69 research outputs found
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
Radio Polarisation Study of High Rotation Measure AGNs
As radio polarised emission from astrophysical objects traverse through
foreground magnetised plasma, the physical conditions along the lines of sight
are encrypted in the form of Rotation Measure (RM). We performed broadband
spectro-polarimetric observations of high Rotation Measure (|RM| >~ 300 rad
m-2) sources away from the Galactic plane (|b| > 10 deg) selected from the NVSS
RM catalogue. The main goals are to verify the NVSS RM values, which could be
susceptible to n{\pi}-ambiguity, as well as to identify the origin of the
extreme RM values. We show that 40 % of our sample suffer from n{\pi}-ambiguity
in the NVSS RM catalogue. There are also hints of RM variabilities over ~20
years epoch for most of our sources, as revealed by comparing the RM values of
the two studies in the same frequency ranges after correcting for
n{\pi}-ambiguity. At last, we demonstrate the possibility of applying
QU-fitting to study the ambient media of AGNs.Comment: 6 pages, 2 figures; Accepted by MDPI Galaxies; Conference Proceedings
for the Polarised Emission from Astrophysical Jets meeting on June 12-16
2017, Ierapetra, Greec
New insights into the interstellar medium of the dwarf galaxy IC 10 : connection between magnetic fields, the radio--infrared correlation and star formation
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. Available at doi: https://doi.org/10.1093/mnras/stx1567.We present the highest sensitivity and angular resolution study at 0.32 GHz of the dwarf irregular galaxy IC\,10, observed using the Giant Metrewave Radio Telescope, probing pc spatial scales. We find the galaxy-averaged radio continuum spectrum to be relatively flat, with a spectral index (), mainly due to a high contribution from free--free emission. At 0.32 GHz, some of the H{\sc ii} regions show evidence of free--free absorption as they become optically thick below GHz with corresponding free electron densities of . After removing the free--free emission, we studied the radio--infrared relations on 55, 110 and 165 pc spatial scales. We find that on all scales the non-thermal emission at 0.32 and 6.2 GHz correlates better with far-infrared (FIR) emission at m than mid-infrared emission at m. The dispersion of the radio--FIR relation arises due to variations in both magnetic field and dust temperature, and decreases systematically with increasing spatial scale. The effect of cosmic ray transport is negligible as cosmic ray electrons were only injected Myr ago. The average magnetic field strength () of G in the disc is comparable to that of large star-forming galaxies. The local magnetic field is strongly correlated with local star formation rate () as , indicating a star-burst driven fluctuation dynamo to be efficient ( per cent) in amplifying the field in IC\,10. The high spatial resolution observations presented here suggest that the high efficiency of magnetic field amplification and strong coupling with SFR likely sets up the radio--FIR correlation in cosmologically young galaxies.Peer reviewedFinal Accepted Versio
Deep uGMRT observations of the ELAIS-North 1 field: statistical properties of radio--infrared relations up to 2
Comprehending the radio--infrared (IR) relations of the faint extragalactic
radio sources is important for using radio emission as a tracer of
star-formation in high redshift () star-forming galaxies (SFGs). Using deep
uGMRT observations of the ELAIS-N1 field in the 0.3--0.5\,GHz range, we study
the statistical properties of the radio--IR relations and the variation of the
`-parameter' up to after broadly classifying the faint sources as SFGs
and AGN. We find the dust temperature (\tdust) to increase with . This
gives rise to \qmir, measured at 24\,\upmum, to increase with as the
peak of IR emission shifts towards shorter wavelengths, resulting in the
largest scatter among different measures of -parameters. \qfir measured at
70\,\upmum, and using total-IR (TIR) emission are largely
unaffected by \tdust. We observe strong, non-linear correlations between the
radio luminosities at 0.4 and 1.4\,GHz with 70\,\upmum luminosity and TIR
luminosity(\ltir). To assess the possible role of the radio-continuum
spectrum in making the relations non-linear, for the first time we study them
at high using integrated radio luminosity (\lrc) in the range
0.1--2\,GHz. In SFGs, the \lrc--\ltir relation remains non-linear with a
slope of , has a factor of 2 lower scatter compared to
monochromatic radio luminosities, and \qtirbol decreases with as
\qtirbol = (2.27 \pm 0.03)\,(1+z)^{-0.12 \pm 0.03}. A redshift variation of
is a natural consequence of non-linearity. We suggest that a redshift
evolution of magnetic field strengths and/or cosmic ray acceleration efficiency
in high- SFGs could give rise to non-linear radio--IR relations.Comment: Accepted for publication in MNRAS (20 pages, 20 Figures and one
Appendix
J021659-044920: a relic giant radio galaxy at z ~ 1.3
We report the discovery of a relic Giant Radio Galaxy (GRG) J021659-044920 at
redshift that exhibits large-scale extended, nearly co-spatial,
radio and X-ray emission from radio lobes, but no detection of Active Galactic
Nuclei core, jets and hotspots. The total angular extent of the GRG at the
observed frame 0.325 GHz, using Giant Metrewave Radio Telescope observations is
found to be 2.4 arcmin, that corresponds to a total projected linear
size of 1.2 Mpc. The integrated radio spectrum between 0.240 and 1.4 GHz
shows high spectral curvature ( 1.19) with sharp steepening above
0.325 GHz, consistent with relic radio emission that is 8
10 yr old. The radio spectral index map between observed frame 0.325 and
1.4~GHz for the two lobes varies from 1.4 to 2.5 with the steepening trend from
outer-end to inner-end, indicating backflow of plasma in the lobes. The
extended X-ray emission characterized by an absorbed power-law with photon
index 1.86 favours inverse-Compton scattering of the Cosmic Microwave
Background (ICCMB) photons as the plausible origin. Using both X-ray and radio
fluxes under the assumption of ICCMB we estimate the magnetic field in the
lobes to be 3.3 G. The magnetic field estimate based on energy
equipartition is 3.5 G. Our work presents a case study of a rare
example of a GRG caught in dying phase in the distant Universe.Comment: 10 pages, 5 figures, 3 tables. Published in MNRAS. Corrected typos
and added a referenc
Statistical properties of Faraday rotation measure from large-scale magnetic fields in intervening disc galaxies
To constrain the large-scale magnetic field strengths in cosmologically
distant galax- ies, we derive the probability distribution function of Faraday
rotation measure (RM) when random lines of sight pass through a sample of disc
galaxies, with axisymmetric large-scale magnetic fields. We find that the width
of the RM distribution of the galaxy sample is directly related to the mean
large-scale field strength of the galaxy population, provided the dispersion
within the sample is lower than the mean value. In the absence of additional
constraints on parameters describing the magneto-ionic medium of the
intervening galaxies, and in the situation where RMs produced in the
intervening galaxies have already been statistically isolated from other RM
contributions along the lines of sight, our simple model of the magneto-ionic
medium in disc galaxies suggests that the mean large-scale magnetic field of
the population can be measured to within ~ 50% accuracy.Comment: 4 pages, Proceedings of FM8 "New Insights in Extragalactic Magnetic
Fields", XXXth General Assembly of the IAU, Vienna, August 20-31, 201
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