34 research outputs found
Radio emission from AGN detected by the VLA FIRST survey
No full textUsing the most recent (April 2003) version of the VLA FIRST survey
radio catalog, we have searched for radio emission from >2800 AGN taken
from the most recent (2001) version of the Veron-Cetty and Veron AGN catalog. These AGN lie in the ~9033 square degrees of sky
already covered by the VLA FIRST survey. Our work has resulted in
positive detection of radio emission from 775 AGN of which 214
are new detections at radio wavelengths
A difference boosting neural network for automated star-galaxy classification
No full textIn this paper we describe the use of a new artificial neural network, called the difference boosting neural network (DBNN), for automated classification problems in astronomical data analysis. We illustrate the capabilities of the network by applying it to star galaxy classification using recently released, deep imaging data. We have compared our results with classification made by the widely used Source Extractor (SExtractor) package. We show that while the performance of the DBNN in star-galaxy classification is comparable to that of SExtractor, it has the advantage of significantly higher speed and flexibility during training as well as classification
Near-infrared bulge-disc correlations of lenticular galaxies
No full textWe consider the luminosity and environmental dependence of structural parameters of lenticular galaxies in the near-infrared K band. Using a 2D galaxy image decomposition technique, we extract bulge and disc structural parameters for a sample of 36 lenticular galaxies observed by us in the K band. By combining data from the literature for field and cluster lenticulars with our data, we study correlations between parameters that characterize the bulge and the disc as a function of luminosity and environment. We find that scaling relations such as the Kormendy relation, photometric plane and other correlations involving bulge and disc parameters show a luminosity dependence. This dependence can be explained in terms of galaxy formation models in which faint lenticulars (MT > -24.5) formed via secular formation processes that likely formed the pseudo-bulges of late-type disc galaxies, while brighter lenticulars (MT < -24.5) formed through a different formation mechanism most likely involving major mergers. On probing variations in lenticular properties as a function of environment, we find that faint cluster lenticulars show systematic differences with respect to faint field lenticulars. These differences support the idea that the bulge and disc components fade after the galaxy falls into a cluster, while simultaneously undergoing a transformation from spiral to lenticular morphologies
Lenticular galaxy formation: possible luminosity dependence
No full textWe investigate the correlation between the bulge effective radius (re) and disk scale length (rd) in the near-infrared K band for lenticular galaxies in the field and in clusters. We find markedly different relations between the two parameters as a function of luminosity. Lenticulars with total absolute magnitude fainter than MT = -24.5 show a positive correlation, in line with predictions of secular formation processes for the pseudobulges of late-type disk galaxies. But brighter lenticulars with MT < -24.5 show an anticorrelation, indicating that they formed through a different mechanism. The available data are insufficient to reliably determine the effect of galaxy environment on these correlations
GMRT mini-survey to search for 21-cm absorption in quasar-galaxy pairs at z∼ 0.1
No full textWe present the results from our 21-cm absorption survey of a sample of 5 quasar-galaxy pairs (QGPs), with the redshift of the galaxies in the range 0.03 ≤ zg ≤ 0.18, selected from the Sloan Digital Sky Survey (SDSS). The H I 21-cm absorption was searched towards the nine sightlines with impact parameters ranging from ∼10 to ∼55 kpc using the Giant Metrewave Radio Telescope (GMRT). 21-cm absorption was detected only in one case, i.e. towards the quasar (zq= 2.625 SDSS J124157.54+633241.6)-galaxy (zg= 0.143 SDSS J124157.26+633237.6) pair with the impact parameter ∼11 kpc. The quasar sightline in this case pierces through the stellar disc of a galaxy having near solar metallicity [i.e. (O/H)+12 = 8.7] and star formation rate uncorrected for dust attenuation of 0.1 M⊙ yr-1. The quasar spectrum reddened by the foreground galaxy is well fitted with the Milky Way extinction curve (with an AV of 0.44) and the estimated H i column density is similar to the value obtained from 21-cm absorption assuming a spin temperature (TS) of 100 K. In the remaining cases, our GMRT spectra provide upper limit on N(H i) in the range (1017-1018) × TS cm-2. Combining our sample with the z ≤ 0.1 data available in the literature, we find the detectability of 21-cm absorption with integrated optical depth greater than 0.1 km s-1 to be 50 per cent for the impact parameter less than 20 kpc. Using the surface brightness profiles and a well-established relationship between the optical size and extent of the H i disc known for nearby galaxies, we conclude that in most of the cases of 21-cm absorption non-detection, the sightlines may not be passing through the H i gas (1σ column density of a few times 1019 cm-2). We also find that in comparison to the absorption systems associated with these QGPs, z < 1 DLAs with 21-cm absorption detections have lower Ca II equivalent widths despite having higher 21-cm optical depths and smaller impact parameters. This suggests that the current sample of DLAs may be a biased population that avoids sight lines through dusty star-forming galaxies. A systematic survey of QGPs over a wider redshift range using a large sample is needed to confirm these findings and understand the nature of 21-cm absorbers
