N. G. Kantharia

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, An = 4) in the cool tenuous medium located in the Perseus arm in front of the supernova remnant, Cassiopeia A (Cas A). Theoretical estimates had shown that atoms which recombined in tenuous media are stable up to quantum levels n ∼ 1500. Our data indicate that we have detected radiation from atoms in states very close to this theoretical limit. We also report high signal-to-noise ratio detections of a, β and y transitions in carbon atoms arising in the same clouds. In these data, we find that the increase in linewidths with quantum number (approximately proportional to n 5 ) due to pressure and radiation broadening of lines is much gentler than that expected from existing models which assume a power-law background radiation field. This discrepancy had also been noted earlier. The model linewidths had been overestimated since the turnover in radiation field of Cas A at low frequencies had been ignored. In this paper, we show that, once the spectral turnover is included in the modelling, the slower increase in linewidth with quantum number is naturally explained.

Search for 150 MHz radio emission from extrasolar planets in the TIFR GMRT Sky Survey

The ongoing radio continuum TIFR GMRT Sky Survey (TGSS) using the Giant Metrewave Radio Telescope (GMRT) at 150 MHz offers an unprecedented opportunity to undertake a fairly deep search for low-frequency radio emission from nearby extrasolar planets. Currently TGSS images are available for a little over a steradian, encompassing 175 confirmed exoplanetary systems. We have searched for their radio counterparts in the TGSS (150 MHz), supplemented with a search in the NRAO VLA Sky Survey (NVSS) and the VLA FIRST survey at 1.4 GHz. For 171 planetary systems, we find no evidence of radio emission in the TGSS maps, placing a3 σ upper limit between 8.7 mJy and 136 mJy (median ∼24.8 mJy) at 150 MHz. These non-detections include the 55 Cnc system for which we place a 3σ upper limit of 28 mJy at 150 MHz. Nonetheless, for four of the extrasolar planetary systems, we find TGSS radio sources coinciding with or located very close to their coordinates. One of these is 61 Vir: for this system a large radio flux density was predicted in the scenario involving magnetosphere-ionosphere coupling and rotation-induced radio emission. We also found 150 MHz emissions toward HD 86226 and HD 164509, where strong radio emission can be produced by the presence of a massive satellite orbiting a rapidly rotating planet. We also detected 150 MHz emission within a synthesized beam from 1RXS1609 b, a pre-main-sequence star harboring a ∼14 Jupiter mass planet (or a brown dwarf). With a bright X-ray-UV star and a high mass, the planet 1RXS1609 b presents the best characteristics for rotation-induced emissions with high radio power. Deeper high-resolution observations toward these planetary systems are needed to discriminate between the possibilities of background radio-source and radio-loud planets. At 1.4 GHz, radio emission toward the planet-harboring pulsar PSR B1620-26 is detected in the NVSS. Emissions at 1.4 GHz are also detected toward the very-hot-Jupiter WASP-77A b (in the FIRST survey) and the giant planet HD 43197 b in a highly eccentric orbit (in NVSS). We briefly discuss these detections, as well as the non-detection of the vast majority of exoplanets.

GMRT observations of the group Holmberg 124: Evolution by tidal forces and ram pressure?

