2009 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/332
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Item Magnetic quenching of alpha and diffusity tensors in helical turbulence(2011-07-06) Brandenburg, Axel; Biman, B.; Subramanian, Kandaswamy; et al.We study the implications of primordial magnetic fields for the thermal and ionization history of the post-recombination era. In particular we compute the effects of dissipation of primordial magnetic fields owing to ambipolar diffusion and decaying turbulence in the intergalactic medium (IGM) and the collapsing halos and compute the effects of the altered thermal and ionization history on the formation of molecular hydrogen.We show that, for magnetic field strengths in the range 2×10−10 G < ∼ B0 < ∼ 2× 10−9 G, the molecular hydrogen fraction in IGM and collapsing halo can increase by a factor 5 to 1000 over the case with no magnetic fields. We discuss the implication of the increased molecular hydrogen fraction on the radiative transfer of UV photons and the formation of first structures in the universeItem Primordial magnetic fields and formation of molecular hydrogen(2009-04-01) Sethi, Shiv K.; Nath, B. B.; Subramanian, Kandaswamy; et al.We study the implications of primordial magnetic fields for the thermal and ionization history of the post-recombination era. In particular we compute the effects of dissi- pation of primordial magnetic fields owing to ambipolar diffusion and decaying tur- bulence in the intergalactic medium (IGM) and the collapsing halos and compute the effects of the altered thermal and ionization history on the formation of molecular hy- drogen.We show that, for magnetic field strengths in the range 2×10−10 G < ∼ B0 < ∼ 2× 10−9 G, the molecular hydrogen fraction in IGM and collapsing halo can increase by a factor 5 to 1000 over the case with no magnetic fields. We discuss the implication of the increased molecular hydrogen fraction on the radiative transfer of UV photons and the formation of first structures in the universeItem SDSS J092712.64+294344.0: recoiling black hole or merging galaxies(2009-08-01) Vivek, M.; Srianand, R.; Noterdaeme, P.; et al.We report long-slit spectroscopic observations of SDSS J092712+294344 carried-out at the recently commissioned 2m telescope in IUCAA Girawali Observatory, India. This AGN-like source is known to feature three sets of emission lines at zem = 0.6972, 0.7020 and 0.7128. Different scenarios such as a recoiling black hole after asymmetric emission of gravitational waves, binary black holes and possible merging systems are proposed for this object. We test these scenarios by comparing our spectra with that fromthe Sloan Digital Sky Survey (SDSS), obtained 4 years prior to our observations. Comparing the redshifts of [Oiii]λλ4960,5008 we put a 3σ limit on the relative acceleration to be less than 32 km s−1 yr −1 between different emitting regions. Using the 2D spectra obtained at different position angles we show that the [Oiii]λ5008 line from the zem = 0.7128 component is extended beyond the spectral point spread function.We infer the linear extent of this line emitting region is ∼ 8 kpc.We also find a tentative evidence for an offset between the centroid of the [Oiii]λ5008 line at zem = 0.7128 and the QSO trace when the slit is aligned at a position angle of 299◦ . This corresponds to the zem = 0.7128 system being at an impact parameter of ∼1 kpc with respect to the zem = 0.6972 in the north west direction. Based on our observations we conclude that the binary black hole model is most unlikely. The spatial extent andItem TELICS-A Telescope Instrument Control System for Small/Medium Sized Astronomical Observatories(2009-10-01) Srivastava, Mudit K.; Ramaprakash, A. N.; Burse, Mahesh P.; et al.For any modem astronomical observatory, it is essential to have an efficient interface between the telescope and its back-end instruments. However, for small and medium-sized observatories, this requirement is often limited by tight financial constraints. Therefore a simple yet versatile and low-cost control system is required for such observatories to minimize cost and effort. Here we report the development of a modem, multipurpose instrument control system TELlCS (Telescope Instrument Control System) to integrate the controls of various instruments and devices mounted on the telescope. TELlCS consists of an embedded hardware unit known as a common control unit (CCU) in combination with Linux-based data acquisition and user interface. The hardware of the CCU is built around the ATmega 128 microcontroller (Atmel Corp.) and is designed with a backplane, master-slave architecture, A Qt-based graphical user interface (GUI) has been developed and the back-end application software is based on C/C++. TELlCS provides