arXiv subjects

Ravi Subrahmanyan

Publications and source records attributed to Ravi Subrahmanyan.

64 records · Page 4Linked to original sources

arXiv2003

The temperature of the WNM in the Milky Way

We report high spectral resolution Australia Telescope Compact Array HI 21 cm observations resulting in the detection of the warm neutral medium (WNM) of the Galaxy in absorption against two extragalactic radio sources, PKS 1814-637 and PKS 0407-658. The two lines of sight were selected on the basis of the simplicity of their absorption profiles and the strength of the background sources; the high velocity resolution of the spectra then enabled us to estimate the kinetic temperatures of the absorbing gas by fitting multiple Gaussians to the absorption profiles. Four separate WNM components were detected toward the two sources, with peak optical depths $τ_{max} = (1.0 \pm 0.08) \times 10^{-2}$, $(1.4 \pm 0.2) \times 10^{-3}$, $(2.2 \pm 0.5) \times 10^{-3}$ and $(3.4 \pm 0.5) \times 10^{-3}$ and kinetic temperatures $T_{k} = 3127 \pm 300$ K, $3694 \pm 1595$ K, $3500 \pm 1354$ K and $2165 \pm 608$ K respectively. All four components were thus found to have temperatures in the thermally unstable range $500 < T_{k} < 5000$ K; this suggests that thermal equilibrium has not been reached throughout the WNM.

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arXiv2003

Radio Interferometers with wide bandwidths

The Australia Telescope Compact Array and the Very Large Array are currently being upgraded to operate with wide bandwidths; interferometers dedicated to the measurement of cosmic microwave background anisotropies are being designed with large instantaneous bandwidths for high sensitivity. Interferometers with wide instantaneous bandwidths that do not operate with correlators capable of decomposing the bands into narrow channels suffer from `bandwidth smearing' effects in wide-field imaging. The formalism of mosaic imaging is extended here to interferometers with finite bandwidths to examine the consequences for the imaging of wide fields if very wide instantaneous bandwidths are used. The formalism presented here provides an understanding of the signal processing associated with wide-band interferometers: mosaicing may be viewed as decomposing visibilities over wide observing bands and, thereby, avoiding `bandwidth smearing' effects in wide-field imaging. In particular, the formalism has implications for interferometer measurements of the angular power spectrum of cosmic microwave background anisotropies: mosaic-mode observing with wide-band radio interferometers decompose wide-band data and synthesize narrow filters in multipole space.

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arXiv2003

Renewed activity in the radio galaxy PKS B1545-321: twin edge-brightened beams within diffuse radio lobes

Australia Telescope Compact Array (ATCA) images of the giant radio galaxy PKS B1545-321 show a pair of oppositely directed beams emerging from a radio core and ending in bright components that are symmetrically located on either side. These inner beams are embedded within edge-brightened outer lobes of lower surface brightness and the bright ends of the inner beams are well recessed from the ends of the outer lobes. The inner beams and diffuse surrounding lobes share a common central core and radio axis. We propose that the observed inner beams are double lobes which have been created within relic outer lobes as a consequence of a restarting of the central activity; therefore, PKS B1545-321 is a rare opportunity for examining the development of restarted beams within a relic synchrotron plasma cocoon. The inner double representing the new episode has among the highest axial ratios found in typical edge-brightened radio galaxies. The low radio luminosity of the inner double, the narrow and constant transverse extent of its cocoon and the relatively low brightness of the hotspots at its ends are consistent with the almost ballistic propagation expected for a beam that has a low density contrast and is advancing within a relatively light ambient medium.

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arXiv2002

A Case for Renewed Activity in the Giant Radio Galaxy J0116-473

We present ATCA radio observations of the giant radio galaxy J0116-473 at 12 and 22 cm wavelengths in total intensity and polarization. The images clearly reveal a bright inner-double structure within more extended edge-brightened lobe emission. The lack of hotspots at the ends of the outer lobes, the strong core and the inner-double structure with its edge-brightened morphology lead us to suggest that this giant radio galaxy is undergoing a renewed nuclear activity: J0116-473 appears to be a striking example of a radio galaxy where a young double source is evolving within older lobe material. We also report the detection of a Mpc-long linear feature which is oriented perpendicular to the radio axis and has a high fractional polarization.

