A Wide-Field High-Resolution H I Mosaic of Messier 31. I. Opaque Atomic Gas and Star Formation Rate Density

Abstract

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 ⊙ .

Dynamical signatures of a ΛCDM-halo and the distribution of the baryons in M 33

“…At a resolution of 3 kpc and 18 km s −1 , the achieved rms flux sensitivity was 7.2 mJy/beam, corresponding to an HI column density of 2.5 ×10 17 cm −2 in a single channel averaged over the beam. Joint deconvolution of all of the VLA interferometric data was carried out after inclusion of the appropriately scaled A23, page 2 of 18 GBT total power images following the method employed for the reduction of the M 31 data described in detail by Braun et al (2009).…”

A wide-field H I mosaic of Messier 31

“…1 we show the first moment map (i.e. the intensityweighted mean velocity along the line of sight) of our 21-cm Andromeda survey (see Braun et al 2009, for more details). Because of the large angular extent of Andromeda, it is necessary to consider the correct transformation between angular and cartesian coordinates in order to derive galactocentric distances.…”

“…The assignment of a measure of the goodness of the fit in this modeling procedure is done using a χ 2 test on the difference between V obs and V mod . The noise is uniform in our map (see Braun et al 2009) and we assign uniform uncertainties σ i to the observed values V obs . In order to keep the model sensitive to variations of parameters of the outermost rings, each pixel in the map is assigned equal weight.…”

“…The corresponding total HI mass is 5.4 × 10 9 M . To consider the possible presence of opaque clumps we can multiply the HI surface brightness by 1.3 since this is the correction inferred by Braun et al (2009). This correction has however no noticeable effect for the dynamical analysis carried out in the next section.…”

“…Our first aim is to use the new WRST HI survey of M 31 (Braun et al 2009) to define the amplitude and orientation of the warp using a tilted ring model. A set of free rings will be considered for which the following parameters need to be determined: the orbital center, the systemic velocity, the inclination and position angle with respect to our line of sight, and the rotational velocity.…”