arXiv subjects

N. M. McClure-Griffiths

Publications and source records attributed to N. M. McClure-Griffiths.

179 records · Page 10Linked to original sources

arXiv2004

A Distant Extended Spiral Arm in the Fourth Quadrant of the Milky Way

Using data from the Southern Galactic Plane Survey we present a possible distant spiral arm in the fourth quadrant of the Milky Way. The very distinct and cohesive feature can be traced for over 70 deg as the most extreme positive velocity feature in the longitude-velocity diagram. The feature is at a Galactic radius between 18 and 24 kpc and appears to be the last major structure before the end of the HI disk. We compare the feature with a Galactic spiral model and show that it is well reproduced by a spiral arm of pitch angle, i ~ 9 deg. The arm is quite well confined to the Galactic plane, dropping at most 1 kpc below the Galactic equator. Over most of its length the arm is 1 - 2 kpc thick.

astro-ph↗

arXiv2004

Magnetic Fields and Ionized Gas in the inner Galaxy: An Outer Scale for Turbulence and the Possible Role of HII Regions

We present an analysis of rotation measure (RM) fluctuations from the Test Region of the Southern Galactic Plane Survey (SGPS), along with emission measure (EM) fluctuations in the same field taken from the Southern H-Alpha Sky Survey Atlas. The structure function of RM fluctuations shows a relatively steep slope at small scales (1-5 arcmin), a break in slope to a flatter structure function at intermediate scales (5-60 arcmin), and a systematic variation of the strength of fluctuations as a function of position angle on the sky at the largest scales (60-200 arcmin). The structure function of EM fluctuations shows similar behavior, although the lower resolution of the data prevents detection of a possible break in the spectrum. We interpret the anisotropy in RM/EM structure on large scales as resulting from a large-scale gradient in electron density (and possibly magnetic field) across the region. The break in the slope of the RM structure function at scales of about 5 arcmin can be explained by contributions from two spatially distinct magneto-ionized screens, most likely in the Local and Carina spiral arms. The observed structure function then implies that the outer scale of RM fluctuations in these screens is about 2 pc. We conclude that we have identified an additional source of enhanced turbulence, injected on scales of a few pc, possibly seen only in the Galactic plane. The most likely source of such turbulence is individual HII regions from relatively low-mass stars, whose characteristic scale size is similar to the outer scale of turbulence inferred here. These sources may be the dominant source of density and velocity fluctuations in warm ionized gas in the Galactic plane.

astro-ph↗

arXiv2004

The Complex Neutral Gas Dynamics Of The Dwarf Starburst Galaxy NGC 625

We present new multi-configuration HI aperture synthesis imaging of the nearby dwarf starburst galaxy NGC 625 obtained with the Australia Telescope Compact Array. Total HI column density images show gas well-aligned with the optical major axis, and low-column density HI extending to > 6 optical scale lengths. The HI velocity field, on the other hand, is highly disturbed, with neutral gas at nearly all detected velocities within the central region. After considering various interpretations, we find that a blowout scenario most accurately describes the data. Since at our resolution we do not detect any large evacuated holes in the HI disk, we interpret this blowout to be the result of the extended (both spatially and temporally) star formation event which NGC 625 has undergone in the last 100 Myr. This is one of the clearest examples of HI outflow detected in a dwarf galaxy. We find no obvious external trigger for this extended star formation event. We detect strong radio continuum emission from the largest HII regions; comparing to our HST and ground-based H Alpha fluxes suggests either appreciable amounts of extinction toward the star formation regions, or the contribution of non-thermal sources to the radio continuum luminosity.

astro-ph↗

arXiv2004

GSH23.0-0.7+117, a neutral hydrogen shell in the inner Galaxy

GSH23.0-0.7+117 is a well-defined neutral hydrogen shell discovered in the VLA Galactic Plane Survey (VGPS). Only the blueshifted side of the shell was detected. The expansion velocity and systemic velocity were determined through the systematic behavior of the HI emission with velocity. The center of the shell is at (l,b,v)=(23.05,-0.77,+117 km/s). The angular radius of the shell is 6.8', or 15 pc at a distance of 7.8 kpc. The HI mass divided by the volume of the half-shell implies an average density n_H = 11 +/- 4 cm^{-3} for the medium in which the shell expanded. The estimated age of GSH23.0-0.7+117 is 1 Myr, with an upper limit of 2 Myr. The modest expansion energy of 2 * 10^{48} erg can be provided by the stellar wind of a single O4 to O8 star over the age of the shell. The 3 sigma upper limit to the 1.4 GHz continuum flux density (S_{1.4} < 248 mJy) is used to derive an upper limit to the Lyman continuum luminosity generated inside the shell. This upper limit implies a maximum of one O9 star (O8 to O9.5 taking into account the error in the distance) inside the HI shell, unless most of the incident ionizing flux leaks through the HI shell. To allow this, the shell should be fragmented on scales smaller than the beam (2.3 pc). If the stellar wind bubble is not adiabatic, or the bubble has burst (as suggested by the HI channel maps), agreement between the energy and ionization requirements is even less likely. The limit set by the non-detection in the continuum provides a significant challenge for the interpretation of GSH23.0-0.7+117 as a stellar wind bubble. A similar analysis may be applicable to other Galactic HI shells that have not been detected in the continuum.

