Spiral structure of M51: Streaming motions across the spiral arms

The atomic hydrogen (HI) and the H alpha emission line in the grand-design spiral galaxy M51 have been observed with the Westerbork Synthesis Radio Telescope and the Taurus Fabry-Perot imaging spectrometer, respectively. Across the inner spiral arms significant tangential and radial velocity gradients are detected in the H alpha emission after subtraction of the axi-symmetric component of the velocity field. The shift is positive on the inside and negative on the outside of the northern arm. Across the southern arm this situation is reversed. The direction of the shifts is such that the material is moving inward and faster compared to circular rotation in both arms, consistent with the velocity perturbations predicted by spiral density wave models for gas downstream of a spiral shock. The observed shifts amount to 20 to 30 km (s-1), corresponding to streaming motions of 60 to 90 km (s-1) in the plane of the disk (inclination angle 20 degrees). Comparable velocity gradients have also been observed by Vogel et al. in the CO emission from the inner northern arm of M51. The streaming motions in M51 are about 2 to 3 times as large as the ones found in HI by Rots in M81, and successfully modelled by Visser with a self-consistent density wave model. Researchers have not been able to detect conclusively streaming motions in the HI emission from the arms, perhaps due to the relatively poor angular resolution (approx. 15 seconds) of the HI observations

Waardoor chirurgen leven

Afscheidsrcolege prof. dr. H.W. Tilanus, hoogleraar heelkunde, Erasmus MC, uitgesproken op 18 oktober 201

Detection of Neutral Carbon in the M 31 Dark Cloud D478

Emission from the 492 GHz CI tranition was detected towards the dark cloud D478 in M31. Using existing 12CO and 13CO measurements, models for the gas properties of D478 are discussed. The observed CO and C line ratios can be explained by two-component models (dense cores and tenuous envelopes); single-density models appear less likely. The models indicate temperatures T(kin) = 10 K. The beam-averaged C column density is 0.3 - 0.8 times that of CO, whereas the total carbon to hydrogen ratio N(C)/N(H) = 5-3 times 10**-4. The resulting CO-to-H2 conversion factor X is about half that of the Solar Neighbourhood. With temperatures of about 10 K and projected mass densities of 5-10 M(sun)/pc**2 there appears to be no need to invoke the presence of very cold and very massive clouds. Rather, D478 appears to be comparable to Milky Way dark cloud complexes such as the Taurus-Auriga dark cloud complex.Comment: 7 Pages, 1 Figure; accepted by A&

Large scale dissociation of molecular gas in the sprial arms of M51

The distribution of the atomic and ionized hydrogen along the inner spiral arms of M51 are compared. As is the case in M83, the location of both these phases of the interstellar medium with respect to the major dust lanes suggests that molecular hydrogen is dissociated on kpc scales in active star-forming regions, and that this dissociation process may strongly affect the observed morphology of atomic hydrogen in spiral arms

The galaxy-wide distributions of mean electron density in the HII regions of M51 and NGC 4449

Using ACS-HST images to yield continuum subtracted photometric maps in H\alpha of the Sbc galaxy M51 and the dwarf irregular galaxy NGC 4449, we produced extensive (over 2000 regions for M51, over 200 regions for NGC4449) catalogues of parameters of their HII regions: their H\alpha luminosities, equivalent radii and coordinates with respect to the galaxy centers. From these data we derived, for each region, its mean luminosity weighted electron density, , determined from the H\alpha luminosity and the radius, R, of the region. Plotting these densities against the radii of the regions we find excellent fits for varying as R^{-1/2}. This relatively simple relation has not, as far as we know, been predicted from models of HII region structure, and should be useful in constraining future models. Plotting the densities against the galactocentric radii, r, of the regions we find good exponential fits, with scale lengths of close to 10 kpc for both galaxies. These values are comparable to the scale lengths of the HI column densities for both galaxies, although their optical structures, related to their stellar components are very different. This result indicates that to a first approximation the HII regions can be considered in pressure equilibrium with their surroundings. We also plot the electron density of the HII regions across the spiral arms of M51, showing an envelope which peaks along the ridge lines of the arms.Comment: Accepted for publication in ApJ Letters. 5 pages, 5 figures. Formatted with emulateap

Automated reduction of submillimetre single-dish heterodyne data from the James Clerk Maxwell Telescope using ORAC-DR

With the advent of modern multi-detector heterodyne instruments that can result in observations generating thousands of spectra per minute it is no longer feasible to reduce these data as individual spectra. We describe the automated data reduction procedure used to generate baselined data cubes from heterodyne data obtained at the James Clerk Maxwell Telescope. The system can automatically detect baseline regions in spectra and automatically determine regridding parameters, all without input from a user. Additionally it can detect and remove spectra suffering from transient interference effects or anomalous baselines. The pipeline is written as a set of recipes using the ORAC-DR pipeline environment with the algorithmic code using Starlink software packages and infrastructure. The algorithms presented here can be applied to other heterodyne array instruments and have been applied to data from historical JCMT heterodyne instrumentation.Comment: 18 pages, 13 figures, submitted to Monthly Notices of the Royal Astronomical Societ