Publications (13)10.06 Total impact
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Article: The Discovery of Host Galaxy HI Absorption in CTA 21
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ABSTRACT: We report the discovery of HI 21-cm absorption towards the well-studied GHz Peaked-Spectrum source CTA 21 (4C 16.09) using the Arecibo Telescope on 2009 September 20 and 21. Recently, the frequency band between 700 and 800 MHz was temporarily opened up to radio astronomy when US TV stations were mandated to switch from analog to digital transmissions, with new frequency allocations. The redshifted HI frequency for CTA 21 falls within this band. CTA 21 has a complex radio structure on a range of scales. The innermost prominent components are separated by ~12 mas while weak diffuse emission extends for up to ~300 mas. The HI absorption profile that we find has two main components, one narrow, the other wider and blue-shifted. The total HI column density is 7.9 x 10^20 cm^-2, assuming a covering factor of unity and a spin temperature of 100 K. This HI absorption confirms the recently determined optical redshift of this faint galaxy of z ~ 0.907. We discuss this new detection in the light of HI absorption studies towards compact radio sources, and also the possibility that CTA 21 may be exhibiting multiple cycles of nuclear activity. This new detection in CTA 21 is consistent with a strong trend for detection of HI absorption in radio galaxies with evidence of episodic nuclear/jet activity. Comment: Accepted for publication in ApJL04/2010; -
Article: The Arecibo Arp 220 Spectral Census I: Discovery of the Pre-Biotic Molecule Methanimine and New Cm-wavelength Transitions of Other Molecules
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ABSTRACT: An on-going Arecibo line search between 1.1 and 10 GHz of the prototypical starburst/megamaser galaxy, Arp 220, has revealed a spectrum rich in molecular transitions. These include the ``pre-biotic'' molecules: methanimine (CH$_{2}$NH) in emission, three $v_{2}=1$ direct l-type absorption lines of HCN, and an absorption feature likely to be from either $^{18}$OH or formic acid (HCOOH). In addition, we report the detection of two, possibly three, transitions of $\lambda$4-cm excited OH not previously detected in Arp~220 which are seen in absorption, and a possible absorption feature from the 6.668-GHz line of methanol. This marks the first distant extragalactic detection of methanimine, a pre-biotic molecule. Also, if confirmed, the possible methanol absorption line presented here would represent the first extragalactic detection of methanol at a distance further than 10 Mpc. In addition, the strong, previously undetected, cm-wave HCN $v_{2}=1$ direct l-type lines will aid the study of dense molecular gas and active star-forming regions in this starburst galaxy. Comment: 25 pages, 10 figures. Accepted for publication in AJ05/2008; -
Article: Probing radio source environments via HI and OH absorption
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ABSTRACT: We present the results of HI and OH absorption measurements towards a sample of radio sources using the Arecibo 305-m telescope and the GMRT. In total, 27 radio sources were searched for associated 21-cm HI absorption. One totally new HI absorption system was detected against the radio galaxy 3C258, while five previously known HI absorption systems, and one galaxy detected in emission, were studied with improved frequency resolution and/or sensitivity. Our sample included 17 GPS and CSS objects, 4 of which exhibit HI absorption. This detection rate of ~25% compares with a value of ~40% by Vermeulen et al. for similar sources. We detected neither OH emission nor absorption towards any of the sources that were observed at Arecibo, and estimate a limit on the abundance ratio of N(HI)/N(OH)>4x10^6 for 3C258. We have combined our results with those from other available HI searches to compile a heterogeneous sample of 96 radio sources consisting of 27 GPS, 35 CSS, 13 flat spectrum and 21 large sources. The HI absorption detection rate is highest (~45%) for the GPS sources and least for the large sources. We find HI column density to be anticorrelated with source size, as reported earlier by Pihlstr\"om et al. The HI column density shows no significant dependence on either redshift or luminosity, which are themselves strongly correlated. These results suggest that the environments of radio sources on GPS/CSS scales are similar at different redshifts. Further, in accordance with the unification scheme, the GPS/CSS galaxies have an HI detection rate of ~40% which is significantly higher than the detection rate (~20%) towards the GPS/CSS quasars. Also, the principal (strongest) absorption component detected towards GPS sources appears blue-shifted in ~65% of the cases, in agreement with the growing evidence for jet-cloud interactions. Comment: Abridged abstract, 22 pages, 21 figures, moderately revised, accepted for publication in MNRAS05/2006; -
Article: Probing radio source environments via H�i and OH absorption
Monthly Notices of The Royal Astronomical Society - MON NOTIC ROY ASTRON SOC. 01/2006; 373(3):972-992. -
Article: Late Time Observations of the Afterglow and Environment of GRB 030329