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arXiv2002

CMB observations using the SKA

We examine the prospects for observations of CMB anisotropy with the SKA; we discuss the advantages of interferometric SKA imaging, observing strategies, calibration issues and the achievable sensitivity. Although the SKA will probably operate at cm wavelengths, where discrete source confusion dominates the CMB anisotropy, its extreme sensitivity to point sources will make it possible to subtract the source contamination at these wavelengths and thereby image the low surface brightness CMB anisotropies on small angular scales. The SKA, operating at 10-20 GHz, may usefully make high-l observations of the CMB anisotropy spectrum and survey the sky for Sunyaev-Zeldovich decrements.

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arXiv2001

ATCA and CMB anisotropies

Australia Telescope Compact Array (ATCA) observations in sky regions selected to be low in foreground confusion have been used to infer limits on arcmin-scale CMB anisotropy in total intensity and polarization. These deep searches have, hitherto, been made using the East-West ATCA in an ultra-compact 1-D array configuration and in the highest available frequency band at 3-cm wavelength. The ATCA is being upgraded for operation at mm wavelengths and in 2-D array configurations: the enhanced capabilities present a new opportunity for pursuing CMB anisotropy related observations with reduced foreground confusion and at smaller angular scales.

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arXiv2001

Observing the CMB with the AMiBA

I discuss the capabilities and limitations of the AMiBA for imaging CMB anisotropies. Michael Kesteven (ATNF-CSIRO) has proposed drift-scanning as an observing strategy for measuring and rejecting any instrumental response that the close-packed interferometers may have to the local environment. The advantages of mosaic imaging CMB anisotropies using a co-mounted interferometric array in a drift-scanning observing mode are discussed. A particular case of mosaic imaging a sky strip using a two-element AMiBA prototype interferometer is considered and the signal-to-noise ratio in the measurement of sky anisotropy using this observing strategy is analysed.

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arXiv2000

ATCA search for 21 cm emission from a candidate damped Ly-$α$ absorber at z = 0.101

We report a deep search for 21 cm emission/absorption from the $z \sim 0.101$ candidate damped Lyman-$α$ system towards PKS 0439-433, using the Australia Telescope Compact Array (ATCA). The spectrum shows a weak absorption feature --- at the $3.3 σ$ level --- which yields a lower limit of 730 K on the spin temperature of the system. No HI emission was detected: the $3σ$ upper limit on the HI mass of the absorber is $2.25 \times 10^9 M_{\odot}$, for a velocity spread of $\sim 70$ km s$^{-1}$. The low HI mass and the high spin temperature seem to rule out the possibility that the absorber is a large gas-rich spiral galaxy.

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arXiv1999

Constraints on structure formation models from Sunyaev-Zel'dovich Effect

In the context of cold dark matter (CDM) cosmological models, we have simulated images of the brightness temperature fluctuations in the cosmic microwave background (CMB)sky owing to the Sunyaev Zel$^{\prime}$dovich (S-Z) effect in a cosmological distribution of clusters. We compare the image statistics with recent ATCA limits on arcmin-scale CMB anisotropy. The S-Z effect produces a generically non-Gaussian field and we compute the variance in the simulated temperature-anisotropy images, after convolution with the ATCA beam pattern, for different cosmological models. All the models are normalised to the 4-year {\it COBE} data. We find an increase in the simulated-sky temperature variance with increase in the cosmological density parameter $Ω_0$. A comparison with the upper limits on the sky variance set by the ATCA appears to rule out our closed-universe model: low-$Ω_0$ open-universe models are preferred. The result is independent of any present day observations of $σ_8$.

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arXiv1998

Australia Telescope Search for Cosmic Microwave Background Anisotropy

We have made an 8.7 GHz image of a sky region with a resolution of 8 arcmin and high brightness sensitivity using the Australia Telescope Compact Array (ATCA). in an ultra compact configuration. The foreground discrete-source confusion was estimated from observations with higher resolution at the same frequency and in a scaled array at a lower frequency. Following the subtraction of the foreground confusion, the field shows no features in excess of the instrument noise. This limits the CMB anisotropy flat-band power to Q_flat < 23.6 microK with 95 per cent confidence; the ATCA filter function F_l in multipole l-space peaks at l_eff = 4700 and has half maximum values at l = 3350 and 6050.

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