feedback mechanisms that give the operator good visibility and a quick-look display of the status and modes of instruments as well as data. TELICS has been used for regular science observations since 2008 March on the 2 m, fllO IUCAA Telescope located at Girawali in Pune, India.Item Reconnecting Flux Rope Dynamo(2009-10-01) Subramanian, Kandaswamy; Baggaley, Andrew W; Barenghi, Carlo F.; et al.We develop a new model of the fluctuation dynamo in which the magnetic field is confined to thin flux ropes advected by a multi-scale model of turbulence. Magnetic dissipation occurs only via reconnection of the flux ropes. This model can be viewed as an implementation of the asymptotic limit Rm → ∞ for a continuous magnetic field, where magnetic dissipation is strongly localized to small regions of strong field gradients. We investigate the kinetic energy release into heat, mediated by the dynamo action, both in our model and by solving the induction equation with the same flow. We find that a flux rope dynamo is an order of magnitude more efficient at converting mechanical energy into heat. The probability density of the magnetic energy release in reconnections has a power-law form with the slope −3, consistent with the Solar corona heating by nanoflares.Item Radio observations of ZwCl 2341.1+0000: a double radio relic cluster(2009-10-01) Weeren, R.; Miniati, F.; Bagchi, Joydeep; et al.infall and mergers of smaller subclusters and galaxy groups. Diffuse radio emission, in the form of radio halos and relics, is found in clusters undergoing a merger, indicating that shocks or turbulence associated with the merger are capable of accelerating electrons to highly relativistic energies. Double relics are a rare class of radio sources found in the periphery of clusters, with the two components located symmetrically on the opposite sides of the cluster center. These relics are important probes of the cluster periphery as (i) they provide an estimate of the magnetic field strength, and (ii) together with detailed modeling can be used to derive information about the merger geometry, mass, and timescale. Observations of these double relics can thus be used to test the framework of LSS formation. Here we report on radio observations of ZwCl 2341.1+0000, a complex merging structure of galaxies located at z = 0.27, using Giant Metrewave Radio Telescope (GMRT) observations. Aims. The main aim of the observations is to study the nature of the diffuse radio emission in the galaxy cluster ZwCl 2341.1+0000. Methods. We carried out GMRT 610, 241, and 157 MHz continuum observations of ZwCl 2341.1+0000. The radio observations are combined with X-ray and optical data of the cluster. Results. The GMRT observations show a double peripheral radio relic in the cluster ZwCl 2341.1+0000. The spectral index is −0.49 ± 0.18 for the northern relic and −0.76 ± 0.17 for the southern relic. We have derived values of 0.48 − 0.93 µGauss for the equipartition magnetic field strength. The relics are probably associated with outward traveling merger shock waves.Item Quasars probing intermediate redshift star-forming galaxies(2009-12-01) Noterdaeme, P.; Srianand, R.; Mohan, V.; et al.We present a sample of 46 [Oiii]-emitting galaxies at z < 0.8 detected in the fibre spectra of quasars from the Sloan Digital Sky Survey, Data Release 7 (SDSS-DR7) through an automatic search procedure. We also detect [Oii] and Hβ emission lines from most of these galaxies in the SDSS spectra. We study both the emission and absorption properties of a sub sample of 17 galaxies in the redshift range z = 0.4-0.7, where Mg ii lines are covered by the SDSS spectra. The measured lower-limits on the star-formation rates of these galaxies are in the range 0.2-20 M⊙ yr −1 . The emission line luminosities and (O/H) metallicities from R23 measured in this sample are similar to what is found in normal galaxies at these redshifts. Thus, this constitutes a unique sample of intermediate redshift star-forming galaxies where we can study the QSO absorber - galaxy connection. Strong Mg ii (Wλ2796> ∼ 1 Å) as well as Mg i absorption lines are detected in the QSO spectra at the redshift of most of these galaxies. Strong Fe ii (Wλ2600 > 1 Å) absorption lines are also generally detected whenever the appropriate wavelength ranges are covered. This suggests that most of these systems could be bona-fide Damped Lyman-α systems. We investigate various possible relations between the Mg ii rest equivalent widths and the emission line properties.We find a possible (2σ) correlation between the emission-line metallicity of the galaxies and the Mg ii rest equivalent width of the absorbers (log(O/H) + 12 = 0.1Wλ2796 + 8.27), which could be a consequence of an underlying mass-metallicity relation. However, [Oiii]-selected Mg ii