A high galactic latitude HI 21 cm-line absorption survey using the GMRT: II. Results and interpretation

We have carried out a sensitive high-latitude (|b| > 15°) HI 21 cm-line absorption survey towards 102 sources using the GMRT. With a 3σ 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_1sr ∼ 0 km s⁻¹ with velocity dispersions of 7.6 ± 0.3 km s⁻¹ and 21 ± 4 km s⁻¹ 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) × 10²⁰ 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) × 10¹⁹ 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.     

GMRT observations of interstellar clouds in the 21cm line of atomic hydrogen

Nearby interstellar clouds with high (|ν|≥10km s⁻¹) 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 ∼10km s⁻¹, consistent with absorption arising from CNM clouds.     

Filling factors and scale heights of the diffuse ionized gas in the Milky Way

The combination of dispersion measures of pulsars, distances from the model of Cordes & Lazio (2002) and emission measures from the WHAM survey enabled a statistical study of electron densities and filling factors of the diffuse ionized gas (DIG) in the Milky Way. The emission measures were corrected for absorption and contributions from beyond the pulsar distance. For a sample of 157 pulsars at |b | > 5. and 60° < ℓ < 360°, located in mainly interarm regions within about 3 kpc from the Sun, we find that: (1) The average volume filling factor along the line of sight and the mean density in ionized clouds are inversely correlated: ( ) = (0.0184 ± 0.0011) ^{–1.07 ± 0.03} for the ranges 0.03 < < 2 cm^–3 and 0.8 > > 0.01. This relationship is very tight. The inverse correlation of and causes the well‐known constancy of the average electron density along the line of sight. As (z ) increases with distance from the Galactic plane |z |, the average size of the ionized clouds increases with |z |. (2) For |z| < 0.9 kpc the local density in clouds n _c(z ) and local filling factor f (z ) are inversely correlated because the local electron density n _e(z ) = f (z )n _c(z ) is constant. We suggest that f (z ) reaches a maximum value of >0.3 near |z | = 0.9 kpc, whereas n _c(z ) continues to decrease to higher |z |, thus causing the observed flattening in the distribution of dispersion measures perpendicular to the Galactic plane above this height. (3) For |z | < 0.9 kpc the local distributions n _c(z ), f (z ) and (z ) have the same scale height which is in the range 250 < h ≲ 500 pc. (4) The average degree of ionization of the warm atomic gas (z ) increases towards higher |z | similarly to (z ). Towards |z | = 1 kpc, (z ) = 0.24 ± 0.05 and (z ) = 0.24 ± 0.02. Near |z | = 1 kpc most of the warm, atomic hydrogen is ionized. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the origin of the wide HI absorption line towards Sgr A*

We have imaged a region of ∼ 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 ∼ 2′ (∼ 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_1sr = −4 ± 15 km s{−1}. The mean full width at half maximum is 119 ± 42 km s⁻¹. Such a wide line is absent in the spectra at positions beyond ∼ 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 ∼ 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.     

Hydrogen recombination lines near 327 MHz−II: A Galactic plane survey with a 2°×6′ beam

In a previous paper we presented a low-resolution (2°×2°) survey of radio recombination lines (RRLs) at 327 MHz in the longitude rangel=330° to 0° to 89°. In this paper, we present the results of a higher resolution (2°×6′) survey of RRLs from seven 2°-wide fields and two 6°-wide fields in the same longitude range. Observations were made using the Ooty Radio Telescope (ORT). A total of 252 spectra that were obtained are presented. RRLs were detected in almost all the individual positions within the fields withl<35° and at several individual positions within the fields in the longitude rangel=35° to 85°. Detailed analysis of the data towards the field centered at G45.5+0.0, shows that the line emission consists of discrete zones of ionized gas. The angular extent of these zones are likely to be one degree or more corresponding to a linear size of >110 pc at the kinematic distance.     