astro-ph↗

arXiv2003

Observations of SNR G328.4+0.2 at 19 GHz

We report on the first polarimetric observations at 19 GHz made with the upgraded Australia Telescope Compact Array. Observations were made of the Galactic supernova remnant (SNR) G328.8+0.2. We find the SNR has circular morphology with a strong central bar, similar to that seen at lower frequencies. The SNR has high linear polarization throughout, with fractional polarization in the bar up to 50 per cent. The orientation of the magnetic field lines follow the filamentary structure of the SNR. The magnetic field at the edge of the SNR is generally toroidal, interspersed with radial fingers, likely caused by Rayleigh-Taylor instabilities. Although the SNR has been identified as Crab-like, we prefer an interpretation in which the bar is a pulsar powered wind nebula with the rest of the SNR consisting of the shell. The proposed pulsar parameters make the SNR / pulsar system more like SNR G11.2--0.3 than the Crab Nebula.

astro-ph↗

arXiv2003

Radio Polarimetry in the Southern Galactic Plane Survey

We present radio polarimetric observations of a test region for the Southern Galactic Plane Survey (SGPS), a radio survey in the Galactic plane in HI and polarized continuum at 1.4 GHz with the Australia Telescope Compact Array (ATCA) and the Parkes 64m dish. Ubiquitous structure in linearly polarized intensity is often uncorrelated with structure in total intensity, indicating Faraday rotation and depolarization. The one-dimensional second order structure function of rotation measure for different position angles of the distance lag shows a very shallow slope of the structure function on small scales and an anisotropy in the slope on large scales. The anisotropy on large scales is indicative of a gradient or large filament across the field. The shallow slope can be explained by contributions of two separated Faraday screens. Assuming that the two Faraday screens are the local and Carina spiral arms, the structure function suggests an outer scale of structure in the local arm of about 2 pc. This scale is similar to the typical scale of a Stroemgren sphere of a B1-2 star. Therefore, we suggest that HII regions possibly dominate the scale of structure in spiral arms.

astro-ph↗

arXiv2003

A Neutral Hydrogen Self-Absorption Cloud in the SGPS

Using data from the Southern Galactic Plane Survey (SGPS) we analyze an HI self-absorption cloud centered on l = 318.0 deg, b = -0.5 deg, and velocity, v = -1.1 km/s. The cloud was observed with the Australia Telescope Compact Array (ATCA) and the Parkes Radio Telescope, and is at a near kinematic distance of less than 400 pc with derived dimensions of less than 5 x 11 pc. We apply two different methods to find the optical depth and spin temperature. In both methods we find upper limit spin temperatures ranging from 20 K to 25 K and lower limit optical depths ~ 1. We look into the nature of the HI emission and find that 60-70% originates behind the cloud. We analyze a second cloud at the same velocity centered on l = 319 deg and b = 0.4 deg with an upper limit spin temperature of 20 K and a lower limit optical depth of 1.6. The similarities in spin temperature, optical depth, velocity, and spatial location are evidence the clouds are associated, possibly as one large cloud consisting of smaller clumps of gas. We compare HI emission data with 12CO emission and find a physical association of the HI self-absorption cloud with molecular gas.

