arXiv subjects

K. S. Dwarakanath

Publications and source records attributed to K. S. Dwarakanath.

At least 73 records · Page 4Linked to original sources

arXiv2007

HI imaging of galaxies in X-ray bright groups

Environment plays an important role in the evolution of the gas contents of galaxies. Gas deficiency of cluster spirals and the role of the hot intracluster medium (ICM) in stripping gas from these galaxies is a well studied subject. Loose groups with diffuse X-ray emmision from the intragroup medium (IGM) offer an intermediate environment between clusters and groups without a hot IGM. These X-ray bright groups have smaller velocity dispersion and lower temperature than clusters, but higher IGM density than loose groups without diffuse X-ray emission. A single dish comparative study of loose groups with and without diffuse X-ray emission from the IGM, showed that the galaxies in X-ray bright groups have lost more gas on average than the galaxies in non X-ray bright groups. In this paper we present GMRT HI observations of 13 galaxies from 4 X-ray bright groups: NGC5044, NGC720, NGC1550 and IC1459. The aim of this work is to study the morphology of HI in these galaxies and to see if the hot IGM has in any way affected their HI content or distribution. In addition to disturbed HI morphology, we find that most galaxies have shrunken HI disks compared to the field spirals. This indicates that IGM assisted stripping processes like ram pressure may have stripped gas from the outer edges of the galaxies.

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arXiv2007

HI imaging of Galaxy Clusters at z=0.2; a Pilot Survey of Abell 963 and Abell 2192

A pilot study with the powerful new backend of the Westerbork Synthesis Radio Telescope (WSRT) of two galaxy clusters at z=0.2 has revealed neutral hydrogen emission from 39 galaxies. The volume probed for each cluster is 1.7x10^4 Mpc^3, with spatial and velocity resolutions of 54x86 kpc^2 and 19.7 km/s, covering both clusters and the large scale structure in which they are embedded. The spatial distribution of the HI detected galaxies is very different for the two clusters. In Abell 963, most of the gas-rich galaxies are located to the northeast, at 1-3 Mpc from the cluster center in projection. Their velocities are slightly redshifted with respect to the cluster mean. This could be a gas-rich group falling in from the front. Abell 2192 is less massive and more diffuse, with the gas rich galaxies more uniformly spread over a large region around the cluster. The HI masses of the detected galaxies range from 5x10^9 to 4x10^10 Msun. Some HI-rich galaxies are spatially resolved and rudimentary rotation curves are derived, showing the prospect for Tully-Fisher studies of different galaxy populations in these environments. Only one galaxy is detected within a 1 Mpc radius from the center of the Butcher-Oemler cluster Abell 963, and none of the blue B-O galaxies which are all located within the central Mpc. The HI detected galaxies outside the central Mpc are of similar colour and magnitude as the non-detected B-O galaxies, indicating that the blue B-O galaxies are gas-poor compared to their counterparts in the field.

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arXiv2006

Limits on the AGN activities in X-ray underluminous galaxy groups

We have observed four X-ray underluminous groups of galaxies using the Giant Meterwave RadioTelescope. The groups NGC 524, 720, 3607, and 4697 are underluminous in relation to the extrapolation of the Lx - T relation from rich clusters and do not show any evidence of current AGN activities that can account for such a departure. The GMRT observations carried out at low frequencies (235 and 610 MHz) were aimed at detecting low surface brightness, steep-spectrum sources indicative of past AGN activities in these groups. No such radio emissions were detected in any of these four groups. The corresponding upper limits on the total energy in relativistic particles is about 3 X 10$^{57}$ erg. This value is more than a factor of 100 less than that required to account for the decreased X-ray luminosities (or, enhanced entropies) of these four groups in the AGN-heating scenario. Alternatively, the AGN activity must have ceased about 4 Gyr ago, allowing the relativistic particles to diffuse out to such a large extent (about 250 kpc) that their radio emission could have been undetected by the current observations. If the latter scenario is correct, the ICM was pre-heated before the assembly of galaxy clusters.

