arXiv subjects

K. R. Anantharamaiah

Publications and source records attributed to K. R. Anantharamaiah.

At least 19 recordsLinked to original sources

arXiv2010

Detection of the H92alpha recombination line from NGC 4945

Context. Hydrogen ionized by young, high-mass stars in starburst galaxies radiates radio recombination lines (RRLs), whose strength can be used as a diagnostic of the ionization rate, conditions and gas dynamics in the starburst, without problems of dust obscuration. However, the lines are weak and only few extragalactic starburst systems have been detected. Aims. We aimed to increase the number of known starburst systems with detectable RRLs for detailed studies. Methods. We searched for the RRLs H91alpha and H92alpha at 8 GHz in the nearby Seyfert galaxy NGC 4945 using the ATCA with resolution of 3". This yielded a detection from which we derived conditions in the starburst. Results. We detected RRLs from the nucleus of NGC 4945 with a peak strength of 17.8 mJy, making it the strongest extragalactic RRL emitter known at this frequency. The line emission from NGC 4945 can be matched by a model consisting of a collection of 10 to 300 H II regions with temperatures of 5000 K, densities of 10^3 cm^-3 to 10^4 cm^-3 and a total effective diameter of 2 pc to 100 pc. The Lyman continuum production rate required to maintain the ionization requires 2000 to 10000 O5 stars to be produced in the starburst, inferring a star formation rate of 2 Msun/yr to 8 Msun/yr. We resolved the rotation curve within the central 70 pc region and this is well described by a set of rotating rings that were coplanar and edge on with a simple flat rotation curve. The rotation speed of 120 km/s within the central 1" (19 pc) radius infers an enclosed mass of 3 x 10^7 Msun. Conclusion. We discovered RRLs from NGC 4945. It is the strongest known extragalactic RRL emitter and is suited to high-quality spectroscopic study. We resolved the dynamics of the ionized gas in the central 70 pc and derived conditions and star formation rates in the ionized gas.

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arXiv2008

Detection of the H92alpha recombination line from the starbursts in the Circinus galaxy and NGC 1808

Context. Gas ionized by starburst activity radiates radio recombination lines (RRLs), from which one can derive the plasma conditions and the number of massive stars formed in the burst, free of dust obscuration effects. Aims. We aimed to find RRL emission from additional starburst systems and to use the line properties to estimate the properties of the ionized gas. Methods. We conducted a search for RRLs in the nearby extragalactic starburst or Seyfert galaxies NGC 1808, the Circinus galaxy, NGC 4038/9, II Zw 40, NGC 6221, NGC 7552, IRAS 18325-5926, IC 5063, and VV 114. We used the VLA and the ATCA to search for the RRLs H91alpha and H92alpha with rest frequencies of 8.6 GHz and 8.3 GHz. Results. We detected for the first time RRLs from the starburst nuclei in the Circinus galaxy and NGC 1808. The Circinus galaxy was detected with a line strength integrated over the source of 3.2 mJy, making it the fourth-strongest extragalactic RRL emitter known at this frequency (after NGC 4945, M 82, and NGC 253). The line and continuum emission can be matched by a model consisting of a collection of 50 to 10 000 H II regions with 300 to 9000 O5 stars required to maintain the ionization, inferring a star formation rate of 0.2 Msun/yr to 6 Msun/yr. NGC 1808 was detected in RRL emission at 3.9 sigma with a line strength of 0.47 mJy at the expected velocity. Conclusion. We have detected RRLs from two galaxies, adding to the small but growing number of known extragalactic RRL emitters. The Circinus galaxy is strong and especially suited to high-quality follow-up spectroscopic study. We derived conditions and star formation rates in the starbursting regions. Uncertainties can be reduced by future multi-transition studies.