We report new radio continuum and 21 cm HI observations using the Giant Metrewave Radio Telescope (GMRT) of the group Holmberg 124 ( Ho 124) comprising four late-type galaxies, namely NGC 2820, Mrk 108, NGC 2814 and NGC 2805. The three galaxies, NGC 2820, Mrk 108 and NGC 2814 which are closely located in the sky plane have clearly undergone tidal interactions as seen from the various morphological tidal signatures and debris. Moreover we note various features in the group members which we believe might be due to ram pressure. In this paper, we describe four interesting results emerging from our observations: a) detection of the tidal radio continuum bridge at 330 MHz connecting the galaxies NGC 2820+ Mrk 108 with NGC 2814. The radio bridge was discovered at 1465 MHz by van der Hulst & Hummel ( 1985, A& A, 150, 17). We find that the bridge has a fairly steep spectrum with a spectral index alpha(S proportional to nu(alpha)) of - 1.8(-0.2)(+0.3) which is much steeper than the - 0.8 quoted by van der Hulst & Hummel ( 1985); b) detection of other tidal features like the tilted HI and radio continuum disk of NGC 2814, a HI streamer and a radio continuum tail arising from the south of NGC 2814. We also report the detection of a possible tidal dwarf galaxy in HI; c) sharp truncation in the HI distribution in the south of NGC 2820 and in the HI and radio continuum distribution in the north of NGC 2814. The optical disks in both the cases look undisturbed. As pointed out by Davis et al. ( 1997, AJ, 114, 613), ram pressure affects different components of the interstellar medium to varying degrees. Simple estimates of pressure in different components of the interstellar medium ( radio continuum, Ha and HI) in NGC 2820 indicate that ram pressure will significantly influence HI; d) detection of a large one-sided HI loop to the north of NGC 2820. No radio continuum emission or Ha emission is associated with the HI loop. We discuss various scenarios for the origin of this loop including a central starburst, ram pressure stripping and tidal interaction. We do not support the central starburst scenario since the loop is not detected in ionized gas. Using the upper limit on X-ray luminosity of Ho 124 (Mulchaey et al. 2003, ApJS, 145, 39), we estimate an upper limit on the intragroup medium (IGrM) density of 8.8 x 10(-4) cm(-3). For half this electron density, we estimate the ram pressure force of the IGrM to be comparable to the gravitational pull of the disk of NGC 2820. Since tidal interaction has obviously influenced the group, we suggest that the loop could have formed by ram pressure stripping if tidal effects had reduced the surface density of HI in NGC 2820. From the complex observational picture of Ho 124 and the numerical estimates, we suggest that the evolution of the Ho 124 group may be governed by both tidal forces due to the interaction and the ram pressure due to motion of the member galaxies in the IGrM and that the IGrM densities should not be too low (i.e. >= 4 x 10(-4)). However this needs to be verified by further observations.

Carbon recombination lines from the Galactic plane at 34.5 & 328 MHz

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:Te=20–40 K,ne ∼ 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 (Te ∼ 60–300 K) with lower electron densities (ne ∼ 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.

Giant Metrewave Radio Telescope Observations of the 2006 Outburst of the Nova RS Ophiuchi: First Detection of Emission at Radio Frequencies <1.4 GHz

The first low radio frequency (<1.4 GHz) detection of the outburst of the recurrent nova RS Ophiuchi is presented in this Letter. Radio emission was detected at 0.61 GHz on day 20 with a flux density of ~48 mJy and at 0.325 GHz on day 38 with a flux density of ~44 mJy. This is in contrast with the 1985 outburst, when it was not detected at 0.327 GHz even on day 66. The emission at low radio frequencies is clearly nonthermal and is well explained by a synchrotron spectrum of index α ~ -0.8 (S ∝ να) suffering foreground absorption due to the preexisting, ionized, warm, clumpy red giant wind. The absence of low-frequency radio emission in 1985 and the earlier turn-on of the radio flux in the current outburst are interpreted as being due to higher foreground absorption in 1985 compared to that in 2006, suggesting that the overlying wind densities in 2006 are only ~30% of those in 1985.

Radio Continuum and H I Study of Blue Compact Dwarf Galaxies

The multifrequency radio continuum and 21 cm H I observations of five blue compact dwarf (BCD) galaxies, Mrk 104, Mrk 108, Mrk 1039, Mrk 1069, and I Zw 97, using the Giant Meterwave Radio Telescope (GMRT) are presented here. Radio continuum emission at 610 MHz and 325 MHz is detected from all the observed galaxies whereas only a few are detected at 240 MHz. In our sample, three galaxies (Mrk 104, Mrk 108, and Mrk 1039) are members of groups and two galaxies (Mrk 1069 and I Zw 97) are isolated galaxies. The radio emission from Mrk 104 and Mrk 108 is seen to encompass the entire optical galaxy whereas the radio emission from Mrk 1039, Mrk 1069, and I Zw 97 is confined to massive H u regions. This, we suggest, indicates that the star formation in the latter group of galaxies has recently been triggered and that the environment in which the galaxy is evolving plays a role. Star formation rates (SFRs) calculated from 610 MHz emission are in the range 0.01-0.1 M ⊙ yr ―1 ; this is similar to the SFR obtained for individual star-forming regions in BCDs. The integrated radio spectra of four galaxies are modeled over the frequency range where data is available. We find that two of the galaxies, Mrk 1069 and Mrk 1039, show a turnover at low frequencies, which is well fitted by free―free absorption whereas the other two galaxies, Mrk 104 and Mrk 108, show a power law at the lowest GMRT frequencies. The flatter spectrum, localized star formation, and radio continuum in isolated galaxies lend support to stochastic self-propagating star formation. The H I observations of four galaxies, Mrk 104, Mrk 108, Mrk 1039, and Mrk 1069, show extended disks as large as ∼1.1―6 times the optical size. All the observed BCDs (except Mrk 104) show rotating disk with a half power width of ∼50―124 km s ―1 . Solid body rotation is common in our sample. We note that the tidal dwarf origin is possible for two of the BCDs in our sample.

