatter in the TF relations for the Eridanus
group is related to the loose structure of the group. If the TF relations are constructed using the baryonic mass (stellar
+HI + Helium mass) instead of the stellar luminosity, nearly identical slopes are obtained in the R and in the near-infrared
bands. The baryonic TF (baryonic massvs log2V
flat) slope is in the range 3.5–4.1. 相似文献