We study the relation between molecular gas and star formation in a volume‐limited sample of 222 galaxies from the COLD GASS survey, with measurements of the CO(1–0) line from the IRAM 30‐m telescope. The galaxies are at redshifts 0.025 < z < 0.05 and have stellar masses in the range 10.0 < log M★/M⊙ < 11.5. The IRAM measurements are complemented by deep Arecibo H i observations and homogeneous Sloan Digital Sky Survey and GALEX photometry. A reference sample that includes both ultraviolet (UV) and far‐infrared data is used to calibrate our estimates of star formation rates from the seven optical/UV bands. The mean molecular gas depletion time‐scale [] for all the galaxies in our sample is 1 Gyr; however,  increases by a factor of 6 from a value of ∼0.5 Gyr for galaxies with stellar masses of ∼1010 M⊙ to ∼3 Gyr for galaxies with masses of a few ×1011 M⊙. In contrast, the atomic gas depletion time‐scale remains constant at a value of around 3 Gyr. This implies that in high‐mass galaxies, molecular and atomic gas depletion time‐scales are comparable, but in low‐mass galaxies, the molecular gas is being consumed much more quickly than the atomic gas. The strongest dependences of  are on the stellar mass of the galaxy [parametrized as ], and on the specific star formation rate (sSFR). A single  versus sSFR relation is able to fit both ‘normal’ star‐forming galaxies in our COLD GASS sample and more extreme starburst galaxies (luminous infrared galaxies and ultraluminous infrared galaxies), which have  yr. Normal galaxies at z = 1–2 are displaced with respect to the local galaxy population in the  versus sSFR plane and have molecular gas depletion times that are a factor of 3–5 times longer at a given value of sSFR due to their significantly larger gas fractions.

We are conducting COLD GASS, a legacy survey for molecular gas in nearby galaxies. Using the IRAM 30‐m telescope, we measure the CO(1−0) line in a sample of ∼350 nearby ( Mpc), massive galaxies (log(M*/M⊙) > 10.0). The sample is selected purely according to stellar mass, and therefore provides an unbiased view of molecular gas in these systems. By combining the IRAM data with Sloan Digital Sky Survey (SDSS) photometry and spectroscopy, GALEX imaging and high‐quality Arecibo H i data, we investigate the partition of condensed baryons between stars, atomic gas and molecular gas in 0.1–10L* galaxies. In this paper, we present CO luminosities and molecular hydrogen masses for the first 222 galaxies. The overall CO detection rate is 54 per cent, but our survey also uncovers the existence of sharp thresholds in galaxy structural parameters such as stellar mass surface density and concentration index, below which all galaxies have a measurable cold gas component but above which the detection rate of the CO line drops suddenly. The mean molecular gas fraction  of the CO detections is 0.066 ± 0.039, and this fraction does not depend on stellar mass, but is a strong function of (NUV − r) colour. Through stacking, we set a firm upper limit of  for red galaxies with NUV − r > 5.0. The average molecular‐to‐atomic hydrogen ratio in present‐day galaxies is 0.3, with significant scatter from one galaxy to the next. The existence of strong detection thresholds in both the H i and CO lines suggests that ‘quenching’ processes have occurred in these systems. Intriguingly, atomic gas strongly dominates in the minority of galaxies with significant cold gas that lie above these thresholds. This suggests that some re‐accretion of gas may still be possible following the quenching event.

