Context. The mechanisms governing the star formation rate in spiral galaxies are not yet clear. The nearby, almost face-on, and interacting galaxy M 51 offers an excellent opportunity to study at high spatial resolutions the local star formation laws.Aims. In this first paper, we investigate the correlation of H 2 , H i, and total gas surface densities with the star forming activity, derived from the radio continuum (RC), along radial averages out to radii of 12 kpc. Methods. We have created a complete map of M 51 in 12 CO 2−1 at a resolution of 450 pc using HERA at the IRAM-30 m telescope.These data are combined with maps of H i and the radio-continuum at 20 cm wavelength. The latter is used to estimate the star formation rate (SFR), thus allowing to study the star formation efficiency and the local Schmidt law Σ SFR ∝ Σ n gas . The velocity dispersion from CO is used to study the critical surface density and the gravitational stability of the disk. Results. The total mass of molecular material derived from the integrated 12 CO 2−1 intensities is 2 × 10 9 M . The 3σ detection limit corresponds to a mass of 1.7 × 10 5 M . The global star formation rate is 2.56 M yr −1 and the global gas depletion time is 0.8 Gyr.H i and RC emission are found to peak on the concave, downstream side of the outer south-western CO arm, outside the corotation radius. The total gas surface density Σ gas drops by a factor of ∼20 from 70 M pc −2 at the center to 3 M pc −2 in the outskirts at radii of 12 kpc. The fraction of atomic gas gradually increases with radius. The ratio of H i over H 2 surface densities, Σ HI /Σ H2 , increases from ∼0.1 near the center to ∼20 in the outskirts without following a simple power-law. Σ HI starts to exceed Σ H2 at a radius of ∼4 kpc. The star formation rate per unit area drops from ∼400 M pc −2 Gyr −1 in the starburst center to ∼2 M pc −2 Gyr −1 in the outskirts. The gas depletion time varies between 0.1 Gyr in the center and 1 Gyr in the outskirts, and is shorter than in other non-interacting normal galaxies. Neither the H i surface densities nor the H 2 surface densities show a simple power-law dependence on the star formation rate per unit area. In contrast, Σ gas and Σ SFR are well characterized by a local Schmidt law with a power-law index of n = 1.4 ± 0.6. The index equals the global Schmidt law derived from disk-averaged values of Σ gas and Σ SFR of large samples of normal and starburst galaxies. The critical gas velocity dispersions needed to stabilize the gas against gravitational collapse in the differentially rotating disk of M 51 using the Toomre criterion, vary with radius between 1.7 and 6.8 km s −1 . Observed radially averaged dispersions derived from the CO data vary between 28 km s −1 in the center and ∼8 km s −1 at radii of 7 to 9 kpc. They exceed the critical dispersions by factors Q gas of 1 to 5. We speculate that the gravitational potential of stars leads to a critically stable disk.

Context. The Carina region is an excellent astrophysical laboratory for studying the feedback mechanisms of newly born, very massive stars within their natal giant molecular clouds (GMCs) at only 2.35 kpc distance. Aims. We use a clumpy PDR model to analyse the observed intensities of atomic carbon and CO and to derive the excitation conditions of the gas.Methods. The NANTEN2-4 m submillimeter telescope was used to map the [C i] 3 P 1 − 3 P 0 , 3 P 2 − 3 P 1 and CO 4-3, 7-6 lines in two 4 × 4 regions of Carina where molecular material interfaces with radiation from the massive star clusters. One region is the northern molecular cloud near the compact OB cluster Tr 14, and the second region is in the molecular cloud south of η Car and Tr 16. These data were combined with 13 CO SEST spectra, HIRES/IRAS 60 µm and 100 µm maps of the FIR continuum, and maps of 8 µm IRAC/Spitzer and MSX emission. Results. We used the HIRES far-infrared dust data to create a map of the FUV field heating the gas. The northern region shows an FUV field of a few 10 3 in Draine units while the field of the southern region is about a factor 10 weaker. While the IRAC 8 µm emission lights up at the edges of the molecular clouds, CO and also [C i] appear to trace the H 2 gas column density. The northern region shows a complex velocity and spatial structure, while the southern region shows an edge-on PDR with a single Gaussian velocity component. We constructed models consisting of an ensemble of small spherically symmetric PDR clumps within the 38 beam (0.43 pc), which follow canonical power-law mass and mass-size distributions. We find that an average local clump density of 2 × 10 5 cm −3 is needed to reproduce the observed line emission at two selected interface positions. Conclusions. Stationary, clumpy PDR models reproduce the observed cooling lines of atomic carbon and CO at two positions in the Carina Nebula.