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arXiv2006

Origin of Disc Lopsidedness in the Eridanus Group of Galaxies

The HI surface density maps for a sample of 18 galaxies in the Eridanus group are Fourier analysed. This analysis gives the radial variation of the lopsidedness in the HI spatial distribution. The lopsidedness is quantified by the Fourier amplitude $A_1$ of the $m=1$ component normalized to the average value. It is also shown that in the radial region where the stellar disc and HI overlap, their $A_1$ coefficients are comparable. All the galaxies studied show significant lopsidedness in HI. The mean value of $A_1$ in the inner regions of the galaxies (1.5 - 2.5 scale lengths) is $\geq 0.2$. This value of $A_1$ is twice the average value seen in the field galaxies. Also, the lopsidedness is found to be smaller for late-type galaxies, this is opposite to the trend seen in the field galaxies. These two results indicate a different physical origin for disc lopsidedness in galaxies in a group environment compared to the field galaxies. Further, a large fraction ($\sim$ 30%) shows a higher degree of lopsidedness ($A_1 \geq 0.3$). It is also seen that the disk lopsidedness increases with the radius as demonstrated in earlier studies, but over a radial range that is two timeslarger than done in the previous studies. The average lopsidedness of the halo potential is estimated to be $\sim 10$%, assuming that the lopsidedness in HI disc is due to its response to the halo asymmetry.

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arXiv2006

The Tully-Fisher relations of the Eridanus group of galaxies

The Tully-Fisher (TF) or the luminosity line-width relations of the galaxies in the Eridanus group are constructed using the HI rotation curves and the luminosities in the optical and in the near-infrared bands. The slopes of the TF relations (absolute magnitude vs log2V_{flat}) are -8.6\pm1.1, -10.0\pm1.5, -10.7\pm2.1, and -9.7\pm1.3 in the R, J, H, and K bands respectively for galaxies having flat HI rotation curves. These values of the slopes are consistent with those obtained from studies of other groups and clusters. The scatter in the TF relations is in the range 0.5 - 1.1 mag in different bands. This scatter is considerably larger compared to those observed in other groups and clusters. It is suggested that the larger scatter in the TF relations for the Eridanus group is related to the loose structure of the group. If the TF relations are constructed using the baryonic mass (stellar + HI + Helium mass) instead of the stellar luminosity, nearly identical slopes are obtained in the R and in the near-infrared bands. The baryonic TF (baryonic mass vs log2V_{flat}) slope is in the range 3.5 - 4.1.

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arXiv2005

GMRT HI observations of the Eridanus group of galaxies

The GMRT HI 21cm-line observations of galaxies in the Eridanus group are presented. The Eridanus group, at a distance of ~23 Mpc, is a loose group of \~200 galaxies. The group extends more than 10 Mpc in projection. The velocity dispersion of the galaxies in the group is ~240 km/s. The galaxies are clustered into different sub-groups. The overall population mix of the group is 30% (E+S0) and 70% (Sp+Irr). The observations of 57 Eridanus galaxies were carried out with the GMRT for ~200 hour. HI emission was detected from 31 galaxies. The channel rms of ~1.0 mJy beam^{-1} was achieved for most of the image-cubes made with 4 hour of data. The corresponding HI column density sensitivity (3-sigma) is ~1x10^{20} cm^{-2} for a velocity-width of ~13.4 km/s. The 3-sigma detection limit of HI mass is ~1.2x10^{7} M_sun for a line-width of 50 km/s. Total HI images, HI velocity fields, global HI line profiles, HI mass surface densities, HI disk parameters and HI rotation curves are presented. The velocity fields are analysed separately for the approaching and the receding sides of the galaxies. This data will be used to study the HI and the radio continuum properties, the Tully-Fisher relations, the dark matter halos, and the kinematical and HI lopsidedness in galaxies.

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arXiv2005

The HI content of the Eridanus group of galaxies

The HI content of galaxies in the Eridanus group is studied using the GMRT observations and the HIPASS data. A significant HI deficiency up to a factor of 2-3 is observed in galaxies in the high galaxy density regions. The HI deficiency in galaxies is observed to be directly correlated with the local projected galaxy density, and inversely correlated with the line-of-sight radial velocity. Furthermore, galaxies with larger optical diameters are predominantly in the lower galaxy density regions. It is suggested that the HI deficiency in Eridanus is due to tidal interactions. In some galaxies, evidences of tidal interactions are seen. An important implication is that significant evolution of galaxies can take place in the group environment. In the hierarchical way of formation of clusters via mergers of groups, a fraction of the observed HI deficiency in clusters could have originated in groups. The co-existence of S0's and severely HI deficient galaxies in the Eridanus group suggests that galaxy harassment is likely to be an effective mechanism for transforming spirals to S0's.

