Publications
8 items found page 1 of 1
|
|
| The gas-to-dust mass ratio of Centaurus A as seen by Herschel |
We present photometry of the nearby galaxy NGC 5128 (Centaurus A) observed with the Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) instruments on board the Herschel Space Observatory, at 70, 160, 250, 350 and 500 micron, as well as new CO J=3-2 observations taken with the HARP-B instrument on the James Clerk Maxwell Telescope (JCMT). Using a single-component modified blackbody, we model the dust spectral energy distribution within the disc of the galaxy using all five Herschel wavebands and find dust temperatures of ~30 K towards the centre of the disc and a smoothly decreasing trend to ~20 K with increasing radius. We find a total dust mass of (1.59 ± 0.05) × 107 MSun and a total gas mass of (2.7 ± 0.2) × 109 MSun. The average gas-to-dust mass ratio is 103 ± 8, but we find an interesting increase in this ratio to approximately 275 towards the centre of Cen A. We discuss several possible physical processes that may be causing this effect, including dust sputtering, jet entrainment and systematic variables such as the XCO factor. Dust sputtering by X-rays originating in the active galactic nucleus or the removal of dust by the jets is our most favoured explanation.
Published online on 27 March 2012, to appear in forthcoming issue of MNRAS |
| Publication date: 27 Mar 2012 |
|
|
| Herschel observations of Cen A: stellar heating of two extragalactic dust clouds |
| We present the first results of a multiwavelength survey, incorporating Herschel-inline image, Spitzer, GALEX and Australian Telescope Compact Array (ATCA) observations, of a 1° × 1° field centred on Centaurus A. As well as detecting the inner lobes of the active galactic nucleus (AGN) jet and counterjet, we have found two clouds, bright at submillimetre wavelengths, ~15 kpc from the centre of Cen A that are co-aligned with the jets. Flux measurements at Herschel wavelengths have proved vital in constraining fits to the spectral energy distributions (SEDs). The clouds are well fitted by a single-temperature, modified blackbody spectrum (beta=2) indicating that we are looking at two cold dust clouds on the outskirts of Cen A. The temperature and masses of the clouds are T_north=12.6(+1.1, -1.2) K, T_south=15.1(+1.7, -1.6) K; log(M_north/M_Sun)=5.8(+0.2, -0.2); log(M_south/M_Sun)=5.6(+0.2, -0.2) and the gas-dust ratio for both clouds is ~100. The measured values for the northern dust cloud are consistent with previous measurements from ISO while the southern cloud is a new submillimetre detection. The two dust clouds are located at the termini of the partial HI ring that surrounds Cen A which is also where the gas column density peaks. The Herschel survey encompasses the partial HI ring yet we find no evidence of dust emission in any other part of the ring. Assuming that the gas-dust ratio is the same in the rest of the ring, dust mass upper limits in the HI ring are consistent with low column density dust being present but falling below the SPIRE detection limit. We have discussed the origin of these clouds and various possible heating mechanisms. The observations favour a scenario in which the gas and dust were once part of a late-type galaxy, which has since merged with Cen A. The dominant heating mechanism which adequately explains the observed temperatures in both clouds is heating from the evolved stellar population within Cen A. |
| Publication date: 15 Mar 2012 |
|
|
| New insight into short wavelength solar wind fluctuations from Vlasov theory |
| The nature of solar wind (SW) turbulence below the proton gyroscale is a topic that is investigated extensively nowadays, both theoretically and observationally. Although recent observations gave evidence of the dominance of kinetic Alfvén waves (KAWs) at sub-ion scales with w < w_ci, other studies suggest that the KAW mode cannot carry the turbulence cascade down to electron scales and that the whistler mode (i.e., w > w_ci) is more relevant. Here we study key properties of the short-wavelength plasma modes under limited, but realistic, SW conditions, typically b_i > b_e ~1 and for high oblique angles of propagation 80° d theta_kB d 90° as observed from Cluster spacecraft data. The linear properties of plasma modes under these conditions are poorly known, which contrasts with the well-documented cold plasma limit and/or moderate oblique angles of propagation (theta_kB < 80°). Based on linear solutions of the Vlasov kinetic theory, we discuss the relevance of each plasma mode (fast, Bernstein, KAW, whistler) in carrying the energy cascade down to electron scales. We show, in particular, that the shear Alfvén mode (known in the magnetohydrodynamic limit) extends at scales k-rho_i e1 to frequencies either larger or smaller than w_ci, depending on the anisotropy k_para/k_perp. This extension into small scales is more readily called whistler (w > w_ci) or KAW (w < w_ci), although the mode is essentially the same. This contrasts with the well-accepted idea that the whistler branch always develops as a continuation at high frequencies of the fast magnetosonic mode. We show, furthermore, that the whistler branch is more damped than the KAW one, which makes the latter the more relevant candidate to carry the energy cascade down to electron scales. We discuss how these new findings may facilitate resolution of the controversy concerning the nature of the small-scale turbulence, and we discuss the implications for present and future spacecraft wave measurements in the SW. |
| Publication date: 13 Mar 2012 |
|
|
| The Herschel view of the on-going star formation in the Vela-C molecular cloud |
Aims: As part of the Herschel guaranteed time key programme "HOBYS", we present the PACS and SPIRE photometric survey of the star-forming region Vela-C, one of the nearest sites of low-to-high-mass star formation in the Galactic plane. Our main objectives are to take a census of the cold sources and to derive their mass distribution down to a few solar masses.