H i line measurements of pulsars towards the Galactic Centre and the electron density in the inner Galaxy

We have measured 21-cm absorption and emission spectra in the direction of a further seven southern pulsars with the Parkes telescope to derive their kinematic distances and to study the interstellar medium. For the first time we have successfully obtained H  i absorption measurements for PSRs J1602–5100, J1740–3015 and J1745–3040. We have also significantly improved the sensitivity and resolution on PSRs J1600–5044, J1752–2806 and J1825–0935, the spectra of which have previously been measured, and have corrected an error in the published distance to PSR J1824–1945.
Since the Frail & Weisberg summary of pulsar distances in 1990, a further 23 pulsars now have measured H  i distances, mainly through the efforts of the current group. We discuss the Taylor & Cordes electron density model in light of these new measurements and find that, although the model towards the Galactic Centre appears good, the line of sight through the Carina spiral arm is poorly fitted by the model.     

Galactic H i on the 50-au scale in the direction of three extragalactic sources observed with MERLIN

We present MERLIN observations of Galactic 21-cm H  i absorption at an angular resolution of  ∼0.1–0.2  arcsec and a velocity resolution of 0.5 km s⁻¹, in the direction of three moderately low latitude  (−8° < b < −12°)  extragalactic radio sources, 3C 111, 3C 123 and 3C 161, all of which are heavily reddened. H  i absorption is observed against resolved background emission sources up to ∼2 arcsec in extent and we distinguish details of the opacity distribution within 1–1.5 arcsec regions towards 3C 123 and 3C 161. This study is the second MERLIN investigation of small-scale structure in interstellar H  i (earlier work probed Galactic H  i in the directions of the compact sources 3C 138 and 3C 147). The 0.1-arcsec scale is intermediate between H  i absorption studies made with other fixed element interferometers with resolution of 1–10 arcsec and very long baseline interferometry studies with resolutions of 10–20 mas. At a scale of 1 arcsec (about 500 au), prominent changes in Galactic H  i opacity in excess of 1–1.5 are determined in the direction of 3C 161 with a signal-to-noise ratio of at least 10σ. Possible fluctuations in the H  i opacity at the level of about 1 are detected at the  2.5–3σ  level in the direction of 3C 123.     

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⁻¹. The estimated column density of atomic Hydrogen is 0.7×10²²(T _s /100) cm⁻². The HI absorption is redshifted by ∼400km s⁻¹ compared to the [OIII] emission line from this system. We attribute this to an infalling 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.     

H i 21-cm absorption at z∼ 3.39 towards PKS 0201+113

We report the Giant Metrewave Radio Telescope detection of H  i 21-cm absorption from the z ∼ 3.39 damped Lyman α absorber (DLA) towards PKS 0201+113, the highest redshift at which 21-cm absorption has been detected in a DLA. The absorption is spread over ∼115 km s⁻¹ and has two components, at   z = 3.387 144(17)  and   z = 3.386 141  (45). The stronger component has a redshift and velocity width in agreement with the tentative detection of Briggs, Brinks & Wolfe, but a significantly lower optical depth. The core size and DLA covering factor are estimated to be ≲100 pc and f ∼ 0.69, respectively, from a Very Long Baseline Array 328-MHz image. If one makes the conventional assumption that the H  i column densities towards the optical and radio cores are the same, this optical depth corresponds to a spin temperature of T _s∼[(955 ± 160) × ( f /0.69)] K. However, this assumption may not be correct, given that no metal-line absorption is seen at the redshift of the stronger 21-cm component, indicating that this component does not arise along the line of sight to the optical quasi-stellar object (QSO), and that there is structure in the 21-cm absorbing gas on scales smaller than the size of the radio core. We model the 21-cm absorbing gas as having a two-phase structure with cold dense gas randomly distributed within a diffuse envelope of warm gas. For such a model, our radio data indicate that, even if the optical QSO lies along a line of sight with a fortuitously high (∼50 per cent) cold gas fraction, the average cold gas fraction is low, ≲17 per cent, when averaged over the spatial extent of the radio core. Finally, the large mismatch between peak 21-cm and optical redshifts and the complexity of both profiles makes it unlikely that the z ∼ 3.39 DLA will be useful in tests of fundamental constant evolution.     