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arXiv2007

VLA H53alpha radio recombination line observations of the ultraluminous infrared galaxy Arp 220

We present high angular resolution (0.7'') observations made with the Very Large Array (VLA) of the radio recombination line (RRL) H53alpha and radio continuum emission at 43 GHz from the ultraluminous infrared galaxy (ULIRG) Arp 220. The 43 GHz continuum emission shows a compact structure (~2'') with two peaks separated by ~1'', the East (E) and West (W) components, that correspond to each galactic nucleus of the merger. The spectral indices for both the E and W components, using radio continuum images at 8.3 and 43 GHz are typical of synchrotron emission (alpha ~ -1.0). Our 43 GHz continuum and H53alpha line observations confirm the flux densities predicted by the models proposed by Anantharamaiah et al. This agreement with the models implies the presence of high-density (~ 100,000 cm^-3) compact HII regions (~ 0.1 pc) in Arp 220. The integrated H53alpha line emission is stronger toward the non-thermal radio continuum peaks, which are also coincident with the peaks of molecular emission of the H2CO. The coincidence between the integrated H53alpha and the H2CO maser line emission suggests that the recent star forming regions, traced by the high density gas, are located mainly in regions that are close to the two radio continuum peaks. A velocity gradient of ~ 0.30 km/s/pc in the H53alpha RRL is observed toward the E component and a second velocity gradient of ~ 0.15 km/s/pc is detected toward the W component. The orientations of these velocity gradients are in agreement with previous CO, HI and OH observations. The kinematics of the high-density ionized gas traced by the H53alpha line are consistent with two counter rotating disks as suggested by the CO and HI observations.

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arXiv2006

VLA H53alpha and H92alpha line observations of the central region of NGC 253

We present new Very Large Array (VLA) observations toward NGC 253 of the recombination line H53alpha (43 GHz) at an angular resolution of 1.''5 X 1.''0. The free-free emission at 43 GHz is estimated to be ~100 mJy, implying a star formation rate of ~1.3 Msun/yr in the nuclear region of this starburst galaxy. A reanalysis is made for previously reported H92alpha observations carried out with angular resolution of 1.''5 X 1.''0 and 0.''36 X 0.''21. Based on the line and continuum emission models used for the 1.''5 X 1.''0 angular resolution observations, the RRLs H53alpha and H92alpha are tracers of the high-density (~10^5 cm-3) and low-density (~10^3 cm^-3) thermally ionized gas components in NGC 253, respectively. The velocity fields observed in the H53alpha and H92alpha lines 1.''5 X 1.''0) are consistent. The velocity gradient in the central ~18 pc of the NE component, as observed in both the H53alpha and H92alpha lines, is in the opposite direction to the velocity gradient determined from the CO observations. The enclosed virial mass, as deduced from the H53alpha velocity gradient over the NE component, is ~5 X 10^6 Msun in the central ~18 pc region. The H92alpha line observations at high angular resolution (0.''36 X 0.''21) reveal a larger velocity gradient, along a P.A. ~ -45 deg on the NE component, of ~110 km/s/arcsec. The dynamical mass estimated using the high angular resolution H92alpha data (~7 X 10^6 Msun) supports the existence of an accreted massive object in the nuclear region of NGC 253.

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arXiv2006

VLA Observations of Carbon 91$α$ Recombination Line Emission in W49 North

We have detected C91$α$ (8.5891 GHz) emission toward 4 ultra-compact \HII regions (\UCHII s; W49G, J, L & C) in the W49 North massive star forming region with the Very Large Array (VLA) at 3\arcsec resolution. No carbon line emission was detected toward \UCHII s W49F, A, O, S and Q at this frequency to a 3$σ$ level of 2 mJy. We also observed the same region in the C75$α$ line (15.3 GHz) with no detection at a 3$σ$ level of 6 mJy with a 1\arcsec.7 beam. Detection of line emission toward these sources add supporting data to the earlier result of \nocite{retal05a}Roshi et al (2005a) that many \UCHII s have an associated photo-dissociation region (PDR). Similarity of the LSR velocities of carbon recombination lines and H$_2$CO absorption toward \UCHII s in W49 North suggests that the PDRs reside in the dense interface zone surrounding these \HII regions. Combining the observed carbon line parameters at 8.6 GHz with the upper limits on line emission at 15.3 GHz, we obtain constraints on the physical properties of the PDRs associated with W49G and J. The upper limit on the number density of hydrogen molecule obtained from carbon line models is $\sim$ $5 \times 10^6$ \cmthree.

