We have carried out a survey of compact star clusters (apparent size 3 ′′ ) in the southwest part of the M31 galaxy, based on the high-resolution Suprime-Cam images (17.5 ′ × 28.5 ′ ), covering ∼15% of the deprojected galaxy disk area. The U BV RI photometry of 285 cluster candidates (V 20.5 mag) was performed using frames of the Local Group Galaxies Survey. The final sample, containing 238 high probability star cluster candidates (typical half-light radius r h ∼ 1.5 pc), was selected by specifying a lower limit of r h 0.15 ′′ ( 0.6 pc). We derived cluster parameters based on the photometric data and multiband images by employing simple stellar population models. The clusters have a wide range of ages from ∼5 Myr (young objects associated with 24 µm and/or Hα emission) to ∼10 Gyr (globular cluster candidates), and possess mass in a range of 3.0 log(m/m ⊙ ) 4.3 peaking at m ∼ 4000 m ⊙ . Typical age of these intermediate-mass clusters is in the range of 30 Myr t 3 Gyr, with a prominent peak at ∼70 Myr. These findings suggest a rich intermediate-mass star cluster population in M31, which appears to be scarce in the Milky Way galaxy.

We present DART-RAY, a new ray-tracing 3D dust radiative transfer (RT) code designed specifically to calculate radiation field energy density (RFED) distributions within dusty galaxy models with arbitrary geometries. In this paper we introduce the basic algorithm implemented in DART-RAY which is based on a pre-calculation of a lower limit for the RFED distribution. This pre-calculation allows us to estimate the extent of regions around the radiation sources within which these sources contribute significantly to the RFED. In this way, ray-tracing calculations can be restricted to take place only within these regions, thus substantially reducing the computational time compared to a complete ray-tracing RT calculation. Anisotropic scattering is included in the code and handled in a similar fashion. Furthermore, the code utilizes a Cartesian adaptive spatial grid and an iterative method has been implemented to optimize the angular densities of the rays originated from each emitting cell. In order to verify the accuracy of the RT calculations performed by DART-RAY, we present results of comparisons with solutions obtained using the DUSTY 1D RT code for a dust shell illuminated by a central point source and existing 2D RT calculations of disc galaxies with diffusely distributed stellar emission and dust opacity. Finally, we show the application of the code on a spiral galaxy model with logarithmic spiral arms in order to measure the effect of the spiral pattern on the attenuation and RFED.

Abstract. We present a preliminary study of a fragment of the radial color profile of the spiral galaxy M 31 in terms of 2-D model accounting for internal extinction in the disk. The two stellar population disk model was assumed. The old dust-free disk population is represented by the double exponential law, and the young disk population, well mixed with the dust, resides in spiral arms of various scale-heights. We find a good agreement among the radial color B-R profiles produced by this simple model and the profile measured around the spiral arm S4 of M 31.

Context. An automatic tool to derive structural parameters of semi-resolved star clusters located in crowded stellar fields in nearby galaxies is needed for homogeneous processing of archival frames. Aims. We have developed a program that automatically derives the structural parameters of star clusters and estimates errors by accounting for individual stars and variable sky background. Methods. Models of observed frames consist of the cluster's surface brightness distribution, convolved with a point spread function; the stars, represented by the same point spread function; and a smoothly variable sky background. The cluster's model is fitted within a large radius by using the Levenberg-Marquardt and Markov chain Monte Carlo algorithms to derive structural parameters, the flux of the cluster, and individual fluxes of all well-resolved stars. Results. FitClust, a program to derive structural parameters of semi-resolved clusters in crowded stellar fields, was developed and is available for free use. The program was tested on simulated cluster frames, and was used to measure clusters of the M31 galaxy in Subaru Suprime-Cam frames. Conclusions. Accounting for bright resolved stars and variable sky background significantly improves the accuracy of derived structural parameters of star clusters. However, their uncertainty remains dominated by the stochastic noise of unresolved stars.