astro-ph↗

arXiv2002

Fitting Together the HI Absorption and Emission in the SGPS

In this paper we study 21-cm absorption spectra and the corresponding emission spectra toward bright continuum sources in the test region (326deg< l < 333 deg) of the Southern Galactic Plane Survey. This survey combines the high resolution of the Australia Telescope Compact Array with the full brightness temperature information of the Parkes single dish telescope. In particular, we focus on the abundance and temperature of the cool atomic clouds in the inner galaxy. The resulting mean opacity of the HI, , is measured as a function of Galactic radius; it increases going in from the solar circle, to a peak in the molecular ring of about four times its local value. This suggests that the cool phase is more abundant there, and colder, than it is locally. The distribution of cool phase temperatures is derived in three different ways. The naive, ``spin temperature'' technique overestimates the cloud temperatures, as expected. Using two alternative approaches we get good agreement on a histogram of the cloud temperatures, T(cool), corrected for blending with warm phase gas. The median temperature is about 65 K, but there is a long tail reaching down to temperatures below 20 K. Clouds with temperatures below 40 K are common, though not as common as warmer clouds (40 to 100 K). Using these results we discuss two related quantities, the peak brightness temperature seen in emission surveys, and the incidence of clouds seen in HI self-absorption. Both phenomena match what would be expected based on our measurements of and T(cool).

astro-ph↗

arXiv2002

Polarization angular spectra of Galactic synchrotron emission on arcminute scales

We study the angular power spectra of the polarized component of the Galactic synchrotron emission in the 28-deg^2 Test Region of the Southern Galactic Plane Survey at 1.4 GHz. These data were obtained by the Australia Telescope Compact Array and allow us to investigate angular power spectra down to arcminute scales. We find that, at this frequency, the polarization spectra for E- and B-modes seem to be affected by Faraday rotation produced in compact foreground screens. A different behavior is shown by the angular spectrum of the polarized intensity PI=\sqrt{Q^2+U^2}. This is well fitted by a power law with slope \~1.7, which agrees with higher frequency results and can probably be more confidently extrapolated to the cosmological window.

astro-ph↗

arXiv2002

The Galactic Distribution of Large HI Shells

We report the discovery of nineteen new HI shells in the Southern Galactic Plane Survey (SGPS). These shells, which range in radius from 40 pc to 1 kpc, were found in the low resolution Parkes portion of the SGPS dataset, covering Galactic longitudes l=253 deg to l=358 deg. Here we give the properties of individual shells, including positions, physical dimensions, energetics, masses, and possible associations. We also examine the distribution of these shells in the Milky Way and find that several of the shells are located between the spiral arms of the Galaxy. We offer possible explanations for this effect, in particular that the density gradient away from spiral arms, combined with the many generations of sequential star formation required to create large shells, could lead to a preferential placement of shells on the trailing edges of spiral arms. Spiral density wave theory is used in order to derive the magnitude of the density gradient behind spiral arms. We find that the density gradient away from spiral arms is comparable to that out of the Galactic plane and therefore suggest that this may lead to exaggerated shell expansion away from spiral arms and into interarm regions.

astro-ph↗

arXiv2001

High resolution polarimetry of the inner Galaxy

We present our results from the Southern Galactic Plane Survey, an effort to map the fourth quadrant of the Milky Way in linear polarization at a frequency of 1.4 GHz and at a resolution of 1-2 arcmin. These data are a powerful probe of both the turbulence and large-scale structure of magneto-ionic gas, and have revealed a variety of new features in the interstellar medium.

astro-ph↗

arXiv2001

HI Shells Behind the Coalsack

We report the discovery of two new large HI shells in the direction of the Coalsack nebula. Both shells were observed with the Parkes Radio Telescope as part of the Southern Galactic Plane Survey. The largest shell, GSH 304-00-12, is at a distance of ~1.2 kpc and has derived physical dimensions of 280 x 200 pc. The second shell, GSH 305+01-24, is at a distance of ~2.2 kpc and has derived dimensions of 280 x 440 pc. We present a simple numerical model to show that GSH 305+01-24 most likely formed from stellar winds in the Centaurus OB1 stellar association. There is associated radio, infrared and H-alpha continuum emission. Both shells are situated in the Sagittarius-Carina arm, with GSH 305+01-24 more distant. The far edge of GSH 304-00-12 is at the near side of the arm and opens into the interarm region. We find no evidence for closure at the near side of the shell and therefore describe the geometry as conical. Emission from the near side of the shell may be lost in absorption by the Coalsack Nebula.

astro-ph↗

arXiv2001

Southern Galactic Plane Survey Measurements of the Spatial Power Spectrum of Interstellar H I in the Inner Galaxy

Using data from the Southern Galactic Plane Survey we have measured the spatial power spectrum of the interstellar neutral atomic hydrogen in the fourth Galactic quadrant. This function shows the same power law behavior that has been found for H I in the second quadrant of the Milky Way and in the Magellanic Clouds, with the same slope. When we average over velocity intervals broader than the typical small-scale velocity dispersion, we find that the slope steepens, from approx. -3 to -4 for the warm gas, as predicted by theories of interstellar turbulence if the column density fluctuations are dominated by variations in the gas density on small spatial scales. The cool gas shows a different increase of slope, that suggests that it is in the regime of turbulence dominated by fluctuations in the velocity field. Overall, these results confirm that the small scale structure and motions in the neutral atomic medium are well described by a turbulent cascade of kinetic energy.