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arXiv2005

Multi-wavelength carbon recombination line observations with the VLA toward an UCHII region in W48: Physical properties and kinematics of neutral material

Using the Very Large Array (VLA) the C76$α$ and C53$α$ recombination lines (RLs) have been detected toward the ultra-compact \HII\ region (UCHII region) G35.20$-$1.74. We also obtained upper limits to the carbon RLs at 6 cm (C110$α$ & C111$α$) and 3.6 cm (C92$α$) wavelengths with the VLA. In addition, continuum images of the W48A complex (which includes G35.20$-$1.74) are made with angular resolutions in the range 14\arcsec to 2\arcsec. Modeling the multi-wavelength line and continuum data has provided the physical properties of the UCHII region and the photodissociation region (PDR) responsible for the carbon RL emission. The gas pressure in the PDR, estimated using the derived physical properties, is at least four times larger than that in the UCHII region. The dominance of stimulated emission of carbon RLs near 2 cm, as implied by our models, is used to study the relative motion of the PDR with respect to the molecular cloud and ionized gas. Our results from the kinematical study are consistent with a pressure-confined UCHII region with the ionizing star moving with respect to the molecular cloud. However, based on the existing data, other models to explain the extended lifetime and morphology of UCHII regions cannot be ruled out.

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arXiv2005

Radio Recombination Lines from the Starburst Galaxy NGC 3256

We have detected the radio recombination lines H91-alpha and H92-alpha with rest frequencies of 8.6 GHz and 8.3 GHz from the starburst nucleus NGC 3256 at an angular resolution of 16.4" x 9.6" using the Australia Telescope Compact Array and at an angular resolution of 12.0" x 2.9" using the VLA. The line was detected at ~ 1 mJy/beam peak with a width of 160 km/s with the ATCA and at ~ 0.5 mJy/beam peak with a width of 114 km/s with the VLA. Modelling the line emitting region as a collection of HII regions, we derive constraints on the required number of HII regions, their temperature, density, and distribution. We find that a collection of 10 to 300 HII regions with temperatures of 5000 K, densities of 1000 cm^-3 to 5000 cm^-3 and diameters of 15 pc produced good matches to the line and continuum emmission. The Lyman continuum production rate required to maintain the ionization is 2 x 10^52 s^-1 to 6 x 10^53 s^-1, which requires 600 to 17000 O5 stars to be produced in the starburst.

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arXiv2004

Multi-density model of the ionised gas in NGC 253 using radio recombination lines

We have imaged the H92alpha (8.3 GHz), H75alpha (15 GHz), and H166alpha (1.4 GHz) Radio Recombination Lines (RRLs) from NGC 253 at resolutions of 4.5 pc (0.4"), 2.5 pc (0.2") and 53 pc (4.5") respectively. The H92alpha line arises from individual compact sources, most of which possess radio continuum counterparts. The line widths range from ~200 km/s for the sources near the radio nucleus to 70-100 km/s for the extranuclear ones. These lines are emitted by gas at a density ~10000 /cc. The remainder of the cm-wave RRLs arise in lower density gas (~500 /cc) with a higher area filling factor and with ten times highermass. A third component of higher density gas (>10000 /cc) is required to explain the mm-wave RRLs.

