A WIDE-FIELD HIGH-RESOLUTION H i MOSAIC OF MESSIER 31. I. OPAQUE ATOMIC GAS AND STAR FORMATION RATE DENSITY

By Robert Braun, D. Thilker, R. Walterbos, E. Corbelli. 2009

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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 nonthermal 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 molecular mass 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 downturn in star formation rate density which may represent a real local threshold for massive star formation at a cloud mass of about 5 × 104 M☉.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/462cc930ee7a7c5a5df5589a09a90d6e9ab6243d

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Citations in the corpus

On the fraction of star formation occurring in bound stellar clusters (2012, background): “The adopted circular velocity is vc = 250 km s−1 (Braun 1991; Chemin et al. 2009), and the gas velocity dispersion is σ g = 8 km s−1 (Braun et al. 2009).”
THE STAR FORMATION RATE AND GAS SURFACE DENSITY RELATION IN THE MILKY WAY: IMPLICATIONS FOR EXTRAGALACTIC STUDIES (2010, background): “These studies have found power law indices of N ≈ 0.8− 1.6 (Kennicutt et al. 2007; Thilker et al. 2007; Bigiel et al. 2008; Braun et al. 2009; Blanc et al. 2009; Verley et al. 2010).”
The CO-to-H2 Conversion Factor (2013, methodology): “…line experiments from our galaxy and others suggest that ∼ 20% HI may be missed due to opacity effects (Heiles & Troland 2003) and observations of M 31 suggest that it may lie in a filamentary, dense component (Braun et al. 2009), making it perhaps a more likely contaminant in XCO determinations.”
Three-Dimensional Dust Radiative Transfer ∗ (2013, methodology): “Panel b shows the complex structure of the ISM in M31 from Hi observations (green; see also Braun et al. 2009), embedded star formation using Spitzer 24-µm data (red; see also Gordon et al. 2006), and unobscured young stars from the GALEX far-UV images (blue; see also Thilker et al. 2005).”
Cloud-scale Molecular Gas Properties in 15 Nearby Galaxies (2018, background): “The gas budget in M31 is dominated by atomic gas at all radii (Braun et al. 2009), while the ISM in M33 may be dominated by molecular gas only within the inner kiloparsec (Druard et al. 2014).”
A wide-field H I mosaic of Messier 31 - II. The disk warp, rotation, and the dark matter halo (2009, methodology): “We determine the rotation curve of the M 31 disk from 8 to 37 kpc using a tilted ring model fit to the velocity field mapped in the full-disk, 21-cm imaging survey of Braun et al. (2009).”
THE CO-TO-H2 CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES (2012, background): “If opaque HI exists at the level Braun et al. (2009) have found in M 31, it would have two main effects on our solutions for αCO.”
HI4PI: a full-sky H i survey based on EBHIS and GASS (2016, background): “(2) provides only a lower limit on NH i (e.g., Radhakrishnan 1960; Gibson et al. 2005; Braun et al. 2009; Martin et al. 2015).”
ANDROMEDA'S DUST (2013, background): “Braun et al. (2009) mapped the H i 21 cm emission out to ∼25 kpc from the center of M31.”