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 disc 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 magnetized plasma pushed out of the disc 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 α < -1 (S ∝ ν α ) arises in the gas beyond the optical disc. This lends support to the ram pressure scenario by indicating that the extended emission is not from the disc 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 21-cm 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 7 x 10 22 W Hz -1 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 α 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.

Studying the radio continuum from nuclear activity and star formation in giant low surface brightness galaxies

We present a multifrequency radio continuum study of seven giant low surface brightness (GLSB) galaxies using the Giant Metrewave Radio Telescope (GMRT). GLSB galaxies are optically faint, dark-matter dominated systems that are poorly evolved and have large HI gas disks. Our sample consists of GLSB galaxies that show signatures of nuclear activity in their optical spectra. We detect radio emission from the nuclei of all the seven galaxies. Five galaxies have nuclear spectral indices that range from 0.12 to -0.44 and appear to be core-dominated; the two galaxies have a steeper spectrum. Two of the galaxies, UGC 2936 and UGC 4422 show significant radio emission from their disks. In our 610 MHz observations of UGC 6614, we detect radio lobes associated with the radio-loud active galactic nucleus (AGN). The lobes have a spectral index of -1.06+/-0.12. The star formation rates estimated from the radio emission, for the entire sample range from 0.15 to 3.6 M{solar} yr^{-1} . We compare the radio images with the near-ultraviolet (NUV) images from GALEX and near-infrared (NIR) images from 2MASS. The galaxies present a diversity of relative NUV, NIR and radio emission, supporting an episodic star formation scenario for these galaxies. Four galaxies are classified members of groups and one is classified as isolated. Our multiwavlength study of this sample suggests that the environment plays an important role in the evolution of these galaxies.

C i 492 GHz mapping towards Cas A

We have mapped the [CI] 3 P 1 - 3 P 0 emission at 492 GHz towards the supernova remnant Cas A. We detect [C i] emission from the periphery of the diffuse photon-dominated region (PDR) covering the disc of Cas A, as traced by the carbon recombination lines, as well as from the denser PDRs associated with the molecular clouds towards the south-east. [C i] emission is detected from both the Perseus and Orion arm molecular clouds, with the -47 km s -1 Perseus arm feature being strong enough to be detected at all positions. We estimate the C/CO relative abundance to be 0.2 at the position of the identified CO clouds and > 1 for most of the cloud. Here we show that the distribution of [C I] emitting regions compared to the C + region and molecular cloud is consistent with a scenario involving PDRs. Using physical models for PDRs, we constrain the physical properties of the [C I] line-forming regions. We estimate the densities of the [C I] emitting regions to be between 10 2 and 10 3 cm -3 . Based on rather high volume filling factors (∼50 per cent), we conclude that [C i] emission mainly arises from diffuse neutral gas in the Perseus arm.

GMRT radio continuum study of Wolf–Rayet galaxies – I. NGC 4214 and NGC 4449

We report low frequency observations of Wolf-Rayet galaxies, NGC 4214 and NGC 4449 at 610, 325 and 150 MHz, using the Giant Meterwave Radio Telescope (GMRT). We detect diffuse extended emission from NGC 4214 at and NGC 4449. NGC 4449 is observed to be five times more radio luminous than NGC 4214, indicating vigorous star formation. We estimate synchrotron spectral index after separating the thermal free-free emission and obtain