Aims. Stellar mass distribution in the Andromeda galaxy (M 31) is estimated using optical and near-infrared imaging data. Combining the derived stellar mass model with various kinematical data, properties of the dark matter (DM) halo of the galaxy are constrained. Methods. SDSS observations through the ugriz filters and the Spitzer imaging at 3.6 microns are used to sample the spectral energy distribution (SED) of the galaxy at each imaging pixel. Intrinsic dust extinction effects are taken into account by using far-infrared observations. Synthetic SEDs created with different stellar population synthesis models are fitted to the observed SEDs, providing estimates for the stellar mass surface density at each pixel. The stellar mass distribution of the galaxy is described with a 3-dimensional model consisting of a nucleus, a bulge, a disc, a young disc and a halo component, each following the Einasto density distribution (relations between different functional forms of the Einasto density distribution are given in Appendix B). By comparing the stellar mass distribution to the observed rotation curve and kinematics of outer globular clusters and satellite galaxies, the DM halo parameters are estimated. Results. Stellar population synthesis models suggest that M 31 is dominated by old ( > ∼ 7 Gyr) stars throughout the galaxy, with the lower limit for the stellar mass- (0.6−2.3 TeV/c 2 cm −3 ), depending on the stellar mass model. The central density of the DM halo is comparable to that of nearby dwarf galaxies, low-surface-brightness galaxies and distant massive disc galaxies, thus the evolution of central DM halo properties seems to be regulated by similar processes for a broad range of halo masses, environments, and cosmological epochs.
Context. Superclusters of galaxies and their surrounding low-density regions (cocoons) represent dynamically evolving environments in which galaxies and their systems form and evolve. While evolutionary processes of galaxies in dense environments are extensively studied at present, galaxy evolution in low-density regions has received less attention. Aims. We study the properties, connectivity, and galaxy content of groups and filaments in the A2142 supercluster (SCl A2142) cocoon to understand the evolution of the supercluster with its surrounding structures and the galaxies within them. Methods. We calculated the luminosity-density field of SDSS galaxies and traced the SCl A2142 cocoon boundaries by the lowest luminosity-density regions that separate SCl A2142 from other superclusters. We determined galaxy filaments and groups in the cocoon and analysed the connectivity of groups, the high density core (HDC) of the supercluster, and the whole of the supercluster. We compared the distribution and properties of galaxies with different star-formation properties in the supercluster and in the cocoon. Results. The supercluster A2142 and the long filament that is connected to it forms the longest straight structure in the Universe detected so far, with a length of approximately 75 h −1 Mpc. The connectivity of the cluster A2142 and the whole supercluster is C = 6 − 7; poor groups exhibit C = 1 − 2. Long filaments around the supercluster's main body are detached from it at the turnaround region. Among various local and global environmental trends with regard to the properties of galaxies and groups, we find that galaxies with very old stellar populations lie in systems across a wide range of richness from the richest cluster to poorest groups and single galaxies. They lie even at local densities as low as D1 < 1 in the cocoon and up to D1 > 800 in the supercluster. Recently quenched galaxies lie in the cocoon mainly in one region and their properties are different in the cocoon and in the supercluster. The star-formation properties of single galaxies are similar across all environments. Conclusions. The collapsing main body of SCl A2142 with the detached long filaments near it are evidence of an important epoch in the supercluster evolution. There is a need for further studies to explore possible reasons behind the similarities between galaxies with very old stellar populations in extremely different environments, as well as mechanisms for galaxy quenching at very low densities. The presence of long, straight structures in the cosmic web may serve as a test for cosmological models.
Aims. We create a model for recovering the intrinsic, absorption-corrected surface brightness distribution of a galaxy and apply the model to the nearby galaxy M 31. Methods. We constructed a galactic model as a superposition of axially symmetric stellar components and a dust disc to analyse the intrinsic absorption effects. Dust column density is assumed to be proportional to the far-infrared flux of the galaxy. Along each line of sight, the observed far-infrared spectral energy distribution was approximated with modified black body functions corresponding to dust components with different temperatures, thereby allowing us to determine the temperatures and relative column densities of the dust components. We applied the model to the nearby galaxy M 31 using the Spitzer Space Telescope far-infrared observations for mapping dust distribution and temperature. A warm and a cold dust component were distinguished.Results. The temperature of the warm dust in M 31 varies between 56 and 60 K and is highest in the spiral arms, while the temperature of the cold component is mostly 15−19 K and rises up to about 25 K at the centre of the galaxy. The intensity-weighted mean temperature of the dust decreases from T ∼ 32 K in the centre to T ∼ 20 K at R ∼ 7 kpc and outwards. The scalelength of the dust disc is (a 0 ) dust ≈ 1.8 (a 0 ) stars . We also calculated the intrinsic U, B, V, R, I, and L surface brightness distributions and the spatial luminosity distribution. The intrinsic dust extinction in the V-colour rises from 0.25 m at the centre to 0.4 m −0.5 m at R 6−13 kpc and decreases smoothly thereafter. The calculated total extinction-corrected luminosity of M 31 is L B = (3.64 ± 0.15) × 10 10 L , corresponding to an absolute luminosity M B = −20.89 ± 0.04 mag. Of the total B-luminosity, 20% (0.24 mag) is obscured from us by the dust inside M 31. The intrinsic shape of the bulge is slightly prolate in our best-fit model.
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