A statistical study of Galactic SNRs using the PMN survey

The Parkes–MIT–NRAO (PMN) radio survey has been used to generate a quasi all-sky study of Galactic Supernova Remnants (SNRs) at a common frequency of 4.85 GHz (λ=6 cm). We present flux densities estimated for the sample of 110 Southern Galactic SNRs (up to δ=−65°) observed with the Parkes 64-m radio telescope and an additional sample of 54 from the Northern PMN (up to δ=+64°) survey undertaken with the Green Bank 43-m (20 SNRs) and 91-m (34 SNRs) radio telescopes. Out of this total sample of 164 selected SNRs (representing 71% of the currently 231 known SNRs in the Green catalogue) we consider 138 to provide reliable estimates of flux density and surface brightness distribution. This sub-sample represents those SNRs which fall within carefully chosen selection criteria which minimises the effects of the known problems in establishing reliable fluxes from the PMN survey data. Our selection criteria are based on a judicious restriction of source angular size and telescope beam together with careful evaluation of fluxes on a case by case basis. Direct comparison of our new fluxes with independent literature values gives excellent overall agreement. This gives confidence in the newly derived PMN fluxes when the selection criteria are respected. We find a sharp drop off in the flux densities for Galactic SNRs beyond 4 Jy and then a fairly flat distribution from 5 to 9 Jy, a slight decline and a further flat distribution from 9 to 20 Jy though the numbers of SNR in each Jy bin are low. We also re-visit the contentious Σ–D (radio surface brightness–SNRs diameter) relation to determine a new power law index for a sub-sample of shell type SNRs which yields β=−2.2±0.6. This new evaluation of the Σ–D relation, applied to the restricted sample, provides new distance estimates and their Galactic scale height distribution. We find a peak in the SNR distribution between 7–11 kpc with most restricted to ±100 pc Galactic scale height.     

Carbon recombination lines from the Galactic plane at 34.5 & 328 MHz

We present the results of a search for carbon recombination lines in the Galaxy at 34.5 MHz (C575α) made using the dipole array at Gauribidanur near Bangalore. Observations made towards 32 directions resulted in detections of lines, in absorption at nine positions. Followup observations at 328 MHz (C272α) using the Ooty Radio Telescope detected these lines in emission. A VLA D-array observation of one of the positions at 330 MHz yielded no detection implying a lower limit of 10′ for the angular size of the line forming region. The longitude-velocity distribution of the observed carbon lines indicate that the line forming regions are located mainly between 4 kpc and 7 kpc from the Galactic centre. Combining our results with published carbon recombination line data near 76 MHz (Erickson, McConnell & Anantharamaiah 1995), we obtain constraintson the physical parameters of the line forming regions. We find thatif the angular size of the line forming regions is ≥ 4°, then the range of parameters that fit the data are:T _e =20–40 K,n _e ∼ 0.1–0.3 cm⁻³ and pathlengths ∼ 0.07–0.9 pc which may correspond to thin photodissociated regions around molecular clouds. On the other hand, if the line forming regions are ∼ 2° in extent, then warmer gas (T _e ∼ 60–300 K) with lower electron densities (n _e ∼ 0.03–0.05 cm⁻³) extending over several tens of parsecs along the line of sight and possibly associated with atomic HI gas can fit the data. Based on the range of derived parameters, we suggest that the carbon line regions are most likely associated with photo-dissociation regions.     

Does the Milky Way launch a large-scale wind?