astro-ph↗

arXiv2000

HI Emission and Absorption in the Southern Galactic Plane Survey

We present preliminary results from the Southern Galactic Plane Survey (SGPS) Test Region and Parkes data. As part of the pilot project for the Southern Galactic Plane Survey, observations of a Test Region (325.5 deg < l < 333.5 deg; -0.5 deg < b < 3.5 deg) were completed in December 1998. Single dish observations of the full survey region (253 deg < l < 358 deg; |b| <1 deg) with the Parkes Radio Telescope were completed in March 2000. We present a sample of SGPS HI data with particular attention to the smallest and largest scale structures seen in absorption and emission, respectively. On the large scale, we detect many prominent HI shells. On the small scale, we note extremely compact, cold clouds seen in HI self-absorption. We explore how these two classes of objects probe opposite ends of the HI spatial power spectrum.

astro-ph↗

arXiv2000

The Southern Galactic Plane Survey: The Test Region

The Southern Galactic Plane Survey (SGPS) is a project to image the HI line emission and 1.4 GHz continuum in the fourth quadrant of the Milky Way at high resolution using the Australia Telescope Compact Array (ATCA) and the Parkes Radio Telescope. In this paper we describe the survey details and goals, present lambda 21-cm continuum data, and discuss HI absorption and emission characteristics of the SGPS Test Region (325.5 deg < l < 333.5 deg; -0.5 deg < b < +3.5 deg). We explore the effects of massive stars on the interstellar medium (ISM) through a study of HI shells and the HI environments of HII regions and supernova remnants. We find an HI shell surrounding the HII region RCW 94 which indicates that the region is embedded in a molecular cloud. We give lower limits for the kinematic distances to SNRs G327.4+0.4 and G330.2+1.0 of 4.3 kpc and 4.9 kpc, respectively. We find evidence of interaction with the surrounding HI for both of these remnants. We also present images of a possible new SNR G328.6-0.0. Additionally, we have discovered two small HI shells with no counterparts in continuum emission.

astro-ph↗

arXiv2000

Two Large HI Shells in the Outer Galaxy near l=279 degrees

As part of a survey of HI 21-cm emission in the Southern Milky Way, we have detected two large shells in the interstellar neutral hydrogen near l=279 deg. The center velocities are +36 and +59 km/s, which puts the shells at kinematic distances of 7 and 10 kpc. The larger shell is about 610 pc in diameter and very empty, with density contrast of at least 15 between the middle and the shell walls. It has expansion velocity of about 20 km/s and swept up mass of several million solar masses. The energy indicated by the expansion may be as high as 2.4 X 10^53 ergs. We estimate its age to be 15 to 20 million years. The smaller shell has diameter of about 400 pc, expansion velocity about 10 km/s and swept up mass of about 10^6 solar masses. Morphologically both regions appear to be shells, with high density regions mostly surrounding the voids, although the first appears to have channels of low density which connect with the halo above and below the HI layer. They lie on the edge of the Carina arm, which suggests that they may be expanding horizontally into the interarm region as well as vertically out of the disk. If this interpretation is correct, this is the first detection of an HI chimney which has blown out of both sides of the disk.

astro-ph↗

arXiv1997

Diffractive Scintillation of the Pulsar PSR B1259-63

PSR B1259-63 is in a highly eccentric 3.4 yr orbit around the Be-star SS2883. The system is located in the direction of the Sagittarius-Carina spiral arm at a distance of $\sim$1.5 kpc. We have made scintillation observations of the pulsar far from periastron at 4.8 GHz and 8.4 GHz, determining the diffractive bandwidth, $Δν_{\rm d}$, and timescale, $Δt_{\rm d}$, at both frequencies. We find no dependency on orbital phase until within 30 days of periastron. The data indicate that the scintillation is caused, not by the circumstellar environment, but by an HII region within the Sagittarius-Carina spiral arm located at least three-quarters of the way to the pulsar. Close to periastron, when the line-of-sight to the pulsar intersects the disk of the Be star, the electron densities within the disk are sufficient to overcome the `lever-arm effect' and produce a reduction in the scintillation bandwidth by six orders of magnitude.

astro-ph↗