systems represent only a minor fraction of the strongMg ii absorbers.We find this cannot be attributed to biases related either to the spectral signal-to-noise ratio or to the brightness of the QSOs. We measure the average observed fluxes (collected into the SDSS fibre) of the [Oii] and [Oiii] lines associated to Mg ii-selected systems through stacking technique.We find that the average lumiosities of emission lines are higher for systems with larger Wλ2796. The stacked luminosities are found to be below the typical detection limit in individual spectra, indicating that faint galaxies can contribute appreciably to the observed population of strong Mg ii absorbers at intermediate redshifts.We also present long-slit spectroscopic observations of SDSS J113108+202151, the most luminous line-emitting galaxy in our z ≥ 0.4 sample. Surprisingly, we find that the line-emitting region does not coincide with the nearby extended bright galaxy with consistent photometric redshift seen in the SDSS image.Item Optical identification of XMM sources in the CFHTLS(2009-05-01) Stalin, C. S.; Petitjean, Patrick; Srianand, R.; et al.We summarize the attempts by our group and others to derive constraints on variations of fundamental constants over cosmic time using quasar absorption lines. Most upper limits reside in the range 0.5 1.5 10 5 at the 3 level over a redshift range of approximately 0:5 2:5 for the fine-structure constant, , the proton-to-electron mass ratio, and a combination of the proton gyromagnetic factor and the two previous constants, gp( 2= ) , for only one claimed variation of . It is therefore very important to perform new measurements to improve the sensitivity of the numerous methods to at least <0.1 10 5 which should be possible in the next few years. Future instrumentations on ELTs in the optical and/or ALMA, EVLA and SKA pathfinders in the radio will undoutedly boost this field by allowing to reach much better signal-to-noise ratios at higher spectral resolution and to perform measurements on molecules in the ISM of high redshift galaxiItem Diffuse molecular gas at high redshift: Detection of CO molecules and the 2175 A* dust feature at z=1.64(2009-06-01) Noterdaeme, P.; Ledoux, C.; Srianand, R.; et al.We present the detection of carbon monoxide molecules (CO) at z = 1.6408 towards the quasar SDSS J160457.50+220300.5 using the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph. CO absorption is detected in at least two components in the first six A-X bands and one d-X(5-0) inter-band system. This is the second detection of this kind along a quasar line of sight. The CO absorption profiles are well modelled assuming a rotational excitation of CO in the range 6 < Tex < 16 K, which is consistent with or higher than the temperature of the Cosmic Microwave Background Radiation at this redshift. We derive a total CO column density of N(CO) = 4 × 1014 cm−2 . The measured column densities of S i, Mg i, Zn ii, Fe ii and Si ii indicate a dust depletion pattern typical of cold gas in the Galactic disc. The background quasar spectrum is significantly reddened (u−K ∼ 4.5 mag) and presents a pronounced 2175 Å dust absorption feature at the redshift of the CO absorber. Using a control sample of ∼500 quasars we find the chance probability for this feature to be spurious is ∼0.3%. We show that the spectral energy distribution (SED) of the quasar is well fitted with a QSO composite spectrum reddened with a Large Magellanic Cloud supershell extinction law at the redshift of the absorber. It is noticeable that this quasar is absent from the colour-selected SDSS quasar sample. This demonstrates our current view of the Universe may be biased against dusty sightlines. These direct observations of carbonaceous molecules and dust open up the possibility of studying physical coItem On distribution of CMEs speed in solar cycle 23(2009-02-01) Mittal, Nishant; Sharma, Joginder; Tomar, Vivek; et al.We have analyzed the data for more than 12900 CMEs which were obtained by SOHO/LASCO during the period of 1996-2007. The online CME catalogue contains all major CMEs detected by LASCO C2 and C3 coronagraphs. Basically we determine the CME speeds from the linear and quadratic fits to the height-time measurements. It is found that linear (constant speed) fit is preferable for 90% of the CMEs. The distribution of speeds of CMEs in solar cycle 23 is presented along with those obtained by others. As expected, the speeds decrease in the decay phase of the cycle 23. There is an unusual drop in speed in the year 2001 and an abnormal increase in speed in the year 2003 due to the high concentration of CMEs, X-class soft X-ray flares, solar energetic particle (SEP) events and interplanetary shocks observed during October-November period called Halloween events.