The ROSAT All-Sky Survey revealed soft X-ray emission on kiloparsec scales towards the Galactic center. Separately, it has also been observed that the cosmic ray intensity (measured via γ-ray emission) rises only very slowly towards the center of the Galaxy, counter to expectations based on the greater number of cosmic ray sources there. A thermal and cosmic-ray driven wind could potentially explain both of these observations. We find that a cosmic-ray and thermally driven wind fits the X-ray observations well; in fact, a wind fits significantly better than an earlier-proposed static-polytrope gas model.     

The Galactic disc distribution of planetary nebulae with warm dust emission features – I

We investigate the Galactic disc distribution of a sample of planetary nebulae characterized in terms of their mid-infrared spectral features. The total number of Galactic disc PNe with 8–13 μm spectra is brought up to 74 with the inclusion of 24 new objects, the spectra of which we present for the first time. 54 PNe have clearly identified warm dust emission features, and form a sample that we use to construct the distribution of the C/O chemical balance in Galactic disc PNe. The dust emission features complement the information on the progenitor masses brought by the gas-phase N/O ratios: PNe with unidentified infrared emission bands have the highest N/O ratios, while PNe with the silicate signature have either very high N enrichment or close to none. We find a trend for a decreasing proportion of O-rich PNe towards the third and fourth Galactic quadrants. Two independent distance scales confirm that the proportion of O-rich PNe decreases from     per cent inside the solar circle to     per cent outside. PNe with warm dust are also the youngest. PNe with no warm dust are uniformly distributed in C/O and N/O ratios, and do not appear to be confined to     They also have higher 6-cm fluxes, as expected from more evolved PNe. We show that the IRAS fluxes are a good representation of the bolometric flux for compact and IR-bright PNe, which are probably optically thick. Selection of objects with     should probe a good portion of the Galactic disc for these young, dense and compact nebulae, and the dominant selection effects are rooted in the PN catalogues.     

The temperature of the warm neutral medium in the Milky Way

We report high-spectral-resolution Australia Telescope Compact Array (ATCA) H  i 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 towards the two sources, with peak optical depths  τ_max= (1.0 ± 0.08) × 10⁻², (1.4 ± 0.2) × 10⁻³, (2.2 ± 0.5) × 10⁻³  and  (3.4 ± 0.5) × 10⁻³  and kinetic temperatures   T _k= 3127 ± 300, 3694 ± 1595, 3500 ± 1354  and  2165 ± 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.     

The Cygnus X region: XXIII. Is 18P87 galactic or extragalactic?

The radio source 18P87, previously thought to be a point source, has been serendipitously found to be resolved into a core‐jet geometry in VLA maps. H I absorption of continuum emission (in data from the Canadian Galactic Plane Survey) appears in gas with radial velocities >+2 km/s but not in brightly emitting gas at lower radial velocity. Examination of further archival observations at radio, infrared and optical wavelengths suggests that the “obvious” interpretation as a radio galaxy requires a rather unusual object of this kind and a highly unusual local line of sight. We argue that 18P87 may be a Galactic object, a local astrophysical jet. If this is correct it could have arisen from outbursts of a microquasar (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A GLIMPSE of new and possible planetary nebulae at mid-infrared wavelengths

We have undertaken a mid-infrared (MIR) search for new planetary nebulae (PNe) using the Spitzer Space Telescope GLIMPSE Galactic plane survey. This has involved searching extant GLIMPSE data products for morphologically appropriate structures, and investigating sources having IRAS colours similar to those of Galactic PNe. We have found 12 sources which have a high probability of being high-extinction PNe, and which possess MIR and IRAS colours, and shell morphologies similar to those of previously identified Galactic nebulae. Calibrated mapping of these structures and profiles in all four of the IRAC bands (3.6, 4.5, 5.8 and  8.0 μm  ) suggests that many (if not all) of the nebulae possess at least two primary structures: an interior high surface brightness shell, corresponding to what is probably the primary ionized zone, and a much weaker halo extending to very much greater distances from the nucleus. These latter regimes are particularly evident at longer MIR wavelengths (5.8 and  8.0 μm  ), and it is probable that they trace the nebular photodissociative regimes, where emission derives from small-grain continua and/or polycyclic aromatic hydrocarbon molecular bands. This latter behaviour has also been noted in previous analyses of Galactic PNe.     