Evolution of galactic disks and spheroids
No full textIn this Thesis the structural properties of galaxies using quantitative 2-dimensional bulge to disk decompositions are analysed across a range of environments and red- shifts. The study of morphological properties of galaxies is undergoing a significant progress since large galaxy surveys such as the Sloan Digital Sky Survey are producing a vast amounts of data enabling properties such as galaxy structure to be analysed on a statistical level. The wide-field imaging available today makes the studies of the nearby and high redshift environments comparable on a similar physical scale and depth. The advances in the theoretical studies and understanding the physics behind processes that govern galaxy formation and evolution make studies such as this one important since they provide basic estimates of galaxy structure that can be used to constrain the current theoretical models. By comparing the structural properties of bright cluster galaxies between z ~ 0.5 and present we are able to confirm that galaxies have undergone a high degree of evolution although the exact processes that govern this evolution still remain unresolved. In order to account for the evolution of z ~ 0.5 cluster spirals into present-day SOs a significant number of spiral galaxies (factor of 3) must have had their disks faded to make them drop .out of our magnitude limited sample. However, the structural properties between the faded-disk spirals and present day SOs are not consistent. One possibility is that the progenitor galaxies might have been accreted recently from the field and that these have also been previously pre-processed. The study of the structural properties of field galaxies has resulted in a potential evidence that galaxies in the field environment have an additional structural component or structural property that cannot be accounted for by the current 2- dimensional decomposition methods. This study has found that disks contribute about 3 times as much light as bulges to the total luminosity density of the local universe. Tighter constraints in terms of bulge and disk mass densities would help to constrain the current theoretical predictions
Deep GMRT 150-MHz observations of the LBDS-Lynx region: ultrasteep spectrum radio sources
No full textMulti-frequency characterisation of remnant radio galaxies in the Lockman Hole field
No full textContext. Remnant radio galaxies represent an important phase in the life-cycle of radio active galactic nuclei. It is suggested that in this phase, the jets have switched off and the extended emission is fading rapidly. This phase is not well-studied due to the lack of statistical samples observed at both low and high frequencies.
Aims. In this work, we study a sample of 23 candidate remnant radio galaxies previously selected using the Low Frequency Array at 150 MHz in the Lockman Hole field. We examine their morphologies and study their spectral properties to confirm their remnant nature and revise the morphological and spectral criteria used to define the initial sample.
Methods. We present new observations with the Karl G. Jansky Very Large Array at 6000 MHz at both high and low resolution. These observations allowed us to observe the presence or absence of cores and study the spectral curvature and steepness of the spectra of the total emission expected at these high frequencies for the remnant candidates.
Results. We confirm 13 out of 23 candidates as remnant radio sources. This corresponds to 7% of the full sample of active, restarted, and remnant candidates from the Lockman Hole field. Surprisingly, only a minority of remnants reside in a cluster (23%). The remnant radio galaxies show a range of properties and morphologies. The majority do not show detection of the core at 6000 MHz and their extended emission often shows ultra-steep spectra (USS). However, there are also remnants with USS total emission and a detection of the core at 6000 MHz, possibly indicating a variety of evolutionary stages in the remnant phase. We confirm the importance of the combination of morphological and spectral criteria and this needs to be taken into consideration when selecting a sample of remnant radio sources
A low-frequency study of recently identified double-double radio galaxies
No full textIn order to understand the possible mechanisms of recurrent jet activity in radio galaxies and quasars, which are still unclear, we have identified such sources with a large range of linear sizes (220-917 kpc), and hence timescales of episodic activity. Here we present high-sensitivity 607-MHz Giant Metrewave Radio Telescope (GMRT) images of 21 possible double-double radio galaxies (DDRGs) identified from the FIRST survey to confirm their episodic nature. These GMRT observations show that none of the inner compact components suspected to be hotspots of the inner doubles are cores having a flat radio spectrum, confirming the episodic nature of these radio sources. We have identified a new DDRG with a candidate quasar, and have estimated the upper spectral age limits for eight sources which showed marginal evidence of steepening at higher frequencies. The estimated age limits (11-52 Myr) are smaller than those of the large-sized (̃1 Mpc) DDRGs