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arXiv2005

Radio continuum and far-infrared emission from the galaxies in the Eridanus group

The Eridanus galaxies follow the well-known radio-FIR correlation. Majority (70%) of these galaxies have their star formation rates below that of the Milky Way. The galaxies having a significant excess of radio emission are identified as low luminosity AGNs based on their radio morphologies obtained from the GMRT observations. There are no powerful AGNs (L{20cm} > 10^{23} W Hz^{-1}) in the group. The two most far-infrared and radio luminous galaxies in the group have optical and HI morphologies suggestive of recent tidal interactions. The Eridanus group also has two far-infrared luminous but radio-deficient galaxies. It is believed that these galaxies are observed within a few Myr of the onset of an intense star formation episode after being quiescent for at least a 100 Myr. The upper end of the radio luminosity distribution of the Eridanus galaxies (L_{20cm} ~ 10^{22} W Hz^{-1}) is consistent with that of the field galaxies, other groups, and late-type galaxies in nearby clusters.

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arXiv2004

A High Galactic Latitude HI 21cm-line Absorption Survey using the GMRT: I. Observations and Spectra

We have used the Giant Meterwave Radio Telescope (GMRT) to measure the Galactic HI 21-cm line absorption towards 102 extragalactic radio continuum sources, located at high (|b| >15deg.) Galactic latitudes. The Declination coverage of the present survey is Decl. ~ -45deg.. With a mean rms optical depth of ~0.003, this is the most sensitive Galactic HI 21-cm line absorption survey to date. To supplement the absorption data, we have extracted the HI 21-cm line emission profiles towards these 102 lines of sight from the Leiden Dwingeloo Survey of Galactic neutral hydrogen. We have carried out a Gaussian fitting analysis to identify the discrete absorption and emission components in these profiles. In this paper, we present the spectra and the components. A subsequent paper will discuss the interpretation of these results.

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arXiv2004

A High Galactic Latitude HI 21cm-line Absorption Survey using the GMRT: II. Results and Interpretation

We have carried out a sensitive high-latitude (|b| > 15deg.) HI 21cm-line absorption survey towards 102 sources using the GMRT. With a 3-sigma detection limit in optical depth of ~0.01, this is the most sensitive HI absorption survey. We detected 126 absorption features most of which also have corresponding HI emission features in the Leiden Dwingeloo Survey of Galactic neutral Hydrogen. The histogram of random velocities of the absorption features is well-fit by two Gaussians centered at V(lsr) ~ 0 km/s with velocity dispersions of 7.6 +/- 0.3 km/s and 21 +/- 4 km/s respectively. About 20% of the HI absorption features form the larger velocity dispersion component. The HI absorption features forming the narrow Gaussian have a mean optical depth of 0.20 +/- 0.19, a mean HI column density of (1.46 +/- 1.03) X 10^{20} cm^{-2}, and a mean spin temperature of 121 +/- 69 K. These HI concentrations can be identified with the standard HI clouds in the cold neutral medium of the Galaxy. The HI absorption features forming the wider Gaussian have a mean optical depth of 0.04 +/- 0.02, a mean HI column density of (4.3 +/- 3.4) X 10^{19} cm^{-2}, and a mean spin temperature of 125 +/- 82 K. The HI column densities of these fast clouds decrease with their increasing random velocities. These fast clouds can be identified with a population of clouds detected so far only in optical absorption and in HI emission lines with a similar velocity dispersion. This population of fast clouds is likely to be in the lower Galactic Halo.

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arXiv2004

Kinematics of Diffuse Interstellar Clouds :Recent GMRT Results

A high latitude HI 21 cm-line absorption survey toward extragalactic sources was recently completed using the Giant Meterwave Radio Telescope (GMRT). A total of 104 sources with mod(b) > 15 deg. and with a 21 cm flux density greater than 1 Jy were observed for about 120 hours. With an optical depth detection limit of about 0.01 this is the most sensitive high-latitude survey as yet. Most of the detected HI 21 cm-line absorption features belong to a population with a velocity dispersion of about 7.6 km/s. These are the 'standard' HI clouds and have been well-studied for decades. However, we also detect a second population of absorbing clouds with a velocity dispersion of about 21 km/s. About 20 percent of the total number of absorbing clouds belong to this population. This new population of 'fast' clouds can be identified with a similar velocity dispersion Ca II absorbing clouds and with the Halo clouds recently detected in HI emission from Green Bank.