A model of diffuse Galactic radio emission from 10 MHz to 100 GHz

Understanding diffuse Galactic radio emission is interesting both in its own right and for minimizing foreground contamination of cosmological measurements. cosmic microwave background experiments have focused on frequencies ≳10 GHz, whereas 21-cm tomography of the high-redshift universe will mainly focus on ≲0.2 GHz, for which less is currently known about Galactic emission. Motivated by this, we present a global sky model derived from all publicly available total power large-area radio surveys, digitized with optical character recognition when necessary and compiled into a uniform format, as well as the new Villa Elisa data extending the 1.42-GHz map to the entire sky. We quantify statistical and systematic uncertainties in these surveys by comparing them with various global multifrequency model fits. We find that a principal component based model with only three components can fit the 11 most accurate data sets (at 10, 22, 45 and 408 MHz and 1.42, 2.326, 23, 33, 41, 61, 94 GHz) to an accuracy around 1–10 per cent depending on frequency and sky region. Both our data compilation and our software returning a predicted all-sky map at any frequency from 10 MHz to 100 GHz are publicly available at http://space.mit.edu/home/angelica/gsm .     

Ultra-high-resolution observations of CH in Southern Molecular Cloud envelopes

We present a mini-survey of ultrahigh-resolution spectroscopy (UHRS) of CH towards three southern molecular cloud envelopes. The sightlines are selected to probe physically similar gas in different Galactic environments. With a velocity resolution of ∼0.5 km s⁻¹  ( R =575 000)  these observations resolve most kinematic components of the absorption lines. We do, however, detect one line component in the Lupus region, which is not resolved and for which an upper limit of   b <0.3 km s^-1  is found. We find a correlation between distance of the absorbing gas from the Galactic mid-plane and the fractional abundance of CH. We show that this correlation can be explained as being a result of a fall-off in the ultraviolet radiation field intensity and propose that CH observations in carefully selected sightlines might allow a mapping of the variations in the interstellar radiation field.     

Extragalactic sources towards the central region of the Galaxy

We have observed a sample of 64 small-diameter sources towards the central  −6° < l < 6°, −2° < b < 2°  of the Galaxy with the aim of studying the Faraday rotation measure near the Galactic Centre region. All the sources were observed at 6- and 3.6-cm wavelengths using the ATCA and the VLA. 59 of these sources are inferred to be extragalactic. The observations presented here constitute the first systematic study of the radio polarization properties of the background sources towards this direction and increase the number of known extragalactic radio sources in this part of the sky by almost an order of magnitude. Based on the morphology, spectral indices and lack of polarized emission, we identify four Galactic H  ii regions in the sample.     

High-resolution H i and radio continuum observations of the SNR G290.1−0.8

We have observed the supernova remnant (SNR) G290.1−0.8 in the 21-cm H  i line and the 20-cm radio continuum using the Australia Telescope Compact Array (ATCA). The H  i data were combined with data from the Southern Galactic Plane Survey to recover the shortest spatial frequencies. In contrast, H  i absorption was analysed by filtering extended H  i emission, with spatial frequencies shorter than 1.1 kλ. The low-resolution ATCA radio continuum image of the remnant shows considerable internal structure, resembling a network of filaments across its 13-arcmin diameter. A high-resolution ATCA radio continuum image was also constructed to study the small-scale structure in the SNR. It shows that there are no structures smaller than ∼17 arcsec, except perhaps for a bright knot to the south, which is probably an unrelated object. The H  i absorption study shows that the gas distribution and kinematics in front of SNR G290.1−0.8 are complex. We estimate that the SNR probably lies in the Carina arm, at a distance 7 (±1) kpc. In addition, we have studied nearby sources in the observed field using archival multiwavelength data to determine their characteristics.