Bars are elongated structures that extend from the centre of galaxies, and about one-third of disk galaxies are known to possess bars. These bars are thought to form either through a physical process inherent in galaxies, or through an external process such as galaxy–galaxy interactions. However, there are other plausible mechanisms of bar formation that still need to be observationally tested. Here we present the observational evidence that bars can form via cluster–cluster interaction. We examined 105 galaxy clusters at redshift 0.015 < z < 0.060 that are selected from the Sloan Digital Sky Survey data, and identified 16 interacting clusters. We find that the barred disk-dominated galaxy fraction is about 1.5 times higher in interacting clusters than in clusters with no clear signs of ongoing interaction (42% versus 27%). Our result indicates that bars can form through a large-scale violent phenomenon, and cluster–cluster interaction should be considered an important mechanism of bar formation.
Several millimeter and submillimeter facilities with linear polarization observing capabilities have started operating during the last years. These facilities, as well as other previous millimeter telescopes and interferometers, require bright and stable linear polarization calibrators to calibrate new instruments and to monitor their instrumental polarization. The current limited number of adequate calibrators implies difficulties in the acquisition of these calibration observations.
Looking for additional linear polarization calibrators in the millimeter spectral range, we started monitoring 3C 286 in mid-2006. This source is a standard and highly stable polarization calibrator for radio observations.
Here we present the 3 mm and 1 mm monitoring observations obtained between September 2006 and January 2012 with the XPOL polarimeter on the IRAM 30 m Millimeter Telescope.
Our observations show that 3C 286 is a bright source of constant total flux with 3 mm flux density S3 mm = (0.91 ± 0.02) Jy. The 3 mm linear polarization degree (p3 mm = [13.5 ± 0.3] %) and polarization angle (χ3 mm = [37.3 ± 0.8] °, expressed in the equatorial coordinate system) were also constant during the time span of our observations. Although with poorer time sampling and signal-to-noise ratio, our 1 mm observations of 3C 286 are also reproduced by a constant source of 1 mm flux density (S1 mm = [0.30 ± 0.03] Jy), polarization fraction (p1 mm = [14.4 ± 1.8] %), and polarization angle (χ1 mm = [33.1 ± 5.7] °).
This, together with the previously known compact structure of 3C 286 – extended by ~3.5′′ in the sky – allow us to propose 3C 286 as a new calibrator for both single-dish and interferometric polarization observations at 3 mm, and possibly at shorter wavelengths.
C-type asteroids are among the most pristine objects in the Solar System, but little is known about their interior structure and surface properties. Telescopic thermal infrared observations have so far been interpreted in terms of a regolith-covered surface with low thermal conductivity and particle sizes in the centimetre range. This includes observations of C-type asteroid (162173) Ryugu. However, on arrival of the Hayabusa2 spacecraft at Ryugu, a regolith cover of sand- to pebble-sized particles was found to be absent (R.J. et al., manuscript in preparation). Rather, the surface is largely covered by cobbles and boulders, seemingly incompatible with the remote-sensing infrared observations. Here we report on in situ thermal infrared observations of a boulder on the C-type asteroid Ryugu. We found that the boulder’s thermal inertia was much lower than anticipated based on laboratory measurements of meteorites, and that a surface covered by such low-conductivity boulders would be consistent with remote-sensing observations. Our results furthermore indicate high boulder porosities as well as a low tensile strength in the few hundred kilopascal range. The predicted low tensile strength confirms the suspected observational bias in our meteorite collections, as such asteroidal material would be too frail to survive atmospheric entry.