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arXiv2004

Radio Recombination Lines from Starbursts: NGC 3256, NGC 4945 and the Circinus Galaxy

A renewed attempt to detect radio recombination lines from external galaxies has resulted in the measurement of lines from several bright starburst galaxies. The lines are produced by hydrogen ionized by young, high-mass stars and are diagnostic of the conditions and gas dynamics in the starburst regions without problems of dust obscuration. We present here detections of the lines H91alpha and H92alpha near 8.6 GHz from the starburst nuclei in NGC 3256, NGC 4945, and the Circinus galaxy using the ATCA and VLA. Modelling the line emitting region as a collection of H II regions, we derive the required number of H II regions, their temperature, density, and distribution.

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arXiv2004

VLA H92 Alpha and H53 Alpha Radio Recombination Line Observations of M82

We present high angular resolution (0.6'') observations made with the VLA of the radio continuum at 8.3 and 43 GHz as well as H92 Alpha and H53 Alpha radio recombination lines from the nearby (3 Mpc) starburst galaxy M82. In the continuum we report 19 newly identified sources at 8.3 GHz and 5 at 43 GHz that were unknown previously. The spatial distribution of the H92 Alpha line is inhomogeneous; we identify 27 features. The line and continuum emission are modeled using a collection of HII regions at different distances from the nucleus assuming a single-density component and two-density components. The high-density component has a density of 4 X 10^{4} cm^{-3}. However, the bulk of the ionization is in regions with densities which are typically a factor 10 lower. The gas kinematics, using the H92 Alpha line, confirms the presence of steep velocity gradient (26 km s^{-1} arcsec^{-1}) in the nuclear region. As this steep gradient is observed not only on the major axis but also at large distances along a band of PA of 150 degrees, the interpretation in terms of x2 orbits elongated along the minor axis of the bar, which would be observed at an angle close to the inclination of the main disk, seems inadequate. Ad-hoc radial motions must be introduced to reproduce the pattern of the velocity field. Different families of orbits are indicated as we detect a signature in the kinematics at the transition between the two plateaus observed in the NIR light distribution. The H92 Alpha line also reveals the base of the outflow where the injection towards the halo on the Northern side occurs. The kinematical pattern suggests a connection between the gas flowing in the plane of M82 towards the center; this behavior most likely originates due to the presence of a bar and the outflow out of the plane.

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arXiv2003

GMRT detection of HI 21 cm associated absorption towards the z=1.2 red quasar 3C 190

We report the GMRT detection of associated HI 21 cm-line absorption in the z=1.1946 red quasar 3C 190. Most of the absorption is blue-shifted with respect to the systemic redshift. The absorption, at $\sim$ 647.7 MHz, is broad and complex, spanning a velocity width of $\sim$ 600 \kms. Since the core is self-absorbed at this frequency, the absorption is most likely towards the hotspots. Comparison of the radio and deep optical images reveal linear filaments in the optical which overlap with the brighter radio jet towards the south-west. We therefore suggest that most of the HI 21 cm-line absorption could be occurring in the atomic gas shocked by the south-west jet.

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arXiv2002

A VLBA Search for a Stimulated Recombination Line from the Accretion Region in NGC1275

The radio source 3C84, in NGC1275, has a two sided structure on parsec scales. The northern feature, presumed to be associated with a jet moving away from the Earth, shows strong evidence for free-free absorption. The ionized gas responsible for that absorption would be a source of detectable stimulated recombination line emission for a wide range of physical conditions. The VLBA has been used to search for the H65$α$ hydrogen recombination line. The line is only expected to be seen against the northern feature which contains a small fraction of the total radio flux density. This spatial discrimination significantly aids the search for a weak line. No line was seen, with upper limits of roughly 15% of the continuum over a velocity range of 1486 km/s with resolutions up to 6.6 km/s. In the absence of a strong radiation field, this would imply that the free-free absorbing gas has a wide velocity width, is moving rapidly relative to the systemic velocity, or is concentrated in a thin, high density structure. All of these possibilities are reasonably likely close to an AGN. However, in the intense radiation environment of the AGN, even considering only the radiation we actually observe passing through the free-free absorbing gas, the non-detection is probably assured by a combination of saturation and radiation damping.