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ABSTRACT: We present Very Long Baseline Interferometry (VLBI) observations 217 days after the gamma-ray burst of 2003 March 29. These observations provide further measurements of the size and position of GRB 030329 that are used to constrain the expansion rate and proper motion of this nearby GRB. The expansion rate appears to be slowing down with time, favoring expansion into a constant density interstellar medium, rather than a circumstellar wind with an r^-2 density profile. We also present late time Arecibo observations of the redshifted HI and OH absorption spectra towards GRB 030329. No absorption (or emission) is seen allowing us to place limits on the atomic neutral hydrogen of N_H < 8.5 x 10^20 cm^-2, and molecular hydrogen of N_H_2 < 1.4 x 10^22 cm^-2. Finally, we present VLA limits on the radio polarization from the afterglow of <2% at late times. Comment: 6 pages, 5 figures, Accepted to ApJ on Dec. 17, 200412/2004; -
Article: The VSOP 5 GHz AGN Survey I. Compilation and Observations
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ABSTRACT: The VSOP mission is a Japanese-led project to image radio sources with sub-milliarcsec resolution by correlating the signal from the orbiting 8-m telescope, HALCA, with a global array of telescopes. Twenty-five percent of the scientific time of this mission is devoted to a survey of 402 bright, small-diameter extra-galactic radio sources at 5 GHz. The major goals of the VSOP Survey are statistical in nature: to determine the brightness temperature and approximate structure; to provide a source list for use with future space VLBI missions; and to compare radio properties with other data throughout the EM spectrum. This paper describes: the compilation of a complete list of radio sources associated with active galactic nuclei (AGN); the selection of the subsample of sources to be observed with VSOP; the extensive ground resources used for the Survey; the status of the observations as of 2000 July; the data-analysis methods; and several examples of results from the VSOP Survey. More detailed results from the full sample will be given in future papers.Publications- Astronomical Society of Japan 12/2000; 52:997-. · 2.44 Impact Factor -
Article: Parsec-scale radio structures in the nuclei of four Seyfert galaxies
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ABSTRACT: We present 18-cm radio maps of four Seyfert nuclei, Mrk 1, Mrk 3, Mrk 231 and Mrk 463E, made with the European VLBI Network (EVN). Linear radio structures are present in three out of four sources on scales of ~100 pc to ~1 kpc, and the 20-mas beam of the EVN enables us to resolve details within the radio structures on scales of <10 pc. Mrk 3 was also imaged using MERLIN and the data combined with the EVN data to improve the sensitivity to extended emission. We find an unresolved flat-spectrum core in Mrk 3, which we identify with the hidden Seyfert 1 nucleus in this object, and we also see marked differences between the two highly-collimated radio jets emanating from the core. The western jet terminates in a bright hotspot and resembles an FRII radio structure, whilst the eastern jet has more in common with an FRI source. In Mrk 463E, we use the radio and optical structure of the source to argue that the true nucleus lies approximately 1 arcsec south of the position of the radio and optical brightness peaks, which probably represent a hotspot at the working surface of a radio jet. The EVN data also provide new evidence for a 100-pc radio jet powering the radio source in the Type 1 nucleus of Mrk 231. However, the Seyfert 2 galaxy Mrk 1 shows no evidence for radio jets down to the limits of resolution (~10 pc). We discuss the range of radio source size and morphology which can occur in the nuclei of Seyfert galaxies and the implications for Seyfert unification schemes and for radio surveys of large samples of objects. Comment: 23 pages, 7 postscript figures (supplied as separate files), uses AAS aaspp4 LaTeX style file, to appear in the 10 June 1999 issue of The Astrophysical Journal04/1999; -
Article: High-Resolution Radio Observations of MARKARIAN:3
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ABSTRACT: We present high-sensitivity 5-GHz MERLIN and 15-GHz VLA observations of the Seyfert nucleus of Markarian 3, with angular resolutions of 40 and 250 mas respectively. Our 5-GHz MERLIN map reveals the best example to date of a pair of highly collimated radio jets in a Seyfert nucleus. The jet system is 2 arcsec (~600 pc) in extent and less than 50 mas across, and lies along P.A. 84^deg^. Embedded in the jets are a number of compact components, and the western side terminates in a bright lobe containing a hotspot. The component at RA (1950) 06^h^09^m^ 48.423^s^, Dec. (1950) 71^deg^03'10.42" is unresolved and is the closest to the peak of the optical continuum emission. We therefore identify this as the radio core. Overall the source has a steep spectrum (α ~ 1, where S is proportional to v^-α^. The jet shows a slight `S'-shaped curvature, and this is also seen in images of the optical narrow-line emission. We discuss these observations in relation to starburst models for Seyfert activity, which we feel are difficult to reconcile with the structure of Mrk 3, the high degree of collimation being more consistent with the idea of a compact `central engine'.Monthly Notices of the Royal Astronomical Society 09/1993; 264:893. · 4.90 Impact Factor -
Article: The Ooty Summer Training Program 1990
Bulletin of the Astronomical Society of India 02/1991; 19:109. · 2.72 Impact Factor -
Article: The Ooty summer training programme, 1990
