Semi-major and semi-minor axes

In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the longest semidiameter or one half of the major axis, and thus runs from the centre, through a focus, and to the perimeter. The semi-minor axis (minor semiaxis) of an ellipse or hyperbola is a line segment that is at right angles with the semi-major axis and has one end at the center of the conic section. For the special case of a circle, the lengths of the semi-axes are both equal to the radius of the circle. The length of the semi-major axis a of an ellipse is related to the semi-minor axis's length b through the eccentricity e and the semi-latus rectum \ell, as follows: The semi-major axis of a hyperbola is, depending on the convention, plus or minus one half of the distance between the two bra

The Cas A-Like Snr 1E 0102.2-7219 In The Small Magellanic Cloud: An Asymmetric Bipolar Explosion

Frédéric Vogt

We have used the Wide Field Spectrograph on the 2.3 m telescope at Siding Spring Observatory to map the [O III] lambda 5007 dynamics of the young oxygen-rich supernova remnant 1E 0102.2-7219 in the Small Magellanic Cloud. From the resultant data cube, we have been able to reconstruct the full three-dimensional structure of the system of [O III] filaments. From this we are able to deduce that the ejecta trace an asymmetric bipolar structure. The redshifted gas contains fainter clumps but with higher velocities than the blueshifted gas. The major axis of the explosion is inclined at similar to 40 degrees to the line of sight. This structure shows that the supernova explosion has been influenced both by the rotation of the progenitor star and, presumably, by a global rotational instability following core collapse.

2010

Tightening weak lensing constraints on the ellipticity of galaxy-scale dark matter haloes

Anand Stéphane Raichoor

Cosmological simulations predict that galaxies are embedded into triaxial dark matter haloes, which appear approximately elliptical in projection. Weak gravitational lensing allows us to constrain these halo shapes and thereby test the nature of dark matter. Weak lensing has already provided robust detections of the signature of halo flattening at the mass scales of groups and clusters, whereas results for galaxies have been somewhat inconclusive. Here we combine data from five weak lensing surveys (NGVSLenS, KiDS/KV450, CFHTLenS, CS82, and RCSLenS, listed in order of most to least constraining) in order to tighten observational constraints on galaxy-scale halo ellipticity for photometrically selected lens samples. We constrain f(h), the average ratio between the aligned component of the halo ellipticity and the ellipticity of the light distribution, finding f(h) = 0.303(-0.079)(+0.080) f h = 0 . 303 - 0.079 + 0.080 for red lens galaxies and f(h) = 0.217(-0.159)(+0.160) f h 217 - 0.159 + 0.160 for blue lens galaxies when assuming elliptical Navarro-Frenk-White density profiles and a linear scaling between halo ellipticity and galaxy ellipticity. Our constraints for red galaxies constitute the currently most significant (3.8 sigma) systematics-corrected detection of the signature of halo flattening at the mass scale of galaxies. Our results are in good agreement with expectations from the Millennium Simulation that apply the same analysis scheme and incorporate models for galaxy-halo misalignment. Assuming these misalignment models and the analysis assumptions stated above are correct, our measurements imply an average dark matter halo ellipticity for the studied red galaxy samples of |E-h|=0.174 +/- 0.046, where |E-h| = (1-q)/(1+q) relates to the ratio q=b/a of the minor and major axes of the projected mass distribution. Similar measurements based on larger upcoming weak lensing data sets can help to calibrate models for intrinsic galaxy alignments, which constitute an important source of systematic uncertainty in cosmological weak lensing studies.

EDP SCIENCES S A2021

Observations of CO in the eastern filaments of NGC 1275

Yves Revaz

We recently found extended CO(2-1) emission from cold molecular gas embedded in the network of Ha filaments surrounding the galaxy NGC 1275 (Salome et al. 2006). We now present CO(2-1) interferometer maps of the eastern filaments, at high spatial and spectral resolutions. The cold molecular gas is detected by the Plateau de Bure Interferometer along the eastern filaments over an extent of 15'', or with a projected length of 5 kpc. In our 2.5'' beam, the main CO filament is mostly unresolved along its minor axis. The multiple peaks along the CO filaments and the low values of the observed CO brightness temperatures imply further unresolved structures that may be giant molecular clouds. These clouds have very narrow line-width emission lines (similar to 30 km s(-1)). The CO emission is optically thick. It very likely traces cold clouds bound under their own self-gravity that may be falling back in the gravitational potential well of the galaxy. Such a picture would agree with current models of "positive feedback" in which some of the hot gas around NGC 1275 (a) is trapped by buoyantly rising bubbles inflated by the energy input of the 3C 84 AGN, (b) subsequently cools effciently at a larger radius around the edges of the hot bubbles, and (c) then falls back in self-gravitating clouds of molecular gas toward the center of the galaxy.

2008

Tightening weak lensing constraints on the ellipticity of galaxy-scale dark matter haloes

Anand Stéphane Raichoor

EDP SCIENCES S A2021