70 research outputs found
Radio Observations of AGN in Low Surface Brightness Galaxies
We present preliminary results of a study of the low frequency radio
continuum emission from the nuclei of Giant Low Surface Brightness (LSB)
galaxies. We have mapped the emission and searched for extended features such
as radio lobes/jets associated with AGN activity. LSB galaxies are poor in star
formation and generally less evolved compared to nearby bright spirals. This
paper presents low frequency observations of 3 galaxies; PGC 045080 at 1.4 GHz,
610 MHz, 325MHz, UGC 1922 at 610 MHz and UGC 6614 at 610 MHz. The observations
were done with the GMRT. Radio cores as well as extended structures were
detected and mapped in all three galaxies; the extended emission may be
assocated with jets/lobes associated with AGN activity. Our results indicate
that although these galaxies are optically dim, their nuclei can host AGN that
are bright in the radio domain.Comment: To appear in proceedings IAU Symp 244, 'Dark Galaxies and Lost
Baryons', June 2007, 2 pages including 1 figur
Low radio frequency signatures of ram pressure stripping in Virgo spiral NGC 4254
We report the detection of extended low radio frequency continuum emission
beyond the optical disk of the spiral galaxy NGC 4254 using the Giant Metrewave
Radio Telescope. NGC 4254, which has an almost face-on orientation, is located
in the outskirts of the Virgo cluster. Since such extended emission is uncommon
in low inclination galaxies, we believe it is a signature of magnetised plasma
pushed out of the disk by ram pressure of the intracluster medium as NGC 4254
falls into the Virgo cluster. The detailed spectral index distribution across
NGC 4254 shows that the steepest spectrum alpha < -1 (S propto nu^{alpha})
arises in the gas beyond the optical disk. This lends support to the ram
pressure scenario by indicating that the extended emission is not from the disk
gas but from matter which has been stripped by ram pressure. The steeper
spectrum of the extended emission is reminiscent of haloes in edge-on galaxies.
The sharp fall in intensity and enhanced polarization in the south of the
galaxy, in addition to enhanced star formation reported by others provide
evidence towards the efficacy of ram pressure on this galaxy. HI 21cm
observations show that the gas in the north lags in rotation and hence is
likely the atomic gas which is carried along with the wind. NGC 4254 is a
particularly strong radio emitter with a power of 7x10^{22} Watts/Hz at 240
MHz. We find that the integrated spectrum of the galaxy flattens at lower
frequencies and is well explained by an injection spectrum with
alpha_0=-0.45+-0.12. We end by comparing published simulation results with our
data and conclude that ram pressure stripping is likely to be a significant
contributor to evolution of galaxies residing in X-ray poor groups and cluster
outskirts.Comment: 10 pages, 7 figures (figures 1,2 and 4 are in jpg format) and 2
tables; Accepted for publication in MNRA
Radio Observations of the AGN and Gas in Low Surface Brightness Galaxies
LSB galaxies have low metallicities, diffuse stellar disks, and massive HI
disks. We have detected molecular gas in two giant LSB galaxies, UGC 6614 and
F568-6. A millimeter continuum source has been detected in UGC 6614 as well. At
centimeter wavelengths we have detected and mapped the continuum emission from
the giant LSB galaxy 1300+0144. The emission is extended about the nucleus and
is most likely originating from the AGN in the galaxy. The HI gas distribution
and velocity field in 1300+0144 was also mapped. The HI disk extends well
beyond the optical disk and appears lopsided in the intensity maps.Comment: one page; submitted to proceedings of IAU Symposium 235: Galaxy
Evolution across the Hubble Tim
The AGN and Gas Disk in the Low Surface Brightness Galaxy PGC045080
We present radio observations and optical spectroscopy of the giant low
surface brightness (LSB) galaxy PGC 045080 (or 1300+0144). PGC 045080 is a
moderately distant galaxy having a highly inclined optical disk and massive HI
gas content. Radio continuum observations of the galaxy were carried out at 320
MHz, 610 MHz and 1.4 GHz. Continuum emission was detected and mapped in the
galaxy. The emission appears extended over the inner disk at all three
frequencies. At 1.4 GHz and 610 MHz it appears to have two distinct lobes. We
also did optical spectroscopy of the galaxy nucleus; the spectrum did not show
any strong emission lines associated with AGN activity but the presence of a
weak AGN cannot be ruled out. Furthermore, comparison of the H flux and
radio continuum at 1.4 GHz suggests that a significant fraction of the emission
is non-thermal in nature. Hence we conclude that a weak or hidden AGN may be
present in PGC 045080. The extended radio emission represents lobes/jets from
the AGN. These observations show that although LSB galaxies are metal poor and
have very little star formation, their centers can host significant AGN
activity. We also mapped the HI gas disk and velocity field in PGC 045080. The
HI disk extends well beyond the optical disk and appears warped. In the HI
intensity maps, the disk appears distinctly lopsided. The velocity field is
disturbed on the lopsided side of the disk but is fairly uniform in the other
half. We derived the HI rotation curve for the galaxy from the velocity field.