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arXiv2002

GMRT and VLA observations of HI and OH from the Seyfert galaxy Mrk 1

We present Giant Meterwave Radio Telescope (GMRT) observations of the HI 21 cm line and Very Large Array (VLA) observations of the OH 18 cm line from the Seyfert 2 galaxy Mrk 1. HI emission is detected from both Mrk 1 and its companion NGC 451. The HI emission morphology and the velocity field of Mrk 1 are disturbed. We speculate that the nuclear activities of Mrk 1 are triggered by tidal interactions. We have also detected two-component HI 21 cm line and the OH 18 cm line in absorption toward the nucleus of Mrk 1 at blueshifted velocities with respect to its systemic velocity indicating an outflow of atomic and molecular gas. The profile of the OH absorption is qualitatively similar to that of the HI absorption. The higher velocity components of the HI and OH (1667 MHz) absorption lines are blueshifted from the O[III]5007, O[I]6300, and the systemic velocity by ~ 100 km s^{-1}, but are consistent with the O[II]3727 velocity. We explain these velocity discrepancies as due to shock ionization of a region which is pushed forward due to shocks in front of the radio nucleus thereby giving apparent blueshift to HI, OH, and O[II] velocities. The optical depth ratios tau{HI}/tau{OH1667} of both the components of the HI and OH absorption are ~3, indicating their origin in dense molecular clouds. Using OH/A_{v} values for the Galactic molecular clouds, we obtain 9 < A_{v} < 90 toward the line of sight of Mrk 1.

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arXiv2002

Carbon Recombination Lines near 327 MHz I: "Diffuse" CII regions in the Galactic Disk

In earlier papers (Roshi & Anantharamaiah 2000, 2001a), we presented extensive surveys (angular resolution -- 2°$\times$ 2°& 2°$\times$ 6\arcmin) of radio recombination lines (RRLs) near 327 MHz in the longitude range $l = $ 332°$\to$ 89°using the Ooty Radio Telescope. These surveys have detected carbon lines mostly between $l = $ 358°$\to$ 20°and in a few positions at other longitudes. This paper presents the observed carbon line parameters in the high-resolution survey and a study of the galactic distribution and angular extent of the line emission observed in the surveys. The carbon lines detected in the surveys arise in ``diffuse'' \CII regions. The \lv diagram and radial distribution constructed from our carbon line data shows similarity with that obtained from hydrogen recombination lines at 3 cm from \HII regions indicating that the distribution of the diffuse \CII regions in the inner Galaxy resembles the distribution of the star-forming regions. We estimated the [CII] 158 micron emission from diffuse \CII regions and find that upto 95 % of the total observed [CII] 158 micron emission can arise in diffuse \CII regions if the temperature of the latter $\sim 80$ K. Our high-resolution survey data shows that the carbon line emitting regions have structures on angular scale $\sim$ 6\arcmin. Analysis of the dual-resolution observations toward a 2°wide field centered at $l = $ 13°.9 and toward the longitude range $l = $ 1°.75 to 6°.75 shows the presence of narrow ($ΔV \le $ 15 \kms) carbon line emitting regions extending over several degrees in $l$ and $b$. The physical size perpendicular to the line-of-sight of an individual diffuse \CII region in these directions is $>$ 200 pc.

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arXiv2002

GMRT HI imaging survey of ERIDANUS group of galaxies

This project aims to image a subset of galaxies in the nearby group Eridanus in HI emission and in radio continuum using the GMRT. The typical resolution of HI images will be $\sim$1 kpc with a 3$σ$ sensitivity to HI mass of a few times 10$^{7}$ M$_\odot$. The rotation curves obtained using the HI velocity field together with the 2MASS magnitudes in the near infrared will be used to construct the Tully-Fisher relation for this group. High resolution (sub-kpc) multi-frequency radio continuum images will be used to study star-formation rates, radio morphologies and spectral indices of Eridanus galaxies. About half of the galaxies have already been observed and further observations on some late type galaxies are proposed with the GMRT. We present here the HI image and the rotation curve for a member of this group viz. NGC 1385.