Power spectra of deprojected images of late-type galaxies in gas or dust emission are very useful diagnostics of the dynamics and stability of their interstellar medium. Previous studies have shown that the power spectra can be approximated as two power laws, a shallow one on large scales (larger than 500 pc) and a steeper one on small scales, with the break between the two corresponding to the line-of-sight thickness of the galaxy disk. The break separates the 3D behavior of the interstellar medium on small scales, controlled by star formation and feedback, from the 2D behavior on large scales, driven by density waves in the disk. The break between these two regimes depends on the thickness of the plane, which is determined by the natural self-gravitating scale of the interstellar medium. We present a thorough analysis of the power spectra of the dust and gas emission at several wavelengths in the nearby galaxy M 33. In particular, we use the recently obtained images at five wavelengths by PACS and SPIRE onboard Herschel. The wide dynamical range (2-3 dex in scale) of most images allows us to clearly determine the change in slopes from −1.5 to −4, with some variations with wavelength. The break scale increases with wavelength from 100 pc at 24 and 100 μm to 350 pc at 500 μm, suggesting that the cool dust lies in a thicker disk than the warm dust, perhaps because of star formation that is more confined to the plane. The slope on small scales tends to be steeper at longer wavelength, meaning that the warmer dust is more concentrated in clumps. Numerical simulations of an isolated late-type galaxy, rich in gas and with no bulge, such as M 33, are carried out to better interpret these observed results. Varying the star formation and feedback parameters, it is possible to obtain a range of power spectra, with two power-law slopes and breaks, that nicely bracket the data. The small-scale power-law does indeed reflect the 3D behavior of the gas layer, steepening strongly while the feedback smoothes the structures by increasing the gas turbulence. M 33 appears to correspond to a fiducial model with an SFR of ∼0.7 M /yr, with 10% supernovae energy coupled to the gas kinematics.

In the framework of the open-time key program "Herschel M 33 extended survey (HerM33es)", we study the far-infrared emission from the nearby spiral galaxy M 33 in order to investigate the physical properties of the dust such as its temperature and luminosity density across the galaxy. Taking advantage of the unique wavelength coverage (100, 160, 250, 350, and 500 μm) of the Herschel Space Observatory and complementing our dataset with Spitzer-IRAC 5.8 and 8 μm and Spitzer-MIPS 24 and 70 μm data, we construct temperature and luminosity density maps by fitting two modified blackbodies of a fixed emissivity index of 1.5. We find that the "cool" dust grains are heated to temperatures of between 11 K and 28 K, with the lowest temperatures being found in the outskirts of the galaxy and the highest ones both at the center and in the bright HII regions. The infrared/submillimeter total luminosity (5-1000 μm) is estimated to be 1.9 × 10 9 +4.0×10 8 −4.4×10 8 L . Fifty-nine percent of the total infrared/submillimeter luminosity of the galaxy is produced by the "cool" dust grains (∼15 K), while the remaining 41% is produced by "warm" dust grains (∼55 K). The ratio of the cool-to-warm dust luminosity is close to unity (within the computed uncertainties), throughout the galaxy, with the luminosity of the cool dust being slightly higher at the center than the outer parts of the galaxy. Decomposing the emission of the dust into two components (one emitted by the diffuse disk of the galaxy and one emitted by the spiral arms), we find that the fraction of the emission from the disk in the mid-infrared (24 μm) is 21%, while it gradually rises up to 57% in the submillimeter (500 μm). We find that the bulk of the luminosity comes from the spiral arm network that produces 70% of the total luminosity of the galaxy with the rest coming from the diffuse dust disk. The "cool" dust inside the disk is heated to temperatures in a narrow range between 18 K and 15 K (going from the center to the outer parts of the galaxy).

We unveil the stellar wind–driven shell of the luminous massive star-forming region of RCW 49 using SOFIA FEEDBACK observations of the [C ii] 158 μm line. The complementary data set of the 12CO and 13CO J = 3 → 2 transitions is observed by the APEX telescope and probes the dense gas toward RCW 49. Using the spatial and spectral resolution provided by the SOFIA and APEX telescopes, we disentangle the shell from a complex set of individual components of gas centered around RCW 49. We find that the shell of radius ∼6 pc is expanding at a velocity of 13 km s−1 toward the observer. Comparing our observed data with the ancillary data at X-ray, infrared, submillimeter, and radio wavelengths, we investigate the morphology of the region. The shell has a well-defined eastern arc, while the western side is blown open and venting plasma further into the west. Though the stellar cluster, which is ∼2 Myr old, gave rise to the shell, it only gained momentum relatively recently, as we calculate the shell’s expansion lifetime of ∼0.27 Myr, making the Wolf–Rayet star WR 20a a likely candidate responsible for the shell’s reacceleration.