The rotation curve has a flat rotation speed of ~ 190 km/s.Comment: Paper contains 14 figures and 4 tables. Figures 8, 10 (color) and 13
supplied separately. Accepted for publication in MNRA
Radio recombination lines from the largest bound atoms in space
In this paper, we report the detection of a series of radio recombination
lines (RRLs) in absorption near 26 MHz arising from the largest bound carbon
atoms detected in space. These atoms, which are more than a million times
larger than the ground state atoms are undergoing delta transitions (n~1009,
Delta n=4) in the cool tenuous medium located in the Perseus arm in front of
the supernova remnant, Cassiopeia A. Theoretical estimates had shown that atoms
which recombined in tenuous media are stable up to quantum levels n~1500. Our
data indicates that we have detected radiation from atoms in states very close
to this theoretical limit. We also report high signal-to-noise detections of
alpha, beta and gamma transitions in carbon atoms arising in the same clouds.
In these data, we find that the increase in line widths with quantum number
(proportional to n^5) due to pressure and radiation broadening of lines is much
gentler than expected from existing models which assume a power law background
radiation field. This discrepancy had also been noted earlier. The model line
widths had been overestimated since the turnover in radiation field of
Cassiopeia A at low frequencies had been ignored. In this paper, we show that,
once the spectral turnover is included in the modeling, the slower increase in
line width with quantum number is naturally explained.Comment: 5 pages, 4 figures, accepted for publication in MNRA
Radio emission at the centre of the galaxy cluster Abell 3560: evidence for core sloshing?
Previous radio observations of the galaxy cluster A3560 in the Shapley
Concentration showed complex radio emission associated with the brightest
cluster member.To understand its origin we observed it with the GMRT, the VLA
and ATCA at 240 and 610 MHz, 1.28,1.4, 2.3,4.8 and 8.4 GHz, and performed a
detailed morphological and spectral study of the radio emission associated with
the BCG. We also observed the cluster with XMM-Newton and Chandra to derive the
properties of the ICM. The radio emission of the N-E nucleus of the dumb-bell
BCG shows an active radio galaxy, plus aged diffuse emission, which is not
refurbished at present. Our Chandra data show that the radio active nucleus of
the BCG has extended X-ray emission, which we classify as a low-luminosity
corona. A residual image of the XMM-Newton brightness shows the presence of a
spiral-like feature, which we interpret as the signature of gas sloshing. The
presence of a subgroup is clear in the surface brightness residual map, and in
the XMM-Newton temperature analysis. The optical 2D analysis shows substructure
in A3560. A galaxy clump was found at the location of the X-ray subgroup, and
another group is present south of the cluster core, close to the spiral-like
feature. The aged part of the radio emission closely follows the spiral pattern
of the X-ray residual brightness distribution, while the two active radio lobes
are bent in a completely different direction. We conclude that the complex
radio emission associated with the cluster BCG is the result of a minor merger
event in A3560. The aged diffuse emission is strongly affected by the sloshing
motion in the ICM. On the other hand, the bent jets and lobes of the current
radio AGN activity may reflect a complex gas velocity field in the innermost
cluster regions and/or sloshing-induced oscillations in the motion of the cD
galaxy.Comment: 15 pages, 8 figures, 5 tables, A&A in pres
Extragalactic radio sources with sharply inverted spectrum at metre wavelengths
We present the first results of a systematic search for the rare
extragalactic radio sources showing an inverted (integrated) spectrum, with
spectral index , a previously unexplored spectral domain. The
search is expected to yield strong candidates for , for which
the standard synchrotron self-absorption (characterized by a single power-law
energy distribution of relativistic electron population) would not be a
plausible explanation, even in an ideal case of a perfectly homogeneous source
of incoherent synchrotron radiation. Such sharply inverted spectra, if found,
would require alternative explanations, e.g., free-free absorption, or
non-standard energy distribution of relativistic electrons which differs from a
power-law (e.g., Maxwellian).
The search was carried out by comparing two sensitive low-frequency radio
surveys made with sub-arcminute resolution, namely, the WISH survey at 352 MHz
and TGSS/DR5 at 150 MHz. The overlap region between these two surveys contains
7056 WISH sources classified as `single' and brighter than 100 mJy at 352 MHz.
We focus here on the seven of these sources for which we find .
Two of these are undetected at 150 MHz and are particularly good candidates for
. Five of the seven sources exhibit a
`Gigahertz-Peaked-Spectrum' (GPS).Comment: 8 pages, 2 figures, accepted for publication in MNRA
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