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arXiv2002

VLA Detection of RRLs from the radio nucleus of NGC 253 : Ionization by a weak AGN, an obscured SSC or a compact SNR ?

We have imaged the H92alpha and H75alpha radio recombination line (RRL) emissions from the starburst galaxy NGC 253 with a resolution of ~4 pc. The peak of the RRL emission at both frequencies coincides with the unresolved radio nucleus. Both lines observed towards the nucleus are extremely wide, with FWHM of ~200 km /s. Modeling the RRL and radio continuum data for the radio nucleus shows that the lines arise in gas whose density is ~10^4 \cc and mass is few thousand Msun, which requires an ionizing flux of (6-20)x10^{51} photons /s. We consider a SNR expanding in a dense medium, a star cluster and also an AGN as potential ionizing sources. Based on dynamical arguments, we rule out an SNR as a viable ionizing source. A star cluster model was considered and the dynamics of the ionized gas in a stellar-wind driven structure was investigated. Such a model is consistent with the properties of the ionized gas only for a cluster younger than ~10^5 years. The existence of such a young cluster at the nucleus seems improbable. The third model assumes the ionizing source to be an AGN at the nucleus. In this model, it was shown that the observed X-ray flux is too weak to account for the required ionizing photon flux. However, the ionization requirement can be explained if the accretion disk is assumed to have a Big Blue Bump in its spectrum. Hence, we favor an AGN at the nucleus as the source responsible for ionizing the observed RRLs. A hybrid model consisting of a inner ADAF disk and an outer thin disk is suggested, which could explain the radio, UV and the X-ray luminosities of the nucleus.

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arXiv2001

VLA detection of OH absorption from the elliptical galaxy NGC 1052

VLA observations of OH absorption towards the elliptical galaxy NGC 1052 are presented. Both OH lines, at 1665 and 1667 MHz, were detected in absorption towards the center of NGC 1052. The hyperfine ratio of the two OH lines ($τ$$_{1667}$/$τ$$_{1665}$) is 2.6$\pm$0.8 as compared to 1.8 expected for the excitation under LTE conditions for an optically thin cloud. The column density of OH is estimated to be $2.73(\pm0.26)\times10^{14} cm^{-2}$ assuming $T_{ex} \sim$10 K. The centers of both the OH lines are redshifted from the systemic velocity of the galaxy by $\sim$173 km s$^{-1}$. The velocity of OH line coincides with the velocity corresponding to the strongest HI absorption. We suggest that OH absorption is arising from a molecular cloud falling towards the nucleus. The OH line, though narrower, is found to be within the much broader and smoother H$_2$O megamaser emission. The possible link between OH/HI and $H_2O$ emission is discussed.

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arXiv2001

Low-frequency Carbon Recombination Lines towards HI self-absorption features in the Galactic Plane

A survey of radio recombination lines (RLs) in the Galactic plane ($l = $ 332°$\to$ 89°) near 327 MHz made using the Ooty Radio Telescope (ORT) has detected carbon RLs from all the positions in the longitude range 0°$< l < $ 20°and from a few positions at other longitudes. The carbon RLs detected in this survey originate from ``diffuse'' \CII regions. Comparison of the \lv diagram and the radial distribution of carbon line emission with those obtained from hydrogen RLs near 3 cm from \HII regions and ``intense'' \CO emission indicates that the distribution of the diffuse \CII regions in the inner Galaxy is similar to that of spiral arm components. Towards several observed positions, the central velocities of the carbon RLs coincide with \HI self-absorption features suggesting an association of diffuse \CII regions with cool \HI. The observed widths of the lines from the two species are, however, different. We discuss possible reasons for the difference in the line widths.

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