We have studied the morphology–density relation and morphology–cluster‐centric‐radius relation using a volume‐limited sample (0.05 < z < 0.1, Mr* < −20.5) of the Sloan Digital Sky Survey (SDSS) data. Major improvements compared with previous work are: (i) automated galaxy morphology classification capable of separating galaxies into four types; (ii) three‐dimensional local galaxy density estimation; and (iii) the extension of the morphology–density relation into the field region. We found that the morphology–density and morphology–cluster‐centric‐radius relation in the SDSS data for both of our automated morphological classifiers, Cin and Tauto, as fractions of early‐type galaxies increase and late‐type galaxies decrease toward increasing local galaxy density. In addition, we found that there are two characteristic changes in both the morphology–density and the morphology–radius relations, suggesting that two different mechanisms are responsible for the relations. In the sparsest regions (below 1 Mpc−2 or outside of 1 virial radius), both relations become less noticeable, suggesting that the physical mechanisms responsible for galaxy morphological change require a denser environment. In the intermediate‐density regions (density between 1 and 6 Mpc−2 or virial radius between 0.3 and 1), intermediate‐type fractions increase toward denser regions, whereas late‐disc fractions decrease. Considering that the median size of intermediate‐type galaxies is smaller than that of late‐disc galaxies, we propose that the mechanism is likely to stop star formation in late‐disc galaxies, eventually turning them into intermediate‐type galaxies after their outer discs and spiral arms become invisible as stars die. For example, ram‐pressure stripping is one of the candidate mechanisms. In the densest regions (above 6 Mpc−2 or inside 0.3 virial radii), intermediate‐type fractions decrease radically and early‐type fractions increase in turn. This is a contrasting result to that in intermediate regions and it suggests that yet another mechanism is more responsible for the morphological change in these regions. We also compared the morphology–density relation from the SDSS (0.01 < z < 0.054) with that of the MORPHS data (z ∼ 0.5). Two relations lie on top of each other, suggesting that the morphology–density relation was already established at z∼ 0.5 as in the present Universe. A slight sign of an excess elliptical/S0 fraction in the SDSS data in dense regions might suggest the additional formation of elliptical/S0 galaxies in the cluster core regions between z= 0.5 and 0.05.

Context. AKARI is the first Japanese astronomical satellite dedicated to infrared astronomy. One of the main purposes of AKARI is the all-sky survey performed with six infrared bands between 9 μm and 200 μm during the period from 2006 May 6 to 2007 August 28. In this paper, we present the mid-infrared part (9 μm and 18 μm bands) of the survey carried out with one of the on-board instruments, the infrared camera (IRC). Aims. We present unprecedented observational results of the 9 μm and 18 μm AKARI all-sky survey and detail the operation and data processing leading to the point source detection and measurements. Methods. The raw data are processed to produce small images for every scan, and the point sources candidates are derived above the 5σ noise level per single scan. The celestial coordinates and fluxes of the events are determined statistically and the reliability of their detections is secured through multiple detections of the same source within milli-seconds, hours, and months from each other. Results. The sky coverage is more than 90% for both bands. A total of 877 091 sources (851 189 for 9 μm, 195 893 for 18 μm) are confirmed and included in the current release of the point source catalog. The detection limit for point sources is 50 mJy and 90 mJy for the 9 μm and 18 μm bands, respectively. The position accuracy is estimated to be better than 2 . Uncertainties in the in-flight absolute flux calibration are estimated to be 3% for the 9 μm band and 4% for the 18 μm band. The coordinates and fluxes of detected sources in this survey are also compared with those of the IRAS survey and are found to be statistically consistent.

AKARI, the first Japanese satellite dedicated to infrared astronomy, was launched on 2006 February 21, and started observations in May of the same year. AKARI has a 68.5 cm cooled telescope, together with two focal-plane instruments, which survey the sky in six wavelength bands from mid–to far-infrared. The instruments also have a capability for imaging and spectroscopy in the wavelength range 2-180$\mu$m in the pointed observation mode, occasionally inserted into a continuous survey operation. The in-orbit cryogen lifetime is expected to be one and a half years. The All-Sky Survey will cover more than 90% of the whole sky with a higher spatial resolution and a wider wavelength coverage than that of the previous IRAS all-sky survey. Point-source catalogues of the All-Sky Survey will be released to the astronomical community. Pointed observations will be used for deep surveys of selected sky areas and systematic observations of important astronomical targets. These will become an additional future heritage of this mission.

In previous work on galaxy clusters, several authors reported the discovery of an unusual population of galaxies, which have spiral morphologies, but do not show any star-formation activity. These galaxies are called "passive spirals", and have been interesting since it has been difficult to understand the existence of such galaxies. Using a volume-limited sample (0.05< z <0.1 and Mr * < −20.5; 25813 galaxies) of the Sloan Digital Sky Survey data, we found 73 (0.28±0.03%) passive spiral galaxies and studied their environments. It is found that passive spiral galaxies exist in a local galaxy density of 1-2 Mpc −2 and have a 1-10 cluster-centric virial radius. Thus, the origins of passive spiral galaxies are likely to be cluster-related. These characteristic environments coincide with a previously reported environment where the galaxy star-formation rate suddenly declines and the so-called morphology-density relation turns. It is likely that the same physical mechanism is responsible for all of these observational results. The existence of passive spiral galaxies suggests that a phys-1 ical mechanism that works calmly is preferred to dynamical origins such as major merger/interaction since such a mechanism would destroy the spiral-arm structures. Compared with the observed cluster galaxy evolution such as the Butcher-Oemler effect and the morphological Butcher-Oemler effect, passive spiral galaxies are likely to be a key galaxy population in transition between red, elliptical/S0 galaxies in lowredshift clusters and blue, spiral galaxies more numerous in higher-redshift clusters.