Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA.

We use new Herschel multiband imaging of the Andromeda galaxy to analyse how dust heating occurs in the central regions of galaxy spheroids that are essentially devoid of young stars. We construct a dust temperature map of M31 through fitting modified blackbodies to the Herschel data, and find that the temperature within 2 kpc rises strongly from the mean value in the disc of 17 ± 1 to ∼35 K at the centre. Ultraviolet (UV) to near-infrared (IR) imaging of the central few kpc shows directly the absence of young stellar populations, delineates the radial profile of the stellar density and demonstrates that even the near-UV dust extinction is optically thin in M31's bulge. This allows the direct calculation of the stellar radiation heating in the bulge, U * (r), as a function of radius. The increasing temperature profile in the centre matches that expected from the stellar heating, i.e. that the dust heating and cooling rates track each other over nearly two orders of magnitude in U * . The modelled dust heating is in excess of the observed dust temperatures, suggesting that it is more than sufficient to explain the observed IR emission. Together with the wavelength-dependent absorption cross-section of the dust, this demonstrates directly that it is the optical, not UV, radiation that sets the heating rate. This analysis shows that neither young stellar populations nor stellar near-UV radiation is necessary to heat dust to warm temperatures in galaxy spheroids. Rather, it is the high densities of Gyr-old stellar populations that provide a sufficiently strong diffuse radiation field to heat the dust. To the extent which these results pertain to the tenuous dust found in the centres of early-type galaxies remains yet to be explored.

We present the Bayesian Extinction And Stellar Tool (BEAST), a probabilistic approach to modeling the dust extinguished photometric spectral energy distribution of an individual star while accounting for observational uncertainties common to large resolved star surveys. Given a set of photometric measurements and an observational uncertainty model, the BEAST infers the physical properties of the stellar source using stellar evolution and atmosphere models and constrains the line of sight extinction using a newly developed mixture model that encompasses the full range of dust extinction curves seen in the Local Group. The BEAST is specifically formulated for use with large multi-band surveys of resolved stellar populations. Our approach accounts for measurement uncertainties and any covariance between them due to stellar crowding (both systematic biases and uncertainties in the bias) and absolute flux calibration, thereby incorporating the full information content of the measurement. We illustrate the accuracy and precision possible with the BEAST using data from the Panchromatic Hubble Andromeda Treasury. While the BEAST has been developed for this survey, it can be easily applied to similar existing and planned resolved star surveys.

The [C ii] 158 µm line is one of the strongest emission lines observed in star-forming galaxies, and has been empirically measured to correlate with the star formation rate (SFR) globally and on ∼ kpc scales. However, due to the multi-phase origins of [C ii], one might expect this relation to break down at small scales. We investigate the origins of [C ii] emission by examining high spatial resolution observations of [C ii] in M 31, with the Survey of Lines in M 31 (SLIM). We present five ∼ 700×700 pc (3 ×3 ) Fields mapping the [C ii] emission, Hα emission, combined with ancillary infrared (IR) data. We spatially separate star-forming regions from diffuse gas and dust emission on ∼ 50 pc scales. We find that the [C ii] -SFR correlation holds even at these scales, although the relation typically has a flatter slope than found at larger (∼ kpc) scales. While the Hα emission in M 31 is concentrated in the SFR regions, we find that a significant amount (∼ 20-90%) of the [C ii] emission comes from outside star-forming regions, and that the total IR (TIR) emission has the highest diffuse fraction of all SFR tracers. We find a weak correlation of the [C ii]/TIR to dust color in each Field, and find a large scale trend of increasing [C ii]/TIR with galactocentric radius. The differences in the relative diffuse fractions of [C ii], Hα and IR tracers are likely caused by a combination of energetic photon leakage from H ii regions and heating by the diffuse radiation field arising from older (B-star) stellar populations. However, we find that by averaging our measurements over ∼ kpc scales, these effects are minimized, and the relation between [C ii] and SFR found in other nearby galaxy studies is retrieved.

We present a [C ii] 158 µm map of the entire M51 (including M51b) grand-design spiral galaxy observed with the FIFI-LS instrument on SOFIA. We compare the [C ii] emission with the total far-infrared (TIR) intensity and star formation rate (SFR) surface density maps (derived using Hα and 24µm emission) to study the relationship between [C ii] and the star formation activity in a variety of environments within M51 on scales of 16 corresponding to ∼660 pc. We find that [C ii] and the SFR surface density are well correlated in the central, spiral arm, and inter-arm regions. The correlation is in good agreement with that found for a larger sample of nearby galaxies at kpc scales. We find that the SFR, and [C ii] and TIR luminosities in M51 are dominated by the Pineda, J.L. et al.extended emission in M51's disk. The companion galaxy M51b, however, shows a deficit of [C ii] emission compared with the TIR emission and SFR surface density, with [C ii] emission detected only in the S-W part of this galaxy. The [C ii] deficit is associated with an enhanced dust temperature in this galaxy. We interpret the faint [C ii] emission in M51b to be a result of suppressed star formation in this galaxy, while the bright midand far-infrared emission, which drive the TIR and SFR values, are powered by other mechanisms. A similar but less pronounced effect is seen at the location of the black hole in M51's center. The observed [C ii] deficit in M51b suggests that this galaxy is a valuable laboratory to study the origin of the apparent [C ii] deficit observed in ultra-luminous galaxies.