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ABSTRACT: Report of a hands-on month-long national level training programme organized in May-June 1990 using Ooty Radio Telescope for 20 MSc level students selected from all over India. Many students who took this training continued in astronomy & astrophysics.Bulletin of the Astronomical Society of India. 01/1991; 19:109-122. -
Article: The Arecibo Arp 220 Spectral Survey. I: Discovery of the Pre-Biotic Molecule Methanamine and New cm-Wavelength Transition of Other Molecules
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ABSTRACT: An on-going Arecibo line search between 1.1 and 10 GHz of the prototypical starburst/megamaser galaxy, Arp 220, has revealed a spectrum rich in molecular transitions. These include the "pre-biotic" molecules: methanimine (CH 2 NH) in emission, three v 2 = 1 direct l-type absorption lines of HCN, and an absorption feature from either 18 OH or formic acid (HCOOH). In addition, we report the detection of two, possibly three, transitions of λ4-cm excited OH not previously detected in Arp 220 which are seen in absorption, and a possible absorption feature from the 6.668-GHz line of methanol. Our results mark the first distant extragalactic detection of methanimine, a molecule with high relevance to the origins of life. Further, the strong, previously undetected, cm-wave HCN v 2 = 1 direct l-type lines can aid the study of dense molecular gas and active star-forming regions in this starburst galaxy. -
Article: Probing radio source environments using absorption lines
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ABSTRACT: We present the results of 21-cm HI absorption measurements towards a sample of extragalactic radio sources using the Arecibo 305-m telescope and the Giant Metrewave Radio Telescope (GMRT). Our sample includes gigahertz peaked spectrum (GPS), compact steep spectrum (CSS), compact flat spectrum (CFS) and large (LRG) sources. These observa-tions, along with other 21-cm searches published in the literature and archival data at other wavebands, allow us to probe the environments of these radio sources on a variety of scales. Our results suggest the following: (a) the HI column den-sity as probed by 21-cm absorption is anticorrelated with the source size, (b) the environments of radio sources on GPS and CSS scales are similar at different redshifts and (c) the 21-cm absorption detection rates of GPS and CSS galaxies and quasars are consistent with the unification scheme. We also note that the principal (strongest) absorption component detected towards GPS sources appears to be blue-shifted in ∼75% of the cases. Results of the photoionization modelling of outflowing material detected in the UV and optical spectrum of the CSS quasar 3C48 are also presented. These results are consistent with the growing evidence that jet-cloud interactions play an important role in determining kinematical and ionization properties of the emitting and absorbing gas observed in these sources. -
Article: The Deep Interferometric VSOP--Arecibo Survey (DIVAS)
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ABSTRACT: The Deep Interferometric VSOP-Arecibo Survey (DIVAS) program used space VLBI observations of a faint sample of flat-spectrum sources made with the HALCA satellite and the Arecibo telescope to provide the highest possible space VLBI sensitivity at 5 GHz. The main aim of the survey is to compare statistical results on the source structures (e.g., brightness temperatures, sizes, visibilities) with results from the VLBI Space Observatory Programme (VSOP) Survey, to determine whether there are systematic differences in the compact structures of sources selected from samples with limiting flux densities differing by over an order of magnitude. -
Article: Excited-OH And Methanol Absorption In The ULIRG Arp220
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ABSTRACT: We are presently conducting a cm-wave molecular line census in Arp 220, the nearest Ultraluminous Infrared Galaxy (ULIRG) and the prototype OH-megamaser source, using the Arecibo 305-m telescope. The main ground-state lines of the OH radical have long been know to show strong maser emission. Our observations confirm that the satellite ground-state lines show a mixture of emission and absorption. Among the initial detections are also 6-, 5- and 4-cm lines of excited-OH. All appear in absorption, and have brightness ratios within a given multiplet that are very close to those expected in local thermodynamic equilibrium. An unidentified absorption appearing near 1611 MHz could either be due to the pre-biotic molecule, formic acid, or the 18OH equivalent of the 1667-MHz OH main line. In view of a possible adjacent absorption that could be the 18OH equivalent of the 1665-MHz OH line, and greater consistency in velocity with other Arp-220 molecular lines, identification of the 1611-MHz line with 18OH is more likely. Also, a possible detection of the 6668-GHz methanol molecule in absorption has been made. The implied total column density of methanol is derived assuming the reality of the feature. -
Article: Towards an Understanding of the Galactic Distribution of Electron-Density Fluctuations
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ABSTRACT: VLBA observations of 32 compact sources within 30 degrees <= l <= 75 degrees, |b| < 3 degrees are being used to test whether the galactic electron-density fluctuations contain a spiral component.144:287.
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2006
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University of Puerto Rico at Arecibo
Arecibo, Arecibo, Puerto Rico
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