Context. To date the onset of large-scale star formation in galaxies and its link to gravitational stability of the galactic disk have not been fully understood. The nearby face-on spiral galaxy M 51 is an ideal target for studying this subject.Aims. This paper combines CO, dust, H i, and stellar maps of M 51 and its companion galaxy to study the H 2 /H i transition, the gas-to-dust ratios, and the stability of the disk against gravitational collapse. Methods. We combine maps of the molecular gas using 12 CO 2−1 map HERA/IRAM-30 m data and H i VLA data to study the total gas surface density and the phase transition of atomic to molecular gas. The total gas surface density is compared to the dust surface density from 850 μm SCUBA data. Taking into account the velocity dispersions of the molecular and atomic gas, and the stellar surface densities derived from the 2MASS K-band survey, we derive the total Toomre Q parameter of the disk. Results. The gas surface density Σ gas in the spiral arms is ∼2−3 higher compared to that of the interarm regions. The ratio of molecular to atomic surface density shows a nearly power-law dependence on the hydrostatic pressure P hydro . The Σ gas distribution in M 51 shows an underlying exponential distribution with a scale length of h gas = 7.6 kpc representing 55% of the total gas mass, comparable to the properties of the exponential dust disk. In contrast to the velocity widths observed in H i, the CO velocity dispersion shows enhanced line widths in the spiral arms compared to the interarm regions. The contribution of the stellar component in the Toomre Q-parameter analysis is significant and lowers the combined Q-parameter Q tot by up to 70% towards the threshold for gravitational instability. The value of Q tot varies from 1.5−3 in radial averages. A map of Q tot shows values around 1 on the spiral arms indicating self-regulation at play.

Context. Star formation at earlier cosmological times took place in an interstellar medium with low metallicity. The Large Magellanic Cloud (LMC) is ideally suited to study star formation in such an environment. Aims. The physical and chemical state of the ISM in a star forming environment can be constrained by observations of submm and FIR spectral lines of the main carbon carrying species, CO, C i and C ii, which originate in the surface layers of molecular clouds illuminated by the UV radiation of the newly formed, young stars. Methods. We present high-angular resolution sub-millimeter observations in the N159W region in the LMC obtained with the NANTEN2 telescope of the 12 CO J = 4 → 3, J = 7 → 6, and 13 CO J = 4 → 3 rotational and [C i]3 P 1 − 3 P 0 and 3 P 2 − 3 P 1 finestructure transitions. The 13 CO J = 4 → 3 and [C i] 3 P 2 − 3 P 1 transitions are detected for the first time in the LMC. We derive the physical and chemical properties of the low-metallicity molecular gas using an escape probability code and a self-consistent solution of the chemistry and thermal balance of the gas in the framework of a clumpy cloud PDR model. Results. The separate excitation analysis of the submm CO lines and the carbon fine structure lines shows that the emitting gas in the N159W region has temperatures of about 80 K and densities of about 10 4 cm −3 . The estimated C to CO abundance ratio close to unity is substantially higher than in dense massive star-forming regions in the Milky Way. The analysis of all observed lines together, including the [C ii] line intensity reported in the literature, in the context of a clumpy cloud PDR model constrains the UV intensity to about χ ≈ 220 and an average density of the clump ensemble of about 10 5 cm −3 , thus confirming the presence of high density material in the LMC N159W region.

No abstract