We estimate the conversion factor relating CO emission to H 2 mass, α CO , in five Local Group galaxies that span approximately an order of magnitude in metallicity -M 31, M 33, the Large Magellanic Cloud (LMC), NGC 6822, and the Small Magellanic Cloud (SMC). We model the dust mass along the line of sight from infrared (IR) emission and then solve for the α CO that best allows a single gas-to-dust ratio (δ GDR ) to describe each system. This approach remains sensitive to COdark envelopes H 2 surrounding molecular clouds. In M 31, M 33, and the LMC we find α CO ≈ 3-9 M ⊙ pc −2 (K km s −1 ) −1 , consistent with the Milky Way value within the uncertainties. The two lowest metallicity galaxies in our sample, NGC 6822 and the SMC (12 + log(O/H) ≈ 8.2 and 8.0), exhibit a much higher α CO . Our best estimates are α NGC6822 CO ≈ 30 M ⊙ pc −2 (K km s −1 ) −1 and α SMC CO ≈ 70 M ⊙ pc −2 (K km s −1 ) −1 . These results are consistent with the conversion factor becoming a strong function of metallicity around 12 + log(O/H) ∼ 8.4 − 8.2. We favor an interpretation where decreased dust-shielding leads to the dominance of CO-free envelopes around molecular clouds below this metallicity. 11 Hubble Fellow 12 We work with α CO , the conversion from integrated CO intensity to mass of molecular gas. A linear scaling relates α CO

We present the 21‐cm rotation curve of the nearby galaxy M33 out to a galactocentric distance of 16 kpc (13 disc scalelengths). The rotation curve keeps rising out to the last measured point and implies a dark halo mass ≳5×1010 M⊙. The stellar and gaseous discs provide virtually equal contributions to the galaxy gravitational potential at large galactocentric radii, but no obvious correlation is found between the radial distribution of dark matter and the distribution of stars or gas. Results of the best fit to the mass distribution in M33 picture a dark halo which controls the gravitational potential from 3 kpc outward, with a matter density which decreases radially as R−1.3. The density profile is consistent with the theoretical predictions for structure formation in hierarchical clustering cold dark matter (CDM) models, and favours lower mass concentrations than those expected in the standard cosmogony.

In this paper we present new measurements of the gas kinematics in M33 using the CO J=1-0 line. The resulting rotational velocities complement previous 21-cm line data for a very accurate and extended rotation curve. The implied dark matter mass, within the total gaseous extent, is a factor 5 higher than the visible baryonic mass. Dark matter density profiles with an inner cusp as steep as R^{-1}, are compatible with M33 data. The dark matter concentrations required for fitting the M33 rotation curve are very low but marginally consistent with halos forming in a standard Cold Dark Matter cosmology. The M33 virialized dark halo is at least 50 times more massive than the visible baryons and its size is comparable with the M33-M31 separation. Inner cusps as steep as R^{-1.5} are ruled out, while halo models with a large size core of constant density are consistent with the M33 data. A central spheroid of stars is needed and we evaluate its dynamical mass range. Using accurate rotational velocity gradients and the azimuthally averaged baryonic surface densities, we show that a disk instability can regulate the star formation activity in M33. Considering the gaseous surface density alone, the predicted outer star formation threshold radius is consistent with the observed drop of the H-alpha surface brightness if a shear rate criterion is used. The classical Toomre criterion predicts correctly the size of the unstable region only when the stellar or dark halo gravity, derived in this paper, is added to that of the gaseous disk.Comment: 14 pages, MNRAS in pres

We have undertaken a deep, wide-field H I imaging survey of M31, reaching a maximum resolution of about 50 pc and 2 km s −1 across a 95×48 kpc region. The H I mass and brightness sensitivity at 100 pc resolution for a 25 km s −1 wide spectral feature is 1500 M ⊙ and 0.28 K. Our study reveals ubiquitous H I self-opacity features, discernible in the first instance as filamentary local minima in images of the peak H I brightness temperature. Local minima are organized into complexes of more than kpc length and are particularly associated with the leading edge of spiral arm features. Just as in the Galaxy, there is only patchy correspondence of self-opaque features with CO(1-0) emission. We have produced images of the best-fit physical parameters; spin temperature, opacity-corrected column density and non-thermal velocity dispersion, for the brightest spectral feature along each line-of-sight in the M31 disk. Spectroscopically opaque atomic gas is organized into filamentary complexes and isolated clouds down to 100 pc. Localized opacity corrections to the column density exceed an order of magnitude in many cases and add globally to a 30% increase in the atomic gas mass over that inferred from the integrated brightness under the usual assumption of negligible self-opacity. Opaque atomic gas first increases from 20 to 60 K in spin temperature with radius to 12 kpc but then declines again to 20 K beyond 25 kpc. We have extended the resolved star formation law down to physical scales more than an order of magnitude smaller in area and mass than has been possible previously. The relation between total-gas-mass-and star-formation-rate-density is significantly tighter than that with molecularmass and is fully consistent in both slope and normalization with the power law index of 1.56 found in the molecule-dominated disk of M51 at 500 pc resolution. Below a gas-mass-density of about 5 M ⊙ pc −2 , there is a down-turn in star-formation-rate-density which may represent a real local threshold for massive star formation at a cloud mass of about 5·10 4 M ⊙ .

Green Bank Telescope (GBT) λ21cm observations have revealed a faint, yet extensive H I cloud population surrounding the Andromeda Galaxy (M31). The newfound objects are likely analogs to the high-velocity H I clouds (HVCs) seen around the Milky Way. At least 20 discrete features are detected within 50 kpc of the M31 disk, with radial velocities that are comparable to those of outer disk rotation. In addition, a filamentary "halo" component of at least 30 kpc extent is concentrated at the M31 systemic velocity. Some of the discrete features are organized into elongated systems with velocity continuity, suggestive of tidal streams. The discrete population can be characterized by a steep power-law distribution of number versus H I mass in the range between 10 5 and 10 7 M ⊙ . The velocity line-width of discrete clouds is correlated with the cloud H I mass: such that if the clouds are gravitationally bound this implies a dark-to H I mass ratio of ∼ 100:1. Possible origins for the discrete and "halo" M31 features include: a Local Group "cooling flow", tidal debris from recent mergers or interactions, and the gaseous counterparts of low mass dark-matter halos.