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arXiv2004

On the Origin of the Wide HI Absorption Line Toward Sgr A*

We have imaged a region of about 5' extent surrounding Sgr A* in the HI 21 cm-line absorption using the Very Large Array. A Gaussian decomposition of the optical depth spectra at positions within about 2' (approx. 5 pc at 8.5 kpc) of Sgr A* detects a wide line underlying the many narrow absorption lines. The wide line has a mean peak optical depth of 0.32 +/- 0.12 centered at a mean velocity of V(lsr) = -4 +/- 15 km/s. The mean full width at half maximum is 119 +/- 42 km/s. Such a wide line is absent in the spectra at positions beyond about 2' from Sgr A*. The position-velocity diagrams in optical depth reveal that the wide line originates in various components of the circumnuclear disk (radius approx. 1.3') surrounding Sgr A*. These components contribute to the optical depth of the wide line in different velocity ranges. The position-velocity diagrams do not reveal any diffuse feature which could be attributed to a large number of HI clouds along the line of sight to Sgr A*. Consequently, the wide line has no implications either to a global population of shocked HI clouds in the Galaxy or to the energetics of the interstellar medium as was earlier thought.

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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

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

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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arXiv2001

Detection of HI 21 cm-line absorption in the Warm Neutral Medium and in the Outer Arm of the Galaxy

Using the Westerbork Synthesis Radio Telescope, we have detected HI 21 cm-line absorption in the Warm Neutral Medium of the Galaxy toward the extragalactic source 3C147. This absorption, at an LSR velocity of -29+/-4 km/s with a full width at half maximum of 53+/-6 km/s, is associated with the Perseus Arm of the Galaxy. The observed optical depth is (1.9+/-0.2)*10**(-3). The estimated spin temperature of the gas is 3600+/-360 K. The volume density is 0.4 per cc assuming pressure equilibrium. Toward two other sources, 3C273 and 3C295, no wide HI 21 cm-line absorption was detected. The highest of the 3sigma lower limits on the spin temperature of the Warm Neutral Medium is 2600 K. In addition, we have also detected HI 21 cm-line absorption from high velocity clouds in the Outer Arm toward 3C147 and 3C380 at LSR velocities of -117.3, -124.5 and -113.7 km/s respectively. We find two distinct temperature components in the high velocity clouds with spin temperatures of greater than 1000 K and less than 200 K, respectively.

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arXiv2001

GMRT Detection of HI 21 cm-line Absorption from the Peculiar Galaxy in Abell 2125

Using the recently completed Giant Meterwave Radio Telescope, we have detected the HI 21 cm-line absorption from the peculiar galaxy C153 in the galaxy cluster Abell 2125. The HI absorption is at a redshift of 0.2533, with a peak optical depth of 0.36. The full width at half minimum of the absorption line is 100 km/s. The estimated column density of atomic Hydrogen is 0.7e22(Ts/100K) per sq. cm. The HI absorption is redshifted by ~ 400 km/s compared to the [OIII] emission line from this system. We attribute this to an in-falling cold gas, or to an out-flowing ionised gas, or to a combination of both as a consequence of tidal interactions of C153 with either a cluster galaxy or the cluster potential.

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arXiv2001

GMRT Observations of Interstellar Clouds in the 21cm line of Atomic Hydrogen

Nearby interstellar clouds with high (|v| >/= 10 km/s) random velocities although easily detected in NaI and CaII lines have hitherto not been detected (in emission or absorption) in the HI 21cm line. We describe here deep Giant Metrewave Radio Telescope (GMRT) HI absorption observations toward radio sources with small angular separation from bright O and B stars whose spectra reveal the presence of intervening high random velocity CaII absorbing clouds. In 5 out of the 14 directions searched we detect HI 21cm absorption features from these clouds. The mean optical depth of these detections is ~ 0.09 and FWHM is ~ 10 km/s, consistent with absorption arising from CNM clouds.

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