73 research outputs found
Constraints on distances to Galactic Centre non-thermal filaments from HI absorption
We have studied HI absorption towards three non-thermal filaments (NTFs)
Sgr-C, G359.54+0.18 and G359.79+0.17 using the Giant Metrewave Radio Telescope
(GMRT). Our study, for the first time, constrains the distance of the Sgr C NTF
and the HII region seen associated with the NTF in the sky plane, to within a
few hundred parsecs from the Galactic Centre (GC). A molecular cloud with a
velocity of -100 km/s appears to be associated with the central part of the Sgr
C NTF. Our study also indicates that the Sgr C HII region is relatively farther
away than the NTF along our line of sight, and thereby provides evidence
against any possible interaction between the two objects. The NTF G359.54+0.18
shows weak HI absorption (4 sigma detection) at a velocity of -140 km/s, which
is the velocity of a known dense molecular cloud seen towards the NTF. This
cloud is expected to be located within ~200 pc from the GC and thereby provides
a lower limit to the distance. The upper limit to the distance of this NTF from
the Sun is 10.5 kpc. The distance to the NTF G359.79+0.17 is between 5.1 and
10.5 kpc from the Sun.Comment: 11 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
Magnetic fields in nearby normal galaxies: Energy equipartition
We present maps of total magnetic field using 'equipartition' assumptions for
five nearby normal galaxies at sub-kpc spatial resolution. The mean magnetic
field is found to be ~11 \mu G. The field is strongest near the central regions
where mean values are ~20--25 \mu G and falls to ~15 \mu G in disk and ~10 \mu
G in the outer parts. There is little variation in the field strength between
arm and interarm regions, such that, in the interarms, the field is < 20
percent weaker than in the arms. There is no indication of variation in
magnetic field as one moves along arm or interarm after correcting for the
radial variation of magnetic field. We also studied the energy densities in
gaseous and ionized phases of the interstellar medium and compared to the
energy density in the magnetic field. The energy density in the magnetic field
was found to be similar to that of the gas within a factor of <2 at sub-kpc
scales in the arms, and thus magnetic field plays an important role in pressure
balance of the interstellar medium. Magnetic field energy density is seen to
dominate over the kinetic energy density of gas in the interarm regions and
outer parts of the galaxies and thereby helps in maintaining the large scale
ordered fields seen in those regions.Comment: 12 Pages, 6 Figures, Accepted to be published in MNRA
Synchrotron spectral index and interstellar medium densities of star-forming galaxies
The spectral index of synchrotron emission is an important parameter in
understanding the properties of cosmic ray electrons (CREs) and the
interstellar medium (ISM). We determine the synchrotron spectral index
() of four nearby star-forming galaxies, namely NGC 4736, NGC
5055, NGC 5236 and NGC 6946 at sub-kpc linear scales. The was
determined between 0.33 and 1.4 GHz for all the galaxies. We find the spectral
index to be flatter () in regions with total neutral (atomic +
molecular) gas surface density, , typically in the arms and inner parts of the galaxies. In regions
with , especially in the
interarm and outer regions of the galaxies, the spectral index steepens sharply
to . The flattening of is unlikely to be caused due to
thermal free--free absorption at 0.33 GHz. Our result is consistent with the
scenario where the CREs emitting at frequencies below GHz are
dominated by bremsstrahlung and/or ionization losses. For denser medium
(), having strong magnetic
fields (G), is seen to be flatter than ,
perhaps caused due to ionization losses. We find that, due to the clumpy nature
of the ISM, such dense regions cover only a small fraction of the galaxy
( percent). Thus, the galaxy-integrated spectrum may not show
indication of such loss mechanisms and remain a power-law over a wide range of
radio frequencies (between to 10 GHz).Comment: 10 pages, 4 figures, 2 tables, Accepted to be published in MNRA
Reorganizing Educational Institutional Domain using Faceted Ontological Principles
The purpose of this work is to find out how different library classification
systems and linguistic ontologies arrange a particular domain of interest and
what are the limitations for information retrieval. We use knowledge
representation techniques and languages for construction of a domain specific
ontology. This ontology would help not only in problem solving, but it would
demonstrate the ease with which complex queries can be handled using principles
of domain ontology, thereby facilitating better information retrieval.Comment: 26 pages, 12 figures, KNOWLEDGE ORGANIZATION Journal Pape
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