Methods: Vela-C was observed with PACS and SPIRE in parallel mode at five wavelengths between 70 micron and 500 micron over an area of about 3 square degrees. A photometric catalogue was extracted from the detections in each of the five bands, using a threshold of 5sigma over the local background. Out of this catalogue we selected a robust sub-sample of 268 sources, of which ~75 per cent are cloud clumps (diameter between 0.05 pc and 0.13 pc) and 25 per cent are cores (diameter between 0.025 pc and 0.05 pc). Their spectral energy distributions (SEDs) were fitted with a modified black body function. We classify 48 sources as protostellar, based on their detection at 70 um or at shorther wavelengths, and 218 as starless, because of non-detections at 70 micron. For two other sources, we do not provide a secure classification, but suggest they are Class 0 protostars.
Results: From the SED fitting we derived key physical parameters (i.e. mass, temperature, bolometric luminosity). Protostellar sources are in general warmer (< T > = 12.8 K) and more compact (< diameter > = 0.040 pc) than starless sources (< T > = 10.3 K, < diameter > = 0.067 pc). Both these findings can be ascribed to the presence of an internal source(s) of moderate heating, which also causes a temperature gradient and hence a more peaked intensity distribution. Moreover, the reduced dimensions of protostellar sources may indicate that they will not fragment further.
The remainder of the abstract is truncated. |
| Publication date: 12 Mar 2012 |
|
|
| Enhanced atmospheric oxygen outflow on Earth and Mars driven by a corotating interaction region |
| Solar wind controls non-thermal escape of planetary atmospheric volatiles, regardless of the strength of planetary magnetic fields. For both Earth with a strong dipole and Mars with weak remnant fields, the oxygen ion (O+) outflow has been separately found to be enhanced during corotating interaction region (CIR) passage. Here we compared the enhancements of O+ outflow on Earth and Mars driven by a CIR in January, 2008 when Sun, Earth and Mars were approximately aligned. The CIR propagation was recorded by STEREO, ACE, Cluster and Mars Express (MEX). During the CIR passage, Cluster observed enhanced flux of upwelling oxygen ions above the Earth's polar region, while MEX detected an increased escape flux of oxygen ions in the Martian magnetosphere. We found that, (1) under a solar wind dynamic pressure increase by 2-3 nPa, the rate of increase in Martian O+ outflow flux was one order higher than those on Earth; (2) as response to the same part of the CIR body, the rate of increase in Martian O+ outflow flux was on the same order as for Earth. The comparison results imply that the dipole effectively prevents coupling of solar wind kinetic energy to planetary ions, and the distance to the Sun is also crucially important for planetary volatile loss in our inner solar system. |
| Publication date: 09 Mar 2012 |
|
|
| First detection of O2 1.27 μm nightglow emission at Mars with OMEGA/MEX and comparison with general circulation model predictions |
| We report the first detection in the atmosphere of Mars of the nightside O2(a1Delta_g) emission at 1.27 µm from limb observations of the OMEGA imaging spectrometer on board Mars Express (MEX). The emission, detected in three cases out of 40 observations, is due to recombination in a downwelling air parcel of O atoms produced by photodissociation of CO2 on the dayside in the upper atmosphere (O + O + M -> O2* + M), and not from ozone UV photodissociation, as is often seen on the dayside. Observed vertical profiles and total retrieved vertical intensities are compared with models. When detected, the emission is 10 times larger than previous predictions, at ~240 kR. This can be explained in the frame of a general circulation model (GCM) of Mars. As predicted by the GCM, all positive observations were obtained at high latitudes, during the winter night. The model is validated, which simulates the large Hadley cell characterizing the meridional circulation, ascending from the summer pole and descending to the winter pole. This new emission is tracing uniquely a downward advection transport mechanism, and therefore its detailed study will provide important constraints on the overall aeronomy and dynamics of Mars. The impact on long-term stability of methane is examined. It is found that recycling through the mesosphere will not decrease significantly the overall lifetime of CH4 (~300 years), because the descent of air is confined to high latitudes and winter seasons. These observations are demonstrating a new diagnosis of the aeronomy and atmospheric dynamics of Mars. |
| Publication date: 09 Mar 2012 |
|
|
| Recent advances in understanding substorm dynamics |
| Magnetospheric substorms are elemental processes of solar wind energy storage and explosive release in Earth's magnetosphere. They encompass fundamental plasma physics questions, are ubiquitous during all types of geomagnetic conditions, contribute significantly to magnetic storms, and are a key element of Space Weather applications. This paper reviews recent major advances enabled by modern multi-point space-based and ground-based platforms. These datasets have also empowered a system-wide perspective and advanced modeling. We particularly highlight progress in two areas: (1) substorm onset timing and evidence for current sheet preconditioning and destabilization and (2) fast flows and dipolarizations, including the role of entropy in magnetotail plasma propagation. |
| Publication date: 06 Mar 2012 |
|
|
| Relativistic iron K X-ray reverberation in NGC 4151 |
| Recent X-ray observations have enabled the study of reverberation delays in active galactic nuclei (AGN) for the first time. All the detections so far are in sources with a strong soft excess, and the measured delay is between the hard (1-3 keV) direct continuum and the soft excess (0.5-1 keV), interpreted as the reflection continuum smeared by relativistic effects. There is however an inherent ambiguity in identifying and studying the details of the lines in the soft excess. Here we report the first detection of reverberation in the iron K band in any AGN. Using XMM-Newton observations of NGC 4151, we find delays of the order of 2000 s on time-scales of 105 s between the 5-6 keV band and the 2-3 and 7-8 keV bands, with a broad lag profile resembling a relativistically broadened iron line. The peak of the lag spectra shifts to lower energies at higher frequencies, consistent with the red wing of the line being emitted at smaller radii, as expected from reflection off the inner accretion disc. This is a first detection of a broad iron line using timing studies. |
| Publication date: 06 Mar 2012 |
|
|
8 items found page 1 of 1
|
